- Email: [email protected]
Pulsed laser deposition of Sr2FeMoO6 thin films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 294 (2005) e119–e122 www.elsevier.com/locate/jmmm
Pulsed laser deposition of Sr2FeMoO6 thin films D. Sancheza, N. Authb, G. Jakobb, J.L. Martı´ neza, M. Garcı´ a-Herna´ndeza, a Insituto de Ciencia de Materiales, CSIC, Madrid 28049, Spain Intitut fu¨r Physik, Gutenberg Universita¨t, Mainz D-55099, Germany
b
Available online 15 April 2005
Abstract The effect of various deposition conditions and after-growth protocols on the magnetic and transport properties of Sr2FeMoO6 films has been explored. It is found that the saturation magnetization and the magnetoresistance (MR) are dominated by the degree of cationic order, and the strain effects are clearly evidenced in a lower TC. The after-growth annealing of the films and the deposition of a buffer layer has been found to relax the film strains. This translates into a clear increase of the measured low-field magnetoresistance ratios. r 2005 Elsevier B.V. All rights reserved. PACS: 61.12q; 75.25.+z Keywords: Double perovskite; Colossal magnetoresistance; Thin film; Antisite disorder
1. Introduction The figure of merit of the double perovskite Sr2FeMoO6 (SFMO) as a component of real world devices is the magnetoresistance ratio [1,2]. This compound is a half metal, ferromagnetic oxide, with TC around 415 K, and its synthesis is almost unavoidably linked to the presence of anti-site disorder (ASD) (i.e. Fe cations ideally occupying the B0 positions can be found at the Mo sites (B00 ) and vice versa). There are three basic mechanisms Corresponding author. Tel.: +34 91 3348992; fax: +34 91 3720623. E-mail address: [email protected] (M. Garcı´ a-Herna´ndez).
that give rise to a high magnetoresistive response in this material: the extrinsic tunnel magnetoresistance (TMR), accounting for a large fraction of the observed magnetoresistance in the polycrystal, mainly at low temperatures and low fields; the intrinsic magnetorresistive effect associated to the ferromagnetic transition, appearing also in single crystals and enhanced at temperatures near TC; and also a third component related to the presence of antisite as it has been recently reported [3,4]. This latter contribution can be understood in terms of the existence of antiferromagnetic (AF) correlations at the antisite defects (i.e. between two consecutive Fe cations) [5]. This antiferromagnetism introduces a significant spin disorder in the system, which in turn, leads to an increase in
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.03.066
ARTICLE IN PRESS D. Sanchez et al. / Journal of Magnetism and Magnetic Materials 294 (2005) e119–e122
the material resistivity, due to the enhanced spinpolarized electron scattering. The application of an external magnetic field suppresses this spin disorder to a great extent, lowering the resistivity and thus inducing a remarkable magnetoresistive effect. The aim of this paper is to explore the magnetoresistive behavior of high-quality SFMO films so as to optimize the different contributions to the lowfield magnetoresistance (LFMR). From now onwards, we attach to the usual non-inflactionary definition of magnetoresistance, MR(T,H) (%) ¼ [(R(T,H)-R(T,H ¼ 0)]100/R(T,H ¼ 0).
