Enhancement of giant magnetoresistance by L21 ...

APPLIED PHYSICS LETTERS 103, 042405 (2013)

Enhancement of giant magnetoresistance by L21 ordering in Co2Fe(Ge0.5Ga0.5) Heusler alloy current-perpendicular-to-plane pseudo spin valves S. Li, Y. K. Takahashi, T. Furubayashi, and K. Hono Magnetic Materials Unit, National Institute for Materials Science, Tsukuba 305-0047, Japan

(Received 5 June 2013; accepted 6 July 2013; published online 23 July 2013) We report large magnetoresistance (MR) output in fully epitaxial Co2Fe(Ge0.5Ga0.5)/Ag/ Co2Fe(Ge0.5Ga0.5) current-perpendicular-to-plane pseudo spin valves. The resistance-area product change (DRA) of 12 mXlm2 at room temperature (RT), equivalent to MR ratio of 57%, and DRA ¼ 33 mXlm2 at 10 K, equivalent to MR ratio of 183%, were obtained by using L21-ordered Co2Fe(Ge0.5Ga0.5) ferromagnetic electrodes. The bulk spin scattering asymmetry (b) were estimated to be 0.83 at RT and 0.93 at 10 K for the L21-ordered Co2Fe(Ge0.5Ga0.5) films by the Valet-Fert model, indicating that the L21-ordered Co2FeGe0.5Ga0.5 Heusler alloy is virtually C 2013 AIP Publishing LLC. half-metal at 10 K, but its half-metallicity is degraded at RT. V [http://dx.doi.org/10.1063/1.4816382] Considering the issues such as impedance matching, data transfer frequency, and signal to noise ratio in a hard disk drive (HDD), the read sensors with low device resistance and high magnetoresistance (MR) output are desired to match the continuous increase in the areal density in magnetic recording.1,2 Due to the intrinsic difficulties in reducing the device resistance of MgO-based magnetic tunnel junctions (MTJs),3–6 metallic current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices have received attention as an alternative reader candidate for future ultrahigh density HDDs.7,8 The latter features a low device resistance-area product (RA) much smaller than 300 mXlm2 that is considered to be the upper limit for achieving the recording density exceeding 2 Tbit/in2 in HDDs.2 Starting from the early report on MR ratios of several percent of CPP-GMR,9–11 notable developments on achieving high MR output have been made by using highly spin-polarized ferromagnetic layers and appropriate spacer materials.12–18 These investigations have shown that the usage of Co-based Heusler alloy is an effective way to enhance the MR output in CPP-GMR devices due to its half-metallic nature at least at low temperature as predicted by density state calculations.19,20 Co-based Heusler alloy films usually have the A2 structure in the as sputter-deposited state, and post-deposition thermal annealing is needed to make it order to either B2 or L21 structure that causes high spin polarization. However, the interdiffusion, a side effects of annealing, has been found to degrade the MR output distinctly as reported in Co2MnSi21 and Co2Mn(Ga,Sn)22 based CPP-GMR devices. In order to improve the robustness of multilayered structure against high temperature annealing, small misfit strains and low solubility among constituent layer materials are necessary. Compared to Co2MnZ-type Heusler alloy, the Co2FeZ type shows better thermal tolerance for a Ag spacer.14 We have previously reported a large MR output (MR ratio ¼ 41.7% with DRA ¼ 9.5 mXlm2 at room temperature (RT), and MR ratio ¼ 129.1% with DRA ¼ 26.4 mXlm2 at 10 K) in CPP-GMR devices using Co2Fe(Ge0.5Ga0.5) (hereafter CFGG) Heusler alloy.16,23 The spin polarization of 0003-6951/2013/103(4)/042405/4/$30.00

