The effect of magnetic order on longitudinal Tomonaga-Luttinger liquid spin dynamics in weakly coupled spin-1 2 chains
Physical Review B American Physical Society 107 (2023) 134425
Abstract:
The quantum many-body interactions in one-dimensional spin- 1 2 systems are subject to Tomonaga-Luttinger liquid (TLL) physics, which predict an array of multi-particle excitations that form continua in momentum-energy space. Here we use inelastic neutron spectroscopy to study the TLL spin dynamics in SrCo2V2O8, a compound which contains weakly coupled spin- 1 2 chains of Co atoms, at 0.05 K and in a longitudinal magnetic field up to 9.0 T. The measurements were performed above 3.9 T, where the ground state N´eel antiferromagnetic (AFM) order is completely suppressed, and the multi-particle excitations are exclusively of TLL type. In this region and below 7.0 T, the longitudinal TLL mode – psinon/antipsinon (P/AP) – is unexpectedly well described by a damped harmonic oscillator (DHO) while approaching the zone center defining the static spin-spin correlations. A non-DHO-type, continuum-like signal is seen at higher fields, but deviations from the ideal one-dimensional TLL still remain. This change in the P/AP mode coincides with the phase transition between the longitudinal spin density wave (LSDW) and transverse AFM order. Inside the LSDW state, the DHO-type P/AP spectral weight increases and the linewidth broadens as the magnetic order parameter decreases. These results reveal the impact of three-dimensional magnetic order on the TLL spin dynamics; they call for beyond the mean-field treatment for the interchain exchange interactions.Impact of mixed anion ordered state on the magnetic ground states of S=1/2 square-lattice quantum spin antiferromagnets, Sr2NiO3Cl and Sr2NiO3F
Physical Review Materials American Physical Society 6:11 (2022) 114404
Abstract:
The magnetic properties of the S=1/2 two-dimensional square-lattice antiferromagnets Sr2NiO3X (X=Cl, F) with the trivalent nickel ions in a low-spin state were studied by magnetic susceptibility, heat capacity, neutron powder diffraction, high-field electron spin resonance (ESR), muon spin rotation and relaxation (μ+SR) measurements, and density functional theory (DFT) calculations. Both oxyhalides are isostructural to an ideal quantum square-lattice antiferromagnet Sr2CuO2Cl2, but the chlorine/fluorine anion exclusively occupies an apical site in an ordered/disordered manner with an oxygen anion, resulting in the formation of highly distorted NiO5X octahedra with an off-center nickel ion. Magnetic susceptibility measurements revealed a remarkable difference between these two compounds: the magnetic susceptibility of Sr2NiO3Cl exhibited a broad maximum at approximately 35 K, which is typical of low-dimensional antiferromagnetic behavior. In contrast, the magnetic susceptibility of Sr2NiO3F exhibited spin-glass-like behavior below 12 K. No anomaly associated with long-range magnetic ordering was observed in the heat capacity, ESR, and neutron powder diffraction experiments. However, μ+SR measurements revealed the emergence of a static magnetic ordered state below TN=28K in Sr2NiO3Cl and a short-range magnetic state below TN=18K in Sr2NiO3F. The DFT calculations suggested that the unpaired electron occupied a d3z2-r2 orbital, and ferromagnetic couplings between the nearest-neighbor nickel spins were energetically favored. The mechanism of ferromagnetic superexchange interactions and the reason for the difference between the magnetic ground states in these nickel oxyhalides are discussed.Room-temperature type-II multiferroic phase induced by pressure in cupric oxide
Physical Review Letters American Physical Society 129 (2022) 217601
Abstract:
According to previous theoretical work, the binary oxide CuO can become a room temperature multiferroic via tuning of the superexchange interactions by application of pressure. Thus far, however, there has been no experimental evidence for the predicted room-temperature multiferroicity. Here, we show by neutron diffraction that the multiferroic phase in CuO reaches 295 K with the application of 18.5 GPa pressure. We also develop a spin Hamiltonian based on density functional theory and employing superexchange theory for the magnetic interactions, which can reproduce the experimental results. The present study provides a stimulus to develop room-temperature multiferroic materials by alternative methods based on existing low temperature compounds, such as epitaxial strain, for tunable multifunctional devices and memory applications.Magnetic structure of the topological semimetal Co3Sn2 S2
Physical Review B American Physical Society 105:9 (2022) 094435
Abstract:
Co3Sn2S2 has recently been predicted to be a Weyl semimetal in which magnetic order is key to its behavior as a topological material. Here, we report unpolarized neutron diffraction and spherical neutron polarimetry measurements, supported by magnetization and transport data, which probe the magnetic order in Co3Sn2S2 below TC=177 K. The results are fully consistent with ferromagnetic order in which the spins on the Co atoms point along the crystal c axis, although we cannot rule out some canting of the spins. We find no evidence for a type of long-ranged (k=0) in-plane 120° antiferromagnetic order which had previously been considered as a secondary phase present at temperatures between ∼90 K and TC. A discontinuous change in bulk properties and neutron polarization observed at T=125 K when samples are cooled in a field and measured on warming is found to be due to a sudden reduction in ferromagnetic domain size. Our results lend support to the theoretical predictions that Co3Sn2S2 is a magnetic Weyl semimetal.Model for coupled 4 f-3d magnetic spectra: a neutron scattering study of the Yb-Fe hybridization in Yb3Fe5 O12
Physical Review B American Physical Society 105:10 (2022) 104422