- Email: [email protected]
Pt)n multilayers with tuned magnetic properties
Journal of Magnetism and Magnetic Materials 324 (2012) 2298–2300
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Ultrathin (CoFe/Pt)n multilayers with tuned magnetic properties J.F. Feng n, K. Rode, K. Ackland, P.S. Stamenov, M. Venkatesan, J.M.D. Coey CRANN and School of Physics, Trinity College, Dublin 2, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 September 2011 Received in revised form 7 February 2012 Available online 14 March 2012
Perpendicular magnetic anisotropy (PMA) has been investigated in ultrathin (CoFe [0.2] nm/Pt [0.2] nm)n multilayers. The Pt layers show an fcc crystal structure with a preferred [111] orientation. The multilayers with n¼ 3, 4 show PMA in the as-grown state, which can be enhanced by thermal annealing. However, no PMA is observed in the as-grown state with higher repetitions (n 4& ¼ 5), although it is observed after thermal annealing. For 1¼ & o n¼ &o 8, the anisotropy energy is around 105 J/m3 for all (CoFe [0.2]/Pt [0.2])n stacks. The perpendicular anisotropy is related to layer thickness and interface roughness. & 2012 Elsevier B.V. All rights reserved.
Keywords: Perpendicular magnetic anisotropy Ultrathin multilayer Annealing effect
Perpendicular magnetic anisotropy (PMA) has been investigated in ultrathin (CoFe [0.2] nm/Pt [0.2] nm)n multilayers. The Pt layers show an fcc crystal structure with a preferred [111] orientation. The multilayers with n ¼3, 4 show PMA in the asgrown state, which can be enhanced by thermal annealing. However, no PMA is observed in the as-grown state with higher repetitions (n Z5), although it is observed after thermal annealing. For 1rn r8, the anisotropy energy is around 105 J/m3 for all (CoFe [0.2]/Pt [0.2])n stacks. The perpendicular anisotropy is related to layer thickness and interface roughness. Magnetic thin film devices with perpendicular magnetic anisotropy (PMA) are of interest because of their potential application in spin-transfer torque magnetic random access memory (STT-MRAM), and high density magnetic recording media [1–7]. These applications usually require a low coercivity in order to have a manageable switching field. A magnetic multilayer is a convenient way to achieve controllable PMA. Co, Fe or CoFe, and Pd, or Pt are often used as the magnetic and metallic layers in these multilayers [1–4,7–12]. The number (n) of layer repetition and the relative thickness of the magnetic and the non-magnetic layers influences the magnetic properties of the stacks, since PMA is a surface/interface effect. The thickness of the magnetic layer is usually in the range of 0.3–0.6 nm [1,2,7–12]. Recently, a few reports have suggested that the nominal thickness of the magnetic layer can be reduced to around 0.14 nm (less than a monolayer), in which case the bulk anisotropy of a partially ordered crystal structure may be dominant [4]. In such multilayers, the magnetic properties are easily tunable to match the requirements of STT-MRAM.
