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Magnetic and ESR investigation of amorphous (FeNi)BSi reentrant spin glasses and FeB Invar alloys
Journal of Magnetism and Magw~ic Mat&Is
MO-141
WYS) 221-224
EISEVIER
Magnetic and ESR investigation of amorphous(FeNi)BSi reentrantspin glassesand FeB Invar alloys G.A. Takzei a**, M.V. Gavrilenko a, A.B. Surzhenko ‘, S.I. Tarapov b ’ Insfirute of Me/al Physics, h’o!ionol Academy ofSciences of Ukraine, 36 Ver’ernudskago aoe., 252142 Kiev, irkaine b Institute of Rodiopkysim d Elecrmaics, Notional Academy of Sciences of Ukrairre, 12 Proskwy sh., 3!0085 Kharkooc, Ub,ar$e Abstract The amorphous IFe,Ni, .r)77B13Si10 (a-FeNi) and lnvar Fe,Et,,*-, alloys have been studied. It was shown that a-F+& alloys with x z 0.08 undergo reentrant paramagnetic-ferromagnetic-spin glass (PM-FM-SG) transition. A ‘narrow’ resonance peak has been observed in the ESR spedra of a-FeNi in a limited temperature range. I1 was shown that when a frustrated FM is cooled, the formaGon of reenkanr SG Slate is preceded by the appearance of canted state.
1. Introduction
2. Experimental
The problem of reentrant transitions from a magnetically ordered state @no(FM) or antiferromagnetic (AFM)) to spin glass (SG) state is of special importance for SG physics. Several studies kwve shown that both in
We studied ribbons of the alloys (Fe,Ni, -r)7,B,3Si1,, (I= 0.08,0.09, and0.10) and Fe,B,,-, (x= 79, &I, 82, 83 and 85 at%). AH samples turned out IO be X-ray amorphous. A mutual induction bridge was used to measure the ac magnetic susceptibility. DC magnetization was measured by a vibrating-sample magnetometer. Magnetic resonance studies were carried out at a frequency of 75 GHz by a ‘BURAN’ speclrometer [6].
kc [l] and amorphous FM 121 and AFM [3] frustrated alloys c[ose to the onset of long-range magnetic order, the formation of the reenlranl SG (RSG) is preceded by the appearance of a noncollinear (canted) magnetic state when the temperature is lowered. This slate is characlerized by local distortions of the regular magnetic structure expanded to some interatomic dislances. Theorelical studies can not perfectly answer the question about the nature of this state. It was shown in the framework of mean-field theory of SG with infinite-range interaction that when a frustrated magnet is cooled the Gabay-Toulouse phase appears 141. This Slate is characterized by the coexistence of magnetic ordering along a certain direclion and SC freezing of transverse spin componcnts. It is extremely important that the last state is the result of a true phase transition and it must bc characrerized al least by order parameters of FM and SG. Clearly the transition from PM to the Gabay-Toulouse phase is not possible in this last case. At the same time tbe X-Y model of RSG transitions [S] shows the possibility of an appearance of spin canted State from PM one. The purpose of this work is the experimental investigation of the noncollinear slates in amorphous frustrated FM alloys and the passibiliry of its formation horn PM stale immediately.
’ Corresponding
author. Fax: t7-W-444
0304.8853/95/$09.50 SSDI 0304-8853(94)00992-9
1020.
