JOURNAL OF APPLIED PHYSICS 108, 073916 共2010兲
Enhanced perpendicular magnetic anisotropy in Co/Ni multilayers with a thin seed layer H. Kurt,a兲 M. Venkatesan, and J. M. D. Coey School of Physics and CRANN, Trinity College, Dublin 2, Ireland
共Received 1 May 2010; accepted 26 July 2010; published online 7 October 2010兲 Perpendicular magnetic anisotropy 共PMA兲 is induced in Co/Ni multilayers when they are grown on a 共111兲 textured Au seed layer, provided it is at least 2 nm thick. The anisotropy increases with increasing Au thickness due to improved crystallinity. Postannealing treatments of as-grown 关Co共0.3兲 / Ni共0.6兲兴5 共thicknesses in nanometer兲 multilayers enhance the coercivity and PMA up to an annealing temperature of 250 ° C, but higher temperatures lead to intermixing of Co and Ni which diminishes PMA. The easy axis becomes in-plane for samples annealed at 400 ° C. The improvement in PMA in Co/Ni layers due to annealing is limited by the Au seed layer thickness. Annealing also reduces the saturation magnetization by ⬃15% due to the formation of superparamagnetic islands. Our results show that the PMA in Co/Ni multilayers can be improved by annealing up to 350 ° C, which is required for the thermal stability of spin transfer torque memories. © 2010 American Institute of Physics. 关doi:10.1063/1.3481452兴 I. INTRODUCTION
Ultrathin Co layers are known to give perpendicular magnetic anisotropy 共PMA兲 when combined with layers of X = Pt, Pd, Au, and Ni. The explanation is the large interface anisotropy energy at the Co/X interface, when this is a 共111兲 plane.1,2 Recent studies of spin transfer torque 共STT兲 have focused on magnetic layers with PMA due to their higher spin transfer efficiency and thermal stability.3,4 The PMA in layered systems is lower than that in L10 ordered hard magnetic layers or rare earth amorphous magnetic alloys, which are unsuitable as free layers for STT applications such as spin torque magnetic random access memories 共ST-MRAMs兲 because of the diffulty in switching them. Perpendicular Co/Ni multilayers are special due to their all-ferromagnetic nature, higher giant magnetoresistance 共GMR兲, and lower uniaxial anisotropy compared to other layered perpendicular systems.1 In recent studies of GMR in magnetic nanopillars with perpendicular magnetic layers, a Co/Pt multilayer has been used as a fixed layer and the Co/Ni layers are grown on top of the Cu spacer, which follows the 共111兲 oriented growth of the underlying Co/Pt superlattice.3–5 Various growth methods and parameters have been investigated to optimize PMA in Co/Ni multilayers6,7 and bilayers.8 The highest anisotropy reported to date was seen in single crystalline Co/Ni multilayers.9 There is a tradeoff between lower critical switching currents and high anisotropy, which would ultimately determine the minimum thermally stable bit size in ST-MRAMs. STT can also be used to shift domain walls in magnetic nanowires for novel storage applications such as magnetic shift register memory.10 Such studies have focused mainly on lithographically patterned permalloy 共Ni80Fe20兲 nanowires which exhibit in-plane anisotropy with very low depinning fields.10 Magnetic nanowires with PMA could be more advantageous for domain wall shift register a兲
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type devices if the depinning field can be reduced, which in turn requires lower threshold currents for domain wall motion.11,12 Co/Ni nanowires are shown to be good candidates for this type of applications due to their low depinning fields.13,14 However, this system usually requires a thick seed layer with 共111兲 texture, which will draw off most of the injected current.15–17 To improve the current injection through the perpendicular Co/Ni layers in a nanowire, the seed layers should be as thin as possible to improve the current injection through the magnetic layers. While very thin Pt on SiOx already provides a 共111兲 oriented seed layer, it may not be desirable for the Co/Ni system because of the higher anisotropy of the Pt/Co 共111兲 interface compared to the Au/Co共111兲 interface, which can increase the overall anisotropy of the Co/Ni multilayers on Pt.1 In addition, to stabilize the PMA in Co/Ni superlattices the crystalline order has to be thermally stable, which can be achieved by annealing. Therefore, it is advantageous to use Au rather than Pt as the seed layer, which is immiscible with Co.18 Here, we implement a Pt/Au bilayer system on thermal SiOx as a seed layer, which serves to reduce the interface anisotropy and also provides an immiscible bottom interface that is advantageous for the thermal stability of the PMA. We have also investigated the effects of postannealing on the anisotropy of the Co/Ni multilayer structures. II. EXPERIMENT
