Dilute Magnetic Impurities Research Articles

Impurities in Magnetic Materials Studied by PAC Spectroscopy

Perturbed gamma-gamma angular correlation (PAC) spectroscopy is a precise and highly efficient tool to follow the temperature dependence of local magnetic fields in any material. Its resolution and efficiency does not depend on temperature and therefore can measure local fields at low as well as high temperature with the same accuracy. Due its versatility in using different probe nuclei it can sense the local magnetic fields at different sites in the crystalline structure of materials. In this review, important results obtained with PAC spectroscopy are shown in two classes of materials: transition metal and transition-metal based compounds and rare earth elements and rare-earth-element based compounds using mainly three different probe nuclei: 111Cd, 181Ta and 140Ce. PAC spectroscopy has contributed to the systematic study of the magnetic hyperfine field in impurities in matrices of Fe, Co and Ni as well as in transition-metal based Heusler alloys. It has also provided important contribution to the investigation of magnetism in rare-earth elements and intermetallic compounds. An still open issue concerning the local fields in metallic magnetic compounds and elements is the exchange interaction between the magnetic ions of the host and a dilute magnetic impurity, which acts as a defect in the magnetic lattice. PAC spectroscopy has been contributing to study this problem with success. Also shown in this review is the crucial role of ab-initio first principle calculations in the interpretation of PAC results.

Critical Metal Phase at the Anderson Metal-Insulator Transition with Kondo Impurities

It is well known that magnetic impurities can change the symmetry class of disordered metallic systems by breaking spin and time-reversal symmetry. At low temperature, these symmetries can be restored by Kondo screening. It is also known that at the Anderson metal-insulator transition, wave functions develop multifractal fluctuations with power-law correlations. Here, we consider the interplay of these two effects. We show that multifractal correlations open local pseudogaps at the Fermi energy at some random positions in space. When dilute magnetic impurities are at these locations, Kondo screening is strongly suppressed. When the exchange coupling J is smaller than a certain value J;{*}, the metal-insulator transition point extends to a critical region in the disorder strength parameter and to a band of critical states.

Nonperturbative Scaling Theory of Free Magnetic Moment Phases in Disordered Metals

The crossover between a free magnetic moment phase and a Kondo phase in low-dimensional disordered metals with dilute magnetic impurities is studied. We perform a finite-size scaling analysis of the distribution of the Kondo temperature obtained from a numerical renormalization group calculation of the local magnetic susceptibility for a fixed disorder realization and from the solution of the self-consistent Nagaoka-Suhl equation. We find a sizable fraction of free (unscreened) magnetic moments when the exchange coupling falls below a critical value Jc. Between the free moment phase due to Anderson localization and the Kondo-screened phase we find a phase where free moments occur due to the appearance of random local pseudogaps at the Fermi energy whose width and power scale with the elastic scattering rate 1/tau.

Magnetic characterization of superconducting La–Pr alloys

In this work we present detailed magnetization measurements in La and in La-2 at% Pr alloy, from which we determine, in particular, the influence of dilute magnetic impurities on the upper critical magnetic field amplitude [and then on the superconducting coherence length amplitude ξ ( 0 ) ] and on the Ginzburg–Landau parameter, κ .

Theoretical study of magnetism in transition-metal-dopedTiO2andTiO2−δ

In recent years there has been an intense search for room temperature ferromagnetism in semiconductors doped with dilute magnetic impurities, in particular, oxides. In this work we study the structural, electronic, and magnetic properties of doped rutile $\mathrm{Ti}{\mathrm{O}}_{2}$ for two different impurity concentrations (25% and 6.25%), considering different distributions of the impurities in the host lattice. Calculations were performed with ab initio methods, assuming that the magnetic impurities substitutionally replace the Ti ions. Our results show that a local magnetic moment appears in the cases of Mn, Fe, and Co impurities, but not in the cases of Ni and Cu impurities. They also show that in the system ${\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{2}$ the magnetic ions align ferromagnetically, while in ${\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{2}$ and ${\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{2}$ the antiferromagnetic alignment is energetically favorable. We have also studied the effect of oxygen vacancies, which turned out to be very important. Their presence decreases the energy required to introduce the impurities in the host lattice and reciprocally, the presence of impurities is related to a higher vacancy concentration. The pairs impurity-nearest-neighbor oxygen vacancy seem to be the energetically preferred structures and to produce the highest local magnetic moments. Ni and even Cu impurities acquire magnetic moments in this environment.

