The magnetic subsystem of nabokoite, KCu7(TeO4)(SO4)5Cl, is constituted by copper ions forming a buckled square kagomé lattice decorated by quasi-isolated ions. This combination determines peculiar physical properties of this compound evidenced in electron spin resonance (ESR) spectroscopy, dielectric permittivity ɛ, magnetization M and specific heat Cp measurements. At lowering temperature, the magnetic susceptibility χ=M/H passes through a broad hump inherent for low-dimensional magnetic systems at about 150 K and a sharp peak at antiferromagnetic phase transition at TN=3.2 K. The Cp(T,H) curves demonstrate additional peak-like anomaly at Tpeak=5.7 K robust to magnetic field. The latter can be ascribed to low-lying singlet excitations filling the singlet-triplet gap in magnetic excitation spectrum of the square kagomé lattice (Richter et al., 2022). ESR spectroscopy provides indications that antiferromagnetic structure below TN is non-collinear. Separate issue is the observation of antiferroelectric-type behavior in ɛ at low temperatures, which tentatively reduces the symmetry and partially lifts frustration of magnetic interactions of decorating copper ions with buckled square kagomé lattice. These complex thermodynamic and resonant properties signal the presence of two weakly coupled magnetic subsystems in nabokoite, namely a spin-liquid in square kagomé lattice layers and an antiferromagnet represented by decorating ions.

Nabokoite family compounds ACu7(TeO4)(SO4)5Cl (A = Na, K, Cs, Rb) are candidates for the experimental realization of highly-frustrated 2D square kagomé lattice (SKL). Their magnetic subsystem includes SKL layers decorated by additional copper ions. All members of this family are characterized by quite high Curie–Weiss temperatures (∼ 80–200 K), but magnetic ordering was reported only for Na and K compounds at a much lower temperatures below 4 K. We report here results of the study of high-frequency (∼10 GHz) dielectric properties of this family of compounds. Our study revealed presence of the strong dielectric anomaly both in the real and imaginary parts of high-frequency dielectric permittivity for Na and K compounds approx. 100 and 26 K, correspondingly, presumably related to antiferroelectric ordering. Additionally, much weaker anomalies were observed at approx. 5 K indicating possible interplay of magnetic and lattice degrees of freedom. We discuss possible relation between the structure rearrangements accompanying dielectric anomalies and a delayed magnetic ordering in the nabokoite family compounds.

Nabokoite family compounds ACu7(TeO4)(SO4)5Cl (A = Na, K, Cs, Rb) are candidates for the experimental realization of highly-frustrated 2D square kagome lattice (SKL). Their magnetic subsystem includes SKL layers decorated by additional copper ions. All members of this family are characterized by quite high Curie-Weiss temperatures (similar to 80-200 K), but magnetic ordering was reported only for Na and K compounds at a much lower temperatures below 4 K. We report here results of the study of high -frequency (similar to 10 GHz) dielectric properties of this family of compounds. Our study revealed presence of the strong dielectric anomaly both in the real and imaginary parts of high -frequency dielectric permittivity for Na and K compounds approx. 100 and 26 K, correspondingly, presumably related to antiferroelectric ordering. Additionally, much weaker anomalies were observed at approx. 5 K indicating possible interplay of magnetic and lattice degrees of freedom. We discuss possible relation between the structure rearrangements accompanying dielectric anomalies and a delayed magnetic ordering in the nabokoite family compounds.

We report the detailed study of the magnetocaloric effect (MCE) in a dipolar-Heisenberg magnet LiGdF4 using magnetization measurements performed on a single crystal sample. Entropy variation on isothermal demagnetization from the magnetic field up to 3 T is determined in the temperature range 2–10 K for two principal directions of the applied field (parallel and perpendicular to the tetragonal c-axis of the crystal). The MCE is found to be highly anisotropic, with the cooling efficiency being up to twice higher at H∥c. The results are nicely interpreted in the frame of a conventional molecular field approach taking into account considerable anisotropy of the paramagnetic Curie–Weiss temperature. These results are compared to earlier studies of MCE in powder samples of LiGdF4 (Numazawa et al., 2006) as well as with analogous data for other well known magnetocaloric materials. Our findings may open new possibilities to enhance the efficiency of magnetic refrigeration in the liquid helium-4 temperature range.

