3634 IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 5, SEPTEMBER 2000

Pulsed Field Magnetometer for Low-TemperatureStudy of High-Performance Permanent Magnets

Kato Seiichi and Kido Giyuu

Abstract—A pulsed magnetometer is useful for testinghigh-performance magnets because it can produce high magneticfields that are sufficient to magnetically saturate them. Weinstalled a cryostat in a pulsed magnetometer and tested severalprevailing permanent magnets at low temperatures. The mea-surements were successfully performed down to 4.2K, and it wasshown that a pulsed magnetometer is useful for low-temperaturemeasurement on permanent magnets as well as room-temperaturemeasurements.

Index Terms—High-performance magnets, magnetic properties,magnetization curve, NdFeB, pulsed magnetometer, SmCo.

I. INTRODUCTION

A T PRESENT, the standard method for measuring mag-netic properties of permanent magnets is the closed flux

method using an iron-cored electromagnet. This method is notsuitable for high-performance permanent magnets, because itsmaximum applied magnetic field is not high enough to saturatethem. Either a superconducting magnet or a pulsed magnet canproduce sufficient fields for sample saturation. We, however,consider a method using a pulsed magnet to be better than a su-perconducting magnet, because a superconducting magnet is ex-pensive, needs liquid helium, and requires longer measurementtime than a pulsed magnet. Recently, superconducting magnetswith a closed cycle refrigeration system, which does not needadditional liquid helium, are becoming more popular. Their run-ning costs can be decreased, but the cost of equipment becomesmore expensive.

We have suggested a measuring method using a pulsedmagnet and developed it to take high temperature measure-ments up to 200C [1]–[4]. In this paper, we combine a pulsedmagnetometer with a cryostat, and show our method to workat low temperatures as well as room temperatures. There aresome reports on a pulsed magnetometer for low-temperaturemeasurement. The magnetometer reported by Juszczyket al.[5] has a pulsed magnet which is cooled in liquid nitrogen. Inthe magnetometer reported by Boomet al. [6], pick-up coilsare cooled in a liquid helium dewar with a sample. In oursystem, the pulsed magnet is cooled by flowing water and thetemperature of the pick-up coils is constant because they arewound outside the cryostat. Our system does not need liquidnitrogen and does not need to consider variation of the coilconstants of pick-up coils caused by thermal contraction.

Manuscript received February 15, 2000; revised May 15, 2000.The authors are with the National Research Institute for Metals, Tsukuba,

305-0047, Japan (e-mail: {kato; kido}@nrim.go.jp).Publisher Item Identifier S 0018-9464(00)08307-2.

Fig. 1. Schematic drawing of the pulsed magnetometer combined with acryostat.

Fig. 2. Schematic drawing of the J-coil.

II. EQUIPMENT AND EXPERIMENT

Fig. 1 shows the schematic drawing of our system. The ca-pacitor bank provides the pulsed magnet with a pulsed currentwhose direction can be changed by a remote-controlled switch.The two pick-up coils, H-coil and J-coil, generate voltages pro-portional to the variation of the magnetic field provided by thepulsed magnet and the magnetization of the sample induced bythe field, respectively. As shown in Fig. 2, the J-coil consistsof two concentrical coils which are connected in series opposi-tion. The product of the number of turns and the closs-section ofeach coil is almost the same, so that the induced electromotiveforce caused by the external magnetic fields can be canceled.

0018–9464/00$10.00 © 2000 IEEE

SEIICHI AND GIYUU: PULSED FIELD MAGNETOMETER FOR LOW-TEMPERATURE STUDY 3635

The magnetization of a sample can be simply calculated regard-less of the influence of the applied field on the signal from theJ-coil.

In order to make the cancellation more completely, a smalladjusting coil is connected in series with the J-coil. It is locateda little off the central axis of the pulsed magnet and above themagnet. It can be rotated on the axis which is perpendicular tothe side of itself, and the axis of rotation is perpendicular to thecentral axis of the pulsed magnet. As it rotates, the area pene-trated by the stray field changes, therefore, the induced voltageof the adjusting coil which is proportional to the field changes.Although the J-coil is designed to cancel the influence of an ap-plied field on measurement of magnetization, it is difficult tocancel it completely. By adding the adjustable small signal gen-erated by the small coil to the signal of the J-coil, one can cancelit almost completely.

