Recent micro-nano electronic and magnetic researches about functional oxide materials were great attentions to investigate the novel multiferroics properties such as puzzled coupling effect between electric dipole- and magnetic spin-moment as well as their mutual interaction in one single-phase material. The prototype coupling examples were already known as electro-mechanical (Piezoelectric) and/or magneto-mechanical (Magnetostrictive) effect. However, in the single phase multiferroics; representing simultaneous Ferroelectricity/Ferromagnetism and their mutual spin/dipole interaction in one-phase was still not-fully exploited field in crystallographic, physical, and chemical point of views. In addition, combining of spontaneous electrical dipolar polarization and spontaneous magnetic spin momentum in a single-phase must be tremendous interesting research field not only in a crystallographic point of view but also the investigation of fundamental solid-state-physics of spin-dipole interactions and perturbations theory. The coupling effect; intrinsic turnover of electrical polarization by the disturbance of the magnetization or vice versa in single phase multiferroics, allows another additional degree of freedom for designing of the novel purpose of new device applications.
In this thesis, we restrained the “multiferroics” as the single-phase multiferroics to distinguish the magneto-electric composite approach, which were investigated elsewhere likewise $Pb(Zr,Ti)O_3$ /Terfenol-D laminated composites. Therefore, this thesis demonstrated that the compositional systematic study of multiferroic solid solution of x $(Bi_{0.1}La_{0.9})FeO_3-(1-x)A(Fe_{0.5}Nb_{0.5})O_3$ (A=Ba, Sr, and Pb) (BLFO-BFNO, BLFO-SFNO, and BLFO-PFNO, respectively) ceramic system as room temperature stable multiferroic materials. For developing of natural multiferroic $BiFeO_3$ ceramics itself; which had cyclical spin canting of Antiferromagnetism and very unstable Ferroelectricity at room temperature, was intended to transform the aligned spin distribution of Ferro/Ferrimagnetism, and enhanced the stable ferroelectric switching property at room temperature through the A- and B-site cation substitutional ordering by conventional solid state reaction method. It can be also said as the development of solid solutions by utilizing perovskite end member of complex double perovskite $A(Fe_{1/2}Nb_{1/2})O_3$ (A=Ba, Sr, and Pb) into the La-doped BFO matrix ceramics.
The solid solution approach for developing room temperature multiferroicity of BFO could be extremely effective at adjusting the critical temperatures of both Ferroelectric and Ferromagnetic ordering above room temperature. Because strong Ferroelectric behavior at room temperature requires a Curie temperature $(T_{CE})$ slightly above room temperature (i.e. near 400~500K), whereas a strong spontaneous magnetization as Ferro/Ferrimagnetic state at room temperature requires a Curie (or Neel) temperature $(T_{CM}, T_{NM})$ as high as possible, $BiFeO_3$ was considered first for solid solution matrix with the other perovskite end members since it has the highest magnetic critical temperature (650K of $T_{NM}$ and1100K of $T_{CE}$). And also the solid solution approach has long been demonstrated to be capable of enhancing the Ferroelectricity by establishing B-site cation order. For example, the prototype relaxor ferroelectric compound of weak B-site cation ordered $Pb(Mg_{1/3}Nb_{2/3})O_3$ with $La(Mg_{2/3}Nb_{1/3})$ or strongly B-site cation ordered $Pb(Mg_{1/3}Nb_{2/3})O_3$ with $Pb(Sc_{1/2}Nb_{1/2})O_3$. Another compound of $Pb(Fe_{2/3}W_{1/3})O_3$, solid solution with $Li(Fe_{1/3}W_{2/3})O_3$ strength the B-site ordering dramatically with parallel enhancement of physical properties. In addition the Fe rich stoichiometry in the $Pb(Fe_{2/3}W_{1/3})O_3$ system have been found to favor B-site cation ordering. Based on the above previous results, an enhancement in cation ordering of BLFO-AFNO solid solution system would be resulted in the enhancement of multiferroic characteristics.
