Coexistence of Spin-Canting, Metamagnetism, and Spin-Flop in a (4,4) Layered Manganese Azide Polymer

Abstract

A novel molecule-based magnetic polymer Mn(N3)2(btr)2 1 (btr = 4,4‘-bi-1,2,4-triazole) was synthesized and characterized crystallographically and magnetically. 1 crystallizes in the monoclinic system, space group P21/c, formula C8H8N18Mn, with a = 12.2831(4) Å, b = 6.3680(1) Å, c = 10.2245(3) Å, β = 105.064(1)°, and Z = 2. Bridged by end-to-end azides, the Mn2+ ions form a two-dimensional layer with (4,4) topology; the layers are further connected to the three-dimensional network by the weak hydrogen bonds between ligands of btr. Magnetic studies on a polycrystalline sample show the existence of strong antiferromagnetic couplings between the adjacent Mn2+ ions, and the Neél temperature is TN = 23.7 K. In the ordered state below TN, detailed investigations on an oriented single-crystal sample of 1 reveal that the hidden spin-canting, metamagnetic transition, and spin-flop transition can appear in different circumstances. The ground state is of an antiferromagnet with hidden spin-canting. An external field applied along the b direction parallel to the manganese azide layer can lead to a first-order metamagnetic phase transition, while a spin-flop transition may occur when the field is applied along the a* direction that is perpendicular to the manganese azide layer. Magnetic phase diagrams in both the T−Ha* and the T−Hb planes were determined. Possible spin configurations before and after the transitions were proposed. Analyses on the experimental data give the following intrinsic parameters: the intra- and interlayer coupling J ≈ −3.5 cm-1 and Ja* = 6 × 10-4 cm-1, the anisotropy field HA = 0.2 kOe, the exchange field HE = 387.8 kOe, and the anisotropy parameter α = 5 × 10-4. The small Ja* and α show 1 to be a good example of a two-dimensional Heisenberg system.