The emerging field of spintronics would be dramatically boosted if room-temperature ferromagnetism could be added to semiconductor nanostructures that are compatible with silicon technology. Here, we report a high-TC (>400K) ferromagnetic phase of (Ge,Mn) epitaxial layer. The manganese content is 6%, and careful structural and chemical analyses show that the Mn distribution is strongly inhomogeneous: we observe eutectoid growth of well-defined Mn-rich nanocolumns surrounded by a Mn-poor matrix. The average diameter of these nanocolumns is 3nm and their spacing is 10nm. Their composition is close to Ge2Mn, which corresponds to an unknown germanium-rich phase, and they have a uniaxially elongated diamond structure. Their Curie temperature is higher than 400K. Magnetotransport reveals a pronounced anomalous Hall effect up to room temperature. A giant positive magnetoresistance is measured from 7,000% at 30K to 200% at 300K and 9T, with no evidence of saturation.

/pdf/high-curie-temperature-ferromagnetism-in-self-organized-gemn-2hwli7oe2v.pdf

High Curie temperature ferromagnetism in self-organized GeMn nanocolumns.

We present a comprehensive study of ferromagnetism and magnetotransport in Mn-doped germanium, grown with molecular-beam epitaxy. Ferromagnetism in ${\mathrm{Mn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ $(0lxl0.09)$ is characterized by two different ordering temperatures ${T}_{C}$ and ${T}_{C}^{*}$ with ${T}_{C}⪡{T}_{C}^{*}$. The onset of global ferromagnetic order at ${T}_{C}$ coincides with the percolation threshold for (activated) charge transport. Magnetism between ${T}_{C}$ and ${T}_{C}^{*}$ originates from ``clustered dopants'' associated with inhomogeneities. The ferromagnetic ordering temperature within the clusters is of order ${T}_{C}^{*}$ while the coupling between the clusters is mediated by thermally activated carriers moving in an impurity band. The magnetoresistance exhibits nonmonotonic temperature and magnetic field dependence; both negative and positive magnetoresistance contributions are observed. The anomalous Hall effect between ${T}_{C}$ and ${T}_{C}^{*}$ appears to be influenced heavily by the large magnetoresistance. The normal and anomalous Hall coefficients both diverge at low temperature. All these observations indicate that ${\mathrm{Mn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ is most adequately described within an impurity band model where the ratio $J∕t$ of the Mn hole exchange $J$ and hole hopping $t$ is large.

Magnetism in MnxGe1-x Semiconductors Mediated by Impurity Band Carriers

Dopant segregation in a ${\mathrm{Mn}}_{\mathrm{x}}{\mathrm{Ge}}_{1\ensuremath{-}x}$ dilute magnetic semiconductor leads to a remarkable self-assembly of Mn-rich nanocolumns, embedded in a fully compensated Ge matrix. Samples grown at $80\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ display a giant positive magnetoresistance that correlates directly with the distribution of magnetic impurities. Annealing at $200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ 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.

Dopant segregation and giant magnetoresistance in manganese-doped germanium

Development of new materials for spintronics

The structural and electronic properties of MnxGe1−x alloys (x⩽0.15) fabricated by ion implantation are investigated by means of x-ray diffraction and synchrotron radiation photoemission spectroscopy. The diffraction patterns point to the presence of ferromagnetic Mn5Ge3 nanoparticles; however, valence band spectra, interpreted by means of accurate ab initio calculations including Hubbard-like correlations, show clear fingerprints of an effective substitutional Mn dilution in the Ge semiconducting host.

/pdf/phase-separation-and-dilution-in-implanted-mnxge1-x-alloys-zibvqwtyt8.pdf

Phase separation and dilution in implanted MnxGe1-x alloys

We have investigated the chemical distributions and the local electronic structure of potential diluted magnetic semiconductor Ge0.94Mn0.06 single crystals using scanning photoelectron microscopy (SPEM), x-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). The SPEM image shows the stripe-shaped microstructures, which arise from the chemical phase separation between the Mn-rich and Mn-depleted phases. The Mn 2p XAS shows that the Mn ions in the Mn-rich region are in the divalent high-spin Mn2+ states but that they do not form metallic Mn clusters. The Mn 3d PES spectrum exhibits a peak centered at approximately 4 eV below E(F) and the negligible spectral weight near E(F). This study suggests that the observed ferromagnetism in Ge1-xMnx arises from the phase-separated Mn-rich phase.

Spatial chemical inhomogeneity and local electronic structure of Mn-doped Ge ferromagnetic semiconductors.

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Spatial chemical inhomogeneity and local electronic structure of Mn-doped Ge ferromagnetic semiconductors.