A.C.-induced D.C. magnetization of a ferromagnetic amorphous alloy below the critical field

Materials Science and Engineering, B7 (1990) 183-187

183

A.C.-induced D.C. Magnetization of a Ferromagnetic Amorphous Alloy Below the Critical Field D.-R. HUANG and J. C. M. LI

Materials Science Program, Department of Mechanical Engineering, University of Rochester, Rochester, NY 14627 (U.S.A.) (Received April 4, 1990)

Abstract The ferromagnetic amorphous alloy Fe78Bt3Si 9 (Allied 2605 $2) can be magnetized below the critical magnetic field of about 0.025 Oe by superposing a suitable a.c. magnetic field such as 0.05 Oe at 100 Hz. With this additional a.c. field, the d.c. magnetic induction is 5.6 kG in a d.c. field of 0.02 Oe as opposed to 60 G without the a.c. field. 1. Introduction The critical magnetic field Her of magnetic materials is the threshold magnetic field needed to cause the Barkhausen jump [1, 2]. If the applied magnetic field Hm is less than Her, the magnetic domain walls move slightly but are pinned by restoring forces of the energy barriers. If H m>/-/~r, the walls are depinned from the energy barriers and move easily. Since the movement of a domain wall is dominated by the rotation of spins in it, the spin relaxation time r for the spontaneous reversal will affect the coercive force H~ and the critical magnetic field Her (generally, H~ t> H , > 0; for ferromagnetic ribbons, H~ = Hcr [3]). For soft ferromagnetic materials, the d.c. or low frequency B - H loops could be narrowed by superposing an alternating field [4, 5]. Spooner and co-workers [6-8] considered this apparent reduction in the B - H hysteresis loop as a combination of many minor loops, and the trace of the reduced loop was connected by the centres of the minor loops. They also supposed that the total hysteresis loss was equal to that of the major loop plus the sum of all the minor loops being traced. The increased effective permeability has been used to improve the d.c. or low frequency shielding factor of magnetic shielding materials. Cohen [9] used a cylinder shell of 4-79 molybdenum permalloy by superposing a magnetic 10921-5107/90/$3.50

field of 60 Hz, a total shielding factor increase of eight at 10 Hz and the maximum possible increased shielding factor was 29. Kelh~ et al. [10] found that the increased shielding factor of a single-layer Mumetal, with a shaking (superposing) field of 5 A m - 1 (r.m.s.) and 50 Hz, was 7 dB in the presence of the Earth's magnetic field. Sasada et al. [11] measured the shielding factor of a cylindrical shield with two-layer 2705M amorphous ribbons against low-level disturbing magnetic fields of 10 Hz; the shielding factor was increased from 3.5 without shaking to higher than 150 under the shaking field of 1 kHz at 2.9 A m-1.

The purpose of our research was to investigate the a.c.-induced d.c. magnetization below the critical magnetic fields of soft ferromagnetic amorphous alloys. The d.c. B - H loops and induced voltages below the critical magnetic field of the amorphous alloy Fe78B13Si9 (Allied 2605 $2) were obtained by superposing an a.c. magnetic field. From the theory of magnetic domain wall motion, the walls swing reversibly about a stable position under a weak alternating field and then they pass into the next-favourable energy state should the a.c. magnitude be increased [12]. It was thus supposed that superposing a weak a.c. magnetic field causes the domain walls to oscillate back and forth from the original positions, so that the domain wall may be easier to move and can be pushed forward by a small additional d.c. field.

