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.

No abstract