Relevance of ferromagnetic correlations for the electron spin resonance in Kondo lattice systems

Electron spin resonance (ESR) measurements of the ferromagnetic (FM) Kondo lattice system CeRuPO show a well defined ESR signal which is related to the Ce3+ magnetism. In contrast, no ESR could be observed in the antiferromagnetic (AFM) homologue CeOsPO. Additionally, we detect an ESR signal in ferromagnetic YbRh while it was absent in a number of Ce or Yb intermetallic compounds with dominant AFM exchange. Thus, the observation of an ESR signal in a Kondo lattice is neither specific to Yb nor to the proximity to a quantum critical point, but seems to be connected to the presence of FM fluctuations. These conclusions not only provide a basic concept to understand the ESR in Kondo lattice systems even well below the Kondo temperature (as observed in YbRh2Si2) but point out ESR as a prime method to investigate directly the spin dynamics of the Kondo ion.     

ESR of Yb3+ ions in a single crystal of the fluctuating-valence compound YbB12

The temperature and angular dependences of electron spin resonance (ESR) spectra of Yb³⁺ ions in a single crystal of fluctuating-valence compound YbB₁₂ were studied. The existence of Yb?Yb ion pairs was observed in a cubic-symmetry crystal. The ions forming the pairs are coupled by the isotropic exchange but interact also with the other pairs by the dipole and exchange coupling. The occurrence of a slight anisotropy in a cubic semiconductor may be the result of a spontaneous break of symmetry specific for the ground state of the Kondo dielectric. A strong temperature dependence of the amplitude of the ESR signals is found at 1.6–4.2 K and interpreted as a result of the capture of electrons by Yb³⁺ ions from electron traps with a binding energy of 18 K. ESR spectra of Yb³⁺ single ions in the Γ₆ state were observed also. The decrease of temperature from 4.2 to 1.6 K indicates a tendency to the ferromagnetic ordering of Yb?Yb pairs.     

Kondo lattice with heavy fermions: peculiarities of spin kinetics

A model of spin relaxation of Kondo lattices is proposed to explain the angular dependence of the electron spin resonance (ESR) parameters in the heavy fermion compounds Y bIr(2)Si(2) and Y bRh(2)Si(2). A perturbational scaling approach reveals a collective spin motion of Yb?ions with conduction electrons in the bottleneck regime. A common energy scale due to the Kondo effect regulates the temperature dependence of different kinetic coefficients to result in a mutual cancelation of all divergent parts in a collective spin mode. The angular dependence of the ESR intensity, linewidth and resonant frequency is shown to be in good agreement with experimental data on Y bIr(2)Si(2) and Y bRh(2)Si(2). In particular, the unexpectedly weak dependence of the ESR intensity on the orientation of the microwave magnetic field agrees with the properties of the discussed model.     

Why could electron spin resonance be observed in a heavy fermion Kondo lattice?

We develop a theoretical basis for understanding the spin relaxation processes in Kondo lattice systems with heavy fermions as experimentally observed by electron spin resonance (ESR). The Kondo effect leads to a common energy scale that regulates a logarithmic divergence of different spin kinetic coefficients and supports a collective spin motion of the Kondo ions with conduction electrons. We find that the relaxation rate of a collective spin mode is greatly reduced due to a mutual cancellation of all the divergent contributions even in the case of the strongly anisotropic Kondo interaction. The contribution to the ESR linewidth caused by the local magnetic field distribution is subject to motional narrowing supported by ferromagnetic correlations. The developed theoretical model successfully explains the ESR data of YbRh₂Si₂ in terms of their dependence on temperature and magnetic field.     

Observation of a new magnetic anomaly below the ferromagnetic Curie temperature in Yb14MnSb11

Yb14MnSb11 is an unusual ferromagnet with a Curie temperature of 52 +/- 1 K. Recent optical, Hall, magnetic, and thermodynamic measurements indicate that Yb14MnSb11 may be a rare example of an underscreened Kondo lattice. We report the first experimental observation of a new magnetic anomaly in this system at around 47 K, a few degrees below T(c). Systematic investigations of the ac and dc susceptibilities of Yb14MnSb11 single crystals reveal features associated with possible spin reorientation at this temperature. This new anomaly is extremely sensitive to the applied measurement field and is absent in temperature-dependent dc magnetization data for fields above 50 Oe. The origin of this could be due to decoupling of two distinct magnetic sublattices associated with MnSb4 tetrahedra.     