B 1995 Elscvier Science B.V. All 6ghts reserved
3. Experimental
techniques
resoks
and discussion
3. I. AC stweptibili?y
In Fig. 1 the temperature dependences of real ,y; and imaginary ,yg components of ac suscep~ibilily are pnsented far the alloy (Fe,,sNio,~~)nB1,Si,, as an example, These measurements are conventional for lhe sludy of in disordered FM alloys. In to define corresponding critical temperatures correctly. In our case it is very difficu:! !o determine the RSG freezing temperature rr and the Curie lemperature Tc from curves &VI’), &VI. Thcrefore both T, and T, were determined not only from anomaties of ,$, and x;’ but also horn respective anomalies of derivatives d$,(Tl/dT (inserts in Figs. la, b). As seen from Figs. la, b strong peaks are observed in the megnetic absorption x: at temperature T, s TA _C T,. TA slrongly depends upon the driving magnetic field. Such dependence of xi(T) at T* was observed previously in some cryslalline and amorphous systems with PM-FM-SG transitions [I]. In the framework of X-Y model [S] il was explained by developing of local ttoncollinerr regions in a
magnetic
phase
transitions
some cases they permit
222
GA. Takei et al. /Journal
0
10
of Mapetism
10
30
andMagnetic
40
T(K)
Materials
10
140-144
20
24
(199s) 221-224
40
T(K)
Fig. 1. Temperature dependenees of the real xl, and imaginary ,& components of the ac susceptibility of the amorphous alloy (FeQ,8BNi,,,),,B,,Si,o in driving magnetic tields of vtious intensities: 0.5 (a) and 6.0 Oe (b). In inserts: temperature dependencesof the derivative dX;l/dT’. Measuring frequency = 84 Hz.
magnetoordered matrix in the temperature range Tr < T s Tc,. According to Ref. [S] this state may be formally identified as a homogeneous noncollinear magetic state of the whole system, which is characterized both by the spontaneous magnetic moment M, and the so called noncollinear parameter. Just this state will be implied below. 3.2. Spontaneous
magnetization
Fig. 2 demonstrates the temperature dependence of the spontaneous magnetization M. for the amorphous (Peo,,Nio,),B~Si,, alloy. The order parameter & of FM develops just below T,, where an anomaly is observed in the ac susceptibilities (see Fig. 1). Ibis unambiguously
indicates the appearance of a long-range FM order in the alloy. For Tk< T< T,, M, increases according to a quasi-Brillouin function whereas further cooling Tr < T < TA leads to a departure from this dependence. It is worth to note that 7” is the same temperature that corresponds to the maximum of ,$ measured in the lowest driving field ho = 0.5 Oe. Finally, M, drops to zero for T I T,. All these phenomena agree well with the model for the reentrant systems described by Saslow and Parker [s]. The usual process for developing of FM order is partly compensated by the simultaneous growth of a noncollinearity due to competing exchange interactions. This leads to deviation of MAT) from the quasi-Brillouin dependence down IO Tr where RSG appears.
3.3. Unirftiecfionrrl anisotropy
T(K) Fig. 2. Temperature dependences of the spontaneous M, and thermoremanent M, magnetizations of the sample (Feo,os NiQ,&TB13Si~o alloy.
The Dzyaloshinskii-Moriya anisotropy plays an impor. tant role in metallic systems. It is manifested by a displacement of the hysteresis loop toward negative magnetic fields after the system is cooled in a magnetic field. This type of anisotropy is seen only in magnetic systems where the spins are not parallel to each other, i.e. in canted and SG states [7]. Fig. 3 shows the shift of the hysteresis loop of the (Feo,,Nie.,zIB,sSi,o alloy after cooling from T> Tc down to 10 K in a magnetic field of 100 Oe. At 10 K, the shift amounts to SH = 1 Oe in the negative field direction. This means that a noncollinear magnetic state (in the case at hand the RSG state) is realized in the alloy at this temperature. It should be noted that the displacement of the hysteresis loop is still observed even in the temperature range Tt < T,. The shift SH decreases with increasing tempera-
GA. Takzei et al./Journal
ofMagnetism and Magnaic Mute&Is 140-144 fl!V$223-224
223
.
