THERMAL-ANALYSIS OF FREEDOM OF SPIN IN PARTIALLY DISORDERED STATE OF THE ANTIFERROMAGNETIC TRIANGULAR LATTICE IN CUFEO2

The partially disordered spins in the triangular antiferromagnetic lattice in CuFeO2 have been studied by heat capacity measurements. A cusp and a sharp peak in the heat capacity curve were observed at T(N1) = 14.0 +/- 0.5 K and T(N2) = 11.0 +/- 0.5 K, respectively. The analysis of heat capacity revealed that the magnetic entropy associated with the anomaly in the heat capacity around T(N2) is attributable to the disordering of one-fifth of Fe3+ spins. The present result is discussed in comparison with partially disordered states of CeSb and PrCo2Si2, anisotropic spin systems with frustration. The heat capacity cusp around T(N1) is compared with the characteristic thermal behavior for frustrated spin systems.