Samples were grown on 25⫻ 25 mm2 thermally oxidized Si wafers at ambient temperature by electron beam evaporation in a UHV chamber with a base pressure of 2.1 ⫻ 10−10 Torr. The source to substrate distance was 45 cm, which leads to good thickness uniformity. Detailed deposition conditions for all layers are given in Table I. We prepared 关Co共0.3兲 / Ni共0.6兲兴5 multilayers on varying thicknesses of Pt 共2兲/Au 共t兲 seed layers 共all thicknesses are in nanometer兲 and capped them with 3 nm thick Au to prevent oxidation. We chose 0.3 nm thick Co to make sure that all layers are
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TABLE I. Deposition parameters for Pt, Au, Co, and Ni. Material
Pt
Au
Co
Ni
Deposition rate 共pm/s兲 10 20 10 20 Pressure during evaporation 共Torr兲 2 – 3 ⫻ 10−8 2 – 3 ⫻ 10−8 2 – 3 ⫻ 10−9 2 – 3 ⫻ 10−9
continuous with well defined interfaces due to the roughness of the substrate and seed layers. After growth, samples were diced into 4 ⫻ 4 mm2 squares for the magnetization measurements. To investigate annealing effects on the PMA in Co/Ni multilayers, we annealed a set of diced samples up to 400 ° C under high vacuum 共⬃10−6 Torr兲 for 1 h and remeasured their magnetization. Magnetization measurements were made at 300 K using a quantum design MPMS XL SQUID magnetometer with the samples mounted either perpendicular or parallel to the field. III. RESULTS AND DISCUSSION
Earlier studies of Co/Pt multilayers have shown that there is an optimum thickness of Pt seed layer for Co/Pt PMA layers. It has also been shown that granular growth of Co/Pt multilayers on pre-etched Pt seed layers exhibit very high perpendicular anisotropy.19 The case of Co/Ni is quite different from that of the Co/Pt and Co/Pd multilayers due to all-ferromagnetic structure and also the absence of normally 共111兲–textured constituents such as Pt and Pd. In this case the 共111兲 order of the bottom seed layer plays a crucial role in determining the texture and therefore the magnetic anisotropy of the films. To grow the Co/Ni 共111兲 multilayers we deposited them on Pt 共2兲/Au 共2–14兲 bilayers, where Pt 共2兲 is used to induce all 共111兲 growth of the thin Au. The use of just Au on thermal SiOx did not result in a proper 共111兲 alignment for Au thicknesses of up to 20 nm, and the Co/Ni films deposited on these films did not show PMA. Very thick 共⬃100 nm兲 Au is needed for the 共111兲 growth of Co/Ni perpendicular multilayers.15 The use of 2 nm thick Pt helps to induce all 共111兲 growth of thin Au on SiOx and also improves the adhesion of the stack to the substrate. As the Au thickness increases, the 共111兲 growth becomes more pronounced due to increased grain size, and magnetization switching of Co/Ni layers becomes sharper with increased coercivity and remanence ratio as shown in Fig. 1. The thinnest Au thickness to give PMA was found to be 2 nm, when the Pt 共2兲/Au 共2兲 seed layer was annealed at 200 ° C for 1 h before the deposition of Co/Ni multilayers. den Broeder et al.20 investigated the interface sharpening effect of Co/Au multilayers and they succeeded in promoting the PMA in Co/Au multilayers after postannealing. The immiscibility of Co and Au was essential in their studies, as it led to interface sharpening upon annealing that turned the easy axis out of plane. We selected the Pt共2兲 / Au共t兲 / 关Co共0.3兲 / Ni共0.6兲兴5 / Au共3兲 system to study the postannealing effects and the persistence of a superlattice structure. Much thinner Co layers have the disadvantage of being discontinuous due to surface roughness. In our case, we obtained interface sharpening between the bottom Co and Au similar to the results of Ref. 20, which improved the
FIG. 1. 共Color online兲 Room temperature magnetization of 关Co共0.3兲 / Ni共0.6兲兴5 multilayers with different seed layers, measured with the field applied perpendicular to the plane of the film.
PMA and coercivity of the Co/Ni multilayers up to an annealing temperature of 250 ° C. Annealing results in significant changes in the shape of the perpendicular hysteresis loop. The remanence ratio Mr / Ms of the Co/Ni multilayers increased considerably, and the coercivity increased by a factor of nearly 2. However, the coercivity decreases on annealing at 300 ° C, although the easy axis remains out of plane as shown in Fig. 2. Further increase in the annealing temperature leads to drastic reduction in degree of PMA and eventually destroys it above 350 ° C due to interfacial alloying between Co and Ni. In addition, the value of room temperature magnetization is reduced by 15 % upon annealing at 250 ° C as shown in Fig. 2共a兲. To understand whether the the cause of the improvement
FIG. 2. 共Color online兲 Annealing temperature dependence of 共a兲 magnetization and 共b兲 coercive field at 300 K for 关Co共0.3兲 / Ni共0.6兲兴5 on Pt 共2兲/Au 共14兲 seed layer.
