Coexistence of localized and collective magnetism in the coupled-spin-tetrahedra systemCu4Te5O12Cl4

Abstract

We report high-field magnetization, electron spin resonance (ESR), and Raman scattering measurements of the coupled spin-tetrahedra system ${\mathrm{Cu}}_{4}{\mathrm{Te}}_{5}{\mathrm{O}}_{12}{\mathrm{Cl}}_{4}$ with magnetic ordering at ${T}_{N}=13.6$ K. We find thermodynamic and spectroscopic signatures for the concomitant occurrence of localized and collective magnetism. Magnetization measurements up to 60 T exhibit a spin-flop transition at ${\ensuremath{\mu}}_{0}{H}_{\mathrm{SF}}=16$ T only for $H\ensuremath{\parallel}c$ as well as periodic magnetization steps at ${\ensuremath{\mu}}_{0}H=16.5,\phantom{\rule{0.28em}{0ex}}24.8,\phantom{\rule{0.28em}{0ex}}33.8,\phantom{\rule{0.28em}{0ex}}42.3,\phantom{\rule{0.16em}{0ex}}\text{and}\phantom{\rule{0.16em}{0ex}}49.7$ T, which are independent of the crystallographic orientations. For $T>{T}_{N}$, the temperature dependence of ESR linewidth is described by a critical power law, $\ensuremath{\Delta}{B}_{pp}(T)\ensuremath{\propto}{(T\ensuremath{-}{T}_{N})}^{\ensuremath{-}0.56\ifmmode\pm\else\textpm\fi{}0.02}$. For $T<{T}_{N}$, an antiferromagnetic resonance mode is observed for $H\ensuremath{\parallel}c$, and its linewidth is given by $\ensuremath{\Delta}{B}_{pp}(T)\ensuremath{\propto}{T}^{3.13\ifmmode\pm\else\textpm\fi{}0.04}$, being close to ${T}^{4}$ expected for a classical magnet. Raman spectra show three one-magnon-like excitations superimposed on a broad two-magnon continuum. While the two higher frequency modes show an intensity variation in accordance to a three-dimensional Heisenberg antiferromagnet, the lower frequency mode clearly deviates. These results suggest that ${\mathrm{Cu}}_{4}{\mathrm{Te}}_{5}{\mathrm{O}}_{12}{\mathrm{Cl}}_{4}$ is a unique material which shows a dual character of zero-dimensional, localized and three-dimensional, collective magnetic behaviors.