Enhancement of Magnetic Anisotropy in Ultrathin Epitaxial La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ Thin Films via Nanostructure Engineering

We report a more than ten-fold enhancement of magnetic anisotropy in nanostructured La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) thin films grown epitaxially on (001) SrTiO$_{3}$ substrates. We have etched periodic linear trenches in 6 nm LSMO films, and investigated magnetic anisotropy in these nanostructured thin films via the planar Hall effect (PHE). These trenches have depth of 2 nm and periodicities of 200 - 400 nm. The PHE resistance of the un-patterned LSMO films exhibits sinusoidal angular dependence in an in-plane magnetic field, and shows four-fold sharp resistance switching below a critical magnetic field of 400 Oe, corresponding to a biaxial magnetic anisotropy of $\sim$ 1x10$^{5}$ erg/cm$^{3}$ along \textless 110\textgreater directions. In the nanostructured samples, we observe an additional two-fold resistance switching feature, which persists in magnetic fields higher than 4000 Oe, corresponding to a uniaxial magnetic anisotropy \textgreater 1x10$^{6}$ erg/cm$^{3}$ along one of the biaxial magnetic easy axes. This significant enhancement of magnetic anisotropy cannot be accounted for by shape anisotropy or a uniform strain modulation. We also discuss the effects of the orientation and periodicity of the nano-trenches on the anisotropy enhancement.