Spin relaxation rates in CuFe measured by neutron scattering

The absolute value and the temperature dependence of the spin relaxation rate of the Kondo system $\text{Cu}$Fe has been deduced from the magnetic scattering of unpolarized neutrons with energy analysis.     

Observation of novel coexistence of Kondo effect and room temperature magnetism in AlN/Al/AlN trilayer thin film

In this work for the first time, we are reporting the unusual observation of the Kondo effect with the coexistence of room temperature ferromagnetism in AlN/Al/AlN trilayer thin film. The grown film shows resistivity minimum at a temperature of ∼48K, which shifts to the lower temperature on the application of magnetic fields. After considering various possibilities for an upturn in resistivity, we found that the Kondo scattering is responsible for upturn at low temperature. The simultaneous presence of ferromagnetism and Kondo scattering is explained by spatial variation of nitrogen vacancy defects from the film surface to the Al sandwich layer. Furthermore, magneto-transport properties of the film measured at different temperature exhibits both negative and positive components described by localized magnetic moment model for the spin scattering of carriers and two-band model, respectively. This work provides insight into the novel co-existence of ferromagnetism and Kondo effect in crystalline AlN.     

Diffusion,trapping and detrapping of positive muon in Al-0.047% Mg

Positive muon spin relaxation was measured in Al-0.047% Mg quenched from 873 K under zero external magnetic field from 6.3 K to 270 K. The observed spectra were analysed with the calculated muon spin relaxation function which included the static relaxation rates, the trapping rates, the detrapping rates and thet=0 initial trapping fractions. Due to the precise measurements and the realistic expression of spin relaxation function, above four parameters could be determined distinctly. The trapping rates and the distortions around a muon were determined from the values of static relaxation rates at a trapping site. The diffusion features were clearly described by the trapping and detrapping rates. The diffusion process was determined by the temperature dependence of these parameters.     

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.     

Scaling of the low-temperature dephasing rate in Kondo systems

We present phase coherence time measurements in quasi-one-dimensional Ag wires doped with Fe Kondo impurities of different concentrations n_{s}. Because of the relatively high Kondo temperature T_{K} approximately 4.3 K of this system, we are able to explore a temperature range from above T_{K} down to below 0.01T_{K}. We show that the magnetic contribution to the dephasing rate gamma_{m} per impurity is described by a single, universal curve when plotted as a function of T/T_{K}. For T>0.1T_{K}, the dephasing rate is remarkably well described by recent numerical results for spin S=1/2 impurities. At lower temperature, we observe deviations from this theory. Based on a comparison with theoretical calculations for S>1/2, we discuss possible explanations for the observed deviations.     

Magnetic field of bifilar lead

Results of cw⁶³Cu NMR measurements in dilute CuFe alloys containing from 0.01 up to 0.2 at.-% Fe are presented. The temperature was well above the Kondo temperature of this specimen. The broadening of the NMR line width is caused by the localized magnetic moment of the Fe impurities. The decreasing signal amplitude with increasing iron content is due to the quadrupolar interactions appearing simultaneously. The line broadening correspond strongly to that of the impurity susceptibility behaviour in CuFe. The quadrupolar disturbances are temperature independent. The influence of the simultaneously appearing magnetic and electric interactions on the spin lattice nuclear relaxation time is discussed.     

Experimental test of the numerical renormalization-group theory for inelastic scattering from magnetic impurities

We present measurements of the phase coherence time taupsi in quasi-one-dimensional Au/Fe Kondo wires and compare the temperature dependence taupsi of with a recent theory of inelastic scattering from magnetic impurities [Phys. Rev. Lett. 93, 107204 (2004)10.1103/PhysRevLett.93.107204]. A very good agreement is obtained for temperatures down to 0.2T(K). Below the Kondo temperature T(K), the inverse of the phase coherence time varies linearly with temperature over almost one decade in temperature.     

Rare earth paramagnetism,as studied by μSR

Some recent result of muon spin relaxation measurements in rare earth metals and intermetallic compounds are reviewed. Special emphasis is put on measurements that relate to the properties of correlated regions of spins existing relatively far above the ordering temperature in the rare earth ions. As far as comparable data from paramagnetic neutron scattering exist, they will be discussed in the same framework. For each temperature the correlated regions (or short-lived magnetic clusters) are characterized by their size, possible anisotropy with respect to the crystalline axes and their lifetime. The actual form of the interaction between the rare earth spins themselves and with the crystal fields determine the temperature dependence of these properties; a strong dipole interaction can, for instance, be expected to change the critical behaviour nearT _c . Much of the time will be devoted to experiments on Gd-metal where there are experimental indications that several interesting phenomena occur: (1) a strong effect of a cross-over from a non-conserved dynamics (dipolar) regime to a conserved (exchange dominated) regime some 10 K aboveT _c , (2) an anisotropy of the magnetic clusters with respect to the hexagonalc-axis, and (3), a persistence of spin correlations far aboveT _c . Some attempts to correlate the rare earth spin relaxation times measured in this region with cluster lifetimes deduced from neutron scattering will be reviewed, as well as a model for understanding these lifetimes in terms of temperature dependent cluster wall motion, which is determined by exchange and magnetic anisotropy parameters. Effects of possible quantum correlations originating from the “spin system+bath” interaction will be mentioned.     

