Imaging the suppression of ferromagnetism in LaMnO$_3$ by metallic overlayers

Abstract

LaMnO$_3$ (LMO) thin films epitaxially grown on SrTiO$_3$ (STO) usually exhibit ferromagnetism above a critical layer thickness. We report the use of scanning SQUID microscopy (SSM) to study the suppression of the ferromagnetism in STO/LMO/metal structures. By partially covering the LMO surface with a metallic layer, both covered and uncovered LMO regions can be studied simultaneously. While Au does not significantly influence the ferromagnetic order of the underlying LMO film, a thin Ti layer induces a strong suppression of the ferromagnetism, over tens of nanometers, which increases with time on a timescale of days. Detailed EELS analysis of the Ti-LaMnO$_3$ interface reveals \textcolor{black}{the presence of Mn$^{2+}$ and} an evolution of the Ti valence state from Ti$^0$ to Ti$^{4+}$ over approximately 5 nanometers. Furthermore, we demonstrate that by patterning Ti/Au overlayers, we can locally suppress the ferromagnetism and define ferromagnetic structures down to sub-micrometer scales.