Large tunneling magnetoresistance (TMR) values reported from the magnetic tunneling junctions (MTJs) using Co₂FeAl_0.5Si_0.5 (CFAS) [1] and Co₂MnSi (CMS) Heusler alloys [2] indicate high spin polarization of the electrode materials. Highly spin polarized materials are believed to be effective also for enhancing current-perpendicular-to-plane giant magnetoresistance (CPP GMR). Because of its intrinsically low resistance compared to TMR, CPP GMR has a potential as read heads applications in hard disk drives with the recording density over 1 Tbits/inch². Thus, it is a critical issue for CPP-GMR devices to realize high MR ratios with low resistance.　In this work, we are developing high MR ratios ΔR/R in CPP-type spin valves using Heusler alloys of CFAS and CMS.

Structure and MR characteristics of epitaxial layers of CFAS/Ag/CFAS [3] and CMS/Cu/CMS. Multilayered films were grown by dc and rf magnetron sputtering. The films were deposited on MgO (001) single crystalline substrates kept at room temperature. The films for CPP-GMR devices using CFAS have the stacking structure of Cr(10)/Ag(200)/CFAS(20)/Ag(5)/CFAS(5)/Co75Fe25(2)/Ir₂₂Mn₇₈(10)Ru(8), where the numbers indicate the thicknesses in nm. The films were microfabricated by electron beam lithography to form pillars with submicron sizes for measuring the resistance in the CPP geometry. Then the samples were annealed at 250 °C in the magnetic field of 5 kOe for attaining the exchange bias by the IrMn layer. It has been reported that the MR ratio of MTJs is strongly enhanced by improving the structural ordering of the CFAS layers. Thus, three samples with different annealing conditions were prepared for examining the effects of annealing CFAS layers: (A) no annealing, (B) annealing only the lower CFAS layer at 400 °C after deposition, and (C) annealing both CFAS layers at 400 °C. The structure of sample (B) was closely examined by cross sectional TEM. Epitaxial growth of the layers up to the top CFAS layer in the [001] direction was confirmed. The bottom CFAS layer was in the B2 ordered structure, while the top CFAS layer was in the A2 disordered structure. The MR ratio was 3.4 % for (A), 6.9% for (B) and 12.4% for (C) with the resistance area product of RA≈0.1 Ω(μm)2 for each sample.

Figure 1 shows the MR curves of sample (C). The MR ratio increased to 31% at 12 K. The results suggest that the MR characteristic is improved by improving the structural ordering of the CFAS Heusler alloy. We prepared the stacking structure of Cr(10)/Ag(200)/Cr(10)/CMS(20)/Cu(4)/CMS(5)/Co₇₅Fe₂₅(2)/Ir22Mn78(10)/Ru(5) by combining CMS with the Cu spacer layer. The bottom CMS layer was annealed at 350 °C after the deposition. The MR ratio of 8.6 % RA=0.16 Ω(μm)2 was observed. The MR ratio increased to 30.5 % at 7 K. Figure 2 shows the cross sectional TEM image of the film. The layers up to the tom CMS layer were found to grow epitaxially in the [001] direction despite of the large lattice mismatch of 9.7 % between CMS and Cu. A lot of defects including lattice distortions and dislocations are observed in the Cu layer. The deformation within the Cu spacer layer is considered to accommodate the large lattice mismatch, thereby enabling the epitaxial growth of the top CMS layer. We have shown that large CPP MR ratios were obtained by using epitaxial layers of Heusler alloys of CFAS and CMS combined with a spacer layer of Ag or Cu. The origin of the large CPP GMR at low temperatures is obviously the half metallicity of CFAS and CMS. However, the MR ratio significantly decreases with increasing the temperature to RT similarly with MTJs using Heusler alloys. One possible explanation for the temperature dependence of the spin polarization of the Heusler alloys due to magnon excitations. The rather poor structural ordering may lead to the narrow energy gap of the minority-spin band and this might enhance the magnon excitation at room temperature. It would be still promising to improve the MR characteristics by improving the structural order by optimizing the preparing condition of the Heusler layers.


Figure The stacking structure of the CPP-GMR and its MR curves.

