Multiferroic transition metal oxides: structural, magnetic, ferroelectric, and thermal properties

Author

Yi Du, University of WollongongFollow

Year

2011

Degree Name

Doctor of Philosophy

Department

University of Wollongong. Institute for Superconducting & Electronic Materials

Recommended Citation

Du, Yi, Multiferroic transition metal oxides: structural, magnetic, ferroelectric, and thermal properties, Doctor of Philosophy thesis, University of Wollongong. Institute for Superconducting & Electronic Materials, University of Wollongong, 2011. https://ro.uow.edu.au/theses/3247

Abstract

In this thesis, the multiferroic compounds, BiFeO₃, La doped BiFeO₃, Cr doped BiFeO₃, La doped NdCrO₃, Bi₂FeMnO₆, Bi₂NiMnO₆, and La doped DyFeO₃, have been systemically investigated. Their structural, magnetic, ferroelectric, thermal, and dielectric properties are studied in details.

A family of bismuth ferrites (BFO), including Bi₂Fe₄O₉, BiFeO₃, and Bi₂₅FeO₃₉ with different morphologies, has been prepared by the hydrothermal method assisted by different alkaline mineralizers. X-ray diffraction refinement calculations are carriedout to study the crystal structures of bismuth ferrites. A thermodynamic calculation based on the dissolution-precipitation model was carried out to analyze the hydrothermal synthesis of BFO powders. Magnetic measurements of the obtained bismuth ferrites show different magnetic properties from 5 K to 350 K. The physical properties of rare earth and transition metal elements doped BiFeO₃ have been studied through La and Cr doping. Bi_1-xLa_xFeO₃ (x= 0.0, 0.1, 0.2, and 0.3) micro-particles were synthesised by a hydrothermal technique. All the samples were phase pure crystallizing in a perovskite structure with a space group of R3c. XRD refinement revealed that the lattice parameters increased along with increase of La content, whilethe Fe-O octahedra became more distorted. It was found that the morphologies of as obtained micron-sized particles turned from spheroidal to octahedral according to different doping levels. The dielectric constant of Bi_1-xLa_xFeO₃ sample increased after La doping, and reached the largest value for the sample of x= 0.2, both in low and high frequency range at room temperature. All the as-prepared samples exhibited magnetic moments starting above room temperature. It was found that the magnetic moment was significantly enhanced from 0.264 emu/g of BiFeO₃ to 0.658 emu/g of Bi_0.9La_0.1FeO₃ in a field of 3 T at 77 K. Both enhancements of ferromagnetic and dielectric properties possibly attribute to the changes of lattice parameters and Fe-Obond lengths caused by lanthanum substitution. Multiferroic BiFe_1-xCr_xO₃ (x = 0.025,0.05, 0.075, and 0.1) is prepared by the hydrothermal method. Samples are systematically characterized by X- ray diffraction, Rietveld refinement, SEM, dielectric and magnetic measurements. It is found that the lattice parameters, themorphology and the size of the obtained particles strongly depend on the Cr dopinglevel. The lattice parameters a, c and the Fe-O (1) bond lengths decrease, while the Fe-O (2) bond lengths increase, as x values rise from 0.025 to 0.1. The particlemorphology changes from spherical shape for pure BiFeO3 to octahedral shape for x =0.1. The dielectric constant and the magnetization increase greatly with the increase ofthe Cr doping level. Both the M-T and the M-H results reveal that the Cr dopingenhances the ferromagnetic state in the doped BiFeO₃. The possible reasons for the enhanced magnetic and dielectric properties are discussed briefly. Hollow BiFeO3 nanoparticles were synthesized by an electrospray route for the first time. The phasepurity and structure have been investigated by X-ray diffraction and Ramanspectroscopy. Transmission and scanning electron microscope investigations revealed that the as-obtained BiFeO₃ hollow spheres were polycrystalline, with a shell thickness of 35 nm. The formation mechanism can be possibly explained by Ostwald ripening. Raman spectra have verified decreased vibrational frequencies in BiFeO3 nanoparticles. These hollow and core-shell multiferroic nanoparticles exhibit significantly enhanced ferromagnetism from 5 K to 600 K due to a broken spiral spinstructure. The ferroelectricity of hollow BiFeO₃ particles exhibits a lower switching electric field which is confirmed by Kelvin probe force microscopy.

