Size dependence of structure and magnetic properties of CoCr_2O_4 nanoparticles synthesized by hydrothermal technique

摘要

The size dependence of structure and magnetic behavior has been investigated on spinel chromite CoCr_2O_4 nanoparticles (NPs) with different particles from ～2.8 to ～19.7 nm, synthesized by a hydro-thermal technique. The microstructure analysis shows that the lattice parameter increases monotonically with reduced particle sizes. Compared with the bulk sample, all synthesized NPs exhibit lower Curie temperature (T_C～87 K) from paramagnetic (PM) to collinear short-range ferrimagnetic (FiM) state, and this PM-FiM transition is gradually degraded with decreased particle sizes and nearly vanishes for ～2.8 nm NPs. No magnetic transition from short-range FiM order to long-range spiral spin structure is found at low temperatures contrast to the bulk ones. Instead, a cluster spin-glass (SG) transition appears for NPs with D ≤ 5.4 nm. The dynamic behavior of the cluster SG can be described by a power law with the spin relaxation time in the range of 10~(-10)-10~(-8) s and critical exponent zv′=9.9 ±0.6, in accordance with a three-dimensional Ising SG behavior. The glass transition temperature (T_g) decreases from 16.3 K for ～5.4 nm to 12.8 K for ～2.8 nm NPs.