2. Experiments The specimens used in our experiments were amorphous Fe78B13Si9 alloys (Allied 2605 $2) of toroidal shapes. The toroidal specimen consisted of a five-layer core wound from an amorphous © Elsevier Sequoia/Printed in The Netherlands

184

ribbon 60.5 cm long and 7.5 cm wide on a pyrex tube. To measure the B - H loops of toroidal specimens, three coils were wound around the toroidal core. The primary coil (Nt) was connected to a d.c. bipolar power supply or a function generator to produce the applied magnetic field (H), the second coil (N2) was connected to a fluxmeter (or integrator) to measure the magnetic flux density (B), and the third coil (N3) was connected to an a.c. power supply to produce an a.c. magnetic field H 3 along the length direction of the specimen. Then, connecting the output terminals of H and B to an X-Y recorder, the B - H hysteresis loop was obtained. 3. Results and discussion

From the initial magnetization curves of as-cast Fe78BI3Sig, the critical magnetic field was found to be H , = 0.025 Oe. When the applied magnetic field H m is lower than the critical magnetic field H~r, the magnetic induction is very small (gener-

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ally lower than 0.1 kG). However, if we apply simultaneously an additional a.c. magnetic field H~ along the length direction of the amorphous core, a marked d.c. B - H loop with significant induced voltage can be obtained. Figure 1 shows such B - H curves measured under the applied magnetic field H m= + 0.016 Oe. Without the a.c. magnetic field, the d.c. B - H hysteresis loop almost cannot be induced as shown in Fig. l(a), and the magnetic induction is lower than 0.02 kG. However, if we apply a square a.c. magnetic field of frequency f = 2 kHz and magnitude H 3 --0.004-0.04 Oe, we may obtain a.c.-induced d.c. magnetization as shown in Figs. l(b)-(i), and the magnetic induction increases as the a.c. magnetic field H 3 increases. Figure 2 shows another example of B - H curves of as-cast Fe78B13Si9 under the applied magnetic field H m = ± 0 . 0 2 O e . Without the a.c. magnetic field, the d.c. B - H hysteresis loop almost cannot be induced, and the magnetic induction is lower than 0.06 kG. However, if we apply a square-shaped a.c. magnetic field of frequency f = 100 Hz and H 3 = 0.05 Oe, we can get a marked d.c. B - H loop with a magnetic induction B~ = 5.6 kG, coercive force He= 0.005 Oe and the maximum permeability can reach 720 000. Figure 2(b) shows the induced voltage (unintegrated switching signal) with a peak value of about 220 mV. Such an induced voltage could be used in devices like security sensors or magnetic switches. The d.c. magnetic properties of the amorphous alloys can be modified by superposing a.c. magnetic fields of different frequencies and magnitudes. Figure 3 shows the variation in magnetic induction of a s - c a s t F e 7 8 B 138i9 as a function of the

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Fig. 4. Variation of coercive force of as-cast amorphous alloy Fe78B]3Si 9 (2605 $2) by applying a.c. magnetic fields of different frequencies and magnitudes.

Fig. 6. Induced voltage as a function of the frequency of sine wave a.c. magnetic field H 3 = 0.052 Oe for as-cast Fe78Bt3Si9 under the applied magnetic field H m= 0.02 Oe.

a.c. magnetic field H 3 at three different frequencies, namely 100 Hz, 400 Hz and 1 kHz. Initially, the magnetic induction n m increased to quasisaturation as the a.c. magnetic field increased to H 3 = 0.024 Oe, then the magnetic induction stayed almost constant for a suitable range of a.c. field (for examples, f = 100 Hz, H 3 = 0.025-0.08 Oe; f = 4 0 0 Hz, H 3 = 0 . 0 2 5 - 0 . 1 2 Oe; f = l kHz, / - / 3 = 0 . 0 2 5 - 0 . 2 0 e ) , and finally the magnetic induction decreased as the a.c. magnetic field increased. Figure 4 shows the variation of coercive force of a s - c a s t Fe78B13Si9 as a function of the a.c. magnetic field H 3 at the three frequencies 100 Hz, 400 Hz and 1 kHz. Generally, the coercive force He decreases as the a.c. magnetic field H 3 is increased. The coercive force decreases more slowly at high frequency ( 1 kHz) than at low

