Ladder-based two-dimensional spin model in a radical-Co complex

Abstract

We successfully synthesize a verdazyl-based complex, ${(p\text{\ensuremath{-}}\mathrm{Py}\text{\ensuremath{-}}\mathrm{V}\text{\ensuremath{-}}p\text{\ensuremath{-}}\mathrm{F})}_{2}[\mathrm{Co}{(\mathrm{hfac})}_{2}]$. Molecular-orbital calculations of this complex indicate that spin ladders composed of radical spins are coupled to Co spins via intramolecular interactions, forming a ladder-based two-dimensional (2D) spin model. The complex exhibits paramagnetic behavior associated with Co spins and the 3/5 magnetization plateau; numerical analysis demonstrates that this originates from a rung-singlet-like ground state with fully polarized Co spins. The peculiar behavior of the linear magnetization curve after the magnetization plateau is attributed to the nature of the 2D quantum spin system, which demonstrates a quantum phase transition from the gapped quantum state to the 2D magnetic state. Furthermore, we examine the magnetic anisotropy originating from ${\mathrm{Co}}^{2+}$ in the distorted octahedral environment of the complex. The anisotropic $g$ values are determined by electron-spin resonance (ESR) powder pattern analysis. Considering the spin-orbit coupling and crystal-field distortion of the ${\mathrm{Co}}^{2+}$ ion, we explain the anisotropic $g$ values of the fictitious spin-1/2 on the ${\mathrm{Co}}^{2+}$ ion. We also evaluate the anisotropy of the effective exchange interaction between the radical spin and the fictitious spin on the ${\mathrm{Co}}^{2+}$ ion, thus revealing the existence of an Ising-like exchange interaction.