Ferromagnetism in ultrathin double perovskite La₂NiMnO₆ thin films

Double perovskite oxides with chemical formula A₂BB’O₆ possess the prototypical perovskite structure integrated by two different cations (B, B’) ordered in a rock-salt fashion. In the R₂NiMnO₆ (RNMO) family, R being a rare-earth cation, the positive superexchange interaction between the Ni²⁺ and Mn⁴⁺ electronic states results in the coexistence of ferromagnetic order with insulating behaviour, a combination of features that is rarely found in nature. La₂NiMnO₆ (LNMO), in particular, has a bulk ferromagnetic Curie temperature of circa 280K. Although an insulating ferromagnetic oxide with near room temperature T_C would be ideal for novel spintronic devices, only a few attempts of growing ultrathin LNMO films have been reported so far.
We grew LNMO films by RF off-axis magnetron sputtering and employed AFM, XRD, STEM and EDX for structural characterization. The results indicate high crystalline quality, sharp interfaces and minimal degree of antisite disorder. The valence state of the Ni and Mn ions has been also inferred by XAS. Bulk SQUID magnetometry and element sensitive XMCD reveal that the ferromagnetic order of our films is robust against epitaxial strain and is retained down to few unit cells (less than 2 nm).