$H$-$T$ diagram of 3D diluted Ising antiferromagnet Ni$_{ c}$Mg$_{ 1-c}$(OH)$_{ 2}$ with $c$ = 0.8 has been determined from measurements of SQUID DC magnetization and AC magnetic susceptibility. At $H$ = 0, this compound undergoes two magnetic phase transitions: an antiferromagnetic (AF) transition at the N\'{ e}el temperature $T_{ N}$ (= 20.7 K) and a reentrant spin glass (RSG) transition at $T_{ RSG}$ ($\approx$ 6 K). The $H$-$T$ diagram consists of the RSG, spin glass (SG), and AF phases. These phases meet a multicritical point $P_{ m}$ ($H_{ m}$ = 42 kOe, $T_{ m}$ = 5.6 K). The irreversibility of susceptibility defined by $\delta$ (= $\chi_{ FC} - \chi_{ ZFC}$) shows a negative local minimum for 10 $\leq H \leq$ 35 kOe, suggesting the existence of possible glassy phase in the AF phase. A broad peak in $\delta$ and $\chi^{ \prime \prime}$ at $H \geq$ 20 kOe for $T_{ N}(c=0.8,H) \leq T \leq T_{ N}(c=1,H=0)$ (= 26.4 K) suggests the existence of the Griffiths phase.
Categories: cond-mat.dis-nn