An Effective-field Theoretical Study On Ising Metamagnet

Recently, more attention has been paid on the study of magnetic material, which can be theoretically well described by a two-sublattice Ising model. So far as we know, the magnetic property of metamagnet material in transverse fields within the effective-field theory has yet not been theoretically studied.In this paper, the phase diagrams of two-sublattice Ising metamagnet in external field are studied within the effective-field theory approach. The phase ground state diagrams with a mixed longitudinal and transverse magnetic field, the phase diagrams at finite temperature with longitudinal crystal field and the phase diagrams at finite temperature with a mixed longitudinal and transverse magnetic field are discussed, respectively. The main results are as follows.(1) The ground state magnetic properties of a two-sublattice Ising metamegnet in a mixed longitudinal hz and transverse magnetic field hx are studied within the effective-field theory. A parameter v=J2/J1 is introduced, which reflects the strength ratio of spin coupling between adjacent planes and in each plane. The ground state phase diagrams in hx-hz and magnetization curves are presented. The results show that when -1000≤v<0 the phase transition of the system is always first-order forO4.36. The reentrant phenomenon occurs in the range ofv<-11.89,hx>14.71. There is no fourth-order critical point in the phase diagrams given by using effective-field theory as found by using mean-field theory.(2) The phase diagrams and magnetization curves of a two-sublattice Ising metamagnet at finite temperature with longitudinal crystal field hz are investigated by the use of an effective-field theory with correlations. The results show that the phase transitions between these phases are the first-order for low temperatures and high longitudinal fields, and the phase transitions are the second-order for high temperatures and low longitudinal fields, being separated by the tricritical point. In region (0