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Hard magnetic SmCo thin films prepared by pulsed laser deposition
Journal of Magnetism and Magnetic Materials 242–245 (2002) 1290–1293
Hard magnetic SmCo thin films prepared by pulsed laser deposition V. Neu*, J. Thomas, S. F.ahler, B. Holzapfel, L. Schultz IFW Dresden, Helmholtzstr. 20, D-01069 Dresden, Germany
Abstract Hard magnetic SmCo films were prepared by pulsed laser deposition in UHV and at moderate background pressures on polycrystalline Al2O3. Structure and magnetic properties are discussed for varying Sm-content. The perpendicular anisotropy observed at a deposition pressure of 0.06 mbar is compared with texturing models known for sputtered samples. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Ferromagnetic films; Rare earth–transition metal compounds; CoercivityFcomposition dependence; Texture; Pulsed laser deposition
Permanent magnet films based on highly anisotropic rare earth–transition metal phases like SmCo5 are promising materials for structured microelectromagnetic devices, when strong magnetic fields and high coercivities are needed. They may also find application in magnetic recording, as future miniaturization requires higher anisotropies to avoid a superparamagnetic switching of a magnetic bit. Besides coercivity, a pronounced texture is desirable for permanent magnet application. Via magnetron sputtering, highly coercive SmCo films can be prepared with a distinct in-plane texture and therefore almost square-shaped in-plane hysteresis loops [1]. In these experiments, the highly anisotropic SmCo5 phase was formed upon direct crystallization on heated substrates. Typical values for the coercive field m0 Hc are 0.7–1.5 T, varying monotonically with a Sm-content of 15–19 at% [2]. The in-plane texture is explained in Ref. [1] as a result of energetic considerations. At deposition temperatures below the Curie temperature of SmCo (Tc ¼ 70029001C, depending on the Co-content), a minimization of the stray field energy favors the orientation of the c-axisFthe easy magnetization axisFparallel to the film plane. The high kinetic energy *Corresponding author. Tel.: +49-351-4659-237; fax: +49351-4659-541. E-mail address: [email protected] (V. Neu).
of the plasma particles may disrupt this growth mode, so that a background pressure of around 0.1 mbar is used in magnetron sputtering to thermalize the plasma. Pulsed laser deposition (PLD) is a strong nonequilibrium process, with plasma energies ranging from 10 eV when operated in a background pressure of 0.1 mbar to 100 eV in UHV [3]. But unlike magnetron sputtering, the ablation process in PLD is hardly changed by the deposition pressure, so that the influence of an ambient gas on the growth mode and texture can be tested over a wide pressure range. In a recent paper, Cadieu et al. [4] used PLD to prepare SmCo-based films. The deposition conditions were chosen close to those used in the sputtering process. A background Ar pressure of 0.13 mbar and an additional shadow mask led to films with a distinct in-plane texture and coercive fields up to 2.2 T for a Sm-content of 20–22 at%. The intention of our work is to explore the possibility of the PLD process to influence the growth of highly coercive SmCo films by adjusting the plasma energy via the background pressure. We have prepared SmCo films by PLD in UHV and in an Ar gas pressure of 0.06 mbar on polycrystalline Al2O3 substrates with a Sm-content varying from 14–22 at%. We investigated the influence of composition and background pressure on structure, texture and magnetic properties.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 2 6 4 - 1
V. Neu et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1290–1293
The UHV deposition chamber (base pressurep2 109 mbar) is equipped with a multiple target holder and the SmCo films are prepared from alternating laser pulses on elemental Sm and Co targets. The composition can therefore be adjusted by the pulse sequence, which is typically repeated after 60 pulses. The excimer laser (Lambda Physik LPX305) was operated with KrF (248 nm), a pulse width of 25 ns, a repetition rate of 10 Hz and energy density of 7 J/cm2. The film architecture consists of a 10 nm thick Cr buffer, a 150–250 nm thick SmCo film and a 10 nm Cr protection layer. All layers were deposited at a temperature Tsub ¼ 4301C on polycrystalline Al2O3 substrates. Film thickness and film composition are measured with an electron beam