TIME-RESOLVED MAGNETIC MICROSCOPY USING NEAR- AND FAR-FIELD PICOSECOND HEATING

Abstract

Advanced magnetic microscopies are key to advancing our understanding and application of novel magnetic phenomenon such as skyrmions, spinwaves, and domain walls. However, due to the diffraction-limit of light, achieving the 10 – 100 nanometer spatial resolution and 10 – 100 picosecond temporal resolution required to image these phenomena is beyond the reach of table-top techniques. My dissertation research has been to develop stroboscopic magnetic microscopy techniques that use picosecond thermal gradients to transduce magnetization into a voltage. In magnetic metals, this is accomplished via the anomalous Nernst effect and in ferromagnetic insulator/heavy metal bilayers the signal is due to the longitudinal spin Seebeck effect detected via the inverse spin Hall effect. Using focused, 3 ps laser pulses to heat cobalt and permalloy films, I demonstrate that the anomalous Nernst effect can image magnetization with 10-100 ps temporal resolution, sub-micron spatial resolution, and sensitivity to the in-plane moment of 0.1°/√Hz. I then show how this sensitivity and resolution can be applied for phase-sensitive ferromagnetic resonance imaging in ultrathin YIG/Pt bilayers in which we observe spatial variation of the resonance field, amplitude, phase, and linewidth. To conclude, I present the development of a near-field scanning optical microscope to create nanoscale thermal gradients and achieve spatial resolution of magnetic textures below the diffraction limit. The advent of these far- and near-field magneto-thermal imaging techniques will enable the table-top measurement of nanoscale magnetization dynamics in thin film devices.