Ic.0
Tf
\ I
0.4.
i
TG
of the alloy Fig. 3. Temperature dependence of the hysteresis loop shift far (Feo.osNi,,~,),,BI~Si,o
the
alloy.
ture. This result confirms the appearance of a noncoltinear FM (AFM) slate in temperature range Tf < T< T,. ‘I’hus the analysis of all results permits us to conclude that in the strongly frustrated amorphous FM alloy (Fe,,J$,,,),, B,,Si,, the noncollinear magnetic state develops before the appearance of a RSG state and this process takes place in a close vicinity of the Curie temperature. 3.4. Magnetic
resonance
The experimental studies shown above confirm that when a frustrated FM is cooled, the formation of RSG phase is preceded by the appearance of canted state, which is local in nature. Distortions of the collinear magnetic structure are connected with ‘frustrated’ sites [5], which are occupied by atoms coupled with their neighbors via not only FM but also AFM exchange coupling. Significantly, the spins centered at these sites feel a far weaker molecular field than those at the spins of the surrounding FM matrix [5]. It might th ere f ore be expected that under certain conditions a resonance of ESR is observed for nearly free spins. Fig* 4 shows magnetic resonance lines of amorphous alloy (Fea,Ni,,.,),,B,,Si,~ at various temperatures. Besides the usual magnetic-resonance peak the alloy exhibits another resonance peak with a half-width H = 200 Oe. Within the experimental error, this width is independent of the temperature. We will refer to the latter resonance as the ‘narrow’ one. To the best of our knowledge, this effect has not been seen previously in systems which exhibit reentrant PM-FM-SG transitions, According to Ref. [5] frustrated sites appear at temperatures Tr < T < T, because of competing exchange interactions existence in alloys near the critical concentration for the onset of a long-range FM order. The associated spins are in a molecular field far weaker than that for spins of the FM surroundings. If these nearly free spins interact weakly with each other and with
the surrounding FM matrix, then they can apparently be regarded as a nearly inoependent ESR system. Quite clearly this resonance should disappear when the alloy undergoes a transition to a frozen RSG state at a temperature T(H) < T(0) = 18 K where H is the external mapetic field. We wish to stress that this narrow resonance is not observed in the alloys with x = 0.09 and x = 0.10. A possible reason is that on the magnetic phase diagram these alloys locate farther from the critical concentration x0 = 0.75 for the onset of FM order inside the FM phase, so that at temperatures T, < T < T, they have fewer frustrated sites than the allay with x = 0.08. 3.5. The magnetic ground state
ofInvar
alloys FexBloO _ x
In order to explain the nature of the magnetic ground state of Fe,B,,-,,,_, alloys, we measured the temperature dependence of the real xl, and imaginary x0” parts of the
10
20
30
T(K)
Fig. 5. Temperature depcndences of the real X; CA, B, C) and imaginary ~6 (A’, B’, C’) pasts of the ac susceptibility of the amorphous alloy Fe,B, in driving magnetic fields of various intensities; 0.08 (A), 0.30 (B), 0.60 Oe (C). Measuring frequency =73Hz.
224
GA. Takzei et al. /Journal
of Magnetism and Magnetic Materials 140-144 (1995) 221-224
ac susceptibility. Fig. 5 shows the results for the alloy Fe,,&,. It can be seen that in the minimum driving field h, = 0,08 Oe, the x$T) curves have an inflection whereas x$(T) curves have a peak at the temperature rA = 14.5 K. These dependences are reminiscent of the behavior of x;(r) and ,&?I for those systems in which reentrant temperature-induced FM-RSG transitions occur (see Section 3.1). However, in such a case, below the temperature Tr for the formation of the RSG phase, &(T) and xi(T) + 0 as T + 0 [1,2]. At the same time it can be seen that the relative changes in x;(T) and x:(T) are very small below T& Moreover, in contrast to the present alloy, in the systems with RSG transitions the anomalies in xi(T) around Tr are almost independent of the amplitude of the driving field [l-3]. Therefore, just like in the case of the classical lnvar alloys [S] no RSG transition occurs in the alloy Fe,B,, at liquid-helium temperature. Thus, as in the case of the classical Invar alloys Fe,Ni,,-, @I, the magnetic ground state of the amorphous Invar alloys FeX13,0,,-x is inhomogeneous in nature, and its spin structure can be regarded as a FM matrix with regions of canted spins distributed in it which have an effective size of several interatomic spacings. Nevertheless, such a state as mentioned above can be formally Llentified with a uniform noncollinear magnetic order that extends over the entire volume of the alloys.
