- About this Journal ·
- Abstracting and Indexing ·
- Aims and Scope ·
- Annual Issues ·
- Article Processing Charges ·
- Author Guidelines ·
- Bibliographic Information ·
- Citations to this Journal ·
- Contact Information ·
- Editorial Board ·
- Editorial Workflow ·
- Free eTOC Alerts ·
- Publication Ethics ·
- Recently Accepted Articles ·
- Reviewers Acknowledgment ·
- Submit a Manuscript ·
- Subscription Information ·
- Table of Contents
Journal of Nanomaterials
Volume 2012 (2012), Article ID 814162, 7 pages
Temperature-Driven Spin Reorientation Transition in CoPt/AlN Multilayer Films
1Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
2Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, Oh-okayama, Meguro-ku, Tokyo 152-8552, Japan
3School of Materials Science and Engineering, Beihang University, Beijing 100191, China
Received 10 May 2012; Accepted 23 May 2012
Academic Editor: Weichang Hao
Copyright © 2012 Wupeng Cai et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Spin reorientation transition phenomena from out-of-plane to in-plane direction with increasing temperature are observed for the 500°C annealed CoPt/AlN multilayer films with different CoPt layer thicknesses. CoPt-AlN interface and volume anisotropy contributions, favoring out-of-plane and in-plane magnetization, respectively, are separately determined at various temperatures. Interface anisotropy exhibits much stronger temperature dependence than volume contribution, hence the temperature-driven spin reorientation transition occurs. Interface anisotropy in this work consists of Néel interface anisotropy and magnetoelastic effect. Magnetoelastic effect degrades rapidly and changes its sign from positive to negative above 200°C, because of the involvement of stress state in CoPt films with temperature. By contrast, Néel interface anisotropy decays slowly, estimated from a Néel mean field model. Thus, the strong temperature dependence of CoPt-AlN interface anisotropy is dominated by the change of magnetoelastic effect.
Spin reorientation transition (SRT) is one of the most interesting phenomena occurring in thin ferromagnetic films. During this process, the orientation of the magnetization changes spontaneously and often reversibly from one direction to another. This process can be induced by varying related parameters such as measuring temperature and film thickness. Tremendous efforts have been made to investigate SRT in various systems such as Fe/Ag(100) [1–4], Fe/Cu(100) [1, 3, 5–7], Gd/W(110) [8–10], and Co/Au(111) [11–14]. Generally, with increasing temperature and film thickness, SRT from perpendicular to the in-plane magnetic direction has been observed. However, Ni/Cu(001) thin films exhibit the reversed SRT, namely, from in-plane to perpendicular orientation [15–17]. That is due to the different signs of the surface anisotropy between Ni and Fe, Co, Gd, which favors in-plane magnetization for Ni and out-of-plane magnetization for Fe, Co, Gd.
SRT phenomena are generally caused by the competition between several anisotropy contributions favoring different directions of magnetization. For example, different temperature dependences of these anisotropy contributions may make a temperature-driven SRT occur. For a thin ferromagnetic film, the surface or interface anisotropy () becomes comparable with the other effects like shape and magnetocrystalline anisotropies. Hence, it is important to study the temperature dependence of to understand the SRT process. Farle et al. found an almost linear decrease of with increasing temperature for Gd/W(110), Ni/W(110), and Ni/Cu(001) thin films [10, 18–20]. Similar linear temperature dependence of has been observed for Ni/Re(0001)  and Fe/Cu(001) . On the other hand, Pechan found a much more abrupt decrease of for Ni/Mo multilayers, which is inexplicable in terms of a simple Néel mean field model . The author then supposed such strong temperature dependence of was due to the involvement of Ni-Mo interface strain, which generates a magnetoelastic term in . But quantitative study of the magnetoelastic effect has not been conducted.
For the metal/metal films, the interlayer mixing or interfacial reaction at elevated temperatures will destroy the interface and make an irreversible change to the magnetic properties. In our previous work, to avoid such structural changes, AlN instead of metal was used to form CoPt/AlN multilayer films [23–25]. Flat and continuous CoPt-AlN interface has been observed even after high-temperature postdeposition annealing at temperatures as high as 500°C [23, 24]. CoPt layers remain FCC structure when the annealing temperature is below 600°C . High crystallinity, strong (111) texture in CoPt layers and (002) texture in AlN layers were exhibited. Moreover, a large perpendicular magnetic anisotropy was obtained mainly due to the tensile stress in the CoPt layers . In the present work, we have investigated the temperature dependence of CoPt-AlN interface anisotropy up to 350°C and its relationship with the temperature-driven SRT in CoPt/AlN multilayer films. The temperature dependence of the stress-induced magnetoelastic term in interface anisotropy has also been quantitatively studied.
