Magnetocaloric effect: From materials research to refrigeration devices

A review of new developments in theoretical and experimental electronic-structure investigations of half-metallic ferromagnets (HMFs) is presented. Being semiconductors for one spin projection and metals for another, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMFs by the peculiarities of their electronic structure and chemical bonding is discussed. The effects of electron-magnon interaction in HMFs and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Special attention is paid to the appearance of nonquasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-the-art electronic calculations for correlated d-systems are discussed, and results for specific HMFs (Heusler alloys, zinc-blende structure compounds, CrO2, and Fe3O4) are reviewed.

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Half-metallic ferromagnets: From band structure to many-body effects

Organic semiconductors are attractive candidates for spintronics applications because of their long spin lifetimes. But few studies have investigated how to optimize the injection of spin into these materials. A new study suggests that the metal/organic interface is key.

Unravelling the role of the interface for spin injection into organic semiconductors

Data analysis for spectroscopic ellipsometry

Graphene based adsorbents for remediation of noxious pollutants from wastewater.

Electrical and optical properties of In2Se3 thin films

The growth of large and uniform graphene layers remains very challenging to this day due to the close correlation between the electronic and transport properties and the layer morphology. Here, we report the synthesis of uniform large-scale mono- and bilayers of graphene on off-axis 6H-SiC(0001) substrates. The originality of our approach consists of the fine control of the growth mode of the graphene by precise control of the Si sublimation rate. Moreover, we take advantage of the presence of nanofacets on the off-axis substrate to grow a large and uniform graphene with good long-range order. We believe that our approach represents a significant step toward the scalable synthesis of graphene films with high structural qualities and fine thickness control, in order to develop graphene-based electronic devices.

Large-area and high-quality epitaxial graphene on off-axis SiC wafers.

This paper investigates the magnetoresistance of micron-sized MnAs/GaAs(AlAs)/MnAs magnetic tunnel junctions. We measure an asymmetric bias dependence of the magnetoresistance in which the negative contribution is attributed to resonant tunneling through a midgap defect band. Within this model we find a spin polarization of 60% for MnAs at the interface with GaAs. Moreover, we show that spin-dependent tunneling is a powerful technique for spectroscopic measurements of defects in a very thin layer.

Resonant tunneling magnetoresistance in MnAs/III-V/MnAs junctions

The structural and magnetic properties of MnAs epilayers grown on $\mathrm{Ga}\mathrm{As}(111)B$ have been investigated by x-ray diffraction and magnetometry. The strain evolution of the epilayer was followed during the phase transition by x-ray diffraction. The results show a high fraction of the $\ensuremath{\alpha}$ ferromagnetic phase at room temperature and an elevated value of the structural phase transition temperature. The system is strongly affected by residual strains that are mainly located inside the $\ensuremath{\beta}$ phase. We attribute this behavior to an increase in the elastic energy density associated with the tight link between epilayer and substrate. It comes out that magnetic and structural properties of MnAs thin films are strongly dependent on the crystallographic relation between epilayer and substrate.

Enhancement of critical temperature and phases coexistence mediated by strain in MnAs epilayers grown on GaAs(111)B

Magnetic and structural properties of CrAs epilayers grown on GaAs(001) by molecular beam epitaxy have been studied. CrAs epilayers are orthorhombic for all thicknesses investigated but show a structural transition from a metastable phase for very thin films, to the usual bulk MnP-type orthorhombic phase at higher thicknesses. At intermediate thicknesses, there is a predominance of the new phase, although a contribution from the usual CrAs bulk phase remains clearly present. These results strongly suggest that the ferromagnetic signal measured at room temperature comes from the new metastable orthorhombic structure with an expanded b-axis induced by the substrate strain.

M. Eddrief

Papers

Electrical and optical properties of In2Se3 thin films

Large-area and high-quality epitaxial graphene on off-axis SiC wafers.

Resonant tunneling magnetoresistance in MnAs/III-V/MnAs junctions

Enhancement of critical temperature and phases coexistence mediated by strain in MnAs epilayers grown on GaAs(111)B

Structure of ferromagnetic CrAs epilayers grown on GaAs(001).