arXiv subjects

V. Kataev

Publications and source records attributed to V. Kataev.

At least 37 records · Page 2Linked to original sources

arXiv2020

Electron spin resonance and ferromagnetic resonance spectroscopy in the high-field phase of the van der Waals magnet CrCl$_3$

We report a comprehensive high-field/high-frequency electron spin resonance (ESR) study on single crystals of the van der Waals magnet CrCl$_3$. This material, although being known for quite a while, has received recent significant attention in a context of the use of van der Waals magnets in novel spintronic devices. Temperature-dependent measurements of the resonance fields were performed between 4 and 175 K and with the external magnetic field applied parallel and perpendicular to the honeycomb planes of the crystal structure. These investigations reveal that the resonance line shifts from the paramagnetic resonance position already at temperatures well above the transition into a magnetically ordered state. Thereby the existence of ferromagnetic short-range correlations above the transition is established and the intrinsically two-dimensional nature of the magnetism in the title compound is proven. To study details of the magnetic anisotropies in the field-induced effectively ferromagnetic state at low temperatures, frequency-dependent ferromagnetic resonance (FMR) measurements were conducted at 4 K. The observed anisotropy between the two magnetic-field orientations is analyzed by means of numerical simulations based on a phenomenological theory of FMR. These simulations are in excellent agreement with measured data if the shape anisotropy of the studied crystal is taken into account, while the magnetocrystalline anisotropy is found to be negligible in CrCl$_3$. The absence of a significant intrinsic anisotropy thus renders this material as a practically ideal isotropic Heisenberg magnet.

cond-mat.mtrl-sci↗

arXiv2020

Ground state and low-temperature magnetism of the quasi-two-dimensional honeycomb compound InCu$_{2/3}$V$_{1/3}$O$_3$

We report a combined $^{115}$In NQR, $^{51}$V NMR and $μ$SR spectroscopic study of the low-temperature magnetic properties of InCu$_{2/3}$V$_{1/3}$O$_3$, a quasi-two dimensional (2D) compound comprising in the spin sector a honeycomb lattice of antiferromagnetically coupled spins $S=1/2$ associated with Cu$^{2+}$ ions. Despite substantial experimental and theoretical efforts, the ground state of this material was has not been ultimately identified. In particular, two characteristic temperatures of about $\sim 40$ K and $\sim 20$ K manifesting themselves as anomalies in different magnetic measurements are discussed controversially. A combined analysis of the experimental data complemented with theoretical calculations of exchange constants enabled us to identify below 39 K an ``intermediate'' quasi-2D static spin state. This spin state is characterized by a staggered magnetization with a temperature evolution that agrees with the predictions for the 2D XY model. We observe that this state gradually transforms at 15 K into a fully developed 3D antiferromagnetic Néel state. We ascribe such an extended quasi-2D static regime to an effective magnetic decoupling of the honeycomb planes due to a strong frustration of the interlayer exchange interactions which inhibits long-range spin-spin correlations across the planes. Interestingly, we find indications of the topological Berezinsky-Kosterlitz-Thouless transition in the quasi-2D static state of the honeycomb spin-1/2 planes of InCu$_{2/3}$V$_{1/3}$O$_3$.

cond-mat.str-el↗

arXiv2019

Coherent spin dynamics of solitons in the organic spin chain compounds ($o$-DMTTF)$_2X$ ($X$ = Cl, Br)