1.0 H = 200 Oe 0.8 M (emu)
e120
0.6 0.4 0.2 0.0 0
100
200
300
400
Temperature (K)
1.0
2. Experiments and results H = 200 Oe
M (emu)
0.8
SFMO films were grown on SrTiO3 (0 0 1)oriented substrates by pulsed laser deposition. We recall here that SrTiO3 (0 0 1) is a cubic perovskite with a lattice parameter a, such that when doubled ( is smaller than the corresponding 2a ¼ 7:81 A ( Therefore, SFMO lattice constant (a ¼ 7:89 A). our SFMO films sustain an in-plane compressive strain around 1%. Details on the substrate preparation and film growth are given elsewhere [4]. Our sample labeled A was prepared with an Ar+O2 mixture as ablation atmosphere at 0.1 mbar total pressure, with the only restriction of O2 content limited to be o0.5%. However, a more restrictive control of the oxygen content in the deposition atmosphere followed by ‘‘in-situ’’ annealing at 950 1C in oxygen flow renders best quality films, among them the film labeled B in this work [4]. X-ray diffractograms point out the epitaxy of the grown films. Four-circle XRD data give a cationic order fraction of 51% and 64% for samples A and B, respectively. Both are metallic, exhibit saturated moments M s ðAÞ ¼ 2:24 mB and M s ðBÞ ¼ 2:71 mB , and Curie temperatures T C ðAÞ ¼ 340 K and T C ðBÞ ¼ 335 K, respectively, as shown in Fig. 1. As previously reported [3,4] there is a clear correlation between the levels of antisite disorder present in the samples and the measured LFMR. It appears that moderate levels of antisite disorder lead to good films that show an accountable ASD contribution to the LFMR, while high ASD levels
0.6 0.4 0.2 0.0 0
100
200
300
400
Temperature (K) Fig. 1. Magnetization of samples A (upper) and B (lower). Solid (open) symbols are for films before (after) annealing.
lead to a complete loss of spin polarization in the system and, hence, low LFMR ratios. Fig. 2 shows the evolution of the magnetoresistance with the applied field for both samples A and B. From our previous observation [4], it also appears that high levels of cationic ordering seem to be linked to films presenting little strain (i.e exhibiting a lattice parameter close to that corresponding to the bulk material). In order to optimize the magnetoresistive properties of the films, we have followed different routes aiming to relax the substrate-induced strain and to enhance the Fe/Mo ordering. First, we have applied a thermal treatment to the films, following the protocols used in the bulk material. Samples A and B were annealed for 10 h at 950 1C in a flow of H2/N2 (5%/95%), in a furnace outside the ablation chamber. The structural changes can be seen
ARTICLE IN PRESS D. Sanchez et al. / Journal of Magnetism and Magnetic Materials 294 (2005) e119–e122
Sample B
4x104
-5
3x104 % MR
-10 -15
SFMO (004)
Sample A
SFMO (004)
0
e121
cbefore= 7.977 Å cafter= 7.883 Å
2x104
-20
1x104
-25 T=5K -30 -8
-6
-4
-2
0
2
4
6
8
0 44
µ0H (T)
Fig. 2. Magnetoresistance (%) for samples A (left) and B (right), before (solid circles) and after ( open circles). Solid circles: as grown; open circles: +12 h, 950 1C, H2/N2 (5%/ 95%).
45
46
47
46
47
4x104
3x104
in Fig. 3. Epitaxy is maintained while the strain relaxation is apparent from the shift of the SFMO reflections to higher angles. The degree of ordering seems to be unaffected by the annealing pointing out the need of higher temperature or longer annealing times to enable the diffusion of the heavy Fe and Mo ions. The magnetic measurement of the films, as grown and after thermal treatment, are given in Fig. 1. Notice that TC increases up to 365 K in sample A, while it does not vary with the annealing for sample B. This is probably related to the fact that sample B was previously annealed in situ after deposition, while sample A was cooled down to room temperature after deposition. In Fig. 2, the magnetoresistance ratios for both films A and B before and after annealing are shown. It can be concluded that in both cases annealing strongly increases the MR and, as ASD is hardly affected by the annealing, this enhancement can be only due to an increase of boundaries and morphological changes in the films during thermal treatment. As an alternative approach to reduce the strain, we have deposited a buffer layer prior to the SFMO deposition so as to minimize the lattice mismatch between SFMO and the SrTiO3 (0 0 1)
cbefore= 7.936 Å cafter= 7.901 Å
2x104
1x104
0 44
45 2Θ (°)
Fig. 3. Epitaxy remains after annealing of samples A (upper) and B (lower). Strain is released as seen from the shift of the Bragg peaks of the annealed samples to higher angles (thick line).