Heusler alloy is generally degraded by the presence of chemical disorder from the ideal L21 structure. The CFGG layer used in the previous work was B2-disordered with respect to the L21 order. The CPP-GMR with L21-ordered CFGG ferromagnetic (FM) layers has not been investigated so far because of the sluggish kinetics of the L21 ordering from the B2 structure. For sputter-deposited CFGG films, an annealing temperature above 500 C was needed to attain high degree of the L21 order, but the MR outputs drop by annealing above 500 C due to the diffusion of Ga and Ge to the Ag/Ru interface for capping.24 In this letter, we report further enhancement of the MR output in CPP-GMR devices with L21-ordered CFGG Heusler alloy. Fully epitaxial pseudo spin valves (PSVs) of CFGG (tF)/ Ag(5 nm)/CFGG(tF), where tF is the thickness of the CFGG layer in nm, were deposited on Cr(10 nm)/Ag(100 nm) buffer layers that were grown on (001)MgO single crystalline substrates. All the film layers were deposited at RT using an ultrahigh vacuum magnetron sputtering machine. tF was varied from 2.5 to15 nm in order to deduce bulk and interfacial scattering spin asymmetries using the Valet-Fert fitting from the DRA dependence on tF. The Cr/Ag bi-layers served as buffer layers for getting a smooth growth for CFGG layers, and as a bottom electrode for CPP-GMR measurement. The film stacks were then annealed in situ at 500–650 C for 30 min to induce the chemical order in the CFGG films. Ag(5 nm)/Ru(8 nm) capping layers were then deposited after cooling down the sample to RT. Our previous study has shown that the Ge and Ga diffuse to the Ag/Ru interface of the capping layers during high temperature annealing, thereby degrading MR outputs. In order to suppress this undesired interlayer diffusion, in-situ annealing treatment was applied before depositing the Ag/Ru capping layers in this study. The film stacks were patterned into elliptical pillars ranging from 70 140 to 200 400 nm2 through conventional electron beam lithography and argon ion milling methods. The actual pillar area was measured by a scanning electron microscopy to estimate DRA accurately. The CPPGMR output was measured by the dc four-probe method in a

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temperature range of 300–10 K. 4-axis x-ray diffraction (XRD) was used to examine the crystalline structure and transmission electron microscopy (TEM) was employed for microstructure analysis. Figure 1 shows DRA measured at RT as a function of the in-situ annealing temperature Ta. tF was fixed to 10 nm for the samples annealed at different temperature. The DRA dependence on in-situ Ta (solid line) is different from our previous results for ex-situ annealed samples (broken line), which were annealed with the Ag/Ru capping layers.24 The DRA increases with Ta up to 600 C, above which it vanishes at Ta ¼ 650 C. The average DRA obtained at Ta ¼ 600 C is 11.361.3 mXlm2, which is much larger than that obtained from ex-situ annealed sample, 8.860.9 mXlm2 with Ta ¼ 500 C that was the optimum temperature for the ex-situ annealed samples. Note that the MR output of the ex-situ annealed samples began to drop at Ta ¼ 550 C, while the optimum temperature for the in-situ annealed samples is 600 C. We found the diffusion of Ge and Ga to the Ag/Ru interface of the capping layers in the sample ex-situ annealed at 550 C.24 We did not expect that the interface of capping layers would influence the structure and magnetoresistive properties of the CPP-GMR devices. One possible reason for the Ge and Ga diffusion is the high interfacial energy at the Ag/Ru interface in the capping layer. Ge and Ga accumulate at the interface of Ag/Ru to relax the high misfit strains during the ex-situ annealing process. With this assumption, we expected that the Ge and Ga diffusion would be suppressed if we anneal the device without the capping layers. Thus, we annealed the CFGG/Ag/CFGG PSVs before depositing the capping layer (in-situ annealing), resulted in higher MR output by annealing the film stack at 600 C. Figure 2(a) shows the h–2h x-ray diffraction patterns of {111} plane of CFGG films after annealed in situ at different temperature. The intensity of the superlattice (111) peak reflects the degree of the L21 order in the CFGG Heusler alloy layers. For the sample annealed at 500 C, no (111) peak was detected. Distinct (111) peaks were clearly observed for the samples annealed above 550 C. The intensity of superlattice (200) and fundamental (400) peaks was also determined for {001} plane of CFGG layers (data was not shown here). The integral intensity ratio I200/I400 and

FIG. 1. DRA as function of in-situ annealing temperature Ta (solid line) for CFGG(10 nm)/Ag(5 nm)/CFGG(10 nm) PSVs. DRA vs ex-situ Ta (broken line) was attached for comparison after Ref. 24.

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FIG. 2. (a) XRD profiles of h–2h scan of MgO(001)sub./Cr(10 nm)/ Ag(100 nm)/CFGG(10 nm)/Ag(5 nm)/CFGG(10 nm)/Ag(5 nm)/Ru(8 nm) films after annealing at Ta ¼ 500–650 C. Sample holder was tilted to v ¼ 54.7 to make the CFGG {111} plane in horizontal. (b) Integral intensity ratio I200/I400 and I111/I444 as functions of Ta, which qualitatively indicated the degrees of the chemical ordering of B2 (circles) and L21 (squares).

