"Magnetic ferroelectric" has been found in a wide range of spiral magnets. However, these materials all suffer from low critical temperatures, which are usually below 40 K, due to strong spin frustration. Recently, CuO has been found to be multiferroic at much higher ordering temperature (∼ 230K). To clarify the origin of the high ordering temperature in CuO, we investigate the structural, electronic and magnetic properties of CuO via first-principles methods. We find that CuO has very special nearly commensurate spiral magnetic structure, which is stabilized via the Dzyaloshinskii-Moriya interaction. The spin frustration in CuO is relatively weak, which is one of the main reasons that the compound have high ordering temperature. We propose that high Tc magnetic ferroelectric materials can be found in double sublattices of magnetic structures similar to that of CuO. PACS numbers: 75.85.+t, Magnetic ferroelectric materials in which ferroelectricity is induced by magnetic ordering, have attracted intensive interests [1,2]. The strong magnetoelectric (ME) coupling in these materials opens up a new path to the design of multifunctional devices that allow the control of charges by the application of magnetic fields or spins by applying voltages. So far, almost all magnetic ferroelectric materials are strongly frustrated magnets [2]. Frustrated magnets have very low ordering temperatures (∼ 30 -40 K), several times smaller than the temperatures expected from their spin interaction strengths. Low critical temperature is one of the major factors that limit the applications of these important materials. Therefore a new mechanism that allows high temperature magnetic ferroelectric materials is critical.Recently, CuO was found to be multiferroic at T c =230 K, which is much higher than the critical temperatures of all other magnetic ferroelectric materials [3]. However, the mechanism for the high ordering temperature were not clear. So far, CuO is the only binary compound that has been found to be multiferroic [3]. CuO undergoes two successsive magnetic phase transitions upon cooling from room temperature to near zero temperature. Neutron scattering experiments [4] show that below T N 1 =213 K, the spin structure is collinear antiferromagnetic (AFM1) [see Fig. 1(a)]. Between T N 1 and T N 2 =230 K, the spin structure becomes non-collinear and slightly incommensurate (AFM2) [see Fig. 1(b)], with a modulation vector of Q= (0.006, 0, 0.017). Remarkably, an electric polarization of 160 µC·m −2 , which can be reversed by applying an electric field of about 55 kV/m, develops in the AFM2 phase. The electric polarization was attributed to the spiral spin structure [3,5] which was assumed to result from spin frustration and whereas the high ordering * Email address: helx@ustc.edu.cn temperature is believed to come from the strong exchange interactions [3].To clarify the mechanism behind its high ordering temperature and the origin of its ferroelectricity, we carry out first-principles studies of the multiferroism of CuO. We find ...

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The electronic structure and magnetic properties of the strongly correlated material La2O3Fe2Se2 are studied by using both the density function theory plus U (DFT+U ) method and the DFT plus Gutzwiller (DFT+G) variational method. The ground-state magnetic structure of this material obtained with DFT+U is consistent with recent experiments, but its band gap is significantly overestimated by DFT+U , even with a small Hubbard U value. In contrast, the DFT+G method yields a band gap of 0.1 -0.2 eV, in excellent agreement with experiment. Detailed analysis shows that the electronic and magnetic properties of of La2O3Fe2Se2 are strongly affected by charge and spin fluctuations which are missing in the DFT+U method. 71.27.+a,