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Magnetic behavior of the diluted antiferromagnet Mn0.39Zn0.61F2 at strong fields
Journal of Magnetism and Magnetic Materials 226}230 (2001) 1343}1345
Magnetic behavior of the diluted antiferromagnet Mn Zn F at strong "elds
A. Rosales-Rivera *, J.M. Ferreira, F.C. Montenegro, C.A. Ramos Departamento de Fn& sica, Universidad Nacional de Colombia, A.A. 127, Manizales, Colombia Departamento de Fn& sica, Universidade Federal de Pernambuco, 50670-901, Recife, PE, Brazil Centro Ato& mico Bariloche, 8400, Bariloche, Rn& o Negro, Argentina
Abstract The magnetic phase boundaries of the random-"eld Ising model (RFIM) system Mn Zn F are determined using magnetization measurements, under "nite DC applied "elds (H). At low "elds (H(12 kOe), our results support a critical phase boundary, separating an ordered antiferromagnetic (AF) phase from the paramagnetic (P) one. For intermediate "elds (12(H(18 kOe), the AF ordering coexists with a spin-#op (SF) clustering. For higher H, a spin-#op phase dominates the upper part of the (H,T) phase diagram. 2001 Published by Elsevier Science B.V. Keywords: Magnetic phase boundaries; Static properties
Mn Zn F has been largely studied as an experiV \V mental realization of the d"3 RFIM [1}8]. At H"0, AF long-range order (LRO) is present below T for all , measured samples with x above the percolation threshold (x "0.24) [3}5]. The critical behavior at T is represent , ed by the random-exchange Ising model (REIM) universality class [3,4]. For low "elds applied parallel to the easy direction, the AF LRO is maintained below T (H). A REIM to RFIM crossover scaling (T } T (H)&H (, , with
1.4) governs the critical boundary for samples with x'0.4 [8,9]. Results from AC susceptibility [6] and neutron scattering [3,4] studies in a sample with x"0.35 indicate that AF LRO coexists with short-range nonequilibrium clusters at H"0. For H(10 kOe, a pseudophase transition boundary separates a partially ordered AF phase from the P one. For higher H, the phase boundaries are no longer visible by neutron scattering studies. The compound Mn Zn F is studied here by magnetization measurements, performed in a SQUID MPMS-5 (Quantum Design), in the temper-
* Corresponding author. Tel.: #57-68-867-312; fax: #5768-810-123. E-mail address: [email protected] (A. Rosales-Rivera).
ature range 2)T)40 K, for DC-"elds 0(H)50 kOe. In addition to the opportune comparison with samples of Mn Zn F , with higher and lower x, the magnetic V \V behavior is explored here particularly at high H, to examine the similarities and di!erences with the one recently reported [10] for the isostructural, but more anisotropic compound Fe Zn F , in the strong RFIM regime. Fig. 1 shows the temperature dependence of the DC susceptibility (M/H) , in the "eld interval 1)H( & 21.5 kOe, using the ZFC and FC protocols. For H(12 kOe, ZFC M curves of Fig. 1 are those typical of a predominant AF order taking place at low temperatures. For 12(H(18 kOe, a spin-#op clustering appears in addition to the AF ordering. The AF}SF phase boundary is signalized by the minimum of the ZFC M curves in Fig. 1, as explained in Ref. [7]. For H'18 kOe the maximum in (M/H) is washed out and & a typical SF behavior appears at low T. Fig. 2 shows the temperature dependence of ZFC d(M/H)/dT for applied "elds in the interval 0(H(25 kOe. For H(12 kOe, the ZFC d(M/H)/dT displays a sharp and symmetric peak at the critical temperature T (H). The position of the ZFC peak shifts to lower temperatures with increasing "eld, following the predicted [9] REIM to RFIM crossover scaling, which is limited in the present case to the "eld range 0(H(12 kOe. Contrasting with the critical behavior
0304-8853/01/$ - see front matter 2001 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 8 4 8 - 9
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A. Rosales-Rivera et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1343}1345
Fig. 1. (M/H) vs. T of Mn Zn F for several H. Filled & symbols are ZFC data and open symbols are FC.
