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The acoustic emission along the hysteresis loop of various ferro and ferrimagnets
Journal of Magnetism and Magnetic Materials lOt (1991)256-262 North-Holland
~ ~
~
Invited paper
The acoustic emission along the hysteresis loop of various ferro and ferrimagnets M. Guyot and V. Cagan Laboratoire de Magn~tisme et Mat~riaux Magn~tiques, CNRS, 1, Place Aristide Briand, 92195 Meudon, France
This paper, which brings together many earlier results, is devoted to demonstrating that the process of acoustic emission (AE) is related to the irreversible character of the domain wall creation/annihilation process. This proposition is supported qualitatively by using previously published data and the authors' data on AE profiles. As expected from our interpretation, a general quantitative original law is established from measurements on series of polycrystalline ferrimagnets: the AE activity is proportional to the hysteresis losses, whatever the grain size, the composition or the measuring temperature are. The AE dependences on grain size and temperature are discussed.
1. Introduction The acoustic emission (AE) process - bursts of stress waves internally generated during dynamical processes [1] - is generally considered to be a defectrelated phenomenon. A similar approach guided the early studies of A E in magnetic materials [1-10]. A theory based on this work has been proposed [2-4]: the A E activity was attributed to the release of the magnetoelastic energy associated with the abrupt motion of non-180 ° domain walls (DW) - since for these authors the motion of a 180 ° D W does not involve, a priori, any change in the magnetoelastic energy. This theory underlies a relation between A E and the "defects" (precipitates, residual stress etc.), which has led several research teams [3-10] to try using the A E p h e n o m e n o n in magnetism, as a non-destructive evaluation technique ( N D E ) of defects, as in the case of non-magnetic materials. Such a "defect-related" approach of the A E in magnetism was also guided by what we could call a dominant idea: the domain wall (DW) motion is essentially controlled by its pinning at the various defects (grain boundaries, pores, precipitates, etc.). In other words, if there were no D W pinning centers, i.e. no defects, there would be no hysteresis. We are of the opinion that the hysteresis is better described if, in addition to the classical D W pinning, one also takes into account the "necessary" process of D W c r e a t i o n / a n n i h i l a t i o n . Let us prove the necessity of this type of process: everybody agrees that in ferromagnetic materials, on the one hand, there is no D W in a saturated state and, on the other hand, the D W are present and observable in a demagnetized state (at the coercive field, for example). Therefore, there might be a field region where the D W are created and another region where they are annihilated. This "reasonable", even "trivial" statement, implied for many years that the domain wall c r e a t i o n / a n n i h i l a t i o n process is a reversible, i.e. that it is a lossless process. Nevertheless in the early 1970s several papers were published, independently, in which the irreversibility of
the D W c r e a t i o n / a n n i h i l a t i o n was stated [11-13]. At this time, in our group we have indeed already shown that the D W c r e a t i o n / a n n i h i l a t i o n process is a dissipative process responsible for an important contribution to the hystercsis losses [13-15]. Even more, from our intensive studies, carried out on polycrystalline insulating ferrimagnets, it turned out that the pinning coefficient is always proportionnal to the domain wall energy, which means that the p i n n i n g / u n p i n n i n g mechanism can be considered as the c r e a t i o n / a n n i h i l a t i o n of small portions of D W around the pinning centers. So, at least in polycrystalline insulating ferrimagnets, the D W c r e a t i o n / a n n i h i l a t i o n processes are responsible for all the hysteresis losses, which are equal to the D W energy involved [13-15]. Although at the time of these earlier studies we were not able to propose a physical mechanism to account for the conversion of magnetic energy into heat, it does seem now that the A E is a good candidate for such a mechanism. Onc of the main reasons in favour is the observation by M o h a m m a d et al. [16] of A E bursts correlated with the collapse or creation of D W in ferroelectrics. In this paper we shall present typical qualitative results, taken from the most pertinent articles already published, as well as from our recent work, to show how the A E profiles along the hysteresis loop can be seriously affected by the shape demagnetizing effect and why A E can be attributed to the D W c r e a t i o n / a n n i h i l a t i o n process. Next, the authors' original quantitative results, including the dependence of A E on grain size, composition and temperature, will seriously contest the previous non-180 ° D W motion magnetostriction-based theory [3,4] and support our D W c r e a t i o n / a n n i h i l a t i o n explanation of the AE.
2. Experimental conditions In general the A E process consists of bursts of ultrasonic waves the central frequency as well as the spectral repartition of which are not known.The gen-
0312-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
257
M. Guyot, V. Cagan / Acoustic emission along hysteresis loop eral methods for AE detection use piezoelectric sensors and, according to the weakness of the signals, the systems are generally of a resonance type (200 kHz in our case). Our room- and variable-temperature systems have been described elsewhere [18,21]. All the previous studies, except our own, were performed exclusively on ferromagnetic metals (either single crystals or polycrystals) [1-10]. Although we have also observed AE along the hysteresis loop of conducting materials [17], we present only the results we have obtained on insulating ferrimagnets, in particular because they show no complicated effects related to eddy currents. These results were obtained mostly on several series of polycrystalline ferrimagnets: YIG, Y I G : M n , Ni and N i - Z n ferrites; a disk-shaped YIG single crystal, nearly 0.4 mm thick, cut parallel to a [110] plane, is also used to support the qualitative analysis.
