- Email: [email protected]
soft bilayers
Journal of Magnetism and Magnetic Materials 226}230 (2001) 1251}1253
Preisach analysis of epitaxial hard/soft bilayers D.R. Cornejo , F.M. Rhen , F.P. Missell *, E.E. Fullerton Instituto de Fn& sica, Univ. de SaJ o Paulo, C.P.66318, 05315-970 SaJ o Paulo, SaJ o Paulo, Brazil Facultad de Matema& tica, Astronomn& a y Fn& sica, Univ. Nac. de Co& rdoba, H. de la Torre y M. Allende, C.P. 5000, Co& rdoba, Argentina IBM Almaden Research Center, 650 Harry Road., San Jose, CA 95120-6099, USA
Abstract We used the moving Preisach model to study the magnetization-reversal process in epitaxial SmCo (1 1 0 0)/Fe bilayers prepared by magnetron sputtering. The SmCo(20 nm)/Fe(t nm) (t"0, 5, 10, 20) bilayers were grown onto single-crystal (1 1 0)MgO substrates with an epitaxial 20 nm Cr(2 1 1) bu!er layer. The second-quadrant magnetization curves were strictly reversible up to a "eld H , close to the "eld H at which the magnetization switched irreversibly. Henkel plots for these bilayers indicated very strong magnetizing interactions for all "lms. Magnetization reversal in these "lms re#ects their epitaxial structure. 2001 Elsevier Science B.V. All rights reserved. Keywords: Epitaxial bilayer; Rare earth}transition metal alloys; Preisach model; Magnetization*reversal
Exchange-spring magnets are composed of hard- and soft-magnetic phases coupled by the exchange interaction and show enhanced remanence as well as reversible demagnetization curves [1]. Although applications of two-phased magnets are currently based upon random, nanoscale geometries, coupled bilayer "lms provide convenient model systems because the relevant length scales can be conveniently controlled. Previously [2], we discussed the reversible (M ) and irreversible (M ) mag netization for SmCo/Fe bilayers, where the SmCo layer consisted of randomly-oriented nanometer grains of the disordered SmCo phase. In that case, the most impor tant source of M is the reversible rotation of the magnetization in the SmCo grains. The soft Fe layer, when well-coupled to the SmCo layer, contributes mainly to M , but is also an important source of M . In this paper, we present results on epitaxial SmCo/Fe bilayers. The magnetization behavior of these "lms is quite di!erent from that observed in Ref. [2]. Henkel plots for these bilayers indicate very strong magnetizing interactions for all "lms. The strong interactions are due to the size and orientation of the SmCo grains in the "lm plane which result in improved coupling between the layers. * Corresponding author. Tel.: #55-11-818-6884; fax: #5511-3818-6984. E-mail address: [email protected] (F.P. Missell).
Epitaxial SmCo (1 1 0 0)/Fe bilayers were prepared by magnetron sputtering onto single-crystal (1 1 0)MgO substrates with an epitaxial 20 nm Cr(2 1 1) bu!er layer [3,4]. Bilayers A, B, C, and D have structures SmCo(20 nm)/Fe(t nm) (t"0, 5, 10, 20), respectively. Major hysteresis loops and "rst-order reversal curves were measured at 300 K using a 7 T superconducting quantum interference device (SQUID) magnetometer. Due to space limitations, results are shown in Fig. 1 for samples A and C only. For sample C, major hysteresis loops are strictly reversible for "elds less than H "7.2 kOe. (Upon returning the "eld to zero from H , the remanence is 97% of the major loop remanence.) The reversible susceptibility was determined as the mean slope of a small minor loop starting from a "eld on the major loop. Films were demagnetized for remanence measurements using the DC procedure. A graphical interpretation of bilayer interactions can be obtained in terms of the Henkel plot, which displays m ,M (H)/M (R) vs. m ,M (H)/M (R), M and 0 0 M being the demagnetizing and magnetizing remanen ces. On the Henkel plot for a DC demagnetization, interactions are called `magnetizinga or `demagnetizinga according to whether the experimental points fall above or below the line de"ned by M (H)"M (R)!M (H) 0 [5]. Henkel plots for the four bilayers are shown in Fig. 2, along with the Wohlfarth line m "1!m . For most of