n
Corresponding author. E-mail addresses: [email protected], [email protected] (J.F. Feng).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2012.02.119
In this work, we investigate the magnetic properties of ultrathin (CoFe [0.2]/Pt [0.2])n monolayer stacks in order to obtain a better understanding of the interfacial PMA. The CoFe composition is Co90Fe10. We focus on the correlation between the number of the repetitions (3rn r8) and PMA. In the as-grown state, PMA appears when n is 3 or 4, but disappears for nZ5. All multilayers with nr8, and t¼0.2 or 0.3 nm show PMA after thermal annealing. A very tiny increase in the layer thickness can greatly improve the PMA, and the anisotropy is enhanced at high annealing temperature. The samples were grown under P 10 3 mBar Ar, at room temperature on thermally oxidized Si wafers using a Shamrock sputtering tool [12]. All (CoFe [0.2] nm/Pt [0.2] nm)n samples were deposited on a Ta 5 nm/Pt 3 nm buffer layer. A Pt 3 nm cap layer is used. The repetition n was varied from 3 to 10. For comparison, one sample with Ta [5]/Pt [3]/(CoFe [0.3]/Pt [0.3])4/Pt [3] was also prepared (thickness in nm). High vacuum annealing in the temperature range 150oTa o375 1C was carried out in an out-of-plane magnetic field of 800 mT for half an hour. The film thickness and its crystal structure were characterized by X-ray Reflectivity (XRR) and X-ray Diffraction (XRD), respectively. The surface morphology of the samples has been done by the scanning electron microscope (SEM). The magnetic properties were measured using the extraordinary Hall Effect (EHE) and a Quantum Design MPMS XL5 magnetometer. For EHE measurements, square samples were contacted at the four corners in the Van der Pauw geometry. Fig. 1 shows XRD data for (CoFe/Pt)n multilayers with different repetition and layer thickness in the as-grown state. There are three reflections for n ¼3–8, t ¼0.2 nm. The strongest reflection is [111]—oriented Pt in all cases, the other two peaks belong to Ta. Despite different texture for Ta, Pt is always [111] textured. In Table 1, the Bragg angle yB and the FWHM of a diffraction peak B are listed for these multilayers. The Bragg angle increases with
J.F. Feng et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2298–2300
2299
Fig. 2. In-plane and out-of-plane magnetic hysteresis loops of (CoFe [0.2]/Pt [0.2])5 multilayers at Ta ¼250 1C. Fig. 1. High angular XRD for different (CoFe/Pt)n multilayers.
Table 1 (111)-oriented Pt diffraction angle y, and the variation of with the n and t. n t (nm) 2yB (deg.) B (deg.)
3 0.2 39.77 1.39
4 0.2 39.89 1.39
5 0.2 39.95 1.41
6 0.2 40.06 1.36
8 0.2 40.24 1.34
4 0.3 40.15 1.33
increasing n for t ¼0.2 nm, which gradually deviates from the [111] textured Pt. Moreover, yB for (CoFe [0.3]/Pt [0.3])4 is close to that for (CoFe [0.2]/Pt [0.2])6, which may reflect that the deviation of yB is related to the total CoFe thickness. Besides, the XRR data show that the roughness of CoFe and Pt for (CoFe [0.2]/Pt [0.2])5 is similar, 0.37 nm, which is higher than the layer thickness. However, the roughnesses of CoFe and Pt for (CoFe [0.3]/Pt [0.3])4 are different, 0.26 and 0.45 nm, respectively. As shown in Table 1, the different B may result from a different grain size [13]. Although the change is rather small, it may be responsible for the different magnetic properties for these (CoFe [0.2]/Pt [0.2])n samples. It has been reported that differences in Pt (111) texture, grain size, and interface roughness have been responsible for the magnetic properties for perpendicular multilayers [14,15]. However, all the parameters, like thickness, roughness, and the FWHM of a diffraction peak change little in these ultrathin (CoFe [0.2]/Pt [0.2])n multilayers. Based on the SEM images, the grain size at the surface for different samples is also close to each other, less than 10 nm. Fig. 2 presents in-plane and out-of-plane M–H loops for (CoFe [0.2]/Pt [0.2])5 at Ta ¼250 1C. The out-of-plane saturation magnetization Ms of this sample is 1.3 106 A/m, which is higher than that reported by Yakushiji et al. [4]. The uniaxial anisotropy energy constant Ku (J/m3) is defined as K u ¼ Hk M s =2
ð1Þ
where Hk is the anisotropy field deduced from the M–H curves. Using Eq. (1), the Ku value is 6.4 104 J/m3 with Hk ¼98.5 mT at Ta ¼250 1C for (CoFe [0.2]/Pt [0.2])5. Moreover, for (CoFe [0.2]/Pt [0.2])3, Ku is 1.3 105 J/m3 (Hk ¼163 mT) in the as-grown state, 1.46 105 J/m3 (Hk ¼200 mT) at Ta ¼250 1C, and 1.1 105 J/m3 (Hk ¼211 mT) at Ta ¼350 1C. We found that the Ku values of these (CoFe [0.2]/Pt [0.2])n multilayers are similar to those given in Ref. [4], where they attributed high Ku to magnetocrystalline
Fig. 3. Out-of-plane EHE hysteresis loops of (CoFe [0.2]/Pt [0.2])4 multilayers at different annealing temperatures.
