Towards a magnetic core-shell nanostructure: A novel composite made by a citrate-nitrate auto-ignition process

The magnetic properties and microstructure in nanocrystalline ferrimagnetic (FIM) Y₃Fe₃Al₂O₁₂ (Al-YIG) powder precipitated during an auto-ignition of a citrate-nitrate gel precursor onto commercial ferromagnetic (FM) CrO₂ powders were studied. The synthesis objective was to create an ideal core-shell structure of CrO₂ surrounded by Al-YIG with possible application for magnetocaloric applications; however, agglomeration of the acicular CrO ₂ particles prevented individual particles from complete encapsulation by Al-YIG. This result was confirmed by electron microscopy. Due to the metastability of CrO₂, magnetic interaction between the ferromagnetic CrO₂ and the ferrimagnetic Al-YIG is found to be mediated by antiferromagnetic (AFM) Cr₂O₃. Low-temperature magnetic measurement provides distinct evidence of an exchange bias shift in the major hysteresis loop with features that are consistent with interfacial exchange from two kinds of interfaces: CrO₂ (FM)/Cr ₂O₃ (AFM) and Y₃Fe₃Al ₂O₁₂ (FIM)/Cr₂O₃ (AFM). Subsequent ex situ heat treatment of the reacted particles accelerates the CrO ₂/Cr₂O₃ transformation and produces additional alterations to the low-temperature magnetic hysteresis loops. The results of this work provide evidence that the magnetic couplings at the FM/AFM and FIM/ AFM interfaces are flexible under the action of applied fields, a conclusion that may be traced to the unique spin arrangement expected at the phase interfaces.