When a Polar Mismatch is Just What You Need
Oxide materials have attracted scientists’ attention for the variety of properties they exhibit, be that magnetism, superconductivity, or a host of other phenomena. When two such materials meet, that interface gives scientists a micro-laboratory to tune their interactions. Assistant Professor Jian Liu and Graduate Student Clayton Frederick are among the researchers who took two electrical insulating materials—one polar (LaMnO3) and one non-polar (SrTiO3)—and induced a ferromagnetic state.
The LaMnO3 material comprised layers of LaO and MnO2, which have alternating positive and negative charges. In contrast, SrTiO3 is a stack of charge-neutral layers of SrO and TiO2. Using pulsed laser deposition, scientists grew thin films of LaMnO3 layer by layer on a substrate of SrTiO3, creating a discontinuity of the charge polarity at the interface. Availing themselves of beamlines at the Advanced Light Source at Lawrence Berkeley Laboratory, they applied x-ray absorption spectroscopy and magnetic circular dichroism to investigate what effect such a polar mismatch might have on the electronic and magnetic structure of the polar, antiferromagnetic insulator LaMnO3 grown on non-polar SrTiO3. What they found was that at the interface, LaMnO3 self-reconstructs its charge distribution (a change in valence) to accommodate the mismatch. By engineering this reconstruction, they were able to induce ferromagnetism in films as thin as three unit cells.
The work demonstrates the state of the art in ultrafine control of materials and was featured as a Science Brief on the ALS website.
The results were published in Physical Review Letters: Electron Accumulation and Emergent Magnetism in LaMnO3/SrTiO3 Heterostructures.