Magnetism is something we are all familiar with from everyday life–we see it in action on refrigerator doors, computer hard drives, credit cards, and a hundred other gadgets. And yet, there is much about magnetism that physicists do not fully understand even today. We are currently developing the magnetic pair distribution function (mPDF) technique as a new tool to investigate magnetic structure and its relationship to novel magnetic phenomena that continue to excite the condensed matter physics and materials science communities.
In the same way that complex materials often exhibit short-range atomic correlations that help determine their properties, complex magnetic materials can possess short-range correlations among the magnetic moments that arise from the electrons’ quantum mechanical spin and orbital angular momentum. Neutron scattering is an excellent technique to determine the magnetic structure of materials, since the magnetic moment of the neutron can interact with the spins in the material. However, just like x-ray or nuclear neutron diffraction, conventional magnetic diffraction techniques fail when the magnetic structure becomes highly disordered or correlated only on a local length scale, resulting in diffuse scattering rather than sharp Bragg peaks. In analogy to atomic PDF, magnetic PDF allows us to overcome these challenges by Fourier transforming the total magnetic scattering intensity into real space. This real-space mPDF function provides a highly intuitive view of long-range and short-range magnetic correlations directly in real space and can be used to perform rapid refinements and solutions of magnetic structures.
Recent work in the Billinge group has led to the derivation of the mPDF equations for the first time [Frandsen et al (2014) Acta Crystallogr. A70, 3-11; here]. Now, we are hard at work developing the experimental aspects of the technique and applying it to actual materials. We hope that mPDF can be helpful in learning about the complex magnetism found in unconventional superconductors, frustrated magnetic materials, multiferroics, magnetic semiconductors, nanomagnets, and more.
The result of repeated mPDF refinements of the magnetic structure of MnO, showing a clear preference for the spin axis to align along specific directions in real space.