Nucleation, instability and discontinuos phase transitions in the monoaxial helimagnet with oblique fields

Abstract

The phase diagram of the monoaxial chiral helimagnet as a function of temperature T and magnetic field with components perpendicular (Hx) and parallel (Hz) to the chiral axis has been theoretically studied. The starting point is a general Hamiltonian with long-range ferromagnetic Heisenberg and Dzyaloshinskii-Moriya interactions. The equilibriurn properties are studied via the variationa/ mean field approach. In the continuum limit the mean field free energy contains only a few parameters which are deterrnined by the many parameters of the Hamiltonian. A phase transition surface in the three dimensional therrnodynamic space separates a chiral spatially modulated phase from a homogeneous forced ferromagnetic phase. The phase boundary is divided into three parts: two surfaces of second order transitions of instability and nucleation type, in De Gennes terrninology, are separated by a surface of first order transitions. Two lines of tricritical points separate tbe first order surface from the second order surfaces. One of them, the instability tricritical line, ends at the zero field ordering transition point, which, therefore, presents tricritical behavior. In figure 1 the calculated 3D phase diagram is depicted. The structure of the spin texture in the chiral phase and the singularities of several quantities on the transition surface are analyzed. The divergence of the period of the modulated state on the nucleation transition surface has the logarithmic bebavior typical of a chiral soliton lattice. The specific heat diverges on the nucleation surface as a power law with logarithmic corrections, while it shows a finite discontinuity on the other two surfaces. The soliton density curves are described by a universal function of Hx if the values of T and Hz determine a transition point lying on the nucleation surface; otherwise, they are not universal.