Influence of the magnetic field and impurity on the commensurate–incommensurate phase transition in a quasi-two-dimensional magnetic system

Abstract

A theory of thermodynamic properties of a spin density wave (SDW) in a quasi-two-dimensional system (with a preset impurity concentration x) is constructed. We choose an anisotropic dispersion relation for the electron energy and assume that external magnetic field H has an arbitrary direction relative to magnetic moment MQ. The system of equations defining order parameters , , Mz, and Mσ is constructed and transformed with allowance for umklapp scattering (U-processes). Special cases when H || MQ and H ⊥ MQ (HzHσ = 0) are considered in detail as well as cases of weak fields H of arbitrary direction. The condition for the transition of the system to the commensurate and incommensurate states of the SDW is analyzed. The concentration dependence of magnetic transition temperature TM is calculated, and the components of the order parameter for the incommensurate phase are determined. The phase diagram (T, ) is constructed