Magnetic ground state and multiferroicity in BiMnO3_3

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 state, where the ferromagnetic order along one crystallographic axis coexists with the the hidden AFM order and related to it ferroelectric polarization along two other axes. The $C2/c$ symmetry can be restored by the magnetic field $B \sim 35$ Tesla, which switches off the ferroelectric polarization. Our analysis is based on the solution of the low-energy model constructed for the 3d-bands of BiMnO$_3$, where all the parameters have been derived from the first-principles calculations. Test calculations for isostructural BiCrO$_3$ reveal an excellent agreement with experimental data.Comment: 5 pages, 5 figure

Синтез и свойства силикатов висмута, приготовленных методом механической активации

В настоящей работе были синтезированы серии силикатов висмута методоммеханической активации, а также исследованы их свойства такими методами, как ТГ-ДСК анализ и РФА-спектроскопия. Установлено, что для серии, приготовленной с соотношением реагентов Bi:Si = 2:1 после 800 °С наступает плавление, тогда как для серии с эквиатомным соотношением Bi и Si в исходных реагентах плавления в этом температурном интервале не наблюдается. Показано, что при увеличении температуры прокаливания с 500˚С до 700 °С фаза метасиликата висмута (Bi2SiO5) полностью переходит в ортосиликат висмута (Bi4Si3O12), а доля фазы силленита (Bi12SiO20) увеличивается при увеличении соотношения Bi:Si

Universal magneto-orbital ordering in the divalent AA-site quadruple perovskite manganites AAMn7_7O12_{12} (AA = Ca, Sr, Cd, and Pb)

Through analysis of variable temperature neutron powder diffraction data, we present solutions for the magnetic structures of SrMn$_7$O$_{12}$, CdMn$_7$O$_{12}$, and PbMn$_7$O$_{12}$ in all long-range ordered phases. The three compounds were found to have magnetic structures analogous to that reported for CaMn$_7$O$_{12}$. They all feature a higher temperature lock-in phase with \emph{commensurate} magneto-orbital coupling, and a delocked, multi-\textbf{k} magnetic ground state where \emph{incommensurate} magneto-orbital coupling gives rise to a constant-moment magnetic helix with modulated spin helicity. CdMn$_7$O$_{12}$ represents a special case in which the orbital modulation is commensurate with the crystal lattice and involves stacking of fully and partially polarized orbital states. Our results provide a robust confirmation of the phenomenological model for magneto-orbital coupling previously presented for CaMn$_7$O$_{12}$. Furthermore, we show that the model is universal to the $A^{2+}$ quadruple perovskite manganites synthesised to date, and that it is tunable by selection of the $A$-site ionic radius