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 0 0 6 - 4
1252
D.R. Cornejo et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1251}1253
the "eld range, magnetizing interactions are very strong, especially for sample C, corresponding to SmCo/ Fe(10 nm). This is due to the epitaxial structure of these bilayers. Bilayers consisting of disordered SmCo and Fe previously [2] showed weak interactions, as the structural disorder canceled the e!ect of exchange}related magnetizing interactions. The contrasting behavior of these epitaxial bilayers and those involving disordered SmCo [2] is also apparent in the behavior of M and M . In principle, M can be determined by integrating the reversible susceptibility However, Crew et al. [6] showed that it is necessary to consider the dependence of M on M while integrating. Subsequently, the moving
Preisach model (MPM) was used [7,8] to show that M can be calculated as
M (H )" ( # ) dH ,
(1)
where H is the internal magnetic "eld and the interrelation function "(M /M )H is related to the reversible susceptibility : "k . In
the MPM, the e!ective "eld acting on an elementary loop is H "H #k M, where the moving parameter
k is a measure of long-range interactions in the system.
k can be estimated [8] from measured values of
. Once M has been calculated from Eq. (1), then M "M!M .
Fig. 1. First-order reversal curves for samples A and sample C. For C, the second-quadrant magnetization curves are strictly reversible for "elds up to H "7.2 kOe.
Fig. 2. Henkel plots (m vs. m ) for all samples studied.
D.R. Cornejo et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1251}1253
1253
Fig. 3. Total (M) and irreversible (M ) magnetization vs. internal magnetic "eld H for samples A and C.
The total magnetization M, as well as M , for samples A and C are shown in Fig. 3. The magnetization of sample A, consisting of epitaxial SmCo without an exchange!coupled Fe "lm, is entirely irreversible. On the contrary, for the disordered SmCo "lm of Ref. [2], M was nearly 50% of M at high "elds. This is ex pected from the Stoner}Wohlfarth model. For sample C, a sizeable reversible component of the magnetization has appeared due to the presence of the Fe "lm and M is only about 75% of M. However, the coercive properties of the "lm are dominated by the hard layer. Both H and the maximum value of dM /dH are very close to H "7.5 kOe for the "lm, indicating that the coupling between the epitaxial SmCo and the Fe layers is very strong. (This can also be seen in the major hysteresis loop of sample C in Fig. 1.) The sizeable M associated with the soft layer and a magnetization reversal being determined by the hard layer is similar to the situation encountered in well-coupled "lms in Ref. [2]. For the epitaxial "lms, however, strong coupling of hard and soft layers is observed for much thicker Fe layers. Thus, the epitaxial "lms permit a study of exchange}coupled bilayers over a wider range of material parameters.
A numerical determination of the Preisach function is underway for detailed simulations of M and M . The authors wish to acknowledge the "nancial support of FAPESP, CNPq, and FINEP.
References [1] E.F. Kneller, R. Hawig, IEEE Trans. Magn. 27 (1991) 3588. [2] D.R. Cornejo, F.P. Missell, J. Appl. Phys. 87 (2000) 4741. [3] E.E. Fullerton, J.S. Jiang, C. Rehm, C.H. Sowers, S.D. Bader, J.B. Patel, X.Z. Wu, Appl. Phys. Lett. 71 (1997) 1579. [4] E.E. Fullerton, J.S. Jiang, M. Grimsditch, C.H. Sowers, S.D. Bader, Phys. Rev. B 58 (1998) 12 193. [5] D. R. Cornejo, Ph.D. Thesis, Universidade de Sa o Paulo, 1998. [6] D.C. Crew, S.H. Farrant, P.G. McCormick, R. Street, J. Magn. Magn. Mater. 163 (1996) 299. [7] D.R. Cornejo, F.P. Missell, J. Magn. Magn. Mater. 203 (1999) 41. [8] M. Emura, D.R. Cornejo, F.P. Missell, J. Appl. Phys. 87 (2000) 1387.
