Giant enhancement of magnetocrystalline anisotropy in ultrathin manganite films via nanoscale 1D periodic depth modulation

We report a unusual giant enhancement of in-plane magnetocrystalline anisotropy (MCA) in ultrathin colossal magnetoresistive oxide films due to 1D nanoscale periodic depth modulation. High quality epitaxial thin films of La$_{\mathrm{0.67}}$Sr$_{\mathrm{0.33}}$MnO$_{\mathrm{3}}$ (LSMO) of thickness 6 nm were grown on (001) SrTiO$_{\mathrm{3}}$ substrates via off-axis radio frequency magnetron sputtering. The top 2 nm of LSMO films are patterned into periodic nano-stripes using e-beam lithography and reactive ion etching. The resulting structure consists of nano-stripes of 2 nm height and 100-200 nm width on top of a 4 nm thick continuous base layer. We employed planar Hall effect measurements to study the in-plane magnetic anisotropy of the unpatterned and nanopatterned films. The unpatterned films show a biaxial anisotropy with easy axis along [110]. The extracted anisotropy energy density is \textasciitilde 1.1 x 10$^{\mathrm{5}}$ erg/cm$^{\mathrm{3}}$, comparable to previously reported values. In the nanopatterned films, a strong uniaxial anisotropy is developed along one of the biaxial easy axes. The corresponding anisotropy energy density is \textasciitilde 5.6 x 10$^{\mathrm{6}}$ erg/cm$^{\mathrm{3\thinspace }}$within the nano-striped volume, comparable to that of Co. We attribute the observed uniaxial MCA to MnO$_{\mathrm{6}}$ octahedral rotations/tilts and the enhancement in the anisotropy energy density to the strain gradient within the nano-stripes.