Successive magnetic phase transitions of a Co vermiculite intercalation compound: An Ising-like Heisenberg antiferromagnet on a triangular lattice

A Co vermiculite intercalation compound (VIC) magnetically behaves as an Ising-like Heisenberg antiferromagnet on a triangular lattice. Magnetic properties of a Co VIC have been studied using superconducting quantum interference device (SQUID) dc magnetization and SQUID ac magnetic-susceptibility measurements. This compound undergoes magnetic phase transitions at $T_{N1\mathrm{}}$ (=16.3 K) and $T_{N2\mathrm{}}$ (=10.7 K), and shows magnetic anomalies at $T_{p1\mathrm{}}$ (=7.7 K), $T_{p2\mathrm{}}$ (=5.9 K), and $T_{p3\mathrm{}}$ (≈3 K). The c-axis spin order and the in-plane spin order occur at $T_{N1\mathrm{}}$ and $T_{N2\mathrm{}},$ respectively. The magnetic phase diagram for $H\Vert c$ consists of four phases whose boundaries meet at a multicritical point ${(T}_m\approx 12\mathrm{K}{,H}_m\approx 25\mathrm{kOe}).\mathrm{}$ The freedom of rotation around an axis perpendicular to the spin plane where three sublattice-magnetization vectors lie could be frozen out below $T_{p1\mathrm{}}$ or $T_{p2\mathrm{}}.$ A superparamagnetic behavior of Arrhenius type is observed around $T_{p3\mathrm{}}.$