Superlattices

Materials that exhibit simultaneous order in their electric and magnetic ground states hold tremendous promise for use in next-generation, low-power memory and logic devices in which electric fields control magnetism. Such materials are, however, rare as a consequence of the competing requirements for ferroelectricity and magnetism, and until recently BiFeO₃ was the only material with this functionality at room temperature. Interface materials are a way to overcome these competing requirements, as was recently demonstrated for (LuFeO₃)_m/(LuFe₂O₄)₁ superlattices [J.A. Mundy et al. Nature 537 (2016) 523–527.]. The rumpling imposed by the geometric ferroelectric hexagonal LuFeO₃ imposes a local distortion on the neighboring LuFe₂O₄—a distortion that removes the mirror symmetry that the LuFe₂O₄ layers would otherwise have. This breaking of symmetry enables the LuFe₂O₄ to become simultaneously ferrimagnetic and ferroelectric. This rumpling is distinct from strain engineering because no macroscopic strain is involved. In this presentation we extend this atomically engineered design methodology to LuFeO₃/CoFe₂O₄ superlattices producing a robust ground state that is simultaneously ferroelectric and ferrimagnetic at temperatures well above room temperature.

* The work reported was performed in collaboration with the groups of Elke Arenholz (Advanced Light Source, LBNL), Julie A. Borchers (NIST), Craig J. Fennie (Cornell), Lena F. Kourkoutis (Cornell), Steven A. McGill (National High Magnetic Field Laboratory), David A. Muller (Cornell), Julia A. Mundy (Harvard), Janice L. Musfeldt (University of Tennessee), Ramamoorthy Ramesh (UC Berkeley), William D. Ratcliff (NIST), Peter Schiffer (Yale), and Andreas Scholl (Advanced Light Source, LBNL).