Measuring low coercive force using a vibrating sample magnetometerJoseph A. Pesch Citation: Review of Scientific Instruments 54, 480 (1983); doi: 10.1063/1.1137395 View online: http://dx.doi.org/10.1063/1.1137395 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/54/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Design of a compensated signal rod for low magnetic moment sample measurements with a vibrating samplemagnetometer Rev. Sci. Instrum. 79, 035107 (2008); 10.1063/1.2901602 Anisotropy characterization of garnet films by using vibrating sample magnetometer measurements J. Appl. Phys. 93, 7065 (2003); 10.1063/1.1540142 Measurement of texture in magnetic recording media using a biaxial vibrating sample magnetometer J. Appl. Phys. 79, 4746 (1996); 10.1063/1.361658 Microvibration sample magnetometer: A forcetype measurement Rev. Sci. Instrum. 56, 411 (1985); 10.1063/1.1138313 Vibrating Sample Magnetometer for Use at Very Low Temperatures and in High Magnetic Fields Rev. Sci. Instrum. 41, 1764 (1970); 10.1063/1.1684405

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Measuring low coercive force using a vibrating sample magnetometer Joseph A. Pesch

IBM System Products Division. Rochester. Minnesota 55901

(Received 6 October 1982; accepted for publication 8 December 1982)

The coercive force He of commercially pure iron and an 80% NiFe alloy were measured on toroid samples using a hysteresigraph and on nontoroid samples of the same materials using a vibrating sample magnetometer. Within experimental error, the results were equal for each case. Values of He appeared to be insensitive to specimen shape. It appears that the demagnetizing field, which for nontoroid specimens affects the gross shape of the hysteresis loop, can be ignored in the region of low-field measurements for these materials.

P ACS numbers: 07.55. + x

INTRODUCTION

Heat treatment of soft magnetic cores, armatures, and shields is done to increase the magnetic permeability of the components. It is necessary to monitor the quality of the parts by measuring a magnetic parameter such as permeabil­ity or coercive force. Coercive force is often used as a mea­sure of magnetic quality of components because, typically, as the coercive force decreases, the permeability increases. Un­heat-treated iron sheet as received from the mill usually has a coercive force of over 3 Oe. Proper heat treatment of the iron results in a coercive force ofless than 0.75 Oe. Unheat-treat­ed 80% NiFe sheet as received from the mill can have a coercive force of over 8 Oe. After heat treatment, the coer­cive force is less than 0.05 Oe. It is, therefore, desirable to have available quick and accurate measurement techniques for determining coercive force of components. A Vibrating Sample Magnetometer (VSM) is being used to measure the coercive force of soft magnetic components.

The VSM has been commonly used l,2 to measure both

hysteresis curves of permanent magnetic materials and satu­ration magnetization of soft magnetic materials. However, because of the demagnetizing field caused by the poles on soft magnetic material, it has been difficult to measure the entire hysteresis curve.

Tests have shown that the coercive force of soft magnet­ic materials can be measured using a VSM, and the values obtained agree quite closely with hysteresigraph measure­ments made on toroids. This observation is based on the fact that at the coercive point on the hysteresis curve the average magnetization of the sample is zero, and therefore, its de­magnetizing field may be negligible compared to He.

I. EXPERIMENTS WITH TOROID SAMPLES

A. Experimental procedure

A toroid of commercially pure iron 2.68 cm outer diam­eter, 2.43 cm inner diameter, and 0.16 cm thick was ma­chined. It was then heat treated at 1120 °C in a vacuum at­mosphere. Also, a toroid of 80% NiFe 0.877 cm outer diameter 0.822 cm inner diameter, and 0.369 cm long was heat treated in vacuum at 790°C. Magnetizing and flux windings were applied to the toroids and their coercivities

were measured with a hysteresigraph. Maximum magnetiza­tion in the iron toroid was 16000 G, and in the 80% NiFe toroid, it was 7200 G.

After the hysteresigraph measurements were taken, the windings were removed from the toroids. Using a low-speed diamond saw,3 the iron toroid was cut into 12 sections. It was thought that this method of cutting would create low residu­al stresses in the pieces.

A section of the iron specimen was attached to the specimen rod on the VSM4 with double-coated adhesive tape. Orientation of the specimen was such that the largest chord was parallel with the pole axis of the electromagnet. The sweep generator was set at zero sweep range and + 100% drive. The sweep range was increased until the

magnetizing field in the gap of the electromagnet was + 1500 Oe. This field was strong enough to overcome the

demagnetizing field of the specimen and magnetize it to 18 500 G. The sweep range was then reduced to less than 10 Oe. At this point, the field in the gap of the electromagnet was about 50 Oe. Further reduction of the gap field to less than 10 Oe was accomplished with the midpoint zero control on the bipolar power supply. Care was taken so the field was decreased only and no minor hysteresis loops were generat­ed.

