Anharmonic Magnon Excitations in Noncollinear and Charge-Ordered RbFe^{2+}Fe^{3+}F_{6}

Crystal Growth and Magnetism of Transition Metal Pyrochlore Fluorides

Geometric magnetic frustration arises when the geometry of a structure prevents the simultaneous fulfillment of nearest-neighbor antiferromagnetic interactions and is commonly observed in lattices that exhibit a triangular topology, such as those found in the pyrochlore structure. Via a mild hydrothermal route, we have synthesized seven quaternary β-pyrochlore-related fluorides AxM2+xM3+(2-x)F6 (A = Cs and Rb; M2+ = Co2+, Ni2+, and Zn2+; and M3+ = V3+ and Fe3+). Crystal structures and compositions were determined using a combination of single-crystal X-ray diffraction and energy-dispersive spectroscopy. After adjusting the reaction conditions, phase-pure products of AxM2+xM3+(2-x)F6 were obtained. The magnetic susceptibility and isothermal magnetization data for all seven compounds were collected to interpret the magnetic behavior, which ranged from paramagnetic to antiferromagnetic with and without a ferromagnetic component. We found that the magnetic behavior of the AxM2+xV3+(2-x)F6 pyrochlore structures strongly depends on the presence or absence of unpaired electrons on the M2+ position. The titled pyrochlore compounds, with the exception of the Zn-analogue, can be considered frustrated materials, with frustration indices in the range of 6-13.

Chloride Reduction of Mn3+ in Mild Hydrothermal Synthesis of a Charge Ordered Defect Pyrochlore, CsMn2+Mn3+F6, 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 AM²⁺M³⁺F₆ offer a convenient platform for probing the interplay between electron distribution over M²⁺ and M³⁺ sites and structural distortions; however, they are limited to compounds with M^2+/3+ = 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 Mn₂O₃ in concentrated HF to obtain the CsMn²⁺Mn³⁺F₆ composition as a phase pure sample and study its properties. The magnetism of CsMn₂F₆ was characterized by measuring the magnetic susceptibility and isothermal magnetization data, and a magnetic transition to a canted antiferromagnet state was found at 24.1 K. We determined the magnetic structure of CsMn₂F₆ using powder neutron diffraction, which revealed successive long-range ordering of the Mn²⁺ and Mn³⁺ sites that is accompanied by a second transition. The role and strength of magnetic exchange interactions were characterized using DFT calculations.

From One- to Two-Magnon Excitations in the S=3/2 Magnet β-CaCr_{2}O_{4}

We apply neutron spectroscopy to measure the magnetic dynamics in the S=3/2 magnet β-CaCr_{2}O_{4} (T_{N}=21  K). The low-energy fluctuations, in the ordered state, resemble large-S linear spin waves from the incommensurate ground state. However, at higher energy transfers, these semiclassical and harmonic dynamics are replaced by an energy and momentum broadened continuum of excitations. Applying kinematic constraints required for energy and momentum conservation, sum rules of neutron scattering, and comparison against exact diagonalization calculations, we show that the dynamics at high-energy transfers resemble low-S one-dimensional quantum fluctuations. β-CaCr_{2}O_{4} represents an example of a magnet at the border between classical Néel and quantum phases, displaying dual characteristics.