We studied the magnetic properties, in particular dynamics, of the correlated spins associated with natural defects in the organic spin chain compounds ($o$-DMTTF)$_2X$ ($X$ = Br, Cl) by means of electron spin resonance (ESR) spectroscopy. Both materials exhibit spin-Peierls transitions at temperatures around 50 K [P. Foury-Leylekian et al. Phys. Rev. B 84, 195134 (2011)], which allow a separation of the properties of defects inside the chains from the magnetic response of the spin chains. Indeed, continuous wave ESR measurements performed over a wide temperature range evidence the evolution of the spin dynamics from being governed by the spins in the chains at elevated temperatures to a low-temperature regime which is dominated by defects within the spin-dimerized chains. Such defects polarize the antiferromagnetically coupled spins in their vicinity, thereby leading to a finite local alternating magnetization around the defect site which can be described in terms of a soliton, i.e. a spin 1/2 quasiparticle built of many correlated spins, pinned to the defect. In addition, contributions of triplon excitations of the spin-dimerized state to the ESR response below the transition temperature were observed which provides a spectroscopic estimate for the spin-gap of the studied systems. Moreover, details of spin dynamics deep in the spin-Peierls phase were investigated by pulse ESR experiments which revealed Rabi-oscillations as signatures of coherent spin dynamics. From a comparison of the characteristic damping times of the Rabi oscillations with measurements of the spin relaxation times by means of primary-echo decay and CPMG methods it becomes evident that inhomogeneities in local magnetic fields strongly contribute to the soliton decoherence.

cond-mat.str-el↗

arXiv2019

Superconducting spin-valve effect in heterostructures with ferromagnetic Heusler alloy layers

We report a comparative analysis and theoretical description of the superconducting properties of two spin-valve-valve structures containing the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al$_{y}$ as one of two ferromagnetic (F1 or F2) layers of the F1/F2/S structure, where S stands for the superconducting Pb layer. In our experiments we used the Heusler alloy layer in two roles: as a weak ferromagnet on the place of the F2 layer and as a half-metal on the place of the F1 layer. In the first case, we obtained a large ordinary superconducting spin-valve effect $ΔT_c$ assisted by the triplet superconducting spin-valve effect $ΔT_c^{trip}$. In the second case, we observed a giant magnitude of $ΔT_c^{trip}$ reaching 0.5 K. An underlying theory based on the solution of the Usadel equations using Kupriyanov-Lukichev boundary conditions with arbitrary material parameters for all layers and arbitrary boundary parameters for all interfaces is presented in Appendix. We find a good agreement between our experimental data and theoretical results.

cond-mat.supr-con↗

arXiv2019

Superconducting switching due to triplet component in the Pb/Cu/Ni/Cu/Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$ spin-valve structure

We report the superconducting properties of the Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$/Cu/Ni/Cu/Pb spin-valve structure which magnetic part comprises the Heusler alloy layer HA = Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$ with a high degree of spin polarization (DSP) of the conduction band and the Ni layer of variable thickness. We obtained that the separation between the superconducting transition curves measured for the parallel ($α= 0^\circ$) and perpendicular ($α= 90^\circ$) orientation of the magnetizations of the HA and Ni layers reaches up to 0.5 K ($α$ is an angle between the magnetization of two ferromagnetic layers). For all studied samples the dependence of the superconducting transition temperature $T_c$ on $α$ demonstrates a deep minimum in the vicinity of the perpendicular configuration of magnetizations. This suggests that the observed minimum and the corresponding full switching effect of the spin valve is caused by the long-range triplet component of the superconducting condensate in the multilayer. Such a large effect can be attributed to a half-metallic nature of the HA layer which in the orthogonal configuration efficiently draws off the spin-polarized Cooper pairs from the space between the HA and Ni layers. Our results indicate a significant potential of the concept of the superconducting spin-valve multilayer comprising a half-metallic ferromagnet recently proposed by A. Singh et al., Phys. Rev. X 5, 021019 (2015) in achieving large values of the switching effect.

cond-mat.supr-con↗

arXiv2019

Magnetic anisotropy and spin-polarized two-dimensional electron gas in the van der Waals ferromagnet Cr$_2$Ge$_2$Te$_6$