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3. Shape demagnetizing effect and AE profiles
3.1. Typical experimental A E profiles In the search for an understanding of the physical origin of the AE phenomenon, many authors have i
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Fig. 2. The demagnetizing effect arising from a gap made in the sample of fig. 1, results in a tilting of t~e hysteresis loop (top) and a splitting of the AE profile (bottom).
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recorded and analysed the field (or time) dependence of the AE activity along the hysteresis loop: we shall call such curves "AE profiles". Fig. 1 shows a typical recording (for simplicity only the ascending branches are shown) of the induction (top) and of the AlE activity (bottom) as a function of the magnetizing field (triangular, 100 Hz) obtained on a toroidal polycrystalline YIG sample (mean grain size D m = 8.5 Ixm): this AE profile exhibits a broad maximum around the coercive field. A toroid is the ideal model of a closed magnetic circuit, i.e. there is no shape demagnetizing effect. Fig. 2 reports the effect of a small demagnetizing effect obtained by making a small gap (0.4 mm thick) in the previous toroidal sample. The most striking consequence of the demagnetizing effect is that the AE activity profile (bottom) now shows a minimum around the coercive field and exhibits two maxima in a field region roughly centered around the so-called "knee" of the corresponding "tilted" hysteresis loop (top). The field expansion arising from the demagnetizing effect reveals that the true AE-active zones are located around the knees, not in the coercive field region. In the case of the virgin toroid, the field values for these
M. Guyot, V. Cagan / Acoustic emission along hysteresis loop
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a relative minimum in the vicinity of the zero field (in fact close to the coercive field). The separation of the active and non-active AE zones is the result, as above, of the quite large shape demagnetizing effect of this thick disk. Similar "double-peak" AE profiles on a metallic single crystal (Incoloy 904) have been observed by Buttle et al. [6-8]. The demagnetizing effect reveals the fine structure of the AE which can be hidden on fast switching hysteresis loops.
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Fig. 3. For a YIG single-crystal thick disk, induction B (top) and AE activity(bottom) as a function of H.
knee zones are very close to each other and close to the coercive field, which causes an overlapping of the active and non-active zones, resulting in the broad AE maximum observed on the virgin toroid (fig. 1, bottom), The literature has examples of similar demagnetizing effects: the experiment of Lord et al. [2] was carried out on a long Ni wire, i.e. the demagnetizing effect was negligible so the AE activity appeared to be at a maximum around H c. May Man Kwan et al. [5] have published typical results of AE on an Fe sheet (190 x 10 x 0.45 ram3), which show two maxima close together separated by a shallow minimum around the coercive field. According to our explanation, the shape demagnetizing effect for such a sample geometry is rather small but large enough to produce the observed splitting of the two zones. This is more pronounced on the AE profile obtained by Buttle et al. on a polycrystalline c~-iron rod (length 50 mm, diameter 5 mm) [7]: the demagnetizing effect is very large there and consequently the AIE profile consists of two large maxima separated by a relative minimum. Other papers have mentioned such two-peak AE profiles, which can be explained by a more or less important shape demagnetizing factor [9,10,22]. Fig. 3 reports on our measurements on a thick YIG single-crystalline disk. The maximum amplitude of the magnetic field (triangular, 0.1 Hz), parallel to the [111] direction, is high enough to approach the saturation of the sample (top). From this figure it is obvious that there is no AIE activity close to the saturation. There exist two pronounced but broad AE maxima, roughly located in the "knee" zones of the hysteresis loop and
All the research groups above cited attribute the AE activity to the release of the elastic energy associated with the abrupt motion of non-180 ° DW [1,10]. Nevertheless, all the AE observations can also be described by the mechanism of DW creation/annihilation. Let us describe half an hysteresis loop (fig. 3), starting from the maximum negative field - H r , up to the maximum positive field + H m. As classically, decreasing the field amplitude results in (reversible) rotations of the spins towards the closest easy axis this may not give rise to any AE effect (see fig. 3 bottom, for H < - 8 0 Oe). Then comes the field zone were DW have to be created: this can explain the important AE activity observed ( - 80 Oe < H < - 10 Oe). Then, when crossing the zero-field region the DW move through the sample without appreciable DW surface variation, so the AE should decrease: this justifies the AE minimum around H c. When approaching the positive knee zone, the DW have to be annihilated: this can explain the second AE maximum. When approaching the positive saturation there are no longer DW, but only magnetization rotations: no AE is seen.
4. The quantitative study of AE As seen above, samples with a shape demagnetizing effect are very useful for qualitative analysis, but are more difficult to magnetize. Our quantitative studies were carried out on series of toroidally shaped polycrystalline insulating ferrimagnets.
4.1. A E and hysteresis losses The link we have previously introduced between the DW creation/annihilation process and the hysteresis losses and, now, the AE, leads us to compare the total cumulative activity A E ( H m) to the hysteresis losses Wh(HnO during any symmetrical hysteresis loop of maximum field H m.