Possible multiferroic compound, Nd_1-xLa_xCrO₃ (0≤ x≤ 1.0), has been synthesizedby solid state reaction and been characterized by a broad range of measurements. Xray diffraction Rietveld refinement calculation and Raman spectra indicate that CrO₆ octahedra are sequentially distorted with decreasing bond angles of Cr-O-Cr caused by the La doping. The temperature dependence of magnetization (M-T) shows two magnetic transition temperatures originating from the Cr³⁺ ordering (TN1) and the Nd3+ orderings (TN2), respectively. The increasing TN1 and decreasing T_N2 by the increasing doping level can be explained by the chemical pressure and non-magnetic property ofLa³⁺. The substitution of La³⁺ induces a small ferromagnetic moment due to the coupling between spin and lattice which confirmed by the magnetic field dependence of magnetization (M-H) and Raman spectra. The Curie- Weiss fitting of M-T curves indicates that the spin state for Nd3+ is stable in Nd_1-xLa_xCrO₃. Heat capacity measurements on Nd_1-xLa_xCrO₃ samples reveal no discernible electronic term and the Debye temperatures near 700K.

Double perovskite multiferroic Bi₂FeMnO₆ was synthesized on Si substrates by anelectrospray method. Three peaks were observed in the temperature dependence of magnetization curve, which is attributed to the inhomogeneous distribution of Fe³⁺ andMn³⁺. The observed magnetic peaks at 150 K, 260 K, and 440 K correspond to orderings of the ferrimagnetic Fe-O-Mn, and antiferromagnetic Mn-O-Mn and Fe-O-Fe,respectively. Heat capacity measurements were carried out to confirm these magnetic transitions. The Debye temperature of Bi₂FeMnO₆ is 339 K, calculated from Debye-Einstein fitting. Single phase Bi2FeMnO6 hollow particles have been fabricated on siliconsubstrates from a sol-gel precursor by an electrospray method. The as-prepared Bi₂FeMnO₆ polycrystalline samples are pure-phase, crystallizing in space group R3c.Raman spectra have verified the vibrational frequencies in the Bi₂FeMnO₆ samples. The Bi₂FeMnO₆ nano-/micron-sized hollow particles show a porous shell with large interiorvoid. The shell thickness is ~100 nm. The structure and morphology of the sample were found to be controlled by the deposition time. The formation mechanism can be explained by an Ostwald ripening − Kirkendall model. The temperature dependence of the magnetization curve (M-T) and magnetic hysteresis loops indicate that Bi₂FeMnO₆ particles exhibit weak ferromagnetic moment at both low and room temperature. Another multiferroic double-perovskite Bi₂NiMnO₆ nanoparticles were synthesized by a electrospray method as well. Bi2NiMnO6 nanoparticles crystallize in the monoclinic structure with space group C121. The particles show a uniform spherical shape with adiameter of 100 to 300 nm. The ferromagnetic transition of Bi₂NiMnO₆ is confirmed at 122 K. The room temperature ferroelectricity of the Bi₂NiMnO₆ nanoparticles is verifiedby Kelvin probe force microscopy.

Lanthanum doped multiferroic DyFeO₃ was synthesized by a solid state reaction. X-ray diffraction (XRD) and refinement show that the lattice parameters of Dy_1-xLaxFeO₃ increase linearly with the La content. A Raman spectroscopy study reveals that the short-range force constant in Dy_1-xLaxFeO₃ is decreased by La³⁺ ion substitution. The spin re-orientation phase transition temperature (TSRPT) is observed to decrease along with the doping level. The antiferromagnetic (AFM) ordering temperature TN of Fe³⁺ ions is depressed with increasing doping level. Both decreasing TSRPT and decreasing TN indicate that Fe-Dy and Fe-Fe interactions are weakened by La substitution. It is found that the electron configuration of Fe³⁺ is high spin state and not affected by the La doping in all the samples above TN.