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MAGNETS

DESIGN GUIDE

1.0 IntroductionMagnets are an important part of our daily lives, serving as essential components in everything fromelectric motors, loudspeakers, computers, compact disc players, microwave ovens and the family ca

to instrumentation, production equipment, and research. Their contribution is often overlookedbecause they are built into devices and are usually out of sight.Magnets function as transducers, transforming energy from one form to another, without anypermanent loss of their own energy. General categories of permanent magnet functions are:

Mechanical to mechanical- such as attraction and repulsion.

Mechanical to electrical- such as generators and microphones.

Electrical to mechanical- such as motors, loudspeakers, charged particle deflection.

Mechanical to heat- such as eddy current and hysteresis torque devices.

Special effects- such as magneto resistance, Hall effect devices, and magnetic resonance.

The following sections will provide a brief insight into the design and application of permanentmagnets. The Design Engineering team at Magnet Sales & Manufacturing will be happy to assist youfurther in your applications.2.0 Modern Magnet MaterialsThere are four classes of modern commercialized magnets, each based on their material compositioWithin each class is a family of grades with their own magnetic properties. These general classes ar

Neodymium Iron Boron

Samarium Cobalt

Ceramic

Alnico

NdFeB and SmCo are collectively known as Rare Earth magnets because they are both composed ofmaterials from the Rare Earth group of elements. Neodymium Iron Boron (general compositionNd2Fe14B, often abbreviated to NdFeB) is the most recent commercial addition to the family ofmodern magnet materials. At room temperatures, NdFeB magnets exhibit the highest properties ofall magnet materials. Samarium Cobalt is manufactured in two compositions: Sm1Co5and Sm2Co17often referred to as the SmCo 1:5 or SmCo 2:17 types. 2:17 types, with higher Hcivalues, offergreater inherent stability than the 1:5 types. Ceramic, also known as Ferrite, magnets (generalcomposition BaFe2O3 or SrFe2O3) have been commercialized since the 1950s and continue to beextensively used today due to their low cost. A special form of Ceramic magnet is "Flexible" materiamade by bonding Ceramic powder in a flexible binder. Alnico magnets (general composition Al-Ni-Cwere commercialized in the 1930s and are still extensively used today.These materials span a range of properties that accommodate a wide variety of applicationrequirements. The following is intended to give a broad but practical overview of factors that must bconsidered in selecting the proper material, grade, shape, and size of magnet for a specificapplication. The chart below shows typical values of the key characteristics for selected grades ofvarious materials for comparison. These values will be discussed in detail in the following sections.

Table 2.1 Magnet Material Comparisons


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Material Grade Br Hc Hci BHmax Tmax(Deg C)*

NdFeB 39H 12,800 12,300 21,000 40 150

SmCo 26 10,500 9,200 10,000 26 300

NdFeB B10N 6,800 5,780 10,300 10 150

Alnico 5 12,500 640 640 5.5 540

Ceramic 8 3,900 3,200 3,250 3.5 300

Flexible 1 1,600 1,370 1,380 0.6 100

* Tmax(maximum practical operating temperature)is for reference only. The maximum practical operating temperature ofany magnet is dependent on the circuit the magnet is operating in.

3.0 Units of MeasureThree systems of units of measure are common: the cgs (centimeter, gram,second), SI (meter, kilogram, second), and English (inch, pound, second)systems. This catalog uses the cgs system for magnetic units, unlessotherwise specified.

Table 3.1 Units of Measure Systems

Unit Symbol cgs System SI System English System

Flux maxwell weber maxwell

Flux Density B gauss tesla lines/in2

Magnetomotive Force F gilbert ampere turn ampere turn

Magnetizing Force H oersted ampere turns/m ampere turns/in

Length L cm m in

Permeability of a vacuum v 1 0.4 x 10-6 3.192

Table 3.2Conversion Factors

Multiply By To obtain

inches 2.54 centimeters

lines/in2 0.155 Gauss

lines/in2 1.55 x 10-5 Tesla

Gauss 6.45 lines/in2

Gauss 0-4 Tesla

Gilberts 0.79577 ampere turns

Oersteds 79.577 ampere turns /m

ampere turns 0.4 Gilberts

ampere turns/in 0.495 Oersteds

ampere turns/in 39.37 ampere turns/m

Click here for an interactive version of this conversion table.

4.0 Design ConsiderationsBasic problems of permanent magnet design revolve around estimating thedistribution of magnetic flux in a magnetic circuit, which may includepermanent magnets, air gaps, high permeability conduction elements, andelectrical currents. Exact solutions of magnetic fields require complexanalysis of many factors, although approximate solutions are possible basedon certain simplifying assumptions. Obtaining an optimum magnet design
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often involves experience and tradeoffs.

4.1 Finite Element AnalysisFinite Element Analysis (FEA) modeling programs are used to analyzemagnetic problems in order to arrive at more exact solutions, which can thenbe tested and fine tuned against a prototype of the magnet structure. Using

FEA models flux densities, torques, and forces may be calculated. Resultscan be output in various forms, including plots of vector magnetic potentials,flux density maps, and flux path plots. The Design Engineering team atMagnet Sales & Manufacturing has extensive experience in many types ofmagnetic designs and is able to assist in the design and execution of FEAmodels.

4.2 The B-H CurveThe basis of magnet design is the B-H curve, or hysteresis loop, whichcharacterizes each magnet material. This curve describes the cycling of amagnet in a closed circuit as it is brought to saturation, demagnetized,saturated in the opposite direction, and then demagnetized again under theinfluence of an external magnetic field.

The second quadrant of the B-H curve, commonly referred to as the"Demagnetization Curve", describes the conditions under which permanentmagnets are used in practice. A permanent magnet will have a unique, staticoperating point if air-gap dimensions are fixed and if any adjacent fields areheld constant. Otherwise, the operating point will move about thedemagnetization curve, the manner of which must be accounted for in thedesign of the device.The three most important characteristics of the B-H curve are the points atwhich it intersects the B and H axes (at Br - the residual induction - and Hc-

the coercive force - respectively), and the point at which the product of Band H are at a maximum (BHmax- the maximum energyproduct).Brrepresents the maximum flux the magnet is able to produceunder closed circuit conditions. In actual useful operation permanentmagnets can only approach this point. Hcrepresents the point at which themagnet becomes demagnetized under the influence of an externally appliedmagnetic field. BHmaxrepresents the point at which the product of B and H,and the energy density of the magnetic field into the air gap surrounding themagnet, is at a maximum. The higher this product, the smaller need be the


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volume of the magnet. Designs should also account for the variation of theB-H curve with temperature. This effect is more closely examined in thesection entitled "Permanent Magnet Stability".

When plotting a B-H curve, the value of B is obtained by measuring the totalflux in the magnet ()and then dividing this by the magnet pole area (A) toobtain the flux density (B=/A). The total flux is composed of the fluxproduced in the magnet by the magnetizing field (H), and the intrinsic abilityof the magnet material to produce more flux due to the orientation of thedomains. The flux density of the magnet is therefore composed of twocomponents, one equal to the applied H, and the other created by theintrinsic ability of ferromagnetic materials to produce flux. The intrinsic fluxdensity is given the symbol Biwhere total flux B = H + Bi, or, Bi= B - H. Innormal operating conditions, no external magnetizing field is present, andthe magnet operates in the second quadrant, where H has a negative value.Although strictly negative, H is usually referred to as a positive number, andtherefore, in normal practice, Bi= B + H. It is possible to plot an intrinsic aswell as a normal B-H curve. The point at which the intrinsic curve crosses theH axis is the intrinsic coercive force, and is given the symbol Hci. HighHcivalues are an indicator of inherent stability of the magnet material. Thenormal curve can be derived from the intrinsic curve and vice versa. Inpractice, if a magnet is operated in a static manner with no external fieldspresent, the normal curve is sufficient for design purposes. When external

fields are present, the normal and intrinsic curves are used to determine thechanges in the intrinsic properties of the material.

4.3 Magnet CalculationsIn the absence of any coil excitation, the magnet length and pole area maybe determined by the following equations:

Equation 1and

Equation 2

where Bm= the flux density at the operating point,Hm= the magnetizing force at the operating point,Ag, = the air-gap area,Lg= the air-gap length,Bg= the gap flux density,Am= the magnet pole area,
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The permeance coefficient method using the demagnetization curves allowsfor initial selection of magnet material, based upon the space available in thedevice, this determining allowable magnet dimensions.

