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

TL;DR

This study investigates the magnetic ordering and field-dependent phase transitions in manganese tricyanomethanide, revealing incommensurate spiral structures and a transition towards a commensurate state under external magnetic fields.

Contribution

It provides the first detailed neutron diffraction analysis of magnetic structures in Mn[C(CN)3]2, demonstrating field-induced phase transitions and incommensurate magnetic ordering in a partially frustrated triangular magnet.

Findings

Identified incommensurate spiral magnetic structure with propagation vector Q=(+/-0.622 0 0).

Discovered three magnetic phases under varying magnetic fields, including a spin-flop like phase.

Observed Q approaching 1/3 at critical field H_c=19 kOe, indicating a transition towards a fully frustrated triangular lattice.

Abstract

Manganese tricyanomethanide, Mn[C(CN)3]2, consists 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 = 1.18K, indicative of magnetic ordering. 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=3.3mu_B is about 2/3 of the full Mn2+ moment. From the values of T_N and Q, the exchange parameters J/k = 0.15 K and J'/J = 0.749 are estimated. The magnetic-field dependence of the intensity of the <2Q 0 0> Bragg reflection, measured for external fields H||Q, indicates the presence of three different magnetic phases. We associate them with the incommensurate spiral (H < 13.5 kOe), an intermediate phase (13.5 kOe < H < 16 kOe), and the "2-1" spin-flop like magnetic structure (H > 16 kOe) 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_c = 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_c, the magnetic ordering is incommensurate in all three magnetic phases of Mn[C(CN)3]2.