Growth, microstructure, and magnetic properties of highly textured and highly coercive Nd-Fe-B films

Thin Nd-Fe-B films have been prepared by pulsed laser deposition on sapphire substrates. A Ta buffer layer has been used to promote epitaxial growth. The deposition temperature has been varied between 400 \ifmmode^\circ\else\textdegree\fi{}C and 700 \ifmmode^\circ\else\textdegree\fi{}C and proved to be an important parameter for both, the phase formation and the control of microstructure and coercivity. For low temperatures the Nd-Fe-B films grow in small grains with irregular shape, and the generally observed $c$-axis texture is not fully developed. The coercivity reaches values of around 1.6 T. For medium preparation temperatures of 600\char21{}650 \ifmmode^\circ\else\textdegree\fi{}C the Nd-Fe-B films grow in small rectangular grains which are mostly isolated from each other. Maximum coercivities of 1.9 T together with in-plane to out-of-plane remanence ratios as low as 0.05 have been achieved. These films possess a nearly perfect magnetic texture with the easy magnetization axis perpendicular to the film plane. For higher deposition temperatures the microstructure coarsens substantially and also oxidation of Nd occurs. Additionally, the coercivity drops drastically. Optimally prepared films have been investigated by magnetic force microscopy. The perfect growth of the ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ phase and the special granular microstructure allow us to study the domain structure of isolated, single crystalline ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ grains on a submicrometer scale. The observed domain patterns show stripe domains, which are typical for films with large perpendicular magnetic anisotropy, and resemble the specific shape of the magnetic grains. The domain width varies from 100 to 300 nm depending on the grain height and can, on a first approximation, be described by domain theory of extended films.