anisotropy rather than to the interfacial PMA, due to the layer-bylayer growth mode for the ultrathin multilayers. In Fig. 3, the EHE signal vs applied field m0H is plotted as a function of the annealing temperature (Ta) for (CoFe [0.2]/Pt [0.2])4 multilayers. The 2.5 mT shift in the M–H and EHE curves, as shown in Figs. 2 and 3, is due to the in-plane magnetic field applied during sample growth. The Hc for this sample is 7.7 mT at Ta ¼350 1C. This low Hc may indicate that the interface roughness cannot be neglected in these multilayers. In the as-grown state, the square EHE loop suggests that complete, out-of-plane, uniaxial anisotropy is obtained. PMA is retained up to Ta ¼300 1C for this sample, and is enhanced during annealing which may be due to the interfacial structuring at the CoFe/Pt interfaces (see below). When further increasing Ta, PMA decreases, as seen in the EHE curve at Ta ¼350 1C, which may be due to the elemental diffusion of Pt from the seed and cap layers. Fig. 4(a) summarizes the Ta dependence of coercivity (Hc) for different (CoFe [0.2]/Pt [0.2])n multilayers, taken from the EHE curves. In the as-grown state, only multilayers with n¼3 and 4 show coercivity, and their Hc values are close to each other. Very similar FWHMs of Pt are also found for these two multilayers, as
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J.F. Feng et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2298–2300
value using Eq. (1) is 2.3 105 J/m3 (Hk ¼ 300 mT) at Ta ¼300 1C for (CoFe [0.3]/Pt [0.3])4, which is also higher than that for (CoFe [0.2]/Pt [0.2])n. Moreover, the roughness of CoFe in the (CoFe [0.3]/Pt [0.3])4 is less than the layer thickness, and it also may help to enhance PMA. In conclusion, the evolution of out-of-plane anisotropy of ultrathin (CoFe/Pt)n multilayers with annealing has been investigated. Perpendicular magnetic anisotropy is observed in these systems, which is sustained at high Ta, despite an interface roughness comparable to the layer thickness. For these structures, the amplitudes of the perpendicular coercivity and saturation magnetization can be tuned vis the structural characteristics of the layers (grain size of the metallic layer and roughness) as well as the CoFe and Pt layer thickness.
Acknowledgment
Fig. 4. The Ta dependence of coercivity for (a) (CoFe [0.2]/Pt [0.2])n multilayers and (b) (CoFe [0.3]/Pt [0.3])4 multilayers.
