Particle size dependence of the magnetic and magneto-caloric properties of HoCrO3

Abstract

Magnetic and magneto-caloric properties of polycrystalline powder samples of HoCrO3 with four different particle sizes are reported here. The samples were prepared by citrate method and were annealed at 700, 900, 1100, and 1300 °C to yield average particle sizes of 60 nm, 190 nm, 320 nm, and 425 nm, respectively, as determined by the analysis of X-ray diffraction patterns and images obtained by scanning electron microscopy. Additional structural characterization was done using Raman spectroscopy. Measurements of the magnetization of the samples were done from 5 K to 300 K in magnetic fields up to 70 kOe. Analysis of the temperature dependence of the paramagnetic susceptibility in terms of the modified Curie-Weiss law, including the Dzyaloshinsky-Moriya (DM) interaction, show small but systematic changes in the Néel temperature TNCr of Cr3+ ions, exchange constant J, and the DM interaction with variation in particle size. However, below TNCr the largest size-dependent effects are observed at 5 K, and the measured magnitudes of coercivity field HC are 1930, 2500, 4660, and 7790 Oe for the 60 nm, 190 nm, 320 nm, and 425 nm size particles, respectively, which can be interpreted by a single domain model. Enhancement of the particle size gives about a fourfold increase in the magnitude of the energy product, HC * MS, where MS is the saturation magnetization. However, as the particle size rises, an opposite trend is observed in the max magnetic entropy (ΔSM = 8.73, 7.22, 7.77, and 6.70 J/kg K) and the refrigerant capacity (RC = 388, 354, 330, and 310 J/kg) for the 60 nm, 190 nm, 320 nm, and 425 nm size particles, respectively. These results suggest ways to optimize the properties of HoCrO3 for applications in magnetic storage and magnetic refrigeration.