Structural, Magnetic and Electrical Studies of Multiferroic BiFeO3 and CuO Epitaxial Thin Films
The multiferroics are an important class of multifunctional material which simultaneously possess spontaneous ferroelectric polarization and magnetic ordering. If there exists a coupling between the ferroelectricity and the magnetic ordering, the materials are known as magneto electric (ME) multiferroic materials. The coupling between the magnetic and electric order parameters allows to tune the magnetic properties by an electric field and vice versa. Multiferroic materials are promising candidate for designing new spintronic devices, advanced sensors, high density ferroelectric memory devices and the emerging category of four-state memory devices. In multiferroic memory devices, data can be written electrically using its ferroelectric property and can be read magnetically without causing any Joule heating. Depending on the origin of ferroelectricity and magnetic orderings, multiferroics can be divided into two categories: type I and type II multiferroics. The type I multiferroics have different sources of ferroelectricity and magnetism. On the other hand, ferroelectricity is induced by the magnetic ordering in type II multiferroic materials and they have a strong ME coupling. However, even after extensive investigations into different families of compounds, a multiferroic material with high-enough polarization and magnetization suitable for practical applications has not been realized yet. In order to overcome this problem, composite multiferroics are designed by combining a ferroelectric and a ferromagnetic material. Recently composite multiferroics have drawn significant attention due to its enormous design flexibility which can be used for a wide range of applications. In this thesis, a thorough study of the structural, electrical, and magnetic properties of multiferroic BiFeO3 and CuO epitaxial thin films is carried out. BiFeO3 is a type I multiferroic material with a perovskite distorted rhombohedral (R3c) crystal structure. It is ferroelectric (TC = 1123 K) and G-type antiferromagnetic (TN = 643 K) at room temperature. Antiferromagnetism in BiFeO3 arises from the Fe sublattice having d5 configuration whereas ferroelectricity appears due to the directional orientation of 6s lone pair electrons of the Bi3+ ion. We observed that the crystal structure of BiFeO3 thin film gets altered depending on lattice misfit stress caused by the substrate which in turn modifies its magnetic properties through strong magneto-structural coupling. Furthermore, a signature of magneto-(di)electric coupling and exchange bias effect were observed between the BiFeO3 and SrRuO3 layers of a heterostructure. On the other hand, CuO is a type II multiferroic material where ferroelectricity is generated between 213 K and 230 K due to incommensurate spiral magnetic spin ordering along its crystallographic ‘b’ axis. We found that CuO thin films can be grown in the direction of its static polarization axis by proper choice of substrate and the temperature dependent magnetic properties of CuO thin films vary depending on its crystallographic orientations due to strong magneto-structural coupling. Chapter 1 provides a general introduction to various physical phenomena, such as ferroelectricity, ferromagnetism, antiferromagnetism, multiferroicity, magneto-electric coupling, and different magnetic interactions, like Dzyaloshinskii-Moriya interaction, and exchange bias effect. Basic concepts of impedance spectroscopy, dielectrics and perovskite structures are also discussed. General introductions of different materials, which are studied in the thesis, and the motivation of choosing them are incorporated at the end of the chapter. Chapter 2 contains the description of thin film growth technique and different steps of device fabrication process. Different characterization techniques, the instruments used for the characterizations and the working-principle of those instruments have been summarized in the chapter. Chapter 3 focuses on the variation of magnetic properties and crystal structure with the thickness of BiFeO3 thin films. BiFeO3 thin films of different thicknesses, ranging from 16 nm to 60 nm, were grown on (001) SrTiO3 substrate by PLD technique. Detailed x-ray diffraction studies show that the 16 nm, 20 nm and 30 nm films have “R-like” crystallographic phase with an out-of-plane lattice parameter of 4.06 Å whereas the 45 nm and 60 nm films have “R-like” and ‘T-like” crystallographic phases simultaneously. The “T-like” phase has an out-of-plane lattice parameter of 4.65 Å and a c/a ratio of 1.25, resembling a tetragonal crystal structure. Off-specular reciprocal space mapping and azimuthal φ scan show that the “T-like” phase deviates from an ideal tetragonal crystal structure by a monoclinic tilt. The occurrence of the “T-like” phase is associated with the formation of a very thin layer of parasitic Bi2O3 phase which appears in between two film-thicknesses of 30 nm and 45 nm and BiFeO3 grows in “T-like” phase thereafter. High lattice mismatch between Bi2O3 phase and BiFeO3 phase causes more distorted unit cell in “T-like” phase with a high c/a ration. Parasitic Bi2O3 phase appears because of slightly higher partial oxygen pressure used during the growth which prevents the formation of the parasitic ferrimagnetic γFe2O3 phase in the films. Moreover, our XPS studies confirmed that the films contain Fe3+ only without any trace of Fe2+ within a resolution of few atomic percentages and the magnetic signals measured in our experiments are entirely from the BiFeO3 phase. The saturation magnetizations of the films were found to increase with decreasing thickness. At room temperature, the saturation magnetization of a 16 nm-thick BiFeO3 thin film is 87 emu/cc but it goes down to 9 emu/cc when the thickness increases to 60 nm. Moreover, it was observed that the 16 nm thick film is magnetically more anisotropic in comparison to the 60 nm thick film and there is an apparent out-of-plane magnetic hard axis in the 16 nm film. Summarizing the results obtained from the films with different thicknesses, it can be concluded that the vanishing magnetic anisotropy is related to the structural transformation of the film. Chapter 4 provides a detailed study of the variation of magnetic properties of a BiFeO3 thin film with its crystal structure. BiFeO3 thin films of different thicknesses were grown on orthorhombic (001) NdGaO3 substrate. In-depth x-ray diffraction studies and off-specular reciprocal space mapping show that a 15 nm thick BiFeO3 film grows with monoclinic crystal symmetry (Cm) with an out-of-plane lattice parameter of 4.187 Å on the NdGaO3 substrate. The crystal structure was further verified by the TEM studies which showed a good agreement with the results obtained from x-ray diffraction studies. To probe the ferroelectric nature of the monoclinic BiFeO3 film, piezo response force microscopy was performed. It was found that the oppositely oriented ferroelectric domains have 180° phase contrast and a phase vs. voltage hysteresis loop gets generated when the domains are switched between two antiparallel directions. DC magnetic measurements at room temperature showed that the saturation magnetization of the 15 nm film with Cm crystal symmetry is as high as ~250 emu/cc. Experimental evidence confirmed that the films are free from all magnetic parasitic phases and the high saturation magnetization comes solely from the BiFeO3 phase. For comparative study, BiFeO3 films of similar thickness were deposited on (001) SrTiO3 under identical conditions which grew in “R-like” crystal structures. We saw that “R-like” BiFeO3 films have saturation magnetization 2.5 times lower (~100 emu/cc) than that of the film with Cm structure grown on NdGaO3. Our observation was further supported by density functional theory calculations which show that BiFeO3 has a ferromagnetic ground state in the Cm crystal phase. The theoretically obtained magnetic moment is 266 emu/cc which is very close to magnetization values found experimentally. Chapter 5 deals with the magnetic interaction and the magneto-electric coupling between the BiFeO3 and SrRuO3 layers of a heterostructure. BiFeO3/SrRuO3 heterostructures were grown on (001) SrTiO3 substrate by PLD technique. The ferroelectric nature of the top BiFeO3 layer was probed by out-of-plane piezo response force microscopy technique. Temperature dependent magnetization measurements of the heterostructure show a sharp ferromagnetic to paramagnetic transition at 160 K which arises from the bottom SrRuO3 layer. Therefore, the heterostructure is ferroelectric and ferromagnetic below 160 K. Magnetic interactions between the two layers were investigated by isothermal magnetic hysteresis loop (M-H) measurement in a SQUID magnetometer. The M-H measurements at 10 K showed a two-step magnetic hysteresis loop which implies that magnetic moments of the SrRuO3 layer get pinned by the magnetic