Spintronics is a burgeoning field that endeavors to use the spin of an electron for information processing. Remarkable progress has been made in understanding injection of spin-polarized electron populations/currents into semiconductors, which are typically non-magnetic, as well as manipulation and detection of said spins.
This dissertation presents a combination of numerical analysis and experiments aimed at understanding and developing imaging tools that utilize the direct coupling of spins with the spatially varying magnetic field of a micromagnetic probe. This coupling produces a spin detection technique which relies on the resulting magnetic force on a cantilever to which the magnetic probe is attached. While extensively used in materials research, such an imaging tool has lagged as a characterization tool for spintronics. This is because of the challenges associated with measuring extremely small forces from these samples, and a lack of understanding of the effects of the inhomogeneous vector field from the probe on the sample spin density.
I have developed a numerical framework, based on solving the spin-transport equation, for understanding behavior of spins in the presence of spatially varying parameters. Based on analysis of the analytically solvable case of spin in a uniform magnetic field, but with multiple vector components, I have identified a crucial quantity, θB. This captures the competition between parallel and perpendicular field components dictating the collective spin precession behavior in these spin ensembles.
The numerical simulations show that in the proximity of the micromagnet the Hanle response is broadening due to the field component parallel to the injected spin direction. This result also sheds light on the effect of magnetic injectors used in spintronics devices. Our simulations show that the Hanle response will also be artificially broadened in this case due to the magnetic field from the injector. Such broadening is the most likely cause for the larger than expected Hanle halfwidths measured in recent experiments, and should be taken into account while extracting spin lifetime from such data.
Insights from numerical analysis have led us to propose scanned magnetic perturbation imaging for spatially mapping spin properties. This technique enhances the imaging capabilities of existing detection methods by encoding local information in the field gradients from a micromagnet. The spin density is governed by a convolution of local spin properties and the local magnetic field that perturbs the spins. By scanning this local perturbation a contrast in the globally detected signal, resulting from sample inhomogeneity, can be seen.
On the experimental side I have measured the broadening of the Hanle response due to a micromagnetic tip, using a globally integrated spin photoluminescence signal from optically excited spins in n-GaAs epitaxial membranes. I have also conducted sensitive force detection experiments to measure the magnetic forces from the spins in these samples. I have measured small forces of the order femtonewtons, similar to that predicted from simulations. The Hanle response of this force signal behaves as expected. This may be the first force detection of non-equilibrium itinerant spin-polarized electrons in paramagnetic semiconductors.