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Reentrant phenomena in Invar FeG5(Ni,xMnx)35 lloysG.A. Takzei, Yu. P. Grebenyuk, and I . I . SychInstitute of Metal Physics, Acade my of Sciencesof the Ukrainian SSR, Kiev(Submitted 26 September 1989)Zh. Eksp. Teor. Fiz. 97,1022-1030 (March 1990)Cooling of Fe,, (Ni,_,n, ) alloys with alloying element concentrations close to the criticalvalue for the appearance of the ferromagnetic order produced the following sequence of magnetictransitions: paramagnet-collinear ferromagnet-canted ferromagnet (asperornagnet)-reentrantspin glass. The Invar alloy Fe,,Ni,, exhibited an asperomagnetic state at temperatures below 20K.

1. INTRODUCTIONIt is now firmly established that in the case of fcc InvarFeNi alloys the interaction between the iron atoms separated

by the shortest distances is antiferromagnetic (AFM)whereas the nearest-neighbor Fe-Ni and Ni-Ni interactionsare ferromagnetic (FM). '. ' In other words, these alloys be-long to a class of systems with competing exchange interac-tions. It is therefore necessary to determine their groundmagnetic state. However, the available experimental resultsare contradictory. For example, it is reported in Refs. 3 and 4that Invar FeNi alloys exhibit the long-range AFM order atT=4.2 K. However, neutron-diffraction investigations of asingle crystal of Fe,, Ni,, have failed to confirm the long-range AFM order.5 Other authors (see, for example, the re-view in Ref. 6) are of the opinion that the low-temperaturemagnetic structure of Invar FeNi alloys can be regarded ascollinear ferromagnets with randomly distributed AFM re-gions of dimensions of the order of several lattice constants.Finally, the authors of Ref. 7 report observation of a reen-trant temperature-induced ferromagnetic-spin glass(FM-SS) transition in Invar FeNi alloys at temperaturesT S 30K. However, the occurrence of a reentrant spin glassstate in these alloys is rejected in Ref. 8.It follows from this account that there is as yet no gener-ally accepted point of view on the nature of the ground mag-netic state of Invar FeNi alloys.Our aim was to investigate reentrant temperature-induced PM-FM-SS transitions (PM stands for aferromagnet) in alloys corresponding to the quasibinaryFe,, (Ni,-.n, ),, tie-line, which includes also the classi-cal Invar alloy Fe,,Ni,,, and to determine the nature of theirlow-temperature magnetic state.2. EXPERIMENTAL METHODWe used cylindrical samples with the height-to-diame-ter ratio of- 0, which were quenched from 1200 Kin waterbefore measurements. The static magnetization was deter-mined in the temperature range 4.2-300 K using a vibrating-sample magnetometer. The temperature dependences of thespontaneous magnetization were obtained by the kink meth-od9 and by the Arrott-Belov method.'' In both cases theresults were qualitatively similar.The real (x&and imaginary ( ~ 6 )arts of the dynamicmagnetic susceptibility in magnetization-reversing fields h,=0.3-10.0 Oe were determined at temperatures 1.4-300 K

using apparatus described in Ref. 11. The vertical compo-nent of the magnetic field of the earth was compensated towithin f 0%.

3. EXPERIMENTALRESULTSAND DISCUSSIONWe shall first consider the FM alloys with thealloying element concentrations close to the criticalFe,, (Ni, _, n, ) corresponding to the appearance of thelong-range ferromagnetic order.

3.1. Temp erature dependen ce of the spontaneousmagnetizationFigure 1 shows, by way of example, the temperaturedependence of the spontaneous magnetization I, of theFe,,(Ni, _ ,Mn, ) alloy with x =0.2 and it demonstratesthat the spontaneous moment (which is the FM order pa-rameter) appeared at the Curie point T, == 165K and it in-creased as a result of cooling. However, at a certain tempera-ture TA


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It is natural to attribute this effect, predicted in Ref. 12,to the establishment of a noncollinear FM (asperomagnetic)state in the alloys. Let us assume that in the FM state (attemperatures T, < T< T, ) the direction of the spontaneousmagnetization vector I, coincides with thez axis. The t ransi-tion to the asperomagnetic (ASM) state results in the ap-pearance of randomly frozen x and y spin projections,I2which is equivalent to an effective reduction in the z projec-tion I, ( T ) .This is precisely the situation which occurs inreality (Fig . I ) , since cooling of the alloy below T, makesthe I, (T ) curve "lag" the quasi-Brillouin dependence.We found that stronger cooling of this alloy reducedsteeply the spontaneous magnetization to zero at T, = 50 K(Fig. 1 .Hence, the alloy assumed the reentrant SS state inwhich there was no long-range ferromagnetic order. Thisconclusion was confirmed by the results of recent investiga-tions of low-angle neutron scattering reported for alloys ofthe same class, but with somewhat different compositionsand x close to x , (Ref. 13). It should also be stressed thatthis result conflicted with the predictions of the molecularfield theory for Heisenberg disordered ferromagnet~'~~'~c-cording to which a degenerate SS coexists with the long-range ferromagnetic order at temperatures T < T f .

