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1. Effect of magnetic fields on the methyl rotation in a paramagnetic cobalt( ii ) complex. Quasielastic neutron scattering studies

   https://doi.org/10.1039/C8CP01660G  

   Stavretis, Shelby E. ; Mamontov, Eugene ; Moseley, Duncan H. ; Cheng, Yongqiang ; Daemen, Luke L. ; Ramirez-Cuesta, A. J. ; Xue, Zi-Ling ( January 2018 , Physical Chemistry Chemical Physics)

   Molecular dynamics is a fundamental property of metal complexes. These dynamic processes, especially for paramagnetic complexes under external magnetic fields, are in general not well understood. Quasielastic neutron scattering (QENS) in 0–4 T magnetic fields has been used to study the dynamics of Co(acac) 2 (D 2 O) 2 ( 1-d4 , acac = acetylacetonate). At 80–100 K, rotation of the methyl groups on the acac ligands is the dominant dynamical process. This rotation is slowed down by the magnetic field increase. Rotation times at 80 K are 5.6(3) × 10 −10 s at 0 T and 2.04(10) × 10more »−9 s at 4 T. The QENS studies suggest that methyl groups in these paramagnetic Co( ii ) molecules do not behave as isolated units, which is consistent with results from earlier magnetic susceptibility studies indicating the presence of intermolecular interactions. DFT calculations show that unpaired electron spin density in 1 is dispersed to the atoms of both acac and H 2 O ligands. Methyl torsions in 1-d4 have also been observed at 5–100 K in inelastic neutron spectroscopy (INS). The QENS and INS results here help understand the dynamics of the compound in the solid state.« less
2. Multiphase magnetism in Yb ₂ Ti ₂ O ₇

   https://doi.org/10.1073/pnas.2008791117  

   Scheie, Allen ; Kindervater, Jonas ; Zhang, Shu ; Changlani, Hitesh J. ; Sala, Gabriele ; Ehlers, Georg ; Heinemann, Andre ; Tucker, Gregory S. ; Koohpayeh, Seyed M. ; Broholm, Collin ( October 2020 , Proceedings of the National Academy of Sciences)

   We use neutron scattering to show that ferromagnetism and antiferromagnetism coexist in the low T state of the pyrochlore quantum magnet${\text{Yb}}_2{\text{Ti}}_2{\mathrm{O}}_7$. While magnetic Bragg peaks evidence long-range static ferromagnetic order, inelastic scattering shows that short-range correlated antiferromagnetism is also present. Small-angle neutron scattering provides direct evidence for mesoscale magnetic structure that we associate with metastable antiferromagnetism. Classical Monte Carlo simulations based on exchange interactions inferred from$⟨111⟩$-oriented high-field spin wave measurements confirm that antiferromagnetism is metastable within the otherwise ferromagnetic ground state. The apparent lack of coherent spin wave excitations and strong sensitivity to quenched disorder characterizingmathvariant='normal'>O7is a consequence of this multiphase magnetism.

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3. Quantum liquid from strange frustration in the trimer magnet Ba4Ir3O10

   https://doi.org/10.1038/s41535-020-0232-6  

   Cao, Gang ; Zheng, Hao ; Zhao, Hengdi ; Ni, Yifei ; Pocs, Christopher A. ; Zhang, Yu ; Ye, Feng ; Hoffmann, Christina ; Wang, Xiaoping ; Lee, Minhyea ; et al ( May 2020 , npj Quantum Materials)

