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Effect of magnetic fields on phase transitions in the reentrant spin glass FexMn1−xTiO3
Journal of Magnetism and Magnetic Materials 104-!07 (1992) 1639-1640 North-Holland i
Effect of magnetic fields on phase transitions in the reentrant spin glass Fe.,.Mn i _xTiO 3 H.
Aruga Katori ", T. Goto ~, S. Ebii b and A. lto b
"blstitute for Solid State Physics, Unit'ersit3.,of Tokyo, Roppongi, Minato-ku, Tok),o 106, Japan t, Department of Physics, Faculty of Science, Ochanomizu Universi~,, Bunk3,o-ku, Tokyo 112, Japan Effect of magnetic fields on phase transitions in the exchange competing system Fe,.Mn~_.,TiO 3 with x = 0.60, 0.65 and 0.75, which exhibit the reentrant spin-glass transition from the antiferromagnetic state in weak fields, has been investigated by measuring the magnetization process in pulsed magnetic fields. The spin-flip-like transition (metamagnetic transition) is observed at 4.2 K for each sample. The critical field of the transition decreases monotonically with the increase of temperature and this trat sition vanishes above the Nt~el temperature. The field variations of the reentrant spin-glass transition and N~ei temperatures have been also observed. From these observations the fieid-temperatur., phase diagram is determined for each of these reentrant spin-glass samples. The shape of the phase diagram is qualitatively consistent with the theoretical prediction given for Ising systems exhibiting the reentrant transition from the antiferromagnetic sta,.~.. The magnetic properties of spin-glass systems have bccn widely examined so far. The reentrant phenomenon is one of the most stimulating subjects in these systems. We consider that the experimental investigation of the magnetic field effect on the reentrant spin glass presents some new aspects for understanding the reentrant transition. However, systematic studies of the reentrant spin glass is magnetic fields have scarcely been reported. The Fe.,Mn~_,TiO 3 spin glass is a mixture of the antifcrromagnets FcTiO 3 and MnTiO 3, having easyaxis anisotropy along the hexagonal c-axis. The spins in FcTiO 3 and MnTiO 3 arc coupled antifcrromagnctitally between adjacent c-layers. However, the intralaycr coupling of the spins is ferromagnetic in FcTiO 3 and antifcrromagnctic in MnTiO 3. Thcrcflwc. in the mixed compound Fc,Mn~ .,TiO 3 the competition between the ferromagnetic and antiferromagnetic interactions within a c-layer causes frustrations. In the previous papers, we have shown that Fe, Mn~_.,TiO 3 with intermediate Fe concentration exhibits a typical lsing spin-glass (SG) type of behaviour [1-3]. We have also shown that the Fe-rich samples undergo the transition from the antiferromagnetic (AF) state to the rcentrant spin-glass (RSG) state. We have studied the field-dependent phenomena in the recntrant spin-glass samples with x = 0.60, 0.65 and 0.75 by measuring the temperature variations of the dc magnetizations at various fields up to 6 T [4,5]. Wc have found that magnetic fields have a remarkable influence on the transition temperatures. The temperature giving the maximum magnetization, TM(H), coincides ~ith the N6el temperature in the low-field limit. TM(H) decreases with an increase of magnetic field, but vanishcs above a certain field. Meat, while, the reentrant spin-glass transition temperature TRsa(H) increases with magnetic field and begins to decrease abruptly
after being coincident with TM(H). The field variations of TM(H) and TRsG(H) are qualitatively consistent with the results obtained from the theoretical work [6-10]. We pay attention to the prediction given by one of the theoretical investigations that spin-flip transitions occur below a certain temperature [6]. In order to clarify whether this transition exists or not in our system, we study in detail the mag~ictic pha~c diagram of this system in the field-temperature plane by measuring the high-field magnetization processes at various temperatures. The high-field magnetization processes wcrc measured in pulsed magnetic fields up to about 15 T at various temperatures. The c-axis of the single-crystal sample was set parallel to the measuring field. The magnetization curves for the x = 0.60, 0.65 and 0.75 samples change with temperature in a qualitatively similar way. As an example we show in fig. 1 the magnetization curves ( M ( H ) against H ) at various temperatures for the x = I).65 sample with TM(0.001 T ) ( = T N)=37.1 K and TRstq(0.001 T ) = 2 2 . 0 K. A
:!4oL 60/ 00
0
0
10 ~0 0 Magnetic Field (T)
3'
6
Fig. 1. Magnetization processes of F%65Mno35TiO3 at vari ous temperatures.
