Magnetic RE₂NiMnO₆ double-perovskites: thin films and superlattices

The double-perovskite RE₂NiMnO₆ (RE= rare earth) family is characterized for being insulating ferromagnets, an unusual combination of properties. Ferromagnetism arises through oxygen-mediated superexchange in the rock salt-ordered Ni/Mn sublattice. The Curie Temperature of La₂NiMnO₆ is T_c=280K, and for the other members of the family T_c decreases linearly with the size of the ionic radius of the RE [1].

Here we will show that epitaxial RE₂NiMnO₆ films (RE=La, Nd, Sm), grown by off-axis RHEED magnetron sputtering, display long-range Ni²⁺ and Mn⁴⁺ order and strain-independent bulk-like T_C at a thickness of 30 unit cells [2,3]. We find that the ferromagnetic behavior occurs down to ultra-low thicknesses of (at least) 3 unit cells (~1.2 nm). However, below 10 unit cells, the magnetic properties deteriorate due to an interfacial charge transfer caused by the polar discontinuity at the RE₂NiMnO₆/SrTiO₃ interface [3,4]. For the case of Nd₂NiMnO₆, a detailed x-ray magnetic circular dichroism (XMCD) study allows us to separate the magnetic components into a robust ferromagnetic Ni/Mn sublattice and a paramagnetic Nd sublattice [3].

We will also present our latest efforts in combining different RE₂NiMnO₆ double perovskites into potential multiferroic artificially-layered superlattices as predicted by first principles [5].