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Influence of oxygen and copper in electrodeposited FePt films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 290–291 (2005) 1270–1273 www.elsevier.com/locate/jmmm
Influence of oxygen and copper in electrodeposited FePt films K. Leistner, J. Thomas, S. Baunack, H. Schlo¨rb, L. Schultz, S. Fa¨hler IFW Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany Available online 14 December 2004
Abstract Electrodeposition can be used to prepare hard magnetic films with a coercivity up to 1.1 T. Nevertheless, as-deposited and vacuum-annealed films exhibit a magnetization much lower than expected. This is caused by 30–35 at% oxygen present in the films after electrodeposition. Annealing in hydrogen leads to a decrease of the oxygen content to about 10 at%, explaining the significantly higher magnetization obtained in hydrogen-annealed films. The remaining iron oxide grains are located particularly at grain boundary triple points. During annealing, Cu diffuses from the buffer layer into the film and is detected along the grain boundaries. Hence, this microstructure can be favourable for a high coercivity. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Vv; 75.50.Ww; 75.70. i; 81.15.Pq Keywords: Hard magnetic materials; Thin films—magnetic; Electrodeposition; FePt; L10
1. Introduction FePt is an attractive material for magnetic data storage and micron size permanent magnets due to its high magneto-crystalline anisotropy and high magnetization. Up to now FePt films have been grown mainly by physical vapour deposition techniques. Electrodeposition is a low-cost alternative because of the simple setup requiring no vacuum equipment. In the meantime, FePt films with a defined composition can be electrodeposited [1,2]. Annealing in vacuum leads to coercivites of 0.3 [3] to 0.4 T [4]. In a previous contribution [5] it has been shown that highly coercive FePt films can be obtained by post-annealing in hydrogen rather than in vacuum. Phase formation and magnetic properties of these samples have been described in detail there and
Corresponding author. Tel.: +49 351 4659 665; fax: +49 351 4659 471. E-mail address: [email protected] (K. Leistner).
only a short summary is given here. In hydrogen atmosphere, hard magnetic properties are first observed after annealing at 300 C. Coercivity reaches a maximum of 1.1 T after annealing at 600 C. In contrast, when annealing in vacuum, temperatures higher than 700 C are needed to achieve hard magnetic properties. For example, a coercivity of only 0.4 T is achieved after annealing at 900 C. Polarization (at 2 T) increases strongly from 0.2 T in the as-deposited state to 0.8 T after annealing at 200 C in hydrogen. Higher annealing temperatures lead to a slight decrease in polarization. Vacuum-annealed films only reach a maximum polarization of 0.4 T (at 2 T). The change in polarization in dependence on the annealing conditions was considered to be related to the oxygen content. In this contribution, therefore, the influence of the annealing conditions on the oxygen content is studied. Additionally, the role of Cu used as a conducting buffer will be examined. Films of near stoichiometric composition are used, as these show the best magnetic properties [6].
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.420
ARTICLE IN PRESS K. Leistner et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1270–1273
1271
2. Experiment Films were deposited from a single aqueous bath containing 1 mmol/l H2 PtCl6 ; 0.1 mol/l FeSO4 and 0.5 mol/l of Na2 SO4 : The pH value was adjusted to 3 by adding H2 SO4 : As working electrode, an oxidized Si(0 0 1) wafer coated with a W–Ti barrier and a 100 nm Cu layer was used. The counter electrode was a Pt foil and the reference electrode a saturated calomel electrode (SCE). The solution was mixed thoroughly in front of the working electrode by N2 gas. Films with a Fe/Pt ratio of 50/50 ( 2 at%) and a thickness of 0.7 mm were obtained after depositing for 30 min at 780 mV (vs. SCE). The films were then annealed in 1 bar H2 or in vacuum (10 6 mbar) for 10 min. The integral film composition and the oxygen content were determined by an EDX detector (EDAX system for SEM Philips XL 20) equipped with a super ultra thin window (SUTW) exhibiting 46 % X-ray transmission for Ka line of oxygen. Auger electron spectroscopy (AES) in combination with sputtering was used to examine the in-depth composition. The samples were sputtered by 1.5 keV Ar ions under 60 to the sample surface. The sputter rate was determined to be about 6 nm/min in SiO2 : The microstructure was analysed by transmission electron microscopy on cross-sections using conventional imaging as well as the scanning transmission mode (STEM) with a high-angle annular dark field (HAADF) detector (instrument: Tecnai F30). It was combined with EDX spectroscopy to determine the chemical composition with high spatial resolution.