Effect of Magnetic Impurities on Energy Exchange between Electrons

In order to probe quantitatively the effect of Kondo impurities on energy exchange between electrons in metals, we have compared measurements on two silver wires with dilute magnetic impurities (manganese) introduced in one of them. The measurement of the temperature dependence of the electron phase coherence time on the wires provides an independent determination of the impurity concentration. Quantitative agreement on the energy exchange rate is found with a theory by Göppert et al. that accounts for Kondo scattering of electrons on spin-1/2 impurities.

Aharonov-Bohm Effect as a Probe of Interaction between Magnetic Impurities

We study the effects of the RKKY interaction between magnetic impurities on the mesoscopic conductance fluctuations of a metal ring with dilute magnetic impurities. At sufficiently low temperatures and strong magnetic fields, the loss of electron coherence occurs mainly due to the scattering off rare pairs of strongly coupled magnetic impurities. We establish a relation between the dephasing rate and the distribution function of the exchange interaction within such pairs. In the case of the RKKY exchange interaction, this rate exhibits 1/B(2) behavior in strong magnetic fields. We demonstrate that the Aharonov-Bohm conductance oscillations may be used as a probe of the distribution function of the exchange interaction between magnetic impurities in metals.

Dephasing of electrons in mesoscopic metal wires

We have extracted the phase coherence time $\tau_{\phi}$ of electronic quasiparticles from the low field magnetoresistance of weakly disordered wires made of silver, copper and gold. In samples fabricated using our purest silver and gold sources, $\tau_{\phi}$ increases as $T^{-2/3}$ when the temperature $T$ is reduced, as predicted by the theory of electron-electron interactions in diffusive wires. In contrast, samples made of a silver source material of lesser purity or of copper exhibit an apparent saturation of $\tau_{\phi}$ starting between 0.1 and 1 K down to our base temperature of 40 mK. By implanting manganese impurities in silver wires, we show that even a minute concentration of magnetic impurities having a small Kondo temperature can lead to a quasi saturation of $\tau_{\phi}$ over a broad temperature range, while the resistance increase expected from the Kondo effect remains hidden by a large background. We also measured the conductance of Aharonov-Bohm rings fabricated using a very pure copper source and found that the amplitude of the $h/e$ conductance oscillations increases strongly with magnetic field. This set of experiments suggests that the frequently observed ``saturation'' of $\tau_{\phi}$ in weakly disordered metallic thin films can be attributed to spin-flip scattering from extremely dilute magnetic impurities, at a level undetectable by other means.

T/sub C/ suppression in superconducting films for use in transition edge sensors

Transition edge sensor (TES) microcalorimeters have proven their value as photon detectors in several wavelength regimes. The central element of a TES is a superconducting film with a transition temperature designed to be at a specific temperature, usually 100 - 500 mK. These films are typically fabricated by depositing bilayers or multilayers of superconducting and normal metals (e.g., molybdenum and copper) whose relative thicknesses have been engineered to produce the desired superconducting transition temperature (T/sub C/) through the proximity effect. Although these fabrication methods have been widely applied, bilayer and multilayer deposition is often difficult, and care must be taken in controlling thicknesses and surface interface quality. An alternative method is to dope the superconducting film with dilute magnetic impurities in order to suppress its T/sub C/ to the desired temperature. This technique has been used successfully in tungsten, but tungsten films are not well-suited to all TES applications. We have thus undertaken a project to investigate T/sub C/ suppression in molybdenum and aluminum films. We present our results from impurity doping using iron and manganese in these films.

Dephasing by extremely dilute magnetic impurities revealed by Aharonov-Bohm oscillations.