The magnetic subsystem of nabokoite, KCu7 (TeO4(SO4)(5)Cl, is constituted by copper ions forming a buckled square kagome lattice decorated by quasi-isolated ions. This combination determines peculiar physical properties of this compound evidenced in electron spin resonance (ESR) spectroscopy, dielectric permittivity epsilon, magnetization M and specific heat C p measurements. At lowering temperature, the magnetic susceptibility x = M / H passes through a broad hump inherent for low-dimensional magnetic systems at about 150 K and a sharp peak at antiferromagnetic phase transition at T-N = 3 . 2 K. The C p (T, H curves demonstrate additional peak-like anomaly at T-peak = 5 . 7 K robust to magnetic field. The latter can be ascribed to low-lying singlet excitations filling the singlet-triplet gap in magnetic excitation spectrum of the square kagome lattice (Richter et al., 2022). ESR spectroscopy provides indications that antiferromagnetic structure below T-N is non-collinear. Separate issue is the observation of antiferroelectric-type behavior in epsilon at low temperatures, which tentatively reduces the symmetry and partially lifts frustration of magnetic interactions of decorating copper ions with buckled square kagome lattice. These complex thermodynamic and resonant properties signal the presence of two weakly coupled magnetic subsystems in nabokoite, namely a spin-liquid in square kagome lattice layers and an antiferromagnet represented by decorating ions.

ortho-Pyrovanadate (or ortho-diorthovanadate) K2Mn23+Mn2+O(OH)(VO4)(V2O7) synthesized hydrothermally crystallizes in the orthorhombic space group Pnma with a = 17.9155(5), b = 5.8940(2), c = 10.9971(3) Å, V = 1161.23(6) Å3, and Z = 4. Its crystal structure features linear chains of edge-sharing Mn3+O6 octahedra with every second pair of Mn3+O6 octahedra condensed with a Mn2+O6 octahedron on one side of a chain in a sawtooth pattern so that each sawtooth chain consists of a triangular trimer. These sawtooth chains, running parallel to the b axis and linked by the VO4 and V2O7 groups, form a framework with channels populated by K atoms. The new compound is a structural analogue of the mineral zoisite Ca2Al3O(OH)(SiO4)(Si2O7), showing a striking example of very different chemical compositions. K2Mn3O(OH)(VO4)(V2O7) undergoes a phase transition into an ordered antiferromagnetic (AFM) state at TN = 14.4 K, which was detected by high-frequency electron spin resonance as well as by both specific heat Cp and Fisher's specific heat d(χT)/dT measurements. However, this phase transition was not detected by magnetic susceptibility measurements. The origin of this puzzling observation was resolved by evaluating the spin exchanges of K2Mn3O(OH)(VO4)(V2O7), which revealed that each triangular trimer is a ferromagnetically coupled cluster, and the observed ordering involves an AFM ordering between the ferromagnetic (FM) clusters. This ordering is shrouded in magnetic susceptibility measurements due to the susceptibility contributions from the individual FM triangular trimers even below TN. We showed that the magnetic susceptibility of K2Mn3O(OH)(VO4)(V2O7) between ∼30 K and room temperature is satisfactorily described by an AFM chain made up of ferromagnetically coupled triangular clusters, as described by a few spin-exchange parameters.

We report results of a multi-frequency (0.8–60 GHz) electron spin resonance study of the spin dynamics in the quasi-2D square lattice antiferromagnet Ba 2 MnGe 2 O 7 both in antiferromagnetically ordered and paramagnetic phases. We directly observe two zero-field gaps in the excitation spectrum of the ordered phase, the larger one being due to easy-plane anisotropy, and the smaller one indicates the presence of fourth-order in-plane anisotropy probably related to the multiferroic properties of this compound. We observe effects of hyperfine interaction on the electron spin resonance spectra in the antiferromagnetically ordered state, which turns out to be comparable with in-plane anisotropy. The hyperfine field strength is found from the observed low-temperature electron spin resonance data. The spin dynamics of the paramagnetic phase is characterized by strong broadening of the ESR absorption line, which can be ascribed to the vortex dynamics of a 2D magnet.