micro analyser (ESMA) against pure metallic standards. The film surface is further examined with scanning electron microscopy (SEM) for droplet density and morphology. X-ray diffraction measurements (XRD) were performed with CoKa radiation in Bragg–Brentano geometry. Cross-sectional transmission electron microscopy (TEM) was conducted with a Philips CM20FEG microscope equipped with energydispersive X-ray analysis (EDX) on samples thinned with focussed ion beam (FIB, Ga+ ions). The magnetic properties were measured with a Quantum Design SQUID in a maximum field of m0 H ¼ 4:5 T. Although often a problem in laser ablation, especially when using composition targets, our SmCo films prepared from elemental targets show only a small droplet density. The main fraction has a diameter of 0.25 mm and an area density o0.1/mm2, the density of a larger fraction (Co droplets with 0.5–5 mm diameter) is even smaller (o0.02/mm2). Fig. 1 shows the cross-sectional TEM micrograph of a film with composition Sm17.5Co82.5 deposited in UHV. The Cr buffer thickness (8 nm) and the overall film thickness (180 nm) compare well with the ESMA result obtained for the same sample. The SmCo layer consists of grains with an average size of 34 nm. A thickness profile reveals a low surface roughness Rrms ¼ 5 nm. The EDX line scan (Fig. 1b) shows that the three layers are chemically clearly separated and that no interdiffusion occurs. The fluctuations in the Sm and Co signals are due to statistical errors (see exemplary error bars) and do not represent inhomogeneities in the composition. The crystal structure of the SmCo grains is identified by small area diffraction as the hexagonal SmCo5 phase (Fig. 1c), in agreement with the sample composition. The X-ray scan of the above sample is seen in Fig. 2, upper graph, together with the calculated pattern of isotropic Al2O3, BCC-Cr and SmCo5 for comparison. Due to the small film thickness, the pattern is dominated by substrate peaks, nevertheless, the SmCo5 phase can be identified mainly from the (1 1 0) and (2 0 0) reflections. The magnetic hysteresis of this sample is seen in
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Fig. 3a. The slight shoulder at zero field indicates a small amount of second phase with low coercivity. It could either be that larger Co droplets contribute to the magnetic behavior or that part of the Sm is oxidized, though none of the phases (SmO or Co) can be identified in the X-ray pattern. The hard magnetic SmCo5 phase leads to a high coercivity of m0 Hc ¼ 1:1 T. The out-ofplane hysteresis results in slightly lower magnetization values than the in-plane measurement. The SmCo samples deposited in UHV possess a weak but measurable magnetic in-plane texture, in agreement with the Xray measurement. This behavior changes when SmCo is deposited at a moderate background pressure of 0.06 mbar. The X-ray pattern of a film with composition Sm18Co82 is depicted in the lower part of Fig. 2. The structure again matches the SmCo5 phase but compared to the sample prepared in UHV, the (h k 0) reflexions are reduced in intensity. Indeed, the magnetic behavior (Fig. 4) shows a perpendicular anisotropy for this sample deposited on
Fig. 1. (a) Cross-sectional TEM bright field image of a Cr\Sm17.5Co82.5\Cr film deposited in UHV on a polycrystalline Al2O3 substrate. The surface is covered with a Pt-containing layer for protection against Ga+ ion bombardment during the TEM preparation. The amorphous region visible below this protection layer is most likely due to an insufficient protection and its occurrence is a subject of further investigations; (b) EDX line scan along the arrow as indicated in the bright field image; (c) electron diffraction pattern of a SmCo5 crystal with [2 3 4] zone axis.
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V. Neu et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1290–1293
Fig. 4. Hysteresis loops measured in the film plane and perpendicular to the film plane for the Cr\Sm18.0Co82.0\Cr sample deposited at a background pressure of 0.06 mbar.
Fig. 2. XRD pattern for a Cr\Sm17.5Co82.5\Cr sample deposited in UHV and for a Cr\Sm18.0Co82.0\Cr sample deposited at a background pressure of 0.06 mbar together with the calculated patterns of Al2O3, BCC-Cr and SmCo5.
Fig. 3. (a) Hysteresis loops measured in the film plane and perpendicular to the film plane for the sample shown in Fig. 1; (b) in-plane coercivity for a series of SmCo films with varying Sm content.
