Rearrangement of Uncorrelated Valence Bonds Evidenced by Low-Energy Spin Excitations in YbMgGaO 4

dc-magnetization data measured down to 40 mK speak against conventional freezing and reinstateYbMgGaO4as a triangular spin-liquid candidate. Magnetic susceptibility measured parallel andperpendicular to thecaxis reaches constant values below 0.1 and 0.2 K, respectively, thus indicatingthe presence of gapless low-energy spin excitations. We elucidate their nature in the triple-axis inelasticneutron scattering experiment that pinpoints the low-energy (E≤J0∼0.2meV) part of the excitationcontinuum present at low temperatures (TJ0that is rooted in the breaking ofnearest-neighbor valence bonds and persists to temperatures well aboveJ0=kB, the low-energy oneoriginates from the rearrangement of the valence bonds and thus from the propagation of unpaired spins.We further extend this picture to herbertsmithite, the spin-liquid candidate on the kagome lattice, and arguethat such a hierarchy of magnetic excitations may be a universal feature of quantum spin liquids

Spin dynamics study and experimental realization of tunable single-ion anisotropy in multiferroic Ba 2 CoGe 2 O 7 under external magnetic fields

We report a spin-wave study on multiferroic Ba2CoGe2O7 under magnetic fields up to 12 T using low-energyinelastic neutron scattering. In-plane transverse (T1) spin-wave modes are highly dispersive along (h00) andrather flat but strong in intensity along (30l). In addition, two dispersive electromagnon modes have beenobserved around 3.5 meV. Dispersion of the out-of-plane transverse modes (T2) under fields reveals that thesingle-ion anisotropy constant decreases with increasing magnetic field, which is consistent with the linearspin-wave theory. Our results imply that the field-dependent single-ion anisotropy plays a crucial role indetermining the characteristics of T2 and electromagnon modes in the three-dimensional anisotropic spin-wavespectrum

Anisotropic effect of a magnetic field on the neutron spin resonance in FeSe

We use inelastic neutron scattering to study the effect of a magnetic field on the neutron spin resonance(Er=3.6 meV) of superconducting FeSe (Tc=9 K). While a field aligned along the in-plane direction broadensand suppresses the resonance, ac-axis aligned field does so much more efficiently, consistent with the anisotropicfield-induced suppression of the superfluid density from the heat capacity measurements. These results suggestthat the resonance in FeSe is associated with the superconducting electrons arising from orbital selectivequasiparticle excitations between the hole and electron Fermi surfaces

Structural, magnetic, and magnetocaloric properties of Fe7_7Se8_8 single crystals

The magnetocaloric effect has been studied in high quality single crystals of Fe7Se8 (3c type) grown by using Bridgman’s method. Magnetization and magnetocaloric effect measurements have been carried out in a magnetic field up to 5 T over the temperature range from 2 to 490 K. The spin reorientation transition from the easy c-axis to the easy c-plane, proceeding in an abrupt fashion, as a first-order phase transition, has been observed near the temperature TR ≈ 125 K. The magnetization curves in the vicinity of this transition were shown to have an S-shape with a clear hysteresis. The first order metamagnetic field induced transitions have been identified above and below TR. The conventional magnetocaloric effect related to the metamagnetic transitions has been found above TR, while below TR the inverse magnetocaloric effect was clearly seen. The existence of both kinds of magnetocaloric effect is important from the point of view of large rotating field entropy change in Fe7Se8 single crystals. The refrigeration capacity associated with a second order phase transition from the ferrimagnetic to the paramagnetic state at the Néel temperature TN ≈ 450 K was found to be weaker than that appearing near TR. The giant anisotropy of the magnetocaloric effect was related to the magnetic anisotropy of Fe7Se8 crystals. The one-ion model of the magnetocaloric effect has been developed and its predictions have been compared with experimental data

Magnetic anisotropy in ferromagnetic CrI 3

We use neutron scattering to show that ferromagnetic (FM) phase transition in the two-dimensional (2D)honeycomb lattice CrI3 is a weakly first order transition and controlled by spin-orbit coupling (SOC) inducedmagnetic anisotropy, instead of magnetic exchange coupling as in a conventional ferromagnet. With increasingtemperature, the magnitude of magnetic anisotropy, seen as a spin gap at the Brillouin zone center, decreasesin a power law fashion and vanishes at TC, while the in-plane and c-axis spin-wave stiffnesses associated withmagnetic exchange couplings remain robust at TC.We also compare parameter regimes where spin waves in CrI3can be described by a Heisenberg Hamiltonian with Dzyaloshinskii-Moriya interaction or a Heisenberg-KitaevHamiltonian. These results suggest that the SOC induced magnetic anisotropy plays a dominant role in stabilizingthe FM order in single layer 2D van der Waals ferromagnets