The magnetic moment is calculated as follows:

(1)

where , , , and are the coil constant of the innercoil, the coil constant of the outer coil, the induced voltage of theJ-coil, and the initial magnetization of the sample, respectively.The coil constant is written as

(2)

where , , and are the number of turns per unit length, halfof the height, and the radius of the pick-up coil. In this work,

, , and in the inner coil are m , 4.40 mm,and 5.53 mm and those of the outer coil are m ,5.09 mm, and 6.98 mm. Therefore, and are 9693 mand 4978 m . The H-coil consists of two coils wound aboveand below the J-coil. They are connected in series and locatedat the same distance from the sample. In order to avoid the in-fluence of the magnetization of the sample on the signal of theH-coil, the two coils should not too close to the sample. Theyshould, however, be located in the uniform magnetic field. Theapplied magnetic field is obtained as the integration of the in-duced voltage of the H-coil.

A sample is set up at the center of the pulsed magnet. A ther-mocouple above the sample measures the temperature. The va-porized gas from liquid helium and the heater keep the temper-ature at an intended low temperature in the tube. The outsideof the cryostat remains close to room temperature. The pick-upcoils surround the tube and are not cooled when the sample iscooled. The coil constants which are determined by the size andthe number of turns of the coils remain constant because thermalcontraction does not occur. Consequently, magnetization curvesat various temperatures can be calculated without variations inthe coil constants and the readjustment of the J-coil is not nec-essary even when the sample temperature changes.

Sintered Nd–Fe–B and SmComagnets were provided byseveral Japanese industrial companies. They were cut into cylin-ders of 3 mm diameter and 2.1 mm length and were tested atvarious temperatures in pulsed magnetic fields up to approxi-mately 14 MA/m with a pulse duration of 10 ms. The 14 MA/mfields are sufficient for these tests. SmCowere tested in pulsed

magnetic fields up to 7, 10, 14, and 18 MA/m at 293 K in orderto examine the effect of the sweep rate of the magnetic field.

To obtain a major hysteresis loop, pulsed magnetic fields areapplied to a sample three times. The first field is for the purposeof sample saturation. The second field is applied in the oppositedirection of the first one, and saturates the sample oppositely. Ahalf of the major hysteresis loop can be obtained in the secondfield. The third field is applied in the same direction of the firstone, and the other half can be obtained as well. Because thesample is saturated in the opposite direction of the field beforeeach of the second and third field, the initial magnetization ofthe sample, , of each half loop has the same absolute value.The complete major hysteresis loop can be obtained by com-bining the two half loops into one and putting it in the posi-tion where the absolute value of the initial value of each halfloop is the same. The obtained hysteresis loop is corrected bysubtracting the demagnetizing field from the applied field. Thedemagnetizing factor is determined at the room temperature byusing a Ni sample in the same shape as test samples. The valuewhich makes the beginning of the magnetization curve of the Nisample vertical is regarded as the magnetizing factor. It is 0.32in this work.

Measurements by the sample extraction method in steadymagnetic fields were performed for the purpose of comparison.The same samples that were measured by the pulsed magne-tometer were tested at 293K, 177K and 4.2K in steady fieldsup to more than 10 MA/m by using a superconducting magnet.Smaller samples, less than 10 mg, were tested up to 4 MA/m at293K with a SQUID magnetometer.

III. RESULTS

Fig. 3 shows major hysteresis loops of (a) the sinteredNd–Fe–B and (b) the sintered SmComagnet at varioustemperatures obtained in this work. These loops have kinks inthe second and fourth quadrant. The same kinks are observedin the hysteresis loops obtained in steady fields. These kinks,however, do not influence the values of magnetic propertiessuch as remanence or coercivity, because they are determinedfrom other parts of the hysteresis loop.