4. I. 1. For a given sample For a series of loops, taken at increasing maximum field Hm, on a YIG toroid (mean grain size D m = 5 ~m), at room temperature (RT), fig. 4 shows that, for
M. Guyot, V. Cagan / Acoustic emission along hysteresis loop
259
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Wh {erg/cm 3 ) any loop, the A E activity A E ( H m) and the hysteresis losses Wh(H m) are proportional to each other, which could imply a common origin: the D W c r e a t i o n / a n n i hilation process.
4.1.2. For a given composition For a series of Y I G toroids with mean grain size D m ranging from 2.2 Ixm up to 30 Ixm, measured as above at RT, the proportionality between the A E activity and the hysteresis losses Wh, is always observed for each sample (fig. 5); in addition one can notice a monotonic increase of the maximum A E activity with Dm. A similar grain size d e p e n d e n c e of the A E activity has been previously observed by Rajan et al. [10] on decarburized steel samples.
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Fig. 6. For a polycrystalline YIG sample (Dm = 20 ~m), the AE is proportional to Wh at any temperature.
4.1.3. For a given sample at various temperatures O n e of the previous Y I G toroids (Dm = 20 ~ m ) has been measured as a function of the temperature from liquid nitrogen temperature up to the Curie temperature of Y I G (550 K). As can be seen in fig. 6, the proportionnality law between AlE and W h is maintained at any temperature, but the slope of the lines presents a strange temperature d e p e n d e n c e that will be discussed later. A N i Z n ferrite has also been tested over a similar temperature range [21]. 4.1.4. For various compositions Fig. 7 shows the results obtained at R T on several compounds: two garnets (one of the above YIG, one Y I G : M n ) and two spinels (one Ni ferrite and one NiZn ferrite). These polycrystals have nearly the same grain size (5 txm). The proportionality A E against W h is again maintained, at least in the first approximation: the deviation from linearity seems to be the result of the high AlE activity of some samples, which may tend to saturate the detection system. 4.2. A E and magnetostriction
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Fig. 5. For each of the investigated YIG samples, the total cumulative acoustic emission A E ( H m) is proportional to the hysteresis losses Wh, whatever the mean grain size D m (~m) is: t~, 2.2; z~, 3.7; ©, 5.0; +, 8.5; e, 12; II, 20; A, 30.
N o n e among the previous studies of AIE reports on such a linear relation between the A E and the hysteresis losses, mainly because no one tried to relate the A E activity to the hysteresis losses. One more reason is that the other theory, which relates A E to the non-180 ° D W motion, predicts a relation with the magnetostriction As, via a certain distribution of non-180 ° domains. If we look at the most extensive study in that field [4,5], only a poor relation between A E and the magnetostriction As is seen, although the authors have tried to improve the relation by selecting A100 for their SiFe
260
M. Guyot, V. Cagan / Acoustic emission along hysteresis loop
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samples. In a d d i t i o n t h e s e results were o b t a i n e d on polycrystalline samples: t h e r e f o r e t h e r e m i g h t not b e a u n i q u e value of A E for a given c o m p o u n d b e c a u s e of t h e grain size effect o n A E described above. 4.2.1. For various compounds at R T T h e results r e p o r t e d in section 4.1.4. are in fact a p a r t of a larger study [20] of the influence of A s on A E for which we p r e p a r e d several series of polycrystalline f e r r i m a g n e t s in o r d e r to have a b r o a d r a n g e of As values at RT. Particularly interesting was the well k n o w n M n - s u b s t i t u t e d Y I G series, in which Y3Fe4.g2 Mn0.08012 has a value of nearly As = 0 at RT.
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Fig. 8 shows the m a x i m u m A E activity m e a s u r e d at R T on several series of c o m p o u n d s , as a function of AS. As r e p o r t e d above the grain size effect results in differe n t m a x i m u m A E activities for a given composition, while As is u n i q u e for each composition. This figure d e m o n s t r a t e s the n o n - c o r r e l a t i o n b e t w e e n AlE a n d As: e.g., t h e two extreme cases r e p o r t e d in fig. 8 (A s = 0 a n d As = - 3 0 × 10 - 6 ) show roughly the same A E activity.
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temperature dependences of the maximum values of AE and Wh.
M. Guyot, V. Cagan / Acoustic emission along hysteresis loop maintained when the measuring temperature is changed. Nevertheless the AE activity and the losses do exhibit very different thermal variations, which are more clearly shown on fig. 9 (the decrease of AE and Wh below 130 K is the result of the fact that the maximum magnetic field applied to the sample was not high enough to saturate the material in this temperature range). The losses Wh vary as the effective anisotropy and as the domain wall energy, in agreement with the previous observations [13,14]. The nonmonotonic AE thermal variations have at present no explanation (we are the first group to investigate AE as a function of temperature). In the same study [21], we tested a NiZn ferrite over the same temperature range: the thermal variation of AE for this sample is very different than that for the YIG, which practically eliminates a possible artefact from the experimental set-up. In any case, the AlE thermal variation cannot be explained by the well-known thermal variations of As. 5. D i s c u s s i o n
We have mentionned in the introduction that the origin of AE was attributed to the release of some magnetoelastic energy during abrupt motion of non180 ° DW. It seems that this theory of AE generation is not appropriate, for the following reasons: (i) the predicted relation with magnetostriction is not experimentally verified, (ii) the grain size effect is not taken into account. Very recently, some authors [7,22] have admitted the possibility that AlE could be associated, at least partly, with the DW creation/annihilation processes. From data obtained on a very large SiFe single crystal, a theory has been developed by Kim and Kim [22] assuming that AE is the result of the release of the magnetoelastic energy inside the DW. Such an idea is plausible, but it predicts a relation between AIE and the magnetostriction which, as we have shown, does not exist. In addition, the proportionality between the AIE activity and hysteresis losses cannot be explained by this theory. On the other hand, our explanation does predict a relation between AE and the hysteresis losses: we have shown that the law of proportionality between AE and the losses remains valid whatever the grain size, the composition and the temperature of the sample are. Nevertheless, the grain size dependence and the temperature dependence of the AE are surprising: - when the grain size increases, one expects [13,14] a decrease of the total DW surface per volume unit in the sample, which implies lower hysteresis losses, which is what we observed; consequently the AE was expected to decrease accordingly: experimentally the maximum of AE increases with the grain size (see fig. 5).