Fig. 1. Integral oxygen content of FePt films annealed in hydrogen and in vacuum for 10 min as measured with EDX.
(a)
(b)
3. Results The integral oxygen content of films annealed at 200–900 C in hydrogen or vacuum measured by EDX is presented in Fig. 1. In as-deposited films more than 30 at% oxygen are present. A remarkable decrease of the oxygen content occurs when annealing takes place in hydrogen. Even at low annealing temperatures of 200 C, the oxygen content decreases rapidly to about 10 at%. However, no significant further reduction is observed for higher annealing temperatures. When annealing in vacuum, the oxygen content is not reduced markedly. After annealing at 900 C in vacuum even a strong increase is observed, in agreement with the formation of oxides detected by X-ray measurements [1]. In order to examine the composition across the film thickness, AES depth profiling has been performed. Regarding the as-deposited sample, it can be seen in Fig. 2a that the composition of Fe and Pt in the film interior does not vary depending on the thickness. Thus, deposition rates do not change during deposition. The reason for the deviation of the surface composition from
Fig. 2. Compositional analysis by AES depth profiling for (a) an as-deposited sample and (b) a sample annealed in hydrogen at 600 C for 10 min.
the composition inside the film is not clarified yet. The oxygen content of about 35 at% measured by AES is in agreement with the EDX measurements. The oxygen is distributed over the whole film thickness, as expected when it is incorporated into the film during deposition. Below the FePt film a well defined Cu/W–Ti buffer architecture can be observed. In Fig. 2b the depth profile of the sample annealed in hydrogen at the optimum temperature of 600 C is presented. In the film interior,
ARTICLE IN PRESS 1272
K. Leistner et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1270–1273
Fig. 3. (a) STEM-HAADF image of a sample annealed in hydrogen at 800 C combined with EDX line scans identifying (b) Fe–O and (c) Cu.
about 10 at% oxygen is distributed quite uniformly. In the depth profile it is additionally seen that Cu has diffused completely into the film after annealing. About 10 at% Cu are then present in the FePt film. The W-Ti barrier is not affected by the heat treatment. Different sputtering rates of the as-deposited and annealed sample may be due to the incorporation of Cu, the reduced number of defects and the transformation into the ordered phase, which may affect the sputter yield. In order to understand the local element distribution, STEM imaging combined with EDX line scans was used. Fig. 3a shows the cross-section of a film annealed at 800 C, as, after annealing at this temperature, the larger grains allow a more precise phase identification. The EDX line scan in Fig. 3b shows that the dark appearing grains, found particularly at grain boundary triple points, can be identified as iron oxides. Cu accumulates along the grain boundaries, as shown by the line scan in Fig. 3c.
4. Discussion During electrodeposition, oxygen is incorporated. Following a concept derived for electrodeposition of
Fe and Fe–Ni alloys [7], this might be related to hydrogen codeposition, which also accompanies Fe–Pt electrodeposition. A change of the pH value in front of the cathode caused by hydrogen codeposition can lead to the formation of Fe–O–H species, which can then be incorporated into the film. In the present FePt films, Fe–O–H species are not clearly identified yet. It is assumed, however, that the oxygen is bound to iron, as Pt as a noble metal does not form stable oxides. The thermodynamically most stable iron oxide is the diamagnetic hematite (Fe2 O3 ), followed by the ferrimagnetic magnetite (Fe3 O4 ). In both cases the resulting magnetic moment is significantly smaller than that of pure Fe or Fe in FePt. A depletion in metallic Fe due to iron oxide formation explains the low magnetization observed in as-deposited and vacuum-annealed samples. Oxidation of Fe and the consequent loss of magnetization has been reported for FePt nanoparticles as well [8]. In a hydrogen atmosphere, however, the oxygen can be removed due to hydrogen acting as a reducing agent, resulting in a higher magnetization. As seen by EDX, AES and STEM, some oxides are still present, explaining that the magnetization is still smaller than that of bulk FePt. As most of these oxides are found in triple points, pinning of grain boundaries may have helped to hinder grain growth. As non-magnetic Cu is enriched
ARTICLE IN PRESS K. Leistner et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1270–1273
within the grain boundaries, this may cause an attractive pinning of domain walls or even a magnetic decoupling of grains. Both effects could lead to an enhanced coercivity. In addition, Cu might have a positive effect on ordering [9]. On the other side, Cu may also decrease the magnetization. The combined effects of hydrogen, like enhanced ordering and the reduced oxygen content, lead to the formation of a higher amount of better ordered FePt [5]. Combined with the hindered grain growth [5] and the accumulation of the impurity phases in grain boundaries, a high coercivity is obtained.
