SUCCESSIVE MAGNETIC-ORDERING IN CUFEO2 - A NEW-TYPE OF PARTIALLY DISORDERED PHASE IN A TRIANGULAR LATTICE ANTIFERROMAGNET

Magnetic ordering in a rhombohedrally stacked triangular lattice antiferromagnet, CuFeO2 was investigated on powder and single crystals by measurements of neutron diffraction, magnetic susceptibility, Mossbauer effect together with Monte Carlo simulations. CuFeO2 was found to exhibit transitions at T(N1)=16+/-0.5K and T(N2)=11+/-0.5 K. The low temperature phase has an orthorhombic magnetic unit cell with collinear moments along the c-axis, whereas the intermediate temperature phase is a partially disordered phase with a monoclinic magnetic unit cell where 1/5 of moments remain paramagnetic. The Monte Carlo simulations for the Ising spin triangular lattice antiferromagnet can reproduce the observed successive transition in the case of the exchange parameters 4J3 < 2J2 is similar to J1 < 0. This combination of exchange parameters gives an account for the stability of the collinear magnetic structure in CuFeO2.