Systematic study of the synthesis of nanostructured multiferroic films and their structural, electronic and magnetic properties

Abstract

Multiferroics are unique materials that display multiple ferroic properties (ferroelectricity, ferromagnetism and ferroelasticity) simultaneously. A number of materials containing bismuth have intrinsic multiferroic properties, including BiFeO₃ and BiCrO₃. Among them, BiFeO₃ has attracted widespread attention because BiFeO₃ was the first material to display multiferroic behaviour at ambient temperature. A weak ferromagnetism occurs only at low temperatures depending on synthesis conditions. This thesis reports the structural, magnetic and optical properties of nanostructured BiFeO₃ thin films prepared by two novel approaches of ion beam sputtering and ion implantation techniques.

Nanocrystalline BiFeO₃ films were prepared at ambient temperature by sputtering and thermal annealing at 500 °C in an oxygen atmosphere. The annealing resulted in the formation of multiferroic BiFeO₃ phase with a reduction of iron oxide and bismuth phases. Superparamagnetism was observed and could be attributed to magnetite and maghemite nanoparticles. The magnetic properties were mainly due to magnetite and maghemite nanoparticles. The saturation magnetic moment was 60% lower after annealing, which was due to Fe in phases of iron oxide being incorporated into BiFeO₃ nanoparticles. An exchange bias was observed before and after annealing. The exchange bias cannot be attributed to BiFeO₃ structure. Instead, the exchange has likely arisen from magnetite and maghemite cores with spin-disordered shells. Piezoelectric responses measured by piezoelectric force microscopy confirmed the presence of BiFeO₃ ferroelectric material. The Magneto-optical Kerr effect (MOKE) and optical studies were used to calculate an anomalously high Verdet constant. The MOKE and magnetic circular dichroism (MCD) displayed a significant modification in function of the wavelength. Further increasing the annealing temperature lead to an increase in iron oxide phases, while increasing the annealing duration reduced the iron oxide phases, however this increases the fraction of Bi₂Fe₄O₉ and Bi₂O₃.

Another approach to synthesise BiFeO₃ thin film was investigated by bismuth ion implantation into iron oxide thin film. An as-made iron oxide film subsequently implanted with bismuth and annealed showed a 6.5% reduction of the ferromagnetic phase fraction. An annealed iron oxide film subsequently implanted with bismuth and annealed show that the ferromagnetic phase was present at less than 4% while Fe₃O₄ and γ-Fe₂O₃ increased to 7%. The coercive field is affected by annealing. However, this field is not affected by the bismuth implantation.

For the first-time, a preliminary investigation reporting the implantation of Bi then Fe then O into SiO₂:Si was made with the aim to synthesise BiFeO₃ films and magnetic nanoparticles. The implantation of Fe then O then Bi into SiO₂:Si contained a mix of iron oxides: α-Fe₂O₃ and Fe₃O₄, as confirmed by Raman spectroscopy and X-ray diffraction, while γ-Fe₂O₃ was most likely also present in the film. The as-implanted sample displayed a sign of a superparamagnetic phase that was lost with annealing the sample.

Preliminary investigations of another multiferroic material, BiCrO₃, were carried out. Thin films of BiCrO₃ were prepared by ion beam sputtering and annealing the sample in an oxygen atmosphere which lead to BiCrxOy with chromium oxides and bismuth oxide phases. Magnetic enhancement was observed when annealing above 700 °C. Annealing in an oxygen atmosphere followed by an argon atmosphere created a superparamagnetic phase that was not visible under other annealing conditions.