Stress-induced anisotropy in Co-Cr thin films

Abstract

A modified Varian dc magnetron sputtering system was used at an Ar pressure of 2 mTorr to prepare a sequence of 500‐nm‐thick Co₇₈Cr₂₂ films. From run to run the substrate temperature T_s was varied from close to room temperature to an upper limit of 300 °C. With a 33‐GHz ferromagnetic resonance (FMR) spectrometer the effective anisotropy field, H_Keff, was measured for films both as‐deposited and removed from the substrate. The differences in effective field, ΔH_Keff, were attributed to stress‐induced anisotropy. The stress σ was determined by the disk flexure method. From σ and ΔH_Keff we determined the magnetostrictive constant, Aλ_s. Usually A is assigned the value 3/2. p‐type (‘‘bulk’’) Co‐Cr is characterized by a negative λ_s, n₁‐type (‘‘transition layer’’) by a positive λ_s. Consequently, the tensile‐type stress associated with our deposition scheme increases the positive effective anisotropy of p‐type film constituents (by as much as 50%) and also increases (in a negative sense) the negative effective anisotropy of n₁‐type film constituents (by as much as 20%)! Since the relative p‐ and n‐type proportions vary with T_s, the ‘‘average’’ λ_s found by conventional methods would vary correspondingly. Also, with the addition of 4πM_s data, we determine the intrinsic magnetocrystalline anisotropy fields of the film constituents. At T_s =155 °C the intrinsic p‐type magnetocrystalline anisotropy field is 85% that of pure cobalt.