IEEE TRANSACTIONS ON MAGNETICS, VOL 31, NO 6, NOVEMBER 1995 3391

A Multipurpose Magnetometer for Measuring Basic Magnetic Characteristics with a Newly Designed Capacitive Torque Sensor

Y. H. Lee, Y. D. Shin and K. H. Lee Physics Department, Jeonbuk National University, Jeonju 560-756, Korea

M. Y. Kim and J. R Rhee Physics Department, Sookmyung Women’s University, Seoul 140-742, Korea

1) AbsftnctAn economic and versatile magnetometer for measuring basic magnetic characteristics such as hysteresis loops, magnetic anisotropy and magneto- striction with a single system was built. It is based on the principle that most magnetic charaderisticr, could be obtained by analyzing the torque acting on a sample under an appropriate magnetic field. A capacitive torque sensor with 0.1 nJ resolution measures such torque. By limiting the duration of electromagnet excitation only to the measuring period (about 4 E), temperature rise of the magnet becomes negligible BO that a very compact and economic electromagnet and power supply (10 kg weight, 25 mm air gap, 350 W power consumption, and 480 U/m of generated field) is realized

I. INTRODUCTION

There have been many types of magnetometers for measuring hysteresis loops [l, 21, magnetic anisotropy[3] and magnetostriction[4] using different principles. A vibrating sample mapetometer[l] is based on electromagnetic induction and the torque magnetometer[3] on force. We report here a unified measuring method based on the view point that most magnetic characteristics can be derived from torques acting on a sample under appropriate magnetic fields. These torques can be measured using properly designed capacitive torque sensors and a transformer-ratio-arm(TRA) bridge with high resolution. It is desirable to design a measuring system miniaturized for economic reasons and minimizing thermal drift.

11. MEASUREMENT SYSTEM AND THEORY

This multipurpose magnetometer consists of (i) a miniaturized rotatable electromagnet with power

Manuscript received February 13, 1995. Y. H. Lee, fax 82-652-703320, phone 82-652-703324 J. R Rhee, fax 82-2-7l8-2337, phone 82-2-7lO-9404 This work is supported in part by Korean Ministry of Education through Grant BSRl 93-213.

supply, (ii) three capacitive sensors for measuring hysteresis loops, magnetic anisotropy and magnetostriction of thin film and bulk samples, and (iii) a high resolution capacitance bridge.

A. Electromagnet and P o m Supply Short duration (about 4 s) electromagnet excita-

tion as described above led to elimination of cooling.

B. Capacitive Torque Sensor

1) Anisotropy and Magnetization Measurement Sensor

A magnetically anisotropic sample placed under a rotating field H(B) gives a torque TA(@ on the sample, where 8 is the azimuthal angle of H. For measuring magnetization of samples which have a large uniaxial anisotropy field and a much smaller coercive field[5] (thin-film or needle of volume D), an added field H, perpendicular to both H(9 and the easy magnetization axis and produced by an auxiliary small Helmholtz coil is needed. A torque q (= DJH~) is then produced on the sample. These two torques TA and TI can be measured with a capacitive torque sensor shown

2 f

X

I

capaa tive cell

Fig. 1. Capacitive torque sensor.

0018-9464/95$04.00 0 1995 IEEE

3398

Voc

Fig. 7. Transformer-ratiearm (TRA) bridge.

in Fig. 1. Four pieces of phosphor-bronze plate (width 70, length 1 and thickness t) form a cross beam, which is vertically suspended at its upper end. The torque generated on the sample attached at the lower end twists the cross beam by an angle 4. This @ causes a capacitance change AC for the two parallel-plate capacitors (with total capacitance C,) attached to the lower end of the cross beam with average distance r from the beam axis. For either torque TA or TI, the angle 4 can be written Q K ? with system constant K so that one gets Am= ( K r / g ) ~ , g being the air gap of the capacitance. This capacitance change can be measured by a TRA bridge (Fig. 2). The output voltage Vo of the TRA bridge is calculated (neglecting conductance) to be

v, = -- -E K V G ~ , n+l g

where G and (n+Z)V are gain of the lock-in amplifier and total voltage of the secondary coil.

