희토류 산화철 애피택시 박막의 육방정계 다강체 특성에 관한 연구

Abstract

Multiferroics exhibit simultaneous ferroic properties such as ferroelectricity, ferromagnetism, and ferroelasticity with coupled electric, magnetic, and structural order parameters. Multiferroic materials are particularly appealing because of their potential for enabling entirely new device paradigms, especially in non-volatile random access memory and magnetic data storage. However, there are few multiferroic systems existing in nature at room temperature because transition-metal d electrons reduce the tendency of an off-centering ferroelectric distortion. The rareness of room-temperature multiferroics has then led many workers to engineer the structure for having both electric and magnetic order. In this thesis, we propose a new type of artificial multiferroics derived by a structural modification. First, we have carried out research on the artificially-induced ferroelectricity and spontaneous magnetization reversal in epitaxially grown YbFeO3 thin films. YbFeO3 is known to be characterized by the FeO6 octahedra corner-linked to form a three-dimensional (3D) network in an orthorhombic unit cell (space group Pnma). Thus, it is expected to be non-ferroelectric. In the present study, however, we have synthesized a stable hexagonal YbFeO3 thin-film heterostructure by adopting suitable hexagonal templates. In this way, we are able to artificially impose ferroelectricity in an epitaxially grown YbFeO3. The hexagonal YbFeO3 shows a c-axis-oriented polarization with a non- centrosymmetric P63cm structure. It has been experimentally demonstrated that the hexagonal YbFeO3 is ferroelectric up to 450K with its remanent polarization (Pr) of ~4 uCoul/cm2 at room temperature. First- principle density-functional theory (DFT) calculations suggest that the ferroelectricity originates from the re-hybridization between Yb and O ions as a consequence of the tilting of FeO5 trigonal bipyramids in a two-dimensional hexagonal layer. Temperature-dependent magnetization study reveals that it is weakly ferromagnetic along the c-axis with its NIJel temperature around 120K. Surprisingly, it exhibits a spontaneous magnetization reversal below the compensation temperature T* owing to the competition between two magnetocrystalline anisotropy terms. According to the present DFT calculations, this extraordinary magnetization reversal can be attributed to the ferromagnetic rare-earth moments that are anti-parallel to the direction of canted Fe3+ spins. Seconds, another multiferroic material, epitaxially grown LuFeO3 thin films, which is dealt with hexagonal ferroelectricity and geometric spin frustration is investigated. LuFeO3 is characterized by the three-dimensional network of the FeO6 octahedra in an orthorhombic perovskite unit cell (space group of Pnma). Thus, it is also non-ferroelectric. By adopting suitable hexagonal templates, however, we are able to artificially impose ferroelectricity in an epitaxial thin-film form. This structurally induced ferroelectricity originates from the two-dimensional layered hexagonal structure with the absence of a mirror image on the a-b plane (P63cm symmetry). Interestingly, this artificially derived hexagonal LuFeO3 is c-axis oriented ferroelectric up to 570 K with its remanent polarization (Pr) as high as ~6.5 ┢Coul/cm2 at 300 K. First-principles density functional theory (DFT) calculations also support a structurally induced ferroelectricity with the predicted value of Pr = 7.8 ┢Coul/cm2 at 0 K. Temperature-dependent magnetization study reveals that it is weakly ferromagnetic with its N？？el temperature around 120 K. Because of the spin frustration in a triangular sublattice, the net spin moment on the a-b plane is nearly zero but with a small non-zero residual moment (0.0021 ┢B/Fe) along the c-axis. These experimental findings are consistent with the computed DFT magnetization that Mc = 0.003 ┢B/Fe along the c-axis but Ma-b = 0 due to the geometrical spin frustration effect of a triangular sublattice in the hexagonal FeO5 bi-pyramidal unit. Finally, we report another novel and interesting phenomena about spontaneous magnetization reversal in SmFeO3, a family of rare-earth ferrites. In this chapter, we show our experimental results about temperature-induced magnetization reversal in the SmFeO3 single crystals. Also, we suggest suitable spin structures on the basis of density functional calculations to help further understanding in the microscopic point of views.