Inter- and intratrimer excitations in the multiferroic Ba₃ NbFe₃ Si₂ O₁₄

Elastic and inelastic neutron-scattering experiments have been performed on single crystals of the multiferroic Ba₃ NbFe₃ Si ₂ O₁₄. It is found that the organization of the spin clusters into true long-range ordered states occurs by the following process: (i) by 60 K, there is evidence for short-range ordered triangular Fe clusters of spins, (ii) below 40 K, the clusters begin to form long-range ordered structures as a spin-wave spectrum develops, (iii) at TN =26 K, the sample exhibits coexisting long-range and short-range ordered structures, and (iv) by T=1.5 K, a long-range ordered structure with a well-defined spin-wave spectrum develops. The spin-wave excitation can be described by a Heisenberg Hamiltonian within a distorted triangular lattice including both the intratrimer and intertrimer interactions. Physical property measurements showing anomalies near T=40 K can now be adequately explained as being due to scattering from low-energy spin fluctuations.