Anisotropic magnetic properties ofCa3Co4O9:Evidence for a spin-density-wave transition at 27 K

Abstract

The dc susceptibility $\ensuremath{\chi}$ of single crystal platelets of ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$ was measured in applied magnetic fields H below 55 kOe. At temperatures below 400 K, the crystals exhibited three magnetic transitions successively at ${T}_{\mathrm{SS}}\ensuremath{\sim}380\mathrm{K},$ ${T}_{\mathrm{SDW}}=27\mathrm{K},$ and ${T}_{\mathrm{ferri}}=19\mathrm{K}.$ The highest spin state (SS) transition was found to be discontinuous with the appearance of a thermal hysteresis loop about 20 K wide. At 27 K, a small shoulder in the $\ensuremath{\chi}(T)$ curve was observed only for $H\ensuremath{\perp}ab,$ indicating the formation of an incommensurate spin-density-wave (SDW) state, which was also confirmed by independent muon spin rotation and relaxation experiments. The lowest of ${T}_{\mathrm{ferri}}$ is a transition to a ferrimagnetic state, for which also a ferrimagnetic hysteresis loop was observed for $H\ensuremath{\perp}ab,$ but not for $H\ensuremath{\Vert}\mathrm{ab}.$ This anisotropic behavior indicated that the ferrimagnetic interaction is parallel to the c axis. Considering the alternating stacking structure of ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$ along the c axis, the ferrimagnetism is most likely caused by interlayer coupling between the $[{\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}]$ and $[{\mathrm{CoO}}_{2}]$ subsystems.