Low-Dimensional Metal–Organic Magnets as a Route toward the S = 2 Haldane Phase

Abstract

Metal–organic magnets (MOMs), modular magnetic materials where metal atoms are connected by organic linkers, are promising candidates for next-generation quantum technologies. MOMs readily form low-dimensional structures and so are ideal systems to realize physical examples of key quantum models, including the Haldane phase, where a topological excitation gap occurs in integer-spin antiferromagnetic (AFM) chains. Thus, far the Haldane phase has only been identified for S = 1, with S ≥ 2 still unrealized because the larger spin imposes more stringent requirements on the magnetic interactions. Here, we report the structure and magnetic properties of CrCl2(pym) (pym = pyrimidine), a new quasi-1D S = 2 AFM MOM. We show, using X-ray and neutron diffraction, bulk property measurements, density-functional theory calculations, and inelastic neutron spectroscopy (INS), that CrCl2(pym) consists of AFM CrCl2 spin chains (J 1 = −1.13(4) meV) which are weakly ferromagnetically coupled through bridging pym (J 2 = 0.10(2) meV), with easy-axis anisotropy (D = −0.15(3) meV). We find that, although small compared to J 1, these additional interactions are sufficient to prevent observation of the Haldane phase in this material. Nevertheless, the proximity to the Haldane phase together with the modularity of MOMs suggests that layered Cr(II) MOMs are a promising family to search for the elusive S = 2 Haldane phase.

Controlling noncollinear ferromagnetism in van der Waals metal-organic magnets

“…1c & d ). We have chosen the space group settings so that the a, b and c lattice parameters correspond to the equivalent chemical directions in these four new compounds and previously reported analogues: 26 the MCl 2 chains lying along the a, the btd or pym ligands along b and the vdW layers along c.…”

“…31,35,36 The controllable nature of the structure means they are an ideal family to realise targetted magnetic phases: for example both CuCl 2 (btd) and CrCl 2 (pym) have proven ripe for investiga-tions of 1D quantum magnetism. 26,31 In this paper we report four noncollinear van der Waals magnets, MCl 2 L, where M = Ni and Fe, L = pyrimidine (pym) and 2,1,3benzothiadiazole (btd). In each compound the noncollinearity leads to a net weak ferromagnetic moment within the layer with either a very large canting angle or large coercive field.…”

“…[24][25][26][27][28][29][30][31][32][33][34] These materials are typically collinear antiferromagnets with the strongest superexchange interaction being along the MX 2 chain. The sign of the interaction depends most strongly on the metal: Ni, Co and Fe form ferromagnetic chains 28 and Cu and Cr form antiferromagnetic chains 26,30 The spatial relationship between MX 2 chains is dictated by the organic ligand: the distance between chains is determined by ligand length 25 and the angle is controlled by the bonding geometry of the ligand (as in supramolecular cage chemistry). 31,35,36 The controllable nature of the structure means they are an ideal family to realise targetted magnetic phases: for example both CuCl 2 (btd) and CrCl 2 (pym) have proven ripe for investiga-tions of 1D quantum magnetism.…”

Extended Honeycomb Metal Chloride with Tunable Antiferromagnetic Correlations

“…Several lead-free metal-halide perovskites have emerged as promising candidates, including vacancy-ordered double perovskite (A 2 MX 6 , e.g., A = Cs + , K + ; M = Sn 4+ and Te 4+ ; X = halide ions), , double perovskite (A 2 M I M III X 6 , e.g., A = Cs + , Rb + ; M I = Ag + and Na + ; M III = Bi 3+ , Sb 3+ , and In 3+ ; X = halide ions), − and vacancy-ordered triple perovskite (A 3 M 2 X 9 , e.g., A = K + , Cs + ; M = Bi 3+ and Sb 3+ ; X = halide ions). − However, most of the current research is focused on nonmagnetic systems. In comparison to the extensively studied optoelectronic properties, the investigation of magnetic properties remains relatively limited. − Introducing transition metals into perovskite structures represents a promising approach for altering optoelectronic properties and introducing cooperative magnetism. , …”

Magnetic structure and properties of the honeycomb antiferromagnet [Na(OH2)3]Mn(NCS)3

“…These results illustrate the importance of magnetic structure determination for understanding the function of magnetic molecular framework compounds, as also shown by other recent works on frameworks containing both Mn(II) [43][44][45][46] and other metals. [47][48][49] In conclusion, we have determined the low temperature structure and magnetic properties of [Na(OH 2 ) 3 ]Mn(NCS) 3 by developing crystal growth methodology for this humidity sensitive compound. Neutron diffraction data permitted the hydrogen atoms to be located and hence the key role of the hydrogen bonding network in framework structuration.…”