서로 다른 두 가지 결정구조를 가지는 사방정형 철산화물에서 발현되는 다강성에 관한 연구

Abstract

Multiferroics are an interesting group of materials that simultaneously exhibit ferroelectricity and magnetic ordering with coupled electric, magnetic, and structural orders. Multiferroic materials with a pronounced degree of magnetoelectric (ME) coupling at room temperature are of great scientific and technological importance for their use in various types of electronic devices that include sensors, actuators, and electric-field controllable magnetic memories. Among all the known multiferroics, BiFeO3 is most extensively studied owing to its large room-temperature spontaneous polarization with improved magnetic properties in epitaxially strained thin-film forms. Polar orthorhombic GaFeO3 (o-GFO) is another prominent multiferroic oxide by virtue of its room-temperature piezoelectricity (possibly ferroelectricity as well), near room-temperature ferrimagnetism, and pronounced low-temperature ME effects. Since a linear ME effect was first reported in 1960s by Rado, magnetization-induced second harmonic generation and optical ME effect and other interesting studies keep renewing our research attention to this system. GFO crystallizes in the polar orthorhombic Pna21 (equivalently, Pc21n) space group with a ferrimagnetically ordered spin structure. Especially, orthorhombic Pna21 is well-known ferroelectric structure. Unfortunately, to the best of our knowledge there has not been previous report about ferroelectricity at or above room temperature clearly. Main problem of o-GFO system which hinder to measure electrical properties is high leakage current. Instead of polycrystalline sample or single crystal bulk sample, here we demonstrate polar c-axis oriented o-GFO thin film on hexagonal substrate to synthesize electrically robust sample to measure its spontaneous polarization above room temperature. First, crystal structure of deposited o-GFO on hexagonal substrate is carefully examined. So, we have very carefully investigated crystal structure of o-GFO on hexagonal substrate using X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). We confirmed that our system has lower number of in-plane orientation than that of o-GFO on cubic substrate by half. This can reduce leakage current because the lower number of in-plane orientation, the lower structural domain boundary which can generate crystal defects. Moreover, we examine strain effect which is one of the most important factors when thin film is deposited. From results of 2theta-theta scan and SAED pattern, o-GFO film is fully relaxed, rather than epitaxially strained. Second, electrical and magnetic properties are measured using our grown sample. Our sample shows the highest electrical resistivity among reported data. This indicate that o-GFO on hexagonal substrate successfully reduce leakage current. To measure local ferroelectricity at room temperature, we performed piezoelectric force microscopy (PFM) along polar axis. This experiment didn’t show ferroelectricity, but only shows piezoelectric switching. To study spontaneous polarization, we input higher electrical field above ±1400 kV/cm. Surprisingly, we can get a beautiful polarization versus electrical filed curve. Our remnant polarization value is very close to theoretically expected value. And Positive-Up Negative-Down (PUND) test makes us sure that polarization is intrinsic property of o-GFO. It is for the first time to demonstrate evident polarization in o-GFO system at room temperature. In contrast to electrical properties, magnetic properties of o-GFO are much researched. Ferrimagnetic Neel temperature (~230K) and other magnetic properties of our system resemble with previous results. Third, origin of ferroelectricity has been investigated by density functional theory (DFT) calculation. In case of o-GFO, as I said before it shows polar orthorhombic Pna21 structure. Based on knowledge that all ferroelectric structure should have centrosymmetric paraelectric phase at high temeperature, we have utilized the PSEUDO code of Bilbao crystallographic sever to find centrosymmetric prototypic phase of o-GFO. And according to group theoretical analysis, there exist only one conceivable transition path that connects the prototypic Pnna phase to the ferroelectric Pna21 phase, Γ_4^-. To reveal main site which induce major displacement, we inquire into phase transition from high temperature prototypic phase Pnna to low temperature ferroelectric phase Pna21. We could find that displacement of Ga ion is about 3 times larger than that of Fe ion. Finally we will discuss main driving force to manifest ferroelectricity, effects of polar phonon driven ferroelectricity and orbital hybridization mechanism both. The most of rare-earth ferrites crystallize in orthorhombic structure when they are in ground states. However, a few rare-earth ions which have small ionic radius (Ho, Er, Tm, Yb, Lu and Y) could be artificially tailored hexagonal structure by adopting suitable hexagonal templates. These structurally tailored hexagonal rare-earth ferrite shows room temperature ferroelectricity about 4~7 μC/cm2. Despite of its considerable spontaneous polarization, magnetic Neel temperature is quite low below 120 K. Among many kinds of rare-earth ferrites, orthorhombic LuFeO3 (LFO) is well-known room temperature ferromagnetic materials. If we can synthesize hexagonal-orthorhombic (h-o) morphotropic phase mixture of LFO, we artificially generate multiferroism above room temperature. In the remaining part of thesis consist of synthesis of h-o morphotropic phase mixture of LFO using PLD methods. First, to examine crystal structure of h-o morphotropic phase mixture of LFO, we have very carefully investigated crystal structure on hexagonal substrate using XRD, STEM, SAED and energy-dispersive X-ray spectroscopy (EDS) in TEM. We could successfully observe (001) peak which comes from h-LFO and quite small amplitude of (112) peak which comes from o-LFO using XRD. To investigate local structural of h-o morphotropic phase mixture of LFO in detial, we performed STEM and HR-TEM. Among majority of h-LFO, we can successfully observe o-LFO. We first demonstrate h-o morphotropic phase mixture in same composition. SAED pattern indexing also supports our observation. Second, electrical and magnetic properties are measured at room temperature. We can get very similar value of polarization to only h-LFO using h-o morphotropic phase mixture about 6μC/cm2. Moreover, we can obtain spontaneous magnetization at room temperature which is never seen in only h-LFO. It is solely magnetic properties of o-LFO in morphotropic phase mixture. Third, we explain the reason why h-o morphotropic phase mixture of LFO is possible by DFT calculation. When we compare ground state energy of each h-LFO and o-LFO versus volume, one can notice that small amount of compressive stress can manifest orthorhombic phase. Compared with STEM images, we can conclude that local compressive strain by hexagonal substrate during deposition, orthorhombic phase could be grown naturally.