261
-when the temperature is changed the ratio between the AE activity and the hysteresis losses has a large variation; even more, in the 150 to 270 K region for the YIG sample studied the AE activity falls below the noise level of our detection system. We are of the opinion that such strange types of behavior have a common origin: the very limited bandwidth of our resonant detection chain, which can only detect the frequency component of the AE bursts at the resonant frequency. The AE bursts reflect the dynamics of the formation of DW (or portions of DW); during the "blow off" of a surface portion AS of DW, with specific energy y, the energy lost is yAS: this energy is converted, through the magnetoelastic coupling, into a burst of elastic waves the amplitude and the spectral repartition of which are representative of the energy involved and of the time duration of the process. So the resonant chain that we use (as all the other research groups do!), gives only a small part of the AE spectrum: according to Ono and Shibata [4], who tried different resonance systems on several samples, such a spectrum should be broad. First, if for a given temperature and a given composition, the AE spectral repartition is dependent on grain size, one could understand the grain size effect presented in fig. 5 [18]. Secondly, if for a given sample the AE spectral repartition is dependent on temperature fig. 9 could be understood: in particular, the absence of AE in the 150-270 K region could mean that the total AlE spectrum is outside the 200 kHz band [21]. Detailed studies of the spectral repartition of AE should be carried out to clarify this point and the observed frequency range must be extended as far as possible, particularly up to the 10-100 MHz range where it is known that DW relax: the AE could also contribute to the DW damping, a phenomenon that is still not satisfactory explained. The AE spectrum could also be a good tool for understanding the dynamics of the DW formation/destruction, a field never investigated before, which could be of basic importance in practical applications such as permanent magnets and high-frequency magnetic recording heads.
References
[1] A.E. Lord Jr., in: Physcal Acoustic, vol. X1, ed. Mason (Academic Press, New York, 1975) p. 289. [2] A.E. Lord Jr., R. Usatschew and M. Robinson, Lett. Appl. Eng. Sci. 2 (1974) 1. [3] H. Kusanagi, H. Kimura and H. Sasaki, in: Fundamentals of Acoustic Emission, ed. K. Ono (UCLA, 1978) p. 309. [4] K. Ono and M. Shibata, in: Advances in Acoustic Emission, eds. Dunegan and Hartman (Dunhart, Knoxwille, 1981) p. 154. [5] May Man Kwan, K. Ono and M. Shibata, J. Acoustic Emission 3 (1984) 144.
262
M. Guyot, V. Cagan / Acoustic emission along hysteresis loop
[6] D.J. Buttle, G.A. Briggs, J.P. Jakubovics, E.A. Little and C.B. Scruby, Phil. Trans. Roy. Soc. London A 320 (1986) 363. [7] D.J. Buttle, C.B. Scruby, J.P. Jakubovics and G.A. Briggs, Phil. Mag. 55 (1987) 717. [8] D.J. Buttle, E.A. Little, C.B. Scruby, G.A. Briggs and J.P. Jakubovics, Proc. Roy. Soc. London A 414 (1987) 221. [9] J. Kameda and R. Rajan, Acta Metall. 35 (1987) 1515. [10] R. Rajan, D.C. Jiles and P.K. Rastogi, IEEE Trans. Magn. MAG-23 (1987) 1869. [11] F. Brailsford, Proc. IEE 117 (1970) 1052. [12] T.R. Hailer and J.J. Kramer, J. Appl. Phys. 41 (1970) 1034. [13] M. Guyot and A. Globus, Phys. Stat. Sol. (b) 59 (1973) 447. [14] M. Guyot, Th~se d'Etat, Paris (1975).
[15] M. Guyot and A. Globus, J. de Phys. C1 (1977) 157. [16] J.J. Mohammad, L. Zammit-Mangion, E.F. Lambson and G.A. Saunders, J. Phys. Chem. Solids 43 (1982) 749. [17] M. Guyot, T. Merceron and V. Cagan, J. Appl. Phys. 63 (1988) 3955. [18] M. Guyot, T. Merceron and V. Cagan, in: Proc. ICPMM 4, eds. W. Gorzkowski, H.K. Lachowics and H. Szymczak (Word Scientific, Singapore, 1989) p. 400. [19] M. Guyot, T. Merceron and V. Cagan, in: Proc. ICF 5, eds. C.M. Srivastava and M.J. Patni (Oxford & IBH, New Delhi, 1989) p. 215. [20] M. Guyot, T. Merceron and V. Cagan, J. Magn. Magn. Mater. 83 (1990) 217. [21] M. Guyot, T. Merceron and V. Cagan, IEEE Trans. Magn. MAG-26 (1990) 1828. [22] H.C. Kim and C.G. Kim, J. Phys. D 22 (1989) 192.
