Big Magnetism in an Ultrathin Material

We live in an increasingly digital world, and as such we have a growing need for innovative ways to read and store the tsunami of information that has accompanied the shift from paper to bytes. As scientists and engineers look for new options in that realm, one possibility is multiferroics. Ferroics, as a class, are materials that can suddenly flip their internal alignment. In ferromagnets, a magnetic field can quickly change the alignment of electron spins. In ferroelectrics, an electric field can switch electric dipole moments. Multiferroics can be both magnetic and ferroelectric, offering the promise of tweaking both magnetism and electricity in a single material to harness desired properties.

In research published in Advanced Materials, Joint Faculty Professor Michael Fitzsimmons and colleagues have engineered an ultra-thin material that shows enhanced magnetization that persists to a record temperature of 200 Kelvin, significantly higher than previous findings (293 Kelvin is room temperature and the minimum threshold for practical applications). The combination of BiFeO₃ (bismuth, iron and oxygen) and La_0.7Sr_0.3MnO₃ (lanthanum, strontium manganese, and oxygen) combines ferroelectric (BFO) and ferromagnetic (LSMO) materials. The research not only reveals uncompensated magnetism in the normally non-ferromagnetic BFO at a high temperature but also gives more insight into how a combination of different materials yields a nanocomposite that is a multiferroic.

Both BFO and LSMO are single crystal layers, meaning they’re structured from layers where the atoms follow a repeating lattice pattern. The layers themselves are thin enough to be measured in nanometers. Fitzsimmons and his co-authors found that by interleaving BFO layers between two manganite layers—the same way you’d insert a blank page between two pages of a book—they created a superlattice combination with enhanced magnetization at the interfaces. Because they’re dealing with incredibly small systems and even minor adjustments can significantly alter the outcome, scientists try different combinations of layered materials to discover a solution that achieves a novel function—in this case one useful to meet society’s needs for information technology. The researchers fabricated samples with different thicknesses of BFO layers and found, using neutron scattering, large magnetization of the BFO atomic layers closest to the LSMO. The affected region was only three to four unit cells deep in the BFO layer, indicating that proximity to—as well as the presence of—the manganese layers is key to realizing this phenomenon. But what is its precise origin? The researchers found that it has to do with a reorganization of how orbitals of the Fe and Mn atoms are structured. Combination of a variety of techniques including neutron scattering, resonant soft x-ray scattering and scanning transmission electron microscopy (STEM) measurements led the researchers to conclude strong hybridization (mixing) of manganese and iron orbitals across the BFO/LSMO interfaces produces canting of the magnetic spins and the large interface magnetization.

The research, published in “Spatially Resolved Large Magnetization in Ultrathin BiFeO₃,” gives promise to further work where new materials with geometrically engineered superlattices could provide more opportunities for controlling magnetism in nanomaterials, especially at higher temperatures. Along with Fitzsimmons, the authors are Er-Jia Guo, Jonathan R. Petrie, Manuel A. Roldan, Qian Li, Ryan D. Desautels, Timothy Charlton, Andreas Herklotz, John Nichols, Johan van Lierop, John W. Freeland, Sergei V. Kalinin, Ho Nyung Lee. The full paper is available online: http://onlinelibrary.wiley.com/doi/10.1002/adma.201700790/full.