An electronic coercivitimeter for amorphous ribbon toroidsC. Couroumalos, M. H. Price, and K. J. Overshott Citation: Journal of Applied Physics 53, 8272 (1982); doi: 10.1063/1.330305 View online: http://dx.doi.org/10.1063/1.330305 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/53/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Giant magnetoimpedance of amorphous ribbon/Cu/amorphous ribbon trilayer microstructures J. Appl. Phys. 95, 1364 (2004); 10.1063/1.1634389 Magnetoelastic hysteresis of amorphous ribbons J. Appl. Phys. 93, 7220 (2003); 10.1063/1.1540043 Study of magnetic domains in amorphous ribbons by scanning electron acoustic microscopy Appl. Phys. Lett. 59, 994 (1991); 10.1063/1.106325 Domain wall relaxation in amorphous ribbons J. Appl. Phys. 67, 5589 (1990); 10.1063/1.345893 Abstract: Magnetoelastic modes in amorphous ribbons J. Appl. Phys. 53, 2670 (1982); 10.1063/1.330933

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An electronic coercivitimeter for amorphous ribbon toroids C. Couroumalos, M. H. Price, and K. J. Overshott

Wolfson Centre for Magnetics Technology, University Col/ege, 30 The Parade, Cardiff CF2 3AD, United Kingdom

The constant current flux reset (C.C.F.R.) method has been frequently used to measure the coercivity He' remanence Brem, and saturation flux density Bm .. , of nickel-iron alloys. A new electronic coercivitimeter has been constructed based upon the C.C.F.R. method, which is capable of measuring the properties of amorphous ribbon toroids. The mode of operation and circuit of the coercivitimeter will be fully described. The coercivity and remanence to maximum induction ratio, Brem/Bm"", of amorphous ribbons of CosaNiIOFe,SiIlB'6 and Fe8lB13.,Si3.SC, which have been forward wound, i.e., with the wheel side of the ribbon on the side of the toroid and reverse wound, i.e., with the wheel side of the ribbon on the outside of the toroid, have been measured. The measurements presented have been made over the frequency range 50 to 1000 Hz and for toroids field annealed under different conditions, i.e., temperature from 325 to 425 ·C, in air, nitrogen, and hydrogen atmospheres. It is found that the coercivity increases and the Brem/Bma< ratio decreases as the frequency increases. These experimental results are explained by considering the effect of annealing conditions and frequency on the domain structure, B/H loop, metallurgical properties, manufacturing stresses, and induced anisotropies of amorphous ribbons.

PACS numbers: 07.55. + x, 85.70. - w

CONSTANT CURRENT FLUX RESET METHOD

In this method, a specific predetermined value of sinusoidal-current excitation, Hmax ' is established, which saturates the sample, and the corresponding induction change is measured to determine the value of maximum induction, Bmax'

The excitation. is then changed to a unidirectional half-wave sinusoidal current of the same peak magnitude as that used for determining the value of the maximum induction. The change in induction under this exci­tation is then measured to determine the property designated (Bmax-Brem)' or the difference between the maximum and residual values of induction. Holding the same value of A.C. half-wave Sinusoidal-current exci­tation, an opposing D.C. magnetizing field is applied and the magnetizing current is increased until the induction reaches its maximum value, already determined in the first part of the test. From the magnitude of this opposing D.C. magnetizing current and

the dimensions of the core under test, the coct-cive force, Hc ' can be calculated. This method permits the

determination of maximum i~duction, Bmax ' remanent induction, B m' and coercive force, H , at different

~ c frequencies, in the range 50 to 1000 Hz.

The circuit diagram of the coercivitimeter is shown in Fiq. 1.

In the"first part of the measurement, a full-wave sinusoidal current, supplied by a sine-wave oscillator, is fed to the input terminals of a power amplifier. The output of the amplifier circuit is th2n applied to the primary winding Nl , of the core under test. The input signal to this winding is increased, until the sample output is saturated. The output of the search coil, N2, is the derivative of the flux density, B, (dB/dt).

The magnitude is small, of the order of millivolts, and is therefore amplified and fed to an active integrator circuit which gives an output signal proportional to B. The output signal of the integrator circuit is then fed to two peak detectors, the first of which is used to detect the average value of the positive half of the B waveform. The second peak de­tector measures the average value of the negative half of the B waveform. The two O.C. levels are then summed "algebraically" by a subtractor circuit. A subtractor circuit had to be used because the output of the first peak detector is positive and the output of the second peak detector is negative. Therefore the subtractor converts the negative signal coming from the second peak detector into a positive signal and effec­tively adds it to the positive signal coming from the first peak detector. Two peak detectors had to be used instead of a peak to peak detector because the output waveform of the active integrator circuit is not symmetrical about the x-axis.

In the second part of the test, the excitation is changed to a unidirectional half-wave sinuosida1 cur­rent by means of the switch S/W. This current has the same magnitude, as that used for determining the maxi­mum induction, Bmax' The change in induction nB

under the form of excitation, is then measured to deter mine the change between the maximum and residual values of induction. Bmax-Br = nB from which Br = Mmax - nB. In the third part of the test, in which coercive field strength, Hc' is measured, an opposing D.C. magnetizing current is applied to a third winding of a small number of turns, N3, on the core under test. This current is increased until the output of the fluxmeter corresponds to the value of the maximum induction obtained in the first test. By using this test the coercivity, He' can not be measured directly. If the exact dimensions

8272 J. Appl. Phys. 53(11), November 1982 0021-8979/82/118272-03$02.40 © 1982 American Institute of Physics 8272

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0f the sample under test are known then the coercive force, Hc ' can be calculated by means of the formula:

~ H c = (2 < • N 3' 10. C. ) /k

where N3 is the number of turns of the opposing D.C. magnetizing winding; ID.C. is the current flowing in this winding and k is the magnetic length of the sample under tes t.