4.3.1 Calculation Of Flux Density On A Magnet's Central Line

Click here to calculate flux density of rectangular or cylindrical magnets invarious configurations (equations 4 through 9).For magnet materials with straight-line normal demagnetization curves suchas Rare Earths and Ceramics, it is possible to calculate with reasonableaccuracy the flux density at a distance X from the pole surface (where X>0)on the magnet's centerline under a variety of conditions.a. Cylindrical Magnets

Equation 4

Table 4.1 shows flux density calculations for a magnet 0.500" in diameter by0.250" long at a distance of 0.050" from the pole surface, for variousmaterials. Note that you may use any unit of measure for dimensions; sincethe equation is a ratio of dimensions, the result is the same using any unitsystem. The resultant flux density is in units of gauss.

Table 4.1 Flux Density vs. Material

Material and Grade Residual Flux Density, Br Flux at distance of 0.050" from surface of magnet

Ceramic 1 2,200 629

Ceramic 5 3,950 1,130

SmCo 18 8,600 2,460

SmCo 26 10,500 3,004

NdFeB 35 12,300 3,518

NdFeB 42H 13,300 3,804

b. Rectangular Magnets

Equation 5(where all angles are in radians)
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c. For Ring Shaped Magnets

Equation 6

d. For a Magnet on a Steel Back plateEquation 7Substitute 2L for L in the above formulae.

e. For Identical Magnets Facing Each Other in Attracting PositionsEquation 8The value of Bx at the gap center is double the value of Bx in case 3. At a pointP, Bpis the sum of B(x-p) and B(x-p), where (X+P) and (X-P) substitute for X in case 3.

f. For Identical, Yoked Magnets Facing Each Other in Attracting PositionsEquation 9Substitute 2L for L in case 4, and adopt the same procedure to calculate Bp.

4.3.2 Force CalculationsThe attractive force exerted by a magnet to a ferromagnetic material may be calculated by:

Equation 10

where F is the force in pounds, B is the flux density in Kilogauss, and A is the pole area insquare inches. Calculating B is a complicated task if it is to be done in a rigorous manner.However, it is possible to approximate the holding force of certain magnets in contact with


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a piece of steel using the relationship:

Equation 11

where Bris the residual flux density of the material, A is the pole area in square inches, andLmis the magnetic length (also in inches).

This formula is only intended to give an order of magnitude for the holding force that isavailable from a magnet with one pole in direct contact with a flat, machined, steel surface.The formula can only be used with straight-line demagnetization curve materials - i.e. forrare earth and ceramic materials - and where the magnet length, Lm, is kept within thebounds of normal, standard magnet configurations.

5.0 Permanent Magnet StabilityThe ability of a permanent magnet to support an external magnetic field results fromsmall magnetic domains "locked" in position by crystal anisotropy within the magnetmaterial. Once established by initial magnetization, these positions are held until actedupon by forces exceeding those that lock the domains. The energy required to disturb themagnetic field produced by a magnet varies for each type of material. Permanent

magnets can be produced with extremely high coercive forces (Hc) that will maintaindomain alignment in the presence of high external magnetic fields. Stability can bedescribed as the repeated magnetic performance of a material under specific conditionsover the life of the magnet.Factors affecting magnet stability include time, temperature, reluctance changes, adversefields, radiation, shock, stress, and vibration.5.1 TimeThe effect of time on modern permanent magnets is minimal. Studies have shown thatpermanent magnets will see changes immediately after magnetization. These changes,known as "magnetic creep", occur as less stable domains are affected by fluctuations inthermal or magnetic energy, even in a thermally stable environment. This variation isreduced as the number of unstable domains decreases. Rare Earth magnets are not aslikely to experience this effect because of their extremely high coercivities. Long-term

time versus flux studies have shown that a newly magnetized magnet will lose a minorpercent of its flux as a function of age. Over 100,000 hours, these losses are in the rangeof essentially zero for Samarium Cobalt materials to less than 3% for Alnico 5 materialsat low permeance coefficients.5.2 TemperatureTemperature effects fall into three categories:

Reversible losses.

Irreversible but recoverable losses.

Irreversible and unrecoverable losses.

5.2.1. Reversible losses.These are losses that are recovered when the magnet returns to its original temperature.Reversible losses cannot be eliminated by magnet stabilization. Reversible losses aredescribed by the Reversible Temperature Coefficients (Tc), shown in table 5.1. Tcisexpressed as % per degree Centigrade. These figures vary for specific grades of eachmaterial but are representative of the class of material as a whole. It is because thetemperature coefficients of Brand Hcare significantly different that the demagnetizationcurve develops a "knee" at elevated temperatures.


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Table 5.1 Reversible Temperature Coefficients of Brand Hc

Material Tc of Br Tc of Hc

NdFeB -0.12 -0.6

SmCo -0.04 -0.3

Alnico -0.02 0.01

Ceramic -0.2 0.3

5.2.2. Irreversible but recoverable losses.These losses are defined as partial demagnetization of the magnet fromexposure to high or low temperatures. These losses are only recoverable by

remagnetization, and are not recovered when the temperature returns to itsoriginal value. These losses occur when the operating point of the magnetfalls below the knee of the demagnetization curve. An efficient permanentmagnet design should have a magnetic circuit in which the magnet operatesat a permeance coefficient above the knee of the demagnetization curve atexpected elevated temperatures. This will prevent performance variations atelevated temperatures.5.2.3. Irreversible and unrecoverable losses.Metallurgical changes occur in magnets exposed to very high temperaturesand are not recoverable by remagnetization. Table 5.2 shows criticaltemperatures for the various materials, where

TCurieis the Curie temperature at which the elementary magneticmoments are randomized and the material is demagnetized; and

Tmaxis the maximum practical operating temperatures for general classesof major materials. Different grades of each material exhibit valuesdiffering slightly from the values shown here.

Table 5.2 Critical Temperatures for Various Materials

Material TCurie Tmax*

Neodymium Iron Boron 310 (590) 150 (302)

Samarium Cobalt 750 (1382) 300 (572)

Alnico 860 (1580) 540 (1004)

Ceramic

460 (860)

300 (572)

(Temperatures are shown in degrees Centigrade with the Fahrenheit equivalent in parentheses.)


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*Note that the maximum practical operating temperature is dependent onthe operating point of the magnet in the circuit. The higher the operatingpoint on the Demagnetization Curve, the higher the temperature at whichthe magnet may operate.Flexible materials are not included in this table since the binders that are

used to render the magnet flexible break down before metallurgical changesoccur in the magnetic ferrite powder that provides flexible magnets with theirmagnetic properties.Partially demagnetizing a magnet by exposure to elevated temperatures in acontrolled manner stabilizes the magnet with respect to temperature. Theslight reduction in flux density improves a magnets stability becausedomains with low commitment to orientation are the first to lose theirorientation. A magnet thus stabilized will exhibit constant flux when exposedto equivalent or lesser temperatures. Moreover, a batch of stabilizedmagnets will exhibit lower variation of flux when compared to each othersince the high end of the bell curve which characterizes normal variation willbe brought in closer to the rest of the batch.5.3 Reluctance ChangesThese changes occur when a magnet is subjected to permeance changessuch as changes in air gap dimensions during operation. These changes willchange the reluctance of the circuit, and may cause the magnet's operatingpoint to fall below the knee of the curve, causing partial and/or irreversiblelosses. The extents of these losses depend upon the material properties andthe extent of the permeance change. Stabilization may be achieved by pre-exposure of the magnet to the expected reluctance changes.5.4 Adverse FieldsExternal magnetic fields in repulsion modes will produce a demagnetizingeffect on permanent magnets. Rare Earth magnets with coercive forcesexceeding 15 KOe are difficult to affect in this manner. However, Alnico 5,with a coercive force of 640 Oe will encounter magnetic losses in the

presence of any magnetic repelling force, including similar magnets.Applications involving Ceramic magnets with coercive forces ofapproximately 4KOe should be carefully evaluated in order to assess theeffect of external magnetic fields.5.5 RadiationRare Earth materials are commonly used in charged particle beam deflectionapplications, and it is necessary to account for possible radiation effects onmagnetic properties. Studies (A.F. Zeller and J.A. Nolen, NationalSuperconducting Cyclotron Laboratory, 09/87, and E.W. Blackmore, TRIUMF,1985) have shown that SmCo and especially Sm2Co17withstand radiation 2to 40 times better than NdFeB materials. SmCo exhibits significantdemagnetization when irradiated with a proton beam of 109to 1010rads.NdFeB test samples were shown to lose all of their magnetization at a dose

of 7 x 107rads, and 50% at a dose of 4 x 106rads. In general, it isrecommended that magnet materials with high Hcivalues be used inradiation environments, that they be operated at high permeancecoefficients, Pc, and that they be shielded from direct heavy particleirradiation. Stabilization can be achieved by pre-exposure to expectedradiation levels.5.6 Shock, Stress, and VibrationBelow destructive limits, these effects are very minor on modern magnetmaterials. However, rigid magnet materials are brittle in nature, and can


