Enhanced dielectric,ferroelectric and magnetic properties of Ba4Sm2Fe2Nb8O30 RT multiferroics prepared by microwave sintering

High density Ba₄Sm₂Fe₂Nb₈O₃₀ (BSFN) multiferroics ceramics with tetragonal tungsten bronze structure had been prepared by microwave sintering (MS) for 30min and conventional sintering (CS) methods for 4 h at 1275 °C. Single tungsten bronze phase and equiaxial grains are obtained for the MS BSFN ceramics, while a small amount secondary phase of SmNbO₄ is observed in the CS BSFN ceramics with columnar grains. Compared to Ba₄Sm₂Fe₂Nb₈O₃₀ ceramics prepared by CS method, enhanced dielectric, ferroelectric and magnetic properties are achieved for the MS BSFN ceramics. The values of electric polarization Pr and coercive electric field Ec are 2.11 μC/cm² and 7.14 kV/cm for the MS BSFN ceramics, respectively. Meantime, the magnetic polarization Mr of 0.410emu/g and coercive magnetic field Hc of 2930Oe are also obtained for the MS BSFN ceramics. Based on the density, crystal structure, point defect and grain, the reasons of enhanced dielectric, ferroelectric and magnetic properties are discussed for the MS BSFN ceramics It is indicated that Ba₄Sm₂Fe₂Nb₈O₃₀ is an intrinsic room temperature multiferroic materials.     

Crystal structure,leakage conduction mechanism evolution and enhanced multiferroic properties in Y-doped BiFeO3 ceramics

Ceramics of pure phase Yttrium (Y) doped BiFeO₃ prepared by a solid-state sintering route were characterized by X-ray diffraction, Raman spectroscopy, magnetic and electrical measurements. The results and analysis show that Y substitution greatly reduces the leakage current and enhances the multiferroic properties of BiFeO₃. Leakage conduction mechanism is shown to change from space-charge-limited conduction type in pure BiFeO₃ to a Poole–Frenkel emission behavior in Bi_0.90Y_0.10FeO₃. A Fowler-Nordheim tunneling mechanism in Bi_0.95Y_0.05FeO₃ ceramic is also evidenced under high electric fields. At the same time, enhanced magnetic properties due to Y-doping are confirmed by temperature dependent magnetometry and supported by Raman spectroscopy. An unexpected and sharp switching behavior in the magnetization under low magnetic fields observed in Bi_0.90Y_0.10FeO₃ ceramic, together with its improved ferroelectric property, may trigger such system for promising magneto-electric applications.     

Inapplicability of the Maxwell relation for the quantification of caloric effects in anisotropic ferroic materials

Giant caloric effects were reported in elasto-, electro- and magnetocaloric materials near phase transformations. Commonly, their entropy change is indirectly evaluated by a Maxwell relation. Additionally to the well-known spurious “colossal” results near first-order phase transitions and limitations in the presence of magnetic domains, we report the fundamental failure of this approach for materials that show structural reorientations in external fields. We analyze exemplarily the magnetic shape memory alloy Ni-Mn-Ga. Application of a magnetic field causes magnetically induced reorientation of the crystal structure in the martensite phase. This results in a spurious inverse magnetocaloric effect that disappears when the measurement procedure is repeated. This failure is universal as the vector character of the applied field is not considered in the common scalar evaluation of a Maxwell relation.     

A new room-temperature multiferroic material: Y2FeAlO6

In this paper, we report the finding and basic characterization of a new multiferroic material in the form of the Y₂FeAlO₆ (YFAO) double perovskite with orthorhombic structure. In order to study the structural and multiferroic properties in detail, we prepared the polycrystalline YFeO₃ (YFO) and Y₂FeAlO₆ ceramic samples through traditional high temperature solid-state reaction method. XRD analysis confirmed that these two samples are single-phase and all the peaks in the pattern can be indexed with orthorhombic symmetry of the Pnma space group. Compared to YFO, YFAO is more symmetric, and has a more stable crystalline structure and smaller grain size. The Fe ion in YFO and YFAO are in the mixed oxidation state of Fe²⁺ and Fe³⁺, while the relative amount of Fe²⁺ in YFAO is higher than that of YFO. Al³⁺ with filled 2p orbital and grain refinement may be the main reasons for the greatly enhanced multiferroicity of YFAO. The magnetic properties of YFO and YFAO are mainly dominant antiferromagnetism and weak ferromagnetism, and the exchange bias effect disappears in YFAO. The coexistence of ferroelectricity and ferromagnetic orders at room temperature in YFAO promises new opportunities and improvements in next generation applications for spintronic, information storage, and sensors etc.     