Acknowledgements: This work was supported by the State Committee of Sciences and Technology of Ukraine and, in part, by a Sores Foundation Grant awarded by the American Physical Society.
References [l] G.A. Takzei, Yu.P. Grebenyuk, A.M. Kostyshin et al., Fiz. Tverd, Tela (Leningrad) 29 (1987) 83 [Sov. Phys. Solid State 29 (1987) 461; G.A. Takzei, Yu.P. Grebenyuk and 1.1. Sych, Zh. Eksp. Teor. Phys. 97 (1990) 1022 ISov. Phys. JETP 70 (1990) 5721. [2] G.A. Takzei, M.V. GavriIenko, Yu.P. Grebenyuk et al., Fiz. Tvrrd. Tela (Leningrad) 31 (1989) 1 [Sov. Phys. Solid State 31 (1989) 9151, [3] G.A. Takzei, A.B. Sunhenko, 1.1.Sych et al., J. Magu. Magn. Mater. 118 (1993) 77. [4] M. Gabay and G. Toulouse, Phys. Rev. l&t. 47 (1982) 201. [5] W.M. Saslow and G. Parker, Phys. Rev. Lett. 56 (1986) 1074: G.N. Parker and W.M. Saslow, Phys. Rev. 38 (1988) 11718;
G.N. Parker and W.M. Saslow, J. Appl. Phys. 67 (1990) 5976. [6] A.A, Vertiy, I.V. Ivanchenko, N.A. Popenko et al., Int. J. Infr. MMW 10 (1989) 395. [7] K. Binder and A.P. Young. Rev. Mod. Phys. 58 (1986) 801. [8] G.A. Takzei, 1.1.Sych, A.M. Kostyshin et al., Metallofizika 9 (1987) 47.
MO-141
WYS) 221-224
EISEVIER
Magnetic and ESR investigation of amorphous(FeNi)BSi reentrantspin glassesand FeB Invar alloys G.A. Takzei a**, M.V. Gavrilenko a, A.B. Surzhenko ‘, S.I. Tarapov b ’ Insfirute of Me/al Physics, h’o!ionol Academy ofSciences of Ukraine, 36 Ver’ernudskago aoe., 252142 Kiev, irkaine b Institute of Rodiopkysim d Elecrmaics, Notional Academy of Sciences of Ukrairre, 12 Proskwy sh., 3!0085 Kharkooc, Ub,ar$e Abstract The amorphous IFe,Ni, .r)77B13Si10 (a-FeNi) and lnvar Fe,Et,,*-, alloys have been studied. It was shown that a-F+& alloys with x z 0.08 undergo reentrant paramagnetic-ferromagnetic-spin glass (PM-FM-SG) transition. A ‘narrow’ resonance peak has been observed in the ESR spedra of a-FeNi in a limited temperature range. I1 was shown that when a frustrated FM is cooled, the formaGon of reenkanr SG Slate is preceded by the appearance of canted state.
1. Introduction
2. Experimental
The problem of reentrant transitions from a magnetically ordered state @no(FM) or antiferromagnetic (AFM)) to spin glass (SG) state is of special importance for SG physics. Several studies kwve shown that both in
We studied ribbons of the alloys (Fe,Ni, -r)7,B,3Si1,, (I= 0.08,0.09, and0.10) and Fe,B,,-, (x= 79, &I, 82, 83 and 85 at%). AH samples turned out IO be X-ray amorphous. A mutual induction bridge was used to measure the ac magnetic susceptibility. DC magnetization was measured by a vibrating-sample magnetometer. Magnetic resonance studies were carried out at a frequency of 75 GHz by a ‘BURAN’ speclrometer [6].