Electron spin resonance in the Heusler alloy YbRh2Pb

An electron spin resonance (ESR) signal was observed in a concentrated Kondo lattice, Heusler alloy YbRh₂Pb. It is attributed to the combined effect of the 4f local magnetic moments of Yb³⁺ and conduction electrons. It is shown that the significant broadening and disappearance of the ESR line at temperatures above 20 K is caused by the processes of the spin-lattice relaxation of the Yb³⁺ ions through the first excited Stark doublet with an activation energy Δ ≈ 73.5 K. A comparison of the ESR data for YbRh₂Pb and some other undoped intermetallic compounds based on ytterbium, cerium, and europium indicates that hybridized electronic states occurring as the result of hybridization between the localized 4f electrons and the collectivized conduction electrons constitute a fundamentally new source of ESR.     

Electron spin resonance in the spin-1/2 quasi-one-dimensional antiferromagnet with Dzyaloshinskii-Moriya interaction BaCu2Ge2O7

We have investigated the electron spin resonance (ESR) on single crystals of BaCu2Ge2O7 at temperatures between 300 and 2 K and in a large frequency band, 9.6-134 GHz, in order to test the predictions of a recent theory, proposed by Oshikawa and Affleck (OA) [Phys. Rev. Lett. 82, 5136 (1999)]], which describes the ESR in a spin-1/2 Heisenberg chain with the Dzyaloshinskii-Moriya interaction. We find, in particular, that the ESR linewidth, Delta H, displays a rich temperature behavior. As the temperature decreases from T(max)/2 approximately 170 to 50 K, Delta H shows a rapid and linear decrease, Delta H approximately T. At low temperatures, below 50 K, Delta H acquires a strong dependence on the magnetic field orientation and for H axially c it shows a (h/T)(2) behavior which is due to an induced staggered field h, according to OA's prediction.     

Dzyaloshinsky-Moriya anisotropy in the spin-1/2 kagome compound ZnCu3(OH)6Cl2

We report the determination of the Dzyaloshinsky-Moriya interaction, the dominant magnetic anisotropy term in the kagome spin-1/2 compound ZnCu3(OH)6Cl2. Based on the analysis of the high-temperature electron spin resonance (ESR) spectra, we find its main component |Dz|=15(1) K to be perpendicular to the kagome planes. Through the temperature dependent ESR linewidth, we observe a building up of nearest-neighbor spin-spin correlations below approximately 150 K.     

Quantum critical Kondo quasiparticles probed by ESR in β-YbAlB₄

Electron spin resonance (ESR) can probe conduction electrons (CE) and local moment (LM) spin systems in different materials. A CE spin resonance (CESR) is observed in metallic systems based on light elements or with enhanced Pauli susceptibility. LM ESR can be seen in compounds with paramagnetic ions and localized d or f electrons. Here we report a remarkable and unprecedented ESR signal in the heavy-fermion superconductor β-YbAlB? [S. Nakatsuji et al., Nature Phys. 4, 603 (2008)] which behaves as a CESR at high temperatures and acquires characteristics of the Yb3? LM ESR at low temperature. This dual behavior strikes as an in situ unique observation of the Kondo quasiparticles in a quantum critical regime. The proximity to a quantum critical point may favor the appearance of this dual character of the ESR signal in β-YbAlB?.     

The Spectral and Magnetic Properties of Er3+ and Yb3+ Ions in Y2Ti2O7 Crystals with a Pyrochlore Structure

Physics of the Solid State - Cubic Y2Ti2O7 single crystals doped with Er3+ or Yb3+ ions have been studied by the methods of electron spin resonance (ESR) and selective laser spectroscopy. ESR...     

