Dilute Magnetic Alloys Research Articles

Kondo resonance effects in emergent flat band materials

Macroscopic degrees of freedom that are involved in the transport of carriers through mesoscopic electronic devices are susceptible to the effects of strong many-body correlations. The presence of magnetic impurities in dilute magnetic alloys typically allow for insights into Kondo effect from the scattering of free carriers by localized electron states of the magnetic impurities but this effect is not well understood when there are no d-band electron states. Herein, the signatures of Kondo resonance effect are elucidated in quantum dots derived from a carbon-nanoline embedded monolayer hexagonal boron nitride whose electron states host flat band ferromagnetism as distinct broken symmetry states. Quantum transport state of mesoscopic devices modelled as quantum dots tunnel coupled to metallic leads is computed by direct diagonalization of the Hamiltonian. The possibility of realizing quantum dots with highly tunable electron states in energy interconversion devices is discussed to show the importance of screening effects on single-electron energy levels. The quantum master equation is solved within different formalisms to determine the stationary-state particle and energy currents. Stability diagrams are calculated to show the dependence of the conductance on experimental control variables of the quantum dot device. The computed responses of the stationary-state transport signatures are used to characterize Kondo resonance effects from flat band states of embedded carbon nanoline-based quantum dots. It is found that the local network structure of the hexagonal ring carbon cluster-based quantum dot has a broken particle-hole symmetry in the transport state. This signals the formation of the quasiparticle states expected in second order scattering when the macroscopic “charge” pseudospin symmetry of the tunnelling electron state is broken dynamically due to charging. The results are discussed to show the implications of a vanishing particle-hole symmetry in the carrier transport state of quantum dots for energy conversion applications.

Spin-glass transition in the spin\u2013orbit-entangled Jeff\u2009=\u20090 Mott insulating double-perovskite ruthenate

We have successfully synthesized new Ru4+ double perovskite oxides SrLaInRuO6 and SrLaGaRuO6, which are expected to be a spin–orbit coupled Jeff = 0 Mott insulating ground state. Their magnetic susceptibility is much significant than that expected for a single Ru4+ ion for which exchange coupling with other ions is negligible. Their isothermal magnetization process suggests that there are about 20 percent isolated spins. These origins would be the Ru3+/Ru5+ magnetic defects, while the regular Ru4+ sites remain nonmagnetic. Moreover, SrLaGaRuO6 shows a spin-glass-like magnetic transition at low temperatures, probably caused by isolated spins. The observed spin-glass can be interpreted by the analogy of a dilute magnetic alloy, which can be seen as a precursor to the mobile Jeff = 1 exciton as a dispersive mode as predicted.

Seebeck effect in nanomagnets

We present a theory of the Seebeck effect in nanomagnets with dimensions smaller than the spin diffusion length, showing that the spin accumulation generated by a temperature gradient strongly affects the thermopower. We also identify a correction arising from the transverse temperature gradient induced by the anomalous Ettingshausen effect and an induced spin-heat accumulation gradient. The relevance of these effects for nanoscale magnets is illustrated by ab initio calculations on dilute magnetic alloys.

Enhancement of Spin-Charge Conversion in Dilute Magnetic Alloys by Kondo Screening.

We derive a kinetic theory capable of dealing both with large spin-orbit coupling and Kondo screening in dilute magnetic alloys. We obtain the collision integral nonperturbatively and uncover a contribution proportional to the momentum derivative of the impurity scattering S matrix. The latter yields an important correction to the spin diffusion and spin-charge conversion coefficients, and fully captures the so-called side-jump process without resorting to the Born approximation (which fails for resonant scattering), or to otherwise heuristic derivations. We apply our kinetic theory to a quantum impurity model with strong spin-orbit, which captures the most important features of Kondo-screened Cerium impurities in alloys such as Ce_{x}La_{1-x}Cu_{6}. We find (1)a large zero-temperature spin-Hall conductivity that depends solely on the Fermi wave number and (2)a transverse spin diffusion mechanism that modifies the standard Fick's diffusion law. Our predictions can be readily verified by standard spin-transport measurements in metal alloys with Kondo impurities.