~ ~
~
Invited paper
The acoustic emission along the hysteresis loop of various ferro and ferrimagnets M. Guyot and V. Cagan Laboratoire de Magn~tisme et Mat~riaux Magn~tiques, CNRS, 1, Place Aristide Briand, 92195 Meudon, France
This paper, which brings together many earlier results, is devoted to demonstrating that the process of acoustic emission (AE) is related to the irreversible character of the domain wall creation/annihilation process. This proposition is supported qualitatively by using previously published data and the authors' data on AE profiles. As expected from our interpretation, a general quantitative original law is established from measurements on series of polycrystalline ferrimagnets: the AE activity is proportional to the hysteresis losses, whatever the grain size, the composition or the measuring temperature are. The AE dependences on grain size and temperature are discussed.
1. Introduction The acoustic emission (AE) process - bursts of stress waves internally generated during dynamical processes [1] - is generally considered to be a defectrelated phenomenon. A similar approach guided the early studies of A E in magnetic materials [1-10]. A theory based on this work has been proposed [2-4]: the A E activity was attributed to the release of the magnetoelastic energy associated with the abrupt motion of non-180 ° domain walls (DW) - since for these authors the motion of a 180 ° D W does not involve, a priori, any change in the magnetoelastic energy. This theory underlies a relation between A E and the "defects" (precipitates, residual stress etc.), which has led several research teams [3-10] to try using the A E p h e n o m e n o n in magnetism, as a non-destructive evaluation technique ( N D E ) of defects, as in the case of non-magnetic materials. Such a "defect-related" approach of the A E in magnetism was also guided by what we could call a dominant idea: the domain wall (DW) motion is essentially controlled by its pinning at the various defects (grain boundaries, pores, precipitates, etc.). In other words, if there were no D W pinning centers, i.e. no defects, there would be no hysteresis. We are of the opinion that the hysteresis is better described if, in addition to the classical D W pinning, one also takes into account the "necessary" process of D W c r e a t i o n / a n n i h i l a t i o n . Let us prove the necessity of this type of process: everybody agrees that in ferromagnetic materials, on the one hand, there is no D W in a saturated state and, on the other hand, the D W are present and observable in a demagnetized state (at the coercive field, for example). Therefore, there might be a field region where the D W are created and another region where they are annihilated. This "reasonable", even "trivial" statement, implied for many years that the domain wall c r e a t i o n / a n n i h i l a t i o n process is a reversible, i.e. that it is a lossless process. Nevertheless in the early 1970s several papers were published, independently, in which the irreversibility of
the D W c r e a t i o n / a n n i h i l a t i o n was stated [11-13]. At this time, in our group we have indeed already shown that the D W c r e a t i o n / a n n i h i l a t i o n process is a dissipative process responsible for an important contribution to the hystercsis losses [13-15]. Even more, from our intensive studies, carried out on polycrystalline insulating ferrimagnets, it turned out that the pinning coefficient is always proportionnal to the domain wall energy, which means that the p i n n i n g / u n p i n n i n g mechanism can be considered as the c r e a t i o n / a n n i h i l a t i o n of small portions of D W around the pinning centers. So, at least in polycrystalline insulating ferrimagnets, the D W c r e a t i o n / a n n i h i l a t i o n processes are responsible for all the hysteresis losses, which are equal to the D W energy involved [13-15]. Although at the time of these earlier studies we were not able to propose a physical mechanism to account for the conversion of magnetic energy into heat, it does seem now that the A E is a good candidate for such a mechanism. Onc of the main reasons in favour is the observation by M o h a m m a d et al. [16] of A E bursts correlated with the collapse or creation of D W in ferroelectrics. In this paper we shall present typical qualitative results, taken from the most pertinent articles already published, as well as from our recent work, to show how the A E profiles along the hysteresis loop can be seriously affected by the shape demagnetizing effect and why A E can be attributed to the D W c r e a t i o n / a n n i h i l a t i o n process. Next, the authors' original quantitative results, including the dependence of A E on grain size, composition and temperature, will seriously contest the previous non-180 ° D W motion magnetostriction-based theory [3,4] and support our D W c r e a t i o n / a n n i h i l a t i o n explanation of the AE.
2. Experimental conditions In general the A E process consists of bursts of ultrasonic waves the central frequency as well as the spectral repartition of which are not known.The gen-
0312-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
257
M. Guyot, V. Cagan / Acoustic emission along hysteresis loop eral methods for AE detection use piezoelectric sensors and, according to the weakness of the signals, the systems are generally of a resonance type (200 kHz in our case). Our room- and variable-temperature systems have been described elsewhere [18,21]. All the previous studies, except our own, were performed exclusively on ferromagnetic metals (either single crystals or polycrystals) [1-10]. Although we have also observed AE along the hysteresis loop of conducting materials [17], we present only the results we have obtained on insulating ferrimagnets, in particular because they show no complicated effects related to eddy currents. These results were obtained mostly on several series of polycrystalline ferrimagnets: YIG, Y I G : M n , Ni and N i - Z n ferrites; a disk-shaped YIG single crystal, nearly 0.4 mm thick, cut parallel to a [110] plane, is also used to support the qualitative analysis.
