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

TL;DR

This paper models a helical spin chain with alternating isotropic and anisotropic spins to explain observed magnetization plateaus and finite entropy, using an effective Ising model and transfer matrix approach.

Contribution

It introduces a variable tilt angle pattern in a helical spin chain model to account for specific magnetization plateaus and thermodynamic features observed experimentally.

Findings

Magnetization plateaus occur for certain tilt angle patterns.

Finite entropy persists at low temperatures at plateau edges.

Distinct features in susceptibility and specific heat are identified.

Abstract

We study a model which could explain some of the unusual magnetic properties observed for the one-dimensional helical spin system Co(hfac)_2 NITPhOMe. One of the properties observed is that the magnetization shows plateaus near zero and near one-third of the saturation value if a magnetic field is applied along the helical axis, but not if the field is applied in the plane perpendicular to that axis. The system consists of a spin-1/2 chain in which cobalt ions (which are highly anisotropic with an easy axis e_i) and organic radicals (which are isotropic) alternate with each other. The easy axis of the cobalts e_i lie at an angle theta_i with respect to the helical axis, while the projection of e_{i+1} - e_i on the plane perpendicular to the helical axis is given by 2 pi /3. For temperatures and magnetic fields which are much smaller than the coupling between the nearest-neighbor cobalts and radicals, one can integrate out the radicals to obtain an Ising model for the cobalts; this enables one to compute the thermodynamic properties of the system using the transfer matrix approach. We consider a model in which the tilt angles theta_i are allowed to vary with i with period three; we find that for certain patterns of theta_i, the system shows the magnetization plateaus mentioned above. At the ends of the plateaus, the entropy is finite even at very low temperatures, while the magnetic susceptibility and specific heat also show some interesting features.