kc [l] and amorphous FM 121 and AFM [3] frustrated alloys c[ose to the onset of long-range magnetic order, the formation of the reenlranl SG (RSG) is preceded by the appearance of a noncollinear (canted) magnetic state when the temperature is lowered. This slate is characlerized by local distortions of the regular magnetic structure expanded to some interatomic dislances. Theorelical studies can not perfectly answer the question about the nature of this state. It was shown in the framework of mean-field theory of SG with infinite-range interaction that when a frustrated magnet is cooled the Gabay-Toulouse phase appears 141. This Slate is characterized by the coexistence of magnetic ordering along a certain direclion and SC freezing of transverse spin componcnts. It is extremely important that the last state is the result of a true phase transition and it must bc characrerized al least by order parameters of FM and SG. Clearly the transition from PM to the Gabay-Toulouse phase is not possible in this last case. At the same time tbe X-Y model of RSG transitions [S] shows the possibility of an appearance of spin canted State from PM one. The purpose of this work is the experimental investigation of the noncollinear slates in amorphous frustrated FM alloys and the passibiliry of its formation horn PM stale immediately.
’ Corresponding
author. Fax: t7-W-444
0304.8853/95/$09.50 SSDI 0304-8853(94)00992-9
1020.
B 1995 Elscvier Science B.V. All 6ghts reserved
3. Experimental
techniques
resoks
and discussion
3. I. AC stweptibili?y
In Fig. 1 the temperature dependences of real ,y; and imaginary ,yg components of ac suscep~ibilily are pnsented far the alloy (Fe,,sNio,~~)nB1,Si,, as an example, These measurements are conventional for lhe sludy of in disordered FM alloys. In to define corresponding critical temperatures correctly. In our case it is very difficu:! !o determine the RSG freezing temperature rr and the Curie lemperature Tc from curves &VI’), &VI. Thcrefore both T, and T, were determined not only from anomaties of ,$, and x;’ but also horn respective anomalies of derivatives d$,(Tl/dT (inserts in Figs. la, b). As seen from Figs. la, b strong peaks are observed in the megnetic absorption x: at temperature T, s TA _C T,. TA slrongly depends upon the driving magnetic field. Such dependence of xi(T) at T* was observed previously in some cryslalline and amorphous systems with PM-FM-SG transitions [I]. In the framework of X-Y model [S] il was explained by developing of local ttoncollinerr regions in a
magnetic
phase
transitions
some cases they permit
222
GA. Takei et al. /Journal
0
10
of Mapetism
10
30
andMagnetic
40
T(K)
Materials
10
140-144
20
24
(199s) 221-224
40
T(K)
Fig. 1. Temperature dependenees of the real xl, and imaginary ,& components of the ac susceptibility of the amorphous alloy (FeQ,8BNi,,,),,B,,Si,o in driving magnetic tields of vtious intensities: 0.5 (a) and 6.0 Oe (b). In inserts: temperature dependencesof the derivative dX;l/dT’. Measuring frequency = 84 Hz.
magnetoordered matrix in the temperature range Tr < T s Tc,. According to Ref. [S] this state may be formally identified as a homogeneous noncollinear magetic state of the whole system, which is characterized both by the spontaneous magnetic moment M, and the so called noncollinear parameter. Just this state will be implied below. 3.2. Spontaneous
magnetization
Fig. 2 demonstrates the temperature dependence of the spontaneous magnetization M. for the amorphous (Peo,,Nio,),B~Si,, alloy. The order parameter & of FM develops just below T,, where an anomaly is observed in the ac susceptibilities (see Fig. 1). Ibis unambiguously
indicates the appearance of a long-range FM order in the alloy. For Tk< T< T,, M, increases according to a quasi-Brillouin function whereas further cooling Tr < T < TA leads to a departure from this dependence. It is worth to note that 7” is the same temperature that corresponds to the maximum of ,$ measured in the lowest driving field ho = 0.5 Oe. Finally, M, drops to zero for T I T,. All these phenomena agree well with the model for the reentrant systems described by Saslow and Parker [s]. The usual process for developing of FM order is partly compensated by the simultaneous growth of a noncollinearity due to competing exchange interactions. This leads to deviation of MAT) from the quasi-Brillouin dependence down IO Tr where RSG appears.
