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The CoPt system: a natural exchange spring
Physica B 327 (2003) 190–193
The CoPt system: a natural exchange spring L.H. Lewisa,*, J. Kimb, K. Barmakb,c a
Materials Science Department, Brookhaven National Laboratory, Building 480, Upton, NY 11973, USA b Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA 15213, USA c Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA
Abstract Ferromagnetic ‘‘exchange-spring’’ nanocomposite systems derive their technical magnetic properties, such as the remanence and coercivity, from the details of the interphase coupling between the magnetically soft component and the magnetically hard component. An ideal model material for the study of the interphase interactions in exchange-spring systems is the CoPt system in thin-film form. Depending on the details of post-deposition annealing treatment, CoPt consists of two phases in varying proportions: the chemically disordered A1 phase with low coercivity and the chemically ordered L10 phase with high coercivity. Coupled magnetic and transmission electron microscopy studies reveal simple relationships between the volume percent of ordered L10 phase, coercivity and the magnetic exchange within the samples. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.50.K; 75.50.W; 75.30.E Keywords: Order–disorder transitions; Permanent magnets; Exchange in magnetically ordered materials
1. Introduction Ferromagnetic ‘‘exchange-spring’’ nanocomposite materials are a unique class of materials that are envisioned as the next-generation permanent magnet. They acquire novel technical magnetic properties such as significant remanence enhancement and increased energy product, from the details of the interphase coupling between the magnetically soft component and the magnetically hard component [1]. Computational and experimental studies have demonstrated that variation of the materials’ attributes of the nanocomposite, *Corresponding author. Tel.: +516-344-2861; fax: +516344-4071. E-mail address: [email protected] (L.H. Lewis).
such as relative phase concentration, crystallographic alignment and grain size, can all drastically affect the magnetic behavior of the system [2–4]. Typically, exchange-spring nanosystems are composed of phases with very different chemistries, such as Nd2Fe14B combined with a-Fe or Fe3B, and Sm–Co combined with Co or Fe [3]. A different approach to synthesize magnetic nanocomposites is to exploit the microstructural evolution of ferromagnetic material as it undergoes a first-order crystallographic phase transition. In this manner, a daughter phase forms from the parent phase of identical chemistry by nucleation and growth processes [5,6] to produce a two-phase nanocomposite. This approach has great potential for tailoring the magnetic behavior of the
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 2 5 - 8
L.H. Lewis et al. / Physica B 327 (2003) 190–193
nanocomposite system because, in principle, it is possible to control the nucleation and growth characteristics of the system. The ferromagnetic compound CoPt is an ideal materials system for the creation of a natural magnetic nanocomposite. In this system, a high-anisotropy chemically L10ordered CoPt phase is obtained via a thermodynamically first-order, disorder–order transformation from the chemically disordered FCC solid solution, or A1, phase. When sputter-deposited into thin-film form at room temperature, CoPt forms in the magnetically soft A1 structure with the magnetically hard L10 structure obtained by elevated temperature annealing.
2. Experimental details CoPt films of thicknesses of either 10 or 25 nm were deposited onto water-cooled oxidized silicon wafers using DC magnetron sputtering from equiatomic alloy targets. Ex situ annealing as a function of time in a tube furnace at 7001C in an Ar–4% H2 atmosphere was employed to induce the first-order crystallographic phase transition to the L10 structure. Transmission electron microscopy (TEM) was used to ascertain grain size, texture, and ordered domain size of the film phases. The chemically ordered phase fraction was determined from the illuminated area ratio of the (1 1 0) Bragg diffraction ring to the (2 2 0) diffraction ring in TEM dark field images of appropriately oriented grains, as described elsewhere [5]. Room-temperature magnetic properties were characterized using a SQUID magnetometer with a 50 kOe maximum field. The exchange coupling present in the nanocomposite samples was examined from the room-temperature recoil curves measured from samples subjected to different annealing states. There are two pertinent parameters of the recoil loop: (i) the maximum 0 normalized recoil magnetization Mrec ¼ Mrec =MS ; where Mrec is the recovered magnetization at zero internal field and MS is the saturation magnetization and (ii) the normalized recoil loop area s0 ; which is the area of the recoil loop divided by the area of the second and third quadrants of the 0 major hysteresis loop. Mrec provides an estimate of
191
the ‘‘springiness’’ of the magnetic exchange by quantifying the proportion of magnetization recovered during recoil. The recoil area s0 is a unitless measure of the robustness of the exchange coupling. The recoil loop area is maximized just prior to magnetic reversal of the soft component of the system, and it is assumed that maxima in the 0 Mrec and s0 parameters signal a maximum in the exchange coupling.
