LAUSR

We carried out high magnetic field magnetization and electron spin resonance (ESR) measurements for the alternating dimer-trimer chain compound ${\mathrm{Li}}_{2}{\mathrm{Cu}}_{5}{\mathrm{Si}}_{4}{\mathrm{O}}_{14}$. The magnetization curve measured at 2 K exhibits a spin-flop transition at 2.6 T followed by a nonlinear increase in magnetization at high field up to 55 T. The antiferromagnetic ordering at ${T}_{\mathrm{N}}=22\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is well characterized by the temperature-dependent ESR spectra, and the anisotropic $g$ factors at the paramagnetic phase are determined. Intriguingly, our frequency-dependent ESR spectra at 2 K reveal the development of two collinear antiferromagnetic sublattices with biaxial anisotropy, which is quite unusual because ${\mathrm{Li}}_{2}{\mathrm{Cu}}_{5}{\mathrm{Si}}_{4}{\mathrm{O}}_{14}$ contains five sublattices. Thus, ${\mathrm{Li}}_{2}{\mathrm{Cu}}_{5}{\mathrm{Si}}_{4}{\mathrm{O}}_{14}$ is three-dimensionally ordered and its magnetism might be described by a pseudo-two-sublattice model composed of antiferromagnetically coupled dimer-trimer groups (\ensuremath{\uparrow}\ensuremath{\uparrow}, \ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\uparrow}) and (\ensuremath{\downarrow}\ensuremath{\downarrow}, \ensuremath{\downarrow}\ensuremath{\uparrow}\ensuremath{\downarrow}).