Shujie Sun

Low magnetic field response single-phase multiferroics under high temperature

A single-phase material where ferroelectricity and ferromagnetism coexist at room temperature (RT) is hardly available at present, and it is even more rare for such a material to further have an intrinsic and low magnetic field response magnetoelectric (ME) coupling at temperatures higher than RT. In this communication, a new single-phase Aurivillius compound, SrBi5Fe0.5Co0.5Ti4O18 has been discovered that exhibits a plausible intrinsic ME coupling. Remarkably, this property appears at a high temperature of 100 °C, surpassing all single-phase multiferroic materials currently under investigation. With a magnetocapacitance effect detectable at 100 °C and under a low response magnetic field, a RT functioning device was demonstrated to convert an external magnetic field variation directly into an electric voltage output. The availability of such a single-phase material with an intrinsic and low magnetic field response that is multiferroic at high temperature is important to the fundamental understanding of physics and to potential applications in sensing, memory devices, quantum control, etc.

Synthesis of Ni-substituted Bi7Fe3Ti3O21 ceramics and their superior room temperature multiferroic properties

Layer-structured bismuth complex oxides Bi7Fe3−xNixTi3O21 (0 ≤ x ≤ 2) (BFNT) were synthesized using a low-temperature combustion synthesis method. X-ray diffraction patterns and high-resolution transmission electron microscopy analysis indicated that the samples presented a six-layer Aurivillius structure. Substituting Fe sites by Ni ions inside the lattice was found to be effective in enhancing the multiferroic properties at or above the room-temperature. The sample with a composition of x = 1 exhibited a large remnant magnetization (2Mr = 1.32 emu g−1) that is about five hundred times higher than that in un-substituted Bi7Fe3Ti3O21 ceramics. The work is an important step in the effort to find a single phase and a fully functioning multiferroic material.

Observation of Exchange Anisotropy in Single-Phase Layer-Structured Oxides with Long Periods.

A remarkable exchange bias effect arising from the temperature-dependent interaction among the ferromagnetic-like cluster glasses and antiferromagnetic regions was observed in a newly developed single-phase multiferroic compound of Bi10Fe6Ti3O30 which has a nine-layer Aurivillius structure. Inhomogeneous distribution of magnetic Fe ions inside this long-period layered structure was experimentally identified via the atomic level imaging. The results confirmed the presence of the short-range magnetic ordering (the cluster glassy state) and the canted antiferromagnetism, and then the direct interaction among them was further confirmed. Finding of this new single-phase material accompanying this remarkable exchange bias effect would be beneficial to both basic physics understanding and the potential device development.

Yttrium-modified Bi7Fe1.5Co1.5Ti3O21 ceramics with improved room temperature multiferroic properties

Yttrium-modified Bi7Fe1.5Co1.5Ti3O21 Aurivillius phase oxides were synthesized using a modified Pechini method, and their multiferroic properties were investigated as a function of the Y content in the formula Bi7−xYxFe1.5Co1.5Ti3O21. X-ray diffraction investigation suggests the formation of a pure Aurivillius phase when the Y content is less than 1.25, and a Co-doped BiFeO3 impurity appears when the content exceeds 1.25. When the Y content is 1.25, the sample exhibits significantly improved room temperature multiferroic properties with a remanent polarization 2Pr of 11.72 μC cm−2 and a remanent magnetization 2Mr of 3.2 emu g−1. Derivative thermo-magneto-gravimetry measurement indicates that the measured magnetic properties mainly originate intrinsically from the Y-modified Aurivillius phases when the Y content is less than 1.25.

Structural transformation and multiferroic properties in Gd-doped Bi7Fe3Ti3O21 ceramics

Bismuth layer-structured Bi7−xGdxFe3Ti3O21 (0.00 ≤ x ≤ 1.50) ceramics were synthesized by Pechinis method, in which gadolinium doping was used with a goal to enhance the magnetic response of the material. With increase in the Gd content of x, an obvious structural transformation, changing gradually from the originally designed six-layer structure to five-layers, was initially shown by X-ray diffraction patterns, and then by Raman scattering spectra and high-resolution transmission electron microscopy images. Substituting Bi sites with Gd3+ ions was found to be able to effectively suppress the leakage current, and its resistivity was found to be about two orders of magnitude higher than that of the un-doped sample. Improved magnetic properties and a clear magnetic anomaly were observed in the sample with a composition of x = 1.00, indicating the behaviour of transforming from anti-ferromagnetism with weak ferromagnetism at the room temperature into a complex magnetism at low temperature.

