Normal and superfluid spin dynamics in magnetic insulator heterostructures

Abstract

Summary of thesis:

Normal and superfluid spin dynamics in magnetic insulator heterostructures

In this thesis, the results of six research papers on spintronics-related effects are presented. The main focus is on phenomena related to spin transport in magnetic insulators. Papers I and II address coupled spin waves in heterostructures consisting of two ferromagnetic insulators that sandwich a normal metal. We find that a charge current in combination with the spin Hall effect can induce magnetic torques that can actuate the magnetization through spin transfer. Spin- pumping and spin-transfer torques lead to a long-range coupling of the spin waves in the two layers. We compute the effect of the dynamic coupling on the resonance frequencies and linewidths of the different modes. The spin waves couple either acoustically or optically, meaning that they oscillate in phase or out of phase. We show that the coupling and thus the effect on the modes is strongly dependent on the spin-wave wavelength. For long wavelengths, the spin waves couple efficiently, whereas the coupling vanishes when the wavelength becomes small relative to the spin-diffusion length of the normal metal spacer. Additionally, we show that a surface-anisotropy energy can induce surface-localized modes that couple much more strongly.

The order parameter of an antiferromagnet can strongly interact with the itinerant electrons of a neighboring normal metal. This interaction is mediated by spin pumping and spin transfer, and it is predicted to be as strong in antiferromagnets as in ferromagnets. In Paper III, we calculate the spin-wave-mediated thermal transport between an antiferromagnetic insulator and a normal metal. Although there is no spin Seebeck effect below the spin-flop transition, there is still a flow of thermal magnons when a temperature gradient is applied. We demonstrate that the flow of thermal magnons gives rise to a heat conductivity between an antiferromagnetic insulator and a normal metal that is considerably larger than that in ferromagnets. In Paper IV, we discuss the possibility of superfluid transport of spins in thin-film ferromagnetic insulators. Recent works suggest that the magnetization of a thin-film ferromagnetic insulator with easy-plane anisotropy will behave as a spin superfluid. This would allow for spin transport over long distances, with essentially no dissipation. In our work, we show that a complete description of such a phenomenon must include the long-range dipole interaction. The

inclusion of this interaction leads to completely different transport properties. Using material parameters for the ferromagnetic insulator yttrium iron garnet, we find that spin superfluidity is limited to a few hundred nanometers. Over this distance, the dipole interaction gives rise to significant decay of the spin current. Spin superfluidity can be recouped in a ferromagnetic insulator - normal metal - ferromagnetic insulator trilayer. In this trilayer, the supercurrent can be achieved for system sizes up to ~µm, and we show that for larger systems, there is still spin transport, albeit with some decay. In Paper V, we also discuss the possibility of spin superfluidity in antiferromagnetic insulators. We show that the threshold, which is associated with an easy-axis anisotropy, can be compensated by an external magnetic field. Numerical calculations show that when the easy-axis anisotropy is compensated, long-range spin superfluidity can be achieved.

In Paper VI, we investigate magneto-optical and magnetoelectric phenomena in topological insulators above the linear response regime. Two ac electric fields applied parallel to the surface induce an ac perpendicular polarization, whereas a static polarization can be induced by a circularly polarized electromagnetic wave. We demonstrate that the anomalous Hall current is strongly modulated by an ac electric field applied perpendicular to the surface.

Sammendrag (norsk):

Vanlig og superfluid spinndynamikk i magnetiske isolator heterostrukturer, av Hans Langva Skarsvåg.

Et elektron har både spinn og ladning. I et klassisk bilde kan spinn sees på som en rotasjon om egen akse. Kombinasjonen av ladning og spinn gir opphav til magnetisering. Dette betyr at et elektron oppfører seg som en liten magnet. I ferromagnetiske materialer «spinner» mange av elektronene om samme akse, som gir opphav til en makroskopisk magnetisering. Dette kan for eksempel observeres i en kjøleskapsmagnet eller i en kompassnål. Fordi spinnene i en ferromagnet er koblet til hverandre vil en lokal perturbasjon av spinnene lede til en propagerende forstyrrelse i magnetiseringen. Slike propagerende forandringer i magnetiseringen kalles spinnbølger. Spinnbølger bærer ikke med seg ladning, men kan likevel forflytte informasjon fra et sted til et annet.

I denne avhandlingen beskriver vi spinnbølgedynamikken i ulike systemer bestående av magnetiske isolatorer og metaller. Vi viser at et metall kan koble spinnbølger i to adskilte ferromagneter. Et annet interessant fenomen er spinn-superfluiditet, hvor spinnene i en magnetisk tynnfilm oppfører seg som en væske uten viskositet. Vi viser at under realistiske forutsetninger vil denne eksotiske effekten bare være observerbar i svært små systemer (noen få hundre nanometer). For å få et spinnsuperfluid til å overleve i større systemer foreslår vi et system bestående av to magnetiske tynnfilmer hvor magnetiseringen peker i motsatt retning. Våre beregninger viser at et spinn-superfluid kan eksistere over mye større lengdeskalaer i et slikt system. Vi ser også på spinnbølgedynamikk i antiferromagneter. I antiferromagnetiske materialer har naboelektronene motsatt spinn, slik at det ikke er noen netto magnetisering. Vi viser at antiferromagnetiske spinnbølger kan bidra til en effektiv varmetransport. Det er også foreslått at spinnsuperfluider kan observeres i antiferromagneter. Vi viser at transportegenskapene knyttet til et spinnsuperfluid i en antiferromagnet kan forbedres ved å sette systemet i et magnetfelt med riktig styrke og retning.