Static magnetic properties of (CH3)4NMnxCu1−xCl3, a quantum ferromagnetic chain with classical impurities: Experiment and theory

Abstract

The magnetic susceptibility of the mixed quasi-one-dimensional system ${\left({\mathrm{CH}}_3\right)}_4\mathrm{N}{\mathrm{Mn}}_x{\mathrm{Cu}}_{1-x}{\mathrm{Cl}}_3$ has been measured down to 0.32 K for $x$ ranging between 0 and 1. At 4.2 K the susceptibility starts to increase with $x$ up to $x\simeq 0.1$ and then decreases rapidly. The Néel temperature decreases from 1.24 K for $x\simeq 0$ to 0.35 K for $x=0.22\mathrm{}$. We present theoretical calculations developed for isotropic exchange interactions and we take into account the quantum and classical natures of the ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Mn}}^{2+}$ spins, respectively. We show that to first order in $x$ the problem of the disorder inside the chains can be reduced to the calculation of a transfer matrix. This matrix is calculated at high temperature as a function of a parameter $v$ which can be determined by a more rigorous calculation made in the case of diamagnetic impurities and at lower temperatures. The experimental data at small values of $x$ are well explained by this model.