Magnetic Ordering in a Partially Frustrated Triangular Antiferromagnet RbFeBr3

Specific heat of quasi-one-dimensional RbFeBr 3 has been measured in the temperature range 1.1∼10 K in applied magnetic fields. Two anomalies with slight field dependences have been found at T N =5.61±0.02 K and T t =2.00±0.04 K in zero field. Below 120 K, the spin frustration is partially released by a lattice distorsion. Analysis by molecular field approximation for magnetic ordering process in the triangular spin ( S =1) system with frustration partially relased by lattice modulation is presented. The partial release of the frustration enables us to predict from the analysis that the high temperature anomaly corresponds to the transition from paramagnetic phase with mutually uncorrelated magnetic chains to a partially disordered phase in which one third of magnetic chains remains uncorrelated with the others and the low temperature one to the transition from the partially disorded phase to the triangular phase in the c -plane.