5. Conclusion The oxygen content in electrodedeposited FePt films can be reduced by annealing in hydrogen, resulting in an increase of magnetization. Remaining oxygen and copper impurities still reduce the magnetization compared to bulk samples, but result in a microstructure favorable for a high coercivity.
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References [1] K. Leistner, E. Backen, B. Schu¨pp, M. Weisheit, L. Schultz, H. Schlo¨rb, S. Fa¨hler, J. Appl. Phys. 95 (2004) 7267. [2] F. Wang, S. Doi, K. Hosoiri, T. Watanabe, Mater. Sci. Eng. A 375–377 (2004) 1289. [3] F.M.F. Rhen, G. Hinds, C. O’Reilly, J.M.D. Coey, IEEE Trans. Magn. 39 (2003) 2699. [4] Y.H. Huang, H. Okumura, G.C. Hadjipanayis, D. Weller, J. Appl. Phys. 91 (2002) 6869. [5] K. Leistner, J. Thomas, H. Schlo¨rb, M. Weisheit, L. Schultz, S. Fa¨hler, Appl. Phys. Lett. 85 (2004) 3498. [6] K. Leistner, H. Schlo¨rb, M. Weisheit, L. Schultz, S. Fa¨hler, Proceedings of the 18th International Workshop on High Performance Magnets and their Applications (2004) 866. [7] S. Gadad, T.M. Harris, J. Electrochem. Soc. 145 (1998) 3699. [8] C. Liu, T.J. Klemmer, N. Shukla, X. Wu, D. Weller, M. Tanase, D. Laughlin, J. Magn. Magn. Mater. 266 (2003) 96. [9] T. Maeda, T. Kai, A. Kikitsu, T. Nagase, J. Akiyama, Appl. Phys. Lett. 80 (2002) 2147.
Journal of Magnetism and Magnetic Materials 290–291 (2005) 1270–1273 www.elsevier.com/locate/jmmm
Influence of oxygen and copper in electrodeposited FePt films K. Leistner, J. Thomas, S. Baunack, H. Schlo¨rb, L. Schultz, S. Fa¨hler IFW Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany Available online 14 December 2004
Abstract Electrodeposition can be used to prepare hard magnetic films with a coercivity up to 1.1 T. Nevertheless, as-deposited and vacuum-annealed films exhibit a magnetization much lower than expected. This is caused by 30–35 at% oxygen present in the films after electrodeposition. Annealing in hydrogen leads to a decrease of the oxygen content to about 10 at%, explaining the significantly higher magnetization obtained in hydrogen-annealed films. The remaining iron oxide grains are located particularly at grain boundary triple points. During annealing, Cu diffuses from the buffer layer into the film and is detected along the grain boundaries. Hence, this microstructure can be favourable for a high coercivity. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Vv; 75.50.Ww; 75.70. i; 81.15.Pq Keywords: Hard magnetic materials; Thin films—magnetic; Electrodeposition; FePt; L10
1. Introduction FePt is an attractive material for magnetic data storage and micron size permanent magnets due to its high magneto-crystalline anisotropy and high magnetization. Up to now FePt films have been grown mainly by physical vapour deposition techniques. Electrodeposition is a low-cost alternative because of the simple setup requiring no vacuum equipment. In the meantime, FePt films with a defined composition can be electrodeposited [1,2]. Annealing in vacuum leads to coercivites of 0.3 [3] to 0.4 T [4]. In a previous contribution [5] it has been shown that highly coercive FePt films can be obtained by post-annealing in hydrogen rather than in vacuum. Phase formation and magnetic properties of these samples have been described in detail there and
Corresponding author. Tel.: +49 351 4659 665; fax: +49 351 4659 471. E-mail address: [email protected] (K. Leistner).
