Quantum Phenomena at Magnetic Insulator-Normal Metal Interfaces
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- Institutt for fysikk 
Electrons carry an intrinsic angular momentum that we call spin. Spintronics is a field of research which aims to create new computer technology by making use of the electron spin. This thesis presents the results of four research papers within the field of spintronics. The materials that we focus on are normal metals and magnetic insulators. Magnetic insulators are electrically insulating materials containing many angular momenta that are all free to rotate, but form a magnetic ordering rather than rotate individually. Interfaces between magnetic insulators and normal metals, and the interfacial couplings thereat, are central to all four papers. We use a quantum-mechanical description, where properties such as energy and angular momentum are restricted to discrete values. This discretization is prominent at the atomic scale. The main focus is on quantum-mechanical phenomena that are apparent even at a macroscopic scale, namely Bose-Einstein condensation and superconductivity. In ferromagnetic insulators (FIs), the angular momenta are parallel in the magnetically ordered state. This ordering gives rise to a large external field. We also consider some materials where each angular momentum is antiparallel to its neighbors in the ordered state, which we refer to as bipartite antiferromagnets. Here, the angular momenta can be grouped into two equivalent sublattices with opposite polarizations, and therefore the net external field vanishes. In the first paper, we discuss an antiferromagnetic insulator (AFI) that is sandwiched between two normal-metal (NM) layers and subjected to a temperature gradient. Due to the symmetry of the antiferromagnet, there is no net transfer of angular momentum. However, we show that there is a significant heat flow from the ordered magnetic moments to the conduction electrons. Magnons are quantized vibrations of ordered magnetic moments. When driven by microwave radiation, magnons can form quasiequilibrium Bose-Einstein condensates. The interfacial coupling to adjacent NMs has been proposed as an alternative mechanism to induce magnon condensates in ferromagnetic insulators. In the second paper, we predict that the same approach extends to antiferromagnetic insulators. In the last two papers, we discuss FI-NM-FI and AFI-NM-AFI trilayers. We show that the coupling between the magnons and the electrons in the metal can lead to superconductivity. Because magnetic fields can break superconductivity, magnetism and superconductivity are incompatible in many cases. We focus on the case where the magnetic insulators have opposite polarizations, in which the effective fields experienced by the electrons cancel out.