Low-dimensional metal-organic frameworks: a pathway to design, explore and tune magnetic structures.

The magnetic structure adopted by a material relies on symmetry, the hierarchy of exchange interactions between magnetic ions and local anisotropy. A direct pathway to control the magnetic interactions is to enforce dimensionality within the material, from zero-dimensional isolated magnetic ions, one-dimensional (1D) spin-chains, two-dimensional (2D) layers to three-dimensional (3D) order. Being able to design a material with a specific dimensionality for the phenomena of interest is non-trivial.

Anomalous continuum scattering and higher-order van Hove singularity in the strongly anisotropic S = 1/2 triangular lattice antiferromagnet.

The S = 1/2 triangular lattice antiferromagnet (TLAF) is a paradigmatic example of frustrated quantum magnetism. An ongoing challenge involves understanding the influence of exchange anisotropy on the collective behavior within such systems. Using inelastic neutron scattering (INS) and advanced calculation techniques, we have studied the low and high-temperature spin dynamics of BaLaCoTeO (BLCTO): a Co-based J = 1/2 TLAF that exhibits 120° order below T = 3.

CsFe(MoO)-A Strongly Frustrated Magnet with Orbital Degrees of Freedom and Magnetocaloric Properties.

We report an in-depth study of the thermodynamic and magnetocaloric properties of a strongly frustrated magnet, CsFe(MoO). The underlying structure belongs to the double trillium lattice, which consists of two Fe ( = 2) sites with easy-axis and easy-plane single-ion anisotropy. Detailed Fe Mössbauer spectroscopic investigations along with ligand-field calculations support the existence of disparate ground states.

Controlling Noncollinear Ferromagnetism in van der Waals Metal-Organic Magnets.

Van der Waals (vdW) magnets both allow exploration of fundamental 2D physics and offer a route toward exploiting magnetism in next generation information technology, but vdW magnets with complex, noncollinear spin textures are currently rare. We report here the syntheses, crystal structures, magnetic properties and magnetic ground states of four bulk vdW metal-organic magnets (MOMs): FeCl(pym), FeCl(btd), NiCl(pym), and NiCl(btd), pym = pyrimidine and btd = 2,1,3-benzothiadiazole. Using a combination of neutron diffraction and bulk magnetometry we show that these materials are noncollinear magnets.

Neutron Instruments for Research in Coordination Chemistry.

Neutron diffraction and spectroscopy offer unique insight into structures and properties of solids and molecular materials. All neutron instruments located at the various neutron sources are distinct, even if their designs are based on similar principles, and thus, they are usually less familiar to the community than commercial X-ray diffractometers and optical spectrometers. Major neutron instruments in the USA, which are open to scientists around the world, and examples of their use in coordination chemistry research are presented here, along with a list of similar instruments at main neutron facilities in other countries.

Noncentrosymmetric Triangular Magnet CaMnTeO: Strong Quantum Fluctuations and Role of s versus s Electronic States in Competing Exchange Interactions.

Noncentrosymmetric triangular magnets offer a unique platform for realizing strong quantum fluctuations. However, designing these quantum materials remains an open challenge attributable to a knowledge gap in the tunability of competing exchange interactions at the atomic level. Here, a new noncentrosymmetric triangular S = 3/2 magnet CaMnTeO is created based on careful chemical and physical considerations.

Line-Graph Approach to Spiral Spin Liquids.

Competition among exchange interactions is able to induce novel spin correlations on a bipartite lattice without geometrical frustration. A prototype example is the spiral spin liquid, which is a correlated paramagnetic state characterized by subdimensional degenerate propagation vectors. Here, using spectral graph theory, we show that spiral spin liquids on a bipartite lattice can be approximated by a further-neighbor model on the corresponding line-graph lattice that is nonbipartite, thus broadening the space of candidate materials that may support the spiral spin liquid phases.

Magnetic Interactions of the Centrosymmetric Skyrmion Material Gd_{2}PdSi_{3}.