First-order transition in the spin dynamics of geometrically frustrated Yb2Ti2O7

Using neutron diffraction, 170Yb M?ssbauer and muon spin relaxation spectroscopies, we have examined the pyrochlore Yb2Ti2O7, where the Yb3+S' = 1/2 ground state has planar anisotropy. Below approximately 0.24 K, the temperature of the known specific-heat lambda transition, there is no long range magnetic order. We show that the transition corresponds to a first-order change in the fluctuation rate of the Yb3+ spins. Above the transition temperature, the rate, in the GHz range, follows a thermal excitation law, whereas below, the rate, in the MHz range, is temperature independent, indicative of a quantum fluctuation regime.     

Electron spin resonance in YbRh2Si2: local-moment, unlike-spin and quasiparticle descriptions

Electron spin resonance (ESR) in the Kondo lattice compound YbRh(2)Si(2) has stimulated discussion as to whether the low-field resonance outside the Fermi liquid regime in this material is more appropriately characterized as a local-moment phenomenon or one that requires a Landau quasiparticle interpretation. In earlier work, we outlined a collective mode approach to the ESR that involves only the local 4f moments. In this paper, we extend the collective mode approach to a situation where there are two subsystems of unlike spins: the pseudospins of the ground multiplet of the Yb ions and the spins of the itinerant conduction electrons. We assume a weakly anisotropic exchange interaction between the two subsystems. With suitable approximations our expression for the g-factor also reproduces that found in recent unlike-spin quasiparticle calculations. It is pointed out that the success of the local-moment approach in describing the resonance is due to the fact that the susceptibility of the Yb subsystem dominates that of the conduction electrons with the consequence that the relative shift in the resonance frequency predicted by the unlike-spin models (and absent in the local-moment models) is ? 1. The connection with theoretical studies of a two-component model with like spins is also discussed.     

Magnetic relaxation in UCu4 + xAl8 - x

A quasielastic neutron scattering study has been performed on UCu_4+xAl_8-x, a system which reveals an alloying-induced transition from magnetic order to heavy-fermion behavior. The magnetic response can be described by a broad quasielastic Lorentzian for all concentrations. No crystal field excitations could be detected. Remarkably, the linewidth decreases with increasing hybridization strength and seems to be no measure of the Kondo lattice temperature T*. In addition, electron spin resonance (ESR) experiments were performed which revealed that, even in the heavy-fermion regime, spin fluctuations play an important role. Both experimental findings are in distinct contrast to cerium-based heavy fermion systems.     

Spin dynamics of the S = 5/2 2D triangular antiferromagnet Ba3NbFe3Si2O14

We report pulse-field magnetization, ac susceptibility, and 100 GHz electron spin resonance (ESR) measurements on the S = 5/2 two-dimensional triangular compound Ba3NbFe3Si2O14 with the Néel temperature T(N) = 26 K. The magnetization curve shows an almost linear increase up to 60 T with no indication of a one-third magnetization plateau. An unusually large frequency dependence of the ac susceptibility in the temperature range of T = 20-100 K reveals a spin-glass behavior or superparamagnetism, signaling the presence of frustration-related slow magnetic fluctuations. The temperature dependence of the ESR linewidth exhibits two distinct critical regimes; (i) ΔH(pp)(T) is proportional to (T-T(N))(-p) with the exponent p = 0.2(1)-0.2(3) for temperatures above 27 K, and (ii) ΔH(pp)(T) is proportional to (T-T*)(-p) with T* = 12 K and p = 0.8(1)-0.8(4) for temperatures between 12 and 27 K. This is interpreted as indicating a dimensional crossover of magnetic interactions and the persistence of short-range correlations with a helically ordered state.     

Submillimeter-wave ESR measurements of Ca1−xCuO2 (x = 0.164) with edge-sharing CuO2 chains

Quasi-one-dimensional (1-D) cupric oxide Ca_1?xCuO₂ (x = 0.164) is the system with 25–40% hole-doped edge-sharing CuO₂ chains. However, the holes are almost localized in Ca_1?xCuO₂ and its magnetic susceptibility with a peak at 30 K was explained by the model considering both 1-D antiferromagnetic chains and spin dimers (Z. Hiroi, M. Okumura, Y. Nabeshima, T. Yamada, M. Takano: J. Phys. Soc. Jpn.69, 1824, 2000). To clarify the magnetic nature of Ca_1?xCuO₂, we performed submillimeter-wave electron spin resonance (ESR) measurements on a powder sample of Ca_0.83?6CuO₂. The resonance above 12 K showed typical powder ESR of Cu²⁺ and theg-values were determined to be g∥= 2.33 and g_⊥ = 2.06 from the analysis. The resonance below 12 K changed completely from ESR. The frequency-field relation of ESR at 1.8 K clearly showed the easy-axis type antiferromagnetic resonance.     

