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Title: Tuning strong correlated properties in EuTiO3 thin films grown by pulsed laser deposition
Author: Lin, Yisong
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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Transition metal ABO3 perovskite oxides possess many fascinating and technologically relevant phenomena such as superconductivity, multiferroicity, magnetoresistivity, and photovoltaics. To achieve and enhance these existing functionalities in perovskite oxides, control and tailor of strong coupling between structural degrees of freedom and correlated order parameters is required. One typical example of decently tuned functionalities in transition metal is perovskite EuTiO3 (ETO). This compound has attracted intensive attention for achieving ferromagnetic-ferroelectric (or multiferroic) and magnetoelectric coupling via strain engineering. Besides phase transition induced by strain engineering, there is an increasing interest in exploring the magnetocaloric effect (MCE) in ETO-based bulk materials. First, systematic studies for optimizing the growth condition for ETO-based self-assembled vertically alligned nanocomposite (VAN) thin films were carried out. We carried out investigations for finding a suitable binary oxide second phase to incoporate with ETO. It was challenging since the growth condition suitable for growing ETO would not always be compatible with that of the binary oxide. After various exploration, (ETO)0.5-(Eu2O3)0.5 was successfully fabricated on (0 0 1)-oriented STO under careful control of growth pressure, substrate temperature and laser repetition rate. The optimized growth condition (laser repletion rate of 3 Hz, substrate temperature of 710 oC and growth pressure of 1-2∙10-6 Torr) was confirmed to make the (ETO)1-x-(Eu2O3)x (x = 0.2, 0.25, 0.33, 0.5) epitaxial thin film on STO substrate with preferred OOP orientation. Next, by controlling the molar ratio of Eu2O3 (EO) in EuTiO3 (ETO) VAN films, from 20% to 50% (20%, 25%, 33% and 50%), vertical strain and hence local structure and magnetic properties were systematically changed in ETO. Using scanning transmission electron microscopy and atomic simulations, the Eu-Ti-Eu bond angle change along {111} direction was determined as a function of EO fraction and hence vertical strain level. On-going from 20 to 50% EO, the vertical strain was changed from 2.48% to 3.15%, and the Eu-Ti-Eu bond angle was decreased by more than 1o to 178.70 o, leading to a progressive weakening of the antiferromagnetic interactions along {111} direction. This structural change caused the magnetic moment (M, measured at 2K, 500 Oe) to be decreased from 5.15 μB /Eu to 1.46 μB /Eu and the coercivity filed (Hc) to be increased from 5.74 Oe to 26.23 Oe. At the same time, the ETO switched from being antiferromagnetic to ferromagnetic. Density functional theory calculations confirmed that the magnetism in ETO being modified by vertical strain. Moreover, giant magnetic entropy change (-∆Sm = 31.4 J/kg∙K at ∆H = 2T) was observed in (ETO)0.8-(Eu2O3)0.2 VAN thin film, which is higher than the literature reported single crystal bulk ETO or polycrystal bulk ETO. Finally, Gd-doped ETO, Eu1-xGdxTiO3 (x=0.3, 0.5), thin films were made and found to be ferromagnetic. The valence states change of Ti ions were demonstrated using XPS, showing that 13.41% (x=0.3) and 34.59% (x=0.5) of extra electrons were bonded to Ti4+ effectively. FM in Eu1-xGdxTiO3 (x = 0.3, 0.5) thin films was explained by magnetic interactions between localized Eu4+ 4f spins mediated by itinerant Ti 3d electrons (introduced by Gd doping).
Supervisor: Driscoll, Judith Sponsor: China Scholarship Council ; Cambridge Commonwealth ; European and International Trust
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: nanocomposite thin film ; self-assembled nanostructures ; EuTiO3 ; strain engineering ; spintronics ; multiferroic