Spin Transport in Antiferromagnets and Ferromagnets
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- Institutt for fysikk 
This doctoral thesis presents and supplements the results of three scientific papers [1,2,3] within the field of theoretical condensed matter physics. The three papers investigate several spin-related phenomena in antiferromagnets and ferromagnets. In Paper 1, we consider spin transfer and spin pumping in an antiferromagnetic insulator connected to a normal metal with disorder. Paper 2 studies the spin Hall effect in an antiferromagnetic metal with spin-orbit coupling. Paper 3 describes laser-induced spin transfer between itinerant electrons and localized spins in a ferromagnetic metal. In Paper 1, we focus on an antiferromagnetic insulator adjacent to a normal metal. Spin accumulation in the normal metal induces spin-transfer torques on the localized spins in the antiferromagnet. The spin-transfer torques are calculated within a quantum-mechanical model by considering the microscopic spin currents at the interface between the antiferromagnet and the normal metal. In the ideal case without disorder, we recover the results obtained in seminal papers. Beyond previous studies, we investigate how disorder in the normal metal affects the spin-transfer torques on the antiferromagnet. Spin-conserving disorder in the normal metal reduces the torques on the antiferromagnet in a manner similar to Ohm's law. Magnetic impurities in the normal metal result in spin loss and reduced spin transfer torques. Reciprocal to the process of spin transfer is spin pumping: precessing spins in the antiferromagnet pump spin currents into the adjacent metal. We show how spin pumping and spin transfer in antiferromagnets are related by considering a time-dependent gauge transformation. Paper 2 investigates the spin Hall effect in an antiferromagnetic metal with Rashba spin-orbit coupling and disorder. We calculate the spin Hall effect in antiferromagnets in two ways. First, we utilize the Landauer-Buttiker formalism to calculate the mesoscopic spin Hall conductance for a finite system connected to four leads. The transverse spin current induced by a longitudinal voltage difference determines the spin Hall effect. Second, we calculate the spin Hall conductivity in an infinite system by considering the Berry-phase-induced intrinsic contribution in the Kubo formula. We identify a regime where the results from the two different methods can be compared. In general, the spin Hall effect in mesoscopic antiferromagnets varies considerably for various parameters of the system. Interestingly, in some regimes, the spin Hall effect increases with increasing exchange interactions, which indicates that antiferromagnets might have a greater spin Hall effect compared to the corresponding system in the normal metal state. In contrast, the spin Hall effect computed from the Berry-phase contribution in the Kubo formula yields the opposite result: increasing exchange interactions in antiferromagnets reduces the spin Hall current. Similar to the conclusions for other systems, our results demonstrate that a calculation of the spin Hall effect in antiferromagnets based only on the intrinsic Berry-phase term in the Kubo formula is inadequate and does not recreate the exact numerical results for a system with disorder. In Paper 3, we model laser-induced spin transfer between itinerant electrons and localized spins in a ferromagnetic metal. The conduction electrons and the localized spins are coupled via the exchange interaction. We consider a ferromagnet where magnetic order can occur in two (opposite) directions due to a uniaxial anisotropy. By expressing the localized spins in terms of Schwinger boson operators, our description takes into account both possible equilibrium states of the ferromagnet. Within a mean-field approach, we present a self-consistent description for the combined spin system in quasi-equilibrium, including spin polarization of electrons caused by the (mean-field) magnetization. We consider a scenario where the system is quickly brought out of equilibrium, for example by laser-induced heating of the conduction electrons. We derive rate equations for the spin transfer between the (hot) itinerant electrons and the localized spins. In contrast to many other related studies, our model is suitable for describing scenarios of demagnetization far from the magnetic ordering direction, including possible switching scenarios.