The experimental realization of magnetic skyrmion crystals in centrosymmetric materials has been driven by theoretical understanding of how a delicate balance of anisotropy and frustration can stabilize topological spin structures in applied magnetic fields. Recently, the centrosymmetric material Gd_{2}PdSi_{3} was shown to host a field-induced skyrmion crystal, but the skyrmion stabilization mechanism remains unclear. Here, we employ neutron-scattering measurements on an isotopically enriched polycrystalline Gd_{2}PdSi_{3} sample to quantify the interactions that drive skyrmion formation.

Magnetic phase diagram of (MoRE)AlC, RETb and Dy, studied by magnetization, specific heat, and neutron diffraction analysis.

We report the results of magnetization, heat capacity, and neutron diffraction measurements on (MoRE)AlC with RE = Dy and Tb. Temperature and field-dependent magnetization as well as heat capacity were measured on a powder sample and on a single crystal allowing the construction of the magnetic field-temperature phase diagram. To study the magnetic structure of each magnetic phase, we applied neutron diffraction in a magnetic field up to 6 T.

Chloride Reduction of Mn in Mild Hydrothermal Synthesis of a Charge Ordered Defect Pyrochlore, CsMnMnF, a Canted Antiferromagnet with a Hard Ferromagnetic Component.

Geometrically frustrated systems play an important role in studying new physical phenomena and unconventional thermodynamics. Charge ordered defect pyrochlores F offer a convenient platform for probing the interplay between electron distribution over and sites and structural distortions; however, they are limited to compounds with = V, Fe, Ni, and Cu due to difficulties in the simultaneous stabilization of other 3d elements in the +2 and +3 oxidation states. Herein, we employ Cl anions under hydrothermal conditions for the mild reduction of MnO in concentrated HF to obtain the CsMnMnF composition as a phase pure sample and study its properties.

Influence of the Cubic Sublattice on Magnetic Coupling between the Tetrahedral Sites of Garnet.

We present a study on the nuclear and magnetic structures of two iron-based garnets with magnetic cations isolated on tetrahedral sites. CaYZrFeO and CaLaZrFeO offer an interesting comparison for examining the effect of increasing cation size within the diamagnetic backbone of the garnet crystal structure, and how such changes affect the magnetic order. Despite both systems exhibiting well-pronounced magnetic transitions at low temperatures, we also find evidence for diffuse magnetic scattering due to a competition between the nearest-neighbor, next nearest-neighbor, and so on, within the tetrahedral sites.

Absence of long-range magnetic order in lithium-containing honeycombs in the Li-Cr-Sb(Te)-O phases.

Li((LiCr)(Te/Sb))Ocompounds where Cr atoms along with Li and Te or Sb are part of a honeycomb and are studied using magnetic susceptibility, specific heat, x-ray photoelectron spectroscopy and neutron diffraction. The oxides stoichiometries as determined from the neutron diffraction studies are LiCrTeOand LiCrSbOwith a stable oxidation state of +3 for Cr. Both the compounds crystallize in space group2/with intermixing of cations at the 4sites leaving the 2sites preferentially for Te or Sb.

Canting of the Magnetic Moments on the Octahedral Site of an Iron Oxide Garnet in Response to Diamagnetic Cation Substitution.

Using neutron powder diffraction and magnetic susceptibility measurements, we report on the preparation and characterization of the temperature- and field-dependent properties of CaYZrFeO, a composition closely related to the high-temperature ferrimagnet YFeO. By diluting the concentration of paramagnetic ions on the octahedral sublattice of the garnet structure, we find temperature-dependent canting of the magnetic moments. This reflects the importance of the octahedral sublattice in mediating the magnetic interactions between the tetrahedral sites and offers insight into a large number of competing magnetic interactions in the garnet structure.

Observation of novel charge ordering and spin reorientation in perovskite oxide PbFeO.

PbMO (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO remain unclear. Herein, we reveal that PbFeO crystallizes into an unusual 2a × 6a × 2a orthorhombic perovskite super unit cell with space group Cmcm.

Frustrated Heisenberg model within the stretched diamond lattice of LiYbO.

We investigate the magnetic properties of LiYbO, containing a three-dimensionally frustrated, diamond-like lattice via neutron scattering, magnetization, and heat capacity measurements. The stretched diamond network of Yb ions in LiYbO enters a long-range incommensurate, helical state with an ordering wave vector that "locks-in" to a commensurate phase under the application of a magnetic field. The spiral magnetic ground state of LiYbO can be understood in the framework of a Heisenberg Hamiltonian on a stretched diamond lattice, where the propagation vector of the spiral is uniquely determined by the ratio of .