Experimental study of electrical conduction in RuO2-based thick resistive films

We have found direct evidence for the dynamical instability of Pr, which could be observed in Pr_0.015 Pd by inelastic neutron scattering, and as such is first ever to be found for Pr ions. Crystalfield analysis of the measured dynamic susceptibility reveals that the spin instability is similar to that of Ce Kondo systems. We show that the previously observed resistivity increase belowT=25 K is a true Kondo effect and is not related to crystal field effects. We conclude that Pr diluted in Pd is a Kondo system withT _K =2.5 K.     

Exchange-mediated spin-lattice relaxation of Fe3+ ions in borate glasses

Spin-lattice relaxation times (T1) of two borate glasses doped with different concentrations of Fe2O3 were measured using the Electron Spin-Echo (ESE) technique at X-band (9.630 GHz) in the temperature range 2-6K. In comparison with a previous investigation of Fe3+-doped silicate glasses, the relaxation rates were comparable and differed by no more than a factor of two. The data presented here extend those previously reported for borate glasses in the 10-250K range but measured using the amplitude-modulation technique. The T1 values were found to depend on temperature (T) as T(n) with n approximately 1 for the 1% and 0.1% Fe2O3-doped glass samples. These results are consistent with spin-lattice relaxation as effected by exchange interaction of a Fe3+ spin exchange-coupled to another Fe3+ spin in an amorphous material.     

Doping-induced temperature evolution of a helicoidal spin structure in the MnGe compound

The helicoidal magnetic structure of a MnGe compound doped with 25% Fe is studied by means of small-angle neutron scattering in a wide temperature range of 10–300 K. Analysis of the scattering-function profile demonstrates that magnetic structures inherent to both pure MnGe and its doped compounds are unstable. The doping of manganese monogermanide is revealed to lead to higher destabilization of the magnetic system. In passing from MnGe to Mn_0.75Fe_0.25Ge, the magnetic-ordering temperature T _N decreases from 130 to 95 K, respectively. It is demonstrated that, at temperatures close to 0 K, the intensity of the contribution to scattering from stable spin helices decreases and the intensity of scattering by spin helix fluctuations increases with increasing impurity-metal concentration. An increased intensity of anomalous scattering caused by spin excitations existing in the system is observed. Helicoidal fluctuations and spin excitations corresponding to low temperatures indicate the quantum nature of the instability in the doped compound. However, MnGe doping with Fe atoms has no influence on the compound’s magnetic properties at temperatures of higher than T _N. The temperature range of short-range ferromagnetic correlations is independent of concentrations and is restricted by temperatures T ranging from 175 to 300 K.     

Effect of localized spins in coherent transport through quantum dots

The effect of localized spins on the quantum coherence in solids is discussed. A quantum dot with an odd number of electrons can be a model system for a localized spin. It is experimentally shown that a spin flip scattering by a quantum dot pulls the trigger of quantum decoherence. On the other hand, spin flip scattering is the basic process to construct the Kondo singlet state around a magnetic impurity. Through an interference effect of the Kondo state (the Fano–Kondo effect) in a side-coupled dot system, we show experimentally that the Kondo singlet state is quantum mechanically coherent. The analysis of the Fano–Kondo lineshape indicates the locking of the phase shift to π/2, which is in agreement with theoretical predictions. The Fano–Kondo effect is also observed in an Aharonov–Bohm ring, in which a quantum dot is embedded, and also indicates the phase shift locking to π/2.     

Impurity spin dynamics in the classical Kondo system Cu: Fe. NMR relaxation of impurity neighbour nuclei

The correlation time τ_i of the impurity electronic spin in the Kondo system Cu:Fe has been measured over the temperature range 4.2 K?T?300 K (T_K=27.6 K) by means of NMR relaxation of impurity neighbour nuclei. τ_i being in the sub-picosecond region varies with temperatures as predicted by the model calculation of Götze and Schlottmann.     