Briefly introducing the results and discussions of following chapters, Rhombohedral-R3c perovskite structure of BFO (which can be treated as distorted pseudo-cubic perovskite) was transformed to monoclinic (or pseudo-cubic) like structure by incorporation of $Sr(Fe_{1/2}Nb_{1/2})O_3$ phase with the enlargement of grain size, which was characterized by XRD and SEM. As consistent with structural variation, micro-domain variation was observed by TEM technique, which showed distinct domain structure according to their composition. Dielectric and Ferroelectric properties were observed as not only increasing relative dielectric constant, but also the frequency dependent dispersion behavior at high $Sr(Fe_{1/2}Nb_{1/2})O$ incorporated composition. However, electrical properties were not paralleled with magnetization enhancement unlike the following BLFO-BFNO and BLFO-PFNO case.
In case of $(1-x) (Bi_{0.9}La_{0.1})FeO_3- x Pb(Fe_{0.5}Nb_{0.5})O_3$ (BLFO-PFNO, x=0.2, 0.7, 0.8), from the powder X-ray diffraction analysis, rhombohedral symmetry in this BLFO-PFNO is maintained through the all $xBLFO-(1-x)PFNO$ compositions, and the relative c/a axis ratio gradually increases with the increase in the PFNO content. It is associated with the ionic radii difference of A-site $Bi^{3+} (1.30 \Aring)$ and $Pb^{2+} (1.63 \Aring)$, but B-site ions of $Fe^{3+}$ and $Nb^{5+}$ possessed almost similar ionic radii in octahedral site. The enhanced magnitude of ferroelectric polarization vector is achievable from the elongated c-axis in the 0.2BLFO-0.8PFNO rhombohedral unit cell. Interestingly, gradual disappearance of ferroelectric domains at 0.2BLFO-0.8PFNO was observed by transmission electron microscopy (TEM), unlike the clear macro-ferroelectric domains structure at 0.8BLFO-0.2PFNO. This may result from the local disturbance of atomic site ordering in relaxor-type ferroelectric PFNO with normal ferroelectric BLFO. The nano-scale ordered single grains of ferroelectric domains at high PFNO composition are consistent to the space charge related dielectric dispersion phenomena as a function of frequency, whilst pure BFO doesn’t show the dielectric dispersion characteristics. In terms of magnetic properties, the addition of PFNO enables to obtain progressively developed saturation magnetizations including large reduction in coercive magnetic field and enhanced saturation magnetization. It should be noted that this is a unique characteristic of our BLFO-PFNO system. It can be explained that the cyclical spin distribution of BFO is broken down by incorporating PFNO end member. Temperature dependent magnetization supports these spin frustration phenomena, however that showed no macroscopic transition temperature indicating glass-like transition due to the nano-scale A- and B- site ordering in single grain. Therefore, we successfully demonstrate another multiferroic BLFO-PFNO solid solution system, which enables soft ferromagnetism at room temperature and macroscopic polarization with the ferroelectric domain evolution.
