Stress Effects on the Magnetic Properties of Evaporated Single-Crystal Nickel Films

Abstract

Nickel single‐crystal films grown epitaxially on NaCl at elevated temperatures shown anomalously large values for the crystalline anisotropy constant, K₁, as well as for K₁, the perpendicular anisotropy (i.e., the anisotropy resulting from rotation out of the plane of the film). These values decrease to normal bulk values when the films are floated off the rocksalt. It has been shown by x‐ray diffraction that the as evaporated films exist in a highly strained condition, resulting in a tetragonal distortion of the cubic symmetry. The parameters of the unit cell change from a₀=3.524 A for unstrained nickel to a=b=3.500 A in the plane of the film and c=3.546 A normal to the film. It is also shown that the stress causing this strain is elastic in character and is relieved by floating the film off the rocksalt. A calculation using bulk elasticity data yields a planar compressive stress σ=−1.2×10¹⁰ d/cm². The magnitude and direction of the stress indicates it results from the difference in thermal expansion between the nickel film and the NaCl substrate. This externally applied stress system influences the magnetic state of the film by contributing to the total energy of the system through a magnetoelastic interaction. Employing the five‐constant magnetostrictive equation, it is shown that a planar compressive stress of this magnitude does result in an anomalously high crystalline anisotropy value, as well as an anomalously high value of the anisotropy perpendicular to the plane of the film.