CoPt/AlN multilayer films with the configuration as [AlN (10 nm)/CoPt ( nm)]5/AlN (10 nm) () were prepared at ambient temperature by direct current (dc) magnetron sputtering on fused-quartz substrates. Two pairs of facing targets, one pair composed of Co and Pt targets and the other pair composed of two Al targets, were equipped at two sides in the sputtering chamber. Sample depositions were conducted in an Ar and N2 gas mixture. CoPt/AlN multilayer films were formed by switching the substrate holder alternatively to the two targets sides. The atomic ratio in the CoPt layer is Co44Pt56. As-deposited films were then annealed in a vacuum furnace (below 1 × 10−4 Pa) at 500°C for 3 hours.
Out-of-plane (along the film normal, ) and in-plane (in the film plane, ) magnetization-temperature (-) curves were measured by a vibrating sample magnetometer (VSM) with heating rate as 5°C/min. Before the measurement of - curves, the sample was magnetized in an external field of 1 T, and then the external field was removed. However, a small constant external field about 60 Oe, which is the residual magnetization of the cores inside the electromagnets, still exists. In-plane and out-of-plane hysteresis loops at various temperatures up to 350°C were measured with the same VSM.
3. Results and Discussion
Figure 1 shows the evolutions of in-plane and out-of-plane remnant magnetizations with temperature for CoPt/AlN multilayer films with different CoPt layer thicknesses. SRT from out-of-plane to in-plane direction, indicated by an abrupt decrease of out-of-plane magnetization () followed by rapid increase of in-plane magnetization (), can be observed for the films with CoPt layer thickness larger than 2 nm. Moreover, SRT occurs at lower temperatures when the CoPt layer thickness increases. For the film with CoPt layer thickness nm, starts decreasing rapidly at a relative high temperature of 303°C, while remains a low value (about one tenth of ) in the whole temperature range without obvious increase. When nm, exhibits an abrupt degradation from a much lower temperature of 110°C. On the other hand, starts a sharp increase around 250°C and reaches a considerable magnitude. For the thicker films with as 6 and 8 nm, it seems that the rapid depression of occurs from a temperature below room temperature. The starting temperature of the rapid increase of is about 100°C when nm, while it should be lower than the room temperature when nm. The final decay of in the in-plane - curve is due to the ferromagnetic-paramagnetic transition.
As mentioned in Section 1, the temperature-driven SRT is generally caused by the change of magnetic anisotropy with temperature. Hence, we measured the in-plane and out-of-plane hysteresis loops for each film at various temperatures up to 350°C in order to measure the effective anisotropy energy. Typical results for the CoPt/AlN multilayer film with nm are shown in Figures 2(a)–2(c), which show a smooth transition from perpendicular to in-plane magnetic anisotropy. At 250°C, the film almost exhibits a magnetic isotropic behavior (Figure 2(b)). Figure 2(d) shows the change of saturation magnetization (), obtained from the hysteresis loops, with temperature. The degradation of exhibits a growing rate with increasing temperature.
The total magnetic anisotropy energy per volume (erg/cm3) is calculated by measuring the area enclosed between in-plane and out-of-plane hysteresis loops. When the magnetic anisotropy is extremely high, the saturation of magnetization along the hard axis cannot be reached under the maximum applied field of 1 T. In such cases, the hard axis hysteresis loop is extrapolated to . Figure 3 summarizes the calculated of CoPt/AlN multilayer films with , 4, 6 and 8 nm at different temperatures. At room temperature, a high perpendicular magnetic anisotropy (PMA) energy of erg/cm3 is obtained when nm. With increasing , PMA weakens and turns to a weak in-plane magnetic anisotropy (IMA) when nm. With increasing temperature, PMA rapidly degrades. Due to the strong PMA of 2 nm thick CoPt thin film, out-of-plane easy axis is maintained up to 350°C. On the other hand, when and 6 nm, the transition from PMA to IMA occurred at 250°C and 100°C, respectively. When nm, only IMA is observed. Maximum with a value of erg/cm3 has been obtained at 300°C. The degradation of IMA at high temperatures may arise from the enhancing thermal fluctuations. Magnetic anisotropy could be expected to vanish at the Curie temperature.
