Coexistence of superparamagnetism and superspin glass in non-stoichiometric Zn0.5Ca0.5Fe2O4 nanoferrite

In this manuscript, we report the coexistence of superparamagnetic-super spin glass phase in aggregated non-stoichiometric Zn0.5Ca0.5Fe2O4 ferrite nanoparticles (ZCF NPs). A polycrystalline sample of ZCF NPs was synthesized by the self-ignition sol-gel route and characterized using a variety of techniques. Extensive temperature, field and wait-time dependent dc-magnetization & ac-susceptibility measurements were performed for elucidating the magnetism of ZCF NPs. M-H & Mo center dot ssbauer data at 300 K and M-T curves in ZFC & FC protocols revealed that ZCF NPs exhibit dominant superparamagnetic (SPM) character at 300 K with M-s = 16 emu/g & T-B similar to 120 K. Magnetically disordered surface over a single domain core of ZCF NPs has been established by the shift in coercivity at 5 K in ZFC and FC protocols. ZFC magnetization for a single stop and wait protocol exhibited an ageing dip at the stop temperature (72 K) with waiting time. It is shown that the in-phase peak of acsusceptibility vs. temperature curves shifts towards higher temperature with driving frequencies and obeys dynamic scaling & Vogel-Fulcher law. These findings affirmed presence of glassy states in ZCF NPs due to interacting magnetic nanoparticles. On cooling the random ensemble of ZCF NPs, interparticle dipolar interactions lead to genesis of glassy behaviour, which is identified as superspin glass (SSG). Observation of memory and rejuvenation in a quasi-equilibrium magnetic state due to the collective nature of low-temperature spin dynamics differ ZCF NPs from canonical spin-glass. Extensive magnetic studies on ZCF NPs revealed a crossover of SPM to SSG phase just below T-B at 115 K due to the random distribution of magnetic momenta, anisotropy and interparticle interactions.