Field-induced tricriticality in antiferromagnetic J1−J2 spin systems on body-centered cubic lattice

The influence of the external magnetic field on the phase diagram of the classical antiferromagnetic spin-1/2 system with the nearest-neighbor and the next-nearest-neighbor antiferromagnetic interactions on the body-centered cubic lattice is investigated in detail using the recursive lattice approximation. In the framework of this exactly solvable model the explicit expression for the free energy per site is found and the nature of all phase transitions is determined. It is shown that applied magnetic field significantly changes the phase diagram of the model. First of all, the presence of arbitrarily weak external magnetic field immediately reduces the number of possible different phases of the model from four to two. Moreover, unlike the case with zero magnetic field, where all phase transitions are of the second-order type, the nature of the phase transitions between these two remaining model phases depends on the values of the parameters of the model. More precisely, the region of the first order phase transitions appears when the external magnetic field is switched on, which is separated from the region of the second order phase transitions by the curve of the tricritical points. It is also shown that such magnetic systems can exhibit series of even three successive phase transitions in the temperature dependence. On the other hand, at most one field-induced phase transition of the first-order or second-order type exists in the field dependence. The sublattice magnetization properties of the model are studied and the magnetization properties of the various types of series of successive phase transitions are demonstrated.