Magnetic anisotropy decides the magnetization state at each temperature, illustrated in Figure 1(b). When is extremely high, for example, nm below 303°C and nm below 110°C, a single perpendicular domain can be maintained. Hence, only gradually decreases with increasing temperature. When decreases to a critical value, for example, nm at 303°C and nm at 110°C, the reversed magnetization starts to form. Consequently, the single domain state is destroyed and rapidly decreases. When thin films become magnetic isotropy ( = 0), for example, nm at 250°C and nm at 100°C, perpendicular domains become unstable and in-plane domains starts to form. Thus, starts a sharp increase. The experimental observation of the evolution of magnetic domain with temperature can be seen in . But a detailed experimental study on the correlation between the change of magnetic anisotropy and the change of remanent magnetization was rarely reported previously. Our work reveals that at the starting temperature of the rapid decrease of , thin films still possess considerable PMA; at the starting temperature of the rapid increase of , thin films exhibit magnetic isotropy.
For a ferromagnetic thin film, (erg/cm3) can be phenomenologically written as , where (erg/cm2) is the interface anisotropy energy density, is the film thickness, and (erg/cm3) is the volume contribution, which is the sum of the magnetocrystalline, magnetoelastic, and shape anisotropies. The factor of 2 in takes into account the two interfaces. corresponds to perpendicular anisotropy and in-plane anisotropy. To understand the observed SRT phenomena, temperature dependences of and need to be revealed. The individual values of and can be determined by the linear relationship between and as , where is the intercept at and is the slope. Good linear dependence of is experimentally obtained at each measuring temperature, shown in Figure 4. Hence, the interface and volume anisotropy contributions are separately determined. Note is positive and is negative, indicating the interface and volume anisotropies favor out-of-plane and in-plane magnetization, respectively. The temperature dependences of and are then shown in Figure 5. It can be seen clearly that decreases more rapidly than . Consequently, will become predominant and the magnetic easy axis will turn to the in-plane direction at high temperatures. Thus, an SRT from out-of-plane to in-plane orientation will be induced. When nm, the absolute value of the effective volume contribution remains lower than the interface contribution 2, whereas their difference is rapidly reduced. That explains why there is no obvious increase in with increasing temperature. When and 6 nm, CoPt thin films become magnetic isotropy at 230°C and 100°C, respectively (see the intersections in Figure 5). Hence, starts a sharp increase near these temperatures for the two films (Figures 1(b) and 1(c)). The volume effect overwhelms the interface effect at room temperature when is increased to 8 nm. As a result, only IMA is observed in the whole temperature range and SRT should happen below room temperature, as shown in Figure 1(d).
The abrupt decrease of with temperature is quite different from the linear dependence observed in some other systems, for example, Fe/Cu(100) and Gd/W(110) [7, 10], but similar to the results in Ni/Mo multilayers , denoting the importance of stress effect. always contains the Néel interface anisotropy , which originates from the broken symmetry at the interface . Our previous study found the existence of a tensile stress in CoPt layers after postannealing and demonstrated that such tensile stress promotes a large PMA through magnetoelastic effect . The magnetoelastic anisotropy energy can be expressed as , where is the magnetostriction constant and is the internal stress . When the parameters are independent of the film thickness, can be identified with a volume contribution . However, in some cases, is proportional to due to the accommodation of misfit dislocations . Hence, is proportional to and , defined as , becomes a part of the interface contribution . It has been revealed that for the 500°C annealed CoPt/AlN multilayer films, contributes to when nm . Based on the above discussion, in the present work should consist of the Néel interface and magnetoelastic anisotropy terms . At room temperature, is 1.0 erg/cm2, while and have been determined to be 0.47 and 0.53 erg/cm2 in our previous work, respectively . According to a Néel mean filed model, is proportional to , [22, 29]. The coefficient is chosen to match the model with the experiment at room temperature. Using the values at various temperatures (Figure 2(d)), the change of with temperature is estimated (Figure 6). Subsequently the temperature dependence of is obtained from (Figure 6). exhibits a much milder temperature dependence than , and thus the rapid decrease of with increasing temperature is dominated by the rapid degradation of .