It therefore follows that the experimental results plot-ted in Fig. 1 demonstrate that, firstly, at temperatures TA< T, there is a change in the sta te of a disordered ferromag-net which can be treated as a transition from the collinear tothe noncollinear F M (ASM) state. Secondly, at even lowertemperatures Tf there is a phase transition to the reentrantSS state accompanied by the loss of the long-range magneticorder. Additional arguments in support of these conclusionswill be given below.3.2.Thermoremanentmagnetizations and magnetization-reversal loops

In addition to the spontaneous magnetization, Fig. 1shows the temperature dependence of the thermoremanentmagnetization I, of the alloy under consideration. It followsfrom the results obtained that below Tf the dependence I ,(T ) s extremely steep. This is evidence of the appearance ofstrong longitudinal irreversibilities at T < T f .At higher tem-peratures T


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netization within the limits of ASM fluctuations and at tem-peratures T


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FI G. 5. Magnetic phase diagram of the Fe, , (Ni, .Mn,) , , system ofalloys: 1 ) paramagnetic; 2) ferromagnetic; 3) asperomagnetic; 4 ) spinglass regions. So urce of data: ( X ) Ref. 25; ( + ), ( A ) Ref. 32; our d atawere obtained from the dependences xh ( T) and xb ( T) represented by 0and and from the dependences I, T ) a n d I, T) represented by M.

the ferromagnetic range of compositions (Fig. 5 ) .The line of the Curie temperatures T, represents thephase transition from the PM state (region 1 ) to the collin-ear FM state (region2 ) .At lower temperatures a canted FM(ASM) state appears in these alloys (region 3 ) .The T, linerepresents the appearance of asperomagnetism and it is de-duced from the temperature of the corresponding anomalyof the dependencex; ( T ) t minimum values of the magneti-zation-reversing field h,.It is pointed out above that T, depends strongly on h,. Asimilar result is reported in Ref. 31 for amorphousPd,, , e, Si,, alloys. Hence, it follows that the T, line inthe phase diagram (Fig. 5 ) , obtained from the dynamic mag-netic susceptibility data in the lowest magnetic fields, is notstrictly speaking a phase-transition line. However, there is nodoubt that the state of the alloys in this part of the phasediagram differs greatly from the collinear ferromagnetic stateand can be identified with asperomagnetism. This conclusionis supported by the observation that the experimental dataobtained by other methods on the temperatures of the appear-ance of the subcritical neutron ~cat t e r ing ,~~he temperaturesof deviations of the dependence I, T ) from the quasi-Bril-louin curve, and the temperature dependences of the appear-ance of weak longitudinal irreversibilities (Fig. 1 ) are all ingood agreement with the values of T, obtained from the de-pendencesX (T ) Fig. 5 ) . Moreover, it is in this part of thephase diagram that we have the Dzyaloshinskii-Moriya ani-sotropy (Fig. 2 ) , which is observed in frozen noncollinearmagnetic structures.

It is shown above that in the case of Fe,, (Ni, ,Mn, )alloys with x close to the critical concentration x, at thetemperature Tf here is a phase transition to the state of areentrant SS (region 4 in Fig. 5 ) characterized by the ab-sence of the long-range ferromagnetic order. Consequently,

in this case the T f ( x ) ine can quite properly be plotted in thephase diagram.Figure 5 does not separate the regions in which the SSstate appears from the PM and ASM states. This allows forthe fact that in the case of the alloy with x =0.2 the SS statedoes not exhibit the long-range ferromagnetic order. Conse-quently, in the case of this system of alloys the differencebetween the reentrant SS phase and the SS phase which ap-pears as a result of the PM-SS transition is manifested onlyin the magnitude of the FM correlations over regions of fi -nite size. In all other respects the two states are clearly iden-tical.

Our results thus demonstrate that cooling ofFe,,(Ni, ,Mn, ) alloys with compositions in the rangex


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Tela (Leningrad) 29,83 ( 1987) [Sov. Phys. Solid State 29,46 ( 1987) 1.2'G . A. Takzei, M. V. Gavrilenko, Yu. P. Grebenyuk et al., Fiz. Tverd.Tela (Leningrad) 31(6), 1 (1989) [Sov. Phys. Solid State 31, 91 5(1989) l .22W.M. Saslow and G. Parker, Phys. Rev. Lett. 56, 1074 (19 86 ).23G . arker and W. M. Saslow, J. Appl. Phys. 63, 3998 (19 88 ).24G.N. Parker and W. M. Saslow, Phys. Rev. B 38, 11718 (1988).25B.N. Mokhov, V. I. Goman'kov, V. A. Makarov et at., Zh. Eksp. Teor.Fiz. 72, 1833 (1977) [Sov. Phys. JET P 45,962 (1977) 1.2"u. A. Izyum ov and R. P. Ozerov, Magnetic Neutron Diffraction, Ple-num Press, New York (1970).27G.A. Takzei, A. M. Kostyshin, I. I. Sych et al., Metallofizika 9 ( 2 ) , 47

577 Sov. Phys. JETP 70 (3),March1990

(1987) .'VV. L. Sedov, Pis'ma Zh. Eksp. Teor. Fiz. 14,499 ( 1971) [JE TP Lett . 14,341 (1971) l .29N.N. Kuz'min and A. Z. Men'shikov, Fiz. Met. Metalloved. 49, 433(1980) ."A. V. Deryabin and V. E. Rode, Fiz. Met. Metalloved. 44,8 72 ( 1977)."R. B. Goldfarb, K . V. Rao, and H. S. Chen, J. M agn. Magn. M ater. 54-57. 111 (1986)..32M.Shiga, T . ~atake, . Wada et al., J. Magn. Magn. Mater. 51, 123(1985) .Translated by A. Tybulewicz

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