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Ferromagnetism Emerges to Alleviate Polar Mismatch

Alchemists dreamed of turning boring, base metals into exotic, noble metals. Although such dreams were never realized, scientists today can induce unexpected properties at the interface between two materials—including properties not present in either parent material. For example, researchers have discovered that if they combine SrTiO3 (a dielectric and paramagnetic material) together with LaMnO3 (an insulating and antiferromagnetic material) in just the right way, they can induce an insulating ferromagnetic state at the interface.

At ALS Beamlines 4.0.2 and 6.3.1, researchers studied LaMnO3/SrTiO3 heterostructures using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), techniques that allow for element-specific study of electronic structure and magnetic order. They found that when LaMnO3—composed of alternating charged (LaO)+ and (MnO2)– layers—is put into contact with SrTiO3—composed of neutral SrO and TiO2 layers—the resulting “polar catastrophe” (polar mismatch) drives an electronic reconstruction wherein extra electrons are localized in the LaMnO3. This, in turn, induces interfacial ferromagnetism in a system not expected to be ferromagnetic. The researchers can ultimately control the system in such a way that this emergent magnetism can be turned on after growing just three layers of LaMnO3.

The work highlights how far scientists have come in their ability to produce and study atomically precise versions of materials and how this control provides a pathway to the production of properties that wouldn’t otherwise exist in the materials. Ultimately, this type of work lays the foundation for realizing functions that could enable new types of logic, memory, sensing, and other technologies.

Left: Illustration of the LaMnO3/SrTiO3 heterostructure, showing an LaMnO3 thickness of three unit cells (UC). Center: XAS spectra at the Mn L2 and L3 edges for various sample thicknesses. The technique probes the evolution of the Mn valence electron states as sample thickness changes. Several reference spectra are shown at the bottom for comparison. Right: Mn XMCD spectra for various sample thicknesses indicate a net ferromagnetic moment, arising from Mn in the films. The inset shows a schematic of the experimental configuration.

Publication: Z. Chen, Z.H. Chen, Z.Q. Liu, M.E. Holtz, C.J. Li, X.R. Wang, W.M. Lu, M. Motapothula, L.S. Fan, J.A. Turcaud, L.R. Dedon, C. Frederick, R.J. Xu, R. Gao, A.T. N’Diaye, E. Arenholz, J.A. Mundy, T. Venkatesan, D.A. Muller, L.-W. Wang, J. Liu, and L.W. Martin, “Electronic Accumulation and Emergent Magnetism in LaMnO3/SrTiO3 Heterostructures,” Phys. Rev. Lett. 119, 156801 (2017), doi:10.1103/PhysRevLett.119.156801.