I111/I444, which are proportional to the square of the degree of B2 and L21 orders, were plotted with annealing temperature as shown in Fig. 2(b). I200/I400 was found to around 0.06 in the rage of Ta ¼ 500–650 C. Compared with the theoretical value, it means CFGG films have almost perfect degree of B2 order in the annealing conditions of Ta ¼ 500–650 C. I111/I444 was found to increase with increasing Ta. It starts from 0 to 0.052 by increasing Ta to 650 C, where the degree of L21 order was estimated to around 0.58, although it was not perfectly L21 ordered. The L21 ordering in the CFGG layer leads to the increase in the MR output from Ta ¼ 500 C to 600 C. For the film stack annealed at 650 C, however, cave-like cracks were found to be formed on the film surface using atomic force microscopy, which caused current leakage in the micro-fabricated devices. Because of this, only a very small MR output was obtained in the microfabricated devices in the sample annealed at Ta ¼ 650 C, despite the highest degree of L21 order shown in Fig. 2(b). Figure 3(a) presents a high-angle annular dark-field (HAADF) image taken from the cross-sectional specimen of the PSV(tF ¼ 10 nm) after in-situ annealed at 600 C, where we can see the sharp and flat CFGG/Ag interfaces after annealing at such a high temperature. Figs. 3(b) and 3(c) show the nanobeam electron diffraction patterns of the top and bottom CFGG layers taken from the [011] zone axis. Both CFGG layers show {001} and {111} superlattice spots corresponding to the L21 order. Fig. 3(d) shows the energy dispersive x-ray spectroscopy (EDS) maps taken from the selected region in Fig. 3(a) using a probe diameter of 0.5 nm.


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FIG. 3. (a) HAADF cross-sectional TEM image of MgO(001)sub./Cr(10 nm)/ Ag(100 nm)/CFGG(10 nm)/Ag(5 nm)/CFGG(10 nm)/Ag(5 nm)/Ru(8 nm) films after in-situ annealing at Ta ¼ 600 C. Electron nanobeam diffraction patterns of (b) top and (c) bottom CFGG layer. (d) EDS mapping results for elements of Ru, Ag, Co, Fe, Co, Ga, and Ge.

No significant interdiffusion between CFGG layer and Ag spacer layer was detected within the resolution limit of the EDS mapping, indicating a good thermal stability of CFGG/ Ag/CFGG tri-layered structure even at the annealing temperature of 600 C. Figure 4(a) shows a typical MR loop for CFGG (10 nm)/ Ag/CFGG(10 nm) PSV annealed at Ta ¼ 600 C, measured at

FIG. 4. (a) Typical MR curve of CFGG(10 nm)/Ag(5 nm)/CFGG(10 nm) PSV with Ta ¼ 600 C, measured at RT. (b) DRA dependence on measuring temperature T.

Appl. Phys. Lett. 103, 042405 (2013)

RT. A large DRA ¼ 12.0 mXlm2 was observed at RT. This value is comparable to that was recently reported in CPPGMR PSVs using Co2(Fe0.4Mn0.6)Si Heusler alloy, one of the highest value reported for CPP-GMR devices using Heusler alloy.18 In order to estimate the intrinsic MR ratio, DR/(R–Rparasitic), parasitic lead resistance, Rparasitic ¼ 0.27 X at RT, has been subtracted by measuring the device resistance dependence of pillar size; as a result, we found DRA ¼ 12.0 mXlm2 corresponds to the intrinsic MR ratio of 57% at RT. These values were further enhanced as measurement temperature decreases, and DRA ¼ 33 mXlm2 was recorded at 10 K as shown in Fig. 4(b), which corresponds to the intrinsic MR ratio of 183%. The DRA shows a large dependence on measurement temperature. The DRA of 33 mXlm2 at 10 K decreases to 12 mXlm2 at RT, nearly one third of the value at low temperature. In order to understand the underlying physics of the high MR output and its large temperature dependence, we deduced the bulk spin asymmetry b and interfacial spin asymmetry c based on the Valet-Fert model.24 Figure 5 shows the tF dependence of DRA for two measurement temperature at RT and 10 K, respectively. The Valet-Fert fitting derived quite large b of 0.83 at RT and 0.93 at 10 K, which are larger than the values for our previously reported 0.73 at RT and 0.9 at 10 K in B2-ordered CFGG films. A short spin diffusion length ‘sf of 23 nm derived here is a typical value for the Co-based Heusler alloy.25 The b value is considered to correspond to the current spin polarization of the FM layer. The increase in the b value compared to the previous report on ex-situ annealed device implies an enhanced spin polarization from B2 to L21-ordered CFGG films. b ¼ 0.93 indicates that the L21- ordered CFGG is virtually a halfmetal at 10 K, but its half-metallicity is somehow degraded at RT to b ¼ 0.83. Note that the Curie temperature of CFGG is 1080 K, far above the RT and little change in the saturation magnetization, Ms, was detected at RT compared to that at 10 K. This suggests that the main reason for the degradation of MR at RT is the thermal degradation of the halfmetallicity of L21-ordered CFGG, although its underlying mechanism is still not clear. In order to estimate the contribution from the interfacial spin-dependent scattering asymmetry, c value must be