Preisach analysis of epitaxial hard/soft bilayers D.R. Cornejo , F.M. Rhen , F.P. Missell *, E.E. Fullerton Instituto de Fn& sica, Univ. de SaJ o Paulo, C.P.66318, 05315-970 SaJ o Paulo, SaJ o Paulo, Brazil Facultad de Matema& tica, Astronomn& a y Fn& sica, Univ. Nac. de Co& rdoba, H. de la Torre y M. Allende, C.P. 5000, Co& rdoba, Argentina IBM Almaden Research Center, 650 Harry Road., San Jose, CA 95120-6099, USA
Abstract We used the moving Preisach model to study the magnetization-reversal process in epitaxial SmCo (1 1 0 0)/Fe bilayers prepared by magnetron sputtering. The SmCo(20 nm)/Fe(t nm) (t"0, 5, 10, 20) bilayers were grown onto single-crystal (1 1 0)MgO substrates with an epitaxial 20 nm Cr(2 1 1) bu!er layer. The second-quadrant magnetization curves were strictly reversible up to a "eld H , close to the "eld H at which the magnetization switched irreversibly. Henkel plots for these bilayers indicated very strong magnetizing interactions for all "lms. Magnetization reversal in these "lms re#ects their epitaxial structure. 2001 Elsevier Science B.V. All rights reserved. Keywords: Epitaxial bilayer; Rare earth}transition metal alloys; Preisach model; Magnetization*reversal
Exchange-spring magnets are composed of hard- and soft-magnetic phases coupled by the exchange interaction and show enhanced remanence as well as reversible demagnetization curves [1]. Although applications of two-phased magnets are currently based upon random, nanoscale geometries, coupled bilayer "lms provide convenient model systems because the relevant length scales can be conveniently controlled. Previously [2], we discussed the reversible (M ) and irreversible (M ) mag netization for SmCo/Fe bilayers, where the SmCo layer consisted of randomly-oriented nanometer grains of the disordered SmCo phase. In that case, the most impor tant source of M is the reversible rotation of the magnetization in the SmCo grains. The soft Fe layer, when well-coupled to the SmCo layer, contributes mainly to M , but is also an important source of M . In this paper, we present results on epitaxial SmCo/Fe bilayers. The magnetization behavior of these "lms is quite di!erent from that observed in Ref. [2]. Henkel plots for these bilayers indicate very strong magnetizing interactions for all "lms. The strong interactions are due to the size and orientation of the SmCo grains in the "lm plane which result in improved coupling between the layers. * Corresponding author. Tel.: #55-11-818-6884; fax: #5511-3818-6984. E-mail address: [email protected] (F.P. Missell).
Epitaxial SmCo (1 1 0 0)/Fe bilayers were prepared by magnetron sputtering onto single-crystal (1 1 0)MgO substrates with an epitaxial 20 nm Cr(2 1 1) bu!er layer [3,4]. Bilayers A, B, C, and D have structures SmCo(20 nm)/Fe(t nm) (t"0, 5, 10, 20), respectively. Major hysteresis loops and "rst-order reversal curves were measured at 300 K using a 7 T superconducting quantum interference device (SQUID) magnetometer. Due to space limitations, results are shown in Fig. 1 for samples A and C only. For sample C, major hysteresis loops are strictly reversible for "elds less than H "7.2 kOe. (Upon returning the "eld to zero from H , the remanence is 97% of the major loop remanence.) The reversible susceptibility was determined as the mean slope of a small minor loop starting from a "eld on the major loop. Films were demagnetized for remanence measurements using the DC procedure. A graphical interpretation of bilayer interactions can be obtained in terms of the Henkel plot, which displays m ,M (H)/M (R) vs. m ,M (H)/M (R), M and 0 0 M being the demagnetizing and magnetizing remanen ces. On the Henkel plot for a DC demagnetization, interactions are called `magnetizinga or `demagnetizinga according to whether the experimental points fall above or below the line de"ned by M (H)"M (R)!M (H) 0 [5]. Henkel plots for the four bilayers are shown in Fig. 2, along with the Wohlfarth line m "1!m . For most of
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 0 0 6 - 4
1252
D.R. Cornejo et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1251}1253
the "eld range, magnetizing interactions are very strong, especially for sample C, corresponding to SmCo/ Fe(10 nm). This is due to the epitaxial structure of these bilayers. Bilayers consisting of disordered SmCo and Fe previously [2] showed weak interactions, as the structural disorder canceled the e!ect of exchange}related magnetizing interactions. The contrasting behavior of these epitaxial bilayers and those involving disordered SmCo [2] is also apparent in the behavior of M and M . In principle, M can be determined by integrating the reversible susceptibility However, Crew et al. [6] showed that it is necessary to consider the dependence of M on M while integrating. Subsequently, the moving
Preisach model (MPM) was used [7,8] to show that M can be calculated as
M (H )" ( # ) dH ,
(1)
where H is the internal magnetic "eld and the interrelation function "(M /M )H is related to the reversible susceptibility : "k . In
the MPM, the e!ective "eld acting on an elementary loop is H "H #k M, where the moving parameter
k is a measure of long-range interactions in the system.