Infinite-Volume Limit of Stochastic s-d System with Single-Component Impurity and s-Electron Spins

A Hamiltonian [Formula: see text], with locally smeared Ising-type s-d exchange between s-electrons and magnetic impurities, in a dilute magnetic alloy, is investigated. The Feynman-Kac theorem, Laplace expansion and Bogolyubov inequality are applied to obtain a lower and upper bound (lb and ub) on the system’s free energy per conducting electron [Formula: see text]. The two bounds differ, in the infinite-volume limit by a term [Formula: see text], linear in impurity concentration: lb[Formula: see text], ub[Formula: see text], [Formula: see text] denoting the Hamiltonian of the approximating mean-field s-d system. [Formula: see text] represents randomly positioned impurities interacting with a mean field implemented by the gas of conduction s-electrons, the latter interacting with the field of barriers and wells (according to the s-electron’s spin orientation) localized at the impurity sites. The inequality [Formula: see text] demonstrates increasing accuracy of the mean-field [Formula: see text]-theory, with decreasing impurity concentration.

Epitaxial thin-film Pd1−xFex alloy: a tunable ferromagnet for superconducting spintronics

In the paper we present the results of extensive studies of palladium-rich Pd1-xFex alloy films epitaxially grown on MgO single-crystal substrate. In a composition range of x = 0.01-0.07 these materials are soft ferromagnets, the saturation magnetization and magnetic anisotropy of which can be tuned by its composition. Vibrating sample magnetometry was used to study temperature dependences of spontaneous magnetic moment and to establish the temperature of magnetic ordering (Curie temperature). Ferromagnetic resonance (FMR) measurements at low temperatures in the in-plane and out-of-plane geometries revealed the four-fold in-plane magnetic anisotropy with the easy directions along the <110> axes of the substrate. The modelling of the angular dependence of the field for resonance allowed to extract the cubic and tetragonal contributions to the magnetic anisotropy of the films and establish their dependence on the concentration of iron in the alloy. Experimental data are discussed in the framework of existing theories of dilute magnetic alloys. Using the anisotropy constants established from FMR, the magnetic hysteresis loops are reproduced utilizing the Stoner-Wohlfarth model thus indicating the predominant coherent magnetic moment rotation at low temperatures. The obtained results compile a database of magnetic properties of a palladium-iron alloy considered as a material for superconducting spintronics.

Enhanced magnetic ordering in Sm metal under extreme pressure

The dependence of the magnetic ordering temperature To of Sm metal was determined through four-point electrical resistivity measurements to pressures as high as 150 GPa. A strong increase in To with pressure is observed above 85 GPa. In this pressure range Sm ions alloyed in dilute concentration with superconducting Y exhibit giant Kondo pair breaking. Taken together, these results suggest that for pressures above 85 GPa Sm is in a highly correlated electron state, like a Kondo lattice, with an unusually high value of To. A detailed comparison is made with similar results obtained earlier on Nd, Tb and Dy and their dilute magnetic alloys with superconducting Y.

Higher-order Fermi-liquid corrections for an Anderson impurity away from half filling: Nonequilibrium transport

We extend the microscopic Fermi-liquid theory for the Anderson impurity [Phys. Rev. B 64, 153305 (2001)] to explore nonequilibrium transport at finite magnetic fields. Using the Ward identities in the Keldysh formalism with the analytic and antisymmetric properties of the vertex function, the spin-dependent Fermi-liquid corrections of order ${T}^{2}$ and ${(eV)}^{2}$ are determined at low temperatures $T$ and low bias voltages $eV$. Away from half filling, these corrections can be expressed in terms of the linear and nonlinear static susceptibilities which represent the two-body and three-body fluctuations, respectively. We calculate the nonlinear susceptibilities using the numerical renormalization group, to explore the differential conductance $dI/dV$ through a quantum dot. We find that the two-body fluctuations dominate the corrections in the Kondo regime at zero magnetic field. The contribution of the three-body fluctuations becomes significant far away from half filling, especially in the valence-fluctuation regime and empty-orbital regimes. In finite magnetic fields, the three-body contributions become comparable to the two-body contributions, and play an essential role in the splitting of the zero-bias conductance peak occurring at a magnetic field of the order of the Kondo energy scale. We also apply our microscopic formulation to the magnetoresistance and thermal conductivity of dilute magnetic alloys away from half filling.

