Magnetic evolution from the superparamagnetism in nanospinel chromites Cd1−xCoxCr2O4 (0 ≤ x ≤ 1.0)

Abstract

Magnetic properties of the spinel-type Cd1−xCoxCr2O4 (0 ≤ x ≤ 1.0) nanocrystals are systematically studied. Complementary results of the inductively coupled plasma optical emission spectroscopy (ICP-OES) and the dispersive X-ray spectroscopy (EDX) indicate cations redistribution with increased Cr concentration from the nanoparticle core to the surface. Powder X-ray diffraction (XRD) reveals an obtained single spinel phase of average crystallite size of 7–10 nm that is in consistence with TEM particle size. The lattice constant exhibits a general decrease by incorporating the smaller Co atoms, however its behavior violates Vegard’s law. An observed kink anomaly in the lattice constant behavior at x ~ 0.4 is attributed to the cations redistribution and further preferred site occupations. Static magnetization and AC susceptibility measurements reveal competitive magnetic phases in the solid solution nanocrystals. A superparamagnetic state of the noninteracting and single domain CdCr2O4 nanoparticles expands first to higher blocking temperatures at x < 0.4 before it evolves to a core collinear ferrimagnetic (FIM) state with coexisting surface spin-glass freezing. The low-temperature core spiral spin orders survive in the FIM phase. A tentative magnetic phase diagram is presented and discussed in a frame of effects of the type and redistribution of cations as well as emergent uncompensated surface spins on the nearest and next nearest neighbor exchanges in chromite nanospinels.