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easily be damaged or chipped by improper handling. Samarium Cobalt inparticular is a fragile material and special handling precautions must betaken to avoid damage. Thermal shock when Ceramics and Samarium Cobaltmagnets are exposed to high temperature gradients can cause fractureswithin the material and should be avoided.6.0 Manufacturing Methods

Permanent magnets are manufactured by one of the following methods:

Sintering, (Rare Earths, Ceramics, and Alnicos)

Pressure Bonding or Injection Molding, (Rare Earths and Ceramics)

Casting, (Alnicos)

Extruding, (Bonded Neodymium and Ceramics)

Calendering (Neodymium and Ceramics)

The sintering process involves compacting fine powders at high pressure inan aligning magnetic field, then sintering to fuse into a solid shape. Aftersintering, the magnet shape is rough, and will need to be machined toachieve close tolerances. The intricacy of shapes that can be thus pressed is

limited.

Rare Earth magnets may be die pressed (with pressure being applied in onedirection) or isostatically pressed (with equal pressure being applied in alldirections). Isostatically pressed magnets achieve higher magnetic propertiesthan die pressed magnets. The aligning magnetic field for die pressedmagnets can be either parallel or perpendicular to the pressing direction.Magnets pressed with the aligning field perpendicular to the pressing

direction achieve higher magnetic properties than the parallel pressed form.

Both Rare Earth and Ceramic magnets can also be manufactured by pressurebonding or injection molding the magnet powders in a carrier matrix. Thedensity of magnet material in this form is lower than the pure sintered form,yielding lower magnetic properties. However, bonded or injection moldedmagnets may be made with close tolerances "off-tool" and in relativelyintricate shapes.Alnico is manufactured in a cast or sintered form. Alnicos may be cast inlarge or complex shapes (such as the common horseshoe), while sinteredAlnico magnets are made in relatively small sizes (normally one ounce orless) and in simple shapes.Flexible Rare Earth or Ceramic magnets are made by calendering orextruding magnet powders in a flexible carrier matrix such as vinyl. Magnetpowder densities and therefore magnetic properties in this form of


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manufacture are even lower than the bonded or injection molded form.Flexible magnets are easily cut or punched to shape.7.0 Physical Characteristics and Machining of Permanent MagnetsSintered Samarium Cobalt and Ceramic magnets exhibit small cracks withinthe material that occur during the sintering process. Provided that cracks donot extend more than halfway through a section, they do not normally affect

the operation of the magnet. This is also true for small chips that may occurduring machining and handling of these magnets, especially on sharp edges.Magnets may be tumbled to break edges: this is done to avoid "feathering"of sharp edges due to the brittle nature of the materials. Tumbling canachieve edge breaks of 0.003" to 0.010". Although Neodymium Iron Boron isrelatively tough as compared to Samarium Cobalt and Ceramic, it is stillbrittle and care must be taken in handling. Because of these inherentmaterial characteristics, it is not advisable to use any permanent magnetmaterial as a structural component of an assembly.Rare Earth, Alnico, and Ceramic magnets are machined by grinding, whichmay considerably affect the magnet cost. Maintaining simple geometries andwide tolerances is therefore desirable from an economic point of view.Rectangular or round sections are preferable to complex shapes. Square

holes (even with large radii), and very small holes are difficult to machineand should be avoided. Magnets may be ground to virtually any specifiedtolerance. However, to reduce costs, tolerances of less than +0.001" shouldbe avoided if possible.Cast Alnico materials exhibit porosity as a natural consequence of the castingprocess. This may become a problem with small shapes, which are machinedout of larger castings. The voids occupy a small portion of the larger casting,but can account for a large portion of the smaller fabricated magnets. Thismay cause a problem where uniformity or low variation is critical, and it maybe advisable either to use a sintered Alnico, or another material. In spite ofits slightly lower magnetic properties, sintered Alnico may yield a higher ormore uniform net density, resulting in equal or higher net magnetic output.

In applications where the cosmetic qualities of the magnet are of a concern,special attention should be placed on selecting the appropriate material,since cracks, chips, pores, and voids are common in rigid magnet materials.Magnet Sales & Manufacturing has extensive experience in the machiningand handling of all permanent magnet materials. In house machiningfacilities allow the ability to deliver prototype to production quantities withshort lead times.8.0 CoatingsSamarium Cobalt, Alnico, and Ceramic materials are corrosion resistant, anddo not require to be coated against corrosion. Alnico is easily plated forcosmetic qualities, and Ceramics may be coated to seal the surface, whichwill otherwise be covered by a thin film of ferrite powder (although not aproblem for most applications).

Neodymium Iron Boron magnets are susceptible to corrosion andconsideration should be given to the operating environment to determine ifcoating is necessary. Nickel or tin plating may be used for Neodymium IronBoron magnets, however, the material must be properly prepared and theplating process properly controlled for successful plating. Plating housesexperienced in the plating of NdFeB materials are difficult to locate, andmust be furnished with the necessary information for proper preparation andcontrol of the process. Aluminum chromate or cadmium chromate vacuumdeposition has been successfully tested, with coating thickness as low as


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0.0005". Teflon and other organic coatings are relatively inexpensive andhave also been successfully tested. A further option for critical applications isto apply two types of protective coatings or to encase the magnet in astainless steel or other housing to reduce the chances of corrosion.9.0 Assembly ConsiderationsMagnet Sales & Manufacturing Inc. has manufacturing capabilities to

manufacture complex magnet pole pieces and housings to provide acomplete magnet assembly. The following points should be considered whendesigning magnet assemblies.9.1 Affixing Magnets to HousingsMagnets can be successfully affixed to housings using adhesives.Cyanoacrylate adhesives that are rated to temperatures up to 350 F withfast cure times are most commonly used. Fast cure times avoid the need forfixtures to hold the magnets in place while the bond cures. Adhesives withhigher temperature ratings are also available, but these require oven curing,and fixturing of the magnets to hold them in place. If magnet assemblies areto be used in a vacuum, potential out-gassing of the adhesives should beconsidered.9.2 Housing Design

Magnet Sales & Manufacturing is equipped with state of the art CNC and EDMequipment allowing the manufacture of complex housings. Effective magnetlocating sections should be included in housing designs to support and locatemagnets precisely.9.3 Mechanical FasteningWhen arrays of magnets must be assembled, especially when the magnetsmust be placed in repelling positions, it is very important to consider safetyissues. Modern magnet materials such as the Rare Earths are extremelypowerful, and when in repulsion they can behave as projectiles if adhesiveswere to break down. We strongly recommend that in these situationsmechanical fastening be included in the design in addition to adhesives.Potential methods of mechanical retention include encasement, pinning, or

strapping the magnets in place with non-magnetic metal components. TheDesign Engineering team at Magnet Sales & Manufacturing is experienced inmagnet housing and fastening designs, and we will be pleased to assist inarriving at an appropriate design.9.4 PottingMagnet assemblies may be potted to fill gaps or to cover entire arrays ofmagnets. Potting compounds cure to hard and durable finishes, and areavailable to resist a variety of environments, such as elevated temperatures,water flow, etc. When cured, the potting compounds may be machined toprovide accurate finished parts.9.5 WeldingAssemblies that are required to be hermetically sealed can be welded usingeither laser welding (which is not affected by the presence of magnetic

fields) or TIG welding (using appropriate shunting elements to reduce theeffect of magnetic fields on the weld arc). Special care should be taken whenwelding magnetic assemblies so that heat dissipation of the weld does notaffect the magnets.10.0 MagnetizationPermanent magnet materials are believed to be composed of small regionsor "domains" each of which exhibit a net magnetic moment. Anunmagnetized magnet will possess domains that are randomly oriented withrespect to each other, providing a net magnetic moment of zero. Thus a


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magnet when demagnetized is only demagnetized from the observer's pointof view. Magnetizing fields serve to align randomly oriented domains to givea net, externally observable field.

