Spin glass behavior and exchange bias effect in GaFeO3-type iron oxide

GaFeO₃-type iron oxide is a promising room-temperature multiferroic material due to its large magnetization and polarization. To expand the scope of its application, it is crucial to control the magnetic properties. Based on introducing the ferromagnetic (FM) Fe₃O₄ in the antiferromagnetic (AFM) GaFeO₃ to build the FM-AFM interface by changing the Ga/Fe ratio, Ga_0.69Fe_1.31O₃ (GFO) was successfully grown by the floating zone method. The resulting sample was characterized by X-ray diffraction (XRD), and its magnetic properties were measured using a superconducting quantum interference device (SQUID). The temperature-dependent AC susceptibility measurement shows that the spin glass-like behavior of GFO at temperatures close to 50 K is a manifestation of the geometrical frustration arising from cation site disorder. In addition, the magnetic property measurement shows that the magnetic transition temperature Tc is at 650 K, which is introduced by Fe₃O₄ and suppresses the ferromagnetic transition around 320 K of GFO. Interestingly, the observed exchange bias effect, which does not exist in the bulk GaFeO₃-type family, is attributed to the formation of an FM/AFM interface due to the existence of FM Fe₃O₄ in the GFO. This study provides a new perspective on the properties of the GaFeO₃-type family for potential applications in spintronic devices.     

Some current problems in perovskite nano-ferroelectrics and multiferroics: kinetically-limited systems of finite lateral size

Abstract

We describe some unsolved problems of current interest; these involve quantum critical points in ferroelectrics and problems which are not amenable to the usual density functional theory, nor to classical Landau free energy approaches (they are kinetically limited), nor even to the Landau–Kittel relationship for domain size (they do not satisfy the assumption of infinite lateral diameter) because they are dominated by finite aperiodic boundary conditions.     

Observation of oxygen pyramid tilting induced polarization rotation in strained BiFeO3 thin film

Oxygen octahedral tilting has been recognized to strongly interact with spin, charge, orbital, and lattice degrees of freedom in perovskite oxides. Here, we observe a strain-driven stripe-like morphology of two supertetragonal (monoclinic Cc and Cm ) phases in the strained BiFeO₃/LaAlO₃ thin films. The two supertetragonal phases have a similar giant axial ratio but differences in oxygen pyramid tilting mode. Especially, the competition between polar instability and oxygen pyramid tilting is identified using atomically resolved scanning transmission electron microscopy, leading to the polarization rotation across the phase boundary. In addition, microtwins are observed in the Cc phase. Our findings provide new insights of the coupling between ferroelectric polarization and oxygen pyramid tilting in oxide thin films and will help to design novel phase morphology with desirable ferroelectric polarization and properties for new applications in perovskite oxides.     

Ferroelectric and Magnetic Properties in Room‐Temperature Multiferroic GaxFe2−xO3 Epitaxial Thin Films

GaFeO₃‐type iron oxide is a promising room‐temperature multiferroic material due to its large magnetization. To expand its usability, controlling the ferroelectric and magnetic properties is crucial. In this study, high‐quality Ga_xFe_2–xO₃ (x = 0–1) epitaxial films are fabricated and their properties are systematically investigated. All films exhibit room‐temperature out‐of‐plane ferroelectricity, showing that the coercive electric field (E_c) decreases monotonically with x. Additionally, the films show in‐plane ferrimagnetism with a Curie temperature (T_C) >350 K at x = 0–0.6. The coercive magnetic field (H_c) decreases with x at x ≤ 0.6, but shows a constant value at x > 0.6, whereas the saturated magnetization (M_s) increases with x at x ≤ 0.6, but decreases with x at x > 0.6. X‐ray magnetic circular dichroism reveals that the large magnetization at x = 0.6 is derived from Fe³⁺ (3d⁵) at octahedral sites. The controllable range of the E_c, H_c, and M_s values at room temperature (400–800 kV cm^?1, 1–8 kOe, and 0.2–0.6 µ_B/f.u.) is very wide and differs from those of well‐known multiferroic BiFeO₃. Furthermore, the Ga_xFe_2?xO₃ films exhibit room‐temperature magnetocapacitance effects, indicating that adjusting T_C near room temperature is useful to achieve large room‐temperature magnetocapacitance behavior.     

