Clustering in a Precipitate-Free GeMn Magnetic Semiconductor

We present the first study relating structural parameters of precipitate-free ${\mathrm{Ge}}_{0.95}{\mathrm{Mn}}_{0.05}$ films to magnetization data. Nanometer-sized clusters—areas with increased Mn content on substitutional lattice sites compared to the host matrix—are detected in transmission electron microscopy analysis. The films show no overall spontaneous magnetization at all down to 2 K. The TEM and magnetization results are interpreted in terms of an assembly of superparamagnetic moments developing in the dense distribution of clusters. Each cluster individually turns ferromagnetic below an ordering temperature which depends on its volume and Mn content.

Figure 1

Dark field TEM overview of a ${\mathrm{Ge}}_{0.95}{\mathrm{Mn}}_{0.05}$ film fabricated in MBE at $T_S=60°\mathrm{C}$. Contrast is obtained by the selection of the chemically sensitive (002) Ge reflex. Bright areas correspond to an enhanced Mn content.

Figure 2

Typical high-resolution TEM micrograph of ${\mathrm{Ge}}_{0.95}{\mathrm{Mn}}_{0.05}$. Dark contrast reveals cubic clusters which are coherently bound to the surrounding.

Figure 3

Temperature dependence of the MFC magnetization of ${\mathrm{Ge}}_{0.95}{\mathrm{Mn}}_{0.05}$ for external fields between 0 and 1 T.

Figure 4

Magnetization loops taken between 60 and 160 K. Solid lines represent fits with the Langevin function with $\mu =435{\mu }_B$. (Upper inset) Magnetization loops at 6.5 and 15 K. (Lower inset) $M/M_S$ versus $H/T$. The symbols represent the same temperature series as in the main figure. The solid line is a Langevin fit with $\mu =435{\mu }_B$.

Figure 5

(a) Relaxation of the magnetization of ${\mathrm{Ge}}_{0.95}{\mathrm{Mn}}_{0.05}$ after switching off the external field at different temperatures. Solid lines represent fits to a stretched exponential decay. (b) Temperature-dependent real part of ac susceptibility measured at different frequencies. (c) FC or ZFC temperature-dependent magnetization. ${\mu }_{0}H=0.1\mathrm{T}$.