Ab initio study of magnetism at iron surfaces under epitaxial in-plane strain

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We investigated magnetism at the (001) surface of iron and its response to the epitaxial in-plane strain that corresponds to bcc-fcc (Bain’s) transformation path using ab initio (first-principles) spin-density-functional theory calculations within the generalized gradient approximation. The magnetic moment is enhanced at the surface of a ferromagnetic (FM) film under a strain-free condition. This was caused by electron rearrangement from the minority-t2g to majority-t2g state due to the decrease in nearest neighbors at the surface. Under in-plane strain, the magnetic and structural phase transition from the FM-bcc to double-layer-antiferromagnetic-fcc occurred at the critical strain of ε=−0.09, accompanying directional bond switching from the nearest-to second-nearest neighbors in the minority spin. The transition caused a discontinuous change in the magnetic moments on the inner layers of the film across the transition, while the magnetic moment of the surface layer was rather insensitive. This was because the electron rearrangement from the t2g to eg states during the transition was limited to within the minority spin due to the fully occupied majority spin state.