Magnetic phase diagram of three-dimensional diluted Ising antiferromagnet Ni₀.₈Mg₀.₂(OH)₂

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H-T diagram of 3D diluted Ising antiferromagnet NicMg₁₋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 Tₙ (= 20.7 K) and a reentrant spin glass (RSG) transition at TRSG (≈ 6 K). The H-T diagram consists of the RSG, spin glass (SG), and AF phases. These phases meet a multicritical point Pₘ (Hₘ = 42 kOe, Tₘ = 5.6 K). The irreversibility of susceptibility defined by δ (= χFC - χZFC) 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 Tₙ(c=0.8,H) ≤ T ≤ Tₙ(c=1,H=0) (= 26.4 K) suggests the existence of the Griffiths phase.