3. Results and discussion An increase of the mean grain size with annealing time at 7001C can clearly be seen in the TEM bright-field micrographs of Fig. 1. The selected area (electron) diffraction patterns shown as insets in Fig. 1 also reveal the progress of the transformation in CoPt from the chemically disordered A1 state to the fully ordered L10 state concurrent with the grain growth in the films. The ordered phase fraction increases gradually with annealing time, reaching the fully ordered state within 120 min. The measured magnetic properties change in accordance with the annealing-induced
Fig. 1. TEM micrographs and SADPs for 25-nm-thick CoPt films annealed at 7001C for (a) 0, (b) 5, (c) 120, and (d) 900 min.
L.H. Lewis et al. / Physica B 327 (2003) 190–193
192
4
1
coercivity (Oe)
8000
0.8
6000
0.6
4000
0.4
2000
0.2
0
L10 vol fraction
1 10
0 0
100
200
300
400
500
600
anneal time @700 C Fig. 2. Development of coercivity and L10-ordered fraction in 25-nm-thick CoPt films annealed at 7001C. Dashed lines are drawn to guide the eye.
2.5
0.3
2
rec
maximum M '
1.5 0.2 1
normalized maximum recoil area σ'
0.15
)
0.5
-2
normalized maximum recovered magnetization M rec '
maximum recoil area σ' (x10
0.25
0
0.1 0
20
40
60
80
100
120
anneal time @700 C (min) 0 Fig. 3. Maximum normalized recovered magnetization Mrec and maximum normalized recoil area s0 for the 25-nm-thick CoPt samples annealed at T=7001C. Dashed lines are drawn to guide the eye.
L.H. Lewis et al. / Physica B 327 (2003) 190–193
microstructural evolution described above. Fig. 2 displays the coercivities of the films which increase with increased annealing time and peak at the fully ordered state. As demonstrated in previous work [5] a remarkably consistent and linear relationship between the ordered fraction and the coercivity of the films is noted that remains unexplained. Fig. 2 includes application of this empirical relationship to track the development of the L10-ordered phase fraction with annealing time at 7001C in the 25nm-thick films of this study. At all annealing stages the second quadrant of the films’ major hysteresis loop exhibited single-phase, coherent demagnetization behavior. The attributes of the recoil curves themselves were used to assess the extent of the exchange coupling found in these ‘‘natural’’ CoPt nanocomposites. Fig. 3 displays both the normalized maximum recovered magne0 tization Mrec and the maximum recoil area s0 : A prominent peak in both parameters occurs after 5 min of annealing, which corresponds to a sample that consists roughly of 27 vol% L10-type CoPt and 73 vol% A1-type CoPt. Based on the precepts of what recoil curve parameters signify, it can be concluded that optimum exchange coupling in the ‘‘natural’’ nanocomposite CoPt system of 25-nmthick films is found when the sample contains approximately 30 vol% magnetically hard phase and 70 vol% magnetically soft phase. This experimental estimate of the phase fraction ratio of hard:soft phase in exchange-spring systems for
193
maximum exchange enhancement is in rough agreement with that computed by Fischer et al. [7], albeit for the magnetic nanocomposite system Nd2Fe14B and a-Fe.
Acknowledgements Research was performed in part under the auspices of the US DOE under Contract No. DE-AC02-98CH10886. Support from National Science Foundation grants ECD-8907068, DMR9458000, DMR-9411146 and DMR-9256332, and the Horner Fellowship of Lehigh University are gratefully acknowledged.
References [1] E.F. Kneller, R. Hawig, IEEE Trans. Magn. 27 (1991) 3588. [2] R. Fischer, T. Leineweber, H. Kronmuller, . Phys. Rev. B 57 (17) (1998) 10723. [3] E.E. Fullerton, J.S. Jiang, M. Grimsditch, C.H. Sowers, S.D. Bader, Phys. Rev. B 58 (1998) 12193. [4] D.C. Crew, J. Kim, L.H. Lewis, K. Barmak, J. Magn. Magn. Mater. 233 (2001) 257. [5] R.A. Ristau, K. Barmak, L.H. Lewis, K.R. Coffey, J.K. Howard, J. Appl. Phys. 86 (1999) 4527. [6] J.B. Newkirk, R. Smolochowski, A.H. Geisler, D.L. Martin, J. Appl. Phys. 22 (1951) 290. [7] R. Fischer, T. Schrefl, H. Kronmuller, . J. Fidler, J. Magn. Magn. Mater. 150 (1995) 329.
