Reentrant superconductivity in the orthorhombic system (Tm1−xLux)RuB2

The boundaries between the paramagnetic, superconducting and magnetically ordered phases in the orthorhombic pseudoternary system (Tm_1?xLu_x)RuB₂ have been established by means of ac magnetic susceptibility measurements down to 1.2 K. Reentrant superconductivity occurs between x = 0.52 and x ? 0.68. The absence of coexistence between superconductivity and long range magnetic order in this region suggests a ferromagnetic-like nature of the magnetic state. The initial linear depression of superconducting transition temperature (T_c) gives a coupling constant value N(E_F)$\text{J}$² between conduction electrons and magnetic Tm³⁺ moments of 6.7 × 10^?4 eV-atom-states/ spin direction.     

Proximity effect of a superconductor on an antiferromagnetic metal

There should be two contributions to the pair breaking energy in an antiferromagnetic metal. The first, already discussed byde Gennes andSarma, is due to disorder on the magnetic sites. The second is a temperature dependent contribution from electron magnon scattering. This term is calculated for the temperature rangeT _N(J/μ)²?T?T _N and found to be of orderT ²/T _N. (T _N = Néel temperature,μ = Fermi energy,J = exchange coupling between conduction electrons and magnetic ions.)     

Gd3+ electron spin resonance spectroscopy on LaO1 ? x F x FeAs superconductors

We report an electron spin resonance (ESR) spectroscopy study on polycrystalline samples of the LaO_{1 ? x} F _x FeAs (x = 0 and 0.1) compound with small levels of Gd doping (2% and 5%). The Gd ESR signal is found to be sensitive to the magnetic phase transition from the paramagnetic to the spin density wave (SDW) state occurring in the parent LaO_{1 ? x} F _x FeAs compounds at T _SDW ?? 130 K. Interestingly, the analysis of the low-temperature ESR spectra of the c-axis oriented Gd_{1 ? y} La _y OFeAs samples gives evidence for the magnetically nonequivalent Gd sites and also for sites having a different local charge environment. The analysis of the temperature dependence of the ESR linewidths gives evidence for a coupling of the localized 4f electrons of Gd to the conduction electrons in the FeAs layers. The ESR data reveal that the fluorine substitution, which provides electron doping, suppresses the SDW order and enhances the density of states in the electronic bands stemming from the xz and yz orbital states of Fe to which the 4f electrons are most strongly coupled.     

Enhanced superconductivity by correlations between two Kondo impurities

We study the interplay between magnetic correlations of two Kondo impurities and superconducting singlet pairing. Performing a Schrieffer-Wolff transformation in the zero-bandwidth limit of the two-impurity Anderson model we obtain the Hamiltonian of two magnetic impurities and we add a superconducting term to the conduction electrons. The model allows us to study the effect of the magnetic correlation between the impurities on the superconducting ground state. At zero temperature, different superconducting ground states can be obtained depending on the magnitude of magnetic coupling between S₁ and S₂. For increasing coupling, the superconducting region is enlarged showing an interesting result: in the strong coupling limit, where the impurities are in a very strong ferromagnetic correlation state, half of the conduction electrons are decoupled from the local moments of the impurities and take advantage of the superconducting pairing lowering the ground state energy. On the contrary, when the coupling between S₁and S₂ decreases, the scenario of the two independent Kondo impurities in presence of superconductivity emerges and all the conduction electrons are involved in the pair breaking physics. At finite temperature, we obtain the phase diagram and we observe a region of parameters where the re-entrance phenomenon occurs.     

Coexistence of superconductivity and magnetic ordering in the borocarbide superconducting system

The investigation of the intermetallic compounds Y_1−xRE_xNi₂B₂C and Y_1−xRE_xPd₅B₃C_0.4 (RE=Gd, Dy, Ho, Er; 0≤x≤1.0) demonstrates that the coexistence of the long-range-magnetic-ordering and the superconductivity is possible The normal-state magnetic susceptibility shows a Curie-Weiss-like temperature dependence and a small paramagnetic Curie temperature. The value of the effective moment is close to that of the free RE⁺³ ion. Both results suggest that the coupling between the conduction electron and magnetic ion is not strong, but the coupling is still strong enough to cause the depression of superconductivity. The Abrikosov-Gor‘kov magnetic pair-breaking theory can be applied to those systems. The corresponding magnetic-ordering temperature increases with increasing magnetic moment concentration while the superconducting transition temperature decreases. The long-range-magnetic-ordering is mainly caused by the Ruderman-Kittel-Kasuya-Yosida indirect exchange interaction via conduction electrons. This indirect exchange coupling strongly depends on the concentration of RE⁺³-moment and de Gennes factor.     

