Magneto-optical Kerr probing of LAO/STO interface ferromagnetism*

Interfacial ferromagnetism in LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterostructures has been probed by a variety of techniques. Recently, magnetic force microscopy (MFM) was used to image ferromagnetic domains that are electrically tunable at room temperature\footnote{F. Bi, \textit{et al}. Nat. commun. \textbf{5}, 5019 (2014)} when the samples were grown in certain conditions. Optical techniques provide powerful tools for probing magnetic phenomena, and recently magnetic circular dichroism has been observed in reduced bulk STO crystals\footnote{W. D. Rice, \textit{et al}. Nat. mater. \textbf{13}, 481–487 (2014)}. Here we describe a scanning magneto-optical Kerr imaging system that could achieve sub-micrometer precision and 10$^{-4}$ rad/$\sqrt{\mathrm{Hz}}$ sensitivity with a 150 fs pulsed-laser centered at 425 nm. Such capability would make pump and probe measurement on the gate-tunable LAO/STO ferromagnetism and ultrafast imaging of domain dynamics possible.