We report a comprehensive experimental investigation on the magnetic anisotropy in bulk single crystals of Cr$_2$Ge$_2$Te$_6$, a quasi-two-dimensional ferromagnet belonging to the family of magnetic layered transition metal trichalcogenides that have attracted recently a big deal of interest with regard to the fundamental and applied aspects of two-dimensional magnetism. For this purpose electron spin resonance (ESR) and ferromagnetic resonance (FMR) measurements have been carried out over a wide frequency and temperature range. A gradual change in the angular dependence of the ESR linewidth at temperatures above the ferromagnetic transition temperature $T_{\rm c}$ reveals the development of two-dimensional spin correlations in the vicinity of $T_{\rm c}$ thereby proving the intrinsically low-dimensional character of spin dynamics in Cr$_2$Ge$_2$Te$_6$. Angular and frequency dependent measurements in the ferromagnetic phase clearly show an easy-axis type anisotropy of this compound. Furthermore, these experiments are compared with simulations based on a phenomenological approach, which takes into account results of static magnetization measurements as well as high temperature $g$ factors obtained from ESR spectroscopy in the paramagnetic phase. As a result the determined magnetocrystalline anisotropy energy density (MAE) $K_U$ is $(0.48 \pm 0.02) \times10^6$ erg/cm$^3$. This analysis is complemented by density functional calculations which yield the experimental MAE value for a particular value of the electronic correlation strength $U$. The analysis of the electronic structure reveals that the low-lying conduction band carries almost completely spin-polarized, quasi-homogeneous, two-dimensional states.

cond-mat.str-el↗

arXiv2019

Magnetic interactions and spin dynamics in the bond-disordered pyrochlore fluoride NaCaCo$_2$F$_7$

We report high-frequency/high-field electron spin resonance (ESR) and high-field magnetization studies on single crystals of the bond-disordered pyrochlore NaCaCo$_2$F$_7$. Frequency- and temperature-dependent ESR investigations above the freezing temperature $T_f \sim 2.4$ K reveal the coexistence of two distinct magnetic phases. A cooperative paramagnetic phase, evidenced by a gapless excitation mode, is found as well as a spin-glass phase developing below 20 K which is associated with a gapped low-energy excitation. Effective $g$-factors close to 2 are obtained for both modes in line with pulsed high-field magnetization measurements which show an unsaturated isotropic behavior up to 58 T at 2 K. In order to describe the field-dependent magnetization in high magnetic fields, we propose an empirical model accounting for highly anisotropic ionic $g$-tensors expected for this material and taking into account the strongly competing interactions between the spins which lead to a frustrated ground state. As a detailed quantitative relation between effective $g$-factors as determined from ESR and the local $g$-tensors obtained by neutron scattering [Ross et al., Phys. Rev. B 93, 014433 (2016)] is still sought after, our work motivates further theoretical investigations of the low-energy excitations in bond-disordered pyrochlores.

cond-mat.str-el↗

arXiv2018

Signatures of low-energy fractionalized excitations in $α$-RuCl$_3$ from field-dependent microwave absorption

Topologically ordered states of matter are generically characterized by excitations with quantum number fractionalization. A prime example is the spin liquid realized in Kitaev's honeycomb-lattice compass model where spin-flip excitations fractionalize into Majorana fermions and Ising gauge fluxes. While numerous compounds have been proposed to be proximate to such a spin-liquid phase, clear-cut evidence for fractionalized excitations is lacking. Here we employ microwave absorption measurements to study the low-energy excitations in $α$-RuCl$_3$ over a wide range of frequencies, magnetic fields, and temperatures, covering in particular the vicinity of the field-driven quantum phase transition where long-range magnetic order disappears. In addition to conventional gapped magnon modes we find a highly unusual broad continuum characteristic of fractionalization which -- most remarkably -- extends to energies below the lowest sharp mode and to temperatures significantly higher than the ordering temperature, and develops a gap of a nontrivial origin in strong magnetic fields. Our results unravel the signatures of fractionalized excitations in $α$-RuCl$_3$ and pave the way to a more complete understanding of the Kitaev spin liquid and its instabilities.

cond-mat.str-el↗

arXiv2018

Increasing the performance of the superconducting spin valve using a Heusler alloy