2) Magnetostriction Mensurenmt S d k sample: The structure of the linear

magnetostriction measurement sensor for a bulk sample of length 4 is shown in Fig. 3 (left) and is essentially the same as [4], but we use a metalic thin-film electrode for the capacitive cell to avoid eddy-current effects in the measuring process. The linear magnetostriction A, = d L b / L b is obtained from the relation

DC bias , VDc

Fig. 4. Output voltage V, of the lock-in amplifier as a function of Dc Mas vm

where gb is air gap of the capacitive sensor. Thin-film sample: The sensor's structure is

shown in Fig. 3(right). The linear magnetostriction .If for a thin film evaporated on a substrate is derived[6] from

where t, L, E, gt and 7 are thickness, length, Young's modulus, the capacitive cell's air gap, and a correction factor containing the Poisson's ratio (subscripts f and s for film and substrate).

111. EXPERIMENT

A. Sensitivity of the Capacitive Torque Sensor

The experimental parameters are as follows: f=32, rx8.0, tS0.10, g=0.20 and w=2.2 in m, n=500, and V =0.2 V. In order to determine the system constant K, a DC bias VDC was applied to the capacitive sensor. The output voltage V, of the lock-in amplifier was proportional to V D ~ as shown in Fig. 4. The system constant K is

CO

Fig. 5. Hysteresis Imps of Metglas 2826MR, 2605 S2, SC and CO. Length and thickntrss of all samples are 10 and 0.075 in mm and m a s are 1.49, 1.04. 1.51 and 1.41 in Fig. 3. Magnetostriction transducers for bulkoeft) and thin

film(right) sample. mg, respectively.

3399

Fig. 6.

m/J

azimuthal angle 0 (%I

Anisotropic torque CUNS of a Metglas 2826 MB disk sample (4 mm dia., 25 pm thick). V ~ 2 . 4 V, TA-116 nJ, -3500, and anisotropy constant=0.74 kJ/m'.

since G=100 and the electrostatic torque

V,,=826 mV. When we raise the gain G to an easily obtainable lo", the torque to produce an output Vo of 5 mV is 0.086 nJ.

TE (= t'c v~?/zg) was 1.42 PJ with V ~ p 9 4 . 5 v and

B. Measured Charncteristics

Several examples for measuring hysteresis loops, magnetostriction and magnetic anisotropy for bulk and thin films are shown in Fig. 5-8. The perpendicular field Ha for measuring hysteresis loops was 80 A/m. We calibrated the measured data for magnetization and magnetic anisotropy using the method described in section A. The magnetostriction calibration is carried out by measuring a change of output voltage due to the addition of an equivalent capacitance (0.02 pF) to the capacitive sensor.

a - I I I I I

-90 0 90 180 270 az?muthal angle 8 ( d o g )

Fig. 7. Linear magnetostriction k of a FecKmSitsBlo ribbon sample (Lb-10 mm).

60

110

165

225

285

Fig. 8. Linear magnetostriction As of a N t l F ~ s thin film sample (15.5 nun long 53 mm wide 1.0 thick, Youngs moduli for film and substrate are 210 and 60 GPa). A, is calculated to be 1.36~10~.

IV. CONCLUSION

The resolution of the capacitive torque sensor is at least 0.1 nJ when the amplifier gain is 10'. For measuring hysteresis loops with our sensor, a few mg of ferromagnetic sample is sufficient. For example, the torque generated on 2 mg of MetgIass 2605 SC sample (1Omm x 1 mm x 0.025mm) was about 30 nJ. For magnetostriction measurements, it is evident from Fig. 7 that the resolution is of the order lo-'. Due to the unique design of our system using short duration electromagnet excitation, the magnetometer reduced construction and operation costs as well as thermal drift.

REFERENCES

[l] S. Foner, "Versatile and sensitive vibrating-sample magnetometer," Rm. Sci. Insfr., vol. 30, pp. 548557 (1957).

(21 P. J. Flanders, "An alternating-gradient magnetometer", 1. Appl. Phys., vol. 63 (8), April, pp. 39403945 (1988).

[3] R. F. Penoyer, "An automatic torque balance for the determination of magnetocrystalline anisotropy," Paper presented at the MMM Conference 1956, p. 356 (1957).

[4] N. Tsuya, K. I. Ami, K. Ohmori, and Y. Shiraga, "Magnetostriction measurement by three terminal capacitance method," Iapnn 1. Appl. Pl~ys., vol. 13, pp. 1808-1810 (1974).

[5] P. J. Flanders, and R F. Pearson, "Measuring Magnetization in Aiisotropic Magnetic Sample Using a Torque Method", Brit. 1. AppZ. Phys. vol. 17, pp. 521 (1966).

[6] E Klokholm, T h e measurement of magnetostriction in ferromagnetic thin film," IEEE Trms. Map., vol. MAG= pp. 819-821, November 1976.