A sequence of two ferrotoroidal phase transitions in nickel-bromine boracite Ni3B7O13Br

The phenomenological theory of a sequence of two second-order phase transit ions in Ni-Br boracite is presented. Two different components of the toroid al moment vector T-i are the order parameters of these transitions. Express ions are derived for the temperature dependences of the spontaneous values of T-i, polarization P-i, and magnetization M-i and the dielectric chi (ij) = dP(i) /dE(j), magnetic k(ij) = dM(i) /H-j, and magnetoelectric alpha (ij ) = dP(i) /dH(j) = dM(j) /dE(i) susceptibilities. Some of these susceptibil ities display sharp temperature peaks in the vicinity of phase transitions. (C) 2001 MAIK "Nauka/Interperiodica".