Electronic and magnetic properties of layered LnFePO (Ln=La and Ce)

Effects of the replacement of La with Ce on the electronic and magnetic properties of a layered superconductor LaFePO (T_c=∼5 K) were studied. Polycrystalline samples of CeFePO, prepared by a solid-state reaction, showed metallic conduction down to 2 K without exhibiting superconducting transition, although the resistivity decreased largely at temperatures below 30 K. Further, they showed an apparent positive magnetoresistance (MR) below ∼2 K, superposed on a negative MR. Temperature dependence of magnetic susceptibility is decomposed to a temperature-sensitive Curie-Weiss component presumably due to the Ce³⁺ ions with a magnetic moment of 1.98μ_B and a less temperature-sensitive component attributable to itinerant electrons. The magnetic interaction between Ce³⁺ ions and itinerant electrons in CeFePO likely suppresses the superconducting transition observed in LaFePO.     

Concentration dependence of the transition temperature in spin glasses

The transition temperature T_f of randomly distributed spins interacting via the RKKY interaction (spin glasses) has been calculated numerically within the spherical approximation. The dependence of T_f on the concentration c of magnetic impurities and on the damping of the interaction due to a finite mean free path of the conduction electrons is investigated for 10^?5 < c < 10^?1.     

Phase transitions in a ferromagnetic semiconductor. I

An analysis is given of the conditions for ferromagnetic phase transitions in an idealised semiconductor model containing magnetic ions. The system is described by a constant interaction — 2J/N between the magnetic ions (Spin I) and the electrons, by the energy gapΔ between two infinitely narrow bands and by the concentrationc of the magnetic ions. We find a great variety of ferromagnetic behavior. In particular there exists the possibility for the magnetization to disappear with a first or second order transition as the temperature decreases or increases. Some magnetization curves are evaluated numerically.     

Antiferromagnetic Ordering of Magnetic Clusters Units in Nb6F15

We have studied the magnetic cluster compound Nb₆F₁₅ which has an odd number of 15 valence electrons per (Nb₆F₁₂)³⁺ cluster core, as a function of temperature using nuclear magnetic resonance, magnetic susceptibility, electron magnetic resonance and neutron powder diffraction. Nuclear magnetic resonance of the ¹⁹F nuclei shows two lines corresponding to the apical F^a?a nucleus, and to the inner Fⁱ nuclei. The temperature dependence of the signal from the Fⁱ nuclei reveals an antiferromagnetic ordering at T < 5 K, with a hyperfine field of ~2 mT. Magnetic susceptibility exhibits a Curie–Weiss behavior with T _N ~5 K, and μ _eff ~1.57 μ_B close to the expected theoretical value for one unpaired electron (1.73 μ_B). Electron magnetic resonance linewidth shows a transition at 5 K. Upon cooling from 10 to 1.4 K, the neutron diffraction shows a decrease in the intensity of the low-angle diffuse scattering below Q ~0.27 Å^?1. This decrease is consistent with emergence of magnetic order of large magnetic objects (clusters). This study shows that Nb₆F₁₅ is paramagnetic at RT and undergoes a transition to antiferromagnetic order at 5 K. This unique antiferromagnetic ordering results from the interaction between magnetic spins delocalized over each entire (Nb₆F ₁₂ ⁱ )³⁺ cluster core, rather than the common magnetic ordering.     

Electron-lattice coupling of F centers: Particular case of NaI: F

Absorption and magnetic circular dichroism studies of the F centers in NaI reveal a strong electron-lattice coupling with the vibration modes of Γ₁⁺ symmetry in comparison with those of Γ₃⁺ and Γ₅⁺ symmetry. On the other hand, the F electron exhibits in its first excited state a large spin-orbit coupling, in agreement with the value previously calculated. These results are confirmed by the Raman spectra which show a predominant intensity for the Γ₁⁺ modes, particularly for the resonant mode at 113 cm^?1.First and second-order Raman spectra have been calculated using a “quasi-resonant” theory in which the vibrations of the ions up to the fifth shell have been taken into consideration. We have been able to determine from these calculations absolute values for the contributions of the different types of vibration modes to the broadening of the F-absorption band.     