This work was supported by SFI as part of the MANSE project 2005/IN/1850, and was conducted under the framework of the INSPIRE program, funded by the Irish Government’s Program for Research in Third Level Institutions, Cycle 4, National Development Plan 2007–2013. K. Ackland was supported by SFI under the IRCSET, EMBARK initiative. References
shown in Table 1. However, with the increase of n ( Z5), PMA disappears in the as-grown state. Based on the XRD data, the B value starts to decrease when nZ5. This may confirm that the magnetic properties are related to the texture of the ultrathin multilayers [14,15]. The PMA can be related to the CoFe–Pt intermixing at the interface [16]. The CoFe–Pt mixing may increase gradually with Ta due to the elemental diffusion. Actually, Hc has been enhanced after a thermal annealing for n¼3 and 4. Furthermore, the annealing can also enhance Ku, for example in the n¼ 3 case mentioned above. Based on this point, PMA can occur after annealing samples with nZ5. Actually, PMA appears at Ta ¼ 150 1C for n¼5, while obvious magnetic hysteresis occurs at Ta ¼200 1C for n¼6, and 275 1C for n¼8, but no coercivity is observed even at Ta ¼375 1C for n¼10. PMA can be sustained up to 350 1C for these (CoFe [0.2]/Pt [0.2])n multilayers, despite an interface roughness higher than the layer thickness. When the layer thickness increases to 0.3 nm, the magnetic properties of the multilayers are much different. Fig. 4(b) shows the Hc for the (CoFe [0.3]/Pt [0.3])4 stack as a function of the annealing temperature. For this sample, PMA appears in the as-grown state and remains after annealing, which may suggest that the increase in coercivity and saturation is related to the enhancement in the PMA. The Hc for (CoFe [0.3]/Pt [0.3])4 is at least twice as high as that for (CoFe [0.2]/Pt [0.2])n when Ta is above 250 1C. The calculated Ku
[1] K. Mizunuma, S. Ikeda, J.H. Park, H. Yamamoto, H. Gan, K. Miura, H. Hasegawa, J. Hayakawa, F. Matsukura, H. Ohno, Applied Physics Letters 95 (2009) 232516. [2] T. Hauet, E. Dobisz, S. Florez, J. Park, B. Lengsfield, B.D. Terris, O. Hellwig, Applied Physics Letters 95 (2009) 262504. [3] K. Yakushiji, K. Noma, T. Saruya, H. Kubota, A. Fukushima, T. Nagahama, S. Yuasa, K. Ando, Applied Physics Express 3 (2010) 053003. [4] K. Yakushiji, T. Saruya, H. Kubota, A. Fukushima, T. Nagahama, S. Yuasa, K. Ando, Applied Physics Letters 97 (2010) 232508. [5] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H.D. Gan, M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, H. Ohno, Nature Materials (2010). doi:10.1038/ nmat2804. Letter. [6] M. Albrecht, C.T. Rettner, A. Moser, M.E. Best, B.D. Terris, Applied Physics Letters 81 (2002) 2875. [7] S. Toshio, K. Takanori, H. Naoki, IEEE Transactions on Magnetics 36 (2000) 2418. [8] P.F. Carcia, A.D. Mcinhaldt, A. Sunna, Applied Physics Letters 47 (1985) 178. [9] O. Hellwig, S. Maat, J.B. Kortright, E.E. Fullerton, Physical Review B 65 (2002) 144418. [10] S. van Dijken, J. Moritz, J.M.D. Coey, Journal of Applied Physics 97 (2005) 063907. [11] R. Law, R. Sbiaa, T. Liew, T.C. Chong, Applied Physics Letters 91 (2007) 242504. [12] J.Y. Chen, J.F. Feng, Z. Diao, G. Feng, J.M.D. Coey, X.F. Han, IEEE Transactions on Magnetics 46 (2010) 1401. [13] L.S. Birks, H. Friedman, Journal of Applied Physics 17 (1946) 687. [14] S. Hashimoto, Y. Ochiai, K. Aso, Journal of Applied Physics 66 (1989) 4909. [15] M. Pakala, Y. Huai, G. Anderson, L. Miloslavsky, Journal of Applied Physics 87 (2000) 6653. [16] J. Moritz, F. Garcia, J.C. Toussaint, B. Dieny, J.P. Nozie res, Europhysics Letters 65 (2004) 123.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Ultrathin (CoFe/Pt)n multilayers with tuned magnetic properties J.F. Feng n, K. Rode, K. Ackland, P.S. Stamenov, M. Venkatesan, J.M.D. Coey CRANN and School of Physics, Trinity College, Dublin 2, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 September 2011 Received in revised form 7 February 2012 Available online 14 March 2012
Perpendicular magnetic anisotropy (PMA) has been investigated in ultrathin (CoFe [0.2] nm/Pt [0.2] nm)n multilayers. The Pt layers show an fcc crystal structure with a preferred [111] orientation. The multilayers with n¼ 3, 4 show PMA in the as-grown state, which can be enhanced by thermal annealing. However, no PMA is observed in the as-grown state with higher repetitions (n 4& ¼ 5), although it is observed after thermal annealing. For 1¼ & o n¼ &o 8, the anisotropy energy is around 105 J/m3 for all (CoFe [0.2]/Pt [0.2])n stacks. The perpendicular anisotropy is related to layer thickness and interface roughness. & 2012 Elsevier B.V. All rights reserved.
