Prospective Multiferroicity in Precisely Engineered Oxide Heterostructures

Multiferroic materials combine ferromagnetism and ferroelectricity, making them very interesting for future computing applications. As single-phase multiferroics are extremely rare, engineering them as artificial heterostructures is an enticing alternative approach. Here, we present our latest efforts in combining La₂NiMnO₆ and Sm₂NiMnO₆ double perovskites into multiferroic superlattices. These materials are intrinsically ferromagnetic, and artificial layering is introduced to induce in-plane ferroelectricity, which is predicted by DFT calculations. [1] The in-plane antipolar motion is unequal between layers featuring La and Sm. For superlattices of odd periodicity, the antipolar displacements are expected not to cancel out, hence leading to a macroscopic polarization. Successful fabrication of these superlattices requires highest precision growth techniques. Here, we employ a custom-designed and -built radio frequency magnetron sputtering setup with in situ reflection high energy electron diffraction (RHEED). All our superlattices feature robust ferromagnetism through oxygen-mediated Ni-O-Mn superexchange. Atomic resolution scanning transmission electron microscopy (STEM) clearly reveals unequal antipolar motion in layers with La or Sm on the A-site. This is a successful first step in establishing ferroelectricity, in addition to ferromagnetism, in these artificially layered heterostructures.

[1] H. J. Zhao, et al., Nature Communications 5, 4021 (2014).