Billinge group members and collaborators recently conducted a detailed experimental and theoretical study of the magnetic interactions in the classic antiferromagnet MnO using magnetic pair distribution function (mPDF) analysis, resulting in a publication in Physical Review Letters (Frandsen et al, Phys. Rev. Lett. 116 197204, see here). This work, which has furthered the methodology of the mPDF technique while also answering longstanding questions about MnO, was highlighted by Brookhaven National Laboratory and the Columbia University School of Engineering and Applied Science (see BNL press release here and Columbia press release here).
As one of the simplest transition-metal oxides, MnO has played an important role in helping physicists and materials scientists understand the unique and varied characteristics of transition-metal oxides, such as magnetism and strong electron correlation. Despite its status as a classic, archetypal transition-metal oxide, there are still unresolved issues surrounding MnO, including questions about the precise nature of the magnetic interactions which lead to the antiferromagnetic arrangement of spins in alternating up and down orientations below 118 K.
There are two types of magnetic interactions expected to be strong for MnO and similar transition-metal oxides; the first is called direct exchange and refers to the magnetic interaction between nearest-neighbor Mn ions; the second is called superexchange and refers to the magnetic interaction that occurs between second-nearest-neighbor Mn ions through an intermediary O anion. There has been an ongoing debate about which of these interactions is more important for the magnetism in MnO, and previous experimental data could not provide a clear answer.
To address this question, we used mPDF and ab initio theory to study the short-range magnetic correlations in the paramagnetic state above 118 K. This differs from most previous experiments, which focused on the long-range ordered antiferromagnetic state below 118 K. Although they are more difficult to study, the short-range magnetic correlations in the paramagnetic state can provide rich information about the magnetic interactions in MnO. We collected neutron total scattering data at several temperatures above 118 K and used a theoretical technique called disordered-local-moment, self-interaction-corrected spin density functional theory to predict the short-range magnetic correlations at the temperatures corresponding to our data points. We then generated the theoretical mPDF corresponding to the predicted magnetic correlations and compared them to the experimental data.
The ab initio theory predicted that superexchange would play the dominant role, and we found that the calculated mPDF patterns matched the experimental data with a very high level of agreement. To verify the superexchange scenario, we recalculated the magnetic correlations and mPDFs for different relative strengths of superexchange and direct exchange, and from this we confirmed that the magnetic correlations that would be produced by a dominant direct exchange are inconsistent with the experimental mPDF data. In the best-fit case, the superexchange interaction was found to be nearly twice as much as the direct exchange interaction, effectively settling the longstanding question about the magnetic interactions in MnO.
This work has resulted in the third publication on mPDF in the Billinge group; the first one established the theory of the technique, the second one provided an experimental proof of principle, and this one has used the technique to significantly advance our understanding of a classic antiferromagnetic material. We are encouraged by these results and expect that mPDF will continue to develop into a useful tool for studying magnetism in complex materials.