polycrystalline Al2O3. The coercive field of m0 Hc ¼ 1:2 T is comparable with the sample prepared in UHV. This offers new possibilities for optimization of the magnetic properties of SmCo films with perpendicular anisotropy. The occurrence of this texture, however, is no longer solely explicable with the stray-field energy consideration outlined above, as an initially observed in-plane texture should then become more pronounced with a reduced particle impact at higher background pressure. Further investigations have to include the special
deposition kinetic of the PLD process to clarify the out-of-plane texture. The influence of composition on the in-plane coercivity of SmCo samples prepared in UHV is seen in Fig. 3b. Coercive fields as high as 2.0 T are obtained. The monotonic increase up to 19 at% Sm is well known in sputtered samples and is explained by the superior anisotropy of the SmCo5 phase over that of the Coricher SmCo7 phase [2]. In our PLD samples, this trend continues for higher Sm contents in agreement with m0 Hc ¼ 2:2 T observed by Cadieu et al. [4] for Sm-rich samples (Sm=20–22 at%). Liu et al. [5] have reported a metastable SmCo phase with DO19 structure for sputtered and post-annealed Sm22Co78. This phase also leads to very high coercive fields and may explain our observed composition dependence of Hc : The remanence of the sample with the highest Hc is, however, reduced to 0.3 T. In conclusion, thin (150–250 nm) SmCo films prepared by PLD from elemental targets show a smooth surface with low droplet density even in conventional on-axis deposition geometry. The hexagonal SmCo5 phase is formed by direct crystallization on heated alumina substrates and coercive fields of 1.1 T are obtained for stoichiometric composition. Coercivity can be further improved with higher Sm content. The texture changes from a weak in-plane texture in UHV to out-of-plane at a deposition pressure of 0.06 mbar and offers the possibility for a magnetic property optimization of SmCo films with perpendicular anisotropy. Composition measurements by I. B.acher and TEM preparation by B. Arnold and D. Lohse are gratefully acknowledged. The authors acknowledge financial support by the DFG through SFB 463.
V. Neu et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1290–1293
References [1] F.J. Cadieu, in: M.H. Francombe, J.L. Vossen (Eds.), Physics of Thin Films, Academic Press, Boston, 1992, p. 145. [2] V. Neu, S.A. Shaheen, J. Appl. Phys. 86 (1999) 7006.
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[3] S. F.ahler, K. Sturm, H. Krebs, Appl. Phys. Lett. 75 (1999) 3766. [4] F.J. Cadieu, R. Rani, T. Theodoropoulos, L. Chen, J. Appl. Phys. 85 (1999) 5895. [5] Y. Liu, R.A. Thomas, S.S. Malhotra, Z.S. Shan, S.H. Liou, D.J. Sellmyer, J. Appl. Phys. 83 (1998) 6244.
Hard magnetic SmCo thin films prepared by pulsed laser deposition V. Neu*, J. Thomas, S. F.ahler, B. Holzapfel, L. Schultz IFW Dresden, Helmholtzstr. 20, D-01069 Dresden, Germany
Abstract Hard magnetic SmCo films were prepared by pulsed laser deposition in UHV and at moderate background pressures on polycrystalline Al2O3. Structure and magnetic properties are discussed for varying Sm-content. The perpendicular anisotropy observed at a deposition pressure of 0.06 mbar is compared with texturing models known for sputtered samples. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Ferromagnetic films; Rare earth–transition metal compounds; CoercivityFcomposition dependence; Texture; Pulsed laser deposition
Permanent magnet films based on highly anisotropic rare earth–transition metal phases like SmCo5 are promising materials for structured microelectromagnetic devices, when strong magnetic fields and high coercivities are needed. They may also find application in magnetic recording, as future miniaturization requires higher anisotropies to avoid a superparamagnetic switching of a magnetic bit. Besides coercivity, a pronounced texture is desirable for permanent magnet application. Via magnetron sputtering, highly coercive SmCo films can be prepared with a distinct in-plane texture and therefore almost square-shaped in-plane hysteresis loops [1]. In these experiments, the highly anisotropic SmCo5 phase was formed upon direct crystallization on heated substrates. Typical values for the coercive field m0 Hc are 0.7–1.5 T, varying monotonically with a Sm-content of 15–19 at% [2]. The in-plane texture is explained in Ref. [1] as a result of energetic considerations. At deposition temperatures below the Curie temperature of SmCo (Tc ¼ 70029001C, depending on the Co-content), a minimization of the stray field energy favors the orientation of the c-axisFthe easy magnetization axisFparallel to the film plane. The high kinetic energy *Corresponding author. Tel.: +49-351-4659-237; fax: +49351-4659-541. E-mail address: [email protected] (V. Neu).
