In-plane uniaxial anisotropy rotations in (Ga,Mn)As thin films

We show, by superconducting quantum interference device magnetometry, that in (Ga,Mn)As films the in-plane uniaxial magnetic easy axis is consistently associated with particular crystallographic directions and that it can be rotated from the $\left[\overline{1}10\right]$ direction to the [110] direction by low-temperature annealing. We show that this behavior is hole-density dependent and does not originate from surface anisotropy. The presence of uniaxial anisotropy as well its dependence on the hole concentration and temperature can be explained in terms of the $p\text{‐}d$ Zener model of the ferromagnetism assuming a small trigonal distortion.

Figure 1

High-temperature dependence of the three major in-plane components of the remnant magnetization in ${\mathrm{Ga}}_{0.978}{\mathrm{Mn}}_{0.022}\mathrm{As}$ before (a) and after annealing (b).

Figure 2

High-temperature dependence of the three major in-plane components of the remnant magnetization in ${\mathrm{Ga}}_{0.944}{\mathrm{Mn}}_{0.056}\mathrm{As}$ before (a) and after annealing (b). Insets: magnetization curves at temperatures close to $T_{\mathrm{C}}$ measured for the [110] and $\left[\overline{1}10\right]$ orientations. Note the switch of the magnetic easy axis from $\left[\overline{1}10\right]$ to [110] upon annealing.

Figure 3

Temperature dependence of the $\left[\overline{1}10\right]$ and [110] in-plane components of the remnant magnetization in the ${\mathrm{Ga}}_{0.916}{\mathrm{Mn}}_{0.084}\mathrm{As}$ annealed layer. Note the switch of the uniaxial easy direction at temperature $T_R\simeq 50\mathrm{K}$.

Figure 4

Temperature dependence of the remnant magnetic moment in the ${\mathrm{Ga}}_{0.933}{\mathrm{Mn}}_{0.067}\mathrm{As}$ annealed layer measured along uniaxially easy [110] and hard $\left[\overline{1}10\right]$ directions before (points) and after (lines) surface etching.

Figure 5

The uniaxial easy axis fan chart for (Ga,Mn)As. Circles—as grown, squares—after annealing. Open symbols mark samples with the uniaxial easy axis oriented along the $\left[\overline{1}10\right]$ direction, full symbols denote samples exhibiting the easy axis along [110]. Half-filled squares mark the two samples exhibiting easy axis rotation to [110] on increasing temperature. The dashed line marks the compensation free $p$-type Mn doping level in (Ga,Mn)As. The thick gray line separates the two regions of hole densities where, independently of being annealed or not, at elevated temperatures the layers consistently show the same crystallographic alignment of the uniaxial easy axis.

Figure 6

In-plane uniaxial anisotropy field (in SI units and normalized to sample saturation magnetization) vs hole density computed for various valence-band spin-splittings in (Ga,Mn)As. The thick arrow illustrates the possibility of $\left[\overline{1}10\right]\Rightarrow \left[110\right]$ easy axis rotation on increasing temperature as observed experimentally and depicted in Fig. 3.