   Quantum spin systems such as magnetic insulators usually show magnetic order, but such classical states can give way toquantum liquids with exotic entanglementthrough two known mechanisms of frustration: geometric frustration in lattices with triangle motifs, and spin-orbit-coupling frustration in the exactly solvable quantum liquid of Kitaev’s honeycomb lattice. Here we present the experimental observation of a new kind of frustrated quantum liquid arising in an unlikely place: the magnetic insulator Ba₄Ir₃O₁₀where Ir₃O₁₂trimers form an unfrustrated square lattice. The crystal structure shows no apparent spin chains. Experimentally we find a quantum liquid state persisting down to 0.2 K that is stabilizedmore »by strong antiferromagnetic interaction with Curie–Weiss temperature ranging from −766 to −169 K due to magnetic anisotropy. The anisotropy-averaged frustration parameter is 2000, seldom seen in iridates. Heat capacity and thermal conductivity are both linear at low temperatures, a familiar feature in metals but here in an insulator pointing to an exotic quantum liquid state; a mere 2% Sr substitution for Ba produces long-range order at 130 K and destroys the linear-T features. Although the Ir⁴⁺(5d⁵) ions in Ba₄Ir₃O₁₀appear to form Ir₃O₁₂trimers of face-sharing IrO₆octahedra, we propose that intra-trimer exchange is reduced and the lattice recombines into an array of coupled 1D chains with additional spins. An extreme limit of decoupled 1D chains can explain most but not all of the striking experimental observations, indicating that the inter-chain coupling plays an important role in the frustration mechanism leading to this quantum liquid.

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4. Antiferromagnetic excitonic insulator state in Sr3Ir2O7

   https://doi.org/10.1038/s41467-022-28207-w  

   Mazzone, D. G. ; Shen, Y. ; Suwa, H. ; Fabbris, G. ; Yang, J. ; Zhang, S.-S. ; Miao, H. ; Sears, J. ; Jia, Ke ; Shi, Y. G. ; et al ( December 2022 , Nature Communications)

   Abstract Excitonic insulators are usually considered to form via the condensation of a soft charge mode of bound electron-hole pairs. This, however, presumes that the soft exciton is of spin-singlet character. Early theoretical considerations have also predicted a very distinct scenario, in which the condensation of magnetic excitons results in an antiferromagnetic excitonic insulator state. Here we report resonant inelastic x-ray scattering (RIXS) measurements of Sr 3 Ir 2 O 7 . By isolating the longitudinal component of the spectra, we identify a magnetic mode that is well-defined at the magnetic and structural Brillouin zone centers, but which merges withmore »the electronic continuum in between these high symmetry points and which decays upon heating concurrent with a decrease in the material’s resistivity. We show that a bilayer Hubbard model, in which electron-hole pairs are bound by exchange interactions, consistently explains all the electronic and magnetic properties of Sr 3 Ir 2 O 7 indicating that this material is a realization of the long-predicted antiferromagnetic excitonic insulator phase.« less
5. Magnetism and spin dynamics in room-temperature van der Waals magnet Fe5GeTe2

   https://doi.org/https://doi.org/10.1088/2053-1583/ac2028  

   Alahmed, L. ; Nepal, B. ; Macy, J. ; Zheng, W. ; Casas, B. ; Sapkota, A. ; Jones, N. ; Mazza, A. R. ; Brahlek, M. ; Jin, W. ; et al ( September 2021 , 2D materials)

   Two-dimensional van der Waals (vdWs) materials have gathered a lot of attention recently. However, the majority of these materials have Curie temperatures that are well below room temperature, making it challenging to incorporate them into device applications. In this work, we synthesized a room-temperature vdW magnetic crystal Fe5GeTe2 with a Curie temperature T$_c = 332$ K, and studied its magnetic properties by vibrating sample magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy. The experiments were performed with external magnetic fields applied along the c-axis (H$\parallel$c) and the ab-plane (H$\parallel$ab), with temperatures ranging from 300 to 10 K. We have foundmore »a sizable Landé g-factor difference between the H$\parallel$c and H$\parallel$ab cases. In both cases, the Landé g-factor values deviated from g = 2. This indicates contribution of orbital angular momentum to the magnetic moment. The FMR measurements reveal that Fe5GeTe2 has a damping constant comparable to Permalloy. With reducing temperature, the linewidth was broadened. Together with the VSM data, our measurements indicate that Fe5GeTe2 transitions from ferromagnetic to ferrimagnetic at lower temperatures. Our experiments highlight key information regarding the magnetic state and spin scattering processes in Fe5GeTe2, which promote the understanding of magnetism in Fe5GeTe2, leading to implementations of Fe5GeTe2 based room-temperature spintronic devices.« less