Concentration Of Magnetic Impurities Research Articles

Are ESR lines in spin glasses exchange narrowed?

By using the Kubo-Tomita formalism, we calculated the concentration and frequency dependence of the exchange narrowed dipolar broadened resonance linewidth and shift at high temperatures. We find the linewidth varies as the square root of the magnetic impurity concentration which is in good agreement with recent data. While at low frequencies the line is in the limit of extreme exchange narrowing, we find at high frequencies, 35 GHz, the 10/3 effect is no longer operative.

Modified mean-random-field theory for spin glass—ferromagnetic systems

Spin glass—ferromagnetic systems such as AuFe alloys having both the ferromagnetic and RKKY interactions are studied on the basis of a mean-random-field approximation. We obtain the phase diagram of the systems in terms of the temperature and the magnetic impurity concentration. The results roughly explain the magnetism of AuFe type alloys.

Susceptibility and order parameter for a superconductor with coupled magnetic impurities.

The influence of impurity pair interactions in superconductors is calculated in the spin-glass regime of 1--10 % magnetic-impurity concentrations. We use the s-d Hamiltonian to represent the exchange coupling J of an impurity spin to the conduction electrons; this term leads to the Ruderman-Kittel-Hasuya-Yosida (RRKY) coupling among impurities, and yields major modifications in the superconducting order parameter as well as the conduction-electron susceptibility. As the impurity concentration is increased, ferromagnetic RKKY coupling among impurities causes a reentrant form of the superconductivity order parameter and a corresponding large increase in the susceptibility. Antiferromagnetic RKKY interactions tend to counteract the isolated impurity suppression of the superconducting transition temperature and thus favor the persistence of superconductivity at higher impurity concentrations in contrast to the dilute-limit theory of Abrikosov and Gor'kov. In certain situations, the antiferromagnetic impurity coupling may in fact allow materials to be superconducting which would otherwise be expected to be normal metals. These results are applied to experiments on rare-earth compounds.

PAC study of spin glass freezing in A uFe alloys

We introduce perturbed angular correlation (PAC) on radioactive isotopes as a new and feasible technique to sense the hyperfine field distribution in randomly disordered alloys. This technique is similar to μSR in the sense that it is a nuclear spin precession measurement, but more widely applicable in view of the limited instrumentation. We present our first results obtained using 111Cd as a typical PAC probe in Au–Fe alloys of different magnetic impurity concentration. Our data clearly show the onset of the spin glass freezing and allows one to derive the mean field value as a function of temperature as well as magnetic impurity concentration.

Absence of itinerant-electron ferromagnetism in LaRh6B4

Contrary to previous studies which reported itinerant-electron ferromagnetism for La${\mathrm{Rh}}_{6}$${\mathrm{B}}_{4}$ at 6 K, no evidence for ferromagnetism is found for high-purity La${\mathrm{Rh}}_{6}$${\mathrm{B}}_{4}$ in the ac susceptibility down to 40 mK and in the static magnetization down to 3 K. It would seem probable that the "ferromagnetism" reported previously for this compound was not intrinsic but arose from too high a concentration of magnetic impurities. Similar studies on Ce${\mathrm{Rh}}_{6}$${\mathrm{B}}_{4}$, Y${\mathrm{Rh}}_{6}$${\mathrm{B}}_{4}$, and Lu${\mathrm{Rh}}_{6}$${\mathrm{B}}_{4}$ fail to find any evidence for either ferromagnetism or superconductivity above 40 mK.

Superconductivity and Localization in Disordered Systems: A Local Gauge Invariant Approach

A new tight binding theory for superconductivity in disordered systems is constructed on the basis of local gauge invariant n orbital models. The large n limit realizes and justifies BCS type results for weak coupling and arbitrarily narrow bands as exact solutions including modifications from the variable density of states. The 1/ n expansion provides a systematic treatment of superconducting-, localization-, and magnetic fluctuations and of Coulomb repulsion. As first applications we derive i) deviations from Anderson's theorem due to the density of states variation and ii) logarithmic reduction of the critical concentration of magnetic impurities due to localization in 2D systems and critical reduction of T c in 3D due to critically decreased spinflip lifetime near the mobility edge (diffusion localization). A similar effect is found for the upper critical magnetic field and is compared with recent theories of Fukuyama and Maekawa et al.

Spin-glass transition as a problem of percolation of overlapping spheres

A theory of substitutional as well as amorphous RKKY-type spin glasses has been developed following a qualitative idea first suggested by Smith and more recently by Mydosh. The theory has been developed to a stage where quantitative calculations become possible on realistic systems. The paramagnetic to spin-glass transition has been described as a percolation of temperature dependent clusters of spins in a continuum. The glass transition temperature has been obtained as a function of the magnetic impurity concentration, after taking the effects of frustration and finite mean free path into account. Different 'critical' temperatures for different experimental probes with characteristic time and length scales have also been studied.

