Ferromagnetism in ion implanted amorphous and nanocrystalline MnxGe1-x

The structural, electronic, and magnetic properties of a MnxGe1−x alloy prepared through room-temperature ion implantation 100 keV, 21016 ions/cm2 and subsequent 400 °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 x12 at./% at 55 nm and end of range at 140 nm. The structural investigation shows that the overall implanted Ge layer is amorphous. In particular, up to a depth of 60 nm, the implanted layer is also porous and oxidized, whereas the deepest implanted region 60–140 nm is purely composed of amorphous Ge with Mn atoms diluted in it. This sample manifests magnetic hysteresis up to 20 K and a strong nonlinear S-shaped magnetic response up to 150 K. Upon annealing at 400 °C, the top porous layer remains essentially amorphous, whereas partial reconstruction into Ge nanocrystals 10 nm in diameter occurs in the 60–140-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 nm from the surface, remain trapped in the residual amorphous matrix or in the Ge nanocrystals, whereas formation of Mn-Ge extrinsic phases like Mn11Ge8 and Mn5Ge3 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 MnxGe1−x diluted magnetic semiconductor in nanocrystalline form, manifests magnetic hysteresis up to above 250 K.