Ni₂MnGa-type ferromagnetic shape memory alloy is one of the most promising candidates for smart systems. Thin film formation of Ni₂MnGa alloy is especially effective for the micro-systems application. Recently, we found that the heat-treated Ni-Mn-Ga-Fe sputtered films with Heusler type crystal structure exhibited both magnetic field induced shape memory effects and magnetoresistance (MR) effects [1]. The MR properties of the films are of interest in view of application to the smart systems. Addition of 3d transition element M such as Co or Fe to the Ni-Mn-Ga films has a potential for expansion of the operating range, because it enhances the Curie temperature T_C. Therefore, our purpose is to investigate the effects of Fe and Co addition on the MR in Ni-Mn-Ga films. Films with different Fe and Co composition x were prepared by co-sputtering method. The obtained films were heat-treated at 1073 K for 1 h and aged at 673 K for 1 h in a constrained condition. MR and magnetization σ were measured at fixed temperatures as a function of magnetic fields of up to 15 kOe by using four-point probe and vibrating sample methods, respectively. Martensitic transformation temperature T_M was determined by resistance-temperature measurements. With increasing x, σ enhanced for M = Fe, while it reduced for M = Co. These results are consistent with those in bulk alloys [2]. At 300 K, MR value at 15 kOe MR₁₅ was as low as 1%. The value of MR₁₅ was found to be decreased with increasing Fe content. In the case of Co, MR₁₅ increased with x. From these results, it was suggested that an origin of the MR in the present films is dominated by a s-d scattering model [3].

[1] K. Koike, M. Ohtsuka, Y. Honda, H. Katsuyama, M. Matsumoto, K. Itagaki, Y. Adachi and H. Morita, J. Magn. Magn. Mater. 310 (2007), e996.

[2] D. Kikuchi, T. Kanomata, Y. Yamaguchi, H. Nishihara, K. Koyama, H. Watanabe, J. Alloys Compd. 383 (2004), 184.

[3] M. Kataoka, Phys. Rev. B 63 (2001), 134435.

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