Low-temperature magnetic behavior of ball-milled copper ferrite.

We present a study on magnetic properties of CuFe2O4 nanoparticles, produced by high-energy ball milling. The series of samples obtained, with average particle sizes LFAN alpha d RTAN ranging from 61 nm to 9 nm, display increasing relaxation effects at room temperature. Irreversibility of the magnetization and shifts to negative fields in the hysteresis loops are observed below T-f APEQ 55 K, indicating unidirectional magnetic anisotropy in milled samples. These features could be explained by assuming the formation of a spin-disordered surface layer, which is exchange-coupled to the ferrimagnetic core. The temperature dependence of average hyperfine field, saturation magnetization and coercive field shows a change of slope at T-f, indicating the spin freezing of the disordered phase at T-f. From the H-C(T) curves below T-f, the thickness of the surface layer was found to increase as the average grain size decreased, reaching t EQ 2.6(6) nm for the sample having LFAN alpha d RTAN EQ 9(3) nm. The anisotropy constant value for the disordered phase was estimated to be K-I/EQ 9(1) MUL alpha 10-5 erg cm+3.