Dynamic Detection of Spin Accumulation by Ferromagnetic Resonance

Abstract

This thesis describes dynamical phenomena occurring in ferromagnet (FM)/n-GaAs heterostructures, while the ferromagnet is driven to resonance (FMR). The relevant work was published in Appl. Phys. Lett. 105, 212401 (2014) and Nature Communications 7, 10296 (2016), respectively. In a FM/n-GaAs heterostructure, strong spin-orbit coupling at the FM/n-GaAs interface leads to a tunneling anisotropic magnetoresistance (TAMR) effect, which makes the resistance across the FM/n-GaAs interface depend on the orientation of the magnetization relative to the crystalline axes. When the FM is driven on resonance, the magnetization starts to precess around an effective magnetic field with an elliptical trajectory. Because of the TAMR effect, the onset of FMR modulates the tunneling resistance across the FM/n-GaAs interface, enabling electrical detection of FMR. I demonstrate that through the TAMR mechanism, the FMR precession cone angles can be characterized quantitatively. This study shows that the TAMR effect plays a predominant role in producing a dc voltage in FM/n-GaAs heterostructures at FMR, which has to be taken into account when studying the FMR-induced phenomena in similar systems. When a large forward bias current is applied across the FM/n-GaAs interface, a spin accumulation can be generated in the n-GaAs. I show that, in this spin accumulation regime, FMR can be used to detect the spin accumulation. This technique utilizes the fact that at FMR, the magnetization can precess at a frequency faster than the electron spin decay rate in the n-GaAs. The electrical signal from a FM, in measuring the electrochemical potential of a spin accumulation in the n-GaAs, depends on the relative angle between the magnetization vector in the FM and the spin accumulation in the semiconductor. Traditionally, to detect the spin accumulation, one applies a perpendicular magnetic field to let the spins in the semiconductor precess (the Hanle effect) or an in-plane magnetic field to switch the direction of the magnetizations (spin valve measurement). Neither one works well at room temperature in n-GaAs due to the significant decrease of spin lifetime as temperature increases. In my experiment, the magnetization is forced to precess at the FMR frequency, which is faster than the decay rate of spins in the n-GaAs. Once spins are injected from the precessing magnetization of the FM into the n-GaAs, they precess at a much lower frequency due to the different effective magnetic fields and electron g-factors of FM and n GaAs. This difference in precession frequency creates a phase angle between the magnetization vector in the FM and the spins in the n-GaAs, which enables an effective detection of spin accumulation. My modeling shows that this FMR technique provides a new way to determine the spin lifetime by measuring the FMR frequency dependence of the spin signal, which is verified experimentally, and the spin lifetime is measured at room temperature. The FMR-spin detection technique developed in this work can be applied to other systems in which spin lifetimes are short, such as metals and perhaps topological insulators.