Effect of cold work on anisotropic magnetoresistance of Co 25Ni 75

Abstract

Various states of the polycrystalline Co 25Ni 75 sample have been achieved by cold rolling and heat treatments after cold rolling. At each metallurgical stage, the electrical resistivity ρ S //, measured in the saturated condition with a longitudinal field, and the resistivity ρ S ⊥, obtained under a similar condition with a transverse field, were studied at 77K. We used the quantity Δ ρ ///Δ ρ ⊥, defined as ρ S //−ρ 0 / ρ S ⊥−ρ 0 , where ρ 0 is the resistivity in zero field (or a demagnetized state), to check the magnetic anisotropy in the sample. It is found that cold work can induce an easy axis parallel to the roll direction. This phenomenon is associated with the short-range order and/or the directional Co–Co pair order formed during a cold-rolling process. Recovery and recrystallization were observed, if the rolled samples were annealed at various temperatures T Q. Both stress relief and change of texturing lead to an enhancement of magnetic anisotropy of the longitudinal type. The anisotropic magnetoresistance Δ ρ/ ρ 0 of the cold-rolled sample (CR-0) is the lowest. When T Q is increased, Δ ρ/ ρ 0 becomes recovered. However, Δ ρ/ ρ 0 of a plainly salt water quenched sample is always larger than that of a cold-rolled one.