Nabokoite family compounds ACu_7(TeO_4)(SO_4)_5Cl (A=Na, K, Cs, Rb) are candidates for the experimental realization of highly-frustrated 2D square kagome lattice (SKL). Their magnetic subsystem includes SKL layers decorated by additional copper ions. All members of this family are characterized by quite high Curie-Weiss temperatures (∼ 80-200 K), but magnetic ordering was reported only for Na and K compounds at a much lower temperatures below 4 K. We report here results of the study of high-frequency (∼ 10 GHz) dielectric properties of this family of compounds. Our study revealed presence of the strong dielectric anomaly both in the real and imaginary parts of high-frequency dielectric permittivity for Na and K compounds approx. 100 and 26 K, correspondingly, presumably related to antiferroelectric ordering. Additionally, much weaker anomalies were observed at approximately 5K indicating possible interplay of magnetic and lattice degrees of freedom. We discuss possible relation between the structure rearrangements accompanying dielectric anomalies and a delayed magnetic ordering in the nabokoite family compounds.

The magnetic subsystem of nabokoite, KCu_7(TeO_4)(SO_4)_5Cl, is constituted by copper ions forming a buckled square kagomé lattice decorated by quasi-isolated ions. This combination determines peculiar physical properties of this compound evidenced in electron spin resonance (ESR) spectroscopy, dielectric permittivity ε, magnetization M and specific heat C_p measurements. At lowering temperature, the magnetic susceptibility χ = M/H passes through a broad hump inherent for low-dimensional magnetic systems at about 150 K and a sharp peak at antiferromagnetic phase transition at T_N = 3.2K. The C_p(T,H) curves demonstrate additional peak-like anomaly at T_peak= 5.7K robust to magnetic field. The latter can be ascribed to low-lying singlet excitations filling the singlet-triplet gap in magnetic excitation spectrum of the square kagomé lattice [J.Richter, O.Derzhko and J.Schnack, Phys. Rev. B 105 (2022) 144427]. ESR spectroscopy provides indications that antiferromagnetic structure below T_N is non-collinear. Separate issue is the observation of antiferroelectric-type behavior in ε at low temperatures, which tentatively reduces the symmetry and partially lifts frustration of magnetic interactions of decorating copper ions with buckled square kagomé lattice. These complex thermodynamic and resonant properties signal the presence of two weakly coupled magnetic subsystems in nabokoite, namely a spin-liquid in square kagomé lattice layers and an antiferromagnet represented by decorating ions.

The magnetic subsystem of nabokoite, KCu$_7$(TeO$_4$)(SO$_4$)$_5$Cl, is constituted by copper ions forming a buckled square kagom\'e lattice decorated by quasi-isolated ions. This combination determines peculiar physical properties of this compound evidenced in electron spin resonance (ESR) spectroscopy, dielectric permittivity $\varepsilon$, magnetization $M$ and specific heat $C_p$ measurements. At lowering temperature, the magnetic susceptibility $\chi = M/H$ passes through a broad hump inherent for low-dimensional magnetic systems at about 150 K and a sharp peak at antiferromagnetic phase transition at $T_N = 3.2 $K. The $C_p(T,H)$ curves demonstrate additional peak-like anomaly at $T_{peak}= 5.7$K robust to magnetic field. The latter can be ascribed to low-lying singlet excitations filling the singlet-triplet gap in magnetic excitation spectrum of the square kagom\'e lattice [J.Richter, O.Derzhko and J.Schnack, Phys. Rev. B \textbf{105} (2022) 144427]. ESR spectroscopy provides indications that antiferromagnetic structure below $T_N$ is non-collinear. Separate issue is the observation of antiferroelectric-type behavior in $\varepsilon$ at low temperatures, which tentatively reduces the symmetry and partially lifts frustration of magnetic interactions of decorating copper ions with buckled square kagom\'e lattice. These complex thermodynamic and resonant properties signal the presence of two weakly coupled magnetic subsystems in nabokoite, namely a spin-liquid in square kagom\'e lattice layers and an antiferromagnet represented by decorating ions.