Cluster Frustration in the Breathing Pyrochlore Magnet LiGaCr_{4}S_{8}.

We present a comprehensive neutron scattering study of the breathing pyrochlore magnet LiGaCr_{4}S_{8}. We observe an unconventional magnetic excitation spectrum with a separation of high- and low-energy spin dynamics in the correlated paramagnetic regime above a spin-freezing transition at 12(2) K. By fitting to magnetic diffuse-scattering data, we parametrize the spin Hamiltonian.

Mechanistic Insights into the Superexchange-Interaction-Driven Negative Thermal Expansion in CuO.

The negative thermal expansion (NTE) in CuO is explained via electron-transfer-driven superexchange interaction. The elusive connection between the spin-lattice coupling and NTE of CuO is investigated by neutron scattering and principal strain axes analysis. The density functional theory calculations show as the temperature decreases, the continuously increasing electron transfer accounts for enhancing the superexchange interaction along [101̅], the principal NTE direction.

Ferromagnetism Near Room Temperature in the Cleavable van der Waals Crystal FeGeTe.

Two-dimensional materials with intrinsic functionality are becoming increasingly important in exploring fundamental condensed matter science and for developing advanced technologies. Bulk crystals that can be exfoliated are particularly relevant to these pursuits as they provide the opportunity to study the role of physical dimensionality and explore device physics in highly crystalline samples and designer heterostructures in a routine manner. Magnetism is a key element in these endeavors; however, relatively few cleavable materials are magnetic and none possess magnetic order at ambient conditions.

3d-Metal Induced Magnetic Ordering on U(IV) Atoms as a Route toward U(IV) Magnetic Materials.

Uranium(IV) 5f magnetism is dominated by a transition from a triplet to a singlet ground state at low temperatures. For the first time, we achieved magnetic ordering of U(IV) atoms in a complex fluoride through the incorporation of 3 d transition metal cations. This new route allowed us to obtain an unprecedented series of U(IV) ferrimagnetic materials of the new composition CsMUF (M = Mn, Co, and Ni), which were comprehensively characterized with respect to their structural and magnetic properties.

Retention of a Paramagnetic Ground State at Low Temperatures in a Family of Structurally Related U Phosphates.

A new uranium fluoride phosphate, UFPO, was synthesized via a mild hydrothermal route and characterized optically, thermally, and magnetically. Two thermal transformation products, UO(PO) and UUO(PO), were discovered to be structurally related, and were subsequently synthesized for bulk property measurements. All three materials failed to follow Curie-Weiss behavior at low temperatures, attributed to the nearly ubiquitous singlet ground state of U(IV), transitioning into a Curie-Weiss paramagnetic regime at high temperatures.

Realization of Large Electric Polarization and Strong Magnetoelectric Coupling in BiMn Cr O.

Magnetoelectric multiferroics have received much attention in the past decade due to their interesting physics and promising multifunctional performance. For practical applications, simultaneous large ferroelectric polarization and strong magnetoelectric coupling are preferred. However, these two properties have not been found to be compatible in the single-phase multiferroic materials discovered as yet.

Competing magnetic ground states and their coupling to the crystal lattice in CuFeGe.

Identifying and characterizing systems with coupled and competing interactions is central to the development of physical models that can accurately describe and predict emergent behavior in condensed matter systems. This work demonstrates that the metallic compound CuFeGe has competing magnetic ground states, which are shown to be strongly coupled to the lattice and easily manipulated using temperature and applied magnetic fields. Temperature-dependent magnetization M measurements reveal a ferromagnetic-like onset at 228 (1) K and a broad maximum in M near 180 K.

Melting of Pb Charge Glass and Simultaneous Pb-Cr Charge Transfer in PbCrO3 as the Origin of Volume Collapse.

A metal to insulator transition in integer or half integer charge systems can be regarded as crystallization of charges. The insulating state tends to have a glassy nature when randomness or geometrical frustration exists. We report that the charge glass state is realized in a perovskite compound PbCrO3, which has been known for almost 50 years, without any obvious inhomogeneity or triangular arrangement in the charge system.