|dc.description.abstract||Multiferroics, defined as materials with coexistence of at least two of the electric, elastic, and magnetic orders, have attracted enormous research activities recently. A subsystem of multiferroics is the ferroelectromagnet, which possesses both electric and magnetic orders. One of the natural ferroelectromagnets is BiFeO3, which has ferroelectric (TC~1100K) and antiferromagnetic (TN~640K) orders at room temperature. Even though bulk samples have been synthesized back in 1950s, characterizations of its intrinsic properties have been difficult due to poor sample quality.
This work is the first study on epitaxial BiFeO3 thin films. Highly resistive films have been prepared using Pulsed Laser Deposition. (001), (110) and (111) cut SrTiO3 substrates were used to control the film orientation. Film structures were characterized using both X-ray diffraction and transmission electron microscope. It was found that epitaxial stress changes the film structure. Monoclinic domain splitting was observed from both (101) and (001) oriented films, while (111) films remain rhombohedral similliar to single crystals.
Much larger polarizations were observed for all three orientations (~55 &#61549;C/cm2 for (001) films, ~80 &#61549;C/cm2 for (101) films, and ~100 &#61549;C/cm2 for (111) films). Calculation using the effective charges and reported ion displacements is performed; indicating that the large observed polarization is likely the intrinsic property of BiFeO3. Magnetic measurements reveal that these resistive BiFeO3 thin films show hysteresis behavior at room temperature, which was not observed in bulk single crystal under the same field range. Thickness dependence of the magnetic property was studied. It is proposed that epitaxial stress destroys the cycloidal spin structure of BiFeO3, releasing the weak ferromagnetic property due to spin canting.
In addition, integration of BiFeO3 with Si using SrTiO3 template layer was also studied. Large dielectric constant and piezoelectric coefficients were observed, showing promise for applications in MEMs and actuators.||en_US