Particle size effect on the magnetic and surface heat capacities of β-Co(OH) 2 and Ni(OH) 2 crystals between 1.5 and 300 K

Abstract

The heat capacities of β-Co(OH) 2 of particle size 40 to 50 nm by greater than 100 nm, and of Ni(OH) 2 of particle sizes (i) about 2 nm by about 13 nm, (ii) 3 to 100 nm by 20 to 1000 nm, and (iii) 0.2 to 0.5 μm by 1 to 3 μm (where particle size means average distance in the 〈001〉 direction by average distance in the 〈100〉 direction) were measured between 1.5 and 300 K. These crystals showed heat capacity anomalies associated with spin ordering at (11.6±0.1) K for β-Co(OH) 2, and at (23.0±0.1) K, (24.25±0.05) K, and (24.80±0.05) K for Ni(OH) 2-(i), -(ii), and -(iii). The total entropy of transition was approximately R ln 2 for β-Co(OH) 2 and R ln 3 for Ni(OH) 2, and more than 50 per cent of the entropy was acquired above the Néel points. This fact is characteristic of the stronger two-dimensional interactions predicted from the crystal structure (CdI 2 type). The variation of transition temperature with particle size observed in the Ni salts is exceptionally large and is regarded as being due to the particle size effect on the magnetic heat capacity. An additional effect of particle size was the increased rounding of the heat capacity maxima with decreasing size of the particles. Appreciable changes in magnetic anisotropy with crystal size were also observed. The excess heat capacity due to the surface vibrations was estimated to be about 1.2 J K −1 mol −1 at 100 K for Ni(OH) 2 crystals of surface area 314 m 2 g −1.