References
[1] N. Tezuka, N. Ikeda, S. Sugimoto, and K. Inomata, Appl. Phys. Lett. 89, 252508 (2006).
[2] Y. Sakuraba, M. Hattori, M. Oogane, Y. Ando, H. Kato, A. Sakuma, T. Miyazaki, and H. Kubota: Appl. Phys. Lett. 88, 192508 (2006).
[3] T. Furubayashi, K. Kodama, H. Sukegawa, Y. K. Takahashi, K. Inomata and K. Hono, Appl. Phys. Lett. 93, 122507 (2008).
[4] K. Kodama, T. Furubayashi, T. Nakatani, H. Sukegawa, K. Inomata and K. Hono, J. Appl. Phys. in press, arXiv: 0811.3282.

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Relevant Publiscations

Oscillatory antiferromagnetic interlayer exchange coupling in Co₂Fe(Al_0.5Si_0.5)/Ag/Co₂Fe(Al_0.5Si_0.5) films and its application to trilayer magnetoresistive sensor
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Effect of NiAl underlayer and spacer on magnetoresistance ofcurrent-perpendicular-to-plane spin valves using Co₂Mn(Ga_0.5Sn_0.5) Heusler alloy
N. Hase, T. M. Nakatani, S. Kasai, Y. K. Takahashi, T. Furubayashi and K. Hono, J. Mag. Mag. Mater. (2011), in press.

Large magnetoresistance in current-perpendicular-to-plane pseudo spin valve using a Co₂Fe(Ge_0.5Ga_0.5) Heusler alloy
Y.K. Takahashi, A. Srinivasan, B. Varaprasad, A. Rajanikanth, N. Hase, T.M. Nakatani, S. Kasai, T. Furubayashi and K. Hono, Appl. Phys. Lett. 98, 152501 (2011).

Low-temperature grown quaternary Heusler-compound Co₂Mn_1-xFe_xSi films on Ge(111)
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Enhancement of current-perpendicular-to-plane giant magnetoresistance by insertion of Co₅₀Fe₅₀ layers at the Co₂Mn(Ga_0.5Sn_0.5)/Ag intervace
N. Hase, T. M. Nakatani, S. Kasai, Y. K. Takahashi, and K. Hono, J. Appl. Phys. 109, 07E112 (2011)..

The effect of substitution of Fe with Cr on the giant magnetoresistance of current-perpendicular-to-plane spin valves with Co₂FeSi Heusler alloy
H. S. Goripati, T. furubayashi, S. V. Karthik, T. M. Nakatani, Y. K. Takahashi, and K. Hono, J. Appl. Phys. 109, 043901 (2011).

Interfacial resistance and spin-dependent scattering in current-perpendicular-to-plane giant magnetoresistance using Co₂Fe(Al_0.5Si_0.5)/Ag/Co₂Fe(Al_0.5Si_0.5) pseuso spin valves
T. M. Nakatani, T. Furubayashi, and K. Hono, J. Appl. Phys. 109, 07B724 (2011).

Effect of the Number of Layers on Determination of Spin Asymmetries in Curret Perpendicular-to-Plane Giant Magnetoresistance
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Enhancement of the current-perpendicular-to-plane giant magnetoresistance by insertion of Co₅₀Fe₅₀ layers at Co₂Mn(Ga_0.5Sn_0.5)/Ag interface
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Current-perpendicular-to-plane spin valves with a Co₂MnGa_0.5Sn_0.5 Heusler alloy
N. Hase, B. Varaprasad, T. M. Nakatani, H. Sukegawa, S. Kasai, Y. K. Takahashi, T. Furubayashi and K. Hono, J. Appl. Phys. 108, 093916 (2010).

Bulk and interfacial scatterings in current-perpendicular-to-plane giant magnetoresistance with Co₂Fe(Al_0.5Si_0.5) Heusler alloy layers and Ag spacer
T. Nakatani, T. Furubayashi, S. Kasai, H. Sukegawa, S. Mitani, and K. Hono, Appl. Phys. Lett. 96, 212501 (2010).

Structure and transport properties of current-perpendicular-to-plane spin valves using Co₂FeAl_0.5Si_0.5 and Co₂MnSi Heusler alloy electrodes
T. Furubayashi, K. Kodama, T. M. Nakatani, H. Sukegawa, Y. K. Takahashi, K. Inomata, and K. Hono, J. Appl. Phys. 107, 113917 (2010).