Type of Document	Master's Thesis
Author	Ruette, Benjamin Thibault
Author's Email Address	bruette@vt.edu
URN	etd-05122003-102123
Title	Induced Phase Transition in Magnetoelectric BiFeO3 Crystals, Thin-layers and Ceramics
Degree	Master of Science
Department	Materials Science and Engineering
Advisory Committee	Advisor Name Title Viehland, Dwight D. Committee Chair Claus, Richard O. Committee Member Li, Jie-Fang Committee Member
Keywords	Ferrite Bismuth Perovskite Magnetoelectric Crystals
Date of Defense	2003-05-22
Availability	restricted
Abstract Bismuth ferrite (BiFeO3) is a magneto-electric material which exhibits simultaneously ferroelectric and antiferromagnetic properties. We have used high-field electron spin resonance (ESR) as a local probe of the magnetic order in the magnetic range of 0-25 Tesla. With increasing magnetic field, an induced transition has been found between incommensurately modulated cycloidal antiferromagnetic and homogeneous magnetized spin state. The data reveal a number of interesting changes with increasing field, including: (i) significant changes in the ESR spectra; (ii) hysteresis in the spectra near the critical field. We have analyzed the changes in the ESR spectra by taking into account the magnetic anisotropy of the crystal and the homogeneous anti-symmetric Dzyaloshinsky-Moria exchange. We have also investigated phase induced transition by epitaxial constraint, and substituent and cystalline solution effects. Variously oriented BiFeO3 epitaxial thin films have been deposited by pulsed laser deposition. Dramatically enhanced polarization has been found for (001)c, (110)c, and (111)c films, relative to that of BiFeO3 crystals. The easy axis of spontaneous polarization lies close to (111)c for the variously oriented films. BiFeO3 films grown on (111)c have a rhombohedral structure, identical to that of single crystals. Whereas, films grown on (110)c or (001)c are explained in terms of an epitaxially-induced transition between cycloidal and homogeneous spin states, via magneto-electric interactions. Finally, lanthanum modified BiFeO3-xPbTiO3 crystalline solutions have been found to have a large linear magneto-electric coefficient, ƒpP. The value of ƒpP (2.5x10-9 s/m or C/m2-Oe) is ƒ{10x greater than that of any other material (cg., Cr2O3 ƒ{2.5x10-10 s/m), and many order(s) of magnitude higher than unmodified BiFeO3 crystals. The data also reveal: (i) that ƒpP is due to a linear coupling between polarization and magnetization; and (ii) that ƒpP is independent of dc magnetic bias and ac magnetic field. We show that the ME effect is significantly enhanced due to the breaking of the transitional invariance of a long-period spiral spin structure, via randomly distributed charged imperfections.
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