[en] Cr${}_2$Ge${}_2$Te${}_6$ compound is a quasi-two-dimensional semiconducting magnet belonging to the family of ferromagnetic layered transition metal trichalcogenides. Ferromagnetic order surviving even in the monolayer samples of these compounds renders these materials attractive for the fundamental research and for future technological applications. Such violation of Mermin-Wagner theorem is possible due to magnetic anisotropy present in these materials. Here we report an experimental study of the magnetic anisotropy in bulk single crystals of Cr${}_2$Ge${}_2$Te${}_6$ by means of electron spin resonance (ESR) and ferromagnetic resonance (FMR) using conventional and torque-detected ESR spectrometers. Measurements were carried out in a wide frequency and temperature range, and at various angles between magnetic field and sample plane. Angular and frequency dependent measurements in the ferromagnetic phase clearly show the easy-axis type anisotropy of Cr${}_2$Ge${}_2$Te${}_6$. Furthermore, simulation of the measured data based on a phenomenological approach yielded a precise value for the magnetocrystalline anisotropy energy density.

[en] The discovery of a new class of superconducting materials, ReFeAsO_1-xF_x, stirred up the scientific community. Here we report the Gd³⁺ high field ESR study of differently doped (La,Gd)FeAsO_1-xF_x compounds. In lightly Gd-doped LaFeAsO samples the SDW transition yields line-broadening at the transition temperature, the SDW transition is then suppressed upon F-doping. In the dense compound, GdFeAsO, with SDW transition around 140 K, the Gd-ESR was also studied. With 15% F-doping superconductivity appears at ∝ 21 K. The SDW and SC transitions are clearly seen in ESR and in μ SR as well. Surprisingly the reminiscence of the SDW transition of the undoped material (GdFeAsO) was identified in the doped (15% F) compound at lower temperature (∝ 80 K). This indicates the importance of the the interplay between superconductivity and magnetism in oxypnictides.

[en] We will present our recent results of high-field NMR and sub-THz ESR studies of the quantum magnet LiCuSbO_4 (LCSO) that presents an excellent model system of a one-dimensional spin-1/2 quantum magnet with frustrated exchange interactions. Such networks are predicted to exhibit a plethora of novel ground states beyond classical ferro- or antiferromagnetic phases. In LCSO the absence of a long-range magnetic order down to sub-Kelvin temperatures is suggestive of the realization of a quantum spin liquid state. Our NMR and ESR measurements in strong magnetic fields up to 16 Tesla reveal clear indications for the occurrence of an exotic field-induced hidden phase which we will discuss in terms of multipolar physics.

[en] In the Swedenborgite type compounds YBaCo_3AlO_7 and YBaCo_3FeO_7 the magnetic lattice can be described as a stacking of kagome layers, where unconventional ground states such as a spin liquid state can be expected due to the strong geometrical frustration. We performed a combined experimental study of magnetic properties of single crystals of YBaCo_3AlO_7 and YBaCo_3FeO_7 with high field ESR and high field NMR spectroscopy. The experimental results show the occurrence of short-range quasi static electron spin correlations at T"* ∼ 22 K for YBaCo_3AlO_7 and T"* ∼ 60K for YBaCo_3FeO_7 but not a long-range antiferromagnetic order. We compare our results with AC and DC susceptibility measurements and discuss a possible competition between a spin glass-like state due to intrinsic structural disorder and a spin liquid state arising from strong magnetic frustration in this materials.

[en] Topologically ordered states of matter have recently gained much attention due to their novel physical properties, the signatures of which can be experimentally probed. A prime example is the spin liquid realized in the Kitaev honeycomb lattice compass model, where fractionalization of particles leads to broad continuum-like features in the magnetic response. We will present the high-field microwave absorption results on the Mott-Hubbard-insulating material $\mathrm{\alpha <!--\; ??\; -->}$-RuCl${}_3$ which is, due to its structure and strong spin-orbit coupling, a promising candidate for the realization of Kitaev physics. Measurements on a single-crystal were conducted over a frequency range of $\mathrm{\nu <!--\; ??\; -->}$ = 70-660 GHz at temperatures ranging from 3-30 K. Strikingly, in addition to previously observed conventional gapped magnon modes, we find a highly unusual broad continuum characteristic of fractionalization which extends to energies below the lowest sharp mode and to temperatures significantly higher than the ordering temperature.

[en] Topologically ordered states of matter have recently gained much attention due to their novel physical properties, the signatures of which can be experimentally probed. A prime example is the spin liquid realized in the Kitaev honeycomb lattice compass model, where fractionalization of particles leads to broad continuum-like features in the magnetic response. We will present the high-field microwave absorption results on the Mott-Hubbard-insulating material $\mathrm{\alpha <!--\; ??\; -->}$-RuCl${}_3$ which is, due to its structure and strong spin-orbit coupling, a promising candidate for the realization of Kitaev physics. Measurements on a single-crystal were conducted over a frequency range of $\mathrm{\nu <!--\; ??\; -->}$ = 70-660 GHz at temperatures ranging from 3-30 K. Strikingly, in addition to previously observed conventional gapped magnon modes, we find a highly unusual broad continuum characteristic of fractionalization which extends to energies below the lowest sharp mode and to temperatures significantly higher than the ordering temperature.

[en] In the quest for 2D conducting materials with high ferromagnetic ordering temperature the new family of the layered Fe${}_n$GeTe${}_2$ compounds, especially the near-room-temperature ferromagnet Fe${}_4$GeTe${}_2$, receives a significant attention. Fe${}_4$GeTe${}_2$ features a peculiar spin reorientation transition at T${}_{SR}$ ≈ 110 K suggesting a non-trivial temperature evolution of the magnetic anisotropy (MA)---one of the main contributors to the stabilization of the magnetic order in the low-dimensional systems. An electron spin resonance (ESR) spectroscopic study reported here provides quantitative insights into the unusual magnetic anisotropy of Fe${}_4$GeTe${}_2$. At high temperatures the total MA is mostly given by the demagnetization effect with a small contribution of the counteracting intrinsic magnetic anisotropy of an easy-axis type, whose growth below a characteristic temperature T${}_{shape}$ ≈ 150 K renders the sample seemingly isotropic at T${}_{SR}$. Below one further temperature T${}_d$ ≈ 50 K the intrinsic MA becomes even more complex. Importantly, all the characteristic temperatures found in the ESR experiment match those observed in transport measurements, suggesting an inherent coupling between magnetic and electronic degrees of freedom in Fe${}_4$GeTe${}_2$. This finding together with the observed signatures of the intrinsic two-dimensionality should facilitate optimization routes for the use of Fe${}_4$GeTe${}_2$ in the magneto-electronic devices, potentially even in the monolayer limit. (© 2024 The Authors. Advanced Functional Materials published by Wiley‐VCH GmbH)