k can be estimated [8] from measured values of
. Once M has been calculated from Eq. (1), then M "M!M .
Fig. 1. First-order reversal curves for samples A and sample C. For C, the second-quadrant magnetization curves are strictly reversible for "elds up to H "7.2 kOe.
Fig. 2. Henkel plots (m vs. m ) for all samples studied.
D.R. Cornejo et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1251}1253
1253
Fig. 3. Total (M) and irreversible (M ) magnetization vs. internal magnetic "eld H for samples A and C.
The total magnetization M, as well as M , for samples A and C are shown in Fig. 3. The magnetization of sample A, consisting of epitaxial SmCo without an exchange!coupled Fe "lm, is entirely irreversible. On the contrary, for the disordered SmCo "lm of Ref. [2], M was nearly 50% of M at high "elds. This is ex pected from the Stoner}Wohlfarth model. For sample C, a sizeable reversible component of the magnetization has appeared due to the presence of the Fe "lm and M is only about 75% of M. However, the coercive properties of the "lm are dominated by the hard layer. Both H and the maximum value of dM /dH are very close to H "7.5 kOe for the "lm, indicating that the coupling between the epitaxial SmCo and the Fe layers is very strong. (This can also be seen in the major hysteresis loop of sample C in Fig. 1.) The sizeable M associated with the soft layer and a magnetization reversal being determined by the hard layer is similar to the situation encountered in well-coupled "lms in Ref. [2]. For the epitaxial "lms, however, strong coupling of hard and soft layers is observed for much thicker Fe layers. Thus, the epitaxial "lms permit a study of exchange}coupled bilayers over a wider range of material parameters.
A numerical determination of the Preisach function is underway for detailed simulations of M and M . The authors wish to acknowledge the "nancial support of FAPESP, CNPq, and FINEP.
References [1] E.F. Kneller, R. Hawig, IEEE Trans. Magn. 27 (1991) 3588. [2] D.R. Cornejo, F.P. Missell, J. Appl. Phys. 87 (2000) 4741. [3] E.E. Fullerton, J.S. Jiang, C. Rehm, C.H. Sowers, S.D. Bader, J.B. Patel, X.Z. Wu, Appl. Phys. Lett. 71 (1997) 1579. [4] E.E. Fullerton, J.S. Jiang, M. Grimsditch, C.H. Sowers, S.D. Bader, Phys. Rev. B 58 (1998) 12 193. [5] D. R. Cornejo, Ph.D. Thesis, Universidade de Sa o Paulo, 1998. [6] D.C. Crew, S.H. Farrant, P.G. McCormick, R. Street, J. Magn. Magn. Mater. 163 (1996) 299. [7] D.R. Cornejo, F.P. Missell, J. Magn. Magn. Mater. 203 (1999) 41. [8] M. Emura, D.R. Cornejo, F.P. Missell, J. Appl. Phys. 87 (2000) 1387.