Device Applications of Rare-Earth Nitrides

Abstract

<p>In this thesis the properties of thin film spintronic devices are investigated. These devices incorporate rare-earth nitrides as the active elements in a geometry with vertical transport perpendicular to the layers. Many rare-earth nitrides are ferromagnetic semiconductors with a rich range of magnetic properties arising from their 4-f magnetic moments. These magnetic moments contain both spin and orbital contributions, in contrast to the quenched, spin-based magnetism frequently exploited in spintronic devices based on transition metals. Magnetic tunnel junctions are demonstrated with the ferromagnetic electrodes made from the intrinsic ferromagnetic semiconductor GdN surrounding GaN and AlN barriers. Fitting of the current-voltage characteristics of a GdN/GaN/GdN device determines a barrier height of 1.5 eV at room temperature. This puts the GdN Fermi level close to the GaN mid-gap, consistent with recent theoretical predictions of the band alignment at the GdN/GaN interface [Kagawa et al., Phys. Rev. Applied 2, 054009 (2014)]. This barrier height is found to scale with the band gap of the group-III nitride barrier, being approximately twice as large for AlN barriers. It was observed that the barrier height reduces as the AlN barrier thickness increases, signalling the formation of Schottky barriers at the interface. These polycrystalline junctions exhibit a tunnel magnetoresistance of a few percent but do not show clear signs of homogeneous switching. The transport properties of the GdN/GaN/GdN junctions are heavily influenced by the electronic structure of the semiconducting GdN layers, making junctions based on rare-earth nitrides promising candidates for further investigation. A fully semiconductor-based magnetic tunnel junction that uses spin-orbit coupled materials made of intrinsic ferromagnetic semiconductors is then presented. Unlike more common approaches, one of the electrodes consists of a near-zero magnetic moment ferromagnetic semiconductor, samarium nitride, with the other electrode comprised of the more conventional ferromagnetic semiconductor gadolinium nitride. Fabricated tunnel junctions exhibit magnetoresistances as high as 200%, implying strong spin polarisation in both electrodes. In contrast to conventional tunnel junctions, the resistance is largest at high fields, a direct result of the orbital-dominant magnetisation in samarium nitride that requires the spin in this electrode aligns opposite to that in the gadolinium nitride when the magnetisation is saturated. The magnetoresistance at intermediate fields is controlled by the formation of a twisted magnetisation phase in the samarium nitride, a direct result of the orbital-dominant ferromagnetism. Thus, new functionality can be brought to magnetic tunnel junctions by use of novel electrode materials, in contrast to the usual focus on tuning the barrier properties. Finally, highly resistive GdN films intentionally doped with Mg are demonstrated. These films are found to have increased resistivities and decreased carrier concentrations, with no observed degradation in crystal quality as compared with undoped films. An increase of the Curie temperature in conductive films is observed which is consistent with the existence of magnetic polarons centred on nitrogen vacancies. The prospect of doping rare-earth nitride films in this manner promises greater control of the material properties and future device applications.</p>