Magnetic properties of a helical spin chain with alternating isotropic and anisotropic spins: Magnetization plateaus and finite entropy

摘要

We propose a model which could explain some of the unusual magnetic properties observed for the one-dimensional helical spin system Co(hfac)_2NITPhOMe. One of these properties is that the magnetization shows some plateaus if a magnetic field is applied along the helical axis. The system consists of cobalt ions (which have easy axes which are tilted at an angle θ_i with respect to the helical axis) and organic radicals alternating with each other. We consider a model in which the tilt angles θ_i are allowed to vary with i with period three. Using the, transfer matrix approach, we show that for certain patterns of θ_i, the model exhibits the magnetization plateaus mentioned above. At the ends of the plateaus, we find that the entropy is finite even at very low temperatures, while the magnetic susceptibility and specific heat also show some interesting features.