Imaging the suppression of ferromagnetism in LaMnO_3 by metallic overlayers

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 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.