Low-temperature electron paramagnetic resonance measurements are performed on single crystals of LiY1 ‒xGdxF4 with low x = 0.005 and moderate x = 0.05 concentration of Gd ions. Modeling of the experimental spectra allows us to precisely determine microscopic parameters of the spin Hamiltonian of the parent LiGdF4 material, including the nearest-neighbor exchange constant. The obtained parameters are further tested by comparing a strongly anisotropic Curie–Weiss temperature obtained for LiGdF4 in our static magnetization measurements with theoretically computed values. We find a fine balance between principal magnetic interactions in LiGdF4, which results in a hidden magnetic frustration presumably leading to a delayed magnetic ordering and an enhanced magnetocaloric effect at low temperatures.

Quasi-one-dimensional magnet NiCl2·4SC(NH2)2 denoted as DTN remains disordered in zero magnetic field down to T = 0: the Sz = 0 ground state is separated from Sz =±1 excitations by a gap caused by strong single-ion easy-plane anisotropy acting on the Ni2+ ions. When a magnetic field is applied along the principal axis of anisotropy, the gap closes in a field above Bc1 = 2.18 T and the field-induced antiferromagnetic order arises. There are two excitation branches in this field-induced phase, one of which should be the Goldstone mode. Recent studies of the excitation spectrum in the field-induced ordered phase of the DTN magnet (T. Soldatov et al., Phys. Rev. B 101, 104410 (2020)) have revealed that the Goldstone mode acquires a gap in the excitation spectrum of the field-induced phase at a small deviation of the applied magnetic field from the tetragonal axis of the crystal. In this work, a simple description of both magnetic resonance branches in the ordered phase of a quasi-one-dimensional quantum S = 1 magnet with strong single-ion anisotropy is proposed. This approach is based on a combination of an effective strong coupling model for an anisotropic spin chain and the classical antiferromagnetic resonance theory. This description reproduces the experimental results semi-quantitatively without additional parameters.

Low-temperature magnetic resonance study of the quasi-two-dimensional antiferromagnet Cu(en)(H$_2$O)$_2$SO$_4$ (en = C$_2$H$_8$N$_2$) was performed down to 0.45~K. This compound orders antiferromagnetically at 0.9K. The analysis of the resonance data within the hydrodynamic approach allowed to identify anisotropy axes and to estimate the anisotropy parameters for the antiferromagnetic phase. Dipolar spin-spin coupling turns out to be the main contribution to the anisotropy of the antiferromagnetic phase. The splitting of the resonance modes and its non-monotonous dependency on the applied frequency was observed below 0.6K in all three field orientations. Several models were discussed to explain the origin of the nontrivial splitting and the existence of inequivalent magnetic subsystems in Cu(en)(H$_2$O)$_2$SO$_4$ was chosen as the most probable source.

We report low temperature electron spin resonance experimental and theoretical studies of an archetype S = 1/2 strong-rung spin ladder material (C5H12N)(2)CuBr4. Unexpected dynamics is detected deep in the Tomonaga-Luttinger spin liquid regime. Close to the point where the system is half-magnetized (and believed to be equivalent to a gapless easy plane chain in zero field) we observed orientation-dependent spin gap and anomalous g-factor values. Field theoretical analysis demonstrates that the observed low-energy excitation modes in magnetized (C5H12N)(2)CuBr4 are solitonic excitations caused by Dzyaloshinskii-Moriya interaction presence.

