Nature of magnetic ordering in Ni(OH)$_{2}$ nanoplates

Nickel hydroxides are important for their potential applications in rechargeable batteries and as precursors for NiO and Ni catalysts. $\beta $--Ni(OH)$_{2}$ has the CdI$_{2}$ layered structure with Ni atoms forming a hexagonal unit cell. Here, we report on the magnetic ordering in 17 nm $\times $ 4 nm nanosheets of $\beta $--Ni(OH)$_{2}$ . Measurements of the magnetization M as a function of temperature (2K to 300K) and magnetic field H up to $\pm $65kOe are reported. M vs. T data in H =100 Oe for the ZFC case shows a peak in M at T$_{N}$ = 24 K characteristic of antiferromagnetic (AF) ordering; however for T $>$ T$_{N}$, the Curie-Weiss ($\chi $ = C/(T - $\theta ))$ fit yields $\theta $ = 26K characteristic of ferromagnetism. Following Takada (J. Phys. Soc. Jpn. 21, 2745, 1966), we measured M vs. H loops from T = 2K to 25K and observed a metamagnetic transition at H$_{c}$ = 56 kOe at 2K, with H$_{c}$ decreasing with increasing T. These results suggests strong ferromagnetic coupling among Ni within (001) sheets and a weaker antiferromagnetic coupling in the neighboring (001) sheets, and [001] as the easy axis. This model is used to determine the exchange constants consistent with the observed Curie-Weiss variation.