FIG. 5. FM layer thickness tF dependence of DRA fitted to the Valet-Fert model, measured at RT (squares), and 10 K (circles) for CFGG(tF)/ Ag(5 nm)/CFGG(tF) PSVs.


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deduced as well; however, ferromagnetic layer/nonmagnetic spacer (F/N) interfacial resistance, RF/N, must be measured for the deduction of c.26 The usual way to estimate RF/N is to measure the RA dependence on the number of interfaces in [FM/N]n multilayers. We noticed that the RF/N estimated by this way involves a large experimental uncertainty. We previously showed the c value derived in CFGG-based PSVs after annealed at 500 C was 0.6 with an uncertainty of 60.22 at RT, and 0.8460.14 at 10 K with considering the uncertainty of RF/N. By simply assuming the RF/N value is the same as that in the CFGG/Ag/CFGG PSVs with Ta ¼ 500 C, we adopted RF/N ¼ 0.7660.25 mXlm2 at RT and RF/N ¼ 0.6260.25 mXlm2 at 10 K. By this way, the c values were roughly deduced to be 0.6360.07 at RT, and 0.8860.05 at 10 K, respectively. This indicates that the c value has a correlation with b, and high b gives rise to high c. Note that the c for conventional Co/Cu CPP-GMR was estimated to be 0.77 at 4.2 K,27 so c for L21-ordered CFGG/ Ag interface is relatively large. This work has demonstrated that large DRA of 12 mXlm2 at RT can be obtained from epitaxial CFGG/Ag/ CFGG PSVs with relatively low RA of 21 mXlm2. The low temperature value is 2.75 times larger than the RT value, 33 mXlm2, which is sufficient output required for reader applications. Such large temperature dependence of DRA is usually not observed in the CPP-GMR using the standard FM layers such as CoFe, and this is attributed to the temperature degradation of the half-metallicity of CFGG. This means that if we can find a way to suppress the temperature degradation of the spin polarization, much higher MR is expected at RT. Apart from it, the MR sensors for reader applications must be grown on thermally oxidized Si substrates for industrial viability, and there is a upper limit for annealing temperature (350 C) from the thermal tolerance of soft magnetic shield. So the high MR value demonstrated using the CFGG/Ag/CFGG PSVs epitaxially grown on (001)MgO single crystalline substrates is not immediately applicable to reader applications. However, our recent work on polycrystalline PSVs using Co2Fe(Al,Si) and Co2Fe(Ga,Ge) has shown that coherent interface can be obtained even in the polycrystalline films because low energy orientation relationships tend to be established between Heusler alloy grains and Ag spacer grains.28 This means the MR value demonstrated using epitaxial PSVs should be reproducible if the polycrystalline multi-layered structure can be optimized. Higher degree of L21 order may be achieved by in-situ growth of Heulser alloy films on heated substrates at a much lower temperature. In summary, we achieved large output in CPP-GMR devices by using L21-ordered CFGG Heusler alloy with suppressed chemical diffusion through in-situ annealing process. MR ratio of 57% with DRA ¼ 12 mXlm2 at RT, and MR ratio of 183% with DRA ¼ 33 mXlm2 at 10 K were demonstrated. The Valet-Fert fitting yielded bulk spin scattering asymmetries of b 0.83 at RT and b 0.93 at 10 K, which corresponds to the large MR output in the CFGG Heusler alloy based CPP-GMR devices. This temperature

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dependence of the bulk spin scattering asymmetry explains the large temperature dependence of the MR output; thus, finding a way to keep the half-metallic state of CFGG at RT is essential to further improve the MR output at RT. We thank Dr. Y. Sakuraba for valuable discussions and S. Kasai for assistance on low temperature measurement. This work was in part supported by the Grant-in-Aid for Scientific Research (A) (Grant No. 22246091) and Advanced Storage Technology Consortium (ASTC), IDEMA. 1

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