The dynamic magnetic properties of cluster glass in La0.25Ca0.75MnO3 nanoparticles with average particle size range from 40 to 1000 nm have been investigated by measuring the frequency and dc magnetic field (H) dependencies of the ac susceptibility. The frequency-dependent Tf, the freezing temperature of the ferromagnetic clusters determined by the peak in the real part of the ac susceptibility χ′ versus T curve with H=0, is fit to a power law. The relaxation time constant τ0 decreases as the particle size increases from 40 to 350 nm, which indicates the decrease in the size of the clusters at the surface of the nanoparticle. The relationship between H and Tf(H) deviates from the De Almeida–Thouless-type phase boundary at relatively high fields for the samples with size range from 40 to 350 nm. Moreover, for the samples with particle sizes of 40 and 100 nm, τ0 increases with increasing H, which indicates the increasing cluster size and may be ascribed to the competition between the influence of H and the local anisotropy field in the shell spins. All these results may give rise to a new insight into the behaviors of the cluster glass state in the nanosized antiferromagnetic charge-ordered perovskite manganites.

Structure and magnetic properties of the compounds Zn1−xCuxCr2O4 (ZCCO) are investigated systematically. A structural phase transition from space-group symmetry F d3m to I41/amd is observed in ZCCO. The critical value of the doping, x, appears at 0.58 ∼ 0.62 through the appearance of a splitting of diffraction peaks at room temperature. Measurements of dc magnetization, ac susceptibility, memory effect and exchange bias-like (EB-like) effect have been performed to reveal the glassy magnetic behaviors of ZCCO. The system with x 0.50 is suggested as a spin glass-like (SG-like) of magnetic characterization whereas doping values of 0.58 x 0.90 defines the system as a "cluster glass-like" (CG-like) with unidirectional anisotropy. The Cu content suppresses the geometrical frustration of ZnCr2O4, which may correlate with the pinning effect of Cu sublattice on Cr sublattice to a preferential direction.