0312-88~3/9,/$05.00 ~ 1992 ElsevierScience Publishers B.V. All rights reser','ed
1640
H. Aruga Katori et al. / The reentrant spin glass Fe, Mn t _ , TiO 3 6
'
5
E
Fe 0.¢,5Mn 0.35TiO 3
~,
•~ 4 "~
3
~
'
~)
O Tm(H,
'.~ SG ~
O 'l'rs¢;(tl~ A tlmd'~
p
2
1 0
0
,
,
10
"
' O-
-
O
~
20 30 40 Temperature (K) Fig. 2. Field-temperature phase diagram of Feu.~,5Mn u.35TiO3. The solid lines are a guide to the eye. P, paramagnetic phase:, AF, anti ferromagnetic phase; RSG, reentrant spin-glass phase and SG, spin-glass phase (which is tentative). spin-flip-like transition is observed at 4.2 K with a large amount of hysteresis. This transition field H m ( T ) is determined by the field where d M ( H ) / d H has a maximum value. We obtain Hm(4.2 K) = 2.4 T. As the temperature increases, H m ( T ) decrease and the width of the hysteresis becomes small monotonically up to 37 K, We cannot find any noticeable change in M ( H ) at Trsc~(0,001 T). Above T n, Hm(T)vanishcs, but M ( H ) still changes nonlinearly with magnetic field up to 60 K, which is the highest temperature examined. This indicates that the short-range spin correlation remains above T N, which is supported by thc M6ssbaucr mcasurcmcnt [1 I]. In fig. 2, wc plot H m ( T ) for the x = 0.65 sampic in the ficld-tcmpcraturc plane. Wc also plot in fig. 2 the transition tcmpcraturcs TrotH) and T r s ( ; ( H ) for thc x = 0.65 sample determined by the temperature variations of magnctization at various ficlds [4,5]. Wc find that magnetic fields suppress the AF order "and facilitate the appearance of the RSG state below about (1.7 T where Trs(;(H) coincides with TM(H). Bctwccn about 0.7 and 2 T the sample scems to undergo the transition from the paramagnetic state to the RSG statc directly. Above about 2 T and below about 15 K, H m ( T ) dcviatcs from Trst3(H). In thc prcvious investigations on Fc,Mn~ , T i P 3 we havc found that in thc SG state only the SG ordering exists whcrcas in the RSG state the AF iong-rangc order cocxists with thc SG ordcring [12,13]. From thc results obtained by thc M6ssbaucr and the ncutron scattering measurements [11-14], wc have shown that the transverse spin components exist and fluctuate rather incoherently with characteristic time scales longer than the M6ssbaucr time scale of 10 7 s, and that below T N only the longitudinal spin components form the AF long-range order. From these observations, wc consider that thc
longitudinal spin components contribute to the spinflip-like transition. Thcrcforc, wc infer that below about 15 K this samplc undergoes the spin-flip-likc transition frorn the RSG state to the SG state, although any remarkable diffcrcnccs could not bc found in the magnetization proccsscs above H m ( T ) whcthcr the sample is in the SG state or the paramagnetic state. The high-field magnetization processes of this system up to 100 T will bc reported in a separate paper [15]. Theoretical investigations have been done for a system undergoing the recntrant transition from the A F state [6-10]. Recently, Takayama proposed an extended mean-field model for the present system [6]. Hc presents a field-temperature phase diagram where the AF-to-paramagnetic transition line merges into the first-order spin-flip transition line below a certain temperature that separates the RSG and SG phases at low temperatures. Our experimental result shows a qualitative agreement with this theoretical prediction. A detailed comparison will bc published elsewhere.