The change of should be correlated to the change of stress , because arises from the magnetoelastic effect. At room temperature there exists a large tensile stress in the CoPt film and a compressive strain in the direction of film normal . When the temperature is increasing, the lattice expands along film normal freely . By contrast, the lattice expansion in the film plane would be restrained to a great extent by adjacent AlN layers through interfacial restriction because of the smaller thermal expansion coefficient and higher elastic modulus of AlN than CoPt . Thus with increasing temperature, the compressive strain along film normal will degrade and further changes to a tensile strain. Therefore, as shown in Figure 6, rapidly decreases with temperature and reaches nearly zero at 200°C. After that, becomes negative and favors in-plane magnetization.
Temperature dependence of CoPt-AlN interface anisotropy in 500°C annealed CoPt/AlN multilayer films has been investigated. The interface anisotropy in the present work consists of Néel interface anisotropy and the magnetoelastic contribution. Néel interface anisotropy shows a mild temperature dependence, estimated from a Néel mean field model. By contrast, with increasing temperature, the magnetoelastic contribution decreases abruptly and changes its sign above 200°C. Such temperature dependence is owing to the involvement of the stress state in CoPt films. Accordingly, the total interface anisotropy exhibits a rapid degradation with temperature. Due to the milder temperature dependence of volume anisotropy than that of interface anisotropy, interesting temperature driven spin reorientation transition phenomena from out-of-plane to in-plane direction are clearly observed for CoPt/AlN multilayer films with appropriate CoPt layer thickness.
- D. P. Pappas, C. R. Brundle, and H. Hopster, “Reduction of macroscopic moment in ultrathin Fe films as the magnetic orientation changes,” Physical Review B, vol. 45, no. 14, pp. 8169–8172, 1992.
- Z. Q. Qiu, J. Pearson, and S. D. Bader, “Asymmetry of the spin reorientation transition in ultrathin Fe films and wedges grown on Ag(100),” Physical Review Letters, vol. 70, no. 7, pp. 1006–1009, 1993.
- S. D. Bader, D. Li, and Z. Q. Qiu, “Magnetic and structural instabilities of ultrathin Fe(100) wedges (invited),” Journal of Applied Physics, vol. 76, no. 10, pp. 6419–6424, 1994.
- A. Berger and H. Hopster, “Nonequilibrium magnetization near the reorientation phase transition of Fe/Ag(100) films,” Physical Review Letters, vol. 76, no. 3, pp. 519–522, 1996.
- D. P. Pappas, K. P. Kämper, and H. Hopster, “Reversible transition between perpendicular and in-plane magnetization in ultrathin films,” Physical Review Letters, vol. 64, no. 26, pp. 3179–3182, 1990.
- R. Allenspach and A. Bischof, “Magnetization direction switching in Fe/Cu(100) epitaxial films: temperature and thickness dependence,” Physical Review Letters, vol. 69, no. 23, pp. 3385–3388, 1992.
- A. Enders, D. Peterka, D. Repetto, N. Lin, A. Dmitriev, and K. Kern, “Temperature dependence of the surface anisotropy of Fe ultrathin films on CU(001),” Physical Review Letters, vol. 90, no. 21, Article ID 217203, 4 pages, 2003.
- A. Berger, A. W. Pang, and H. Hopster, “Magnetic reorientation transition in epitaxial Gd-films,” Journal of Magnetism and Magnetic Materials, vol. 137, no. 1-2, pp. L1–L5, 1994.
- A. Berger, A. W. Pang, and H. Hopster, “Magnetic reorientation transition of Gd(0001)/W(110) films,” Physical Review B, vol. 52, no. 2, pp. 1078–1089, 1995.
- G. André, A. Aspelmeier, B. Schulz, M. Farle, and K. Baberschke, “Temperature dependence of surface and volume anisotropy in Gd W(110),” Surface Science, vol. 326, no. 3, pp. 275–284, 1995.
- R. Allenspach, M. Stampanoni, and A. Bischof, “Magnetic domains in thin epitaxial Co/Au(111) films,” Physical Review Letters, vol. 65, no. 26, pp. 3344–3347, 1990.
- S. Pütter, H. F. Ding, Y. T. Millev, H. P. Oepen, and J. Kirschner, “Magnetic susceptibility: an easy approach to the spin-reorientation transition,” Physical Review B, vol. 64, no. 9, Article ID 092409, 4 pages, 2001.
- H. F. Ding, S. Pütter, H. P. Oepen, and J. Kirschner, “Spin-reorientation transition in thin films studied by the component-resolved Kerr effect,” Physical Review B, vol. 63, no. 13, Article ID 134425, 7 pages, 2001.