溶胶-凝胶法制备BiFeO3粉体及其表征

以硝酸铁和硝酸铋为原料,冰醋酸和乙二醇单甲醚为溶剂制备BiFeO3粉体。采用XRD,SEM,EDS,TG-DSC以及FT-IR等手段对产物的物相、形貌、结构等进行分析,研究煅烧温度和保温时间对BiFeO3粉体的影响。结果表明:BiFeO3的最佳煅烧温度为550~600℃,保温时间为0.5~1.5h。另外,在BiFeO3制备过程中对Bi含量进行过量添加处理,发现当Bi过量3%~6%时,制备出的BiFeO3粉体杂相最少。     

Ferroelectric and magnetic properties in (1−x)BiFeO3–x(0.5CaTiO3–0.5SmFeO3) ceramics

Multiferroic ceramics were prepared and characterized in (1?x)BiFeO₃–x(0.5CaTiO₃–0.5SmFeO₃) system by a standard solid‐state reaction process. The structure evolution was investigated by X‐ray diffraction and Raman spectrum analyses. The refinement results confirmed the different phase assemblages with varying amounts of polar rhombohedral R3c and nonpolar orthorhombic Pbnm as a function of the substitution content. In the compositions range of 0.2≤x≤0.5, polar R3c and nonpolar Pbnm coexisted, which was referred to polar‐to‐nonpolar morphotropic phase boundary (MPB). According to the dielectric and DSC analysis results, the ceramics with x≤0.2 changed to diffused ferroelectric, and the ferroelectric properties were enhanced significantly. Two dielectric relaxations were detected in the temperature range of 200‐300 K and 500‐700 K, respectively. The high‐temperature dielectric relaxation was attributed to the grain‐boundary effects. While the low temperature dielectric relaxation obtained in the samples with x=0.3‐0.5 was related to the charge transfer between Fe²⁺ and Fe³⁺. The magnetic hysteresis loops measured at different temperature indicated the enhanced magnetic properties in the present ceramics, which could be attributed to the suppressed cycloidal spin magnetic structure by Ti ions. In addition, the rare‐earth Sm spin moments might also affect the magnetic properties at relatively lower temperature.     

弛豫多铁性材料研究进展

多铁性材料同时具有多种铁性(铁电性、铁磁性或铁弹性)的有序, 可实现电磁信号的相互控制, 成为近年来研究热点。在具有成分无序的复杂体系中, 长程铁性有序有可能被打破, 材料将表现出弛豫特性。我们将至少存在一种铁性弛豫特性的多铁性材料称之为弛豫多铁性材料。这类多铁性材料的极化强度(或磁化强度)在外加电场(或外加磁场)作用下响应更加灵敏, 其磁电耦合机制与长程有序的多铁性材料不同。本文结合国内外最新研究成果, 首先介绍了和弛豫铁性有序相关的物理概念, 重点阐述了多铁性材料在铁电和铁磁双弛豫态下的磁电耦合机制; 然后, 详细介绍了钙钛矿结构(包括PbB₁B₂O₃基和BiFeO₃基材料)和非钙钛矿结构(包括层状Bi结构和非正常铁电体)弛豫多铁性材料的研究进展; 最后, 对该领域亟待解决的问题进行了展望。     

自组织纳米复合薄膜BiFeO3-CoFe2O4的微结构与相成分

采用先进电子显微术在原子尺度研究了(001)单晶SrTiO₃衬底上生长的纳米复合薄膜0.65BiFcO₃-0.35CoFe₂O₄的组织形态以及界面结构.BiFeO₃(BFO)和CoFe₂O₄(CFO)两相在外延生长过程中自发相分离,形成自组织的复合纳米结构.磁性尖晶石CFO呈方块状均匀分布于铁电钙钛矿BFO基体中,并沿[001)1]方向外延生长,形成垂直的柱状纳米结构.两相具有简单的立方-立方取向关系,即[001]_BFO//[001]_CFO和(100)_BFO//(100)_CFO,且界面为{110}晶面.薄膜表面起伏不平,形成CFO{111}小刻面而BFO则为平整的(001)表面.能谱分析结果表明各相成分均匀分布并无明显的元素互扩散发生.     

Nano‐Domain Pinning in Ferroelastic‐Ferroelectrics by Extended Structural Defects

Most ferroelectrics are also ferroelastics (hysteretic stress‐strain relationship and response to mechanical stresses). The interactions between ferroelastic twin walls and ferroelectric domain walls are complex and only partly understood, hindering the technological potential of these materials. Here we study via atomic force microscopy the pinning of 180‐degree ferroelectric domain walls in lead zirconate titanate (PZT). Our observations satisfy all three categories of ferroelectric‐ferroelastic domain interaction proposed by Bornarel, Lajzerowicz, and Legrand.