Structure and magnetism of ${\mathrm{Bi}}_2$(Sr,Ca${)}_2$${\mathrm{MnO}}_{6+\mathit{y}}$ antiferromagnets with ferrimagnetic layers

The layered oxides ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{MnO}}_{6+\mathit{y}}$ and ${\mathrm{Bi}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{MnO}}_{6+\mathit{y}}$, the Mn analogs of the superconductor ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_{6+\mathit{y}}$, have anomalously sharp peaks in their magnetic susceptibility as a function of temperature at 120 and 100 K, respectively. We have studied the crystallographic and magnetic structure of these compounds by x-ray, electron, and neutron diffraction on both powders and single crystals and correlated the structure with the magnetic properties. In the magnetically ordered state, nearest-neighbor Mn moments are antiparallel and point normal to the ${\mathrm{MnO}}_2$ layers. As in the superconducting Cu analog, the crystallographic structure of these compounds is distorted; because of the flexing of the atomic slabs associated with the distortion, not all the Mn in the crystal are crystallographically identical, so the magnetic moment of Mn can vary between these different lattice sites. As a result, the moments on adjacent sites do not exactly cancel, and each layer of ${\mathrm{MnO}}_2$ is a ferrimagnet. A magnetic field can induce a transition from an antiferromagnetic phase, where the net moments of different layers are opposed, to a ferrimagnetic phase, where the net moments are aligned. A simple mean-field theory mimics the shape of the susceptibility versus temperature, and the deficiencies of the mean-field theory suggest the importance of fluctuations and domains.