For the case of BLFO-BFNO, we observed more strong polarization and magnetization at room temperature in all $Ba(Fe_{1/2}Nb_{1/2})O_3$ incorporated system. They were also characterized by XRD, TEM, impedance analyzer, RT66A ferroelectric system, VSM, and SQUID, etc. Especially, it was observed that the complex rhombohedral-type of long-ranged-ordered (LRO) ferroelectric domains in 1.0BLFO at x=1.0 were revolutionized into the mixed LRO domains with the short-range-ordered (SRO) mottled polar domains of 0.8BLFO-0.2BFNO at x=0.8, then, it was finally reached to the single domain of polar grains at x=0.5 of 0.5BLFO-0.5BFNO, which were consisted of SRO mottled polar domains. These evolutions of LRO to SRO ferroelectric domain structure were possibly revealed from the difference of oxygen octahedral distortion in rhombohedral perovskite unit cell without the destruction of rhombohedral symmetry, which were supported by Rietvelt x-ray refinement, oxygen K-edge comparison using NEXAFS spectroscopy, and TEM electron diffraction patterns. Noticeably, the generated $e_g$ like electronic state in oxygen K-edge spectra of each sample showed the clear evidence of oxygen octahedral distortion by cation substitution and Rietvelt x-ray refinement and TEM electron diffraction supported the unchangeable rhombohedral symmetry of BLFO-BFNO unit cell parameters. Temperature dependent magnetization (M-T curve) and magnetic hysteresis (M-H curve) showed the dramatic transformation of magnetization as a function of their compositions x. It was revealed that the canted antiferromagnetic spin states of $(Bi_{0.9}La_{0.1})FeO_3$ at x=1.0 were varied into the spontaneously aligned ferromagnetic-like state of at x=0.8; and then finally developed into the fully saturated magnetic response at x=0.5 even at room temperature. Un-shifted Fe L-edge absorption spectra and Core-level x-ray photoelectron spectra of Fe ions are convinced that the $Fe^{3+}$ valence stabilization correspond to the stable ferromagnetic nature without $Fe^{2+}$ fluctuation. From these results, it was developed the ferromagnetic super-exchange interaction of $Fe^{3+} -O-Nb^{5+} -O-Fe^{3+}$ in BLFO-BFNO solid solutions ceramics by the incorporation of the Nb ion in B-site, and Ba ions in A-site. It was achieved the ferroelectric short range ordered polar domain transition throughout the solid solution approach by utilizing double perovskite end member of $Ba(Fe_{0.5}Nb_{0.5})O_3$.
For a promising future works, some results of epitaxial preparation and characterization of 0.5BLFO-0.5BFNO composition (it can be treated as $(Bi_{0.45}La_{0.05}Ba_{0.5})(Fe_{0.75}Nb_{0.25})O_3$ from the Rietvelt refinement result) thin films was introduced by using Pulsed Laser Deposition method. Artificially cations engineered $(Bi_{0.45}La_{0.05}Ba_{0.5})(Fe_{0.75}Nb_{0.25})O_3$ thin film was epitaxially prepared on the atomically smooth Ir bottom electrode buffered (001) MgO substrate by pulsed laser ablation. The “cube-on-cube” epitaxial relation was [100]BLBFNO//[100]Ir//[100]MgO, (001)BLBFNO//(001)Ir//(001)MgO, which was supported by x-ray diffraction of (111) phi-scan and (111) pole figure, and HRTEM of cross section. These epitaxial relation could be originated from the lattice mismatching less than 3 % between pseudo-cubic BLBFNO $(d_{(001)} = 4.2 \Aring)$, Ir $(d_{(001)} = 3.8 \Aring)$, and rock-salt type MgO $(d_{(001)} = 4.1 \Aring)$. The relative ratio between complex cations in BLBFNO was almost stoichiometric composition, which was checked by RBS and XPS technique. Noticeably, clear $Fe^{3+}$ XPS spectra supported the saturated magnetic hysteresis and ferromagnetic-like magnetization as a function of temperature. The ferroelectric nature of thin film was characterized as a box patterned ferroelectric domain switching and phase/amplitude signal at room temperature by PFM. It was speculated that the oxygen octahedral tilting owing to the cationic size difference of A-site $(Bi^{2+}, La^{2+}, Ba^{2+})$ and B-site $(Fe^{2+}, Nb^{2+})$ in BLBFNO was to develop the room temperature stable multiferroic properties as a thin film form. Thus, we suggested the epitaxial stabilization of complex cations substituted thin films on cubic substrate and confirmed the multiferroic properties at room temperature. As a suggesting future works, (001), (110) and (111) cut MgO substrates are used to control the complex multiferroic film orientation. Film structures are characterized using both X-ray diffraction (XRD) and transmission electron microscope (TEM). Effect of epitaxial stress, orientation, and thickness on the film properties will be discussed later.
