Optically-Induced Persistent Magnetization in Oxygen Deficient Strontium Titanate

Interest in electronics and spintronics based on complex oxide materials has exploded in recent years, fueled by the ability to grow atomically-precise heterostructures of various oxides [1]. A foundational material in this burgeoning field is strontium titanate, a (nominally) non-magnetic wide-bandgap semiconductor. Owing to its ubiquity in oxide materials science, studies of SrTiO$_3$'s interesting dielectric, lattice, and optical properties represent mature research areas. However, renewed interest in SrTiO$_3$ was recently sparked by observations of unexpected {\it spin and magnetization} phenomena at interfaces between SrTiO$_3$ and other nonmagnetic oxides [1]. The formation and distribution of oxygen vacancies ($V_O$) in SrTiO$_3$ are widely thought to play an essential but as-yet-incompletely understood role in these emergent phenomena. Here we demonstrate a surprising new aspect to the phenomenology of magnetism in SrTiO$_3$ by reporting the observation of an optically-induced and persistent magnetization in slightly oxygen-deficient SrTiO$_{3-\delta}$ bulk crystals, using magnetic circular dichroism spectroscopy and optically-coupled SQUID studies [2]. This magnetization appears below 18K, persists for hours below 10K, and is tunable via the polarization and wavelength of sub-bandgap (400-500 nm) light. As such, magnetic patterns can be ``written'' into SrTiO$_{3-\delta}$, and subsequently read out, using light alone. This magnetism occurs only in crystals containing $V_O$, and is consistent with a metastable spin polarization of $V_O$-related defect complexes. These data reveal a detailed interplay between magnetism, lattice defects, and light in an archetypal complex oxide material, which may yield new insights into the recent exciting spin physics observed at oxide interfaces. \\[4pt] [1] see, {\it e.g.}: H.Y. Hwang {\it et al.}, Nat. Mater. {\bf 11}, 103 (2012); J. Mannhart \& D.G. Schlom, Science {\bf 327}, 1607 (2010); MRS Bulletin {\bf 38}, 1017 (2013).\\[0pt] [2] W.D. Rice, P. Ambwani, M. Bombeck, J.D. Thompson, G. Haugstad, C. Leighton \& SC, Nat. Mater. {\bf 13}, 481 (2014); ibid, J.Vac. Sci. Tech. B {\bf 32}, 04E102 (2014).