interaction between the two layers. During magnetization reversal process, the pinned magnetic moments switch at a higher magnetic field and generate the second step of the hysteresis loop whereas the first step appears at a lower magnetic field during the switching of the free SrRuO3 moments. The amount of the pinned SrRuO3 moments depends on the thickness of the BiFeO3 layer as the magnetic properties of a BiFeO3 thin film are related to its thickness. Moreover, evidence of the exchange bias effect was also found in the heterostructure. Field-cooled M-H measurement shows that the second step of the hysteresis loop shifts in two opposite directions along the magnetic field axis depending on the polarity of the cooling field whereas the first step doesn’t respond to the cooling field. This confirms that the exchange bias effect is directly related to the pinned magnetic moments of the SrRuO3 layer. The total amount of pinned moment and hence the exchange bias effect reduces with increasing temperature and disappears completely above 100 K. A strong coupling between the electrical properties of the BiFeO3 layer and the magnetic properties of the SrRuO3 layer was also observed in the heterostructure. To carry out electrical measurements, interdigitated gold electrodes were fabricated on the BiFeO3 layer of the heterostructure by standard photolithography, magnetron sputtering, and lift-off procedure. Temperature dependent resistance and reactance measurements of the heterostructure at different frequencies show anomalies at ferromagnetic TC of the bottom SrRuO3 layer. Moreover, temperature dependent capacitance measurement at 0 T and at 5 T magnetic fields also showed anomalies near 160 K which indicate that the electrical properties of the heterostructure are affected by the magnetic transition of the SrRuO3 layer. Furthermore, impedance spectroscopy measurements were carried out at different constant temperatures and the corresponding Nyquist plots were fitted with an equivalent circuit model. Remarkably, the capacitance and resistance of the equivalent circuit corresponding to the BiFeO3 layer of the heterostructure, show anomalies at 160 K. Absence of any dielectric anomaly at 160 K in pure BiFeO3 confirms that the observed ones appear because of the magnetic phase transition of the bottom SrRuO3 layer. Therefore, the BiFeO3/SrRuO3 heterostructure has ferroelectric and ferromagnetic properties along with a strong magneto-electric coupling between the layers which can be a promising candidate for the composite multiferroic. Chapter 6 describes a correlation between the crystal structure and magnetic properties of CuO thin film. CuO thin films were grown on (001) SrTiO3, (110) SrTiO3, and (111) Si substrate with MgO buffer layers by PLD technique. On (110) SrTiO3 substrate, CuO thin films grow along  direction, which is the direction of ferroelectric polarization of CuO, but growth direction becomes  when (001) SrTiO3 substrate is used. The CuO film becomes polycrystalline when it is grown on (111) Si substrate. To find the in-plane epitaxial relations between the substrate and the two layers, cross-sectional TEM of the heterostructure grown on (110) SrTiO3 was carried out. HRTEM images showed very sharp interfaces between the layers indicating high-quality growth of the heterostructure. The epitaxial relations were deduced from the SAED pattern and the FFT pattern of the HRTEM images. Distinctly different temperature dependent magnetic properties were found for three differently oriented CuO films. Two anomalies at 213 K and 230 K are clearly visible in temperature dependent magnetization (M vs. T) plot of the heterostructure with (010) CuO film which are associated with the two magnetic transitions of CuO. On the other hand, no such anomaly was observed in M vs. T plot of the heterostructure with (111) CuO film. The heterostructure with polycrystalline CuO film shows a very weak magnetic anomaly at 230 K in its M vs. T plot. It can be concluded from our studies that the contrasting magnetic behaviours of these three heterostructures are due to the difference in epitaxial orientations of the CuO layers. Moreover, CuO thin films can be successfully grown in the direction of static ferroelectric polarization which is the ‘b’ axis of its monoclinic crystal structure. Chapter 7 concludes with general findings pertaining to various observations made in the different chapters. Prospects for future work are briefly outlined in this chapter.