Praphase concept for the phenomenological description of magnetoelectrics
- PDF / 323,600 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 30 Downloads / 128 Views
ICE DYNAMICS AND PHASE TRANSITIONS
Praphase Concept for the Phenomenological Description of Magnetoelectrics V. P. Sakhnenko and N. V. TerOganessian Institute of Physics, Southern Federal University, pr. Stachki 194, RostovonDon, 344090 Russia email: [email protected] Received June 29, 2010
Abstract—Complex cascades of phase transitions with the alternation of antiferromagnetic, incommensu rate, and magnetoelectric phases are observed in most crystals having magnetoelectric properties. The entire variety of observed phase transitions in numerous recent works is described by introducing several order parameters which are sequentially “condensed” with a decrease in temperature. The possibility of introduc ing the praphase concept into consideration to reduce the number of order parameters and interpret rich phase diagrams due to the smaller number of instabilities is demonstrated by the examples of MnWO4 and CuO magnetoelectrics. This approach is also used in the phenomenological description of the phase transi tions in the BiFeO3 ferroelectromagnetic with the cubic phase of the perovskite structure as a starting point. DOI: 10.1134/S1063774511060241
INTRODUCTION In the last two decades, classes of compounds have been found in which antiferromagnetic ordering is accompanied by the occurrence of electric polariza tion. In most cases, several phase transitions with sim ilar temperatures occur with the alternation of modu lated magnetic structures and homogeneous antiferro magnetic phases with a multiply increased unit cell. Manganites RMnO3 (R = Eu, Gd, Tb, Dy) [1] with 16 the sp. gr. D2h (Pbnm ) of the paramagnetic phase are the most representative and wellstudied in this respect. The class of RMn2O5 crystals (R is a rare earth element, Y or Bi) [2], which also belongs to the 9 orthorhombic system (sp. gr. D2h ( Pbam )), is another class of manganites showing magnetoelectric proper ties during antiferromagnetic transitions. Among the manganesefree compounds, the most pronounced magnetoelectric properties were found in R3V2O8 crys tals (R = Ni, Co, Cu) [3], which are described by the 18 sp. gr. D2h (Cm ca ) in the paramagnetic state. A number of crystals have a lower symmetry in the initial phase. 4 Among these crystals, wolframite MnWO4 (sp. gr. C2h 6 (P 2/ c )) [4] and copper oxide CuO (sp. gr. C2h (C 2/ c )), in which a transition to a modulated mag netic structure (accompanied by the occurrence of spatially homogeneous electric polarization) was found at 230 K [5], are being actively studied. One of the specific features of crystals revealing magnetoelectric properties is the frequent occurrence of incommensurate phases. The symmetry correlation between the occurrence of a modulated magnetic order and ferroelectric states at antiferromagnetic
phase transitions was shown in [6, 7]. It was men tioned, in particular, that the transformation proper ties of magnetic order parameters in some magneto electrics are determined by irreducible representations that do not satisfy the Lifshitz criterion. This
Data Loading...