References
[I] A. lto, tt. Aruga, E. Torikai, M. Kikuchi, Y. Syono and H. Takei, Phys. Rev. Lett. 57 (1986) 483. [2] 1t. Aruga, T. Tokoro and A. Ito, J. Phys. Soc. Jpn. 57 (1988) 26 !. [3] A. lto, !t. Aruga, E. Torikai, M. Kikuchi, Y. Syono and it. Takei, Solid State Conmaun. 66 (1988) 475. [4] tt. Aruga, A. Ito, H. Wakabayashi and T. Goto, J. Phys. Soc. Jpn, 57 (1988) 2636. [5] tt. Aruga. A. lto, it. Wakabayashi and T. Goto, Physica B 155 (1989) 311. [6] 1I. l'akayama, Prog. Thcor. Phys. 811 (It)88) 827. [7] 1. Ya. Korcnblit and E.F. Shcndcr, Soy. Phys. JETP 62 ( 1985 ) 1030. [8] Ya. V. Fyodorov, 1. Ya. Korenblit and E.F. Shender, J. Phys. C 20 (1987) 1835. [9] 1. Ya. Korenblit, Ya V. Fedorov and E.F. Shender, Soy. Phys. JETP 65 (1987) 400. [10] Ya. V. Fyodorov, I. Ya. Korenblit and E.F. Shender, J. Phys.: Condens. Matter 2 (1990) 1669. [11] A. Ito, S. Morimoto and H. Aruga, Hyperfine Int. 54 (199(I) 567. [12] H. Yoshizawa, S. Mitsuda, H. Aruga and A. Ito, Phys. Re,,,. Lett. 59 (1987) 2364. [13] H. Yoshizawa, S. Mitsu:la, H. Aruga and A. Ito, J. Phys. Soc. Jpn. 58 (1989) 1416. [141 a l,,, tl Aruga, ~ N............. and H. Y,;shizawa, J. ,de Plays. 49 (1988) C8-I 129. [15] tt. Aruga Katori, T. (3oto, S. Ebii and A. lip, Proc. 3rd Int. Syrup. on Research in ltigh Magnetic Fields, to bc published in Physica B 177 (1992).
Effect of magnetic fields on phase transitions in the reentrant spin glass Fe.,.Mn i _xTiO 3 H.
Aruga Katori ", T. Goto ~, S. Ebii b and A. lto b
"blstitute for Solid State Physics, Unit'ersit3.,of Tokyo, Roppongi, Minato-ku, Tok),o 106, Japan t, Department of Physics, Faculty of Science, Ochanomizu Universi~,, Bunk3,o-ku, Tokyo 112, Japan Effect of magnetic fields on phase transitions in the exchange competing system Fe,.Mn~_.,TiO 3 with x = 0.60, 0.65 and 0.75, which exhibit the reentrant spin-glass transition from the antiferromagnetic state in weak fields, has been investigated by measuring the magnetization process in pulsed magnetic fields. The spin-flip-like transition (metamagnetic transition) is observed at 4.2 K for each sample. The critical field of the transition decreases monotonically with the increase of temperature and this trat sition vanishes above the Nt~el temperature. The field variations of the reentrant spin-glass transition and N~ei temperatures have been also observed. From these observations the fieid-temperatur., phase diagram is determined for each of these reentrant spin-glass samples. The shape of the phase diagram is qualitatively consistent with the theoretical prediction given for Ising systems exhibiting the reentrant transition from the antiferromagnetic sta,.~.. The magnetic properties of spin-glass systems have bccn widely examined so far. The reentrant phenomenon is one of the most stimulating subjects in these systems. We consider that the experimental investigation of the magnetic field effect on the reentrant spin glass presents some new aspects for understanding the reentrant transition. However, systematic studies of the reentrant spin glass is magnetic fields have scarcely been reported. The Fe.,Mn~_,TiO 3 spin glass is a mixture of the antifcrromagnets FcTiO 3 and MnTiO 3, having easyaxis anisotropy along the hexagonal c-axis. The spins in FcTiO 3 and MnTiO 3 arc coupled antifcrromagnctitally between adjacent c-layers. However, the intralaycr coupling of the spins is ferromagnetic in FcTiO 3 and antifcrromagnctic in MnTiO 3. Thcrcflwc. in the mixed compound Fc,Mn~ .,TiO 3 the competition between the