Adhikari, Samyog; (2025) Wet chemical synthesis and characterisation of core-shell nanoparticles for magnetic and multiferroic applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Magnetic and multiferroic nanomaterials are of great scientific and technological interest due to their distinctive properties and multifunctionality. These materials are crucial for various applications, particularly in nanoelectronics and biomedicine. Multiferroics exhibit multiple primary order parameters, such as ferromagnetism and ferroelectricity, alongside non-primary parameters like antiferromagnetism, ferrimagnetism, and ferrotoroidicity. Their multifunctionality enables novel technological applications, facilitates device miniaturisation, improves energy efficiency, and offers tuneable properties. Beyond electronics, magnetic and multiferroic nanoparticles hold promise in biomedical applications, where their properties can be optimised to enhance bioavailability and biocompatibility. Such improvements make them particularly relevant for precision medicine and targeted therapies. This thesis explores two multifunctional core-shell nanostructures. The first study focused on the magnetic interaction between the antiferromagnetic and ferrimagnetic core-shell nanoparticles. Colloidally stable Co0.6Fe0.4O-Co1.4Fe1.6O4 nanoparticles with lattice-matched interface were synthesised using a one-step thermal decomposition method. Record-exchange bias was observed due to interactions between the antiferromagnet core and ferrimagnet shell. The Co0.6Fe0.4O phase also exhibited a Néel temperature of 397 K, significantly higher than that of the bulk CoO and FeO at ~293 K and ~198 K, respectively. The second study investigated the coupling of ferrimagnetism in cobalt and nickel-doped ferrites with the ferroelectric properties of barium titanate. To explore the feasibility of synthesising magnetoelectric core-shell nanostructures, various synthesis methods were investigated. This involved a twostep approach where the magnetic cores were prepared via solvothermal or thermal decomposition methods, followed by the growth of the ferroelectric shell using solvothermal or sol-gel techniques. The biphasic nanomaterials were characterised in terms of their morphological, chemical, structural, optical, and magnetic properties, as well as their multifunctional property. A comprehensive characterisation provided key insights into the exchange bias and magnetoelectric effects, helping to understand the influence of synthesis methods on nanomaterial properties.

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