Effect of electron irradiation on the paramagnetic state of La0.67Ca0.33MnO3

The effect of point defects on the magnetic properties of La_0.67Ca_0.33MnO₃ polycrystals and single crystals has been studied. The magnetic susceptibility χ _dc of the initial samples and samples irradiated by electrons to the maximum dose F = 9 × 10¹⁸ cm^?2 has been measured in the temperature region 80 K < T < 650 K. Local variations of Mn-O-Mn bond angles and lengths result in a nonmonotonic dose dependence of the Curie temperature T _C. At high doses of electron irradiation, F ≥ 5 × 10¹⁸ cm^?2, the temperature of the transition from the ferromagnetic to polaron state in a single crystal is found to increase. In the paramagnetic region close to T _C, ferromagnetically ordered polarons are observed to exist, while at T > 1.2T _C, localization of e _g electrons initiates formation of paramagnetic polarons with a higher magnetic moment. Electron irradiation stimulates persistence of magnetic polarons up to higher temperatures T > 2T _C.     

Knight shifts and magnetic susceptibilities of the intermetallic compounds CeCu4 and CeCu5

The magnetic susceptibility and Knight shift of the compounds CeCu₄ and CeCu₅ have been measured over the temperature ranges 80–800 and 140–400 K, respectively. The most important contributions to the magnetic susceptibility are the Curie-Weiss term, expressing the paramagnetism of the localized ?-electrons, and a temperature independent term, which have both been determined. The phenomenological exchange integral F_s? between the 4?-electron spins and conduction electron spins was found to be ?10.43× 10^?3 eV for CeCu₄ and 3.9 × 10^?3 eV for CeCu₅. A reversal in the sign of the s?? coupling for CeCu₅ is noted.     

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.     

Itinerant magnetism in metallic glasses

It is argued that when a ferromagnetic transition metal T (e.g. Fe) is mixed with a metalloid M to form a metallic glass (having a narrow band of conduction electrons) the competing effects of the 3-d intra-atomic Coulomb repulsion and of the interaction between conduction and d-electrons make the magnetic moment of T-atoms to vanish when the concentration x of T-atoms is smaller than a critical concentration x_c. Experimental data for the magnetic moment of iron atoms in Fe_xB_1-x is analyzed along this line and a satisfactory fit is obtained. Inclusion of structural as well as compositional disorder effects could result in a better agreement between theory and experiment.     

The anion and cation effects in the magnetic anisotropy of rare-earth compounds: Charge screening by conduction electrons

The formation of magnetic anisotropy (MA) in rare-earth compounds with transition metals has been analyzed. The screening of the charges creating the crystal field by conduction electrons has been shown to play an important role. The calculations took into account the Friedel charge-density oscillations. The model used for RCo₅ is the point-charge crystal field including nonuniform screening by conduction electrons with an anisotropic Fermi surface. The mechanisms of strong MA due to light-element impurities (hydrogen and nitrogen) are considered. The effective charge of an impurity can heavily depend on its ionic radius and the characteristics of the Fermi surface (in particular, on the Fermi momentum k _F ) of the screening electrons. The screening of the cation and anion charge in hydrides and nitrides based on the R₂Fe₁₇ and RFe₁₁Ti intermetallic compounds is discussed.     

Localization of conduction electrons and the magnetic properties of the molecular metals β″-(BEDT-TTF)4NH4[M(C2O4)3] · DMF (M = Cr3+, Fe3+)

Quasi-two-dimensional organic metals β″-(BEDT-TTF)₄NH₄[M(C₂O₄)₃] · DMF containing the oxalate complexes of Cr³⁺ or Fe³⁺ ions between the conducting organic layers of the BEDT-TTF molecules are studied by EPR spectroscopy, and the contributions of these metallic complexes, conduction electrons, and non-equilibrium lattice defects to the magnetic susceptibility are determined. An analysis of the temperature dependence of the magnetic susceptibility and the EPR line shape has revealed partial localization of conduction electrons at T < 20 K in the crystals with Cr³⁺ ions. The size of the localization region is close to the size of an individual BEDT-TTF molecule. The localization effect weakens as nonequilibrium defects disappear during long-term storage at room temperature. The localization of conduction electrons is found to be accompanied by the appearance of weak antiferromagnetic interaction between the Cr³⁺ ions at T < 20 K, which disappears when Fe³⁺ ions substitute for Cr³⁺ ions.     