10.1 Objective of MagnetizationThe objective of magnetization is initially to magnetize a magnet tosaturation, even if it will later be slightly demagnetized for stabilizationpurposes. Saturating the magnet and then demagnetizing it in a controlledmanner ensures that the domains with the least commitment to orientationwill be the first to lose their orientation, thereby leading to a more stablemagnet. Not achieving saturation, on the other hand, leads to orientation ofonly the most weakly committed domains, hence leading to a less stablemagnet.Anisotropic magnets must be magnetized parallel to the direction of

orientation to achieve optimum magnetic properties. Isotropic magnets canbe magnetized through any direction with little or no loss of magneticproperties. Slightly higher magnetic properties are obtained in the pressingdirection.10.2 Magnetizing EquipmentMagnetization is accomplished by exposing the magnet to an externallyapplied magnetic field. This magnetic field may be created by otherpermanent magnets, or by currents flowing in coils. Using permanentmagnets for magnetization is only practical for low coercivity or thin sectionsof materials. Removal of the magnetized specimen from the permanentmagnet magnetizer can be problematic since the field cannot be turned off,and fringing fields may adversely affect the magnetization of the specimen.The two most common types of magnetizing equipment are the DC and

capacitor discharge magnetizers.10.2.1 DC MagnetizersDC magnetizers employ large coils through which a current is applied for ashort duration by closing a switch. The current flowing through the coilproduces a magnetic field, which is usually directed by the use of iron coresand pole pieces, and magnets are placed in the gap between the pole pieces.DC magnetizers are only practical for magnetizing Alnico materials, whichhave a low magnetizing force requirement, or small sections of Ceramicmaterials.


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10.2.2 Capacitor Discharge MagnetizersCapacitor discharge magnetizers employ capacitor banks that are charged,and then discharged through a coil. Provided the coil has a resistance, R,which is greater than , where L is the inductance and C the capacitance, thecurrent flowing though the coil will be unidirectional. Extremely highmagnetizing fields (in the range of 100 KOe) can be achieved using specialcoils and power supplies.

10.3 Saturation Fields RequiredSome Rare Earth magnets require very high magnetizing fields in the 20 to

50 KOe range. These fields are difficult to produce requiring large powersupplies in conjunction with carefully designed magnetizing fixtures.Isotropic bonded Neodymium materials require fields in the high 60 KOerange to be fully saturated. However, fields in the 30 KOe range may achieve98% of saturation. Ceramics require fields in the order of 10 KOe, whileAlnicos require fields in the range of 3 KOe for saturation. Because of theease by which Alnico 5 can become inadvertently demagnetized, it ispreferable for this material to be magnetized just prior to or even after finalassembly of the magnet into the device.10.4 Multiple Pole MagnetizationIn certain cases, it may be desirable to magnetize a magnet with more thanone pole on a single pole surface. This may be accomplished by constructing

special magnetizing fixtures. Multiple pole magnetizing fixtures are relativelysimple to build for Alnico and Ceramic, but require great care in design andconstruction for Rare Earth materials.Magnetizing with multiple poles will sometimes eliminate the need for severaldiscrete magnets, reducing assembly costs, although a cost will be incurredfor building an appropriate magnetizing fixture. Multiple pole fixtures forRare Earth magnets may cost several thousand dollars to build, dependingon the size of the magnet, the number of poles required, and the fieldsnecessary to achieve saturation.


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10.5 The Orientation DirectionSome applications require magnets oriented in a particular direction with a

high degree of accuracy. This direction may or may not coincide with ageometrical plane of the magnet. For anisotropic materials the orientationdirection can normally be held within 3 of the nominal with no specialprecautions. However, more precise requirements may need specialmeasurement and testing. This is achieved by the use of Helmholtz coils,which measure the total flux in various axes, and thence calculating theresultant magnetic moment vector. Materials must be cut and machinedtaking into account the actual angle of orientation to achieve the requiredaccuracy. Isotropic materials may be magnetized in any direction, andtherefore pose no problem in this regard.11.0 Measurement and TestingIt is important that incoming inspection of magnetic characteristics be clearlyand properly specified. End point characteristics (such as Bror Hc) cannot be

directly observed; therefore inspection personnel should not expect tomeasure 8,500 Gauss on a SmCo 18 magnet even though the Bris specifiedat 8,500 Gauss.A test method or combination of test methods should be based upon thecriticality of the requirement, and the cost and ease of performing tests.Ideally, the test results should be able to be directly translated intofunctional performance of the magnet. A sampling plan should be specifiedwhich inspects the parameters which are critical to the application. A briefdescription of some common test methods follows below.11.1 B-H CurvesB-H curves may be plotted with the use of a permeameter. These curvescompletely characterize the magnetic properties of the material at a specific

temperature. In order to plot a B-H curve, a sample of specific size must beused, then cycled through a magnetization/demagnetization cycle. This testis expensive to perform due to the length of time required to complete. Thetest is destructive to the sample piece in many cases, and is not practical toperform on a large sample of finished magnets. However, when magnets aremachined from a larger block, the supplier may be requested to provide B-Hcurves for the starting raw stock of magnet material.


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11.2 Total FluxUsing a test set up consisting of a Helmholtz coil pair connected to afluxmeter, total flux measurements can be made to obtain total dipolemoments, and interpolated to obtain close estimates of Br, Hc, and BHmax.The angle of orientation of the magnet can also be determined using thismethod. This is a quick and reliable test, and one that is not overly sensitiveto magnet placement within the coil.11.3 Flux DensityFlux density measurements are made using a gaussmeter and anappropriate probe. The probe contains a Hall Effect device whose voltage

output is proportional to the flux density encountered. Two types of probeconstruction (axial, where the lines of flux traveling parallel to the probeholder, andtransversewhere the lines of flux traveling perpendicular to theprobe holder, are measured) allow the measurement of flux density ofmagnets in various configurations. The placement of the probe with respectto the magnet is critical in order to obtain comparable measurements frommagnet to magnet. This is accomplished by building a holding fixture for themagnet and probe, so that their positions are fixed relative to each other.

11.4 Flux MapsUsing special scanners equipped with 3-axis Hall probes, magnetic arrayscan be mapped, to capture flux densities in x, y, and z directions with aspecified number of data points across the entire array. The resulting data

can then be output as a flux contour map, as flux vectors, or as a data tablefor further analysis.11.5 Pull TestsThis is a commonly used test for magnets. The pull of the magnet isproportional to B2, and is therefore very sensitive to the value of B.Variations in B occur due to variations in the inherent properties of themagnet itself, as well as environmental effects such as temperature,composition and condition of the material that the magnet is being tested on,measurement equipment, and operator. Since B decays exponentially from a


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zero air gap, small inadvertently introduced air gaps between the magnetand the test material can have a large effect on the measured pull. It istherefore recommended that pull be tested at a positive air gap. Performingpull tests at a number of air gaps, and plotting results as air gap vs.(pull)1/2, provides a more accurate description of the pull characteristics ofthe magnet. Extrapolating from this pull at zero air-gap may be calculated.

11.6 Other Functional TestsThese should be determined according to the application and after discussionwith the supplier. They may involve complex tests such as a profile of fluxdensity along a specified axis, flux uniformity requirements within a definedvolume, or relatively simple tests such as a torque test.12.0 Handling and StorageHandle magnets with care!Personnel wearing pacemakers should not handle magnets.Magnets should be kept away from sensitive electronic equipment.Modern magnet materials are extremely strong magnetically and somewhatweak mechanically. Any person required to handle magnets should beappropriately trained about the potential dangers of handling magnets.Injury is possible to personnel, and magnets themselves can easily getdamaged if allowed to snap towards each other, or if nearby metal objectsare allowed to be attracted to the magnets.Materials with low coercive forces such as Alnico 5 must be carefully handledand stored when received in a magnetized condition. When stored, thesemagnets should be maintained on a keeper which provides a closed loopprotecting the magnet from adverse fields. Bringing together like poles inrepulsion would lead to irreversible, though re-magnetizable, losses.Samarium Cobalt should be carefully handled and stored due to theextremely brittle nature of the material.Uncoated Neodymium magnets should be stored so as to minimize the risk ofcorrosion.

