Details of structural and magnetic properties of a sample of ␤-Ni͑OH͒ 2 with nanoplate morphology ͑sample A͒ are compared with those of the hydrothermally synthesized bulklike ␤-Ni͑OH͒ 2 , sample B. Transmission electron microscopy, x-ray diffraction, infrared spectroscopy, and thermogravimetric analysis were used for structural characterization whereas studies of magnetic properties covered the temperature range of 2-350 K in magnetic fields upto H = 180 kOe. Temperature dependence of the low-field susceptibilities, ͑ZFC͒ and ͑FC͒, showed that ͑ZFC͒ peak at T P = 24.5 K ͑26.5 K͒ for sample A ͑B͒ with ͑FC͒ Ͼ ͑ZFC͒ below T p . Curie-Weiss fit of ͑ZFC͒ for T Ͼ T P for sample A ͑B͒ yields = 20.5 K ͑19.8 K͒ and magnetic moment = 2.92 B ͑3.30 B ͒ / Ni 2+ . Measurements of the magnetization M vs H for T Ͻ T P for sample A show a two-step transition to ferromagnetism with the first transition at H C1 Ӎ 28 kOe and the second transition at H C2 Ӎ 55 kOe. For sample B, the transition at H C1 is absent and H C2 = 53.5 kOe. Using molecular-field approach and three exchange constants determined from the Curie-Weiss fits, theoretical expressions for H C1 and H C2 are derived from which estimates of H C1 and H C2 are found to be in good agreement with the experimental values. According to this model, bulk ␤-Ni͑OH͒ 2 orders antiferromagnetically ͑AF͒ at T N Ӎ 25.5 K, with the dominant intralayer ferromagnetic ͑FM͒ coupling and a weaker interlayer AF coupling resulting in metamagnetism and FM ordering for H Ն H C2 . Nanosize effects are shown to lower T P and T N while also producing the transition at H C1 due to flipping of the surface layer Ni 2+ spins.

The two layered hexagonal hydroxides of Ni are β-Ni(OH)(2) and α-Ni(OH)(2); β-Ni(OH)(2) is now known to be an antiferromagnet whereas the nature of the magnetism in α-Ni(OH)(2) is not yet well established. Here, the magnetic properties of α-Ni(OH)(2) with lattice parameters a = 3.02 Å and c = 8.6 Å, and flower-like morphology with petal thickness of approximately equal to 50 Å are reported. Temperature (2-300 K) and magnetic field (up to 65 kOe) dependence of the magnetization and ac susceptibility at f = 0.1-1000 Hz were measured. Analysis of the data yields ferromagnetic ordering in the system with T(C) is approximately equal to 16 K. In addition, a nanosize related blocking temperature T(B) = 8 K and spin-glass-like ordering of the surface spins near 3.5 K are inferred from the ac frequency and dc magnetic field dependence of these transitions. Fitting to the high temperature series and quasi-2D nature of the system is used to determine J(1)/k(B) = 4.38 K (J(2)/k(B) = 0.14 K) for the intraplane (interplane) exchange coupling between the Ni(2+) ions.

For the two stable phases of Ni(OH)2, viz. α-Ni(OH)2 and β-Ni(OH)2, structural and magnetic properties are compared employing x-ray diffraction (XRD), scanning electron microscopy/transmission electron microscopy (TEM/SEM), and variation of the magnetization (M) with temperatures (2–350 K) and magnetic fields (up to ≈65 kOe). Both phases crystallize in the layered hexagonal structure with a=3.04 Å (3.12 Å) and c=23.6 Å (4.67 Å) for the α(β) phase with clear evidence for turbostraticity in the α-phase. From TEM/SEM, the β-phase consists of nanoplates of dimensions 30(10)×3(1) nm, whereas the α-phase has flowerlike morphology with petal thickness ≈5(1) nm. The low-field M versus T data for the zero-field-cooled/field cooled cases bifurcates at Tp=25 K (13 K) for the β(α) phase. For T>Tp, Curie–Weiss variation of M versus T is observed yielding positive θ=20.6 K (32 K) with μ=2.95 μB (3.13 μB) for the Ni2+ ions in the β(α) phase. The M versus H data at 2 K shows antiferromagnetic (ferromagnetic) like variation for the β(α) phase with dM/dH showing peaks at Hc1=28 kOe and Hc2=55 kOe for the β-phase. It is concluded that β-Ni(OH)2 is a metamagnet for T<TN=25 K, whereas α-Ni(OH)2 is a hard ferromagnet with Tc=13 K and domain structure for H<5 kOe.

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