A neutron study of the tetragonal antiferromagnet FeGe2 has shown the existence of two continuous magnetic transitions at temperatures of -263 and -289 K. The upper temperature corresponds to a transition from paramagnetism to a basal-plane spiral structure propagating along the cell edges in that plane. At the lower temperature the spiral structure is transformed into the simple collinear structure previously reported in the literature. Typical critical behavior is observed at the upper temperature for individual satellite peaks. T' he spiral propagation vector decreases continuously to zero at the lower critical point, exhibiting power-law behavior with an exponent of 0.407+0.005. Heat-capacity measurements reveal two A.-type anomalies with critical exponents in the expected range. The phase diagram has been analyzed using mean-field and renormalizationgroup considerations. A model based on zero basal-plane spin anisotropy yields a magnetic structure which agrees with the observed structure of the intermediate phase. The effect of an external field has also been treated theoretically.

The nature of filler−polymer and filler−filler interactions in rubber composites under strain remains an open question in soft matter physics. These interactions are key to explaining the rich variety of complex behavior exhibited by particle-filled rubber products. In this paper we demonstrate a simultaneous dielectric/dynamic mechanical analysis technique (SDMS) which provides new insights into the structure−property relationships of filled rubbers. The complex permittivity of carbon black filled natural rubber has been characterized under a simultaneous tensile strain field (from 0.1% to 50%). The complex permittivity exhibits a dramatic nonlinear dependence on strain coupled with features which are analogous to mechanical strain softening and strain history, namely the "Payne" and "Mullins" effects. The sensitivity of the complex permittivity to such effects is several orders of magnitude greater than in corresponding, traditional mechanical tests. In addition, we demonstrate for the first time that it is possible to use both strain and electrical field frequency as "dipole filters" which can be used to selectively probe the dipoles present at the polymer−filler interface.

Significant quenched disorder in crystal structure can break ferroic (magnetic or electric) long-range order, resulting in the development of ferroic glassy states at low temperatures such as magnetic spin glasses, electric dipolar glasses, relaxor ferroelectrics, etc. These states have been widely studied due to novel physical phenomena they reveal. Much less known are the effects of quenched disorder in multiferroics, i.e. the materials where magnetic and electric correlations coexist. Here we report an unusual behaviour in complex perovskite Pb(Fe2/3W1/3)O3 (PFW) crystals: the coexistence of electric relaxor, magnetic relaxor and antiferromagnetic (AFM) states. The most striking finding is the transformation of the AFM phase into a new reentrant-type magnetic glassy phase below Tg ≅ 10 K. We show that the behaviour at this transformation contrasts the typical behaviour of canonical spin glasses and is similar to the behaviour of relaxor ferroelectrics. Magnetoelectric effect is also observed in the AFM phase in the temperature range of the transition into electric relaxor phase at Tf ≅ 200. The mechanism of magnetic relaxor behaviour is supposed to arise from the frustrated interactions among the spins located at the AFM domain walls. Our results should inspire further studies of multirelaxor behaviour in other multiferroic systems.

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Piezo-/ferroelectrics are essential materials for electromechanical sensors and actuators and energy harvesters in a wide range of technological applications. The demand for piezo-/ferroelectric materials with high Curie temperature (TC) arises...

As important multifunctional oxides, BiFeO 3 -PbTiO 3 (BFPT) based perovskite materials have drawn increasing research interests due to their promising roomtemperature magnetoelectric (ME), [1] piezoelectric/ferroelectric, [2,3] photovoltaic, [4] and tuneable thermal expansion performances. [5] The two end members, BiFeO 3 (BFO) and PbTiO 3 (PT), can form complete solid solution in the (1−x)BFO-xPT system, where a morphotropic phase boundary (MPB) with coexisting rhombohedral and tetragonal phases was found in the composition range around x = 0.3. [6] Due to the high Curie temperatures (T C ) in both end members, the T C for the MPB compositions in BFPT is as high as ≈ 630 °C. [7] Moreover, the MPB is of metastable nature in a relatively wide composition range, [8] where a phase transition can be induced by an applied electric field. [1] Such an MPB-related phase transition could result in a giant and stable piezoelectric response. [9] Indeed, enhanced electromechanical effects were obtained recently in the vicinity of MPB of BFPT and Mn-doped BFPT, [1,10] making the BFPT system particularly interesting for high-temperature piezoelectric/ferroelectric applications.In prototype piezoelectrics such as Pb(Zr x Ti 1−x )O 3 (PZT) and Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMN-PT), [11,12] monoclinic phases are found to exist and act as a bridge connecting two phases on both sides of the MPB. In contrast, the MPB compositions of BFPT system show coexistence of two distinct ferroelectric phases inherited from the two respective end members. In addition, it is found in modified BFPT materials [13] that the two different ferroelectric phases also reveal diverse magnetic orderings. In our previous work, the substitution of rare earth Dy for Bi in the BFPT system is proved to induce ferromagnetism in the rhombohedral compositions with magnetic hysteresis loops, while the antiferromagnetic state is retained in the tetragonal compositions [14] at room temperature. These characteristics also make the BFPT system promising materials for the so-called phase-change large ME effects, where large ME Perovskite materials based on BiFeO 3 -PbTiO 3 (BFPT) solid solutions are promising for various applications thanks to the extremely large spontaneous polarization (P s ) and existence of multiferroic morphotropic phase boundary. For applications in piezoelectric and memory devices, complete switching of P s is needed, which is hard to achieve practically. In this work, a simple modified mixed-oxide reaction method is developed allowing to prepare Dy-and Smmodified BFPT ceramics with significantly improved properties. The MPB compositions demonstrate well-saturated ferroelectric hysteresis loops with large switchable remanent polarization of 60 µC cm −2 and enhanced piezoelectric properties with a large-signal piezoelectric coefficient d 33 * = 214 pm V −1 and a direct piezoelectric coefficient d 33 = 128 pC N −1 , which is one and a half times larger than the best d 33 value reported for the BFPT ceramics so far. The Curie temperature reaches...

Morphotropic phase boundary (MPB) in ferroelectric solid solutions can significantly enhance the dielectric and piezoelectric performances of the materials. Similarly, magnetic MPB has been found to exist in a few ferromagnets and is proved to be greatly beneficial to the magnetostrictive response. In this letter, we report the finding of a magnetic MPB in the multiferroic 0.66Bi1−xDyxFeO3-0.34PbTiO3 system, which overlaps the structural MPB of ferroelectric phases. A (weak) ferromagnetic state is induced at room temperature by Dy for the rhombohedral compositions with x ≥ 0.10. Upon cooling down from room temperature, the tetragonal compositions (x ≤ 0.05) show only one magnetic phase transition from the paramagnetic to antiferromagnetic phase, while the rhombohedral compositions exhibit a series of magnetic phase transitions from the (weak) ferromagnetic state to an antiferromagnetic order then to another weak ferromagnetic phase. A magneto-structural phase diagram has been established that reveals the presence of a magnetic MPB and a ferroelectric MPB. More significantly, the ferroelectric and magnetic MPB regions are found to overlap with each other, pointing to interesting kinds of multiferroic couplings.

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