Stress-induced anisotropy in Co-Cr thin films

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 Ts 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, HKeff, was measured for films both as-deposited and removed from the substrate. The differences in effective field, ΔHKeff, were attributed to stress-induced anisotropy. The stress σ was determined by the disk flexure method. From σ and ΔHKeff we determined the magnetostrictive constant, Aλs. Usually A is assigned the value 3/2. p-type (‘‘bulk’’) Co-Cr is characterized by a negative λs, n1-type (‘‘transition layer’’) by a positive λs. 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 Ts, the ‘‘average’’ λs found by conventional methods would vary correspondingly. Also, with the addition of 4πMs data, we determine the intrinsic magnetocrystalline anisotropy fields of the film constituents. At Ts =155 °C the intrinsic p-type magnetocrystalline anisotropy field is 85% that of pure cobalt.