Dopant segregation and giant magnetoresistance in manganese-doped germanium

Dopant segregation in a ${\mathrm{Mn}}_{\mathrm{x}}{\mathrm{Ge}}_{1-x}$ dilute magnetic semiconductor leads to a remarkable self-assembly of Mn-rich nanocolumns, embedded in a fully compensated Ge matrix. Samples grown at $80°\mathrm{C}$ display a giant positive magnetoresistance that correlates directly with the distribution of magnetic impurities. Annealing at $200°\mathrm{C}$ increases Mn substitution in the host matrix above the threshold for the insulator-metal transition, while maintaining the columnar morphology, and results in global ferromagnetism with conventional negative magnetoresistance. The qualitative features of magnetism and transport in this nanophase material are thus extremely sensitive to the precise location and distribution of the magnetic dopants.

Figure 1

(Color online). Nanoscale phase separation in as-grown ${\mathrm{Mn}}_{0.05}{\mathrm{Ge}}_{0.95}$. (a) Cross-sectional TEM image; (b) plan-view TEM image; (c) STEM image highlighting strain contrast ($\sim 35$ mrad inner detector angle); and (d) STEM image highlighting Z-contrast ($\sim 60$ mrad inner detector angle).

Figure 2

(Color online). Channeling RBS angular scans about the $⟨100⟩$ axes for as-grown (a) and postannealed (b) ${\mathrm{Mn}}_{0.1}{\mathrm{Ge}}_{0.9}$ samples.

Figure 3

(Color online). (a) Remanent magnetization of ${\mathrm{Mn}}_{0.05}{\mathrm{Ge}}_{0.95}$ as a function of temperature, before and after postannealing. The inset shows the dc magnetization at $5\mathrm{K}$ with the field perpendicular to the sample surface. (b) Temperature-dependent ac susceptibility of postannealed ${\mathrm{Mn}}_{0.05}{\mathrm{Ge}}_{0.95}$ measured in various dc fields and as-grown ${\mathrm{Mn}}_{0.05}{\mathrm{Ge}}_{0.95}$ (solid line) in zero dc field. $H_{\mathrm{ac}}=5\mathrm{Oe}$, $\mathrm{f}=24\mathrm{Hz}$.

Figure 4

(Color online). (a) Giant MR of as-grown ${\mathrm{Mn}}_{0.05}{\mathrm{Ge}}_{0.95}$ at different temperatures. The solid lines represent theoretical fits, according to Eq. (1). (b) MR of postannealed ${\mathrm{Mn}}_{0.05}{\mathrm{Ge}}_{0.95}$.