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3. Shape demagnetizing effect and AE profiles
3.1. Typical experimental A E profiles In the search for an understanding of the physical origin of the AE phenomenon, many authors have i
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Fig. 2. The demagnetizing effect arising from a gap made in the sample of fig. 1, results in a tilting of t~e hysteresis loop (top) and a splitting of the AE profile (bottom).
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recorded and analysed the field (or time) dependence of the AE activity along the hysteresis loop: we shall call such curves "AE profiles". Fig. 1 shows a typical recording (for simplicity only the ascending branches are shown) of the induction (top) and of the AlE activity (bottom) as a function of the magnetizing field (triangular, 100 Hz) obtained on a toroidal polycrystalline YIG sample (mean grain size D m = 8.5 Ixm): this AE profile exhibits a broad maximum around the coercive field. A toroid is the ideal model of a closed magnetic circuit, i.e. there is no shape demagnetizing effect. Fig. 2 reports the effect of a small demagnetizing effect obtained by making a small gap (0.4 mm thick) in the previous toroidal sample. The most striking consequence of the demagnetizing effect is that the AE activity profile (bottom) now shows a minimum around the coercive field and exhibits two maxima in a field region roughly centered around the so-called "knee" of the corresponding "tilted" hysteresis loop (top). The field expansion arising from the demagnetizing effect reveals that the true AE-active zones are located around the knees, not in the coercive field region. In the case of the virgin toroid, the field values for these
M. Guyot, V. Cagan / Acoustic emission along hysteresis loop
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a relative minimum in the vicinity of the zero field (in fact close to the coercive field). The separation of the active and non-active AE zones is the result, as above, of the quite large shape demagnetizing effect of this thick disk. Similar "double-peak" AE profiles on a metallic single crystal (Incoloy 904) have been observed by Buttle et al. [6-8]. The demagnetizing effect reveals the fine structure of the AE which can be hidden on fast switching hysteresis loops.
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Fig. 3. For a YIG single-crystal thick disk, induction B (top) and AE activity(bottom) as a function of H.
knee zones are very close to each other and close to the coercive field, which causes an overlapping of the active and non-active zones, resulting in the broad AE maximum observed on the virgin toroid (fig. 1, bottom), The literature has examples of similar demagnetizing effects: the experiment of Lord et al. [2] was carried out on a long Ni wire, i.e. the demagnetizing effect was negligible so the AE activity appeared to be at a maximum around H c. May Man Kwan et al. [5] have published typical results of AE on an Fe sheet (190 x 10 x 0.45 ram3), which show two maxima close together separated by a shallow minimum around the coercive field. According to our explanation, the shape demagnetizing effect for such a sample geometry is rather small but large enough to produce the observed splitting of the two zones. This is more pronounced on the AE profile obtained by Buttle et al. on a polycrystalline c~-iron rod (length 50 mm, diameter 5 mm) [7]: the demagnetizing effect is very large there and consequently the AIE profile consists of two large maxima separated by a relative minimum. Other papers have mentioned such two-peak AE profiles, which can be explained by a more or less important shape demagnetizing factor [9,10,22]. Fig. 3 reports on our measurements on a thick YIG single-crystalline disk. The maximum amplitude of the magnetic field (triangular, 0.1 Hz), parallel to the [111] direction, is high enough to approach the saturation of the sample (top). From this figure it is obvious that there is no AIE activity close to the saturation. There exist two pronounced but broad AE maxima, roughly located in the "knee" zones of the hysteresis loop and
All the research groups above cited attribute the AE activity to the release of the elastic energy associated with the abrupt motion of non-180 ° DW [1,10]. Nevertheless, all the AE observations can also be described by the mechanism of DW creation/annihilation. Let us describe half an hysteresis loop (fig. 3), starting from the maximum negative field - H r , up to the maximum positive field + H m. As classically, decreasing the field amplitude results in (reversible) rotations of the spins towards the closest easy axis this may not give rise to any AE effect (see fig. 3 bottom, for H < - 8 0 Oe). Then comes the field zone were DW have to be created: this can explain the important AE activity observed ( - 80 Oe < H < - 10 Oe). Then, when crossing the zero-field region the DW move through the sample without appreciable DW surface variation, so the AE should decrease: this justifies the AE minimum around H c. When approaching the positive knee zone, the DW have to be annihilated: this can explain the second AE maximum. When approaching the positive saturation there are no longer DW, but only magnetization rotations: no AE is seen.
4. The quantitative study of AE As seen above, samples with a shape demagnetizing effect are very useful for qualitative analysis, but are more difficult to magnetize. Our quantitative studies were carried out on series of toroidally shaped polycrystalline insulating ferrimagnets.
4.1. A E and hysteresis losses The link we have previously introduced between the DW creation/annihilation process and the hysteresis losses and, now, the AE, leads us to compare the total cumulative activity A E ( H m) to the hysteresis losses Wh(HnO during any symmetrical hysteresis loop of maximum field H m.