We have probed the magnetic field dependence of the electron phase coherence time tau(phi) by measuring the Aharonov-Bohm conductance oscillations of mesoscopic Cu rings. Whereas tau(phi) determined from the low-field magnetoresistance saturates below 1 K, the amplitude of Aharonov-Bohm h/e oscillations increases strongly on a magnetic field scale proportional to the temperature. This provides strong evidence that a likely explanation for the frequently observed saturation of tau(phi) at low temperature in weakly disordered metallic thin films is the presence of extremely dilute magnetic impurities.

Effect of spin-orbit scattering on the magnetic and superconducting properties of nearly ferromagnetic metals: application to granular Pt.

We calculate the effect of scattering on the static, exchange enhanced, spin susceptibility and show that, in particular, spin-orbit scattering leads to a reduction of the giant moments and spin glass freezing temperature due to dilute magnetic impurities. The harmful spin fluctuation contribution to the intragrain pairing interaction is strongly reduced opening the way for BCS superconductivity. We are thus able to explain the superconducting and magnetic properties recently observed in granular Pt as being due to scattering effects in single small grains.

Nuclear Orientation Studies of Magnetism in Single Crystal Pr0.5Nd0.5Ni

The angular distributions of γ-rays from 142Pr, 147Nd and 144Pm in Pr0.5Nd0.5Ni were investigated by means of low-temperature nuclear orientation. The results show that the Pr and Nd electronic moments order along the c- and a-axis, respectively. Pm ions introduced as a very dilute magnetic impurity, preferentially align with the Nd ions.

Helical spin-density wave inV2−yO3

Recent neutron scattering and nuclear magnetic resonance experiments have revealed that the low temperature phase of doped V_{2-y}O_3 is an itinerant antiferromagnet with a helical spin structure. We use a band structure calculation as the point of departure to show that these experiments are in agreement with mean field results for an Overhauser spin-density wave state. The influences of a finite life-time and of dilute magnetic impurities are discussed.

Renormalization-group study of Anderson and Kondo impurities in gapless Fermi systems

Thermodynamic properties are presented for four magnetic impurity models describing the scattering of fermions from a localized orbital at an energy-dependent rate $\ensuremath{\Gamma}(\ensuremath{\epsilon})$ which vanishes precisely at the Fermi level, $\ensuremath{\epsilon}=0$. Specifically, it is assumed that for small $|\ensuremath{\epsilon}|$, $\ensuremath{\Gamma}(\ensuremath{\epsilon})\ensuremath{\propto}|\ensuremath{\epsilon}{|}^{r}$ with $r>0$. The cases $r=1$ and $r=2$ describe dilute magnetic impurities in unconventional $(d$- and $p$-wave) superconductors, ``flux phases'' of the two-dimensional electron gas, and certain zero-gap semiconductors. For the nondegenerate Anderson model, the main effects of the depression of the low-energy scattering rate are the suppression of mixed valence in favor of local-moment behavior and a marked reduction in the exchange coupling on entry to the local-moment regime, with a consequent narrowing of the range of parameters within which the impurity spin becomes Kondo screened. The precise relationship between the Anderson model and the exactly screened Kondo model with power-law exchange is examined. The intermediate-coupling fixed point identified in the latter model by Withoff and Fradkin (WF) is shown to have clear signatures both in the thermodynamic properties and in the local magnetic response of the impurity. The underscreened, impurity-spin-1 Kondo model and the overscreened, two-channel Kondo model both exhibit a conditionally stable intermediate-coupling fixed point in addition to unstable fixed points of the WF type. In all four models, the presence or absence of particle-hole symmetry plays a crucial role in determining the physics both at strong coupling and in the vicinity of the WF transition. These results are obtained using an extension of Wilson's numerical renormalization-group technique to treat energy-dependent scattering. The strong- and weak-coupling fixed points of each model are identified and their stability is analyzed. Algebraic expressions are derived for the fixed-point thermodynamic properties and for low-temperature corrections about the stable fixed points. Numerical data are presented confirming the algebraic results, identifying and characterizing intermediate-coupling (non-Fermi-liquid) fixed points, and exploring temperature-driven crossovers between different physical regimes.

