The Splitting of SDW State into Commensurable and Incommensurables Ones and the Peculiarities of the Behavior of Thermodynamic Quantities in a Magnetic Field Arbitrarily Oriented to Magnetization in Quasi Two-Dimensional Systems

Abstract

The quasi-two-dimensional system in which magnetism is caused by spin density wave (SDW) with an anisotropic energy spectrum (with defined impurity concentration x) is examined. The wave vector \(\vec{Q}\) is supposed to be different from 2kF and the umklapp scattering (U-processes) is taken into account. The system is placed in a magnetic field arbitrarily oriented with respect to the vector \(\vec{M}_{Q}\). The basic equations for order parameters \(M_{Q}^{z}, M_{Q}', M_{z}, M^{\sigma}\) are obtained and the system of these equations is transformed taking into account the U-processes. The particular cases \(( \tilde{H} \Vert \vec{M}_{Q} )\) and \(( \tilde{H} \bot \vec{M}_{Q} )\) and the case of small arbitrarily oriented magnetic fields \(\vec{\tilde{H}}\) are examined in detail. The conditions of the system transition to commensurable and incommensurable SDW state are analyzed. The phase diagram (T,x) at H=0 is traced. The influence of the magnetic field \(\vec{\tilde{H}}\) on the temperature of magnetic transition is researched and the aspect of the phase diagram in magnetic field in the cases HzHσ=0 is presented. The longitudinal magnetic susceptibility χ∥ which demonstrates that at x xc to a gapless case. At x∼xc in the dependence X∥(T) a local maximum appears. The influence of the energy spectrum anisotropy on the system’s properties is researched. Also the angular anisotropy of the quantity χ∥ at different values of T and x is determined.