We have studied superconducting properties of the spin-valve thin layer heterostructures CoO$_x$/F1/Cu/F2/Cu/Pb where the ferromagnetic F1 layer was standardly made of Permalloy whereas for the F2 layer we have taken a specially prepared film of the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al with a small degree of spin polarization of the conduction band. The heterostructures demonstrate a significant superconducting spin-valve effect, i.e. a complete switching on and off of the superconducting current flowing through the system by manipulating the mutual orientations of the magnetization of the F1 and F2 layers. The magnitude of the effect is doubled in comparison with the previously studied analogous multilayers with the F2 layer made of the strong ferromagnet Fe. Theoretical analysis shows that a drastic enhancement of the switching effect is due to a smaller exchange field in the heterostructure coming from the Heusler film as compared to Fe. This enables to approach almost ideal theoretical magnitude of the switching in the Heusler-based multilayer with the F2 layer thickness of $\sim 1$\,nm.

cond-mat.supr-con↗

arXiv2018

Unraveling the nature of magnetism of the 5$\boldsymbol{d^4}$ double perovskite Ba$_2$YIrO$_6$

We report electron spin resonance (ESR) spectroscopy results on the double perovskite Ba$_2$YIrO$_6$. On general grounds, this material is expected to be nonmagnetic due to the strong coupling of the spin and orbital momenta of Ir$^{5+}$ (5$d^4$) ions. However, controversial experimental reports on either strong antiferromagnetism with static order at low temperatures or just a weakly paramagnetic behavior have triggered a discussion on the breakdown of the generally accepted scenario of the strongly spin-orbit coupled ground states in the 5$d^4$ iridates and the emergence of a novel exotic magnetic state. Our data evidence that the magnetism of the studied material is solely due to a few percent of Ir$^{4+}$ and Ir$^{6+}$ magnetic defects while the regular Ir$^{5+}$ sites remain nonmagnetic. Remarkably, the defect Ir$^{6+}$ species manifest magnetic correlations in the ESR spectra at $T\lesssim 20$ K suggesting a long-range character of superexchange in the double prevoskites as proposed by recent theories.

cond-mat.str-el↗

arXiv2017

Magnetic properties of the spin-1 chain compound NiCl$_3$C$_6$H$_5$CH$_2$CH$_2$NH$_3$

We report experimental results of the static magnetization, ESR and NMR spectroscopic measurements of the Ni-hybrid compound NiCl$_3$C$_6$H$_5$CH$_2$CH$_2$NH$_3$. In this material NiCl$_3$ octahedra are structurally arranged in chains along the crystallographic $a$-axis. According to the static susceptibility and ESR data Ni$^{2+}$ spins $S = 1$ are isotropic and are coupled antiferromagnetically (AFM) along the chain with the exchange constant $J = 25.5$ K. These are important prerequisites for the realization of the so-called Haldane spin-1 chain with the spin-singlet ground state and a quantum spin gap. However, experimental results evidence AFM order at $T_{\rm N} \approx 10$ K presumably due to small interchain couplings. Interestingly, frequency-, magnetic field-, and temperature-dependent ESR measurements, as well as the NMR data, reveal signatures which could presumably indicate an inhomogeneous ground state of co-existent mesoscopically spatially separated AFM ordered and spin-singlet state regions similar to the situation observed before in some spin-diluted Haldane magnets.

cond-mat.str-el↗

arXiv2017

Anisotropic magnetic interactions and spin dynamics in the spin-chain compound Cu(py)$_2$Br$_2$: An experimental and theoretical study