ferromagnetic and antiferromagnetic interactions within a c-layer causes frustrations. In the previous papers, we have shown that Fe, Mn~_.,TiO 3 with intermediate Fe concentration exhibits a typical lsing spin-glass (SG) type of behaviour [1-3]. We have also shown that the Fe-rich samples undergo the transition from the antiferromagnetic (AF) state to the rcentrant spin-glass (RSG) state. We have studied the field-dependent phenomena in the recntrant spin-glass samples with x = 0.60, 0.65 and 0.75 by measuring the temperature variations of the dc magnetizations at various fields up to 6 T [4,5]. Wc have found that magnetic fields have a remarkable influence on the transition temperatures. The temperature giving the maximum magnetization, TM(H), coincides ~ith the N6el temperature in the low-field limit. TM(H) decreases with an increase of magnetic field, but vanishcs above a certain field. Meat, while, the reentrant spin-glass transition temperature TRsa(H) increases with magnetic field and begins to decrease abruptly
after being coincident with TM(H). The field variations of TM(H) and TRsG(H) are qualitatively consistent with the results obtained from the theoretical work [6-10]. We pay attention to the prediction given by one of the theoretical investigations that spin-flip transitions occur below a certain temperature [6]. In order to clarify whether this transition exists or not in our system, we study in detail the mag~ictic pha~c diagram of this system in the field-temperature plane by measuring the high-field magnetization processes at various temperatures. The high-field magnetization processes wcrc measured in pulsed magnetic fields up to about 15 T at various temperatures. The c-axis of the single-crystal sample was set parallel to the measuring field. The magnetization curves for the x = 0.60, 0.65 and 0.75 samples change with temperature in a qualitatively similar way. As an example we show in fig. 1 the magnetization curves ( M ( H ) against H ) at various temperatures for the x = I).65 sample with TM(0.001 T ) ( = T N)=37.1 K and TRstq(0.001 T ) = 2 2 . 0 K. A
:!4oL 60/ 00
0
0
10 ~0 0 Magnetic Field (T)
3'
6
Fig. 1. Magnetization processes of F%65Mno35TiO3 at vari ous temperatures.
0312-88~3/9,/$05.00 ~ 1992 ElsevierScience Publishers B.V. All rights reser','ed
1640
H. Aruga Katori et al. / The reentrant spin glass Fe, Mn t _ , TiO 3 6
'
5
E
Fe 0.¢,5Mn 0.35TiO 3
~,
•~ 4 "~
3
~
'
~)
O Tm(H,
'.~ SG ~
O 'l'rs¢;(tl~ A tlmd'~
p
2
1 0
0
,
,
10
"
' O-
-
O
~
20 30 40 Temperature (K) Fig. 2. Field-temperature phase diagram of Feu.~,5Mn u.35TiO3. The solid lines are a guide to the eye. P, paramagnetic phase:, AF, anti ferromagnetic phase; RSG, reentrant spin-glass phase and SG, spin-glass phase (which is tentative). spin-flip-like transition is observed at 4.2 K with a large amount of hysteresis. This transition field H m ( T ) is determined by the field where d M ( H ) / d H has a maximum value. We obtain Hm(4.2 K) = 2.4 T. As the temperature increases, H m ( T ) decrease and the width of the hysteresis becomes small monotonically up to 37 K, We cannot find any noticeable change in M ( H ) at Trsc~(0,001 T). Above T n, Hm(T)vanishcs, but M ( H ) still changes nonlinearly with magnetic field up to 60 K, which is the highest temperature examined. This indicates that the short-range spin correlation remains above T N, which is supported by thc M6ssbaucr mcasurcmcnt [1 I]. In fig. 2, wc plot H m ( T ) for the x = 0.65 sampic in the ficld-tcmpcraturc plane. Wc