substrate. For a buffer layer of Ba0.4Sr0.6TiO3, ( a perfect matching with the with a ¼ 7:89 A, SFMO lattice parameter is found [6]. However, growing the buffer layer in an oxygen atmosphere we found an ‘‘in-plane’’ lattice constant identical to that of the substrate. Also, due to the restriction imposed by the subsequent deposition of the SFMO layer that is extremely sensitive to the oxygen content in the ablation chamber, the buffer layer was deposited in an oxygen deficient atmosphere. By doing so,
ARTICLE IN PRESS D. Sanchez et al. / Journal of Magnetism and Magnetic Materials 294 (2005) e119–e122
e122
0
-8
-12 0
µ0H (T)
10
1
2
6
0
0
-2
-6
2
SFMO(008)
-1
STO(003)
-2
M (µB/fu)
-6
STO(004)
STO(002) MR (%)
4
12
-4
-10
SFMO(004)
counts
106
STO(001)
108
4
T = 10K
-2
-4
-1.0
102
-12
SFMO/BSTO/STO -0.5
0.0
0.5
1.0
µ0H (T) 100 20
40
60
80
100
2θ
Fig. 4. Diffractogram of a representative sample of SFMO on a buffer layer of Ba0.4Sr0.6TiO3. Inset shows the magnetoresistance (%) of the film vs. the applied field, H.
the bottom layer exhibits an in-plane lattice different from the substrate constant and acts as an efficient oxygen getter which prevents the nucleation of the spurious SrMoO4 phase. Concomitantly, the buffer layer grows textured with grain sizes around 100 nm, enabling the deposition of an unstrained SFMO film at the expense of an enhanced surface roughness. Saturated moments of up to 3.2 mB have been measured and the films are always metallic. The diffraction pattern of a representative film of this series is given in Fig. 4. Also, the inset shows the evolution of the magnetoresistance, with the applied field at 10 K. The origin of such MR behavior can be understood from Fig. 5, where the derivative of the magnetoresistance is given along with the magnetization of the sample versus applied field. As it is apparent, the maximum variation of the resistivity closely follows the magnetization process of the sample, in analogy to the case of polycrystalline samples. This can be explained in terms of an increase of the grain boundaries density in the film. In conclusion, we have developed a clean protocol to grow high-quality epitaxial films of SFMO. It is found that careful post growth annealing of the films in an reducing atmosphere,
Fig. 5. Magnetization (left axis) vs. applied field for a film grown on Ba0.4Sr0.6TiO3 derivative of the resistivity (right axis) with respect to the applied field.
as it is used in the synthesis of the bulk material, strongly increases the MR ratios, as a result of relaxing the strains naturally occurring in the epitaxial film. Also, it has been shown that the use of a buffer layer modifies the morphology of the films enhancing the magnetoresistive properties of the material.
Acknowledgments We acknowledge financial support from Spanish grants, MAT2002-01329, CAM 07N/0080/2002 and German grants BMBF 03N6500 and DAAD D/03/39325.
References [1] K.I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677. [2] Y. Tomioka, T. Okuda, Y. Okimoto, R. Kumai, K.I. Kobayashi, Y. Tokura, Phys. Rev. B 61 (2000) 422. [3] M. Garcı´ a-Herna´ndez, J.L. Martı´ nez, M.J. Martı´ nez-Lope, M.T. Casais, J.A. Alonso, Phys. Rev. Lett. 86 (2001) 2443. [4] D. Sa´nchez, M. Garcı´ a-Herna´ndez, N. Auth, G. Jakob, J. Appl. Phys. 96 (2004) 2736. [5] D. Sa´nchez, J.A. Alonso, M. Garcı´ a-Herna´ndez, M.J. Martı´ nez-Lope, J.L. Martı´ nez, Phys. Rev. B 65 (2002) 104426. [6] H. Asano, Y. Kohara, M. Matsui, Jpn. J. Appl. Phys. 41 (2002) L1081.
