Spin-Flop Transition and Magnetocaloric Effect through Disconnected Magnetic Blocks in CoIII/CoIV Oxybromides

The magnetic properties of the layered Ba n +1 Co n O 3 n −1 Br ( n = 5 and 6) oxybromides have been determined by means of magnetization measurements and neutron diffraction under variable magnetic field. For n = 6, the magnetic phase diagram has been built on the basis of several features, including the notions of short-range ordering, spin reorientation, and spin-flop transitions. In those compounds, the competition between magnetic exchanges arise from the existence of [Ba 2 O 2 Br] − double-layers that separate perovskite 2D-blocks. The latter are dominated by ferromagnetic (FM) intraexchanges while the interblock exchanges are antiferromagnetic (AFM). Interestingly, a perturbation created by an external magnetic field could be on the same order of magnitude than the interblock exchange leading to a complex set of spin reorientations versus H and T . From the point of view of magneto-crystalline anisotropy, the AFM system with moments parallel to the c -axis turns into a FM system with moments aligned in the perpendicular ( a, b ) planes. The magnetic entropy is distributed within at least three phenomena, spread out in a wide range of temperature.Here, the influence of the magneto-crystalline anisotropy on the magnetocaloric effect is unambiguously shown.