Formation process and superparamagnetic properties of (Mn,Ga)As nanocrystals in GaAs fabricated by annealing of (Ga,Mn)As layers with low Mn content

X-ray diffraction, transmission electron microscopy, and magnetization measurements are employed to study the structural and magnetic properties of Mn-rich (Mn,Ga)As nanocrystals embedded in GaAs. These nanocomposites are obtained by moderate-temperature (400 °C) and high-temperature (560 °C and 630 °C) annealing of (Ga,Mn)As layers with Mn concentrations between 0.1% and 2%, grown by molecular beam epitaxy at 270 °C. Decomposition of (Ga,Mn)As is already observed at the lowest annealing temperature of 400 °C for layers with initial Mn content of 1% and 2%. Both cubic and hexagonal (Mn,Ga)As nanocrystals, with similar diameters of 7–10 nm, are observed to coexist in layers with an initial Mn content of 0.5% and 2% after higher-temperature annealing. Measurements of magnetization relaxation in the time span 0.1–10 000 s provide evidence for superparamagnetic properties of the (Mn,Ga)As nanocrystals, as well as for the absence of spin-glass dynamics. These findings point to weak coupling between nanocrystals even in layers with the highest nanocrystal density.

Figure 1

006 reflections in 2θ/ω x-ray diffractometer scans, measured for (a) as-grown and (b) 400 °C-, (c) 560 °C-, and (d) 630 °C-annealed (Ga,Mn)As layers containing 0.1%, 0.3%, 0.5%, 1%, and 2% Mn.

Figure 2

Evolution of strain in the as-grown and annealed (Ga,Mn)As layers. Annealing temperatures ($T_{\mathrm{a}}$) were set to 400 °C, 560 °C, and 630 °C. The dashed lines joining the experimental points are visual guides and do not necessarily follow the true values of strain in layers annealed at intermediate temperatures.

Figure 3

Cross-sectional bright-field TEM images of Ga${}_{0.995}$Mn${}_{0.005}$As layers annealed at (a) 400 °C and (b) 630 °C. (c) Aberration-corrected ADF STEM image of a void, a hexagonal (Mn,Ga)As nanocrystal, and a rhombohedral As nanocrystal in a sample annealed at 560 °C. (d) Aberration-corrected ADF STEM image of a void and a cubic (Mn,Ga)As nanocrystal in a GaAs host. The ADF inner detector semiangle used was 47.4 mrad.

Figure 4

High-resolution aberration-corrected TEM images of hexagonal and cubic (Mn,Ga)As nanocrystals in Ga${}_{0.98}$Mn${}_{0.02}$As layers annealed at (a) 560 °C and (b) 630 °C. (a) Dashed circles mark the location of the nanocrystals. (b) Dashed circles mark cubic (Mn,Ga)As nanocrystals, while arrows indicate the $c$-axes of hexagonal crystals that are parallel with {111} orientation of the GaAs host.

Figure 5

Temperature $T$ dependence of the (a) and (d) FC and (b) and (c) ZFC/FC magnetization $M$ of all layers measured (a) before and (b)–(d) after heat treatment. (d) Temperature dependence of the FC magnetization at higher temperatures for the layers annealed at 630 °C. No significant magnetic signal is observed for the as-grown or annealed layers with 0.1% Mn. In both the as-grown and the annealed cases, $M$ is converted to units of kiloamps per meter by considering the initial volumes of the (Ga,Mn)As layers.

Figure 6

(a) Temperature (main frame) and time (inset) dependence of the ZFC, FC, and TRM magnetization of the layer with 2% Mn, plotted as M/H and [$M$ − $M$($t$ $=$ 0.3 s)]/$H$, respectively. (b) TRM relaxation recorded at measurement temperature $T_{\mathrm{m}}$ $=$ 15 K for different magnetic fields.

Figure 7

Time $t$ dependence of the TRM of the layer with 2% Mn, plotted as [$M$ − $M$($t$ $=$ 0.3 s)]/$H$. The sample was cooled rapidly from a reference temperature $T_{\mathrm{ref}}$ $=$ 120 K to the measurement temperature $T_{\mathrm{m}}$ in a magnetic field $H$ $=$ 1 Oe. After a waiting time $t_{\mathrm{w},}$ the magnetic field was switched off and the magnetization was recorded as a function of time while keeping the temperature constant. (a) The TRM magnetization was recorded at $T_{\mathrm{m}}$ $=$ 15 K for different waiting times (the inset shows examples of measurements recorded over longer time scales). (b) The TRM magnetization was recorded at different temperatures without a waiting time, i.e., for $t_{\mathrm{w}}$ $=$ 0 s.