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FIG. 3. 共Color online兲 Variation in the coercive field vs seed layer thickness for the as-grown 共square兲 and 250 ° C annealed 共circle兲 Pt共2兲 / Au共t兲 / 关Co共0.3兲 / Ni共0.6兲兴5 / Au共3兲 samples.
in anisotropy is due to only the interface sharpening at the bottom Co/Au interface, we annealed a set of samples at 250 ° C for 1 h with different Au seed layer thickness and measured their magnetization at room temperature. Figure 3 shows a plot of room temperature coercivity as a function of seed layer thickness for films annealed at 250 ° C for 1 h. As the Au thickness increases, the coercivity increases for the annealed samples at a higher rate than the as-deposited samples due to a better 共111兲 ordering set during deposition. We speculate that the initial 共111兲 order set during the growth leads to an optimal predefined anisotropy which can only increase slightly upon annealing. Therefore, it is essential to grow a very thin but highly ordered seed layer before depositing Co/Ni multilayers to achieve optimal PMA. However, our results suggest that the most stable PMA could be reached after annealing at 250 ° C. Postannealing Co/Ni layers also has the advantage that they can be exchange biased in a spin valve stack using L12 ordered antiferromagnetic layers such as IrMn due to the all 共111兲 texture and the thermal stability of the Co/Ni at the blocking temperatures of L12 ordered antiferromagnets.21 Postannealing causes a decrease in the saturation mag-
FIG. 5. 共Color online兲 Magnetization data for the as-deposited and 250 ° C-annealed Pt共2兲 / Au共14兲 / 关Co共0.3兲 / Ni共0.6兲兴5 / Au共3兲 samples at 4 K.
netization of Co/Ni layers 共Fig. 2共a兲兲. Interfacial alloying cannot account for this reduction, because the saturation magnetization varies linearly with composition for Co–Ni alloys.22 A possible explanation is the formation of superparamagnetic islands. In the case of a multilayer structure with very thin Co/Ni layers, a random dissolution of the magnetic layers could cause a lateral variation in the layer thickness and lead to a formation of the discontinuous layer structure in the Co/Ni layers. The discrete regions of the magnetic layers can exhibit superparamagnetism due to their reduced dimensions. We detected the presence of these superparamagnetic islands by measuring susceptibility in one of the annealed samples. The zero-field cooled 共ZFC兲 and field cooled 共FC兲 magnetization measurements at 5 mT 共ⰆHc兲 show diverging susceptibility for the annealed sample but no difference for the as-grown sample as shown in Fig. 4. The differences in the ZFC-FC curves clearly indicate the formation of superparamagnetic islands in the annealed sample that can account for the reduction in the saturation magnetization. To confirm this result, we also measured the magnetization of as-deposited and 250 ° C-annealed samples at 4 K, which both saturate at the same value as seen in Fig. 5. IV. CONCLUSIONS
FIG. 4. 共Color online兲 ZFC and FC magnetization for the 共a兲 as grown, 共b兲 250 ° C annealed Pt共2兲 / Au共14兲 / 关Co共0.3兲 / Ni共0.6兲兴5 / Au共3兲 samples.
The PMA in 关Co共0.3兲 / Ni共0.6兲兴5 multilayers grown on thin Pt/Au bilayers with seed layers as thin as Pt 共2兲/Au 共2兲 is due to the good 共111兲 orientation of the Au seed layer. Thinner seed layers give smaller coercivity on account of smaller grain sizes. Ultrathin Pt/Au seed layers could be used to improve current injection through Co/Ni nanowires for storage applications such as sensors and race track memories. Improvements of PMA by post annealing up to 350 ° C are compatible with read head and complementary metal-oxide semiconductor manufacturing processes. Annealing above 350 ° C leads to excessive interfacial alloying that turns the easy axis in-plane. The perpendicular anisotropy set during initial growth can be further enhanced by annealing depending on the seed layer thickness. The annealing of the Co/Ni layers ultimately results in formation of superparamagnetic islands, which re-
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duces the saturation magnetization. The perpendicular anisotropy of Co/Ni multilayers grown on thin Au seed layers could be useful for STT devices. ACKNOWLEDGMENTS
This work was supported by Science Foundation Ireland 共SFI兲 as part of the MANSE project Grant No. SFI 05/IN/ 1850. This work was conducted under the framework of the INSPIRE program, funded by the Irish Government’s Programme for Research in Third Level Institutions, Cycle 4, National Development Plan 2007–2013. 1
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