Bismuth ferrite bilayered thin films of different constituent layer thicknesses

Bilayered thin films consisting of (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃ and (Bi_0.90La_0.10)(Fe_0.90Zn_0.10)O₃ layers have been fabricated by radio frequency sputtering. Both multiferroic layers are well retained in these bilayers. Their leakage current, multiferroic properties, and fatigue behavior are largely dependent on the thicknesses of (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃. With an increase of the thickness in the (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃ layer, the leakage current density of bilayers is degraded due to different grain growth modes and an increase in oxygen vacancies, the dielectric constant (?_r) becomes larger due to the introduction of (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃ with a high ?_r value, and their magnetic properties are deteriorated with increasing the thickness ratios of (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃ with a weaker magnetization. All bilayers exhibit a good ferroelectric behavior regardless of varying thicknesses of the (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃ layer, while their coercive field decreases with increasing the thickness of the (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃ layer. An anomalous enhancement in switchable polarization is demonstrated by these bilayers, owing to the involvement of space charges accumulated at the interfaces between two constituent layers.     

Tuning the dielectric,ferroelectric and electromechanical properties of Ba0.83Ca0.10Sr0.07TiO3–MnFe2O4 multiferroic composites

We report the successful synthesis of Ba_0.83Ca_0.10Sr_0.07TiO₃–MnFe₂O₄ multiferroic composites showing significant improvement in electromechanical and magnetoelectric properties. All the composite samples have formed a diphasic perovskite-ferrite composite without the presence of any impurity or intermediate phase. The bare as well as composite samples have shown classical dielectric behavior even at higher ferrite substituted samples. The electrical characteristics of composite samples have shown slight deterioration, which is mainly attributed to non-ferroelectric MnFe₂O₄. However, the composites still exhibit high enough piezoelectric behavior and the modification in the electromechanical response of composites is mainly caused by a change in applied stress with MnFe₂O₄ addition. The M-H loops of composites have demonstrated a ferrimagnetic behavior with a substantial increase in saturation magnetization on increasing the ferrite concentration. Further, the composites have shown better coupling between the ferroelectric and ferrimagnetic phases, which has resulted in an improved magnetoelectric characteristic. The role of oxygen vacancies on ferroelectric and magnetic properties of prepared composites has been systematically studied.     

Dielectric,multiferroic and magnetodielectric properties of (1-x)BaTiO3-xSr2CoMoO6 solid solution

The magnetoelectric composites of (1-x)BaTiO₃-xSr₂CoMoO₆ (x=0.02, 0.04, 0.06, 0.08, 0.10 and 0.12) are synthesized successfully via solid state reaction. The dielectric, multiferroic and magnetodielectric properties were discussed in details. The XRD data confirm the single perovskite phase among the composites. In addition, as the content of Sr₂CoMoO₆ increase, the diffraction peaks shift to higher angle which indicates the gradually decreased lattice parameters. The observed dielectric relaxation phenomenon could be attributed to Maxwell-Wagner (MW) model. The weak ferromagnetism was observed for x=0.02, 0.04 with the remnant magnetization (M_r) 0.025 memu/g, 0.065 memu/g and coercivity (H_c) 4 Oe, 10 Oe, respectively despite the anti-ferromagnetic nature of Sr₂CoMoO₆. Furthermore, the M − H hysteresis loops of (1-x)BaTiO₃-xSr₂CoMoO₆ change obviously with polarization state, which is related to the ordering degree of the Co and Mo sites. The reason for observed magnetodielectric effect is the influence of magnetoresistance (MR) on Maxwell-Wagner (MW) effect.     

Synthesis and mechanical properties characterization of multiferroic BiFeO3-CoFe2O4 composite nanofibers

Multiferroic nanofibers with excellent mechanical properties have great potential applications in multifunctional nanodevices. BiFeO₃-CoFe₂O₄ (BFO-CFO) composite nanofibers with different molar ratios were successfully synthesized by sol-gel-based electrospinning method. The mechanical properties of BFO-CFO composite nanofibers were examined by nanoindentation technique, and further investigated by amplitude modulation-frequency modulation (AM-FM) method based on atomic force microscopy (AFM). The results of AM-FM showed that the elastic moduli of BFO-CFO composite nanofibers increased with the increase of CFO ratio, which was consistent with the results of nanoindentation. These results indicated that AFM-based AM-FM is a powerful method for nondestructively investigating the mechanical properties of materials at nanoscale, and that the results of BFO-CFO composite nanofibers are also of practical importance for the future applications of multifunctional nanodevices.     

Structural evolution and electrical properties of Na0.5Bi0.5TiO3-CoFe2O4 ceramics with embedded structures

Lead-free 0.9Na_0.5Bi_0.5TiO₃ − 0.1CoFe₂O₄ (0.9NBT-0.1CFO) composite ceramics with novel embedded microstructures were synthesized by an in-situ sol-gel method. The structural evolution, ferroelectricities and dielectricities were studied. The embedded structures were formed around 1000 °C − 1100 °C with contributions of the strong grain boundary mobility and small size of CFO grains, and then were destroyed at 1150 °C. The 0.9NBT-0.1CFO ceramic sintered at 1100 °C showed the optimal ferroelectric hysteresis loop with a remanent polarization of ~ 52 μC/cm². Anomaly peaks in the temperature dependent permittivity curves were observed in all ceramics with embedded structures. These Debye-like type peaks generated by the interface effects of NBT and CFO appeared around ~620 °C, and could reach relatively large values of ~15000 at 100 kHz. The change rate of permittivity between zero magnetic field and 500 Oe reached 2.4%, indicating a noteworthy magnetoelectric effect.