Electrical resistivity in spin glass C uMn

The electrical resistivity is discussed, providing a direct measurement of the critical exponent β for a spin glass CuMn. CuMn alloys with magnetic impurity concentrations ∼0.7–4.5 at. % exhibit the characteristic spin glass cusp in low field ac susceptiblity at temperature TF. The magnetic contribution to the resistance of this alloy increases with temperature below Tf and is constant above Tf. By using the exchange interaction between the conduction electrons and the unpaired electrons localized on the Mn ion as a perturbation and by applying the Boltzmann transport equation, the electrical resistivity is shown to be related to the order parameter q. Below the freezing temperature, scattering can be caused by deviation of the spin from its ground state. As the temperature rises, the resistivity will increase, too. Above the freezing temperature, each spin behaves perfectly independently and gives rise to a constant resistivity. So the critical exponent β of spin glass order parameter q can therefore be determined by fitting the theory to measured data. It also can be shown that specific heat is related to the temperature derivative of ρmag(T) for ordered magnetic moment material.

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, MMo 6X 8 (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 c 2 , 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 c 2 when large concentrations of Eu magnetic impurities are substituted in SnMo 6S 8. 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 GdMo 6S 8 and EuMo 6S 8.

Magnetic Gruneisen parameters for some spin glasses

Values for the magnetic Gruneisen parameter gamma M for CuMn, (Cu-10 at.% Al)Mn and AgMn alloys determined form low-temperature thermal expansion and specific heat capacity measurements are reported. mu M is found to be both temperature- and concentration-dependent in contrast to the simple RKKY model for the spin-glass ordering. The concentration dependence of mu M differs significantly from that reported earlier based upon a combination of thermal expansion and specific heat capacity measurements made independently on different samples. Dipolar anisotropy is discussed as a possible cause of the dependence of mu M on temperature and magnetic impurity concentration.

Pairbreaking in superconductors near and below antiferromagnetic transitions

The Abrikosov-Gorkov theory for pairbreaking in superconductors due to a dilute concentration of magnetic impurities is generalized to the case when the magnetic ions are present in a large concentration or on a regular lattice. The case in which the magnetic ions undergo a transition to an antiferromagnetic phase is considered in detail. The physical considerations for increased or decreased pairbreaking over the Abrikosov-Gorkov value due to the increased range of correlations near the antiferromagnetic transition and due to inelastic scattering of electrons off spin waves in the antiferromagnetic phase are discussed in detail. Recent experimental results are considered in terms of the new theory.

Muon spin depolarization in nonmagnetic metals doped with paramagnettc impurities

The diffusion of muons and their magnetic interactions are treated by describing the physics to be learned from experiments which measure muon depolarization in metallic hosts doped with dilute concentrations of magnetic impurities. (GHT)

Transport Properties of SnTe-MnTe System: Anomalous Hall Effect

Electrical measurements have been made of the Bridgman-grown degenerate magnetic semiconductors Sn 1- x Mn x Te ( x <19 at.%) over a temperature range from 300 to 2 K. The magnetic impurities, Mn ions, have an effect on carrier transport in two different ways. First, they act simply as ionized impurity scattering centers giving rise to a decrease in the Hall mobility; the exponent n in the temperature dependence of the Hall mobility in the range 77–300 K, µ∝ T - n , and the residual resistivity at 4.2 K, ρ 4.2 , are affected appreciably by the magnetic impurity concentration. Second, the localized spins on the Mn ions come into play at low temperatures, which align ferromagnetically at an ordering temperature. As a result, the Hall effect consists of the ordinary component due to the Lorentz force and the anomalous one arising from spin dependent scattering of conduction carriers through a spin-orbit coupling; the second component is found to be strongly temperature-dependent, R 1 ∝ T 2 , in agreement with the Voloshinskii model.

Paramagnetic resonance in a ?dirty? spin-glass

For a spin-glass with nonmagnetic defects (n m 1/3l ≪ 1, where n m is the magnetic impurity concentration and l is the mean free path) an absorption function χ″(Ω) is derived. Three ranges of temperature and external magnetic field are considered. In the vicinity of the “transition” the value of ∫ χ″(Ω) dΩ is estimated as a function of temperature and field.