In general, it is preferable to store magnetized materials under vacuum-sealed film so that the magnets do not collect ferromagnetic dust particlesover time, since cleaning this accumulated dust is time consuming.13.0 Quick Reference Specification ChecklistWhen requesting design assistance, information should establish adverseconditions to which the magnet may be subjected - for example unusualtemperatures, humidity, radiation, demagnetizing fields produced by otherparts of the magnetic circuit, etc. The various magnet materials reactdifferently under different environmental conditions, and it is most likely that


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a material can be selected which will maximize the chances of success,provided that all relevant information is conveyed.The following checklist may be helpful in constructing and communicatingspecifications for permanent magnets:

Material type

Nominal, minimum and/or maximum magnetic properties(Br, Hc, Hci, BHmax)

Geometry and tolerances of magnet

Orientation direction (and tolerance of orientation direction if critical)

Whether to be supplied magnetized or not

Marking requirements

Coating requirements

Acceptance tests or performance requirements

Inspection sampling plan

Packaging and identification


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MAGNETS

MAGNET MATERIAL PROPERTIES

NEODYMIUM IRON BORON

Magnetic Properties

GradeBr

(Gauss)Hc

(Oersteds)Hci

(Oersteds)BHmax(MGOe)

Temp.Coefficient of

Br(%/C)

Max. Op.Temp. (C)

Density(lbs/in3)

4SB 3,460 3,460 9,600 3 -0.10 150 0.217

B10N 6,800 5,780 10,300 10 -0.10 150 0.217

24 9,800 7,500 8,000 24 -0.12 80 0.275

24UH 10,000 9,600 41,000 24 -0.10 210 0.271

27 10,850 9,650 13,500 27 -0.12 80 0.267

27H 10,600 10,100 17,000 27 -0.11 150 0.271

28 10,800 10,100 17,000 28 -0.11 150 0.271

28UH 10,900 10,400 25,000 28 -0.09 190 0.271

30 11,400 10,400 13,500 30 -0.12 150 0.267

30H 11,200 10,700 17,000 30 -0.11 150 0.271

32SH 11,600 11,100 31,000 32 -0.10 180 0.271

35 12,300 11,300 14,000 35 -0.11 150 0.271

35SH 12,200 11,700 26,000 36 -0.10 160 0.271

38H 12,550 11,700 17,000 39 -0.10 130 0.271

39H 12,800 12,300 21,000 40 -0.10 150 0.271

40 12,900 12,400 12,000 40 -0.11 130 0.271

42 13,050 12,500 14,000 41 -0.11 120 0.271

42H 13,300 12,700 17,000 43 -0.10 120 0.271

45 13,550 11,750 11,000 44 -0.12 100 0.271

45H 13,500 12,900 15,000 45 -0.11 100 0.271

48 14,100 12,900 13,500 48 -0.12 80 0.271

Physical and Thermal Properties

Description SmCo 1-5 Alloys SmCo 2-17 Alloys NdFeB

Mechanical Properties:
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Modulus of elasticy 23 x 106psi 17 x 106 psi 22 x 106 psi

Ultimate tensile strength 6 x 103 psi 5 x 103 psi 12 x 103 psi

Density 8.2 g/cc 8.4 g/cc 7.4 g/cc

Coefficient of thermal expansion:

Perpendicular to orientation 13 x 10-6/C 11 x 10-6/C -4.8 x 10-6/C

Parallel to orientation 6 x 10-6/C 8 x 10-6/C 3.4 x 10-6/C

Electrical resistivity 5 ohm cm 86 ohm cm 160 ohm cm

Magnetic Properties:

Curie temperature 750C 825C 310C

Reversible temperaturecoefficient of residual

induction (-100C to +100C)

-0.043%/ C -0.03%/C -0.09 to -0.13%/C

Recoil permeability 1.05 1.05 1.05

Max. service temperature* 250C 300C 150C

* Maximum Service Temperature depends on permeance coefficient of magneticcircuit. Temperatures shown here are guidelines only

SOMARIUM COBALT

Magnetic Properties

GradeBr(Gauss)Hc(Oersteds)Hci(Oersteds)BHmax(MGOe)MaximumOperating

Temperature (C)

TempertureCoefficient of Br

(%/C)

Density(Lb/in3)

B15S 7,950 6,100 10,500 14 150 -0.04 0.253

18 8,600 7,200 9,000 18 250 -0.04 0.300

22 9,850 8,750 12,000 22 250 -0.03 0.300

26 10,500 9,200 10,000 26 350 -0.03 0.300

26H 10,600 9,250 11,500 27 350 -0.03 0.300

26HS 10,600 9,800 18,000 27 380 -0.03 0.300

27H 11,000 10,300 26,000 28 350 -0.03 0.300

28 10,700 10,300 18,000 28 350 -0.03 0.300

32H 11,600 9,500 14,000 31 350 -0.03 0.300


Description SmCo 1-5 Alloys SmCo 2-17 Alloys NdFeB

Mechanical Properties:

Modulus of elasticy 23 x 106psi 17 x 106 psi 22 x 106 psi

Ultimate tensile strength 6 x 103 psi 5 x 103 psi 12 x 103 psi

Density 8.2 g/cc 8.4 g/cc 7.4 g/cc

Coefficient of thermal expansion:
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Perpendicular to orientation 13 x 10-6/C 11 x 10-6/C -4.8 x 10-6/C

Parallel to orientation 6 x 10-6/C 8 x 10-6/C 3.4 x 10-6/C

Electrical resistivity 5 ohm cm 86 ohm cm 160 ohm cm

Magnetic Properties:

Curie temperature 750C 825C 310C

Reversible temperaturecoefficient of residual

induction (-100C to +100C)

-0.043%/ C -0.03%/C -0.09 to -0.13%/C

Recoil permeability 1.05 1.05 1.05

Max. service temperature* 250C 300C 150C

* Maximum Service Temperature depends on permeance coefficient of magneticcircuit. Temperatures shown here are guidelines only

FERRITE

Magnetic Properties

GradeBr(Gauss)Hc(Oersteds)Hci(Oersteds)BHmax(MGOe)

MaximumOperating

Temperature(C)

TempertureCoefficient of

Br(%/C)

Density(Lb/in3)

1 2,200 1,900 3,250 1.1 300 -0.18 0.18

5 3,950 2,400 2,450 3.6 300 -0.20 0.18

8 3,900 3,200 3,250 3.5 300 -0.20 0.18

10 4,200 2,950 3,050 4.2 300 -0.20 0.18


Property Typical Value

Coefficient of thermal expansion (25C to 450C)

Perpendicular to orientation 10 x 10-6cm/cm/C

Parallel to orientation 14 x 10-6cm/cm/C

Thermal conductivity 0.007 cal/cm-secC

Reversible temperature coefficient of residual induction -0.2% /C

Reversible temperature coefficient of intrinsic coerciveforce

0.2 to 0.5% /C

Curie temperature 450C

Maximum service temperature* (without metallurgicalchange)

800C
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ALNICO

Magnetic Properties

GradeBr

(Gauss)Hc

(Oersteds)Hci

(Oersteds)BHmax(MGOe)

Max.Op.

Temp.(C )

RelativeCost by

Wt.