Keywords: Perpendicular magnetic anisotropy Ultrathin multilayer Annealing effect
Perpendicular magnetic anisotropy (PMA) has been investigated in ultrathin (CoFe [0.2] nm/Pt [0.2] nm)n multilayers. The Pt layers show an fcc crystal structure with a preferred [111] orientation. The multilayers with n ¼3, 4 show PMA in the asgrown state, which can be enhanced by thermal annealing. However, no PMA is observed in the as-grown state with higher repetitions (n Z5), although it is observed after thermal annealing. For 1rn r8, the anisotropy energy is around 105 J/m3 for all (CoFe [0.2]/Pt [0.2])n stacks. The perpendicular anisotropy is related to layer thickness and interface roughness. Magnetic thin film devices with perpendicular magnetic anisotropy (PMA) are of interest because of their potential application in spin-transfer torque magnetic random access memory (STT-MRAM), and high density magnetic recording media [1–7]. These applications usually require a low coercivity in order to have a manageable switching field. A magnetic multilayer is a convenient way to achieve controllable PMA. Co, Fe or CoFe, and Pd, or Pt are often used as the magnetic and metallic layers in these multilayers [1–4,7–12]. The number (n) of layer repetition and the relative thickness of the magnetic and the non-magnetic layers influences the magnetic properties of the stacks, since PMA is a surface/interface effect. The thickness of the magnetic layer is usually in the range of 0.3–0.6 nm [1,2,7–12]. Recently, a few reports have suggested that the nominal thickness of the magnetic layer can be reduced to around 0.14 nm (less than a monolayer), in which case the bulk anisotropy of a partially ordered crystal structure may be dominant [4]. In such multilayers, the magnetic properties are easily tunable to match the requirements of STT-MRAM.
n
Corresponding author. E-mail addresses: [email protected], [email protected] (J.F. Feng).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2012.02.119
In this work, we investigate the magnetic properties of ultrathin (CoFe [0.2]/Pt [0.2])n monolayer stacks in order to obtain a better understanding of the interfacial PMA. The CoFe composition is Co90Fe10. We focus on the correlation between the number of the repetitions (3rn r8) and PMA. In the as-grown state, PMA appears when n is 3 or 4, but disappears for nZ5. All multilayers with nr8, and t¼0.2 or 0.3 nm show PMA after thermal annealing. A very tiny increase in the layer thickness can greatly improve the PMA, and the anisotropy is enhanced at high annealing temperature. The samples were grown under P 10 3 mBar Ar, at room temperature on thermally oxidized Si wafers using a Shamrock sputtering tool [12]. All (CoFe [0.2] nm/Pt [0.2] nm)n samples were deposited on a Ta 5 nm/Pt 3 nm buffer layer. A Pt 3 nm cap layer is used. The repetition n was varied from 3 to 10. For comparison, one sample with Ta [5]/Pt [3]/(CoFe [0.3]/Pt [0.3])4/Pt [3] was also prepared (thickness in nm). High vacuum annealing in the temperature range 150oTa o375 1C was carried out in an out-of-plane magnetic field of 800 mT for half an hour. The film thickness and its crystal structure were characterized by X-ray Reflectivity (XRR) and X-ray Diffraction (XRD), respectively. The surface morphology of the samples has been done by the scanning electron microscope (SEM). The magnetic properties were measured using the extraordinary Hall Effect (EHE) and a Quantum Design MPMS XL5 magnetometer. For EHE measurements, square samples were contacted at the four corners in the Van der Pauw geometry. Fig. 1 shows XRD data for (CoFe/Pt)n multilayers with different repetition and layer thickness in the as-grown state. There are three reflections for n ¼3–8, t ¼0.2 nm. The strongest reflection is [111]—oriented Pt in all cases, the other two peaks belong to Ta. Despite different texture for Ta, Pt is always [111] textured. In Table 1, the Bragg angle yB and the FWHM of a diffraction peak B are listed for these multilayers. The Bragg angle increases with
J.F. Feng et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2298–2300
2299
Fig. 2. In-plane and out-of-plane magnetic hysteresis loops of (CoFe [0.2]/Pt [0.2])5 multilayers at Ta ¼250 1C. Fig. 1. High angular XRD for different (CoFe/Pt)n multilayers.
