Influence of oxygen concentration on the magnetic properties of multifunctional La$_{2}$CoMnO$_{6}$ thin films*

The dependence of the magnetic properties on oxygen concentration in epitaxial La$_{2}$CoMnO$_{6 }$(LCMO) thin films has been investigated grown by pulsed laser deposition (PLD). Using x-ray magnetic circular dichroism spectroscopy (XMCD) at Mn-$L_{2,3}$ and Co-$L_{2,3}$ edges, we have determined that the primary ion valence state is Mn$^{4+}$/Co$^{2+}$. Additionally, we see evidence of some low-spin Co$^{3+}$ ions, corresponding to the existence of a second ferromagnetic (FM) phases in our samples. The existence of oxygen vacancies induces the local vibronic Mn$^{3+}$-O-Co$^{3+}$ superexchange interactions in direct competition with the static FM Mn$^{4+}$-O-Co$^{2+}$ interactions. This results in the appearance of a new low temperature FM phase and suppression of the high-temperature FM phase, creating two distinct magnetic phase transitions.