The coercivitimeter can be calibrated by the fol­lowing method.

For small applied input voltages to the primary winding, Nl , of the core under test, the output wave-form is sinusoidal. Therefore an A.C. digital volt­meter can be used to accurately measure the output of the search coil, N2. The induced emf, e, the flux density, B, the area of the sample, G, the number of turns of the search coil, N2, and the frequency, f, are related by the formula:

B = e/(4.44 AN2f)

The D.C. level output of the subtractor circuit is adjusted by means of a mu1titurn potentiometer until the reading is equal to the value of B calculated above. Therefore the output of the coercivitimeter now reads directly in Tes1as.

The coercivitimeter constructed, operates satis­factorily for frequencies in the range from 50 to 1000 Hz. The instrument was built by employing the integra­~es circuits and was calibrated using commercial digital lnstruments. The coercivitimeter presented in this work is simple to operate and its accuracy is approxi­mately 2% in the frequency range 50 to 1000 Hz.

8273 J. Appl. Phys. Vol. 53. No. 11. November 1982

EXPERIMENTAL RESULTS

This electronic coercivitimeter has been used to measure the coercivity, Hc' and remanence to maximum

induction, Brem/Bmax' of toroids of amorphous ribbons

of Co 58 Ni 10 Fe5 Sill B16 and Fe81 B13 .5 5i 3. 5 C2 which have been forward wound, i.e. with the wheel side of the ribbon on the inside of the toroid, and reverse wound. i.e. with the wheel side of the ribbon on the outside of the toroid. The measurements have been made over the freouencv ranoe 50 to 1000 Hz and for toroids field annea1edounder ditferent conditions. i.e. tempera­tures from 325 C to 425 C. in air. nitr00en. and hydro­gen atmospheres. The field applied during annealing is suffi cient to cause saturation.

Fig. 2 shows that for FeS1 B13 .5 5;3.5 C2 a reverse wound toroid has a lower coercivity than a forward wound toroid over the whole frequency range. It has been claimed [1] that this behaviour is becaus~ the forwa~d winding techniques increases the compresslve stress 1n the side of the ribbon which has a compressive stress introduced by the manufacturing process, thereby in­creasing the power loss and coercivity. In a reverse wound toroid the winding compressive stress opposes the tensile stress produced during manufacture and there­fore the magnetic properties are relatively unchanged. It is predicted that when the toroids have been cor­rectly stress relief annealed that there will be no difference in the properties between the two types of toroid. Fig. 3 shows that at 55 Hz the minimum coreci­vity is attained at an annealing temperature of 3650 C for forward wound toroid and 3550C for reverse wound toroid and the difference between the properties of the two types of toroid winding Becomes negligible at an annealing temperature of 385 C. This same character­istic is obtained at 1 kHz. Fig. 4 shows that annealing in nitrogen gives the lowest coercivity since the hydro­gen atmosphere results lie between the nitrogen and air results. The shape of these characteristics for C059 Ni lO Fe5 Sill B16 are similar except the effect of annealing on as-cast is minimal.

Couroumalos. Price. and Overshott 8273

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A minimum coerci~ity at 55 ~z occurs at an annealing temperature of 325 C and at 330 C for 1 kHz operation. It was also found, for both materials, that the Brem/Bmax ratio decreases as the frequency increases. The B/H loop at hiqher fre­quencies is less square with higher values of coercivity which is due to the increase in the number of domain walls present in the material. The reverse wound samples have a less square B-H loop than forward wound toroids. Annealing in a magnetic field which reduces the domain wall anisotropy increases the Brem/Bmax ratio and the squareness of the B-H loop.

CONCLUSIONS

An electronic coercivitimeter, which uses the con­stant flux reset method, has been designed and con­structed which has enabled the properties of amorphous ribbon toroids to be investigated. The coercivity of amorphous ribbons increases with frequency due to the increase in the number of domain walls with frequency which results in an increase in the pinning, and hence coercivity, of the ribbons. It is found that a minimum coercivity can be achieved by annealing at an optimum temperature but the improvement in coercivity due to annealing is only small. The cosrcivity increases at annealing temperatures above 3S5 C when crystallisation begins to occur in the ribbon. It is found that the

effect of annealing temperature on the variation of coercivity with frequency is not aopreciable which has also been observed for power loss. It is found that annealing temperatures below 3S00C that the reverse wound toroids have lower values of coercivity. It is suggested that the difference between winding methods is due to the combination of manufacturing and winding stresses and when complete stress relief has been achieved by annealing then the properties of the toroids become exactly similar. The toroids in this investi­gation were maintained at the annealing temperature for 1 hour and hence an investigation into annealing for longer periods at lower temperatures is being under­taken. It has been found that the amorphous ribbon toroids annealed in the nitrogen atmosphere possess the lowest values of coercivity. It has been found that in oxidizing atmospheres there is a tendency for the boron in the surface of the ribbons to oxidise, which causes localised crystallization and the ribbons become less amorphous and hence their properties deteriorate. The use of an inert nitrogen atmosphere prevents oxidation.

REFERENCE

[1] K.J. Overshott, private communication.

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Fig. 3 Variation of coercivity with annealing temperature for toroids (forward and reverse wound) of FeSl B13 . 5 Si 3. 5 C2

8274 J. Appl. Phys. Vol. 53, No. 11, November 1982

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Couroumalos, Price, and Overshott 8274

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