5 Cast 12,500 640 640 5.5 540 15

5-7Cast

13,500 740 740 7.5 540 30

6 Cast 10,500 780 800 3.9 540 20

8 Cast 8,300 1,650 1,650 5.5 540 18

2Sintered

6,600 550 550 1.4 540 8

5Sintered

10,800 600 600 3.8 540 10

8Sintered

7,000 1,900 1,900 5.0 540 10

Magnetic Properties vs.Temperature

MaterialandGrade

Reversible Temperature Cofficient ofBr (% Change /C)

Curie TemperatureMax. ServiceTemperature

Near BrNear Max. Energy

Prod.Near Hc C F C F

Alnico 2 -0.03 -0.02 -0.02 810 1490 450 840

Alnico 5 -0.02 -0.015 -0.01 860 1580 525 975

Alnico 6 -0.02 -0.015 -0.03 860 1580 525 975

Alnico 8 -0.025 -0.01 -0.01 860 1580 550 1020

Physical Properties

Material andGrade

Density TensileStrength

psi

TransverseModulus of

Rupture

Hardness(RockwellC )

CoefficientOf ThermalExpansion

(Inches x10-6PerC)

ElectricalResistivity(Ohm-

cm x 10-6at20Cg/cm3 lbs/in3
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Alnico 5, Cast 7.3 0.264 5,400 10,500 50 11.4 47

Alnico 6, Cast 7.3 0.265 23,000 45,000 50 11.4 50

Alnico 8, Cast 7.3 0.262 10,000 30,000 55 11 53

Alnico 2,Sintered 6.8

0.246 65,000 70,000 45 12.4 68

Alnico 5,Sintered

6.9 0.25 50,000 55,000 45 11.3 50

Alnico 8,Sintered

7 0.252 50,000 55,000 45 11 54

FLEXIBLE

Material Grade Br(Gauss)Hc(Oersteds)Hci(Oersteds)BHmax(MGOe)Max.Op.

(C)

Temp.Coeff.

(%/C)

Density(Lbs/in3)

Flexible Standard 1,725 1,325 1,340 0.6 100 -0.19 0.133

Flexible HF1 2,200 2,000 2,400 1.1 100 -0.19 0.140

Flexible HF2 2,450 2,200 2,400 1.4 100 -0.19 0.140

Flexible HF3 2,650 2,200 2,400 1.6 100 -0.19 0.140
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MAGNETS

GLOSSARY OF MAGNETIC TERMS

Glossary of Magnetic Terms

Air Gap:

A low permeability gap in the fluxpath of a magnetic circuit. Often air,but inclusive of other materials suchas paint, aluminum, etc.

Anisotropic Magnet:

A magnet having a preferreddirection of magnetic orientation, sothat the magnetic characteristics areoptimum in one preferred direction.

Closed Circuit:

This exists when the flux pathexternal to a permanent magnet isconfined within high permeability

materials that compose the magnetcircuit.

Coercive Force, Hc:

The demagnetizing force, measuredin Oersteds, necessary to reduceobserved induction, B, to zero afterthe magnet has previously beenbrought to saturation.

Site Directory

Curie Temperature, Tc:

The temperature at which the parallel alignment of elementary magneticmoments completely disappears, and the material is no longer able to holdmagnetization.


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Demagnetization Curve:

The second quadrant of the hysteresis loop, generally describing the behavior ofmagnetic characteristics in actual use. Also known as the B-H Curve.

Eddy Currents:

Circulating electrical currents that are induced in electrically conductiveelements when exposed to changing magnetic fields, creating an opposing forceto the magnetic flux. Eddy currents can be harnessed to perform useful work(such as damping of movement), or may be unwanted consequences of certaindesigns, which should be accounted for or minimized.

Electromagnet:

A magnet, consisting of a solenoid with an iron core, which has a magnetic fieldexisting only during the time of current flow through the coil.

Energy Product:

Indicates the energy that a magnetic material can supply to an externalmagnetic circuit when operating at any point on its demagnetization curve.Calculated as Bdx Hd, and measured in Mega Gauss Oersteds, MGOe.

Ferromagnetic Material:

A material whose permeability is very much larger than 1 (from 60 to severalthousand times 1), and which exhibits hysteresis phenomena.

Flux

The condition existing in a medium subjected to a magnetizing force. Thisquantity is characterized by the fact that an electromotive force is induced in aconductor surrounding the flux at any time the flux changes in magnitude. Thecgs unit of flux is the Maxwell.

Fluxmeter:

An instrument that measures the change of flux linkage with a search coil.

Fringing Fields:

Leakage flux particularly associated with edge effects in a magnetic circuit.

Gauss:

Lines of magnetic flux per square centimeter, cgs unit of flux density, equivalent


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to lines per square inch in the English system, and Webers per square meter orTesla in the SI system.

Gaussmeter:

An instrument that measures the instantaneous value of magnetic induction, B.Its principle of operation is usually based on one of the following: the Halleffect, nuclear magnetic resonance (NMR), or the rotating coil principle.

Hysteresis Loop:

A closed curve obtained for a material by plotting corresponding values ofmagnetic induction, B, (on the abscissa) against magnetizing force, H, (on theordinate).

Induction, B:

The magnetic flux per unit area of a section normal to the direction of flux.Measured in Gauss, in the cgs system of units.

Intrinsic Coercive Force,Hci:

Measured in Oersteds in the cgs system, this is a measure of the materialsinherent ability to resist demagnetization. It is the demagnetization forcecorresponding to zero intrinsic induction in the magnetic material after

saturation. Practical consequences of high Hci values are seen in greatertemperature stability for a given class of material, and greater stability indynamic operating conditions.

Intrinsic Induction, Bi:

The contribution of the magnetic material to the total magnetic induction, B. Itis the vector difference between the magnetic induction in the material and themagnetic induction that would exist in a vacuum under the same field strength,H. This relationship is expressed as: Bi = B-H.

Irreversible Loss:

Defined as the partial demagnetization of a magnet caused by external fields orother factors. These losses are only recoverable by re-magnetization. Magnetscan be stabilized to prevent the variation of performance caused by irreversiblelosses.


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Isotropic Magnet:

A magnet material whose magnetic properties are the same in any direction,and which can therefore be magnetized in any direction without loss of magneticcharacteristics.

Keeper:

A piece of soft iron that is placed on or between the poles of a magnet,decreasing the reluctance of the air gap and thereby reducing the flux leakagefrom the magnet.

Knee of the Demagnetization Curve:

The point at which the B-H curve ceases to be linear. All magnet materials, evenif their second quadrant curves are straight line at room temperature, develop aknee at some temperature. Alnico 5 exhibits a knee at room temperature. If theoperating point of a magnet falls below the knee, small changes in H producelarge changes in B, and the magnet will not be able to recover its original fluxoutput without re-magnetization.

Leakage Flux:

That portion of the magnetic flux that is lost through leakage in the magneticcircuit due to saturation or air-gaps, and is therefore unable to be used.

Length of air-gap, Lg:

The length of the path of the central flux line in the air-gap.

Load Line:

A line drawn from the origin of the Demagnetization Curve with a slope of -B/H,the intersection of which with the B-H curve represents the operating point ofthe magnet. Also see Permeance Coefficient.

Magnetic Circuit:

An assembly consisting of some or all of the following: permanent magnets,ferromagnetic conduction elements, air gaps, electrical currents.


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Magnetic Flux:

The total magnetic induction over a given area. When the magnetic induction,B, is uniformly distributed over an area A, Magnetic Flux = BA.

Magnetizing Force, H:

The magnetomotive force per unit length at any point in a magnetic circuit.Measured in Oersteds in the cgs system.

Magnetomotive Force, F:

Analogous to voltage in electrical circuits, this is the magnetic potentialdifference between any two points.

Maximum Energy Product, BHmax:

The point on the Demagnetization Curve where the product of B and H is amaximum and the required volume of magnet material required to project agiven energy into its surroundings is a minimum. Measured in Mega GaussOersteds, MGOe.

North Pole:

That pole of a magnet which, when freely suspended, would point to the northmagnetic pole of the earth. The definition of polarity can be a confusing issue,and it is often best to clarify by using "north seeking pole" instead of "northpole" in specifications.

Oersted, Oe:

A cgs unit of measure used to describe magnetizing force. The English systemequivalent is Ampere Turns per Inch, and the SI systems is Ampere Turns perMeter.

Orientation Direction:

The direction in which an anisotropic magnet should be magnetized in order toachieve optimum magnetic properties. Also known as the "axis", "easy axis", or"angle of inclination".


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Paramagnetic Material:

A material having a permeability slightly greater than 1.

Permeance:

The inverse of reluctance, analogous to conductance in electrical circuits.

Permeance Coefficient,Pc:

Ratio of the magnetic induction, Bd, to its self demagnetizing force, Hd. Pc = Bd/ Hd. This is also known as the "load line", "slope of the operating line", oroperating point of the magnet, and is useful in estimating the flux output of themagnet in various conditions. As a first order approximation, Bd / Hd = Lm/Lg,where Lm is the length of the magnet, and Lg is the length of an air gap thatthe magnet is subjected to. Pc is therefore a function of thegeometryof the

magnetic circuit.