frequency (100 Hz), probably being affected by the eddy current. Figure 5 shows both the magnetic induction B m and the coercive force H e as a function of the frequency of a.c. magnetic field H 3 = 0.052 Oe, when the applied magnetic field H m= 0.02 Oe. The magnetic induction is almost constant from 20 Hz to 20 kHz, while the coercive force increased almost linearly as the fie, quency increased from 20 Hz to 20 kHz. The induced voltage below the critical magnetic field for F e 7 8 B 1 3 5 i 9 is also affected by the frequency and magnitude of the superposing a.c. magnetic field. Figure 6 shows the induced voltage as a function of the frequency of the sine-wave a.c. magnetic field H 3 = 0.052 Oe under the applied., magnetic field Hm--0.02 Oe. The induced voltage increased from 65 mV at 20 H~ to a

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(c) maximum value of 200 mV around 100. Hz and then decreased to a value of about 125 mV in the frequency range from 2 kHz to 20 kHz. Figure 7 shows the induced voltage affected by the magnitude of the superposing a.c. magnetic field /43 (sine wave, 100 Hz) for Fe78B138i 9 under the applied magnetic field Hm=0.02 Oe. The induced voltage increased monotonically from 0 to 200 mV as the a.c. magnetic field increased from 0 to 0.052 Oe, reaching a maximum of about 220 mV and then decreasing as the field increased beyond 0.08 Oe. The effect of superposing an a.c. magnetic field on the specimen can be viewed as causing the magnetic domain walls to oscillate back and forth so that they can be pushed around by a small additional d.c. field. This phenomenon can be shown by superposing a low frequency a.c. magnetic field onto the specimen. If we measure the d.c. B - H loop at 20 s per cycle and the superposing a.c. magnetic field (/43=0.04 Oe) at several different frequencies f = 1-20 Hz, the affected B - H loops shown in Fig. 8 result. The evolution from low to high frequencies and the way in which the two effects are superimposed on each other are shown.

4. Conclusion A method to improve the d.c. soft magnetic properties below the critical magnetic field of a ferromagnetic amorphous alloy Fe78B135i9 by superposing a.c. magnetic fields was investigated. With this method, it is possible to produce a magnetic induction B m and an induced voltage Vp in

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amorphous alloys using an applied magnetic field lower than its original critical value, n c r = 0.025 Oe. At Hm=0.02 Oe, the original magnetic induction B i n < 6 0 G and the permeability /a = B / H < 3000. After applying an a.c. field of 0.052 Oe at 100 Hz, we obtain Bin=5.6 kG, /.tmax=720000 and an induced peak voltage Vp = 220 mV. These properties should be useful for some sensor applications.

Acknowledgment This work was supported by the China Steel Corporation, Taiwan, through Dr. J. Chi Tsou, Vice President for Research.

References 1 H. J. Williams and W. Shockley, Phys. Rev., 79 (1949) 178. 2 H. Barkhausen, Z. Phys., 20(1919) 401.

187 3 R. C. O'Handley, J. Appl. Phys., 46 (1975) 4996. 4 H. J. Williams and M. Goertz, J. Appl. Phys., 23 (3) (1952) 316. 5 R. M. Bozorth, Ferromagnetism, Van Nostrand, New York, 1951, p. 549. 6 T. Spooner, Phys. Rev., 25(1925) 527. 7 L. W. Chubb and T. Spooner, Trans. Am. Inst. Elec. Engrs., 34 (1915) 2671.

8 J.D. Ball, Trans. Am. Inst. Elec. Engrs., 34 (1915) 2693. 9 D. Cohen, Appl. Phys. Lett., 10 (3)(1967) 67. 10 V.O. Kelh~i,R. Peltonen and B. Bantala, IEEE Trans. Magn., MAG-16 (4) (1980) 575. 11 I. Sasada, S. Kubo and K. Harada, J. Appl. Phys. 64 (10) (1988) 5696. 12 C. Heck, Magnetic Materials and Their Applications, Grane, Russak, 1974, p. 110.