Fig. 2. ZFC temperature dependence of d(M/H)/dT for Mn Zn F at several "elds.
found at low "elds, the amplitude of the ZFC d(M/H)/dT peaks decreases as H increases for 12(H(18 kOe. In this intermediate "eld region, the ZFC magnetization becomes time dependent, re#ecting the instability of the AF LRO. For H'18 kOe the peaks in ZFC (dM/dT) & are broad, but reasonably well de"ned, as shown in Fig. 2. Contrary to the behavior at low "elds, the positions of the ZFC (dM/dT) broad peaks at high "elds shift to & higher temperatures with increasing "eld, re#ecting the SF}P phase transition at T,(H). The ZFC (dM/dT) & broad peaks are no longer visible for H above 30 kOe. Another signature of the SF phase has been obtained
Fig. 3. Phase diagram of Mn Zn F . Circles represent the boundaries obtained from M vs. T (H "xed) and triangles were measured from M vs. H (T "xed) cycles.
from "eld cycling measurements. In agreement with previous magnetization results [7] for Mn Zn F , the (dM/dH) curves obtained in the present experiments for 2 x"0.39 display sharp peaks (not shown here) at H(T). The phase boundaries, AF}P at T (H), AF}SF at T (H) and SF}P at T,(H) obtained from the magneti zation data, are mapped in the (H, T) phase diagram of Fig. 3. In the weak RFIM regime (H(12 kOe), T (H) is governed by the expected REIM}RFIM crossover scaling: T "T }bH-T (H)"C H (, with 1.4 being , the REIM}RFIM crossover exponent, and b, C are constants. The extrapolation of T (H) to HP0 gives the NeH el temperature T "14.42$0.08 K. For intermediate , "elds (12(H(18 kOe), The AF LRO becomes unstable and coexists with SF clustering. The dynamic behavior in this regime will be reported elsewhere [11]. For higher "elds (H'18 kOe), the present results indicate that a spin #op phase is present in Mn Zn F , at least V \V for x'0.39. The magnetic features of Mn Zn F at high "elds, presented in this work, are distinct from the random-"eld-induced glassy behavior which emerge at high "elds in samples of Fe Zn F , for 0.31)x)0.60 V \V [10,12,13]. This work was supported by CAPES, CNPq and FINEP (Brazilian agencies). One of us (AR-R) also acknowledges the support of the FundacioH n para la promocioH n de la InvestigacioH n y la TecnologmH a, Banco de la RepuH blica (Colombian agency).
References [1] D.P. Belanger, A.P. Young, J. Magn. Magn. Mater. 100 (1991) 272.
A. Rosales-Rivera et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1343}1345 [2] D.P. Belanger, in: A.P. Young (Ed.), Spin Glasses and Random Fields, World Scienti"c, Singapore, 1997. [3] F.C. Montenegro, D.P. Belanger, Z. Slanic, J.A. Fernandez-Baca, J. Appl. Phys. 87 (2000) 6537. [4] F.C. Montenegro, D.P. Belanger, Z. Slanic, J.A. Fernandez-Baca, Phys. Rev. B 61 (2000) 14681. [5] R.A. Cowley, G. Shirane, R.J. Birgeneau, E.C. Svensson, Phys. Rev. Lett. 39 (1977) 894. [6] F.C. Montenegro, A. Rosales-Rivera, J.C.O. de Jesus, E. Montarroyos, F.L.A. Machado, Phys. Rev. B 53 (1995) 5849. [7] F.C. Montenegro, J.C.O. de Jesus, F.L.A. Machado, E. Montarroyos, S.M. Rezende, J. Magn. Magn. Mater. 104 (1992) 277.
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[8] C.A. Ramos, A.R. King, V. Jaccarino, Phys. Rev. B 37 (1988) 5483. [9] S. Fishman, A. Aharony, J. Phys. C 12 (1979) L729. [10] A. Rosales-Rivera, J.M. Ferreira, F.C. Montenegro, Europhys. Lett. 50 (2000) 264. [11] A. Rosales-Rivera, J.M. Ferreira, F.C. Montenegro, C.A. Ramos, unpublished. [12] F.C. Montenegro, K.A. Lima, M.S. Torikachvili, A.H. Lacerda, J. Magn. Magn. Mater. 177}181 (1998) 145. [13] F.C. Montenegro, K.A. Lima, M.S. Torikachvili, A.H. Lacerda, Mater. Sci. Forum 302}303 (1999) 371.