We compare theoretical results for electron spin resonance (ESR) properties of the Heisenberg-Ising Hamiltonian with ESR experiments on the quasi-one-dimensional magnet Cu(py)$_2$Br$_2$ (CPB). Our measurements were performed over a wide frequency and temperature range giving insight into spin dynamics, spin structure, and magnetic anisotropy of this compound. By analyzing the angular dependence of ESR parameters (resonance shift and linewidth) at room temperature we show that the two weakly coupled inequivalent spin chain types inside the compound are well described by Heisenberg-Ising chains with their magnetic anisotropy axes perpendicular to the chain direction and almost perpendicular to each other. We further determine the full $g$-tensor from these data. In addition, the angular dependence of the linewidth at high temperatures gives us access to the exponent of the algebraic decay of a dynamical correlation function of the isotropic Heisenberg chain. From the temperature dependence of static susceptibilities we extract the strength of the exchange coupling ($J/k_B = 52.0\,\text{K}$) and the anisotropy parameter ($δ\approx -0.02$) of the model Hamiltonian. An independent compatible value of $δ$ is obtained by comparing the exact prediction for the resonance shift at low temperatures with high-frequency ESR data recorded at $4\,\text{K}$. The spin structure in the ordered state implied by the two (almost) perpendicular anisotropy axes is in accordance with the propagation vector determined from neutron scattering experiments. In addition to undoped samples we study the impact of partial substitution of Br by Cl ions on spin dynamics. From the dependence of the ESR linewidth on doping level we infer an effective decoupling of the anisotropic component $Jδ$ from the isotropic exchange $J$ in these systems.

cond-mat.str-el↗

arXiv2016

Evidence for a magnetic field-induced unconventional nematic state in the frustrated and anisotropic spin-chain cuprate LiCuSbO$_4$

Modern theories of quantum magnetism predict exotic multipolar states in weakly interacting strongly frustrated spin-1/2 Heisenberg chains with ferromagnetic nearest neighbor (NN) inchain exchange in high magnetic fields. Experimentally these states remained elusive so far. Here we report the evidence for a long-sought magnetic field-induced nematic state arising above a field of $\sim 13$ T in the edge-sharing chain cuprate LiSbCuO$_4$ $\equiv$ LiCuSbO$_4$. This interpretation is based on the observation of a field induced spin-gap in the measurements of the $^7$Li NMR spin relaxation rate $T_1^{-1}$ as well as a contrasting field-dependent power-law behavior of $T_1^{-1}$ vs. $T$ and is further supported by static magnetization and ESR data. An underlying theoretical microscopic approach favoring a nematic scenario is based essentially on the NN XYZ exchange anisotropy within a model for frustrated spin-1/2 chains. It is investigated by the DMRG technique. The employed exchange parameters are justified qualitatively by electronic structure calculations for LiCuSbO$_4$.

cond-mat.str-el↗

arXiv2016

Non-collinear antiferromagnetism of coupled spins and pseudospins in the double perovskite La2CuIrO6

We report the structural, magnetic and thermodynamic properties of the double perovskite compound La2CuIrO6 from X-ray, neutron diffraction, neutron depolarization, dc magnetization, ac susceptibility, specific heat, muon-spin-relaxation (uSR), electron-spin-resonance (ESR) and nuclear magnetic resonance (NMR) measurements. Below ~113 K, short-range spin-spin correlations occur within the Cu2+ sublattice. With decreasing temperature, the Ir4+ sublattice progressively involves in the correlation process. Below T = 74 K, the magnetic sublattices of Cu (spin s = 1/2) and Ir (pseudospin j = 1/2) in La2CuIrO6 are strongly coupled and exhibit an antiferromagnetic phase transition into a non-collinear magnetic structure accompanied by a small uncompensated transverse moment. A weak anomaly in ac-susceptibility as well as in the NMR and μSR spin lattice relaxation rates at 54 K is interpreted as a cooperative ordering of the transverse moments which is influenced by the strong spin-orbit coupled 5d ion Ir4+. We argue that the rich magnetic behaviour observed in La2CuIrO6 is related to complex magnetic interactions between the strongly correlated spin-only 3d ions with the strongly spin-orbit coupled 5d transition ions where a combination of the spin-orbit coupling and the low-symmetry of the crystal lattice plays a special role for the spin structure in the magnetically ordered state.