4. I. 1. For a given sample For a series of loops, taken at increasing maximum field Hm, on a YIG toroid (mean grain size D m = 5 ~m), at room temperature (RT), fig. 4 shows that, for
M. Guyot, V. Cagan / Acoustic emission along hysteresis loop
259
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Wh {erg/cm 3 ) any loop, the A E activity A E ( H m) and the hysteresis losses Wh(H m) are proportional to each other, which could imply a common origin: the D W c r e a t i o n / a n n i hilation process.
4.1.2. For a given composition For a series of Y I G toroids with mean grain size D m ranging from 2.2 Ixm up to 30 Ixm, measured as above at RT, the proportionality between the A E activity and the hysteresis losses Wh, is always observed for each sample (fig. 5); in addition one can notice a monotonic increase of the maximum A E activity with Dm. A similar grain size d e p e n d e n c e of the A E activity has been previously observed by Rajan et al. [10] on decarburized steel samples.
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4.1.3. For a given sample at various temperatures O n e of the previous Y I G toroids (Dm = 20 ~ m ) has been measured as a function of the temperature from liquid nitrogen temperature up to the Curie temperature of Y I G (550 K). As can be seen in fig. 6, the proportionnality law between AlE and W h is maintained at any temperature, but the slope of the lines presents a strange temperature d e p e n d e n c e that will be discussed later. A N i Z n ferrite has also been tested over a similar temperature range [21]. 4.1.4. For various compositions Fig. 7 shows the results obtained at R T on several compounds: two garnets (one of the above YIG, one Y I G : M n ) and two spinels (one Ni ferrite and one NiZn ferrite). These polycrystals have nearly the same grain size (5 txm). The proportionality A E against W h is again maintained, at least in the first approximation: the deviation from linearity seems to be the result of the high AlE activity of some samples, which may tend to saturate the detection system. 4.2. A E and magnetostriction
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N o n e among the previous studies of AIE reports on such a linear relation between the A E and the hysteresis losses, mainly because no one tried to relate the A E activity to the hysteresis losses. One more reason is that the other theory, which relates A E to the non-180 ° D W motion, predicts a relation with the magnetostriction As, via a certain distribution of non-180 ° domains. If we look at the most extensive study in that field [4,5], only a poor relation between A E and the magnetostriction As is seen, although the authors have tried to improve the relation by selecting A100 for their SiFe
260
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samples. In a d d i t i o n t h e s e results were o b t a i n e d on polycrystalline samples: t h e r e f o r e t h e r e m i g h t not b e a u n i q u e value of A E for a given c o m p o u n d b e c a u s e of t h e grain size effect o n A E described above. 4.2.1. For various compounds at R T T h e results r e p o r t e d in section 4.1.4. are in fact a p a r t of a larger study [20] of the influence of A s on A E for which we p r e p a r e d several series of polycrystalline f e r r i m a g n e t s in o r d e r to have a b r o a d r a n g e of As values at RT. Particularly interesting was the well k n o w n M n - s u b s t i t u t e d Y I G series, in which Y3Fe4.g2 Mn0.08012 has a value of nearly As = 0 at RT.
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Fig. 8 shows the m a x i m u m A E activity m e a s u r e d at R T on several series of c o m p o u n d s , as a function of AS. As r e p o r t e d above the grain size effect results in differe n t m a x i m u m A E activities for a given composition, while As is u n i q u e for each composition. This figure d e m o n s t r a t e s the n o n - c o r r e l a t i o n b e t w e e n AlE a n d As: e.g., t h e two extreme cases r e p o r t e d in fig. 8 (A s = 0 a n d As = - 3 0 × 10 - 6 ) show roughly the same A E activity.
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M. Guyot, V. Cagan / Acoustic emission along hysteresis loop maintained when the measuring temperature is changed. Nevertheless the AE activity and the losses do exhibit very different thermal variations, which are more clearly shown on fig. 9 (the decrease of AE and Wh below 130 K is the result of the fact that the maximum magnetic field applied to the sample was not high enough to saturate the material in this temperature range). The losses Wh vary as the effective anisotropy and as the domain wall energy, in agreement with the previous observations [13,14]. The nonmonotonic AE thermal variations have at present no explanation (we are the first group to investigate AE as a function of temperature). In the same study [21], we tested a NiZn ferrite over the same temperature range: the thermal variation of AE for this sample is very different than that for the YIG, which practically eliminates a possible artefact from the experimental set-up. In any case, the AlE thermal variation cannot be explained by the well-known thermal variations of As. 5. D i s c u s s i o n
We have mentionned in the introduction that the origin of AE was attributed to the release of some magnetoelastic energy during abrupt motion of non180 ° DW. It seems that this theory of AE generation is not appropriate, for the following reasons: (i) the predicted relation with magnetostriction is not experimentally verified, (ii) the grain size effect is not taken into account. Very recently, some authors [7,22] have admitted the possibility that AlE could be associated, at least partly, with the DW creation/annihilation processes. From data obtained on a very large SiFe single crystal, a theory has been developed by Kim and Kim [22] assuming that AE is the result of the release of the magnetoelastic energy inside the DW. Such an idea is plausible, but it predicts a relation between AIE and the magnetostriction which, as we have shown, does not exist. In addition, the proportionality between the AIE activity and hysteresis losses cannot be explained by this theory. On the other hand, our explanation does predict a relation between AE and the hysteresis losses: we have shown that the law of proportionality between AE and the losses remains valid whatever the grain size, the composition and the temperature of the sample are. Nevertheless, the grain size dependence and the temperature dependence of the AE are surprising: - when the grain size increases, one expects [13,14] a decrease of the total DW surface per volume unit in the sample, which implies lower hysteresis losses, which is what we observed; consequently the AE was expected to decrease accordingly: experimentally the maximum of AE increases with the grain size (see fig. 5).
