The recent discovery of skyrmions in Cu$_2$OSeO$_3$ has established a new platform to create and manipulate skyrmionic spin textures. We use high-field electron spin resonance (ESR) spectroscopy combining a terahertz free electron laser and pulsed magnetic fields up to 64 T to probe and quantify its microscopic spin-spin interactions. Besides providing direct access to the long-wavelength Goldstone mode, this technique probes also the high-energy part of the excitation spectrum which is inaccessible by standard low-frequency ESR. Fitting the behavior of the observed modes in magnetic field to a theoretical framework establishes experimentally that the fundamental magnetic building blocks of this skyrmionic magnet are rigid, highly entangled and weakly coupled tetrahedra.
Spin-1/2 Heisenberg antiferromagnets Cs$_2$CuCl$_4$ and Cs$_2$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 Cs$_2$CuCl$_4$, yielding $J/k_B=4.7(2)$ K, $J/k_B=1.42(7)$ K [$J/Jsimeq 0.30$] and revealing good agreement with inelastic neutron-scattering results. For the isostructural Cs$_2$CuBr$_4$, we obtain $J/k_B=14.9(7)$ K, $J/k_B=6.1(3)$ K, [$J/Jsimeq 0.41$], providing exact and conclusive information on the exchange couplings in this frustrated spin system.
We report on high-field electron spin resonance (ESR) studies of magnetic excitations in the spin-1/2 triangular-lattice antiferromagnet Cs$_2$CuBr$_4$. Frequency-field diagrams of ESR excitations are measured for different orientations of magnetic fields up to 25 T. We show that the substantial zero-field energy gap, $Deltaapprox9.5$ K, observed in the low-temperature excitation spectrum of Cs$_2$CuBr$_4$ [Zvyagin $et~al.$, Phys. Rev. Lett. 112, 077206 (2014)], is present well above $T_N$. Noticeably, the transition into the long-range magnetically ordered phase does not significantly affect the size of the gap, suggesting that even below $T_N$ the high-energy spin dynamics in Cs$_2$CuBr$_4$ is determined by short-range-order spin correlations. The experimental data are compared with results of model spin-wave-theory calculations for spin-1/2 triangle-lattice antiferromagnet.
By means of electron spin resonance investigations we revealed the crucial role of the interchain coupling in the spin dynamics of the spin-1/2 Heisenberg antiferromagnetic (AF) chain material copper-pyrazine-dinitrate, Cu(C$_4$H$_4$N$_2$)(NO$_3$)$_2$. We found that the dominating interchain interaction is of a zig-zag type. This interaction gives rise to geometrical frustration effects and strongly influences the character of AF ordering. Combining our experimental findings with the results of a quasiclassical approach we argue that at low temperatures the system orders in an incommensurate spiral state.
We report on the measurement of the magnetic susceptibility and of ESR transitions in the garnet substance Tb$_3$Ga$_5$O$_{12}$ (TGG). The results are compared with a calculation in the framework of crystal field theory for the orthorhombic surroundings of the six inequivalent Tb ions of TGG. We also present a calculation of the magnetization for the three main crystal directions.
Magnetic excitations in copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating $g$-tensor and Dzyaloshinskii-Moriya interactions that exhibits a field-induced spin gap, are probed by means of pulsed-field electron spin resonance spectroscopy. In particular, we report on a minimum of the gap in the vicinity of the saturation field $H_{sat}=48.5$ T associated with a transition from the sine-Gordon region (with soliton-breather elementary excitations) to a spin-polarized state (with magnon excitations). This interpretation is fully confirmed by the quantitative agreement over the entire field range of the experimental data with the DMRG investigation of the spin-1/2 Heisenberg chain with a staggered transverse field.
Low-energy magnetic excitations in the spin-1/2 chain compound (C$_6$H$_9$N$_2$)CuCl$_3$ [known as (6MAP)CuCl$_3$] are probed by means of tunable-frequency electron spin resonance. Two modes with asymmetric (with respect to the $h u=gmu_B B$ line) frequency-field dependences are resolved, illuminating the striking incompatibility with a simple uniform $S=frac{1}{2}$ Heisenberg chain model. The unusual ESR spectrum is explained in terms of the recently developed theory for spin-1/2 chains, suggesting the important role of next-nearest-neighbor interactions in this compound. Our conclusion is supported by model calculations for the magnetic susceptibility of (6MAP)CuCl$_3$, revealing a good qualitative agreement with experiment.
Magnetic excitations in the spin-ladder material (C$_5$H$_{12}$N)$_2$CuBr$_4$ [BPCB] are probed by high-resolution multi-frequency electron spin resonance (ESR) spectroscopy. Our experiments provide a direct evidence for a biaxial anisotropy ($sim 5%$ of the dominant exchange interaction), that is in contrast to a fully isotropic spin-ladder model employed for this system previously. It is argued that this anisotropy in BPCB is caused by spin-orbit coupling, which appears to be important for describing magnetic properties of this compound. The zero-field zone-center gap in the excitation spectrum of BPCB, $Delta_0/k_{B}=16.5$ K, is detected directly. Furthermore, an ESR signature of the inter-ladder exchange interactions is obtained. The detailed characterization of the anisotropy in BPCB completes the determination of the full spin hamiltonian of this exceptional spin-ladder material and shows ways to study anisotropy effects in spin ladders.
Results of magnetization and high-field ESR studies of the new spin-1 Haldane-chain material [Ni(C2H8N2)2NO2](BF4) (NENB) are reported. A definite signature of the Haldane state in NENB was obtained. From the analysis of the frequency-field dependence of magnetic excitations in NENB, the spin-Hamiltonian parameters were calculated, yielding Delta = 17.4 K, g_parallel = 2.14, D = 7.5 K, and |E| = 0.7 K for the Haldane gap, g factor and the crystal-field anisotropy constants, respectively. The presence of fractional S = 1/2 chain-end states, revealed by ESR and magnetization measurements, is found to be responsible for spin-glass freezing effects. In addition, extra states in the excitation spectrum of NENB have been observed in the vicinity of the Haldane gap, which origin is discussed.
