Origin of negative anisotropic magnetoresistance effect in Fe0.75Co0.25 single-crystal thin films upon Ir addition

The anisotropic magnetoresistance (AMR) effect is one of the fundamental spin-dependent transport phenomena in ferromagnets and has been the subject of numerous experimental observations. However, the origin of AMR including the sign change of the magnetoresistance has not been fully clarified theoretically. In this paper, we observe a large negative AMR ratio in Fe0.75Co0.25 single-crystal thin films upon Ir addition and elucidate its origin by a theoretical model. (Fe0.75Co0.25)(100-x)Ir-x composition-spread thin films with x up to 11% are fabricated on MgO(100) substrates by combinatorial sputtering technique. From x-ray diffraction results, the metastable B2-ordered phase of Fe3Co-Ir is detected from x = 2.1%, which does not appear in the bulk equilibrium phase diagram. The B2 ordering is also confirmed by scanning transmission electron microscopy. The AMR ratio of the pure Fe3Co shows a small positive value of 0.3%. In contrast, once the Ir atoms are added, the AMR ratio becomes negative, and it exhibits the largest negative values of -4.7% at 10 K and -3.6% at 300 K for x = 11%. Using a theoretical model, the sign change of the AMR ratio from positive to negative upon B2 ordering is obtained for high-Ir concentration, which agrees with the experimental results. Thus, the metastable B2 ordering would be the key to observing the negative AMR. These findings provide deep insight into the origin of AMR in heavy-metal-doped ferromagnetic ordered alloys.