Room-temperature multiferroic responses arising from 1D phase modulation in correlated Aurivillius-type layer structures

Structural instability induced by chemical substitution in multiferroic materials, especially in layer-structured oxides of the Bi4Ti3O12–BiFeO3 system, gives rise to intriguing phenomena and extraordinary coupling properties. In this work, we carefully studied an analogous morphotropic structural transformation in Co-substituted Bi7Ti3Fe3O21 oxides, similar to a morphotropic phase boundary. Large effects (including ferroelectric, dielectric and ferromagnetic) and abnormal magnetic behaviour were recorded at the analogous morphotropic transformation region (AMTR). 1D phase-modulated structures composed of two different layered phases and structural distortions inside the AMTR were visualized. Analyses of the correlations between oxygen octahedral distortions and multiferroic responses confirmed that the enhanced multiferroic responses inside the AMTR are intrinsic to the 1D correlated interface structures, mainly arising from such structural distortions due to the existence of different lattice strains and the change of crystal symmetry.

Measuring room-temperature intrinsic multiferroic properties by excluding the secondary magnetic inclusion contribution

The assertion that a new material could become a potential single-phase and room-temperature functioning multiferroic material may be confounded by the presence of minor amount of secondary magnetic inclusions, especially in the Aurivilliustype material system. In this study, we demonstrated that the derivative thermo-magneto-gravimetry (DTMG) technique can be a sensitive tool to identify an d quantify the magnetic secondary phases in the Bi7Fe2.25Co0.75Ti3O21 ceramic, which shows the potential to become a single-phase multiferroic material. The accuracy of this DTMG measurement experimentally reaches to ~0.5 wt.%, far below the detection limit of the traditional X-ray diffraction. The impurity identified in the specimen is the ferrimagnetic CoFe2O4 spinel phase with an amount of ~3.6 wt.%. Significantly, the room-temperature intrinsic magnetism of the ceramic was measured, which is sorely from the main phase.中文摘要判断新的材料是否是一个室温单相多铁性材料需要认真的鉴定评价, 特别是对于Aurivillius 相多铁材料. 这类材料中易生 成微量的具有铁磁性的杂质, 从而混淆对其本征磁性能的判断. 本论文介绍了一种磁失重方法, 并应用该方法判断和量化了Aurivillius 相层状结构陶瓷Bi7Fe2.25Co0.75Ti3O21中的磁性杂质. 该方法的测量精度远高于X-射线仪器的精度, 能够辨别出含量仅为0.5%重量的杂质. 最终结果表明陶瓷B i7Fe2.25Co0.75Ti3O21中的磁性杂质是尖晶石相CoFe2O4, 其含量约占总质量的3.6%. 通过该方法同时确定了该陶瓷的室 温固有磁性. 本研究不仅展示了磁失重方法, 而且通过固有磁性和固有铁电性的鉴定, 证明了该陶瓷是一种新的室温单相多铁性材料.

Nanoscale Structural Modulation and Low-temperature Magnetic Response in Mixed-layer Aurivillius-type Oxides

Nanoscale structural modulation with different layer numbers in layer-structured complex oxides of the binary Bi4Ti3O12-BiFeO3 system can give rise to intriguing phenomena and extraordinary properties, originating from the correlated interfaces of two different phases with different strain states. In this work, we studied the nanoscale structural modulation induced by Co-substitution in the Aurivillius-type oxide of Bi11Fe3Ti6O33 with a unique and naturally occurred mixed-layer structure. Nanoscale structural evolution via doping occurred from the phase-modulated structure composed of 4- and 5-layer phases to a homogeneous 4-layer structure was clearly observed utilizing x-ray diffraction and electron micro-techniques. Significantly, magnetic response for the samples under various temperatures was recorded and larger magnetic coercive fields (e.g. Hc ∼ 10 kOe at 50 K) were found in the phase-modulated samples. Analyses of the x-ray absorption spectra and magnetic response confirmed that the low-temperature magnetic behaviour should be intrinsic to the phase-modulated structure inside the structural transformation region, mainly arising from structural distortions at the correlated interfaces.