3.3. Unirftiecfionrrl anisotropy
T(K) Fig. 2. Temperature dependences of the spontaneous M, and thermoremanent M, magnetizations of the sample (Feo,os NiQ,&TB13Si~o alloy.
The Dzyaloshinskii-Moriya anisotropy plays an impor. tant role in metallic systems. It is manifested by a displacement of the hysteresis loop toward negative magnetic fields after the system is cooled in a magnetic field. This type of anisotropy is seen only in magnetic systems where the spins are not parallel to each other, i.e. in canted and SG states [7]. Fig. 3 shows the shift of the hysteresis loop of the (Feo,,Nie.,zIB,sSi,o alloy after cooling from T> Tc down to 10 K in a magnetic field of 100 Oe. At 10 K, the shift amounts to SH = 1 Oe in the negative field direction. This means that a noncollinear magnetic state (in the case at hand the RSG state) is realized in the alloy at this temperature. It should be noted that the displacement of the hysteresis loop is still observed even in the temperature range Tt < T,. The shift SH decreases with increasing tempera-
GA. Takzei et al./Journal
ofMagnetism and Magnaic Mute&Is 140-144 fl!V$223-224
223
.
Ic.0
Tf
\ I
0.4.
i
TG
of the alloy Fig. 3. Temperature dependence of the hysteresis loop shift far (Feo.osNi,,~,),,BI~Si,o
the
alloy.
ture. This result confirms the appearance of a noncoltinear FM (AFM) slate in temperature range Tf < T< T,. ‘I’hus the analysis of all results permits us to conclude that in the strongly frustrated amorphous FM alloy (Fe,,J$,,,),, B,,Si,, the noncollinear magnetic state develops before the appearance of a RSG state and this process takes place in a close vicinity of the Curie temperature. 3.4. Magnetic
resonance
The experimental studies shown above confirm that when a frustrated FM is cooled, the formation of RSG phase is preceded by the appearance of canted state, which is local in nature. Distortions of the collinear magnetic structure are connected with ‘frustrated’ sites [5], which are occupied by atoms coupled with their neighbors via not only FM but also AFM exchange coupling. Significantly, the spins centered at these sites feel a far weaker molecular field than those at the spins of the surrounding FM matrix [5]. It might th ere f ore be expected that under certain conditions a resonance of ESR is observed for nearly free spins. Fig* 4 shows magnetic resonance lines of amorphous alloy (Fea,Ni,,.,),,B,,Si,~ at various temperatures. Besides the usual magnetic-resonance peak the alloy exhibits another resonance peak with a half-width H = 200 Oe. Within the experimental error, this width is independent of the temperature. We will refer to the latter resonance as the ‘narrow’ one. To the best of our knowledge, this effect has not been seen previously in systems which exhibit reentrant PM-FM-SG transitions, According to Ref. [5] frustrated sites appear at temperatures Tr < T < T, because of competing exchange interactions existence in alloys near the critical concentration for the onset of a long-range FM order. The associated spins are in a molecular field far weaker than that for spins of the FM surroundings. If these nearly free spins interact weakly with each other and with
the surrounding FM matrix, then they can apparently be regarded as a nearly inoependent ESR system. Quite clearly this resonance should disappear when the alloy undergoes a transition to a frozen RSG state at a temperature T(H) < T(0) = 18 K where H is the external mapetic field. We wish to stress that this narrow resonance is not observed in the alloys with x = 0.09 and x = 0.10. A possible reason is that on the magnetic phase diagram these alloys locate farther from the critical concentration x0 = 0.75 for the onset of FM order inside the FM phase, so that at temperatures T, < T < T, they have fewer frustrated sites than the allay with x = 0.08. 3.5. The magnetic ground state
ofInvar
alloys FexBloO _ x
In order to explain the nature of the magnetic ground state of Fe,B,,-,,,_, alloys, we measured the temperature dependence of the real xl, and imaginary x0” parts of the
10
20
30
T(K)
Fig. 5. Temperature depcndences of the real X; CA, B, C) and imaginary ~6 (A’, B’, C’) pasts of the ac susceptibility of the amorphous alloy Fe,B, in driving magnetic fields of various intensities; 0.08 (A), 0.30 (B), 0.60 Oe (C). Measuring frequency =73Hz.
