Magnetic freezing and collective behavior of nanostructures

Sayan Chandra (University of South Florida)

Collective magnetic behavior in nanostructures is a phenomenon commonly observed in various magnetic systems. It arises due to competing inter/intra-particle interactions, size distribution and can manifest in phenomena like magnetic freezing, magnetic aging, and exchange bias (EB) effect. In order to probe these rather complex phenomena, both conventional DC and AC magnetic measurements have been performed along with radio-frequency transverse susceptibility (TS) measurements. We also demonstrate the magnetic entropy change as a parameter sensitive to subtle changes in the magnetization dynamics of nanostructures. The focus of this talk is to discuss the collective magnetic behavior in core/shell nanostructures of Fe/γ-Fe2O3 and Co/CoO, and LaMnO3 nanoparticles.

In the case of core/shell Fe/γ-Fe2O3, we found the particles to critically slow down below the glass transition temperature, below which they exhibit aging effects associated with a superspin glass (SSG) state. We demonstrate that it is possible to identify individual magnetic responses of the Fe core and the γ-Fe2O3 shell. Consistently, a systematic study of the magnetocaloric effect (MCE) in the Fe/γ-Fe2O3 system reveals the development of inverse MCE with peaks associated with the individual magnetic freezing of the core and the shell. From these obtained results, we establish a general criterion for EB to develop in core/shell nanostructures, that is when the core is in the frozen state and the magnetic moments in the shell begin to block. This criterion is shown to be valid for both ferromagnetic/ferrimagnetic (FM/FIM) Fe/γ-Fe2O3 and ferromagnetic/antiferromagnetic (FM/AFM) Co/CoO core-shell nanostructures. We also elucidate the physical origin of the occurrence of asymmetric field-cooled hysteresis loops and its dependence on magnetic anisotropy in the Co/CoO system by performing a detailed TS study. Finally, we have studied the collective magnetic behavior of LaMnO3 nanoparticles synthesized by the sol-gel technique. The nanoparticle ensemble shows the unusual co-existence of super-ferromagnetism (SFM), as well as the SSG state, which we term the 'ferromagnetic superglass' (FSG) state. The existence of FSG and the characteristics of its magnetic ground state will be discussed.

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