The CoPt system: a natural exchange spring L.H. Lewisa,*, J. Kimb, K. Barmakb,c a
Materials Science Department, Brookhaven National Laboratory, Building 480, Upton, NY 11973, USA b Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA 15213, USA c Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA
Abstract Ferromagnetic ‘‘exchange-spring’’ nanocomposite systems derive their technical magnetic properties, such as the remanence and coercivity, from the details of the interphase coupling between the magnetically soft component and the magnetically hard component. An ideal model material for the study of the interphase interactions in exchange-spring systems is the CoPt system in thin-film form. Depending on the details of post-deposition annealing treatment, CoPt consists of two phases in varying proportions: the chemically disordered A1 phase with low coercivity and the chemically ordered L10 phase with high coercivity. Coupled magnetic and transmission electron microscopy studies reveal simple relationships between the volume percent of ordered L10 phase, coercivity and the magnetic exchange within the samples. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.50.K; 75.50.W; 75.30.E Keywords: Order–disorder transitions; Permanent magnets; Exchange in magnetically ordered materials
1. Introduction Ferromagnetic ‘‘exchange-spring’’ nanocomposite materials are a unique class of materials that are envisioned as the next-generation permanent magnet. They acquire novel technical magnetic properties such as significant remanence enhancement and increased energy product, from the details of the interphase coupling between the magnetically soft component and the magnetically hard component [1]. Computational and experimental studies have demonstrated that variation of the materials’ attributes of the nanocomposite, *Corresponding author. Tel.: +516-344-2861; fax: +516344-4071. E-mail address: [email protected] (L.H. Lewis).
such as relative phase concentration, crystallographic alignment and grain size, can all drastically affect the magnetic behavior of the system [2–4]. Typically, exchange-spring nanosystems are composed of phases with very different chemistries, such as Nd2Fe14B combined with a-Fe or Fe3B, and Sm–Co combined with Co or Fe [3]. A different approach to synthesize magnetic nanocomposites is to exploit the microstructural evolution of ferromagnetic material as it undergoes a first-order crystallographic phase transition. In this manner, a daughter phase forms from the parent phase of identical chemistry by nucleation and growth processes [5,6] to produce a two-phase nanocomposite. This approach has great potential for tailoring the magnetic behavior of the
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 2 5 - 8
L.H. Lewis et al. / Physica B 327 (2003) 190–193
nanocomposite system because, in principle, it is possible to control the nucleation and growth characteristics of the system. The ferromagnetic compound CoPt is an ideal materials system for the creation of a natural magnetic nanocomposite. In this system, a high-anisotropy chemically L10ordered CoPt phase is obtained via a thermodynamically first-order, disorder–order transformation from the chemically disordered FCC solid solution, or A1, phase. When sputter-deposited into thin-film form at room temperature, CoPt forms in the magnetically soft A1 structure with the magnetically hard L10 structure obtained by elevated temperature annealing.
2. Experimental details CoPt films of thicknesses of either 10 or 25 nm were deposited onto water-cooled oxidized silicon wafers using DC magnetron sputtering from equiatomic alloy targets. Ex situ annealing as a function of time in a tube furnace at 7001C in an Ar–4% H2 atmosphere was employed to induce the first-order crystallographic phase transition to the L10 structure. Transmission electron microscopy (TEM) was used to ascertain grain size, texture, and ordered domain size of the film phases. The chemically ordered phase fraction was determined from the illuminated area ratio of the (1 1 0) Bragg diffraction ring to the (2 2 0) diffraction ring in TEM dark field images of appropriately oriented grains, as described elsewhere [5]. Room-temperature magnetic properties were characterized using a SQUID magnetometer with a 50 kOe maximum field. The exchange coupling present in the nanocomposite samples was examined from the room-temperature recoil curves measured from samples subjected to different annealing states. There are two pertinent parameters of the recoil loop: (i) the maximum 0 normalized recoil magnetization Mrec ¼ Mrec =MS ; where Mrec is the recovered magnetization at zero internal field and MS is the saturation magnetization and (ii) the normalized recoil loop area s0 ; which is the area of the recoil loop divided by the area of the second and third quadrants of the 0 major hysteresis loop. Mrec provides an estimate of
191
the ‘‘springiness’’ of the magnetic exchange by quantifying the proportion of magnetization recovered during recoil. The recoil area s0 is a unitless measure of the robustness of the exchange coupling. The recoil loop area is maximized just prior to magnetic reversal of the soft component of the system, and it is assumed that maxima in the 0 Mrec and s0 parameters signal a maximum in the exchange coupling.