261
-when the temperature is changed the ratio between the AE activity and the hysteresis losses has a large variation; even more, in the 150 to 270 K region for the YIG sample studied the AE activity falls below the noise level of our detection system. We are of the opinion that such strange types of behavior have a common origin: the very limited bandwidth of our resonant detection chain, which can only detect the frequency component of the AE bursts at the resonant frequency. The AE bursts reflect the dynamics of the formation of DW (or portions of DW); during the "blow off" of a surface portion AS of DW, with specific energy y, the energy lost is yAS: this energy is converted, through the magnetoelastic coupling, into a burst of elastic waves the amplitude and the spectral repartition of which are representative of the energy involved and of the time duration of the process. So the resonant chain that we use (as all the other research groups do!), gives only a small part of the AE spectrum: according to Ono and Shibata [4], who tried different resonance systems on several samples, such a spectrum should be broad. First, if for a given temperature and a given composition, the AE spectral repartition is dependent on grain size, one could understand the grain size effect presented in fig. 5 [18]. Secondly, if for a given sample the AE spectral repartition is dependent on temperature fig. 9 could be understood: in particular, the absence of AE in the 150-270 K region could mean that the total AlE spectrum is outside the 200 kHz band [21]. Detailed studies of the spectral repartition of AE should be carried out to clarify this point and the observed frequency range must be extended as far as possible, particularly up to the 10-100 MHz range where it is known that DW relax: the AE could also contribute to the DW damping, a phenomenon that is still not satisfactory explained. The AE spectrum could also be a good tool for understanding the dynamics of the DW formation/destruction, a field never investigated before, which could be of basic importance in practical applications such as permanent magnets and high-frequency magnetic recording heads.
References
[1] A.E. Lord Jr., in: Physcal Acoustic, vol. X1, ed. Mason (Academic Press, New York, 1975) p. 289. [2] A.E. Lord Jr., R. Usatschew and M. Robinson, Lett. Appl. Eng. Sci. 2 (1974) 1. [3] H. Kusanagi, H. Kimura and H. Sasaki, in: Fundamentals of Acoustic Emission, ed. K. Ono (UCLA, 1978) p. 309. [4] K. Ono and M. Shibata, in: Advances in Acoustic Emission, eds. Dunegan and Hartman (Dunhart, Knoxwille, 1981) p. 154. [5] May Man Kwan, K. Ono and M. Shibata, J. Acoustic Emission 3 (1984) 144.
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M. Guyot, V. Cagan / Acoustic emission along hysteresis loop
[6] D.J. Buttle, G.A. Briggs, J.P. Jakubovics, E.A. Little and C.B. Scruby, Phil. Trans. Roy. Soc. London A 320 (1986) 363. [7] D.J. Buttle, C.B. Scruby, J.P. Jakubovics and G.A. Briggs, Phil. Mag. 55 (1987) 717. [8] D.J. Buttle, E.A. Little, C.B. Scruby, G.A. Briggs and J.P. Jakubovics, Proc. Roy. Soc. London A 414 (1987) 221. [9] J. Kameda and R. Rajan, Acta Metall. 35 (1987) 1515. [10] R. Rajan, D.C. Jiles and P.K. Rastogi, IEEE Trans. Magn. MAG-23 (1987) 1869. [11] F. Brailsford, Proc. IEE 117 (1970) 1052. [12] T.R. Hailer and J.J. Kramer, J. Appl. Phys. 41 (1970) 1034. [13] M. Guyot and A. Globus, Phys. Stat. Sol. (b) 59 (1973) 447. [14] M. Guyot, Th~se d'Etat, Paris (1975).
[15] M. Guyot and A. Globus, J. de Phys. C1 (1977) 157. [16] J.J. Mohammad, L. Zammit-Mangion, E.F. Lambson and G.A. Saunders, J. Phys. Chem. Solids 43 (1982) 749. [17] M. Guyot, T. Merceron and V. Cagan, J. Appl. Phys. 63 (1988) 3955. [18] M. Guyot, T. Merceron and V. Cagan, in: Proc. ICPMM 4, eds. W. Gorzkowski, H.K. Lachowics and H. Szymczak (Word Scientific, Singapore, 1989) p. 400. [19] M. Guyot, T. Merceron and V. Cagan, in: Proc. ICF 5, eds. C.M. Srivastava and M.J. Patni (Oxford & IBH, New Delhi, 1989) p. 215. [20] M. Guyot, T. Merceron and V. Cagan, J. Magn. Magn. Mater. 83 (1990) 217. [21] M. Guyot, T. Merceron and V. Cagan, IEEE Trans. Magn. MAG-26 (1990) 1828. [22] H.C. Kim and C.G. Kim, J. Phys. D 22 (1989) 192.