MOLECULAR-FIELD CALCULATIONS OF THE PHASE-TRANSITIONS IN NDCO2SI2

NdCo2Si2 has a collinear antiferromagnetic structure viith a wave vect or Q(1) = (0, 0, 1) for T < T-1 = 16 K, a high-order commensurate one with 0, = (0, 0, 13-14) for T-1 less than or equal to T < T-2 = 24 K a nd with Q(3) = (0, 0, 11/14) for T-2 less than or equal to T < T-N = 3 2 K. Molecular field calculations of the moment arrangements and magne tization processes of the compound were carried out and the held-tempe rature phase diagram was reproduced. The magnetic transition at T-2 is discussed in connection with the fact that the third harmonics 3Q(2) of the magnetic structure for T-1 less than or equal to T < T-2 is equ ivalent to the fundamental wavevector Q(3) for T-2 less than or equal to T < T-N. (C) 1998 Elsevier Science B.V. All rights reserved.