Study on magnetic properties of multiferroic $BiFeO_3$ Films

Abstract

Materials that have coupled electric, magnetic, and structural order parameters that result in simultaneous ferroelectricity, ferromagnetism, and ferroelasticity are known as multiferroics. In general, most ferromagnetics contain the center of symmetry and do not allow an electric polarization. Most of the ferroelectric oxides consist of transition-metal ions without the seed of magnetism, that is active d electrons. Both of them can not be multiferroics. As being rhombohedrally distorted perovskite structure and Fe - O -Fe indirect exchange coupling, $BiFeO_3$ has ferroelectric and antiferromagnetic properties simultaneously at room temperature, so called ferroelectromagnets. With this coexistence they supplement materials in which magnetization can be induced by an electric field and electrical polarization by a magnetic field, a property which is termed the magnetoelectric effect. In this thesis, $BiFeO_3$ thin films are fabricated by UHV rf magnetron sputtering system for magnetization to control by voltage based on magnetoelectric effect. To make epitaxial films, single crystal $SrTiO_3$ (100) with lattice mismatch (0.76 %) is used as substrate. $Bi_2O_3$ and $Fe_2O_3$ targets are prepared for co-sputtering and 3 Fe chips are added on $Fe_2O_3$ target for the stoichiometric compositions with 20 W rf power of $Bi_2O_3$ and 70 W $Fe_2O_3$. Sputtering gas is argon with the pressure of 2 mTorr. The temperature of substrate is between 350 ℃ and 400 ℃. Films are annealed at 850 ℃ in air. After all processes are done, We clearly observe XRD characteristic peaks of rhombohedral $BiFeO_3$. VSM and SQUID measurements reveal weak ferromagnetic properties at both room temperature and especially 5K with 72 Oe of the coercive field.