Magnetic Domain Structure and Magnetic Anisotropy in ${\mathrm{G}\mathrm{a}}_{1-x}{\mathrm{M}\mathrm{n}}_x\mathrm{A}\mathrm{s}$

Large, well-defined magnetic domains, on the scale of hundreds of micrometers, are observed in ${\mathrm{G}\mathrm{a}}_{1-x}{\mathrm{M}\mathrm{n}}_x\mathrm{A}\mathrm{s}$ epilayers using a high-resolution magneto-optical imaging technique. The orientations of the magnetic moments in the domains clearly show in-plane magnetic anisotropy, which changes through a second-order transition from a biaxial mode (easy axes nearly along [100] and [010]) at low temperatures to an unusual uniaxial mode (easy axis along [110]) as the temperature increases above about $T_c/2$. This transition is a result of the interplay between the natural cubic anisotropy of the GaMnAs zinc-blende structure and a uniaxial anisotropy which attribute to the effects of surface reconstruction.

Figure 1

Magnetization curves at 15 K (a) and 35 K (b) along different crystallographic directions. Inset (a): expanded presentation of the data at 15 K; inset (b): temperature dependence of the saturation magnetization, $M_s$.

Figure 2

Magneto-optical domain images taken near a corner of the sample at 15 K [top panels: (a) to (f)] and at 35 K [bottom panels: (g) to (l)] in the indicated fields applied along the [100] direction (vertical axis). The frame width is 1 mm. Positive (negative) perpendicular fields are seen as bright (dark) contrast. Black arrows indicate the orientation of the magnetic moments in the domains.

Figure 3

Magneto-optical images at 35 K in fields applied along the [110] direction. The width of the frames is 1 mm.

Figure 4

(a) Temperature dependence of the in-plane uniaxial and cubic anisotropy fields. The inset defines the experimental geometry. (b) Temperature dependence of the angle of the easy axis with respect to the [110] direction. The inset shows the magneto-optical contrast around an 80-$\mu$m hole at 16 and 34 K, which identifies the moment orientation at $H=0$.