- R. Sellmann, H. Fritzsche, H. Maletta, V. Leiner, and R. Siebrecht, “Spin-reorientation transition and magnetic phase diagrams of thin epitaxial Au(111)/Co films with W and Au overlayers,” Physical Review B, vol. 64, no. 5, Article ID 054418, 2001.
- W. L. O'Brien and B. P. Tonner, “Transition to the perpendicular easy axis of magnetization in Ni ultrathin films found by x-ray magnetic circular dichroism,” Physical Review B, vol. 49, no. 21, pp. 15370–15373, 1994.
- W. L. O'Brien, T. Droubay, and B. P. Tonner, “Transitions in the direction of magnetism in Ni/Cu(001) ultrathin films and the effects of capping layers,” Physical Review B, vol. 54, no. 13, pp. 9297–9303, 1996.
- B. Schulz and K. Baberschke, “Crossover from in-plane to perpendicular magnetization in ultrathin Ni/Cu(001) films,” Physical Review B, vol. 50, no. 18, pp. 13467–13471, 1994.
- M. Farle, W. Platow, A. N. Anisimov, B. Schulz, and K. Baberschke, “The temperature dependence of magnetic anisotropy in ultra-thin films,” Journal of Magnetism and Magnetic Materials, vol. 165, no. 1–3, pp. 74–77, 1997.
- M. Farle, B. Mirwald-Schulz, A. N. Anisimov, W. Platow, and K. Baberschke, “Higher-order magnetic anisotropies and the nature of the spin-reorientation transition in face-centered-tetragonal Ni(001)/Cu(001),” Physical Review B, vol. 55, no. 6, pp. 3708–3715, 1997.
- M. Farle, “Ferromagnetic resonance of ultrathin metallic layers,” Reports on Progress in Physics, vol. 61, no. 7, pp. 755–826, 1998.
- R. Bergholz and U. Gradmann, “Structure and magnetism of oligatomic Ni(111)-films on Re(0001),” Journal of Magnetism and Magnetic Materials, vol. 45, no. 2-3, pp. 389–398, 1984.
- M. J. Pechan, “Temperature dependence of interface anisotropy in Ni/Mo multilayers,” Journal of Applied Physics, vol. 64, no. 10, pp. 5754–5756, 1988.
- Y. Hodumi, J. Shi, and Y. Nakamura, “Controlling the magnetic anisotropy of CoPtAlN multilayer films,” Applied Physics Letters, vol. 90, no. 21, Article ID 212506, 2007.
- Y. Yu, J. Shi, and Y. Nakamura, “Thickness-dependent perpendicular magnetic anisotropy of copt top layer on CoPt/AlN multilayer,” IEEE Transactions on Magnetics, vol. 46, no. 6, pp. 1663–1666, 2010.
- W. P. Cai, J. Shi, Y. Nakamura, W. Liu, and R. H. Yu, “Roles of L10 ordering in controlling the magnetic anisotropy and coercivity of (111)-oriented CoPt ultrathin continuous layers in CoPt/AlN multilayer films,” Journal of Applied Physics, vol. 110, no. 7, Article ID 073907, 2011.
- L. Neel, “Anisotropie magnetique superficielle et surstructures dorientation,” Journal de Physique et Le Radium, vol. 15, no. 4, pp. 225–239, 1954.
- M. T. Johnson, P. J. H. Bloemen, F. J. A. Den Broeder, and J. J. De Vries, “Magnetic anisotropy in metallic multilayers,” Reports on Progress in Physics, vol. 59, no. 11, pp. 1409–1458, 1996.
- Y. X. Yu, Perpendicular magnetic anisotropy in CoPt/AlN layered structures: roles of internal stress and interface roughness, [Ph.D. dissertation], Tokyo Institute of Technology, 2011.
- U. Gradmann and J. Muller, “Flat ferromagnetic epitaxial 48Ni/52Fe(111) films of few atomic layers,” Physica Status Solidi, vol. 27, no. 1, pp. 313–324, 1968.
- W. P. Cai, S. Muraishi, J. Shi, Y. Nakamura, W. Liu, and R. H. Yu, “Curie temperatures of CoPt ultrathin continuous films,” Applied Physics A-Materials Science & Processing, vol. 107, no. 3, pp. 519–523, 2012.