Electronic structure,spin polarization and high critical fields in Chevrel compounds

Results are presented of an extensive theoretical study of the origin of high field superconductivity and/or magnetism in a number of Chevrel phase ternary compounds, MMo₆X₈ (with M=Sn, Eu, Gd and X=S and/or Se) based on self-consistent linear muffin-tin orbital (LMTO) energy band calculations using the local density approach (Hedin et al. exchange correlation) for the paramagnetic structures and local spin density formalism (Gunnarsson and Lundqvist) for the ferromagnetic structures. All electrons and all 15 atoms/cell are included with the core electrons (including the 4f's) recalculated in each iteration in a fully relativistic representation and the conduction electrons treated semirelativistically (all relativistic terms except spin-orbit). Superconductivity is found to be due to the high Mo d-band density of states (DOS) at E_F resulting from the unusual large charge transfer of Mo electrons to the chalcogen sites. There is also a large charge transfer from the metal site to the cluster (≈2 electrons in Sn and Eu) giving essentially no occupied conduction bands, for example, at the Eu site and a divalent ion isomer shift in very good agreement with the experiments of Dunlap et al. The conduction-electron DOS at the Eu site is found to be reduced by an order of magnitude from its metallic state value - in close agreement with their spin - lattice relaxation rate measurements. This low conduction-electron DOS yields very weak coupling of the 4f electrons to the conduction electrons and only a very weak Ruderman-Kittel-Kasuya-Yosida magnetic interaction showing why all the Chevrel rare-earth compounds - except Ce and Eu - are superconducting despite their having large local magnetic moments. The unusually high upper critical fields, H_c2, in these materials is found to be due to the unusully flat energy bands near F_F. The ferromagnetic (spin polarized) results for the Eu- and Gd-compounds show a net small but positive magnetic moment on the metal site and a small but negative induced spin magnetic moment on the Mo site in the Eu compound. Fermi-contact contributions to the hyperfine field are calculated and found to be in good agreement with the Eu Mössbauer results and the negative NMR Knights shift results of Fradin et al. These results demonstrate theoretically for the first time the validity of the Fischer et al. and Fradin et al. conclusion that the Jaccarino-Peter mechanism is responsible for the large increase in the H_c2 when large concentrations of Eu magnetic impurities are substituted in SnMo₆S₈. Finally, calculated Stoner factors for the paramegnetic phase and spin magnetization densities for the ferromagnetic phase are used to discuss qualitatively the origin of the different behavior observed for GdMo₆S₈ and EuMo₆S₈.     

Itinerant electron theory of transition metal compounds of mixed valency: La1−xSrxMnO3

We use a simplified model to calculate the electronic band structure of compounds of the type of La_1?xSr_xMnO₃. The model includes two nonequivalent sites for the transition metal ions as well as strong Coulomb and exchange interactions between d electrons. Using the resulting band structure we discuss the correlation between magnetic order and conductivity in these compounds.     

Characteristic structure in core electron spectra of metals due to the electron-plasmon coupling

The coupling of metallic core electrons to the density fluctuations of the conduction electrons is studied. Due to the strong electron-plasmon coupling there is a characteristic satellite structure in the core electron spectrum, starting at the plasma energy?ω _p below the quasiparticle level and with a maximal spectral weight at (1.6–2.5)×?ω _p below the same level. The total spectral weight in the satellite band is 50–100 percent of the quasiparticle weight, the actual value dependent on the density of the conduction electrons. The possible implications on X-ray photoemission, soft X-ray emission and absorption, and inelastic scattering of electrons are drawn. Particularly, a close correspondence with the location of the fine structure of the L_2,3 absorption spectrum of Al is found. The relation to the cohesive energy is also considered.     

Temperature evolution of spin-transfer switching in nonlocal spin valves with dipolar coupling

The spin-transfer effect has been achieved in nanoscale metallic nonlocal spin valves. A magnetic domain (∼70×150 nm²) in an extended wire can be switched by a pure spin current between 4.5 and 200 K. The dipolar coupling between the magnetic spin injector (F₁) and spin detector (F₂), the surface anisotropy of the thin F₂ layer, and the thermal instability of F₂ generates complex switching characteristics. Analysis of the results allows for detailed understanding of magnetic configurations during the current-sweep and the field-sweep measurements. The critical current (I_c) for spin-transfer switching gradually decreases as the temperature increases. The I_c⁺ for the transition from parallel (P) state to antiparallel (AP) state decreases faster than the I_c^‐ for the transition from AP to P due to the dipolar coupling. Above 200 K, the dipolar coupling and the thermal instability prevents a stable P state in the absence of an external field.