A. V. Semeno

Enhancement of band magnetism and features of the magnetically ordered state in the CeB6 compound with strong electron correlations

Precision measurements of transport and magnetic parameters of high-quality CeB6 single crystals are performed in the temperature range 1.8—300 K. It is shown that their resistivity in the temperature interval 5 K < T < T* ≈ 80 K obeys not a logarithmic law, which is typical of the Kondo mechanism of charge carrier scattering, but the law ρ ∝ T−1/η corresponding to the weak localization regime with a critical index 1/η = 0.39 ± 0.02. Instead of the Curie-Weiss dependences, the asymptotic form χ(T) ∝ T−0.8 is obtained for magnetic susceptibility of CeB6 in a temperature range of 15–300 K. Analysis of the field dependences of magnetization, magnetoresistance, and the Hall coefficient in the paramagnetic and magnetically ordered phases of CeB6 and comparison with the results of measurements of Seebeck coefficient, the inelastic neutron scattering coefficient, and EPR spectroscopy lead to the conclusion that the Kondo lattice model and skew scattering model cannot be used for describing the transport and thermodynamic parameters of this compound with strong electron correlations. On the basis of detailed analysis of experimental data, an alternative approach to interpreting the properties of CeB6 is proposed using (1) the assumption concerning itinerant paramagnetism and substantial renormalization of the density of electron states upon cooling in the vicinity of the Fermi energy, which is associated with the formation of heavy fermions (spin-polaron states) in the metallic CeB6 matrix in the vicinity of Ce sites; (2) the formation of ferromagnetic nanosize regions from spin polarons at 3.3 K < T < 7 K and a transition to a state with a spin density wave (SDW) at TQ ≈ 3.3 K; and (3) realization of a complex magnetic phase H-T diagram of CeB6, which is associated with an increase in the SDW amplitude and competition between the SDW and antiferromagnetism of localized magnetic moments of cerium ions.

Is MnSi an itinerant-electron magnet? Results of ESR experiments

High-frequency (60 GHz) electron spin resonance (ESR) has been studied in manganese monosilicide, MnSi, single crystals. The measurements performed within the 4.2–300 K temperatures range at the applied magnetic field up to 70 kOe have demonstrated that the magnetic resonance in MnSi is due to localized magnetic moments of the Heisenberg type with the g-factor depending only slightly on temperature, g ∼ 1.9–2. At the same time, it has been found that the ESR linewidth is determined by spin fluctuations and can be quantitatively described in the wide temperature range (4.2 K < T < 60 K) in the framework of the Moriya theory using the SL(T) function. The revealed deviations from the model of weak itinerant-electron magnetism commonly used for the description of the magnetic properties of MnSi indicate a possible spin-polaron nature of the unusual magnetic properties of this strongly correlated metal.

Microwave EPR spectroscopy of cobalt-doped germanium cuprate

Resonance magnetoabsorption spectra of CuGeO3 single crystals containing 2% Co impurity have been studied in the frequency range 60–360 GHz in magnetic fields of up to 16 T and in the temperature interval 2–60 K with the magnetic field B aligned parallel to the a crystallographic axis. In addition to the Cu2+ chain resonance, a new absorption line (unobserved previously in doped CuGeO3 and deriving apparently from the Co2+ ions) was detected in EPR spectra. Quantitative analysis of the spectra suggests that the spin-Peierls transition occurs in about 10% Cu2+ chains, while the spin-Peierls state in the remaining 90% chains is completely destroyed by cobalt doping. The results obtained reveal considerable deviations from the universally accepted scenario of CuGeO3 doping and are discussed within alternative theoretical models, namely, the quantum critical behavior (based on the EPR theory for quasi-one-dimensional systems) and a three-dimensional antiferromagnet with the Néel temperature lowered by disorder.

Antiferro-quadrupole resonance in CeB6

We report experimental observation of a new type of magnetic resonance caused by orbital ordering in a strongly correlated electronic system. Cavity measurements performed on CeB 6 single crystals in a frequency range 60-100 GHz show that a crossing of the phase boundary T Q (B) between the antiferro-quadrupole and paramagnetic phases gives rise to development at T< T Q (B) of a magnetic resonance. The observed mode is gapless and correspond to g-factor 1.62.

