Unusual magnetic-field dependence of partially frustrated triangular ordering in manganese tricyanomethanide

Manganese tricyanomethanide, Mn[C(CN)(3)](2), crystallizes in an orthorhombic lattice consisting of two interpenetrating three-dimensional rutile-like networks. In each network, the tridentate C(CN)3 anion gives rise to superexchange interactions between the Mn2+, ions (S = 5/2) that can be mapped onto the 'row model' for partially frustrated triangular magnets. We present heat capacity measurements that reveal a phase transition at T-N = 1.18 K, indicative of magnetic ordering. From magnetic-field dependent heat capacity data a saturation field H-sat = 42 kOe is estimated. The zero-field magnetically ordered structure was solved from neutron powder diffraction data taken between 0.04 and 1.2 K. It consists of an incommensurate spiral with a temperature independent propagation vector Q = [2Q 0 0] = [+/-0.622 0 0], where different signs relate to the two different networks. This corresponds to [+/-0.311 +/-0.311 0] in a quasi-hexagonal representation. The ordered moment mu = 3.3 mu(B) is about two-thirds of the full Mn2+ moment. From the values of TN and Q, the exchange parameters J/k(B) = 0.15 K and J'/J = 0.749 are estimated. The magnetic-field dependence of the intensity of the (2Q00) Bragg reflection, measured for external fields H 11 Q, indicates the presence of three different magnetic phases. We associate them with the incommensurate spiral (H < 13.5 kOe), an intermediate 'up-up-down' phase (115 kOe < H < 16 kOe) and the '2-l' spin-flop-like magnetic structure (H > 16kOe) proposed for related compounds. For increasing fields, Q continuously approaches the value 1/3, corresponding to the commensurate magnetic structure of the fully frustrated triangular lattice. This value is reached at H* = 19 kOe. At this point, the field dependence reverses and Q adopts a value of 0.327 at 26 kOe, the highest field applied in the experiment. Except for H*, the magnetic ordering is incommensurate in all three field dependent magnetic phases of Mn [C(CN)(3)](2).