A reading of the gap field was taken, the pen lowered, and the DOWN button pressed on the sweep function. A long enough sweep time was used (10--20 min) to assure that the VSM was reading accurately. The long sweep rate and small sweep range were used to trace the magnetic moment M vs applied field Ha curve through zero magnetic moment. After the sweep generator reached maximum negative drive, the sweep range was increased to - 1500 Oe. The above procedure was repeated in the opposite direction to again trace M vs Ha through zero magnetic moment. The coercive force He of the specimen was measured from the spacing of the lines along the Ha axis. Calibration of the Ha axis was obtained from the Gaussmeter readings taken at the starting points of the descending and ascending curves and the spac­ing of the points along the H a axis. Some of the sections were etched to remove suspected cold-worked material due to cut­ting, and remeasured in the VSM.

A similar measurement was performed on the 80% NiFe toroid. This sample was small enough that it could fit

480 Rev. Sci. Instrum. 54 (4), April 1983 0034-6748/83/040480-02$01.30 © 1983 American Institute of Physics 480

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1.2

M 1.0

E ~ :> .8 E ~ ::;; .6

.4

-10 -8 -6 -4 4 6 8 10

H. (Oersteds)

-.4

-.6

-.8

-1.0

-1.2

FIG. I. Portion of an M vs Ha curve for an iron sample in the vicinity of low applied field. Measurement was made with a vibrating sample magnetometer. The coercive force H, is 0.47 Oe.

in the gap of the electromagnet without cutting. The M vs H a

curve for the toroid was obtained with its axis parallel with the axis of the poles of the electromagnet.

B. Results

The coercive force He of the iron toroid, as measured with the hysteresigraph, was 0.52 Oe. Hysteresigraph mea­surement on the 80% NiFe toroid gave an He value of 0.067 Oe.

As read with the VSM, a typical M vs Ha plot for a section of the iron toroid is shown in Fig. I. The illustration shows that there are two straight parallel lines which pass through zero moment. The upper descending line starts at + 9.3 Oe and the lower ascending line begins at - 9.7 Oe.

The He value for this particular section is 0.47 Oe. He values of the 12 sections ranged from 0.47 to 0.64 Oe and averaged 0.55 Oe. One section was measured ten times. The average for the ten readings was 0.468 ± 0.008 Oe. Actual horizontal separation between - He and + He on the chart paper was about 1.8 cm. Tests in which the iron sections were etched and remeasured in the VSM showed no detectable change in He due to etching.

He for the 80% NiFe toroid, measured along the axis of the toroid, was 0.070 Oe.

481 Rev. ScI. Instrum., Vol. 54, No.4, April 1983

II. EXPERIMENTS WITH A STEEL WIRE

A. Experimental procedure

A steel wire sample 1.53 cm long and 0.092 cm diameter was prepared with a paper clip. The sample was placed in the VSM with the length of the wire parallel with the axis of the poles of the electromagnet. Following the procedure out­lined above, the He value of the wire was measured. The wire was then removed from the VSM and cut to a length of 0.84 cm. It was again placed in the VSM with the length of the wire parallel with the axis of the poles of the electromagnet. He was determined for the shorter wire. The housing of the VSM was then rotated so that the length of the wire was parallel with the pole faces. He was again measured through a diameter of the wire.

B. Results

The He value of the long wire was 10.2 Oe, of the short wire 10.0 Oe, of the short wire through a diameter 10.3 Oe. Further analysis was made on the hysteresis loop of the short wire. The experiment showed that the width ofthe hysteresis loop remained constant to a field Ha of about 300 Oe. At this point, the intensity of magnetization M was 1080 emu/cm\ the magnetic induction B was 13 670 G, and the field H acting on the center of the specimen was 700e.

III. DISCUSSION

A characteristic of a VSM is that it can sense small magnetic moments and, therefore, is capable of measuring the coercive force of small amounts of material. The lower limit of coercive force that is readily measured with the sys­tem is about 0.02 Oe.

Results of the experiments indicate that for soft mag­netic materials the He values obtained with the VSM are almost the same as He values obtained on toroids. This is probably because the remanent magnetization and, hence, the demagnetization fields are small compared to He. As a result, the He values are also insensitive to sample shape.

IS. Foner, Rev. Sci. Instrum. 30, 548 (1959). 'B. D. Cullity, Introduction to Magnetic Materials (Addison-Wesley, Read­ing, MA, 1972).

'Isomet 11-1180 Low-Speed Saw, A. B. Buehler, Ltd., Evanston, Illinois. 4Model155 Vibrating Sample Magnetometer, Princeton Applied Research Corporation.

Measuring low coercive force 481

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