
Linear perturbation renormalization group method for Isinglike spin systemsKeywords: ANNNI model , renormalization group , isolated critical point Abstract: The linear perturbation group transformation (LPRG) is used to study the thermodynamics of the axial nextnearestneighbor Ising model with four spin interactions (extended ANNNI) in a field. The LPRG for weakly interacting Ising chains is presented. The method is used to study finite field paraferrimagnetic phase transitions observed in layered uranium compounds, UAs1xSex, UPd2Si2 or UNi2Si2. The abovementioned systems are made of ferromagnetic layers and the spins from the nearestneighbor and nextnearestneighbor layers are coupled by the antiferromagnetic interactions J1<0 and J2<0, respectively. Each of these systems exhibits a triple point in which two ordered phases (ferrimagnetic and incommensurate) meet the paramagnetic one, and all undergo the high field phase transition from para to ferrimagnetic (++) phase. However, if in UAs1xSex the paraferri phase transition is of the first order as expected from the symmetry reason, in UT2Si2 (T=Pd, Ni) this transition seems to be a continuous one, at least in the vicinity of the multicritical point. Within the MFA, the critical character of the finite field paraferrimagnetic transition at least at one isolated point can be described by the ANNNI model supplemented by an additional, e.g., fourspin interaction. However, in LPRG approximation for the ratio κ = J2/J1 around 0.5 there is a critical value of the field for which an isolated critical point also exists in the original ANNNI model. The positive fourspin interaction shifts the critical point towards higher fields and changes the shape of the specific heat curve. In the latter case for the fields small enough, the specific heat exhibits twopeak structure in the paramagnetic phase.