Quantum critical behavior induced by Mn impurity in CuGeO3

Results of high frequency (60-315 GHz) studies of ESR in CuGeO3 single crystals containing 0.9% of Mn impurity are reported. Quantitative EPR line shape analysis allowed concluding that low temperature magnetic susceptibility for T<40 K diverges following power law with the critical exponent 0.81 and therefore manifests onset of a quantum critical (QC) regime. We argue that transition into Griffiths phase occurs at TG~40 K and disorder produced by Mn impurity in quantum spin chains of CuGeO3 may lead to co-existence of the QC regime and spin-Peierls dimerisation.

Magnetic resonance in cerium hexaboride caused by quadrupolar ordering

Abstract Experimental evidence of the magnetic resonance in the antiferro-quadrupole phase of CeB 6 is reported. We have shown that below orbital ordering temperature a new magnetic contribution emerges and gives rise to the observed resonant phenomenon.

Anomalous polarization characteristics of magnetic resonance in a quasi-one-dimensional CuGeO3: Co magnet

Doping of CuGeO3 by 2% Co was found to cause a new magnetic resonance, which has anomalous polarization characteristics. In the Faraday geometry, where a microwave field Bω is directed along certain crystallographic directions, this mode is suppressed, which indicates that the character of magnetic oscillations in this mode differs strongly from standard spin precession. This resonance coexists with the EPR on Cu2+ chains and is likely to be caused by an unknown collective mode of magnetic oscillations of an antiferromagnetic quantum S = 1/2 spin chain.

Anomalous magnetism and charge transport in dielectric La1−xCaxMnO3 (x≤xMIT∼0.22)

A study of the charge transport, magnetization, and high frequency (ω/2π=60 GHz) electron spin resonance (ESR) has been carried out on La1−xCaxMnO3 single crystals corresponding to the dielectric side of the concentration driven metal-insulator transition (x≤xMIT∼0.22). For x=0.22 the onset of ferromagnetism below the Curie temperature TC≈183 K was found to induce crossover from holelike (T>TC) to electronlike (T TC. A large enhancement of the Hall effect, which is accompanied by the increase in Hall mobility to the values of μH∼−20 cm2/V s, was observed at intermediate temperatures 150 K TC was shown to evolve into an intricate two-mode ESR spectrum with irregular fine structure characterizing the ferromagnetic insulating...

Magnetic resonance probing of ground state in the mixed valence correlated topological insulator SmB 6

Introducing of topological insulator concept for fluctuating valence compound – samarium hexaboride – has recently initiated a new round of studies aimed to clarify the nature of the ground state in this extraordinary system with strong electron correlations. Here we discuss the data of magnetic resonance in the pristine single crystals of SmB6 measured in 60 GHz cavity experiments at temperatures 1.8–300 K. The microwave study as well as the DC resistivity and Hall effect measurements performed for the different states of SmB6 [110] surface prove definitely the existence of the layer with metallic conductivity increasing under lowering temperature below 5 K. Four lines with the g-factors g ≈ 2 are found to contribute to the ESR-like absorption spectrum that may be attributed to intrinsic paramagnetic centers on the sample’s surface, which are robust with respect to the surface treatment. The temperature dependence of integrated intensity I(T) for main paramagnetic signal is found to demonstrate anomalous critical behavior I(T) ~ (T* − T)ν with characteristic temperature T* = 5.34 ± 0.05 K and exponent ν = 0.38 ± 0.03 indicating possible magnetic transition at the SmB6 [110] surface. Additional resonant magnetoabsorption line, which may be associated with either donor-like defects or cyclotron resonance mode corresponding to the mass mc ~ 1.2m0, is reported.

Anomalous magnetic properties of VOx multiwall nanotubes

Basing on the high frequency (60 GHz) electron spin resonance (ESR) and magnetic susceptibility study of the VOx multiwall nanotubes (VOx-NTs) in the range 4.2–300 K we report the ESR evidence of the presence of the antiferromagnetic V4+ dimers in VOx-NTs and the observation of an anomalous low temperature (T<50 K) growth of the magnetic susceptibility for V4+ quasi-free spins, which obey power law χ(T)~1/Tα with the exponent α≈0.6. The estimates of the concentrations for various spin species (clusters) indicate that the non-interacting dimers should be an essential element in the VOx-NTs structure. The possibility of the disorder driven quantum critical regime in VOx-NTs is discussed.