In some magnets, despite the presence of a strong exchange interaction between magnetic ions, conventional magnetic ordering does not occur, but a collective paramagnetic state is formed. If, due to the particular architecture of the exchange bonds, the ground state turns out to be a singlet state and is separated from the triplet excited states by a gap in the energy spectrum, then this state persists down to T = 0. The spin dynamics of collective paramagnets with gap excitation spectrum (spin-gap magnets) at low temperatures can be described as the behavior of a dilute gas of triplet excitations. The application of a sufficiently strong magnetic field can close the gap in the spectrum, which leads to a gapless spin liquid state, or even to the unusual phenomenon of the formation of field-induced antiferromagnetism. The introduction of impurities into a spin-gap magnet leads to the formation of a paramagnetic center in the vicinity of a defect or exchange bonds randomly distributed in the lattice. This review presents the results of the study of several characteristic representatives of the class of quantum paramagnets with gapped excitation spectrum by the EPR spectroscopy method: a quasi-two-dimensional antiferromagnet (C 4 H 12 N 2 )Cu 2 Cl 6 and quasi-one-dimensional magnets of the spin tube, Cu 2 C l4 ⋅ H 8 C 4 SO 2 , and the spin ladder, (C 7 H 10 N) 2 CuBr 4 , types. It has been shown that the electron spin resonance spectra make it possible to find common features in the behavior of these systems: to detect and characterize the fine structure of the energy levels of triplet excitations, to detect multiparticle relaxation processes in a gas of triplet excitations, and to observe the excitation of spin waves in the field-induced antiferromagnetically ordered state. Individual features of different systems are revealed as well.

We discuss magnetization curves of a toy-model trigonal and tetrahedral clusters. Nonlinearity of magnetization with local minimum of differential susceptibility resembling known magnetization plateaus of triangular-lattice and pyrochlore lattice antiferromagnets is observed at intermediate temperature range J ≲ T ≲ Θ (here, J is the exchange coupling constant and Θ is a Curie–Weiss temperature). This behavior is due to increased statistical weight of the states with intermediate total spin of the cluster, which is related to the “order-by-disorder” mechanism of plateau stabilization of a macroscopic frustrated magnet.

V. N. Glazkov, 2, ∗ Yu. V. Krasnikova, I. K. Rodygina, 3 J. Chovan, 5 R. Tarasenko, and A. Orendáčová P. L. Kapitza Institute for Physical Problems RAS, Kosygin str. 2, 119334 Moscow, Russia International Laboratory for Condensed Matter Physics, National research university “Higher School of Economics”, Myasnitskaya str. 20, 101000 Moscow, Russia Faculty of Physics, National research university “Higher School of Economics”, Myasnitskaya str. 20, 101000 Moscow, Russia IT4Innovations National Supercomputing Center, VSB-Technical University of Ostrava, 17. listopadu 2172/15, CZ 708 33 Ostrava, Czech Republic International Clinical Research Center, St. Anne’s University Hospital, Pekarska 53, 656 91 Brno, Czech Republic Institute of Physics, P. J. Šafárik University, Park Angelinum 9, 040 00 Košice, Slovakia (Dated: July 26, 2019)

We have studied electron spin resonance (ESR) absorption spectra for the nonmagnetically diluted strong-leg spin ladder magnet (C7H10N)(2)Cu(1-x)ZnxBr4 (abbreviated as DIMPY) down to 450 mK. Formation of the clusters with nonzero net magnetization is confirmed; the cluster-cluster interaction is evidenced by the concentration dependence of ESR absorption. High-temperature spin-relaxation time was found to increase with nonmagnetic dilution. The ESR linewidth analysis proves that the Dzyaloshinskii-Moriya (DM) interaction remains the dominant spin-relaxation channel in diluted DIMPY. Experimental data indicate that the dilution results in the weakening of the effective DM interaction, which can be interpreted as total suppression of DM interaction in the close vicinity of impurity atom.

Recently found quasi-two dimensional metalloorganic compound (C4H12N2)(Cu2Cl6) (abbreviated PHCC) is an example of a spin-gap magnet. Its ground state is a nonmagnetic singlet separated from the triplet excitations by an energy gap of approximately 1 meV. This compound allows partial substitution of chlorine ions by bromine, which results in the modulation of the affected exchange bonds. We have found by means of electron spin resonance spectroscopy that this doping results in the formation of the gapless S = 1 paramagnetic centers. These centers can be interpreted as triplet excitations trapped in a potential well created by doping.