Phase coherence of conduction electrons below the Kondo temperature

We have measured the phase decoherence rate tau_{varphi};{-1} of conduction electrons in disordered Ag wires implanted with 2 and 10 ppm Fe impurities, by means of the weak-localization magnetoresistance. The Kondo temperature of Fe in Ag, T_{K} approximately 4 K, is in the ideal temperature range to study the progressive screening of the Fe spins as the temperature T falls below T_{K}. The contribution to tau_{varphi};{-1} from the Fe impurities is clearly visible over the temperature range 40 mK-10 K. Below T_{K}, tau_{varphi};{-1} falls rapidly until T/T_{K} approximately 0.1, in agreement with recent theoretical calculations. At lower T tau_{varphi};{-1} deviates from theory with a flatter T-dependence. Understanding this anomalous dephasing for T/T_{K}<0.1 may require theoretical models with larger spin and number of channels.     

稀磁合金中“电阻极大”现象的双杂质散射理论

本文基于s-d相互作用,考虑杂质之间存在RKKY相互作用,提出了一种新的双杂质散射模型。按照这个模型,当一个杂质作自旋翻转散射时,由于杂质之间存在着RKKY关联,它的自旋作为内部自由度会受到限制。由于这种关联,抑制了杂质的自旋翻转散射,结合Kondo的logT项,能形成电阻极大。本文计算了所有可能的“DIS”图(双杂质自能图),在Kondo电阻公式中加入了A/(T₀²—T²)这样的项。其中A是一个正常数。T₀是一个临界温度。当T≤T₀时,这个公式不再有意义。这个理论和已有的分子场理论在本质上是不同的.因为它并不依赖于合金中的磁有序.因此当T≥T_c时(T_c是磁有序转变温度),这种机制仍起作用,但分子场理论则不行.这是一种顺磁效应.我们和Cd-Mn(杂质浓度从0.01到0.1at./0)的实验曲线进行了比较,发现符合得很好.最后,我们认为即使在极低浓度下这种机制也是消除Kondo logT发散的主要原因. 关键词：     

Spin density wave of itinerant heavy electrons in Ce(Ru0.85Rh0.15)2Si2

The magnetic properties of Ce(Ru_0.85Rh_0.15)₂Si₂ were studied by neutron scattering and measurements of magnetization, susceptibility, specific heat and thermal expansion as a function of temperature. We observe a crossover from a high temperature localized spin to a low temperature heavy electron state. Spin density wave (SDW) behavior appears in the heavy electron state below T_N = 5.5 K and the volume change due to spin quantum fluctuations associated with the SDW and the Kondo screening is reminiscent of moment-volume instabilities of the INVAR and anti-INVAR behavior of 3 d transition metal alloys.     

Electrical resistivity and magnetoresistance of CeB6

Electrical resistivity and magnetoresistance of CeB₆ single crystal have been measured in the temperature range from 1.3 to 300 K under the magnetic field up to 85 kOe. Three characteristic phases are distinguished consistently with other measurements. The Kondo like behaviour in the resistivity observed in the high temperature phase is fitted by the conventional form for the dilute Kondo state with the Kondo temperature T_K = 5 ～ 10K and the unitarity limit resistivity $\text{?}{}_{\mathrm{u}}\text{? 110 μΩ}$ cm/Ce-atom. The negative magnetoresistance in the middle phase is stronger with increasing magnetic field and with decreasing temperature suggesting rapid destruction of the Kondo state. The magnetoresistance in the low temperature phase exhibits some anomalies suggesting sub-phases corresponding to several kinds of spin ordering.     

On the Kondo-effect in dilute alloys of iron and nanoscaled copper

Electric resistivity measurements in nanocrystalline Cu-samples alloyed with Fe in the concentration regime of C _Fe ～ 0.17 - 0.37 at - % and nanocrystallite sizes of 6 nm to 24 nm show a Kondo minimum at ～ 30 K. Resistivity does not saturate at low temperature but passes over a maximum at ～ 10 K, which may be due to a spin glass transition. An applied magnetic field of 0.5T shifts the minimum to lower T, but does not destroy the maximum. The curves measured are rather spiky in contrast to curves obtained from polycrystalline samples which is possibly due to non-selfaveraging effects. Further indication for such effects is a non-monotonuous dependence of the resistivity minimum as a function of average crystallite size.     

Evidence for anisotropic kondo behavior in Ce0.8La0.2Al3

We have performed an inelastic neutron scattering study of the low energy spin dynamics of the heavy fermion compound Ce0.8La0.2Al3 as a function of temperature and external pressure up to 5 kbar. At temperatures below 3 K, the magnetic response transforms from a quasielastic form, common to many heavy fermion systems, to a single well-defined inelastic peak, which is extremely sensitive to external pressure. The scaling of the spin dynamics and the thermodynamic properties are in agreement with the predictions of the anisotropic Kondo model.     

Magnetic correlations and the anisotropic Kondo effect in Ce1-xLaxAl3

By combining the results of muon spin relaxation and inelastic neutron scattering in the heavy fermion compounds Ce1-xLaxAl3 (0.0相似文献