Journal of Magnetism and Magnetic Materials 294 (2005) e119–e122 www.elsevier.com/locate/jmmm
Pulsed laser deposition of Sr2FeMoO6 thin films D. Sancheza, N. Authb, G. Jakobb, J.L. Martı´ neza, M. Garcı´ a-Herna´ndeza, a Insituto de Ciencia de Materiales, CSIC, Madrid 28049, Spain Intitut fu¨r Physik, Gutenberg Universita¨t, Mainz D-55099, Germany
b
Available online 15 April 2005
Abstract The effect of various deposition conditions and after-growth protocols on the magnetic and transport properties of Sr2FeMoO6 films has been explored. It is found that the saturation magnetization and the magnetoresistance (MR) are dominated by the degree of cationic order, and the strain effects are clearly evidenced in a lower TC. The after-growth annealing of the films and the deposition of a buffer layer has been found to relax the film strains. This translates into a clear increase of the measured low-field magnetoresistance ratios. r 2005 Elsevier B.V. All rights reserved. PACS: 61.12q; 75.25.+z Keywords: Double perovskite; Colossal magnetoresistance; Thin film; Antisite disorder
1. Introduction The figure of merit of the double perovskite Sr2FeMoO6 (SFMO) as a component of real world devices is the magnetoresistance ratio [1,2]. This compound is a half metal, ferromagnetic oxide, with TC around 415 K, and its synthesis is almost unavoidably linked to the presence of anti-site disorder (ASD) (i.e. Fe cations ideally occupying the B0 positions can be found at the Mo sites (B00 ) and vice versa). There are three basic mechanisms Corresponding author. Tel.: +34 91 3348992; fax: +34 91 3720623. E-mail address: [email protected] (M. Garcı´ a-Herna´ndez).
that give rise to a high magnetoresistive response in this material: the extrinsic tunnel magnetoresistance (TMR), accounting for a large fraction of the observed magnetoresistance in the polycrystal, mainly at low temperatures and low fields; the intrinsic magnetorresistive effect associated to the ferromagnetic transition, appearing also in single crystals and enhanced at temperatures near TC; and also a third component related to the presence of antisite as it has been recently reported [3,4]. This latter contribution can be understood in terms of the existence of antiferromagnetic (AF) correlations at the antisite defects (i.e. between two consecutive Fe cations) [5]. This antiferromagnetism introduces a significant spin disorder in the system, which in turn, leads to an increase in
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.03.066
ARTICLE IN PRESS D. Sanchez et al. / Journal of Magnetism and Magnetic Materials 294 (2005) e119–e122
the material resistivity, due to the enhanced spinpolarized electron scattering. The application of an external magnetic field suppresses this spin disorder to a great extent, lowering the resistivity and thus inducing a remarkable magnetoresistive effect. The aim of this paper is to explore the magnetoresistive behavior of high-quality SFMO films so as to optimize the different contributions to the lowfield magnetoresistance (LFMR). From now onwards, we attach to the usual non-inflactionary definition of magnetoresistance, MR(T,H) (%) ¼ [(R(T,H)-R(T,H ¼ 0)]100/R(T,H ¼ 0).