최근의 나노 사이즈의 다기능(multi-functional) 물질에 대한 전기/자기 소자로의 응용에 관한 연구는, 독창적인 전자/기계 공학 소자(MEMS 및 Actuator)로의 응용 뿐 만 아니라, 물질의 새로운 고유 특성(intrinsic property)을 증진하는 연구 방향으로도 진행되고 있다. 한 가지 예시로, 다강체(multiferroic materials)는 강유전 특성과 강자성 특성이 동시에, 그리고 자발적으로 나타나는 물질이며, 이 물질의 고유 특성 증진에 관한 연구는 현대 고체 물리학의 주된 관심 분야 일 뿐 만 아니라, 차세대 응용 메모리 소자로의 발전이 기대 되는 소재이다. 기존의 DRAM과 같은 하나의 특성 제어만을 통한 소자 구현과는 다르게, 단일 상 (single phase) 내에서 강유전 특성과 강자성 특성의 고유한 결합 원리 (intrinsic coupling-mechanism)를 내재한 다강체 물질은 증가된 또다른 자유도(degree of freedom)의 '읽기/쓰기'를 가능케 한다. 특히 증가된 자유도의 개념은 전기장이나 자기장중 하나의 구동력이 아닌 상호 별도의 구동력을 이용한 다중항 상태의(multiple-state) 메모리 소자의 구현을 가능하게 한다. 즉, 개별적인 강유전, 강자성, 강탄성 물질에서의 변위, 분극, 그리고 자화의 제어를 위해선 각각 응력, 전기장, 자기장의 구동력이 필요한 반면, 본과제에서 제안하는 다강체는 하나의 인가 전기장 (electric field)을 통한 잔류 분극(pilarization)의 제어가 핵심이 되는 요소라 할 수 있다. 다시 말해, 다강체에서의 분극 특성을 이용하여 data bit을 통해 쓴 정보가, 동시에 강자성 자화 특성을 변화시켜 외부의 자기장 변화를 탐지 하여 읽어 낼 수 있다는 개념이다. 따라서 다강체는, 기존 자성 물질 내에서의 수백 나노 이상의 자기 구역벽 크기에 의한 낮은 밀집도를 수 나노미터 이하의 강유전 도메인 벽으로 대체함으로서 정보 저장의 고밀도를 수 나노미터 이하의 강유전 도메인 벽으로 대체함으로서 정보 저장의 고밀도화와 고집적화를 이룰 수 있을 것으로 예상된다. 그리고 이를 기존의 자기 헤드와 같은 구동 방식을 적용함으로써, 빠른 구동이 가능한 차세대 메모리를 구현할 수 있는 장점을 지닌다.
실제적인 다강체 물질에 관한 탐구는 19세기 초에 제안 되었으나 실험적으로 전기장의 인가에 의한 자구(magnetic domain)의 제어와 자기장 인가에 의한 전기자구(electric domain)의 제어는, 최근 물리학계에서의 큰 관심사이다. 그러나 이러한 전장/자장 상호 작용에 관한 탐구에도 불구하고, 다강체의 실제적인 소자 응용을 위해선 다음과 같은 문제점들이 있다. 첫째로, 현존하는 단일상 다강체($BiFeO_3, BiMnO_3$ 등등)의 경우 위에서 언급한 상호 교환 작용(ME coupling effect)이 너무나 미약하며, 둘째로, 다강체 특성을 나타내는 단일상 물질이 소수의 형태로만 존재한다는 점이며($d_0$ rule), 셋째로, 대부분 상온 이하에서 강유전 전이 온도(Curie Temperature, $T_c$)나 강자성 전이 온도 (Neel temperature, $T_N$)가 나타나는 특성을 지니고 있다는 점이다. 현재 다른 관점에서의 복합 재료 형태의 magneto-electric couping)의 커다란 진전에도 불구하고, 단일상 다강체 물질의 상온에서의 결합 특성(magneto-electric coupling)에 관한 근본적 이해는 여전히 풀리지 않는 문제들을 가지고 있다.