cond-mat.str-el↗

arXiv2016

Spin dynamics and magnetic interactions of Mn dopants in the topological insulator Bi$_2$Te$_3$

The magnetic and electronic properties of the magnetically doped topological insulator Bi$_{\rm 2-x}$Mn$_{\rm x}$Te$_3$ were studied using electron spin resonance (ESR) and measurements of static magnetization and electrical transport. The investigated high quality single crystals of Bi$_{\rm 2-x}$Mn$_{\rm x}$Te$_3$ show a ferromagnetic phase transition for $x\geq 0.04$ at $T_{C}\approx 12$ K. The Hall measurements reveal a p-type finite charge-carrier density. Measurements of the temperature dependence of the ESR signal of Mn dopants for different orientations of the external magnetic field give evidence that the localized Mn moments interact with the mobile charge carriers leading to a Ruderman-Kittel-Kasuya-Yosida-type ferromagnetic coupling between the Mn spins of order 2-3 meV. Furthermore, ESR reveals a low-dimensional character of magnetic correlations that persist far above the ferromagnetic ordering temperature.

cond-mat.str-el↗

arXiv2016

Isolation of proximity-induced triplet pairing channel in a superconductor/ferromagnet spin valve

We have studied the proximity-induced superconducting triplet pairing in CoO$_x$/Py1/Cu/Py2/Cu/Pb spin-valve structure (where Py = Ni$_{0.81}$Fe$_{0.19}$). By optimizing the parameters of this structure we found a triplet channel assisted full switching between the normal and superconducting states. To observe an "isolated" triplet spin-valve effect we exploited the oscillatory feature of the magnitude of the ordinary spin-valve effect $ΔT_c$ in the dependence of the Py2-layer thickness $d_{Py2}$. We determined the value of $d_{Py2}$ at which $ΔT_c$ caused by the ordinary spin-valve effect (the difference in the superconducting transition temperature $T_c$ between the antiparallel and parallel mutual orientation of magnetizations of the Py1 and Py2 layers) is suppressed. For such a sample a "pure" triplet spin-valve effect which causes the minimum in $T_c$ at the orthogonal configuration of magnetizations has been observed.

cond-mat.supr-con↗

arXiv2016

Boosting the superconducting spin valve effect in a metallic superconductor/ferromagnet heterostructure

Superconducting spin valves based on the superconductor/ferromagnet (S/F) proximity effect are considered to be a key element in the emerging field of superconducting spintronics. Here, we demonstrate the crucial role of the morphology of the superconducting layer in the operation of a multilayer S/F1/F2 spin valve. We study two types of superconducting spin valve heterostructures, with rough and with smooth superconducting layers, using transmission electron microscopy in combination with transport and magnetic characterization. We find that the quality of the S/F interface is not critical for the S/F proximity effect, as regards the suppression of the critical temperature of the S layer. However, it appears to be of paramount importance in the performance of the S/F1/F2 spin valve. As the morphology of the S layer changes from the form of overlapping islands to a smooth case, the magnitude of the conventional superconducting spin valve effect significantly increases. We attribute this dramatic effect to a homogenization of the Green function of the superconducting condensate over the S/F interface in the S/F1/F2 valve with a smooth surface of the S layer.

cond-mat.supr-con↗

arXiv2016

Tuning the magnetocrystalline anisotropy in $R$CoPO by means of $R$ substitution: a ferromagnetic resonance study

We report on broad-band electron spin resonance measurements performed within the itinerant ferromagnetic phase of $R$CoPO ($R$ = La, Pr, Nd and Sm). We reveal that the $R$ substitution is highly effective in gradually introducing a sizeable easy-plane magnetocrystalline anisotropy within the Co sublattice. We explain our results in terms of a subtle interplay of structural effects and of indirect interactions between the $f$ and $d$ orbitals from $R$ and Co, respectively.

cond-mat.str-el↗