only a short summary is given here. In hydrogen atmosphere, hard magnetic properties are first observed after annealing at 300 C. Coercivity reaches a maximum of 1.1 T after annealing at 600 C. In contrast, when annealing in vacuum, temperatures higher than 700 C are needed to achieve hard magnetic properties. For example, a coercivity of only 0.4 T is achieved after annealing at 900 C. Polarization (at 2 T) increases strongly from 0.2 T in the as-deposited state to 0.8 T after annealing at 200 C in hydrogen. Higher annealing temperatures lead to a slight decrease in polarization. Vacuum-annealed films only reach a maximum polarization of 0.4 T (at 2 T). The change in polarization in dependence on the annealing conditions was considered to be related to the oxygen content. In this contribution, therefore, the influence of the annealing conditions on the oxygen content is studied. Additionally, the role of Cu used as a conducting buffer will be examined. Films of near stoichiometric composition are used, as these show the best magnetic properties [6].
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.420
ARTICLE IN PRESS K. Leistner et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1270–1273
1271
2. Experiment Films were deposited from a single aqueous bath containing 1 mmol/l H2 PtCl6 ; 0.1 mol/l FeSO4 and 0.5 mol/l of Na2 SO4 : The pH value was adjusted to 3 by adding H2 SO4 : As working electrode, an oxidized Si(0 0 1) wafer coated with a W–Ti barrier and a 100 nm Cu layer was used. The counter electrode was a Pt foil and the reference electrode a saturated calomel electrode (SCE). The solution was mixed thoroughly in front of the working electrode by N2 gas. Films with a Fe/Pt ratio of 50/50 ( 2 at%) and a thickness of 0.7 mm were obtained after depositing for 30 min at 780 mV (vs. SCE). The films were then annealed in 1 bar H2 or in vacuum (10 6 mbar) for 10 min. The integral film composition and the oxygen content were determined by an EDX detector (EDAX system for SEM Philips XL 20) equipped with a super ultra thin window (SUTW) exhibiting 46 % X-ray transmission for Ka line of oxygen. Auger electron spectroscopy (AES) in combination with sputtering was used to examine the in-depth composition. The samples were sputtered by 1.5 keV Ar ions under 60 to the sample surface. The sputter rate was determined to be about 6 nm/min in SiO2 : The microstructure was analysed by transmission electron microscopy on cross-sections using conventional imaging as well as the scanning transmission mode (STEM) with a high-angle annular dark field (HAADF) detector (instrument: Tecnai F30). It was combined with EDX spectroscopy to determine the chemical composition with high spatial resolution.
Fig. 1. Integral oxygen content of FePt films annealed in hydrogen and in vacuum for 10 min as measured with EDX.
(a)
(b)
3. Results The integral oxygen content of films annealed at 200–900 C in hydrogen or vacuum measured by EDX is presented in Fig. 1. In as-deposited films more than 30 at% oxygen are present. A remarkable decrease of the oxygen content occurs when annealing takes place in hydrogen. Even at low annealing temperatures of 200 C, the oxygen content decreases rapidly to about 10 at%. However, no significant further reduction is observed for higher annealing temperatures. When annealing in vacuum, the oxygen content is not reduced markedly. After annealing at 900 C in vacuum even a strong increase is observed, in agreement with the formation of oxides detected by X-ray measurements [1]. In order to examine the composition across the film thickness, AES depth profiling has been performed. Regarding the as-deposited sample, it can be seen in Fig. 2a that the composition of Fe and Pt in the film interior does not vary depending on the thickness. Thus, deposition rates do not change during deposition. The reason for the deviation of the surface composition from
Fig. 2. Compositional analysis by AES depth profiling for (a) an as-deposited sample and (b) a sample annealed in hydrogen at 600 C for 10 min.
the composition inside the film is not clarified yet. The oxygen content of about 35 at% measured by AES is in agreement with the EDX measurements. The oxygen is distributed over the whole film thickness, as expected when it is incorporated into the film during deposition. Below the FePt film a well defined Cu/W–Ti buffer architecture can be observed. In Fig. 2b the depth profile of the sample annealed in hydrogen at the optimum temperature of 600 C is presented. In the film interior,
ARTICLE IN PRESS 1272
K. Leistner et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1270–1273
Fig. 3. (a) STEM-HAADF image of a sample annealed in hydrogen at 800 C combined with EDX line scans identifying (b) Fe–O and (c) Cu.
about 10 at% oxygen is distributed quite uniformly. In the depth profile it is additionally seen that Cu has diffused completely into the film after annealing. About 10 at% Cu are then present in the FePt film. The W-Ti barrier is not affected by the heat treatment. Different sputtering rates of the as-deposited and annealed sample may be due to the incorporation of Cu, the reduced number of defects and the transformation into the ordered phase, which may affect the sputter yield. In order to understand the local element distribution, STEM imaging combined with EDX line scans was used. Fig. 3a shows the cross-section of a film annealed at 800 C, as, after annealing at this temperature, the larger grains allow a more precise phase identification. The EDX line scan in Fig. 3b shows that the dark appearing grains, found particularly at grain boundary triple points, can be identified as iron oxides. Cu accumulates along the grain boundaries, as shown by the line scan in Fig. 3c.