Table 1 (111)-oriented Pt diffraction angle y, and the variation of with the n and t. n t (nm) 2yB (deg.) B (deg.)
3 0.2 39.77 1.39
4 0.2 39.89 1.39
5 0.2 39.95 1.41
6 0.2 40.06 1.36
8 0.2 40.24 1.34
4 0.3 40.15 1.33
increasing n for t ¼0.2 nm, which gradually deviates from the [111] textured Pt. Moreover, yB for (CoFe [0.3]/Pt [0.3])4 is close to that for (CoFe [0.2]/Pt [0.2])6, which may reflect that the deviation of yB is related to the total CoFe thickness. Besides, the XRR data show that the roughness of CoFe and Pt for (CoFe [0.2]/Pt [0.2])5 is similar, 0.37 nm, which is higher than the layer thickness. However, the roughnesses of CoFe and Pt for (CoFe [0.3]/Pt [0.3])4 are different, 0.26 and 0.45 nm, respectively. As shown in Table 1, the different B may result from a different grain size [13]. Although the change is rather small, it may be responsible for the different magnetic properties for these (CoFe [0.2]/Pt [0.2])n samples. It has been reported that differences in Pt (111) texture, grain size, and interface roughness have been responsible for the magnetic properties for perpendicular multilayers [14,15]. However, all the parameters, like thickness, roughness, and the FWHM of a diffraction peak change little in these ultrathin (CoFe [0.2]/Pt [0.2])n multilayers. Based on the SEM images, the grain size at the surface for different samples is also close to each other, less than 10 nm. Fig. 2 presents in-plane and out-of-plane M–H loops for (CoFe [0.2]/Pt [0.2])5 at Ta ¼250 1C. The out-of-plane saturation magnetization Ms of this sample is 1.3 106 A/m, which is higher than that reported by Yakushiji et al. [4]. The uniaxial anisotropy energy constant Ku (J/m3) is defined as K u ¼ Hk M s =2
ð1Þ
where Hk is the anisotropy field deduced from the M–H curves. Using Eq. (1), the Ku value is 6.4 104 J/m3 with Hk ¼98.5 mT at Ta ¼250 1C for (CoFe [0.2]/Pt [0.2])5. Moreover, for (CoFe [0.2]/Pt [0.2])3, Ku is 1.3 105 J/m3 (Hk ¼163 mT) in the as-grown state, 1.46 105 J/m3 (Hk ¼200 mT) at Ta ¼250 1C, and 1.1 105 J/m3 (Hk ¼211 mT) at Ta ¼350 1C. We found that the Ku values of these (CoFe [0.2]/Pt [0.2])n multilayers are similar to those given in Ref. [4], where they attributed high Ku to magnetocrystalline
Fig. 3. Out-of-plane EHE hysteresis loops of (CoFe [0.2]/Pt [0.2])4 multilayers at different annealing temperatures.
