Atomically-engineered heterostructures constitute excellent model systems to investigate fundamental structure-property relations in transition metal oxides and their evolution as the thickness of the constituent layers is reduced to only a few unit cells.
Double perovskite oxides with chemical formula A2BB’O6 possess the prototypical ABO3 perovskite structure additionally characterized by two long-range-ordered cations (B, B’) that provide an additional degree of freedom to tune the desired functionality.
The R2NiMnO6 (R=rare earth ion) family, with rock-salt ordering of Ni and Mn sublattices, is particularly interesting due to the rare property combination of being simultaneously insulating and ferromagnetic. The Curie temperature (Tc) is close to room temperature for La2NiMnO6 (280 K), and it decreases linearly with the size of the rare earth.
Long-range Ni-Mn ordered films of R2NiMnO6 (R= La, Nd, Sm) are successfully grown by off-axis rf magnetron sputtering. The films display bulk-like magnetic properties, with Tc independent of the epitaxial strain for films of 30 unit cells or thicker (t>10 nm) [DeLuca, APL Materials, DOI: 10.1063/5.0055614; Jonathan PRM in press] , opens an external URL in a new window
The films of ferromagnetic insulating La2NiMnO6 (LNMO) are still magnetic down to 2 u.c.. However, the magnetic properties of 2-5 u.c. LNMO films are affected beyond dimensionality effects due to an electronic reconstruction at the interfaces. Using a top LaNiO3 electron-acceptor layer, the electron excess is redistributed, and the magnetism of the ultrathin LNMO films is restored.
Interestingly, in Nd2NiMnO6 thin films, the Nd sublattice embedded in the Ni/Mn ferromagnetic lattice is found to display a paramagnetic behaviour at low temperatures, as revealed by XMCD measurements [Jonathan Spring, in press].
Superlattices using double-perovskites as building blocks
The implementation of multiferroic devices is especially hampered by the scarcity of single-phase compounds exhibiting sizable electric polarization and ferromagnetism at room temperature. Materials by design arise as a powerful strategy to overcome this issue. Interestingly, it has been predicted by first-principles calculations that artificial materials consisting of alternating layers of the double-perovskites La2NiMnO6 and R2NiMnO6 (R=rare earth ion) will exhibit multiferroic behavior close to room temperature.
We are currently growing La2NiMnO6/ R2NiMnO6 superlattices by RHEED-equipped off-axis magnetron sputtering, and investigating their magnetic and potential ferroelectric properties.
Compared to most of the 3d transition metal oxides, the perovskite Cr-family has been little explored. The high pressures and temperatures typically required to stabilize Cr(IV) oxidation state in an octahedral configuration might be the reason. As a result, the properties of these compounds are still debated. We use epitaxy to facilitate the stabilization of the perovskite phase. Transport and magnetic measurements are used to determine the ground state of SrCrO3 but also its strain- and thickness-dependence.