Magnetic structure determination from the magnetic pair distribution function (mPDF): ground state of MnO

An experimental determination of the magnetic pair distribution function (mPDF) defined in an earlier paper (Frandsen, Yang, and Billinge. (2014) Acta Crystallogr. A, 70(1), 3-11) is presented for the first time. The mPDF was determined from neutron powder diffraction data from a reactor and a neutron time-of-flight total scattering source on a powder sample of the antiferromagnetic oxide MnO. A description of the data treatment that allowed the measured mPDF to be extracted and then modelled is provided and utilized to investigate the low-temperature structure of MnO. Atomic and magnetic co-refinements support the scenario of a locally monoclinic ground-state atomic structure, despite the average structure being rhombohedral, with the magnetic PDF analysis successfully recovering the known antiferromagnetic spin configuration. The total scattering data suggest a preference for the spin axis to lie along the pseudocubic [10$\bar{1}$] direction. Finally, $r$-dependent PDF refinements indicate that the local monoclinic structure tends toward the average rhombohedral $R\overline{3}m$ symmetry over a length scale of approximately 100~\AA.