anisotropy rather than to the interfacial PMA, due to the layer-bylayer growth mode for the ultrathin multilayers. In Fig. 3, the EHE signal vs applied field m0H is plotted as a function of the annealing temperature (Ta) for (CoFe [0.2]/Pt [0.2])4 multilayers. The 2.5 mT shift in the M–H and EHE curves, as shown in Figs. 2 and 3, is due to the in-plane magnetic field applied during sample growth. The Hc for this sample is 7.7 mT at Ta ¼350 1C. This low Hc may indicate that the interface roughness cannot be neglected in these multilayers. In the as-grown state, the square EHE loop suggests that complete, out-of-plane, uniaxial anisotropy is obtained. PMA is retained up to Ta ¼300 1C for this sample, and is enhanced during annealing which may be due to the interfacial structuring at the CoFe/Pt interfaces (see below). When further increasing Ta, PMA decreases, as seen in the EHE curve at Ta ¼350 1C, which may be due to the elemental diffusion of Pt from the seed and cap layers. Fig. 4(a) summarizes the Ta dependence of coercivity (Hc) for different (CoFe [0.2]/Pt [0.2])n multilayers, taken from the EHE curves. In the as-grown state, only multilayers with n¼3 and 4 show coercivity, and their Hc values are close to each other. Very similar FWHMs of Pt are also found for these two multilayers, as
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J.F. Feng et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2298–2300
value using Eq. (1) is 2.3 105 J/m3 (Hk ¼ 300 mT) at Ta ¼300 1C for (CoFe [0.3]/Pt [0.3])4, which is also higher than that for (CoFe [0.2]/Pt [0.2])n. Moreover, the roughness of CoFe in the (CoFe [0.3]/Pt [0.3])4 is less than the layer thickness, and it also may help to enhance PMA. In conclusion, the evolution of out-of-plane anisotropy of ultrathin (CoFe/Pt)n multilayers with annealing has been investigated. Perpendicular magnetic anisotropy is observed in these systems, which is sustained at high Ta, despite an interface roughness comparable to the layer thickness. For these structures, the amplitudes of the perpendicular coercivity and saturation magnetization can be tuned vis the structural characteristics of the layers (grain size of the metallic layer and roughness) as well as the CoFe and Pt layer thickness.
Acknowledgment
Fig. 4. The Ta dependence of coercivity for (a) (CoFe [0.2]/Pt [0.2])n multilayers and (b) (CoFe [0.3]/Pt [0.3])4 multilayers.
This work was supported by SFI as part of the MANSE project 2005/IN/1850, and was conducted under the framework of the INSPIRE program, funded by the Irish Government’s Program for Research in Third Level Institutions, Cycle 4, National Development Plan 2007–2013. K. Ackland was supported by SFI under the IRCSET, EMBARK initiative. References
shown in Table 1. However, with the increase of n ( Z5), PMA disappears in the as-grown state. Based on the XRD data, the B value starts to decrease when nZ5. This may confirm that the magnetic properties are related to the texture of the ultrathin multilayers [14,15]. The PMA can be related to the CoFe–Pt intermixing at the interface [16]. The CoFe–Pt mixing may increase gradually with Ta due to the elemental diffusion. Actually, Hc has been enhanced after a thermal annealing for n¼3 and 4. Furthermore, the annealing can also enhance Ku, for example in the n¼ 3 case mentioned above. Based on this point, PMA can occur after annealing samples with nZ5. Actually, PMA appears at Ta ¼ 150 1C for n¼5, while obvious magnetic hysteresis occurs at Ta ¼200 1C for n¼6, and 275 1C for n¼8, but no coercivity is observed even at Ta ¼375 1C for n¼10. PMA can be sustained up to 350 1C for these (CoFe [0.2]/Pt [0.2])n multilayers, despite an interface roughness higher than the layer thickness. When the layer thickness increases to 0.3 nm, the magnetic properties of the multilayers are much different. Fig. 4(b) shows the Hc for the (CoFe [0.3]/Pt [0.3])4 stack as a function of the annealing temperature. For this sample, PMA appears in the as-grown state and remains after annealing, which may suggest that the increase in coercivity and saturation is related to the enhancement in the PMA. The Hc for (CoFe [0.3]/Pt [0.3])4 is at least twice as high as that for (CoFe [0.2]/Pt [0.2])n when Ta is above 250 1C. The calculated Ku
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