Resistance Anomaly Near Superconducting Transition Temperature of Zr–Cu Metallic Glasses

The theoretical analyses of the recent discovery of a new resistance anomaly in Zr–Cu superconducting metallic glasses with dilute magnetic impurities have been made. The resistance peaks are found to occur within 20–60 mK range above the resistive midpoint of the superconducting transition. The influence of Zr concentration on the peak has also been investigated. The results are in good qualitative agreement with the reported experimental findings.

Behavior of magnetic impurities in gapless Fermi systems.

In a number of systems, including certain semiconductors and unconventional superconductors, the effective density of states varies near the Fermi energy like $|E-E_F|^r$. The behavior of dilute magnetic impurities in such systems is studied using a non-perturbative renormalization-group approach. Close to particle-hole symmetry, the Kondo effect is suppressed for the cases of greatest relevance ($r=1$ and $2$). Away from this symmetry, any quenching of the impurity moment is accompanied by a low-temperature decrease in the impurity resistivity, rather than the increase found in metals.

Proximity-effect-induced superconductivity in normal metals which exhibit the Kondo effect

The Meissner effect in the normal metals induced by the proximity effect of Nb has been investigated in relation to the Kondo effect for Cu with dilute magnetic impurities: Mn and Fe, Co, Ni. The thickness ρ of the Meissner region in Cu/Mn becomes nearly constant at low temperatures due to the magnetic scattering. The data provide a spin-flip scattering probability per 1 ppm Mn of about 5·109s−1. A study was also made of the proximity effect for heavy-fermion materials: CeAl3 and CeCu6. No measurable Meissner effect was observed in these materials.

Superconducting and magnetic properties of Y 1− xU xRh 4B 4

The heat capacity, magnetic susceptibility and magnetization of Y 1− x U x Rh 4B 4 were determined within the range from x = 0 to x = 0.65. The influence of the local magnetic moments of the U atoms (d U-U = 5.27 A ̊ ) in the CeCo 4B 4 structure of the tetragonal RERh 4B 4 was studied. A.c. susceptibility measurements showed a rather large value for the T c depression of d T c/d x = −1.88 K per at. % U in the lower concentration range x = 0.0 to x = 0.06. In comparison with the (Bardeen-Cooper-Schrieffer) BCS theory and the Abrikosov-Gorkov model for dilute magnetic impurities, the decrease in the jump in specific heat at T c vs. the depression of T c is rather strong in these samples, so the existence of a demagnetizing Kondo cloud around the U moments seems likely. In the samples containing more U, the magnetization and d.c. susceptibility showed a tendency towards magnetic behaviour, but no evidence for long-range magnetic order was found.

Superconducting proximity effect in Cu-clad Nb wires doped with Mn, Fe, Co and Ni

The superconducting proximity effect in Cu-clad Nb wires, doped with dilute magnetic impurities, has been studied in relation to the Kondo effect. The spin flip scattering by Mn is found to be rapidly suppressed below 0.5 K, probably due to the Mn-Mn interaction, while that by Fe, Co and Ni decreases linearly with temperature below 1 K.

Superconducting Proximity Effect in Cu Clad Nb Wires Doped with Mn, Fe, Co and Ni

The proximity effect in Cu clad Nb wires of which Cu is doped with dilute magnetic impurities, has first been investigated in relation to the Kondo effect. As for Cu doped with dilute Fe, Co and Ni impurities, the probability of the spin flip scattering of conduction electrons, α, by Fe, Co and Ni impurities decreases linearly with decreasing temperature. As for Cu doped with dilute Mn impurities, the probability, α, by Mn impurities is concluded to be suppressed by the RKKY interaction between Mn impurities which depends on the electronic mean free path. The temperature dependence of the resistivity of the Mn alloy may be understood as a result of the coexistence of the dilute Kondo effect and the RKKY interaction between Mn impurities.