A Study of Strain Effects in Ni Alloys

Abstract

A method for producing uniaxially strained films on an unstrained substrate is described. These films were used to measure the strain sensitivity for γ-phase Ni-Fe and Ni-Co alloys. They were also used to investigate the process of strain relaxation in thin films. The experimental strain sensitivity was found to be roughly half of the strain sensitivity predicted from bulk material properties for all alloys measured. The strain sensitivity was predicted from bulk magnetoelastic constants by assuming that a film is uniformly strained when its substrate is bent. In the limit of zero thickness, this assumption of a uniform strain is undoubtedly correct. Since no thick-ness dependence was found for films between 64 and 2800 A, the uniform strain model should apply to all normal thicknesses. The applied strain sensitivity was found to be independent of strain in agreement with the model of uniform strain. All experimental results are consistent with the assumption that thin film elastic constants are roughly half of bulk elastic constants. A simple model for uniaxial strain relaxation by volume diffusional creep, roughly predicted the dependence of strain upon annealing time and temperature. A method for determining a single activation energy for the complex process of strain relaxation was found. This activation energy (2.4 ev) is in good agreement with the model used. It was concluded that the dominant mechanism for strain relaxation in thin films is volume diffusional creep.