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We discuss a simple phenomenological Landau theory of phase transitions with two coupled single-component order parameters and compare the results with available experimental data. The model corresponds to the case of a ferroic system, in which ferromagnetic and ferroelectric transitions originally occur at temperatures $T_M$ and $T_f$, respectively. For $T_f>T_M$ the magnetoelectric coupling strongly renormalizes the magnetic transition temperature, $T_Mto T_{RM}$ (with $T_{RM}>>T_M$), as well as generates an additional anomaly in ferroelectric subsystem $T_{RM}$. Full susceptibility tensor has also been determined. The concept of textit{Arrot plot} is replaced by the textit{Arrot planes} which appear when both types of order coexist. The results are in good overall agreement with experimental data for the ferroelectromagnetic BiMnO$_3$. We also estimate the contribution of Gaussian fluctuations of both order parameters, that lead to corrections to the mean-field specific heat. Those corrections are still insufficient even though other quantities agree quite well with experiment. We calculate the temperature dependence of the coherence length for both types of order as well.
The structural properties of BiMn$O_3$ were determined from neutron powder diffraction data as a function of temperature and magnetic field. The structure at all temperatures was found to be centrosymmetric with space group $C2/c$, which is incompati
Magnetic and magnetoelectric excitations in the multiferroic TbMnO_3 have been investigated at terahertz frequencies. Using different experimental geometries we can clearly separate the electro-active excitations (electromagnons) from the magneto-act
We argue that the centrosymmetric $C2/c$ symmetry in BiMnO$_3$ is spontaneously broken by antiferromagnetic (AFM) interactions existing in the system. The true symmetry is expected to be $Cc$, which is compatible with the noncollinear magnetic ground
We report the results of direct measurement of remanent hysteresis loops on nanochains of BiFeO$_3$ at room temperature under zero and $sim$20 kOe magnetic field. We noticed a suppression of remanent polarization by nearly $sim$40% under the magnetic
For densities above $n=1.6 times 10^{11}$ cm$^{-2}$ in the strongly interacting system of electrons in two-dimensional silicon inversion layers, excellent agreement between experiment and the theory of Zala, Narozhny and Aleiner is obtained for the r