3. Results and discussion An increase of the mean grain size with annealing time at 7001C can clearly be seen in the TEM bright-field micrographs of Fig. 1. The selected area (electron) diffraction patterns shown as insets in Fig. 1 also reveal the progress of the transformation in CoPt from the chemically disordered A1 state to the fully ordered L10 state concurrent with the grain growth in the films. The ordered phase fraction increases gradually with annealing time, reaching the fully ordered state within 120 min. The measured magnetic properties change in accordance with the annealing-induced
Fig. 1. TEM micrographs and SADPs for 25-nm-thick CoPt films annealed at 7001C for (a) 0, (b) 5, (c) 120, and (d) 900 min.
L.H. Lewis et al. / Physica B 327 (2003) 190–193
192
4
1
coercivity (Oe)
8000
0.8
6000
0.6
4000
0.4
2000
0.2
0
L10 vol fraction
1 10
0 0
100
200
300
400
500
600
anneal time @700 C Fig. 2. Development of coercivity and L10-ordered fraction in 25-nm-thick CoPt films annealed at 7001C. Dashed lines are drawn to guide the eye.
2.5
0.3
2
rec
maximum M '
1.5 0.2 1
normalized maximum recoil area σ'
0.15
)
0.5
-2
normalized maximum recovered magnetization M rec '
maximum recoil area σ' (x10
0.25
0
0.1 0
20
40
60
80
100
120
anneal time @700 C (min) 0 Fig. 3. Maximum normalized recovered magnetization Mrec and maximum normalized recoil area s0 for the 25-nm-thick CoPt samples annealed at T=7001C. Dashed lines are drawn to guide the eye.
L.H. Lewis et al. / Physica B 327 (2003) 190–193
microstructural evolution described above. Fig. 2 displays the coercivities of the films which increase with increased annealing time and peak at the fully ordered state. As demonstrated in previous work [5] a remarkably consistent and linear relationship between the ordered fraction and the coercivity of the films is noted that remains unexplained. Fig. 2 includes application of this empirical relationship to track the development of the L10-ordered phase fraction with annealing time at 7001C in the 25nm-thick films of this study. At all annealing stages the second quadrant of the films’ major hysteresis loop exhibited single-phase, coherent demagnetization behavior. The attributes of the recoil curves themselves were used to assess the extent of the exchange coupling found in these ‘‘natural’’ CoPt nanocomposites. Fig. 3 displays both the normalized maximum recovered magne0 tization Mrec and the maximum recoil area s0 : A prominent peak in both parameters occurs after 5 min of annealing, which corresponds to a sample that consists roughly of 27 vol% L10-type CoPt and 73 vol% A1-type CoPt. Based on the precepts of what recoil curve parameters signify, it can be concluded that optimum exchange coupling in the ‘‘natural’’ nanocomposite CoPt system of 25-nmthick films is found when the sample contains approximately 30 vol% magnetically hard phase and 70 vol% magnetically soft phase. This experimental estimate of the phase fraction ratio of hard:soft phase in exchange-spring systems for
193
maximum exchange enhancement is in rough agreement with that computed by Fischer et al. [7], albeit for the magnetic nanocomposite system Nd2Fe14B and a-Fe.
Acknowledgements Research was performed in part under the auspices of the US DOE under Contract No. DE-AC02-98CH10886. Support from National Science Foundation grants ECD-8907068, DMR9458000, DMR-9411146 and DMR-9256332, and the Horner Fellowship of Lehigh University are gratefully acknowledged.
References [1] E.F. Kneller, R. Hawig, IEEE Trans. Magn. 27 (1991) 3588. [2] R. Fischer, T. Leineweber, H. Kronmuller, . Phys. Rev. B 57 (17) (1998) 10723. [3] E.E. Fullerton, J.S. Jiang, M. Grimsditch, C.H. Sowers, S.D. Bader, Phys. Rev. B 58 (1998) 12193. [4] D.C. Crew, J. Kim, L.H. Lewis, K. Barmak, J. Magn. Magn. Mater. 233 (2001) 257. [5] R.A. Ristau, K. Barmak, L.H. Lewis, K.R. Coffey, J.K. Howard, J. Appl. Phys. 86 (1999) 4527. [6] J.B. Newkirk, R. Smolochowski, A.H. Geisler, D.L. Martin, J. Appl. Phys. 22 (1951) 290. [7] R. Fischer, T. Schrefl, H. Kronmuller, . J. Fidler, J. Magn. Magn. Mater. 150 (1995) 329.