Magnetic and structural properties of ${\mathrm{Ge}}_x{\mathrm{Mn}}_{1-x}$ films: Precipitation of intermetallic nanomagnets

We present a comprehensive study relating the nanostructure of ${\mathrm{Ge}}_{0.95}{\mathrm{Mn}}_{0.05}$ films to their magnetic properties. The formation of ferromagnetic nanometer-sized inclusions in a defect-free Ge matrix fabricated by low-temperature molecular beam epitaxy is observed down to substrate temperatures $T_{\mathrm{S}}$ as low as $70°\mathrm{C}$. A combined transmission electron microscopy and electron energy-loss spectroscopy analysis of the films identifies the inclusions as precipitates of the ferromagnetic compound ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$. The volume and amount of these precipitates decrease with decreasing $T_{\mathrm{S}}$. Magnetometry of the films containing precipitates reveals distinct temperature ranges: Between the characteristic ferromagnetic transition temperature of ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ at approximately room temperature and a lower, $T_{\mathrm{S}}$-dependent blocking temperature $T_{\mathrm{B}}$ the magnetic properties are dominated by superparamagnetism of the ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ precipitates. Below $T_{\mathrm{B}}$, the magnetic signature of ferromagnetic precipitates with blocked magnetic moments is observed. At the lowest temperatures, the films show features characteristic of a metastable state.

Figure 1

Cross-sectional TEM micrographs of a sample with $T_{\mathrm{S}}=120°\mathrm{C}$. (a) Bright-field cross-sectional TEM overview image. The dark regions correspond to ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ precipitates. (b) Typical moiré fringe images of precipitates. (c) High-resolution TEM micrograph of a typical partially coherent precipitate. (d) Fourier transform of (c) showing cubic Ge and hexagonal ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ lattice reflection indices.

Figure 2

Cross-sectional TEM micrographs of a sample with $T_{\mathrm{S}}=70°\mathrm{C}$. (a) Bright-field TEM overview image. Arrows mark ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ precipitates. (b) Dark-field TEM micrograph recorded after selecting a characteristic reflex of the hexagonal ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ lattice. The dashed line indicates the interface between GeMn layer and Ge buffer layer. Bright regions correspond to ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ precipitates. (c) High-resolution TEM of a partially coherent, hexagonal ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ precipitate, marked by the white circle. (d) Dark-field TEM micrograph recorded after selecting a characteristic reflex of the cubic Ge lattice. Bright regions represent the coherent clusters. (e) High-resolution TEM of typical coherent, cubic clusters.

Figure 3

Magnetization versus temperature curves of samples fabricated with substrate temperatures $T_{\mathrm{S}}$ between 60 and $120°\mathrm{C}$. External fields were applied in the sample plane. Magnetization was measured during warmup. (a) MFC, ${\mu }_{0}H=0.1\mathrm{T}$. (b) MFC, ${\mu }_{0}H=0\mathrm{T}$. The arrows indicate the onset of magnetization. (c) Field-cooled (FC, open symbols) and zero-field-cooled (ZFC, closed symbols) measurements, ${\mu }_{0}H=0.1\mathrm{T}$.

Figure 4

Real part of the ac susceptibility versus temperature. The measurement field ${\mu }_{0}H=0.5\mathrm{mT}$ was applied perpendicular to the sample plane. (Inset) Shift of the ac susceptibility peak with measurement frequency toward higher temperatures, shown for a sample with $T_{\mathrm{S}}=85°\mathrm{C}$.

Figure 5

Magnetization curves taken for a $T_{\mathrm{S}}=85°\mathrm{C}$ sample with external field applied in the plane of the sample. Solid lines represent Langevin fits. (Inset) Closeup of the same data set. Hysteresis effects gradually appear below $200\mathrm{K}$.