Magnetic phase diagram of three-dimensional diluted Ising antiferromagnet Ni_0.8Mg_0.2(OH)₂

H T diagram of 3D diluted Ising antiferromagnet Ni_cMg _{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 Néel temperature T_N (= 20.7 K) and a reentrant spin glass (RSG) transition at T_RSG (≈ 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 δ (= XFC - XZFC) shows a negative local minimum for 10 ≤ H ≤ 35 kOe, suggesting the existence of possible glassy phase in the AF phase. A broad peak in δ and χ″ at H ≥ 20 kOe for T_N(c = 0.8, H) ≤ T ≤ T _N(c = 1, H = 0) (= 26.4 K) suggests the existence of the Griffiths phase.