1.0 H = 200 Oe 0.8 M (emu)
e120
0.6 0.4 0.2 0.0 0
100
200
300
400
Temperature (K)
1.0
2. Experiments and results H = 200 Oe
M (emu)
0.8
SFMO films were grown on SrTiO3 (0 0 1)oriented substrates by pulsed laser deposition. We recall here that SrTiO3 (0 0 1) is a cubic perovskite with a lattice parameter a, such that when doubled ( is smaller than the corresponding 2a ¼ 7:81 A ( Therefore, SFMO lattice constant (a ¼ 7:89 A). our SFMO films sustain an in-plane compressive strain around 1%. Details on the substrate preparation and film growth are given elsewhere [4]. Our sample labeled A was prepared with an Ar+O2 mixture as ablation atmosphere at 0.1 mbar total pressure, with the only restriction of O2 content limited to be o0.5%. However, a more restrictive control of the oxygen content in the deposition atmosphere followed by ‘‘in-situ’’ annealing at 950 1C in oxygen flow renders best quality films, among them the film labeled B in this work [4]. X-ray diffractograms point out the epitaxy of the grown films. Four-circle XRD data give a cationic order fraction of 51% and 64% for samples A and B, respectively. Both are metallic, exhibit saturated moments M s ðAÞ ¼ 2:24 mB and M s ðBÞ ¼ 2:71 mB , and Curie temperatures T C ðAÞ ¼ 340 K and T C ðBÞ ¼ 335 K, respectively, as shown in Fig. 1. As previously reported [3,4] there is a clear correlation between the levels of antisite disorder present in the samples and the measured LFMR. It appears that moderate levels of antisite disorder lead to good films that show an accountable ASD contribution to the LFMR, while high ASD levels
0.6 0.4 0.2 0.0 0
100
200
300
400
Temperature (K) Fig. 1. Magnetization of samples A (upper) and B (lower). Solid (open) symbols are for films before (after) annealing.
lead to a complete loss of spin polarization in the system and, hence, low LFMR ratios. Fig. 2 shows the evolution of the magnetoresistance with the applied field for both samples A and B. From our previous observation [4], it also appears that high levels of cationic ordering seem to be linked to films presenting little strain (i.e exhibiting a lattice parameter close to that corresponding to the bulk material). In order to optimize the magnetoresistive properties of the films, we have followed different routes aiming to relax the substrate-induced strain and to enhance the Fe/Mo ordering. First, we have applied a thermal treatment to the films, following the protocols used in the bulk material. Samples A and B were annealed for 10 h at 950 1C in a flow of H2/N2 (5%/95%), in a furnace outside the ablation chamber. The structural changes can be seen
ARTICLE IN PRESS D. Sanchez et al. / Journal of Magnetism and Magnetic Materials 294 (2005) e119–e122
Sample B
4x104
-5
3x104 % MR
-10 -15
SFMO (004)
Sample A
SFMO (004)
0
e121
cbefore= 7.977 Å cafter= 7.883 Å
2x104
-20
1x104
-25 T=5K -30 -8
-6
-4
-2
0
2
4
6
8
0 44
µ0H (T)
Fig. 2. Magnetoresistance (%) for samples A (left) and B (right), before (solid circles) and after ( open circles). Solid circles: as grown; open circles: +12 h, 950 1C, H2/N2 (5%/ 95%).
45
46
47
46
47
4x104
3x104
in Fig. 3. Epitaxy is maintained while the strain relaxation is apparent from the shift of the SFMO reflections to higher angles. The degree of ordering seems to be unaffected by the annealing pointing out the need of higher temperature or longer annealing times to enable the diffusion of the heavy Fe and Mo ions. The magnetic measurement of the films, as grown and after thermal treatment, are given in Fig. 1. Notice that TC increases up to 365 K in sample A, while it does not vary with the annealing for sample B. This is probably related to the fact that sample B was previously annealed in situ after deposition, while sample A was cooled down to room temperature after deposition. In Fig. 2, the magnetoresistance ratios for both films A and B before and after annealing are shown. It can be concluded that in both cases annealing strongly increases the MR and, as ASD is hardly affected by the annealing, this enhancement can be only due to an increase of boundaries and morphological changes in the films during thermal treatment. As an alternative approach to reduce the strain, we have deposited a buffer layer prior to the SFMO deposition so as to minimize the lattice mismatch between SFMO and the SrTiO3 (0 0 1)
cbefore= 7.936 Å cafter= 7.901 Å
2x104
1x104
0 44
45 2Θ (°)
Fig. 3. Epitaxy remains after annealing of samples A (upper) and B (lower). Strain is released as seen from the shift of the Bragg peaks of the annealed samples to higher angles (thick line).
