Influence of growth mode \& substrate doping on the reversed remanent magnetic configuration in La$_{0.7}$Sr$_{0.3}$MnO$_3$ films

Charge transfer effects which can occur at oxide interfaces can modify the properties of oxide thin films. In such a system, an unusual reversed orientation of the remanent magnetic state was observed recently for La$_{0.7}$Sr$_{0.3}$MnO$_3$ on Nb-doped SrTiO$_3$(001) deposited via pulsed laser deposition\footnote{J.-S. Lee \emph{et al., Phys. Rev. Lett. } {\bf 105}, 257204 (2010)}. We observe a similar effect for La$_{0.7}$Sr$_{0.3}$MnO$_3$ grown via molecular beam epitaxy, a deposition method with different growth kinetics, onto both Nb-doped and undoped SrTiO$_3$ (STO) substrates. The reversed magnetic state occurrs in both samples, and a region of slightly increased charge density was revealed with x-ray reflectivity. Intriguingly, the onset of the reversed remanent state occurred at different temperatures: $\sim$125K for the Nb:STO substrate and $\sim$240K for the undoped STO substrate. High resolution x-ray diffraction reveals a subtle relationship with the cubic-to-tetragonal structural transition of the STO substrate at $\sim$105 K. Our results point to an additional mechanism for controlling the magnetism in mixed-valence oxide films\footnote{J.-S. Lee \emph{et al., J. Phys. D: Appl. Phys}. {\bf 44}, 245002 (2011)}.