In all loops, the saturated parts of the magnetization curveswhich were obtained with increasing fields are in accord withthose with decreasing fields. This means the effect of eddy cur-rent can be ignored [3]. The hysteresis loops of SmCoat 293Kwhich is obtained in magnetic fields with various sweep rates arepractically in accord with those with steady fields. These factsshow that 10 ms duration is long enough for tests on Nd–Fe–Band SmCo in this size.

The obtained hysteresis loop is a hysteresis loop of magneticpolarization. The hysteresis loop of magnetic flux density is ob-tained by the following transformation.

(3)

where , , and are magnetic flux density, permeability offree space, and magnetic polarization, respectively. From thesetwo loops, various magnetic properties are obtained.

The values of magnetic properties at various temperaturesobtained in this work are listed at Table I. Results with the

3636 IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 5, SEPTEMBER 2000

Fig. 3. Major hysteresis loops of (a) sintered Nd–Fe–B magnet and (b) sinteredSmCo magnet at various temperatures.

pulsed magnetometer are shown without brackets, and thoseobtained by the sample extraction method in steady magneticfields up to more than 10 MA/m are shown in square brackets.The discrepancies between them are small enough, and it showsthat this method is practical. Values in parentheses are resultswith a SQUID magnetometer. The and of Nd–Fe–Bare smaller by more than 10% than those by other methods.The same tendency is observed in SmCoin some degree. Incase of measuring a sample whose magnetization is large, aSQUID magnetometer requires that the sample should be verysmall. The smaller the sample is, the larger the percentage ofthe sample surface in the total of the sample is. It is suggestedthat damage of the sample surface, e.g. oxidation and damageby cutting, affects the total magnetic properties.

IV. CONCLUSIONS

We have developed a pulsed magnetometer forlow-temperature measurements on high-performance

TABLE IMAGNETIC PROPERTIES OF THEMAGNETS AT VARIOUS TEMPERATURES

MEASURED IN THIS PAPER. T , Br, HcJ, HcB AND (BH)max ARE

TEMPERATURE, REMANENCE, COERCIVITY OF MAGNETIC POLARIZATION,COERCIVITY OF MAGNETIC FLUX DENSITY AND MAXIMUM BH PRODUCT,RESPECTIVELY. VALUES IN SQUARE BRACKETS WERE OBTAINED BY THE

SAMPLE EXTRACTION METHOD IN STEADY MAGNETIC FIELDS UP TOMORE

THAN 10 MA/m. VALUES IN PAREMTHESESWERE OBTAINED BY USING A

SQUID MAGNETOMETER

permanent magnets. An adjusting coil connected with theJ-coil reduces the influence of the applied field on measure-ments of magnetization. Not cooling the pick-up coils with thesample cooled makes the procedure of the measurement simple.

Sintered Nd–Fe–B and sintered SmComagnets are testeddown to 4.2K. Clear major hysteresis loops are obtained andvalues of various magnetic properties are easily determined.This method is available at low temperatures as well as roomtemperatures.

ACKNOWLEDGMENT

The authors thank Dr. T. Takamasu, H. Abe, Dr. M. Tujii, andK. Takehana for measurements in steady magnetic fields.

REFERENCES

[1] G. Kido, T. Miyakawa, and Y. Nakagawa,Phisica B, vol. 155, pp.403–406, 1989.

[2] G. Kido, Y. Nakagawa, T. Ariizumi, H. Nishino, and T. Takano,Proc.10th Int. Workshop on Rare-Earth Magnets and Their Applications,Kyoto, 1989, pp. 101–109.

[3] G. Kido, Proc. 13th Int. Workshop on Rare-Earth Magnets and TheirApplications, Birmingham, 1994, pp. 707–715.

[4] S. Kato and G. Kido,J. Mag. Soc. Jpn., vol. 23, pp. 1113–1116, 1999.[5] S. Juszczyk and H. Duda,J. Mag. Mag. Mat., vol. 44, pp. 133–138, 1984.[6] W. Boom, W. Magnus, and L. van Gerven,J. Mag. Mag. Mat., vol. 46,

pp. 95–101, 1984.