Magnetic behavior of the diluted antiferromagnet Mn Zn F at strong "elds
A. Rosales-Rivera *, J.M. Ferreira, F.C. Montenegro, C.A. Ramos Departamento de Fn& sica, Universidad Nacional de Colombia, A.A. 127, Manizales, Colombia Departamento de Fn& sica, Universidade Federal de Pernambuco, 50670-901, Recife, PE, Brazil Centro Ato& mico Bariloche, 8400, Bariloche, Rn& o Negro, Argentina
Abstract The magnetic phase boundaries of the random-"eld Ising model (RFIM) system Mn Zn F are determined using magnetization measurements, under "nite DC applied "elds (H). At low "elds (H(12 kOe), our results support a critical phase boundary, separating an ordered antiferromagnetic (AF) phase from the paramagnetic (P) one. For intermediate "elds (12(H(18 kOe), the AF ordering coexists with a spin-#op (SF) clustering. For higher H, a spin-#op phase dominates the upper part of the (H,T) phase diagram. 2001 Published by Elsevier Science B.V. Keywords: Magnetic phase boundaries; Static properties
Mn Zn F has been largely studied as an experiV \V mental realization of the d"3 RFIM [1}8]. At H"0, AF long-range order (LRO) is present below T for all , measured samples with x above the percolation threshold (x "0.24) [3}5]. The critical behavior at T is represent , ed by the random-exchange Ising model (REIM) universality class [3,4]. For low "elds applied parallel to the easy direction, the AF LRO is maintained below T (H). A REIM to RFIM crossover scaling (T } T (H)&H (, , with
1.4) governs the critical boundary for samples with x'0.4 [8,9]. Results from AC susceptibility [6] and neutron scattering [3,4] studies in a sample with x"0.35 indicate that AF LRO coexists with short-range nonequilibrium clusters at H"0. For H(10 kOe, a pseudophase transition boundary separates a partially ordered AF phase from the P one. For higher H, the phase boundaries are no longer visible by neutron scattering studies. The compound Mn Zn F is studied here by magnetization measurements, performed in a SQUID MPMS-5 (Quantum Design), in the temper-
* Corresponding author. Tel.: #57-68-867-312; fax: #5768-810-123. E-mail address: [email protected] (A. Rosales-Rivera).
ature range 2)T)40 K, for DC-"elds 0(H)50 kOe. In addition to the opportune comparison with samples of Mn Zn F , with higher and lower x, the magnetic V \V behavior is explored here particularly at high H, to examine the similarities and di!erences with the one recently reported [10] for the isostructural, but more anisotropic compound Fe Zn F , in the strong RFIM regime. Fig. 1 shows the temperature dependence of the DC susceptibility (M/H) , in the "eld interval 1)H( & 21.5 kOe, using the ZFC and FC protocols. For H(12 kOe, ZFC M curves of Fig. 1 are those typical of a predominant AF order taking place at low temperatures. For 12(H(18 kOe, a spin-#op clustering appears in addition to the AF ordering. The AF}SF phase boundary is signalized by the minimum of the ZFC M curves in Fig. 1, as explained in Ref. [7]. For H'18 kOe the maximum in (M/H) is washed out and & a typical SF behavior appears at low T. Fig. 2 shows the temperature dependence of ZFC d(M/H)/dT for applied "elds in the interval 0(H(25 kOe. For H(12 kOe, the ZFC d(M/H)/dT displays a sharp and symmetric peak at the critical temperature T (H). The position of the ZFC peak shifts to lower temperatures with increasing "eld, following the predicted [9] REIM to RFIM crossover scaling, which is limited in the present case to the "eld range 0(H(12 kOe. Contrasting with the critical behavior
0304-8853/01/$ - see front matter 2001 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 8 4 8 - 9
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A. Rosales-Rivera et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1343}1345
Fig. 1. (M/H) vs. T of Mn Zn F for several H. Filled & symbols are ZFC data and open symbols are FC.
