Half-metallic ferromagnets (HMFs) have attracted much interest as promising materials in spintronics devices since they generate highly spin-polarized current due to the absence of one spin-channel at the Fermi level (EF). Among the known HMFs, the use of Co2-based Heusler alloys enables enhancing the giant-magnetoresistance, tunnel-magnetoresistance, and spin-accumulation effects, thereby improving the performance of spintronic devices [1-3]. Despite the large magnetoresistance (MR) ratio observed in Heusler based devices, the structural disorder is still one of the remaining issues which lower the P. Therefore, it is essential to have a highly ordered Heusler structure to realize the half-metallicity. The structural order (B2→L21) of the Heusler alloys e.g., Co2MnSi [4] and Co2FeGe0.5Ga0.5 [1] have been promoted by annealing at high temperatures (&gt;500°C) resulting in an enhanced MR ratio. However, for various applications, the permissible maximum annealing temperature is restricted, e.g., for a magnetic read head in a hard disk drive (HDD) the annealing temperature is limited to 300°C because of the temperature tolerance of the NiFe shield. Therefore, it is desirable to search for other Heusler alloys that crystallize in L21-order below 300°C. The present work is motivated by Okubo et al. [5]’s report, where the L21 to B2-order transition temperature of Co2MnZ (Z = Ge, Sn) Heusler alloys is very high (&gt;1500K) suggesting high energetical stability of L21-structure; and hence L21-order is expected to appear even in the as-deposited state or by low-temperature annealing. Co2MnZ (Z = Ge, Sn) and reference Co2FeGe0.5Ga0.5 films were deposited on MgO (001) single crystal substrate using an ultra-high vacuum magnetron sputter system at room temperature and then post-annealed in-situ at Tann = 200-700°C to induce the atomic-ordering. Here, Co50Fe50 (3nm) was used as a buffer layer to deposit Co2MnZ films to suppress a phase separation. Co2MnSn films with two different compositions Co:Mn:Sn = 53.4:29.3:17.3 and 47.4:25.5:27.1 were deposited, which we refer to as Sn-poor and Sn-rich films, respectively. The crystal structure of the films was analyzed using a high-resolution X-ray diffractometer with Cu-Kα radiation. Finally, to quantify the P of the conduction electron in the bulk region, Co2MnZ and Co2FeGe0.5Ga0.5 films were microfabricated into a non-local spin valve (NLSV) device using electron-beam lithography, Ar-ion milling, and the lift-off techniques. X-ray diffraction revealed that Co2MnZ and Co2FeGe0.5Ga0.5 films are epitaxially grown along (001) direction. The degree of L21-ordering (SL21) as a function of Tann of Co2MnZ, Co2FeGe0.5Ga0.5, and Co2MnSi [4] films are shown in Fig.1(a). In the case of Co2FeGe0.5Ga0.5 and Co2MnSi, L21-ordering was observed above 500°C. Interestingly, in Co2MnZ films, L21-ordering appeared even in the as-deposited state and it enhances systematically with increasing Tann. Figure 2(a) shows a scanning electron microscopy (SEM) image of a typical NLSV device using Co2MnGe and Cu and the schematic of non-local measurement. We observed a clear spin-accumulation signal (△RS) in all our NLSV devices, which decays with increasing the gap-length (d); as Fig.2(b) shows a representative △RS-d curve of Co2MnGe film (Tann = 300°C). To evaluate P, the measured △RS in the Cu channel was analyzed by one-dimensional spin diffusion model [6]. The evaluated P of the films are displayed in Fig.1(b), which varies systematically with increasing Tann. The spin-polarization P of Co2MnGe at Tann = 300°C is higher (~0.65) than that of Co2MnSn and Co2FeGe0.5Ga0.5 due to a higher SL21, suggesting that Co2MnGe is a promising material for various practical applications. For Co2MnSn, however, low P was found in both compositions, despite high SL21. Because first-principle calculations predict highly spin-polarized electronic structure in the Sn-rich composition, it is desirable to check the microstructure by transmission-electron microscopy to understand the origin of low P. In conclusion, we elucidated that Co2MnGe is a promising material to obtain high atomic order and large P in the sputtered film without high-temperature annealing. Since the present Co2MnGe film has a slightly Co-rich composition which is not suitable to obtain high P due to the formation of the Co antisite disorder, optimization of the composition ratio is expected to lead to higher P.

**References:**

[1] S. Li et al., Appl. Phys. Lett. 103, 042405 (2013). [2] H. Liu et al., Appl. Phys. Lett. 101, 132418 (2012). [3] Ikhtiar et al., J. Appl. Phys. 115, 173912 (2014). [4] Y. Sakuraba et al., Phys. Rev. B 82, 094444 (2010). [5] A. Okubo et al., Appl. Phys. Lett. 96, 222507 (2010). [6] S. Takahashi and S. Maekawa, Phys. Rev. B 67, 052409 (2003).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8902ea3d787672d1d6b6265fa50d0219.jpg)
Fig.1. Tann dependence of (a). SL21, and (b). bulk spin-polarization P measured in NLSV devices. SL21 exceeding 1 is expected to be caused by off-stoichiometry in the films.

![](https://assets.underline.io/uploads/markdown_image/1/image/1c520a3f468d3d48a7c2ba59af666eb7.jpg)
Fig.2. (a). An SEM image of a typical NLSV device using Co2MnGe and Cu, and the schematic of non-local measurement. (b). The gap-length (d) dependence of △RS of NLSV based on Co2MnGe film (Tann = 300°C). The solid line indicates the fitting curve using one-dimensional spin diffusion model [6].

 Enhancement of L21-atomic order and spin-polarization in Co2MnZ (Z = Ge, Sn) Heusler thin films by low-temperature annealing process

*Session Chair: Julius de Rojas, Universitat Autonoma de Barcelona*

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Enhancement of L21-atomic order and spin-polarization in Co2MnZ (Z = Ge, Sn) Heusler thin films by low-temperature annealing process

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