CoFe2O4/Cr2O3 Nanoparticles: A Candidate for Room-temperature and above Magnetoelectricity

Abstract

In response to the recent scientific and technological interest, and to meet the need for increased power efficiency, miniaturization, and reduced manufacturing costs, new materials need to be considered. Potential candidates include complex oxides that exhibit magnetoelectric (ME) coupling, the ability to control magnetization through the applied electric field, and conversely influence the electric polarization by the application of the magnetic field. They usually are materials with new and enhanced physical and chemical properties combined in one single multi-functional platform. Moreover, materials must exhibit room temperature and above magnetoelectricity if they are to be utilized for practical applications. In this regard, through appropriate material selection, multi-phase materials exhibit enhanced ME coupling, usually absent or found at cryogenic temperature with a lower magnitude in single-phase systems. In this thesis, CoFe2O4 (CFO)/Cr2O3 (CRO) nanoparticles are investigated for their structural, magnetic, electric, and magnetoelectric properties. This thesis work provides an effective approach to increase the ME operating temperature of primordial sesqui oxide CRO. The bi-magnetic CFO/CRO nanoparticles having core/shell architectures as well as in the form of composites are synthesized via the sol-gel auto-combustion method by varying relative molar concentrations of CFO and CRO phases. The presence of distinct X-ray diffraction (XRD) peaks in all of the prepared bi-phasic systems corresponds to the cubic structure of CFO having space group Fd-3m (No. 227) and the rhombohedral structure of CRO with space group R-3c (No. 167). The micro-strain induced in the CFO (compressive strain) and CRO (tensile strain) phases have been validated from the Rietveld refinement of XRD data, and are complementary to the Fourier transform infrared spectroscopy and Raman spectroscopy studies. Field emission scanning electron micrographs display the homogeneous distribution of spherical shape particles with average particle sizes of ~100-120 nm. Transmission electron microscopy measurement confirms the core/shell configuration of the nanoparticles. The magnetic properties of CFO/CRO nanoparticles are significantly different in many aspects from the bare CFO nanoparticles, caused by the magnetic proximity effect. The thermal variations in magnetization measurements reveal a considerable decrease in ferrimagnetic transition temperature (TC) for the nanocomposite samples compared to bare CFO. The magnetic field variation in magnetization measurements suggests screening of ferromagnetic interaction of CFO (core) due to CRO shell over it, such that core/shell nanoparticles respond like single domain particles. A careful inspection of the impedance and modulus data suggests single relaxation in the studied frequency/temperature range for all the compositions and are well described by the impedance-based Havriliak-Negami expression. Both, the relaxation and the conduction processes are found to be polaronic, obeying Mott variable range hopping mechanism. The relaxation spectra are found to be significantly affected by the applied magnetic field, thereby manifesting the signature of the ME effect. Direct ME measurements on nanocomposites display linear ME responses up to ~350 K, above which it is dominated by magnetoconductivity/magnetoloss which results in a ‘U’ shaped nature with negligible magnetoelectricity. Nonetheless, core/shell samples exhibit the presence of linear magnetoelectricity for temperatures as high as 400 K ―a hallmark of enhancement in the ME operating temperature of the parental CRO phase. The results presented in this thesis elucidate the role of magnetic proximity effects on enhancing the magnetoelectric operating temperature and may provide insight into the development of new room temperature and above magnetoelectric devices whose capacity for storage, speed, and energy efficiency may surpass the current state-of-the-art.