Acoustic emission, domain walls and hysteresis in YIG

Volume 120, number 2

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2 February 1987

A C O U S T I C E M I S S I O N , D O M A I N W A L L S A N D H Y S T E R E S I S I N YIG M. G U Y O T , T. M E R C E R O N and V. C A G A N Laboratoire de Magn~tisme, CNRS, 92195 Meudon Bellevue. France

Received 4 August 1986; revised manuscript received 19 November 1986; accepted lbr publication 2 December 1986

The acoustic emission (AE) is experimentally shown to vary along the hysteresis loop of YlG samples with the creation and/or annihilation of domain walls, The AE and the Barkhausen activities are in anticorrelation. AE and hysteresis losses are proportional, which could mean that AE is the fundamental step to convert magnetic energy into heat.

I. Introduction Acoustic emission ( A E ) is a general p h e n o m e n o n observed in m a n y solids when subjected to various stresses. An extensive review o f AE in n u m e r o u s materials has been given by Lord [ 1 ]; m o r e recently Jaffrey [ 2 ] focused his review on AE in metals. Generally the precise nature and origin o f AE are not exactly known. Mostly, AE results in discontinuous and aleatory bursts o f high frequency elastic waves, the frequency o f which is not generally known; in some cases a direct mechanical origin has been proposed, such as dislocation propagation, fractures, macro or microcracks, etc. F o r ferromagnetic materials, some among the few pioneering works [ 3 ] suggested a relation between AE a n d the magnetization processes; m o r e recently the origin o f AE in ferromagnetic metals and alloys (Ni, Fe, N i - F e , F e - S i .... ) has been attributed to the irreversible m o v e m e n t o f non-180 ° d o m a i n walls [4,5]. On the other hand M o h a m a d et al. [6,7] have shown recently in ferroelectric single crystals such as lead germanate, that AE is directly related to the d o m a i n wall ( D W ) creation or a n n i h i l a t i o n which occur during the polarisation reversals. This latter report is o f f u n d a m e n t a l importance, since we h a d previously shown [ 8 - 1 0 ] that the hysteresis losses are directly related to the D W creation or annihilation; a similar idea had been p r o p o s e d i n d e p e n d e n t l y by Brailsford [ 11 ] for ferromagnets. I f AE is also related to D W creation/annihilation in ferrimagnets, that could be the missing step for converting the magnetic energy in heat. In 64

addition, our more recent works on D W d y n a m i c s [ 12,13] have suggested a "'direct" interaction between the D W and the lattice, since neither the spin d a m p i n g nor the diffusion d a m p i n g can account for the observed D W mobilities.

2. Experimental O u r purpose is to relate the AE with the magnetization processes: then the AE and the magnetization signals should be simultaneously recorded. However, due to experimental difficulties, such a condition cannot be fully satisfied, as it will be shown below. 2.1. Samples

F o r this first work, we use y t t r i u m iron garnet ( Y I G ) , a very s t a n d a r d ferrimagnet that we have already investigated to establish the relation between hysteresis and D W creation or annihilation. The very high resistivity of Y I G avoids parasitic effects related to eddy currents (particularly the hysteresis excess losses). Most o f the results are o b t a i n e d on t o r o i d shaped polycrystals which have been p r e p a r e d by ourselves; a d d i t i o n a l qualitative m e a s u r e m e n t s are performed on a thin Y I G single crystal disk :~. The well-known :~ The authors are indebted to J.M. Desvignes for growing the crystal and to D. Rouet and A. Malmanche for slicing and Xrays. 0375-9601/87/$ 03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing D i v i s i o n )

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advantage shown by a toroid is the absence of shape demagnetizing field which produces, for soft magnetic materials, a tilting of the hysteresis loop resulting in a possible screening of some interesting features.

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2, 2. Hysteresis loop and Barkhausen noise recording For toroids, we use the classical transformer arrangement with a primary and a secondary coil. In the case of the single crystal disk, an homogeneous magnetizing field is produced by a solenoid which contains a sample coil and a compensation coil. In both cases the field time dependence is triangular, oscillating between +Hm and -Hm; the sample signal and the field signal are processed by digital acquisition and controlled with a microcomputer [ 14 ]. In order to facilitate the signal integration, the driving frequency is chosen at 200 Hz, a value low enough to keep the hysteresis behavior comparable to the quasi-static situation used for the AE recording. The Barkhausen noise is recorded on the toroid shaped samples using the same arrangement as for hysteresis. Since the sample signal is not integrated, the frequency can be adjusted as low as 0.02 Hz in order to show the aleatory and discrete nature of this classical effect.

