Coexistence of localized and collective magnetism in the coupled-spin-tetrahedra system Cu4 Te5 O12 Cl4

We report high-field magnetization, electron spin resonance (ESR), and Raman scattering measurements of the coupled spin-tetrahedra system Cu4Te5O12Cl4 with magnetic ordering at TN=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 μ0HSF=16 T only for H¥c as well as periodic magnetization steps at μ0H=16.5,24.8,33.8,42.3,and49.7 T, which are independent of the crystallographic orientations. For T>TN, the temperature dependence of ESR linewidth is described by a critical power law, ΔBpp(T) (T-TN)-0.56±0.02. For T<TN, an antiferromagnetic resonance mode is observed for H¥c, and its linewidth is given by ΔBpp(T) T3.13±0.04, being close to T4 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 Cu4Te5O12Cl4 is a unique material which shows a dual character of zero-dimensional, localized and three-dimensional, collective magnetic behaviors.