상온에서 결합 특성이 우수하게 나타나는 다강체 물질을 유도하기 위하여, 본 지원자는 표 1.에서 보인바와 같이, $BiFeO_3$ 와 $Pb(Fe_{0.5}Nb_{0.5}O_3, Ba(Fe_{0.5}W_{0.5})O_3, Sr(Fe_{0.5}Nb_{0.5})O_3$ 등의 물질들 간의 고상 고용체(solid solution) 합성을 통한 접근을 시도 하였다. 본 연구 그룹의 선행 조사에 따르면, 각각의 물질들의 고유특성(intrinsic prolerty)에 관한 몇몇의 보고가 있다. 하지만 이들 간의 고상고용체(solid solution)에 관한 결과는 충분하지 못함을 알 수 있었다. 각각의 물질에 관한 각각의 물성과 본 연구에서 사용하려는 조성 분포에 관한 이유를 다음과 같이 나타내었다.
(1) $Pb(Fe_{0.5}Nb_{0.5})O_3, Ba(Fe_{0.5}W_{0.5})O_3, Sr(Fe_{0.5}Nb_{0.5})O_3, BiFeO_3$ 는 자기/전기 결합 특성을 보이는 다강체 물질이다.
(2) 이들 물질은 다른 다강체들에 비해 상대적으로 높은 자기 전이 온도($T_N$ : Neel temperature)와 전기 전이 온도($T_C:Curie temperature)를 가지는 물질이다.
(3) 상호 비슷한 구조적 특성을 지녀, A-site B-site ordering 을 통하여 새로운 고용체 형성 시 안정화된 고용 상을 얻을 수 있다.
(4) 각각의 물질들은 Tolerence factor 가 1에 가까운 물질들로서, 고용체 형성 시 log-rage-ordering 을 형성함과 동시에 phase boundary 에서 향상된 물성(잔류자화, 잔류 분극)을 기대할 수 있다.
고상 소결법으로 $x(Bi_{0.9}La_{0.1})O_3 - (1-X)Ba(Fe_{0.5}Nb_{0.5})O_3, x(Bi_{0.9}La_{0.1})FeO_3 - (1-X)Sr(Fe_{0.5})O-3, x(Bi_{0.9}La_{0.1})FeO_3 - (1-x)(Pb(Fe_{0.5}Nb_{0.5})O_3$ 를 합성하여 상온에서 안정된 강유전 특성, 자성 특성을 측정하였고, XRD 및 TEM 분석을 통하여 합성된 고용체 물질의 구조적 특성에 대해서 파악할 수 있었다.
$(Bi_{0.9}La_{0.1}FeO_3 - Sr(Fe_{0.5}Nb_{0.5})O_3$ 의 경우, $Sr(Fe_{0.5}Nb_{0.5})O_3$ 의 함량 증가에 따라 점차 대칭성이 강화되고, 유전특성이 약화되는 경향을 보였다. $Sr(Fe_{0.5}Nb_{0.5})O_3$ 의 함량 증가에 따른 자성특성의 강화는 XRD pattern과 SEM 및 TEM 분석을 통해서 파악된 Fe 원자가 풍부한 impurity phase에 의한 것으로 생각된다.
$(Bi_{0.9}La_{0.1})FeO_3 - Pb(Fe_{0.5}Nb_{0.5})O_3$ 와 $(Bi_{0.9}La_{0.1})FeO_3 - Ba(Fe_{0.5}Nb_{0.5})O_3$ 의 경우는 $Pb(Fe_{0.5}Nb_{0.5})O_3$ 함량 증가에 따라 [111] 방향으로 c축 성장이 강화되는 비대칭성의 증가 및 유전특성 및 자성특성이 강화되는 경향을 보였다. $Sr(Fe_{0.5}Nb_{0.5})O_3$ 의 경우와 비교해 보았을 때, 특성 향상 및 자기 spin 정렬에 의한 것으로 판단하였다. 또한 $(Bi_{0.9}La_{0.1})FeO_3 - Ba(Fe_{0.5}Nb_{0.5})O_3$ 의 에피텍셜한 성장을 통하여 인공적인 스트레인이 인가된 상태에서의 다강체 특성과 구조적 특성을 분석하였다.
이러한 새로운 물질의 합성을 통한 다강체 특성 분석을 통하여 다강체 물질 합성에 대한 새로운 가능성을 제시할 수 있었다.