also plot in fig. 2 the transition tcmpcraturcs TrotH) and T r s ( ; ( H ) for thc x = 0.65 sample determined by the temperature variations of magnctization at various ficlds [4,5]. Wc find that magnetic fields suppress the AF order "and facilitate the appearance of the RSG state below about (1.7 T where Trs(;(H) coincides with TM(H). Bctwccn about 0.7 and 2 T the sample scems to undergo the transition from the paramagnetic state to the RSG statc directly. Above about 2 T and below about 15 K, H m ( T ) dcviatcs from Trst3(H). In thc prcvious investigations on Fc,Mn~ , T i P 3 we havc found that in thc SG state only the SG ordering exists whcrcas in the RSG state the AF iong-rangc order cocxists with thc SG ordcring [12,13]. From thc results obtained by thc M6ssbaucr and the ncutron scattering measurements [11-14], wc have shown that the transverse spin components exist and fluctuate rather incoherently with characteristic time scales longer than the M6ssbaucr time scale of 10 7 s, and that below T N only the longitudinal spin components form the AF long-range order. From these observations, wc consider that thc
longitudinal spin components contribute to the spinflip-like transition. Thcrcforc, wc infer that below about 15 K this samplc undergoes the spin-flip-likc transition frorn the RSG state to the SG state, although any remarkable diffcrcnccs could not bc found in the magnetization proccsscs above H m ( T ) whcthcr the sample is in the SG state or the paramagnetic state. The high-field magnetization processes of this system up to 100 T will bc reported in a separate paper [15]. Theoretical investigations have been done for a system undergoing the recntrant transition from the A F state [6-10]. Recently, Takayama proposed an extended mean-field model for the present system [6]. Hc presents a field-temperature phase diagram where the AF-to-paramagnetic transition line merges into the first-order spin-flip transition line below a certain temperature that separates the RSG and SG phases at low temperatures. Our experimental result shows a qualitative agreement with this theoretical prediction. A detailed comparison will bc published elsewhere.
References
[I] A. lto, tt. Aruga, E. Torikai, M. Kikuchi, Y. Syono and H. Takei, Phys. Rev. Lett. 57 (1986) 483. [2] 1t. Aruga, T. Tokoro and A. Ito, J. Phys. Soc. Jpn. 57 (1988) 26 !. [3] A. lto, !t. Aruga, E. Torikai, M. Kikuchi, Y. Syono and it. Takei, Solid State Conmaun. 66 (1988) 475. [4] tt. Aruga, A. Ito, H. Wakabayashi and T. Goto, J. Phys. Soc. Jpn, 57 (1988) 2636. [5] tt. Aruga. A. lto, it. Wakabayashi and T. Goto, Physica B 155 (1989) 311. [6] 1I. l'akayama, Prog. Thcor. Phys. 811 (It)88) 827. [7] 1. Ya. Korcnblit and E.F. Shcndcr, Soy. Phys. JETP 62 ( 1985 ) 1030. [8] Ya. V. Fyodorov, 1. Ya. Korenblit and E.F. Shender, J. Phys. C 20 (1987) 1835. [9] 1. Ya. Korenblit, Ya V. Fedorov and E.F. Shender, Soy. Phys. JETP 65 (1987) 400. [10] Ya. V. Fyodorov, I. Ya. Korenblit and E.F. Shender, J. Phys.: Condens. Matter 2 (1990) 1669. [11] A. Ito, S. Morimoto and H. Aruga, Hyperfine Int. 54 (199(I) 567. [12] H. Yoshizawa, S. Mitsuda, H. Aruga and A. Ito, Phys. Re,,,. Lett. 59 (1987) 2364. [13] H. Yoshizawa, S. Mitsu:la, H. Aruga and A. Ito, J. Phys. Soc. Jpn. 58 (1989) 1416. [141 a l,,, tl Aruga, ~ N............. and H. Y,;shizawa, J. ,de Plays. 49 (1988) C8-I 129. [15] tt. Aruga Katori, T. (3oto, S. Ebii and A. lip, Proc. 3rd Int. Syrup. on Research in ltigh Magnetic Fields, to bc published in Physica B 177 (1992).