Field-induced disorder point in noncollinear Ising spin chains

We perform a theoretical study of a noncollinear Ising ferrimagnetic spin chain inspired by the compound $\mathrm{Co}{\left(\mathrm{hfac}\right)}_2\mathrm{NITPhOMe}$. The basic building block of its structure contains one cobalt ion and one organic radical each with a spin $1∕2$. The exchange interaction is strongly anisotropic and the corresponding axes of anisotropy have a period three helical structure. We introduce and solve a model Hamiltonian for this spin chain. We show that the present compound is very close to a so-called disorder point at which there is a massive ground state degeneracy. We predict the equilibrium magnetization process and discuss the impact of the degeneracy on the dynamical properties by using arguments based on the Glauber dynamics.

Figure 1

The helical structure of the 1D magnet CoPhOMe. Each anisotropic center has a local axis of anisotropy ${\widehat{\mathbf{z}}}_i$. These axes are all tilted by a common angle $\theta$ with respect to the chain axis $\underset{̱}{c}$. The angle between the projections of ${\widehat{\mathbf{z}}}_i$ and ${\widehat{\mathbf{z}}}_{i+1}$ on the plane perpendicular to $\underset{̱}{c}$ is equal to $2\pi ∕3$.

Figure 2

The helical structure of CoPhOMe as seen from the chain axis of symmetry. The anisotropic cobalt ions are depicted as ellipsoids and the radical as spheres.

Figure 3

The critical field between F and AF ground states as a function of the angle $\theta$ for $g_{\mathrm{iso}}=2$ and $g_{\mathrm{ani}}=9.5$, 9, 8.5, 8, 7.5, 7 from bottom to top.

Figure 4

Ground state magnetization curve for $\theta =50°$, 55°, 60°, 65°. We use $g_{\mathrm{ani}}=9$ and $g_{\mathrm{iso}}=2$ as appropriate for CoPhOMe.

Figure 5

Dependence upon the angle $\theta$ of the cobalt correlation length $\xi$ in zero field for temperatures given by $J∕T=2$, 1.5, 1, 0.5 from top to bottom.

Figure 6

Disorder field dependence on $T$ for $\theta =44°$, 46°, 48°, 50°, 52°, 54° from top to bottom.

Figure 7

Correlation length as a function of $h∕J=\mid {\mu }_B\mid B∕J$ for $J∕T=0.5$ and $\theta =52°$, 53°, 54°, ${\theta }_c$.