2. 3. Acoustic emission recording We use partly a standard AE recording set-up as described in ref. [ 1 ]. A small drop of gel insures the transmission of the surface acoustic waves (due to the internal AE) from the sample surface to the detector. The signal is amplified by using a preamplifier and a wideband conditioning amplifier (all these equipments from BruEl & Kjaer). Such an arrangement gives an output signal which does not reproduce the original bursti it is generally assumed that amplitude and number of events (or a mixture of them if several events overlap) are the only preserved informations in the present process while all the others, such as the central frequency, the spectral repartition, etc.., are definitively lost. Then, the signal given by the detected output of the amplifier will contain all the preserved informations. This signal is digitally recorded as a function of time or field with a detector time constant 0.2 s ; it is assumed

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Fig. 1. Hysteresis (top) and AE loops ( bottom ) for various maximum applied fields recorded on a toroidal polycrystal (YIG).

that no event is missed provided a slow sampling rate is used on the digital acquisition system. Hysteresis and AE are recorded at room temperature, at various applied maximum fields Hm, from the reversible region up to values high enough to approach the magnetic saturation.

3. Results

Figs. 1a- 1d show a series of selected recordings of hysteres~ loops and corresponding AE, for several increasing Hm values. All recordings show the discrete and aleatory behavior of AE. For Hm just higher than the critical field (fig. 1a), irreversible magnetic processes are just to appear; the corresponding AE is just above the background noise level. For higher Hm (figs. 1b and 1c) the field dependence is "hysteretic" (as better seen in fig. 3 where arrows indicate the path). A minimum of AE is observed around the coercive field. For the maximum possible H m value 65

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2000

'-

2 February 1987 50(

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Fig. 2. Hysteresis (a) and AE loops (b) recorded on a single crystal disk (YIG).

( 5.60e, fig. ld), the AE still shows its two minimum at the coercive field values; moreover, one can see that the AE decreases when approaching the magnetic saturation. In order to confirm this latter observation, we have measured a 0.3 m m thick YIG single crystal disc cut along a 110 plane using a magnetizing solenoid, since we cannot apply higher fields to a toroid shaped sample. Results are shown in fig. 2. The hysteresis loop recorded in fig. 2a is typically affected by the demagnetizing field, which results in the fact that the coercive field or the remanence cannot be accurately determined. So the high AE values (fig. 2b) which occur in the zero field region of the loop cannot be analyzed. However, the magnetic saturation is easily approached, which allows us to clearly show that AE is almost zero at the magnetic saturation, as expected from fig. 1d. Fig. 3 presents, for a given hysteresis loop recorded on the toroid shaped polycrystal (fig. 3a), a comparison between the AE (fig. 3b) and the Barkhausen noise (fig. 3c). Since the applied field varies slowly enough one can resolve some single Barkhausen jumps; however because the sampling rate used for digital acquisition is 25 ms/sample, the absolute amplitude of the spike is not necessarily good; moreover some sharp spikes could have been missed. Nevertheless, the important point is that, as classically reported [ 16 ], the maxi66

Fig. 3. For a given hysteresis loop (a) recorded on the YIG polycrystal, anticorrelation between acoustic emission (b) and Barkhausen noise t c ).

mum Barkhausen activity is seen in the vicinity of the coercive field while the AE is minimum at that point. Fig. 4 shows a plotting of the AE activity versus the hysteresis losses for various H m values recorded on the polycrystalline sample. The cross points are related to the AE recorded as above described. The circle points correspond to AE measurements performed at the same field frequency as the hysteresis recording, i.e. 200 Hz, which results in a small information lost in the AE amount. Both plottings show

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Fig. 4. Linear relation between the AE activity and the hysteresis losses recorded on the YIG polycrystal ( × : f = 2 x l 0 - - " Hz. o:f=2xlO 2 Hz).

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clearly a linear relation between AE and hysteresis losses.

4. Discussion It is now well admitted, particularly in the case of soft magnetic material such as YIG, that domain walls play a fundamental and dominant role in the magnetization reversals. Numerous works [ 15 ], using the Barkhausen effect among others, have clearly established that the irreversible DW displacement is mostly the main source for the magnetization reversals. The AE appears to be also related to the irreversible magnetization process since AE is seen neither in the Rayleigh region (reversible DW motions) nor at the saturation approach (reversible spin rotations). In a saturated material all the magnetic moments are aligned along the applied field direction and no domain wall exists. But to shift from one saturated state to the opposite one (e.g. to cycle a full half loop) domain walls are necessary; and a process of DW creation and annihilation must exist. In some ferroelectric materials it has been unambiguously established [ 6,7 ] that the AE bursts are due to the creation or annihilation of DW portions. In our case, since we did not get yet simultaneous DW visualisation results, the validity of this mechanism for our magnetic materials is only indirectly backed: we have indeed previously established [ 8-10 ] that the magnetization processes are controlled by the effective DW surface present in the material and that, particularly, the hysteresis losses are due to the irreversible creation or annihilation of DW portions. So the linear relation shown in fig. 4 between AE and hysteresis losses allows us to relate the AE to the DW creation-annihilation mechanism. The aleatory and discrete "noisy" nature of the AE phenomenon leads some authors [2] to describe it as an "acoustic Barkhausen effect". Looking at our results, such a denomination seems to be unappropriated since the Barkhausen effect and the AE are anticorrelated (refer to fig. 3): although they are both related with irreversible effects associated to the DW, they have different origins. The field dependence of AE indicates the regions where DW are created or annihilated, while the Bark-

