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 CrO2 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)/Cr2O3 (AFM) and Y3Fe3Al2O12 (FIM)/Cr2O3 (AFM). Subsequent ex situ heat treatment of the reacted particles accelerates the CrO2/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.