Frustration and 120° Magnetic Ordering in the Layered Triangular Antiferromagnets AFe(PO₃F)₂ (A = K, (NH₄)₂Cl, NH₄, Rb, and Cs)

Abstract

A new family of oxofluorophoshates, AFe(PO 3 F) 2 (A = K, (NH 4 ) 2 Cl, NH 4 , Rb, and Cs), was synthesized via ionothermal methods using PF 6 ionic liquids. Single-crystal and powder X-ray diffraction reveal that AFe(PO 3 F) 2 with A = (NH 4 ) 2 Cl crystallizes in a trigonal structure, while AFe(PO 3 F) 2 with A = NH 4 , Rb, and Cs crystallizes in a triclinic structure. Dimorphic KFe(PO 3 F) 2 crystallizes in both the trigonal and triclinic forms. The structures of all compounds feature Yavapaiitelike Fe(PO 3 F) 2 slabs, which are characterized by triangular Fe layers, planar in the case of the trigonal structure and undulated in the case of the triclinic one. Magnetization measurements reveal all compounds to order antiferromagnetically at low temperatures. The trigonal phases AFe(PO 3 F) 2 (A = K and (NH 4 ) 2 Cl) display complex magnetic H−T phase diagrams. The observation of magnetization plateaus at M sat /3 (M sat = saturation magnetization) indicates the existence of the up−up−down (UUD) and V-phases at applied magnetic fields in the magnetically ordered state. Powder neutron diffraction measurements of KFe(PO 3 F) 2 confirm the 120°spin structure at zero fields. Along c, the magnetic moments form a commensurate spiral since the spins in each plane are rotated by 90°with respect to the adjacent one. To our knowledge, this is the first time such a non-centrosymmetric version of the 120°spin structure with a 90°rotation between nearest planes has been reported.

K2Ni(SeO3)2: A Perfect S = 1 Triangular-Lattice Antiferromagnet with Strong Geometric Frustration and Easy-Plane Anisotropy

“…This feature means that in higher field, the system successively undergoes a magnetic transition from the paramagnetic state to the UUD state and a transition from the UUD state to the AFM state with a 120° spin configuration, showing the Heisenberg character of K 2 Ni­(SeO 3 ) 2 . Similar χ­( T ) curves and changes in spin structure were reported in RbFe­(MoO 4 ) 2 and KFe­(PO 3 F) 2 . ,− …”

“…Among the geometrically frustrated spin systems, the two-dimensional (2D) triangular-lattice antiferromagnet (TLAF) is one of the most studied systems due to its simple structure and wide variety of magnetism. − For TLAFs with classical Ising anisotropy, since all antiferromagnetic (AFM) interactions are not satisfied simultaneously, a disordered, highly macroscopically degenerated ground state emerges. , Recently reported Ising TLAFs include NdTa 7 O 19 and TmMgGaO4. − However, for TLAFs with isotropic Heisenberg exchanges, the long-range magnetic order can survive because the spin-glass or spin-liquid states can be bypassed by forming a noncollinear 120° magnetic order in the triangular-lattice plane. − When a strong enough magnetic field is applied along the plane, the coplanar 120° spin structure can be converted into a collinear up-up-down (UUD) state. − Macroscopically, this UUD state is usually manifested by a 1/3 magnetization plateau in the magnetization curve. , Well-studied materials include RbFe­(MoO 4 ) 2 , CsFe­(SO 4 ) 2 , A Fe­(PO 3 F) 2 [ A = K and (NH 4 ) 2 Cl], and Ba 3 CoSb 2 O 9 . − …”

Single crystal synthesis and properties of the two-dimensional van der Waals frustrated magnets, Mn2In2Se5 and Mn2Ga2S5

“…Magnetic frustration occurs when the magnetic coupling interactions between all neighbouring spins cannot be satisfied simultaneously, leading to a disordered glassy magnetic ground state and/or greatly suppressed magnetic transition temperatures. Numerous geometrical patterns of magnetic elements are known to lead to this frustration, such as triangular [31][32][33][34] and Kagome lattices. 35,36 These disordered phases have attracted considerable interest in the materials community, both in spin-dynamic devices and in quantum spin liquids whereby this frustration inhibits ordering at any temperature.…”