Spin injection from a half-metal at finite temperatures

Spin injection from a half-metallic electrode in the presence of thermal spin disorder is analyzed using a combination of random matrix theory, spin-diffusion theory, and explicit simulations for the tight-binding s-d model. It is shown that efficient spin injection from a half-metal is possible as long as the effective resistance of the normal metal does not exceed a characteristic value, which does not depend on the resistance of the half-metallic electrode, but is rather controlled by spin-flip scattering at the interface. This condition can be formulated as \alpha<(l/L)/T, where \alpha is the relative deviation of the magnetization from saturation, l and L the mean-free path and the spin-diffusion length in the non-magnetic channel, and T the transparency of the tunnel barrier at the interface (if present). The general conclusions are confirmed by tight-binding s-d model calculations. A rough estimate suggests that efficient spin injection from true half-metallic ferromagnets into silicon or copper may be possible at room temperature across a transparent interface.Comment: 9 pages, 4 figures, revtex4-1; expanded introduction, added references, additional comments in Section V, fixed typo

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First-principles analysis of spin-disorder resistivity of Fe and Ni

Spin-disorder resistivity of Fe and Ni and its temperature dependence are analyzed using noncollinear density functional calculations within the supercell method. Different models of thermal spin disorder are considered, including the mean-field approximation and the nearest-neighbor Heisenberg model. Spin-disorder resistivity is found to depend weakly on magnetic short-range order. If the local moments are kept frozen at their zero-temperature values, very good agreement with experiment is obtained for Fe, but for Ni the resistivity at elevated temperatures is significantly overestimated. Agreement with experiment for Fe is improved if the local moments are iterated to self-consistency. The overestimation of the resistivity for paramagnetic Ni is attributed to the reduction of the local moments down to 0.35 Bohr magnetons. Overall, the results suggest that low-energy spin fluctuations in Fe and Ni are better viewed as classical rotations of local moments rather than quantized spin fluctuations that would require an (S+1)/S correction.Comment: 10 pages (RevTeX), 6 eps figure

Translanguaging as a political act with Roma: carving a path between pluralism and collectivism for transformation

Translanguaging claims to advance social justice as a transformative pedagogy. This paper analyses a tension which developed over the life span of a European research project which aimed to improve the educational experience for Eastern European Roma pupils through teachers’ employment of a translanguaging pedagogy. Roma are ethnically and linguistically heterogeneous, but as a minority group face continued racism, whilst Roma pupils face educational exclusion. The voices of Roma parents, pupils and activists and academics alerted us to potential threats in utilising translanguaging as a political act for transformation in education. They revealed a central tension between recognition of linguistic pluralism for emancipation at school level (with possibilities for policy level changes at local or national levels) and unifying endeavours for collective action towards equality and human rights at a (trans)national level. To understand this tension we reframed it in light of the postmodernist positioning of translanguaging, and critiques of the de-politicizing tendencies of postmodernism. In proposing a way forward for research and pedagogy, we carve a path between pluralism and collectivism by placing translanguaging pedagogy and associated research into Nancy Fraser’s integrative model of recognition and redistribution for transformation