Fig. 2. ZFC temperature dependence of d(M/H)/dT for Mn Zn F at several "elds.
found at low "elds, the amplitude of the ZFC d(M/H)/dT peaks decreases as H increases for 12(H(18 kOe. In this intermediate "eld region, the ZFC magnetization becomes time dependent, re#ecting the instability of the AF LRO. For H'18 kOe the peaks in ZFC (dM/dT) & are broad, but reasonably well de"ned, as shown in Fig. 2. Contrary to the behavior at low "elds, the positions of the ZFC (dM/dT) broad peaks at high "elds shift to & higher temperatures with increasing "eld, re#ecting the SF}P phase transition at T,(H). The ZFC (dM/dT) & broad peaks are no longer visible for H above 30 kOe. Another signature of the SF phase has been obtained
Fig. 3. Phase diagram of Mn Zn F . Circles represent the boundaries obtained from M vs. T (H "xed) and triangles were measured from M vs. H (T "xed) cycles.
from "eld cycling measurements. In agreement with previous magnetization results [7] for Mn Zn F , the (dM/dH) curves obtained in the present experiments for 2 x"0.39 display sharp peaks (not shown here) at H(T). The phase boundaries, AF}P at T (H), AF}SF at T (H) and SF}P at T,(H) obtained from the magneti zation data, are mapped in the (H, T) phase diagram of Fig. 3. In the weak RFIM regime (H(12 kOe), T (H) is governed by the expected REIM}RFIM crossover scaling: T "T }bH-T (H)"C H (, with 1.4 being , the REIM}RFIM crossover exponent, and b, C are constants. The extrapolation of T (H) to HP0 gives the NeH el temperature T "14.42$0.08 K. For intermediate , "elds (12(H(18 kOe), The AF LRO becomes unstable and coexists with SF clustering. The dynamic behavior in this regime will be reported elsewhere [11]. For higher "elds (H'18 kOe), the present results indicate that a spin #op phase is present in Mn Zn F , at least V \V for x'0.39. The magnetic features of Mn Zn F at high "elds, presented in this work, are distinct from the random-"eld-induced glassy behavior which emerge at high "elds in samples of Fe Zn F , for 0.31)x)0.60 V \V [10,12,13]. This work was supported by CAPES, CNPq and FINEP (Brazilian agencies). One of us (AR-R) also acknowledges the support of the FundacioH n para la promocioH n de la InvestigacioH n y la TecnologmH a, Banco de la RepuH blica (Colombian agency).
References [1] D.P. Belanger, A.P. Young, J. Magn. Magn. Mater. 100 (1991) 272.
A. Rosales-Rivera et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1343}1345 [2] D.P. Belanger, in: A.P. Young (Ed.), Spin Glasses and Random Fields, World Scienti"c, Singapore, 1997. [3] F.C. Montenegro, D.P. Belanger, Z. Slanic, J.A. Fernandez-Baca, J. Appl. Phys. 87 (2000) 6537. [4] F.C. Montenegro, D.P. Belanger, Z. Slanic, J.A. Fernandez-Baca, Phys. Rev. B 61 (2000) 14681. [5] R.A. Cowley, G. Shirane, R.J. Birgeneau, E.C. Svensson, Phys. Rev. Lett. 39 (1977) 894. [6] F.C. Montenegro, A. Rosales-Rivera, J.C.O. de Jesus, E. Montarroyos, F.L.A. Machado, Phys. Rev. B 53 (1995) 5849. [7] F.C. Montenegro, J.C.O. de Jesus, F.L.A. Machado, E. Montarroyos, S.M. Rezende, J. Magn. Magn. Mater. 104 (1992) 277.
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[8] C.A. Ramos, A.R. King, V. Jaccarino, Phys. Rev. B 37 (1988) 5483. [9] S. Fishman, A. Aharony, J. Phys. C 12 (1979) L729. [10] A. Rosales-Rivera, J.M. Ferreira, F.C. Montenegro, Europhys. Lett. 50 (2000) 264. [11] A. Rosales-Rivera, J.M. Ferreira, F.C. Montenegro, C.A. Ramos, unpublished. [12] F.C. Montenegro, K.A. Lima, M.S. Torikachvili, A.H. Lacerda, J. Magn. Magn. Mater. 177}181 (1998) 145. [13] F.C. Montenegro, K.A. Lima, M.S. Torikachvili, A.H. Lacerda, Mater. Sci. Forum 302}303 (1999) 371.