Strain relaxation in epitaxial γ′-Fe4N ultrathin films

Abstract

To investigate substrate-induced strain effects, we grew epitaxial γ’-Fe4N ultrathin films on three different single crystal substrates, MgO, SrTiO3 (STO), and LaSrAlTa oxide. The lattice mismatch was −9.7%, −2.6%, and −1.5% between the Fe4N layer and the MgO, STO, and LSAT substrates, respectively. Therefore, the strain was varied depending on the substrates. Cross-sectional transmission electron microscopy revealed that the lattice strain of Fe4N layer on the STO and LSAT maintained from the interfaces up to ∼2 nm, note here that we achieved to induce strain for the entire Fe4N film thickness. On the other hand, rapid relaxation appeared through many misfit dislocations in the case on MgO. The strain effects in the ultrathin Fe4N films were confirmed as the different behaviors for both ferromagnetic resonance and anisotropic magnetoresistance. These results broaden the amount of modulation in Fe4N based device properties by strain when we reduce the thickness down to several nanometer, which is a dimension encountered in recent spintronic switching devices.