Spin-$1/2$ Heisenberg antiferromagnets ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ and ${\mathrm{Cs}}_2{\mathrm{CuBr}}_4$ with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The viability of the proposed method was proven by applying it to ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$, yielding $J/k_B=4.7(2)\mathrm{K}$, $J^{\prime }/k_B=1.42(7)\mathrm{K}$, [$J^{\prime }/J\simeq 0.30$] and revealing good agreement with inelastic neutron-scattering results. For the isostructural ${\mathrm{Cs}}_2{\mathrm{CuBr}}_4$, we obtain $J/k_B=14.9(7)\mathrm{K}$, $J^{\prime }/k_B=6.1(3)\mathrm{K}$, [$J^{\prime }/J\simeq 0.41$], providing exact and conclusive information on the exchange couplings in this frustrated spin system.

We report on spectroscopy study of elementary magnetic excitations in an Ising-like antiferromagnetic chain compound ${\mathrm{SrCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ as a function of temperature and applied transverse magnetic field up to 25 T. An optical as well as an acoustic branch of confined spinons, the elementary excitations at zero field, are identified in the antiferromagnetic phase below the Néel temperature of 5 K and described by a one-dimensional Schrödinger equation. The confinement can be suppressed by an applied transverse field and a quantum disordered phase is induced at 7 T. In this disordered paramagnetic phase, we observe three emergent fermionic excitations with different transverse-field dependencies. The nature of these modes is clarified by studying spin dynamic structure factor of a 1D transverse-field Heisenberg-Ising $\left(XXZ\right)$ model using the method of infinite time evolving block decimation. Our work reveals emergent quantum phenomena and provides a concrete system for testifying theoretical predications of one-dimension quantum spin models.

The successful use of picosecond-pulse free-electron-laser (FEL) radiation for the continuous-wave terahertz-range electron spin resonance (ESR) spectroscopy has been demonstrated. The combination of two linac-based FELs (covering the wavelength range of $4–250\mu \text{m}$ ) with pulsed magnetic fields up to 70 T allows for multifrequency ESR spectroscopy in a frequency range of 1.2–75 THz with a spectral resolution better than 1%. The performance of the spectrometer is illustrated with ESR spectra obtained in the 2,2-diphenyl-1-picrylhydrazyl and the low-dimensional organic material $\left({\text{C}}_6{\text{H}}_9{\text{N}}_2\right){\text{CuCl}}_3$ .

We report on an investigation of optical properties of multiferroic CoCr${}_2$O${}_4$ at terahertz frequencies in magnetic fields up to 30 T. Below the ferrimagnetic transition (94 K), the terahertz response of CoCr${}_2$O${}_4$ is dominated by a magnon mode, which shows a steep magnetic-field dependence. We ascribe this mode to an exchange resonance between two magnetic sublattices with different $g$ factors. In the framework of a simple two-sublattice model (the sublattices are formed by Co${}^{2+}$ and Cr${}^{3+}$ ions), we find the inter-sublattice coupling constant, $\lambda =-(18\pm 1)$ K, and trace the magnetization for each sublattice as a function of field. We show that the Curie temperature of the Cr${}^{3+}$ sublattice, ${\Theta }_2$ $=$ $(49\pm 2)$ K, coincides with the temperature range, where anomalies of the dielectric and magnetic properties of CoCr${}_2$O${}_4$ have been reported in literature.

Spin excitations of magnetoelectric ${\mathrm{LiNiPO}}_4$ are studied by infrared absorption spectroscopy in the THz spectral range as a function of magnetic field through various commensurate and incommensurate magnetically ordered phases up to 33 T. Six spin resonances and a strong two-magnon continuum are observed in zero magnetic field. Our systematic polarization study reveals that some of the excitations are usual magnetic-dipole active magnon modes, while others are either electromagnons, being only electric-dipole active, or magnetoelectric, that is both electric- and magnetic-dipole active spin excitations. Field-induced shifts of the modes for all three orientations of the field along the orthorhombic axes allow us to refine the values of the relevant exchange couplings, single-ion anisotropies, and the Dzyaloshinskii-Moriya interaction on the level of a four-sublattice mean-field spin model. This model also reproduces the spectral shape of the two-magnon absorption continuum, found to be electric-dipole active in the experiment.