of the plasma particles may disrupt this growth mode, so that a background pressure of around 0.1 mbar is used in magnetron sputtering to thermalize the plasma. Pulsed laser deposition (PLD) is a strong nonequilibrium process, with plasma energies ranging from 10 eV when operated in a background pressure of 0.1 mbar to 100 eV in UHV [3]. But unlike magnetron sputtering, the ablation process in PLD is hardly changed by the deposition pressure, so that the influence of an ambient gas on the growth mode and texture can be tested over a wide pressure range. In a recent paper, Cadieu et al. [4] used PLD to prepare SmCo-based films. The deposition conditions were chosen close to those used in the sputtering process. A background Ar pressure of 0.13 mbar and an additional shadow mask led to films with a distinct in-plane texture and coercive fields up to 2.2 T for a Sm-content of 20–22 at%. The intention of our work is to explore the possibility of the PLD process to influence the growth of highly coercive SmCo films by adjusting the plasma energy via the background pressure. We have prepared SmCo films by PLD in UHV and in an Ar gas pressure of 0.06 mbar on polycrystalline Al2O3 substrates with a Sm-content varying from 14–22 at%. We investigated the influence of composition and background pressure on structure, texture and magnetic properties.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 2 6 4 - 1
V. Neu et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1290–1293
The UHV deposition chamber (base pressurep2 109 mbar) is equipped with a multiple target holder and the SmCo films are prepared from alternating laser pulses on elemental Sm and Co targets. The composition can therefore be adjusted by the pulse sequence, which is typically repeated after 60 pulses. The excimer laser (Lambda Physik LPX305) was operated with KrF (248 nm), a pulse width of 25 ns, a repetition rate of 10 Hz and energy density of 7 J/cm2. The film architecture consists of a 10 nm thick Cr buffer, a 150–250 nm thick SmCo film and a 10 nm Cr protection layer. All layers were deposited at a temperature Tsub ¼ 4301C on polycrystalline Al2O3 substrates. Film thickness and film composition are measured with an electron beam micro analyser (ESMA) against pure metallic standards. The film surface is further examined with scanning electron microscopy (SEM) for droplet density and morphology. X-ray diffraction measurements (XRD) were performed with CoKa radiation in Bragg–Brentano geometry. Cross-sectional transmission electron microscopy (TEM) was conducted with a Philips CM20FEG microscope equipped with energydispersive X-ray analysis (EDX) on samples thinned with focussed ion beam (FIB, Ga+ ions). The magnetic properties were measured with a Quantum Design SQUID in a maximum field of m0 H ¼ 4:5 T. Although often a problem in laser ablation, especially when using composition targets, our SmCo films prepared from elemental targets show only a small droplet density. The main fraction has a diameter of 0.25 mm and an area density o0.1/mm2, the density of a larger fraction (Co droplets with 0.5–5 mm diameter) is even smaller (o0.02/mm2). Fig. 1 shows the cross-sectional TEM micrograph of a film with composition Sm17.5Co82.5 deposited in UHV. The Cr buffer thickness (8 nm) and the overall film thickness (180 nm) compare well with the ESMA result obtained for the same sample. The SmCo layer consists of grains with an average size of 34 nm. A thickness profile reveals a low surface roughness Rrms ¼ 5 nm. The EDX line scan (Fig. 1b) shows that the three layers are chemically clearly separated and that no interdiffusion occurs. The fluctuations in the Sm and Co signals are due to statistical errors (see exemplary error bars) and do not represent inhomogeneities in the composition. The crystal structure of the SmCo grains is identified by small area diffraction as the hexagonal SmCo5 phase (Fig. 1c), in agreement with the sample composition. The X-ray scan of the above sample is seen in Fig. 2, upper graph, together with the calculated pattern of isotropic Al2O3, BCC-Cr and SmCo5 for comparison. Due to the small film thickness, the pattern is dominated by substrate peaks, nevertheless, the SmCo5 phase can be identified mainly from the (1 1 0) and (2 0 0) reflections. The magnetic hysteresis of this sample is seen in
1291
Fig. 3a. The slight shoulder at zero field indicates a small amount of second phase with low coercivity. It could either be that larger Co droplets contribute to the magnetic behavior or that part of the Sm is oxidized, though none of the phases (SmO or Co) can be identified in the X-ray pattern. The hard magnetic SmCo5 phase leads to a high coercivity of m0 Hc ¼ 1:1 T. The out-ofplane hysteresis results in slightly lower magnetization values than the in-plane measurement. The SmCo samples deposited in UHV possess a weak but measurable magnetic in-plane texture, in agreement with the Xray measurement. This behavior changes when SmCo is deposited at a moderate background pressure of 0.06 mbar. The X-ray pattern of a film with composition Sm18Co82 is depicted in the lower part of Fig. 2. The structure again matches the SmCo5 phase but compared to the sample prepared in UHV, the (h k 0) reflexions are reduced in intensity. Indeed, the magnetic behavior (Fig. 4) shows a perpendicular anisotropy for this sample deposited on
Fig. 1. (a) Cross-sectional TEM bright field image of a Cr\Sm17.5Co82.5\Cr film deposited in UHV on a polycrystalline Al2O3 substrate. The surface is covered with a Pt-containing layer for protection against Ga+ ion bombardment during the TEM preparation. The amorphous region visible below this protection layer is most likely due to an insufficient protection and its occurrence is a subject of further investigations; (b) EDX line scan along the arrow as indicated in the bright field image; (c) electron diffraction pattern of a SmCo5 crystal with [2 3 4] zone axis.
