Superparamagnetic behavior in a Ni vermiculite intercalation compound

The Ni vermiculite intercalation compound (VIC) magnetically behaves like a quasi two-dimensional (2D) Ising-like site-random ferromagnet on the triangular lattice sites, with weak antiferromagnetic interplanar interactions. The magnetic properties of Ni VIC have been studied using superconducting quantum interference device (SQUID) DC magnetization and SQUID AC magnetic susceptibility measurements. The 2D ferromagnetic short-range order of ${\mathrm{Ni}}^{2+}$ spins starts to grow below 45 K. A partially disordered antiferromagnetic phase is established below $T_N(=21.0\mathrm{}\mathrm{K}),$ where 2D ferromagnetic Ni clusters are antiferromagnetically coupled along the c axis. The dispersion ${\chi }_{\mathrm{cc}}^{\prime }$ along the c axis shows a peak around 2–3 K shifting to the low-temperature side with increasing AC frequency. Temperature dependence of the corresponding average relaxation time is well described by a generalized Arrhenius law. The system may be formed of disordered antiferromagnetic clusters, exhibiting a superparamagnetic behavior.