Pole Pieces:

Ferromagnetic materials placed on magnetic poles used to shape and alter theeffect of lines of flux.

Relative Permeability:

The ratio of permeability of a medium to that of a vacuum.In the cgs system,

the permeability is equal to 1 in a vacuum by definition. The permeability of airis also for all practical purposes equal to 1 in the cgs system.

Reluctance, R:

Analogous to resistance in an electrical circuit, reluctance is related to themagnetomotive force, F, and the magnetic flux by the equation R = F/(MagneticFlux), paralleling Ohm's Law where F is the magnetomotive force (in cgs units).

Remanence, Bd:

The magnetic induction that remains in a magnetic circuit after the removal ofan applied magnetizing force. If there is an air gap in the circuit, the remanencewill be less than the residual induction, Br.


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Residual Induction, Br:

This is the point at which the hysteresis loop crosses the B axis at zeromagnetizing force, and represents the maximum flux output from the givenmagnet material. By definition, this point occurs at zero air gap, and thereforecannot be seen in practical use of magnet materials.

Return Path:

Conduction elements in a magnetic circuit which provide a low reluctance pathfor the magnetic flux.

Reversible Temperature Coefficient:

A measure of the reversible changes in flux caused by temperature variations.

Saturation:

The condition under which all elementary magnetic moments have becomeoriented in one direction. A ferromagnetic material is saturated when anincrease in the applied magnetizing force produces no increase in induction.Saturation flux densities for steels are in the range of 16,000 to 20,000 Gauss.

Search Coil:

A coil conductor, usually of known area and number of turns that is used with afluxmeter to measure the change of flux linkage with the coil.

Stabilization:

Exposure of a magnet to demagnetizing influences expected to be encounteredin use in order to prevent irreversible losses during actual operation.Demagnetizing influences can be caused by high or low temperatures, or byexternal magnetic fields.

Temperature Coefficient:

A factor, which describes the change in a magnetic property with change intemperature. Expressed as percent change per unit of temperature.


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Weber:

The practical unit of magnetic flux. It is the amount of magnetic flux which,when linked at a uniform rate with a single-turn electric circuit during aninterval of 1 second, will induce in this circuit an electromotive force of 1 volt.


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MAGNETS

FREQUENTLY ASKED QUESTIONS

What does a magnet do?

Magnets do the following things:

Attract certain materials - such as iron, nickel, cobalt, certain steels andother alloys;

Exert an attractive or repulsive force on other magnets (opposite polesattract, like poles repel);

Have an effect on electrical conductors when the magnet and conductorare moving in relation to each other;

Have an effect on the path taken by electrically charged particlestraveling in free space.

Based on these effects, magnets transform energy from one form toanother, without any permanent loss of their own energy. Examples ofmagnet functions are:

A. Mechanical to mechanical - such as attraction and repulsion.

B. Mechanical to electrical - such as generators and microphones.

C. Electrical to mechanical - such as motors, loudspeakers, chargedparticle deflection.

D. Mechanical to heat - such as eddy current and hysteresis torquedevices.

E. Special effects - such as magneto-resistance, Hall effect devices,and magnetic resonance.

What are permanent magnets made of?

Modern permanent magnets are made of special alloys that have been

found through research to create increasingly better magnets. The mostcommon families of magnet materials today are ones made out ofAluminum-Nickel-Cobalt (Alnicos), Strontium-Iron (Ferrites, also knownas Ceramics), Neodymium-Iron-Boron (Neo magnets, sometimes referredto as "super magnets"), and Samarium-Cobalt. (The Samarium-Cobaltand Neodymium-Iron-Boron families are collectively known as the RareEarths.)
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How are magnets made?

Modern magnet materials are made through casting, pressing and

sintering, compression bonding, injection molding, extruding, orcalendering processes.

How permanent is a magnet's strength?

If a magnet is stored away from power lines, other magnets, hightemperatures, and other factors that adversely affect the magnet, it willretain its magnetism essentially forever.

Will magnets lose their power over time?

Modern magnet materials do lose a very small fraction of their magnetismover time. For Samarium Cobalt materials, for example, this has beenshown to be less that 1% over a period of ten years.

What might affect a magnet's strength?

The factors can affect a magnet's strength:

Heat

Radiation

Strong electrical currents in close proximity to the magnet

Other magnets in close proximity to the magnet

(Neo magnets will corrode in high humidity environments unless theyhave a protective coating.)

Shock and vibration do not affect modern magnet materials, unlesssufficient to physically damage the material.
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How does a magnet's strength drop off over distance?

The strength of a magnetic field drops off roughly exponentially over

distance.

Here is an example of how the field (measured in Gauss) drops off withdistance for a Samarium Cobalt Grade 18 disc magnet which is 1" indiameter and 1/2 " long.

Distance, x Field at Distance x

0.063 2,690

0.125 2,320

0.188 1,970

0.250 1,660

0.313 1,390

0.375 1,160

0.438 970

0.500 810

0.563 680

0.625 580

0.688

490

0.750 420

0.813 360

0.875 310

0.938 270

1.000 240

What is the governing equation for field strength relative todistance?

For a circular magnet with a radius of R and Length L, the field at thecenterline of the magnet a distance X from the surface can be calculatedby the following formula (where Br is the Residual Induction of thematerial):
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Can a magnet that has lost its magnetism be re-magnetized?

Provided that the material has not been damaged by extreme heat, the

magnet can be re-magnetized back to its original strength.

Can I make a magnet that I already have any stronger?

Once a magnet is fully magnetized, it cannot be made any stronger - it is"saturated". In that sense, magnets are like buckets of water: once theyare full, they can't get any "fuller".

How do you measure the strength or power of a magnet?

Most commonly, Gaussmeters, Magnetometers, or Pull-Testers are usedto measure the strength of a magnet. Gaussmeters measure the strengthin Gauss, Magnetometers measure in Gauss or arbitrary units (so its easyto compare one magnet to another), and Pull-Testers can measure pull inpounds, kilograms, or other force units. Special Gaussmeters can costseveral thousands of dollars. We stock several types of Gaussmeters thatcost between $400 and $1,500 each.

If I have a Neo magnet with a Br of 12,300 Gauss, should I beable to measure 12,300 Gauss on its surface?

No. The Br value is measured under closed circuit conditions. A closedcircuit magnet is not of much use. In practice, you will measure a fieldthat is less than 12,300 Gauss close to the surface of the magnet. Theactual measurement will depend on whether the magnet has any steelattached to it, how far away from the surface you make themeasurement, and the size of the magnet (assuming that the

measurement is being made at room temperature). For example, a 1"diameter Grade 35 Neo magnet that is 1/4"long, will measureapproximately 2,500 Gauss 1/16" away from the surface, and 2,200Gauss 1/8" away from the surface.
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What are Magnetic Poles?

Magnetic Poles are the surfaces from which the invisible lines of magneticflux emanate and connect on return to the magnet.

What are the standard industry definitions of "North" and "South"Pole?

The North Pole is defined as the pole of a magnet that, when free torotate, seeks the North Pole of the Earth. In other words, the North Poleof a magnet seeks the North Pole of the Earth. Similarly, the South Poleof a magnet seeks the South Pole of the Earth.

Can a particular pole be identified?

Yes, the North or South Pole of a magnet can be marked if specified.

How can you tell which is the North Pole if it is not marked?

You can't tell by looking. You can tell by placing a compass close to themagnet. The end of the needle that normally points toward the NorthPole of the Earth would point to the South Pole of the magnet.

What are the different types of magnets available?

There are 2 types of magnets: permanent magnets and electro-magnets.

Permanent magnets emit a magnetic field without the need for any

external source of power. Electro-magnets require electricity in order tobehave as a magnet.

There are various different types of permanent magnet materials, eachwith their own unique characteristics. Each different material has a familyof grades that have properties slightly different from each other, thoughbased on the same composition.


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What are Rare Earth Magnets?

Rare Earth magnets are magnets that are made out of the Rare Earth

group of elements. The most common Rare Earth magnets are theNeodymium-Iron-Boron and Samarium Cobalt types.

Which are the strongest magnets?

The most powerful magnets available today are the Rare Earths types. Ofthe Rare Earths, Neodymium-Iron-Boron types are the strongest.

