“Super-ferromagnetic” clusters in spinel oxide

Abstract We report the effect of large magnetic moment of Ho3+ (free ion moment ≈10 μ B ) ions and interactions between 4f (for Ho3+ ions) and 3d (for Fe3+ ions) spins in Co0.2Zn0.8Fe1.95Ho0.05O4 spinel oxide. Co0.2Zn0.8Fe2O4 is a typical cluster glass system where B site Fe3+ moments form finite clusters. On substituting small amount of Fe3+ ions by Ho3+ ions, the system shows re-entrant like magnetic phases with paramagnetic to ferromagnetic phase transition at TC ≈188 K and ferromagnetic to cluster glass freezing below Tf≈120 K. The magnetic complexity in the ferromagnetic phase can be attributed to the existence of different size clusters and associated relaxation time distribution. To simplify the problem, we have considered a random distribution of large clusters containing RE3+–Fe3+ moments along with small clusters of Fe3+ moments. The larger clusters with long relaxation time can show a spontaneous magnetic contribution at ≈170 K in temperature dependence of thermoremanent magnetization (TRM) data. We have defined the large clusters as “super-ferromagnetic” in comparison with the small Fe3+ clusters.