Magnetic phase diagram of three-dimensional diluted Ising antiferromagnet Ni${}_{0.8}$Mg${}_{0.2}$(OH)${}_2$

$H$-$T$ 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 $\delta$ (= $\chi_{FC} - \chi_{ZFC}$)showsanegativelocalminimumfor10$\leq H \leq$ 35 kOe, suggesting the existence of possible glassy phase in the AF phase. A broad peak in $\delta$ and ${\chi }^{\mathrm{\prime }\mathrm{\prime }}$ at $H\ge$ 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.