4. Discussion During electrodeposition, oxygen is incorporated. Following a concept derived for electrodeposition of
Fe and Fe–Ni alloys [7], this might be related to hydrogen codeposition, which also accompanies Fe–Pt electrodeposition. A change of the pH value in front of the cathode caused by hydrogen codeposition can lead to the formation of Fe–O–H species, which can then be incorporated into the film. In the present FePt films, Fe–O–H species are not clearly identified yet. It is assumed, however, that the oxygen is bound to iron, as Pt as a noble metal does not form stable oxides. The thermodynamically most stable iron oxide is the diamagnetic hematite (Fe2 O3 ), followed by the ferrimagnetic magnetite (Fe3 O4 ). In both cases the resulting magnetic moment is significantly smaller than that of pure Fe or Fe in FePt. A depletion in metallic Fe due to iron oxide formation explains the low magnetization observed in as-deposited and vacuum-annealed samples. Oxidation of Fe and the consequent loss of magnetization has been reported for FePt nanoparticles as well [8]. In a hydrogen atmosphere, however, the oxygen can be removed due to hydrogen acting as a reducing agent, resulting in a higher magnetization. As seen by EDX, AES and STEM, some oxides are still present, explaining that the magnetization is still smaller than that of bulk FePt. As most of these oxides are found in triple points, pinning of grain boundaries may have helped to hinder grain growth. As non-magnetic Cu is enriched
ARTICLE IN PRESS K. Leistner et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1270–1273
within the grain boundaries, this may cause an attractive pinning of domain walls or even a magnetic decoupling of grains. Both effects could lead to an enhanced coercivity. In addition, Cu might have a positive effect on ordering [9]. On the other side, Cu may also decrease the magnetization. The combined effects of hydrogen, like enhanced ordering and the reduced oxygen content, lead to the formation of a higher amount of better ordered FePt [5]. Combined with the hindered grain growth [5] and the accumulation of the impurity phases in grain boundaries, a high coercivity is obtained.
5. Conclusion The oxygen content in electrodedeposited FePt films can be reduced by annealing in hydrogen, resulting in an increase of magnetization. Remaining oxygen and copper impurities still reduce the magnetization compared to bulk samples, but result in a microstructure favorable for a high coercivity.
1273
References [1] K. Leistner, E. Backen, B. Schu¨pp, M. Weisheit, L. Schultz, H. Schlo¨rb, S. Fa¨hler, J. Appl. Phys. 95 (2004) 7267. [2] F. Wang, S. Doi, K. Hosoiri, T. Watanabe, Mater. Sci. Eng. A 375–377 (2004) 1289. [3] F.M.F. Rhen, G. Hinds, C. O’Reilly, J.M.D. Coey, IEEE Trans. Magn. 39 (2003) 2699. [4] Y.H. Huang, H. Okumura, G.C. Hadjipanayis, D. Weller, J. Appl. Phys. 91 (2002) 6869. [5] K. Leistner, J. Thomas, H. Schlo¨rb, M. Weisheit, L. Schultz, S. Fa¨hler, Appl. Phys. Lett. 85 (2004) 3498. [6] K. Leistner, H. Schlo¨rb, M. Weisheit, L. Schultz, S. Fa¨hler, Proceedings of the 18th International Workshop on High Performance Magnets and their Applications (2004) 866. [7] S. Gadad, T.M. Harris, J. Electrochem. Soc. 145 (1998) 3699. [8] C. Liu, T.J. Klemmer, N. Shukla, X. Wu, D. Weller, M. Tanase, D. Laughlin, J. Magn. Magn. Mater. 266 (2003) 96. [9] T. Maeda, T. Kai, A. Kikitsu, T. Nagase, J. Akiyama, Appl. Phys. Lett. 80 (2002) 2147.