脉冲强磁场高频电子自旋共振装置的研制

国内第一套脉冲强磁场高频电子自旋共振(ESR)装置由武汉国家脉冲强磁场科学中心研制成功.该装置的频率范围为210-370 GHz,样品温度范围为2-300 K, 磁场强度为0-50 T.在脉冲强磁场ESR装置上进行了红宝石ESR实验,获得了Cr³⁺ 离子的ESR谱.     

Development and applications of high-frequency ESR up to 55 T

A multiextreme (high-field, low-temperature, high-pressure and nanoscale) electron spin resonance (ESR) measurement system is under development in Kobe. In this connection, our recent development is introduced and two applications of our high-frequency high-field ESR are described. High-frequency high-field ESR measurements of dioptase (Cu₆Si₆O₁₈·6H₂O), which has an interesting antiferromagnetic Cu²⁺ network, have been performed using a pulsed magnetic field of up to 55 T. Antiferromagnetic resonances (AFMR) are clearly observed at 4.2 K with the light sources of up to 1017 GHz. However, a deviation from the conventional two-sublattice AFMR theory is observed in the high field. Temperature dependence of the X-band and high-frequency ESR has been also observed in the triangular lattice antiferromagnet EtMe₃P[Pd(dmit)₂]₂ which shows the spin-Peierls-like transition below T _c = 25 K. The preliminary field dependence of the spin gap estimated from the analyses of our ESR results has been shown in connection with the previous magnetic susceptibility results.     

Theory of inelastic scattering from magnetic impurities

We use the numerical renormalization group method to calculate the single-particle matrix elements T of the many-body T matrix of the conduction electrons scattered by a magnetic impurity at T=0 temperature. Since T determines both the total and the elastic, spin-diagonal scattering cross sections, we are able to compute the full energy, spin, and magnetic field dependence of the inelastic scattering cross section sigma(inel)(omega). We find an almost linear frequency dependence of sigma(inel)(omega) below the Kondo temperature T(K), which crosses over to a omega(2) behavior only at extremely low energies. Our method can be generalized to other quantum impurity models.     

Defect-induced magnetic fluctuations in YbB12

We present the results of nuclear spin–lattice relaxation rate (1/T₁) measurements in a typical Kondo insulator YbB₁₂ for ^10,11B and ¹⁷¹Yb nuclei. Above 20 K, 1/T₁ at the B sites shows thermally activated temperature dependence with a gap of about 100 K. However, it shows anomalous enhancement below 15 K, which is partially suppressed by magnetic field up to 16 T. No such anomaly was observed at the Yb sites. The ratio of 1/T₁ for ¹¹B and ¹⁰B nuclei indicates that the anomaly below 15 K is caused by dilute magnetic moments assisted by nuclear spin diffusion. The origin and the nature of the low temperature magnetic fluctuations are discussed.     

Magnetic properties of the charge ordered Nd0.75Na0.25MnO3

Nd_0.75Na_0.25MnO₃ polycrystalline ceramic is prepared via sol-gel process and its magnetic properties and electron spin resonance (ESR) spectra have been investigated experimentally. As the compound is cooled from room temperature, a charge-ordered state first develops below 170 K. A high magnetic field melts the charge ordered state and stabilizes a ferromagnetic (FM) state below 170 K. A field induced transition, analogous to a spin flip transition, is observed between 40 and 170 K. The critical temperature for spin flip increases with increasing temperature. Below 130 K, the compound tends to be intrinsically inhomogeneous, i.e. FM clusters and paramagnetic domains coexist in this system at least, which is confirmed by ESR measurements. When the external magnetic field is zero, long range FM interaction is not developed in this system; however, a tendency of re-entrant FM transition is observed in this compound.