Comparison of the nature of magnetism in α-Ni(OH)2 and β-Ni(OH)2.

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. [ABSTRACT FROM AUTHOR]