An elastic Ising model for a one-dimensional diatomic spin chain is proposed to explain the ferroelectricity induced by the collinear magnetic order with a low-excited energy state. A statistical theory based on this model is developed to calculate the electrical and magnetic properties of Ca3CoMnO6, a typical quasi-one-dimensional diatomic spin chain system. The calculated ferroelectric polarization and dielectric susceptibility show a good agreement with recently reported data on Ca3Co2-xMnxO6 （x ≈ 0.96） （Phys. Rev. Lett. 100 047601 （2008））, although the predicted magnetic susceptibility does not coincide well with experiment. We also address the rationality and deficiency of this model by including a first-order correction which improves the consistency between the model and experiment.
We have investigated the low-temperature magnetism and spin-lattice coupling in （Bio.46Nao.46Bao.os）TiO3 ：Co in order to understand the magnetoelectric effect in such artificially synthesized dilute magnetic ferroelectrics. It is revealed that the as-prepared （Bio.46Nao.46Bao.os）TiO3：Co at Co content of 20%~30% exhibits fascinating ferrimagnetism which is robust against magnetic field, the abnormal spin lattice coupling characterized by a negative magnetostriction effect; and the suppressed magnetic moment within the temperature range of 30 K-50 K is identified. These magnetic behaviours at low temperatures can be explained by the competition between the ferrimagnetic response and the magnetic moment suppression induced by the abnormal spin lattice coupling effect. Finally, the ferroelectric and magnetodielectric properties are also discussed.
The Ginzburg-Landau theory on ferroelectrics with random field induced by dipole defects is studied by using Monte Carlo simulation, in order to investigate the dipole configuration and the dielectric relaxation of relaxor ferro-electrics. With the increase of random field, the dipole configuration evolves from the long-range ferroelectric order into the coexistence of short-range dipole-clusters and less polarized matrix. The dipole-cluster phase above the transition temperature and superparaelectric fluctuations far below this temperature are identified for the relaxor ferroelectrics. We investigate the frequency dispersion and the time-domain spectrum of the dielectric relaxation, demonstrating the Vogel-Fulcher relationship and the multi-peaked time-domain distribution of the dielectric relaxation.
