Temperature and thickness dependence at the onset of perpendicular magnetic anisotropy inFePdthin films sputtered onMgO(001)

Abstract

The onset of chemical order ($L{1}_{0}$ phase) and perpendicular magnetic anisotropy in $\mathrm{FePd}(001)$ thin films sputtered on $\mathrm{MgO}(001)$ at temperatures from room temperature to $700\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and thickness between 1.4 and $22\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ are investigated. It is found that the formation of the $\mathrm{FePd}$ ordered phase exhibiting high perpendicular magnetic anisotropy ($L{1}_{0}$ phase with the $c$ axis in the growth direction) is affected by a two-dimensional to three-dimensional growth mode transition with increasing deposition temperatures, hindering higher chemical ordering at moderate and high temperatures. For 22-nm-thick films, the ordered phase is only obtained in a narrow range of growth temperature centered at $450\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. This fact, together with strong surface morphology dependence on the deposition temperature, determines the magnetic and magneto-optical properties of the studied system. No dependence of the ordering degree on film thickness is found for films with thicknesses of 3, 7 and $22\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ grown at $450\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, with a constant value indicating that chemical ordering occurs since the early stages of growth and does not improve as the growth proceeds. The samples consist of chemically ordered nanostructures that range in size from 30 to $200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ average diameter and $0.5--30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ height as the film becomes thicker, and exhibit perpendicular magnetic anisotropy indicating that the $c$ axis is parallel to the direction of growth. The largest coercive field $(7\phantom{\rule{0.3em}{0ex}}\mathrm{kOe})$ corresponds to the sample with nano-sized particles, and the coercivity drastically decreases down to $1\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ as percolation sets in.