Ferromagnetism in ion implanted amorphous and nanocrystallineMnxGe1−x

The structural, electronic, and magnetic properties of a ${\mathrm{Mn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ alloy prepared through room-temperature ion implantation ($100\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$, $2\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.3em}{0ex}}\text{ions}∕{\mathrm{cm}}^{2}$) and subsequent $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ annealing have been investigated with several experimental techniques. The as-implanted sample shows a quasi-Gaussian Mn concentration depth profile with a projected range (peak Mn concentration $x\ensuremath{\simeq}12\phantom{\rule{0.3em}{0ex}}\mathrm{at.}∕%$) at $55\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and end of range at $140\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The structural investigation shows that the overall implanted Ge layer is amorphous. In particular, up to a depth of $60\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the implanted layer is also porous and oxidized, whereas the deepest implanted region $(60\char21{}140\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ is purely composed of amorphous Ge with Mn atoms diluted in it. This sample manifests magnetic hysteresis up to $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and a strong nonlinear S-shaped magnetic response up to $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Upon annealing at $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, the top porous layer remains essentially amorphous, whereas partial reconstruction into Ge nanocrystals ($\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter) occurs in the $60\char21{}140\text{\ensuremath{-}}\mathrm{nm}$-deep implanted region. Part of the Mn atoms, mainly belonging to the top porous layer, further diffuses toward the surface and forms chemical bonds with O contaminants, becoming magnetically inactive. The other Mn atoms, mainly in the region between 60 and $140\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ from the surface, remain trapped in the residual amorphous matrix or in the Ge nanocrystals, whereas formation of Mn-Ge extrinsic phases (like ${\mathrm{Mn}}_{11}{\mathrm{Ge}}_{8}$ and ${\mathrm{Mn}}_{5}{\mathrm{Ge}}_{3}$) is excluded. The magnetic response of the annealed sample originates from the existence of a soft and a harder magnetic component, assigned to the dilution of Mn atoms in residual amorphous Ge and Ge nanocrystals, respectively. The hard component, attributable to a ${\mathrm{Mn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ diluted magnetic semiconductor in nanocrystalline form, manifests magnetic hysteresis up to above $250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.