Weak ferromagnetism and magnetoelectric coupling through the spin-lattice coupling in (1-x)Pb(Fe2/3W1/3)O3-(x)BiFeO3 (x = 0.1 and 0.4) solid solution.

We report on the structure, spin-lattice and magneto-electric coupling in (1-x)Pb(Fe2/3W1/3)O3-(x)BiFeO3(where x = 0.1 and 0.4) (PBFW) solid solution synthesized through two-step solid-state reaction method. The room temperature (RT) crystallographic studies were carried out using x-ray diffraction and neutron diffraction measurements which show a single-phase Pseudocubic crystal system with Pm-3m space group. Rietveld refinement was carried out to obtain the structural parameters using Fullprof software and the observed structural parameters are in good agreement with the previous reports. Temperature-dependent neutron diffraction measurements reveal the presence of commensurate G-type antiferromagnetic structure. The magnetic structure was analyzed using the propagation wave vector k ∼ (½ ½ ½) for both the solid solutions. The obtained lattice constants increase linearly and the magnetic moment decrease with temperature, which shows a remarkable anomaly around the magnetic (T N ∼ 405 K for x = 0.1 and 531 K for x = 0.4) transition temperatures. This anomaly clearly indicates the existence of spin-lattice and magnetoelectric coupling. The magnetic susceptibility (ZFC and FC at 500 Oe) and M-H hysteresis loop measurements show spontaneous magnetic moment due to the Fe3+-O2--Fe3+ superexchange interaction coexisting with the weak ferromagnetism. Bifurcation of ZFC and FC curve reveals the strong anisotropic nature. Astonishingly, magnetic measurements show the non-zero magnetic moment above T N and broadening of the magnetic transition indicates the presence of short-range uncompensated sublattice weak ferromagnetic clusters in the paramagnetic region. The Mossbauer spectroscopy and electron paramagnetic resonance studies exhibit the RT magnetically ordered system and confirm the +3 state of Fe along with the fraction of Fe2+ ions.

Quantum paraelectricity coexisting with a ferroelectric metastable state in single crystals of NaNbO(3): a new quantum effect.

We experimentally show that, in contrast to the data having been collected so far, some single crystals of NaNbO(3) exhibit a dielectric permittivity of several thousand, at low T, and this value is saturated when approaching 0 K on cooling. Other sodium niobate crystals (having larger dielectric losses) present a first-order phase transition to a ferroelectric phase on cooling (at 80-200 K). The width of the thermal hysteresis in these crystals increases when the temperature of the phase transition obtained on heating decreases. The dielectric permittivity at the phase transition obtained on cooling shows a tendency to increase and saturate, when the thermal hysteresis increases. We identify the ground state of the sodium niobate crystal exhibiting the smallest dielectric losses (in the studied set of crystals) as a novel quantum paraelectric state coexisting with a metastable ferroelectric state. In principle, the crystal presenting the state of quantum paraelectricity can be considered as having the largest (among the crystals studied) thermal hysteresis, for which the low boundary is below 0 K.