Unusual Magnetism in a d-Electron Compound Fe(OH)Cl: A X–Y Stacked-Triangular Antiferromagnet with Characteristic Double-Bridge Bonding, Unquenched Orbital Moment and Coexisting Spin Fluctuations

Single crystals of triangular-lattice Fe(OH)Cl in orthorhombic structure were synthesized and its magnetism was studied thoroughly using multiple magnetic probes of magnetization, muon spin rotation/relaxation (μSR) and neutron diffraction. Antiferromagnetic transition was found at T_N = 13.3 K together with slow dynamics and surprisingly unquenched orbital moments in the paramagnetic state. Residual spin fluctuations coexisting with a long-range order below T_N were witnessed in μSR. A coplanar spin structure of propagation vector \(\boldsymbol{k} = (0,0,0)\) confined within the triangular lattice was revealed by neutron diffraction to have a relatively small frozen moment of 1.96 μ_B. Ferromagnetic superexchange interactions through a characteristic zig-zag path via ∼90° angled double Fe==O==Fe and double Fe==Cl==Fe bridges counted for this spin arrangement, presenting similarity and contrasting difference to the antiferromagnetic zig-zag path in the S = 1/2 triangular lattice Cu(OH)Cl in monoclinic structure. The obtained value of critical exponent, β = 0.2585(16), in Fe(OD)Cl agrees well with the theoretical value 0.253 \( \mp \) 0.01 in the X–Y frustrated spin model for stacked-triangular antiferromagnet. Experimental evidence all consistently demonstrate the existence of magnetic order with extensive spin fluctuations on a wide range from ∼1 to 10⁻⁹–10⁻¹² s. This work presents a unique system for studying geometric frustration on frustrated triangular lattice with unquenched orbital moment.