224
GA. Takzei et al. /Journal
of Magnetism and Magnetic Materials 140-144 (1995) 221-224
ac susceptibility. Fig. 5 shows the results for the alloy Fe,,&,. It can be seen that in the minimum driving field h, = 0,08 Oe, the x$T) curves have an inflection whereas x$(T) curves have a peak at the temperature rA = 14.5 K. These dependences are reminiscent of the behavior of x;(r) and ,&?I for those systems in which reentrant temperature-induced FM-RSG transitions occur (see Section 3.1). However, in such a case, below the temperature Tr for the formation of the RSG phase, &(T) and xi(T) + 0 as T + 0 [1,2]. At the same time it can be seen that the relative changes in x;(T) and x:(T) are very small below T& Moreover, in contrast to the present alloy, in the systems with RSG transitions the anomalies in xi(T) around Tr are almost independent of the amplitude of the driving field [l-3]. Therefore, just like in the case of the classical lnvar alloys [S] no RSG transition occurs in the alloy Fe,B,, at liquid-helium temperature. Thus, as in the case of the classical Invar alloys Fe,Ni,,-, @I, the magnetic ground state of the amorphous Invar alloys FeX13,0,,-x is inhomogeneous in nature, and its spin structure can be regarded as a FM matrix with regions of canted spins distributed in it which have an effective size of several interatomic spacings. Nevertheless, such a state as mentioned above can be formally Llentified with a uniform noncollinear magnetic order that extends over the entire volume of the alloys.
Acknowledgements: This work was supported by the State Committee of Sciences and Technology of Ukraine and, in part, by a Sores Foundation Grant awarded by the American Physical Society.
References [l] G.A. Takzei, Yu.P. Grebenyuk, A.M. Kostyshin et al., Fiz. Tverd, Tela (Leningrad) 29 (1987) 83 [Sov. Phys. Solid State 29 (1987) 461; G.A. Takzei, Yu.P. Grebenyuk and 1.1. Sych, Zh. Eksp. Teor. Phys. 97 (1990) 1022 ISov. Phys. JETP 70 (1990) 5721. [2] G.A. Takzei, M.V. GavriIenko, Yu.P. Grebenyuk et al., Fiz. Tvrrd. Tela (Leningrad) 31 (1989) 1 [Sov. Phys. Solid State 31 (1989) 9151, [3] G.A. Takzei, A.B. Sunhenko, 1.1.Sych et al., J. Magu. Magn. Mater. 118 (1993) 77. [4] M. Gabay and G. Toulouse, Phys. Rev. l&t. 47 (1982) 201. [5] W.M. Saslow and G. Parker, Phys. Rev. Lett. 56 (1986) 1074: G.N. Parker and W.M. Saslow, Phys. Rev. 38 (1988) 11718;
G.N. Parker and W.M. Saslow, J. Appl. Phys. 67 (1990) 5976. [6] A.A, Vertiy, I.V. Ivanchenko, N.A. Popenko et al., Int. J. Infr. MMW 10 (1989) 395. [7] K. Binder and A.P. Young. Rev. Mod. Phys. 58 (1986) 801. [8] G.A. Takzei, 1.1.Sych, A.M. Kostyshin et al., Metallofizika 9 (1987) 47.