Quasiparticle branch mixing rates in superconducting aluminum

The kinetic equation is used to compute the elastic and inelastic quasiparticle branch mixing rates for a superconducting film into which quasiparticles are injected via a tunnel barrier from a second superconducting film. Representative graphs are presented of the steady-state quasiparticle distribution, the quasiparticle charge imbalance ${Q}^{*}$ versus injection current, the charge relaxation rate ${\ensuremath{\tau}}_{{Q}^{*}}^{\ensuremath{-}1}$ vs $\frac{\ensuremath{\Delta}}{{k}_{B}{T}_{c}}$ for several values of elastic scattering rate, and the quasiparticle branch relaxation rate ${\ensuremath{\tau}}_{Q}^{\ensuremath{-}1}$ as a function of energy. The quasiparticle potential developed in the injection film is related to ${\ensuremath{\tau}}_{Q}^{\ensuremath{-}1}$ and thence to ${\ensuremath{\tau}}_{0}^{\ensuremath{-}1}$ a characteristic electron-phonon scattering time. Detailed measurements of ${\ensuremath{\tau}}_{Q}$ are reported for films of superconducting A1, some of which were doped with oxygen to give a range of transition temperatures from 1.2 to 2.1 K. From the dependence of ${\ensuremath{\tau}}_{{Q}^{*}}^{\ensuremath{-}1}$ on $\frac{\ensuremath{\Delta}}{{k}_{B}{T}_{c}}$, values are deduced for the gap anisotropy of the films. In the cleanest samples, ${\ensuremath{\tau}}_{0}=0.10\ifmmode\pm\else\textpm\fi{}0.02$ \ensuremath{\mu} sec, a value that is in good agreement with energy-gap relaxation and $2\ensuremath{\Delta}$- phonon (phonons of energy $\ensuremath{\gtrsim}2\ensuremath{\Delta}$) mean-free-path measurements, but a factor of about 4 smaller than that obtained from recombination time measurements and theoretical calculations. The value of ${\ensuremath{\tau}}_{0}^{\ensuremath{-}1}$ in the A1 films increases with the transition temperature ${T}_{c}$ as ${T}_{c}^{5}$ or ${T}_{c}^{6}$, instead of ${T}_{c}^{3}$ as predicted by simple theory. It is suggested that the rapid increase of ${\ensuremath{\tau}}_{0}^{\ensuremath{-}1}$ with ${T}_{c}$ may arise from either a strong dependence of ${\ensuremath{\alpha}}^{2}F(\ensuremath{\omega})$ on ${T}_{c}$ or from a small concentration of magnetic impurities.

SUSCEPTIBILITIES AND ORDER PARAMETERS OF NEMATIC LIQUID CRYSTALS

SUSCEPTIBILITIES AND ORDER PARAMETERS OF NEMATIC LIQUID CRYSTALS

Spin-glass with nonmagnetic defects

A theory is presented of spin-glasses with n m 1/3l≪1, where n m is the magnetic impurity concentration and l the mean free path (this corresponds to a large concentration of nonmagnetic defects). The specific heat, susceptibility, and resistivity are derived. Five temperature ranges can be distinguished in which all of these exhibit different behavior. The nature of the transition is discussed. The physical conditions for the applicability of the model are considered.

Host Nuclear Magnetic Resonance in Copper‐Based Magnetic Alloys

AbstractThe lineshape of the bulk Cu63 NMR signal of the Kondo systems _Cu—Mn, _Cu—Fe, and _Cu—Co is investigated experimentally as a function of the temperature and of the concentration of the dilute magnetic impurities. The measurements are compared with Monte‐Carlo simulated line‐shapes broadened by charge density oscillations as well as spin density oscillations of the conduction electrons in the surrounding of the impurity atoms. The increasing coupling between the impurity and the conduction electrons with decreasing temperatures (T &lt; TK) leads to a reduction in the amplitude of charge density oscillations in the range of the coherence length. The analysis of the experimental data gives the coupling constant |Jsd| of the different Kondo systems, showing the minimum value of |Jsd| in the middle of the 3d series for _Cu—Mn.

Nuclear spin-lattice relaxation in a Kondo superconductor

The nuclear spin--lattice relaxation rate R/sub s/ in a Kondo superconductor has been calculated numerically in the framework of the Mueller-Hartmann-Zittartz depairing theory. The calculations have been performed in the case of the usual Korringa mechanism for certain concentrations of magnetic impurities and some typical values of T/sub kappa//T/sub c0/, where T/sub k/ is the Kondo temperature and T/sub c0/ is the superconducting transtition temperature in the absence of magnetic impurities. The temperature variation of R/sub s/ shows very interesting behavior for values of T/sub k//T/sub c0/ corresponding to the reentrance behavior of superconductivity. Results are compared with the experimental data on the La(Ce)Al/sub 2/ system obtained by MacLaughlin, Alloul, and Daugherty. Other Possible relaxation mechanisms in the La(Gd, Ce)Al/sub 2/ systems are also discussed.

The effect of impurities on the spin-density-wave transition

Abstract The effect of impurities on the second order phase transition into the commensurate spin-density-wave state is considered for a two-band itinerant-electron antiferromagnet with imperfect nesting. The enhancement of electron-hole pairing by polarized magnetic impurities and the pair destruction effect of scattering by both magnetic and non-magnetic scatters is exhibited in a calculation of the dependence of the Neel temperature on magnetic impurity concentration and on the degree of imperfect nesting. The commensurate SDW is shown to be stabilized by magnetic impurities.