Transverse anisotropic magnetoresistance effects in pseudo-single-crystal γ′-Fe₄N thin films

Transverse anisotropic magnetoresistance (AMR) effects, for which magnetization is rotated in an orthogonal plane to the current direction, were investigated at various temperatures, in order to clarify the structural transformation from a cubic to a tetragonal symmetry in a pseudo-single-crystal Fe₄N film, which is predicted from the usual in-plane AMR measurements by the theory taking into account the spin-orbit interaction and crystal field splitting of 3d bands. According to a phenomenological theory of AMR, which derives only from the crystal symmetry, a cos 2θ component (C₂^tr) exists in transverse AMR curves for a tetragonal system but does not for a cubic system. In the Fe₄N film, the C₂^tr shows a positive small value (0.12%) from 300 K to 50 K. However, the C₂^tr increases to negative value below 50 K and reaches to -2% at 5 K. The drastic increasing of the C₂^tr demonstrates the structural transformation from a cubic to a tetragonal symmetry below 50 K in the Fe₄N film. In addition, the out-of-plane and in-plane lattice constants (c and a) were precisely determined with X-ray diffraction at room temperature using the Nelson-Riely function. As a result, the positive small C₂^tr above 50 K is attributed to a slightly distorted Fe₄N lattice (c/a = 1.002).