However, at elevated temperatures (of approximately 150 C and above),the Samarium Cobalt types can be stronger that the Neodymium-Iron-

Boron types (depending on the magnetic circuit).

What does 'orientation direction' mean?

Most modern magnet materials have a "grain" in that they can bemagnetized for maximum effect only through one direction. This is the"orientation direction", also known as the "easy axis", or "axis".

Unoriented magnets (also known as "Isotropic magnets") are muchweaker than oriented magnets, and can be magnetized in any direction.Oriented magnets (also known as "Anisotropic magnets") are not thesame in every direction - they have a preferred direction in which theyshould be magnetized.

How are magnets rated?

Magnets are characterized by three main characteristics. These are known as the:

1. Residual Induction (given the symbol Br, and measured in Gauss). This is anindication of how strong the magnet is capable of being.

2. Coercive Force (given the symbol Hc, and measured in Oersteds). This is an indicationof how difficult it is to demagnetize the magnet.

3. Maximum Energy Product (given the symbol BHmax, and measured in Gauss-Oersteds). This is an indication of what volume of magnet material is required to project agiven level of magnetic flux.


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What are the properties of commonly used magnet materials?

Here are the three important properties that characterize magnets for some of the mostcommon magnet materials used today.

Material Br Hc BHmax

Flexible 1,725 1,325 0.6

Ceramic 1 2,200 1,900 1.1

Ceramic 5 3,950 2,400 3.6

SmCo 18 8,600 7,200 18

SmCo 26 10,500 9,200 26

NdFeB 35 12,300 11,300 35

NdFeB 35 13,050 12,500 41

How can I use this information?

Given a magnet size, you can estimate how much magnetic flux different materials willproject at a given distance or you can use this information to compare one material to

another.

Examples:

For example, How much more flux will a Neo 35 project as compared to a Ceramic 5 of thesame dimension at a given distance?

Simply divide the Br of Neo 35 by the Br of Ceramic 5 (12300/3950) to get 3.1. This meansthat the Neo 35 would give you 3.1 times the flux a Ceramic 5 the same size would at agiven distance.

Given a certain flux required at some fixed distance from the magnet, you can use thisinformation to estimate what magnet volume will be required for different magnetmaterials.
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For example, what volume of Ceramic 5 magnet would give the same flux as a Neo 35magnet at a given distance?Simply divide the BHmax of Neo 35 by the BHmax of Ceramic 5(35/3.6) to get 9.7. This means that the volume of the Ceramic 5 magnet would have to be9.7 times that of the Neo 35 magnet to give you the same flux.

What is a magnetic assembly?

A magnet assembly consists of one or more magnets, and other components, such as steel,that generally affect the functioning of the magnet.

How should I assemble magnets to my device?

If a magnet needs to be fastened to a device, you can use either mechanical means, oradhesives to secure the magnet in place.

Adhesives are often used to secure magnets in place. If magnets are being adhered touneven surfaces, you will need an adhesive with plenty of 'body' so that it will conform tothe uneven surface. Hot glues have been found to work well for adhering magnets toceramics, wood, cloth, and other materials. For magnets being adhered to metal, 'super-glues' can be used very effectively.

We can supply Flexible magnets with an adhesive already attached to the magnet: all youneed to do is to peel off the liner and attach to your product.

As with all adhesive applications, it is very important to ensure that all surfaces beingbonded are clean and dry before bonding.

What are the maximum recommended operating temperatures for differentmagnet materials?

The maximum temperature that a magnet may be effectively used at depends greatly onthe 'permeance coefficient' - which is a function of the magnetic circuit - the magnet isoperating in. The higher the permeance coefficient (the more 'closed' the circuit), the highertemperature at which the magnet may operate at, without becoming severelydemagnetized. Shown here are approximate maximum operating temperatures for thevarious classes of magnet material. At temperatures close to those listed here, specialattention may be needed in order to ensure that the magnet will not become demagnetized.


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MaterialApprox. Maximum Operating

Temperatures

C F

NdFeB 140 284

SmCo 300 572

Ferrite 300 572

Alnico 540 1,004

Flexible 100 212

Why is the maximum temperature a magnet can operate at not a set value?

Magnets function at different levels of efficiency given different circuits that they operate in.The more closed the circuit the magnet is operating in, the more stable it is, and the less

effect temperature will have on it.

Can I machine magnets?

Magnets can be machined. However, hard magnet materials - as opposed to the flexible orrubber type magnet materials - are extremely difficult to machine. Magnets should bemachined using diamond tools or soft grinding wheels, and in the unmagnetized state as faras possible. In general, it is best not to try to machine hard magnet materials unless youare familiar with these specialized machining techniques.

How much does it cost to machine magnets?

The factors which determine cost to machine magnets are:

Quantity- the larger the quantity, the lower the cost since set-up charges must beamortized over the quantity, and special tooling can be created to machine larger

quantities;

Material- SmCo materials are more costly to machine since they are very brittle, flexiblematerials are very inexpensive to machine because of their physical characteristics;

Shape- complex shapes are more expensive than simple shapes; and,

Tolerances- the closer the required tolerances, the more expensive it will be to machine themagnets.


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What can I use to block a magnetic field?

Only materials that are attracted to a magnet can 'block' a magnetic field. Depending onhow thick the blocking piece is, it will partially or completely block the magnetic field.

Tips on handling and storing magnets

Always take care! Magnets can snap together and injure personnel or damage themselves.

Keep magnets away from magnetic media - such as floppy discs , credit cards and computermonitors.

Store magnets in closed containers, so that they don't attract metal debris.

If several magnets are being stored, they should be stored in attracting positions.

Alnico magnets should be stored with "keepers" (iron or magnetic steel plates that connectthe poles of the magnet) since they can easily become demagnetized.

Magnets should be kept away from pacemakers!

What are eddy currents?

These are electrical currents that are induced when a magnetic field moves in relation to anelectrical conductor, which is placed within reach of the magnetic field. In turn, these eddycurrents create a magnetic field that acts to stop the relative motion of the originalmagnetic field and electrical conductor.

What are some good magnetic reference books?

Permanent Magnet Design handbook, by Lester Moskowitz, a 385-page book aimed at thetechnical layperson, price approximately $150.

Permanent Magnets and their Applications, by Dr. Peter Campbell, a 203-page book aimedat the technical person, price approximately $40.


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The Driving Force, by James Livingston, a 310-page book aimed at the non-technical reader- a very well written and interesting book on the history of magnets and some of their moreexotic applications. Price approximately $20.

What do magnets cost?

The costs of different magnet materials vary significantly from one to the other. Here is anapproximate guide as to what magnets cost.

Material BHmaxRelative Cost($ / pound)

Relative Cost($ / BHmax)

Flexibible

1

$1.00

$0.60

Ceramic 3 $2.00 $0.50

Alnico 5 $20.00 $4.30

SmCo 20 $100.00 $6.00

NdFeB 40 $50.00 $1.40

Note:the costs shown here are relative costs based on high volumes of magnet materialsthat have no special machining or other characteristics.

On a cost-per-pound basis, Neodymium magnets seem very costly. However, on a cost perBHmax basis, they do not seem so costly. Often by using a more powerful magnet, theentire device that the magnet goes into can be miniaturized, yielding cost savings that favorthe more powerful magnet materials.

Are there Industry Standards for Magnets?

Yes. Two industry associations produce standards. The Magnetic Materials ProducersAssociation (MMPA) publishes standards for the production of magnetic materials, and theMagnet Distributors and Fabrications Association (MDFA) produces standards on various

ways of testing magnets and magnetic devices.

Download a free copy of the current MMPA Standard Specifications for Permanent MagnetMaterials here. Adobe Acrobat Reader is required.
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How do I order magnets?

To efficiently order magnets, you need to have a good idea of what you want to accomplish.Here are a few items that you will need to consider:

General nature of application - Holding, moving, lifting, etc.

Shape of magnet desired - Disc, Ring, Rectangle, etc.

Size of magnet desired - Diameter, length, width, height, etc

Tolerances - what variation in dimensions is allowed.

Conditions magnet will be used in - Elevated temperature, humidity, outside, inside,etc.

Strength of magnet required - In pounds of holding force, Gauss, etc.

Magnet should cost no more than? - This will eliminate certain materials fromconsideration.

Quantities you will need.
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