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Fig. 5. Drawn from experimental results, this figure shows the schematic variation of the magnetization (top), the Barkhausen noise (middle) and the AE (bottom) as a function of the internal field along half and hysteresis loop. The field range has been divided (dotted lines) into five regions for which the text explains the magnetization mechanisms.

hausen effect shows the field regions where the DW move irreversibly. Let us increase the internal field starting from - H m (saturated state in negative sense): first, the magnetic moments rotate toward the closest easy magnetization direction (neither AE nor Barkhausen noise are observed); then DW nucleate grow (large AE and very small Barkhausen noise observed); the internal field is then reversed (we are in the vicinity of the coercive field) and the DW move (high Barkhausen effect but no AE are observed); then the DW are destroyed (a large AE and a small BN are observed); finally all the DW disappear: one reaches the opposite saturated state ( + H m ) when the spins rotate off the easy direction (neither AE nor Barkhausen noise are observed). The above description is schematized in fig. 5. Still standing is the question how physically the DW produces an AE? First, one could consider that 67

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a part of the magnetoelastic energy included inside a DW, as suggested by N6el [ 16 ], is released when the DW is annihilated. Another possible mechanism backed also by D W dynamics analysis is a more direct interaction between the D W and the lattice by magnetoelastic coupling, taking into account the elastic constants of the material. The linear relation between the AE activity and the hysteresis losses suggests that the AE phenomenon could be the intermediate step in the energy conversion from its magnetic form into lattice vibrations, i.e. into heat. At present the energy involved in the AE process cannot be quantitatively determined: thus it cannot be said if the total energy or only a part of it, is concerned. Anyway the total process might be very complex. When an AE burst is emitted at some place in the material, it propagates as a sound wave and consequently it is reflected, refracted, dispersed, absorbed and transmitted by all the various singularities such as grain boundary, dislocation, fracture, sample edge, etc. Even the other D W could affect this wave: it has been indeed reported by le Craw el al. [ 17 ], working on a spherical YIG single crystal, that phonons at 9 G H z propagate with remarkable low losses when the sample is magnetically saturated, while they are drastically absorbed when the sphere is no longer saturated, i.e. when D W are present. Then the DW can act both as emitters or absorbers of acoustic waves.

5. Conclusion

Yhe acoustic emission, which is already used as a non-destructive test in various domains, is seen to be an open field in the case of magnetic materials. First, it is of importance to confirm, by using simultaneous

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vizualisation, that in ferromagnets AE results from DW creation/annihilation as it does in ferroelectrics [6]. Using AE as a tool for evaluating D W surface variations could be also of interest in various situations such as magnetic structural phase or time transitions and temperature dependence of the DW structure.

References [ 1 ] A.E. Lord ix]: Physical acoustic. Vol. X1 ed. Mason I Academic Press, New York ~ p. 28cL ] 2 ] D. Jaffrey, Australasian Corros. Eng. 23 ( I q79 i ~, 25. [3] A.E. Lord, Acoustica 18 (1967~ 187. [4) K. Ono and M. Shibata, Advances in acousUc emission, eds. Dunegan and Harlmann ( Dunhart, Knowville, TN, 198 ! ! pp. 154-174. [5] H. Kusanagi, H. Kimura and H. Sasaki, Fundamentals ot acoustic emission, Proc. AE Session, &SA-ASC Meeting, ed. K. Ono (Honolulu. 1978) p. 309. [ 61 l.J. Mohamad, L Zammit-Mangion, E.F. Lamhson and ( i . ~ Saunders, J. Phys. Chem. Solids 43 ( 1982 ) 749 [7] L.J. Zammit-Mangion and G.A Saunders. J. Phys. (' I~ (1984) 2825. [8] M. Guyot and A. Globus, Phys. Star. Sol. (bl 5 ~ (19731 447 [9] M. Guyot, These (Pans, 1975 I [ 10] M. Guyot and ~, Globus, J. Phys. (' I (1977) 157 [ I 1] F. Brailsford, Proc. IEEE I 17 ( 1970 ) 1052. [ 12] T. Merceron, M. Guyot and V. ('agan, J. Phys. D. 17 11984! L83. [ 13] T. Merceron, A. Messekher, M. Guyot and V. Cagan, J Magn. Magn. Mater', 54-57 (1986) 1615. !14] M. Guyot and V. Cagan, Rev. Metall. (Parisl Part I: % (1984) 433. [ 151 R.M. Bozonh, Ferromagnetism ( Van Nostrand, New York. 1951)p. 525. [16] L. NOel, Cah. Phys. 25 11944i I [ 17] R.C le Craw and R.L. Comstock, in: Physical acoustic, Vol. III B, ed. Mason (Academic Press, New York) pp. 127-19q