Magneto and ferroelectric phase transitions in HoMn2O5 monocrystals

From the physical point of view multiferroics present an extremely interesting class of systems and problems. These are essentially of two kinds. One is what are the microscopic conditions, and sometimes constrains, which determine the possibility to combine in one system both magnetic and ferroelectric properties. This turned out to be a quite nontrivial question, and usually, in conventional systems, these two phenomena tend to exclude one another. Why it is the case is an important and still not completely resolved issue. In the present article we report our results from magnetic properties measurements on HoMn2O5 with short discussion about it possible origin

Frustrated minority spins in GeNi2O4

Recently, two consecutive phase transitions were observed, upon cooling, in an antiferromagnetic spinel GeNi$_2$O$_4$ at $T_{N1}=12.1$ K and $T_{N2}=11.4$ K, respectively \cite{matsuno, crawford}. Using unpolarized and polarized elastic neutron scattering we show that the two transitions are due to the existence of frustrated minority spins in this compound. Upon cooling, at $T_{N1}$ the spins on the \kagome planes order ferromagnetically in the plane and antiferromagnetically between the planes (phase I), leaving the spins on the triangular planes that separate the \kagome planes frustrated and disordered. At the lower $T_{N2}$, the triangular spins also order in the $$ plane (phase II). We also present a scenario involving exchange interactions that qualitatively explains the origin of the two purely magnetic phase transitions

Order Parameters and Phase Diagrams of Multiferroics

The symmetry properties, order parameters, and magnetoelectric phase diagrams of multiferroics are discussed. After brief reviews of Ni$_3$V$_2$O$_8$, TbMnO$_3$, and RbFe(MoO$_4$)$_2$, we present a detailed analysis of RMn$_2$O$_5$ (with R=Y, Ho, Dy, Er, Tb, Tm).Comment: 23 pages, 7 figures, 3 tables. For FOCUS ON MULTIFERROICS (IOP