Perpendicular magnetic anisotropy, domains, and misfit strain in epitaxial Ni/Cu1-xNix/Cu/Si (001) thin films.

Abstract

We have investigated the magnetic anisotropy in epitaxial Ni/${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ni}}_{\mathit{x}}$/Cu/Si (001) thin films (0x50%) as a function of Ni thickness h and alloy substrate composition. Also the average in-plane biaxial strain ${\mathit{e}}_{0}$(h) in Ni/Cu (001) (x=0) was measured ex situ versus Ni thickness using optical interferometry. We observed that the preferred direction of magnetization is perpendicular to the films over an exceptionally broad Ni thickness range for films kept under UHV: 10 \AA{}h60 \AA{} in Ni/Cu (001); 20 \AA{}h40 \AA{} in Ni/${\mathrm{Cu}}_{60}$${\mathrm{Ni}}_{40}$/Cu (001). We have also studied the perpendicular magnetic anisotropy by magnetic force microscopy and observed a complex domain pattern characterized by two length scales and very strong contrast. We have analyzed our results using a phenomenological model that includes the bulk magnetoelastic anisotropy energy (${\mathit{B}}^{\mathit{b}}$${\mathit{e}}_{0}$) and both the surface magnetocrystalline (${\mathit{K}}^{\mathit{s}}$/h) and the surface magnetoelastic (${\mathit{B}}^{\mathit{s}}$${\mathit{e}}_{0}$/h) anisotropy energies. Our analysis yields the magnetic surface energies ${\mathit{K}}^{\mathit{s}}$ and ${\mathit{B}}^{\mathit{s}}$ of the vacuum/Ni (001) and Ni/Cu (001) interfaces. The origin of the strong perpendicular magnetic anisotropy lies in the surface energy ${\mathit{K}}^{\mathit{s}}$ corresponding to the Ni/Cu (001) interface and the bulk magnetoelastic anisotropy energy. The surface magnetoelastic anisotropy energy ${\mathit{B}}^{\mathit{s}}$${\mathit{e}}_{0}$ favors an in-plane magnetization. The effective magnetoelastic coupling coefficient depends strongly on h for h\ensuremath{\le}150 \AA{} and changes sign near h\ensuremath{\approxeq}28 \AA{}. The two observed in-plane to out-of-plane magnetization easy-axis transition thicknesses are predicted by our pheomenological model. The lower magnetization easy-axis transition is not due to the onset of misfit dislocations at the Ni/Cu interface.