substrate. For a buffer layer of Ba0.4Sr0.6TiO3, ( a perfect matching with the with a ¼ 7:89 A, SFMO lattice parameter is found [6]. However, growing the buffer layer in an oxygen atmosphere we found an ‘‘in-plane’’ lattice constant identical to that of the substrate. Also, due to the restriction imposed by the subsequent deposition of the SFMO layer that is extremely sensitive to the oxygen content in the ablation chamber, the buffer layer was deposited in an oxygen deficient atmosphere. By doing so,
ARTICLE IN PRESS D. Sanchez et al. / Journal of Magnetism and Magnetic Materials 294 (2005) e119–e122
e122
0
-8
-12 0
µ0H (T)
10
1
2
6
0
0
-2
-6
2
SFMO(008)
-1
STO(003)
-2
M (µB/fu)
-6
STO(004)
STO(002) MR (%)
4
12
-4
-10
SFMO(004)
counts
106
STO(001)
108
4
T = 10K
-2
-4
-1.0
102
-12
SFMO/BSTO/STO -0.5
0.0
0.5
1.0
µ0H (T) 100 20
40
60
80
100
2θ
Fig. 4. Diffractogram of a representative sample of SFMO on a buffer layer of Ba0.4Sr0.6TiO3. Inset shows the magnetoresistance (%) of the film vs. the applied field, H.
the bottom layer exhibits an in-plane lattice different from the substrate constant and acts as an efficient oxygen getter which prevents the nucleation of the spurious SrMoO4 phase. Concomitantly, the buffer layer grows textured with grain sizes around 100 nm, enabling the deposition of an unstrained SFMO film at the expense of an enhanced surface roughness. Saturated moments of up to 3.2 mB have been measured and the films are always metallic. The diffraction pattern of a representative film of this series is given in Fig. 4. Also, the inset shows the evolution of the magnetoresistance, with the applied field at 10 K. The origin of such MR behavior can be understood from Fig. 5, where the derivative of the magnetoresistance is given along with the magnetization of the sample versus applied field. As it is apparent, the maximum variation of the resistivity closely follows the magnetization process of the sample, in analogy to the case of polycrystalline samples. This can be explained in terms of an increase of the grain boundaries density in the film. In conclusion, we have developed a clean protocol to grow high-quality epitaxial films of SFMO. It is found that careful post growth annealing of the films in an reducing atmosphere,
Fig. 5. Magnetization (left axis) vs. applied field for a film grown on Ba0.4Sr0.6TiO3 derivative of the resistivity (right axis) with respect to the applied field.
as it is used in the synthesis of the bulk material, strongly increases the MR ratios, as a result of relaxing the strains naturally occurring in the epitaxial film. Also, it has been shown that the use of a buffer layer modifies the morphology of the films enhancing the magnetoresistive properties of the material.
Acknowledgments We acknowledge financial support from Spanish grants, MAT2002-01329, CAM 07N/0080/2002 and German grants BMBF 03N6500 and DAAD D/03/39325.
References [1] K.I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677. [2] Y. Tomioka, T. Okuda, Y. Okimoto, R. Kumai, K.I. Kobayashi, Y. Tokura, Phys. Rev. B 61 (2000) 422. [3] M. Garcı´ a-Herna´ndez, J.L. Martı´ nez, M.J. Martı´ nez-Lope, M.T. Casais, J.A. Alonso, Phys. Rev. Lett. 86 (2001) 2443. [4] D. Sa´nchez, M. Garcı´ a-Herna´ndez, N. Auth, G. Jakob, J. Appl. Phys. 96 (2004) 2736. [5] D. Sa´nchez, J.A. Alonso, M. Garcı´ a-Herna´ndez, M.J. Martı´ nez-Lope, J.L. Martı´ nez, Phys. Rev. B 65 (2002) 104426. [6] H. Asano, Y. Kohara, M. Matsui, Jpn. J. Appl. Phys. 41 (2002) L1081.