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V. Neu et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1290–1293
Fig. 4. Hysteresis loops measured in the film plane and perpendicular to the film plane for the Cr\Sm18.0Co82.0\Cr sample deposited at a background pressure of 0.06 mbar.
Fig. 2. XRD pattern for a Cr\Sm17.5Co82.5\Cr sample deposited in UHV and for a Cr\Sm18.0Co82.0\Cr sample deposited at a background pressure of 0.06 mbar together with the calculated patterns of Al2O3, BCC-Cr and SmCo5.
Fig. 3. (a) Hysteresis loops measured in the film plane and perpendicular to the film plane for the sample shown in Fig. 1; (b) in-plane coercivity for a series of SmCo films with varying Sm content.
polycrystalline Al2O3. The coercive field of m0 Hc ¼ 1:2 T is comparable with the sample prepared in UHV. This offers new possibilities for optimization of the magnetic properties of SmCo films with perpendicular anisotropy. The occurrence of this texture, however, is no longer solely explicable with the stray-field energy consideration outlined above, as an initially observed in-plane texture should then become more pronounced with a reduced particle impact at higher background pressure. Further investigations have to include the special
deposition kinetic of the PLD process to clarify the out-of-plane texture. The influence of composition on the in-plane coercivity of SmCo samples prepared in UHV is seen in Fig. 3b. Coercive fields as high as 2.0 T are obtained. The monotonic increase up to 19 at% Sm is well known in sputtered samples and is explained by the superior anisotropy of the SmCo5 phase over that of the Coricher SmCo7 phase [2]. In our PLD samples, this trend continues for higher Sm contents in agreement with m0 Hc ¼ 2:2 T observed by Cadieu et al. [4] for Sm-rich samples (Sm=20–22 at%). Liu et al. [5] have reported a metastable SmCo phase with DO19 structure for sputtered and post-annealed Sm22Co78. This phase also leads to very high coercive fields and may explain our observed composition dependence of Hc : The remanence of the sample with the highest Hc is, however, reduced to 0.3 T. In conclusion, thin (150–250 nm) SmCo films prepared by PLD from elemental targets show a smooth surface with low droplet density even in conventional on-axis deposition geometry. The hexagonal SmCo5 phase is formed by direct crystallization on heated alumina substrates and coercive fields of 1.1 T are obtained for stoichiometric composition. Coercivity can be further improved with higher Sm content. The texture changes from a weak in-plane texture in UHV to out-of-plane at a deposition pressure of 0.06 mbar and offers the possibility for a magnetic property optimization of SmCo films with perpendicular anisotropy. Composition measurements by I. B.acher and TEM preparation by B. Arnold and D. Lohse are gratefully acknowledged. The authors acknowledge financial support by the DFG through SFB 463.
V. Neu et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1290–1293
References [1] F.J. Cadieu, in: M.H. Francombe, J.L. Vossen (Eds.), Physics of Thin Films, Academic Press, Boston, 1992, p. 145. [2] V. Neu, S.A. Shaheen, J. Appl. Phys. 86 (1999) 7006.
1293
[3] S. F.ahler, K. Sturm, H. Krebs, Appl. Phys. Lett. 75 (1999) 3766. [4] F.J. Cadieu, R. Rani, T. Theodoropoulos, L. Chen, J. Appl. Phys. 85 (1999) 5895. [5] Y. Liu, R.A. Thomas, S.S. Malhotra, Z.S. Shan, S.H. Liou, D.J. Sellmyer, J. Appl. Phys. 83 (1998) 6244.