Spin-cluster resonance in the pseudo-one-dimensional canted Ising antiferromagnet RbFeCl3·2H2O

Spin-cluster resonance (SCR) is observed in the pseudo-one-dimensional canted Ising antiferromagnet RbFe${\mathrm{Cl}}_3$·2${\mathrm{H}}_2$O. Transitions between clusters up to 9 spins ($|m>\mathrm{}\Rightarrow |m+9>$) could be detected. Since the moments in the chain are canted with respect to each other, SCR were observed which show characteristics of a ferromagnetic chain and an antiferromagnetic chain for the external magnetic field parallel to the $c$ and $a$ axis, respectively. The experimental data for $\overrightarrow{\mathrm{H}}\parallel \overrightarrow{\mathrm{c}}$ could excellently be described by a pure Ising model, with $\frac{J_b}{k}=0.76\mathrm{}$ K, $\frac{J_c}{k}=-0.21\mathrm{}$ K, $\frac{J_{\mathrm{bc}}}{k}=-0.13\mathrm{}$ K, $\overrightarrow{\mu }·\frac{\overrightarrow{\mathrm{H}}}{\left|\overrightarrow{\mathrm{H}}\right|}=1.50\mathrm{}$ ${\mathrm{\mu }}_{\mathrm{B}}$, and a correction for a demagnetizing field. In order to describe the SCR for $\overrightarrow{\mathrm{H}}\parallel \overrightarrow{\mathrm{a}}$ the influence of the crystal-field splitting of the pseudo-ground-doublet of the ${\mathrm{Fe}}^{2+}$ ion had to be taken into account.