Scaling of Memories and Crossover in Glassy Magnets

Glassiness is ubiquitous and diverse in characteristics in nature. Understanding their differences and classification remains a major scientific challenge. Here, we show that scaling of magnetic memories with time can be used to classify magnetic glassy materials into two distinct classes. The systems studied are high temperature superconductor-related materials, spin-orbit Mott insulators, frustrated magnets, and dilute magnetic alloys. Our bulk magnetization measurements reveal that most densely populated magnets exhibit similar memory behavior characterized by a relaxation exponent of 1-napprox 0.6(1). This exponent is different from 1-{boldsymbol{n}}approx 1/3 of dilute magnetic alloys that was ascribed to their hierarchical and fractal energy landscape, and is also different from 1-{boldsymbol{n}}=1 of the conventional Debye relaxation expected for a spin solid, a state with long range order. Furthermore, our systematic study on dilute magnetic alloys with varying magnetic concentration exhibits crossovers among the two glassy states and spin solid.

Evidence for strong enhancement of the magnetic ordering temperature of trivalent Nd metal under extreme pressure

Four-point electrical resistivity measurements were carried out on Nd metal and dilute magnetic alloys containing up to 1 at.% Nd in superconducting Y for temperatures 1.5--295 K under pressures to 210 GPa. The magnetic ordering temperature ${T}_{\text{o}}$ of Nd appears to rise steeply under pressure, increasing ninefold to 180 K at 70 GPa before falling rapidly. Y(Nd) alloys display both a resistivity minimum and superconducting pair breaking $\mathrm{\ensuremath{\Delta}}{T}_{\text{c}}$ as large as 38 K/at.% Nd. The present results give evidence that for pressures above 30--40 GPa, the exchange coupling $J$ between Nd ions and conduction electrons becomes negative, thus activating Kondo physics in this highly correlated electron system. The rise and fall of ${T}_{\text{o}}$ and $\mathrm{\ensuremath{\Delta}}{T}_{\text{c}}$ with pressure can be accounted for in terms of an increase in the Kondo temperature.

Nonlocal anomalous Hall effect in ternary alloys based on noble metals

We present a theoretical study of the nonlocal anomalous Hall effect induced by heavy-metal impurities in dilute magnetic alloys based on noble metals. The results of our first-principles calculations are shown in comparison to those obtained within a model consideration via Matthiessen's rule. Based on the transport properties of the constituent binary alloys, we reveal optimal host-impurity combinations to enhance the phenomenon. In particular, this allows us to explain experimental findings showing a strong effect in Cu-based alloys but a vanishing effect in the case of the Au host.

Aging, memory, and nonhierarchical energy landscape of spin jam

The notion of complex energy landscape underpins the intriguing dynamical behaviors in many complex systems ranging from polymers, to brain activity, to social networks and glass transitions. The spin glass state found in dilute magnetic alloys has been an exceptionally convenient laboratory frame for studying complex dynamics resulting from a hierarchical energy landscape with rugged funnels. Here, we show, by a bulk susceptibility and Monte Carlo simulation study, that densely populated frustrated magnets in a spin jam state exhibit much weaker memory effects than spin glasses, and the characteristic properties can be reproduced by a nonhierarchical landscape with a wide and nearly flat but rough bottom. Our results illustrate that the memory effects can be used to probe different slow dynamics of glassy materials, hence opening a window to explore their distinct energy landscapes.

Tunable exchange bias in dilute magnetic alloys – chiral spin glasses

A unidirectional anisotropy appears in field cooled samples of dilute magnetic alloys at temperatures well below the cusp temperature of the zero field cooled magnetization curve. Magnetization measurements on a Cu(13.5 at% Mn) sample show that this anisotropy is essentially temperature independent and acts on a temperature dependent excess magnetization, ΔM. The anisotropy can be partially or fully transferred from being locked to the direction of the cooling field at lower fields to becoming locked to the direction of ΔM at larger fields, thus instead appearing as a uniaxial anisotropy. This introduces a deceiving division of the anisotropy into a superposition of a unidirectional and a uniaxial part. This two faced nature of the anisotropy has been empirically scrutinized and concluded to originate from one and the same exchange mechanism: the Dzyaloshinsky-Moriya interaction.

First-principles calculation on dilute magnetic alloys in zinc blend crystal structure

Ab-initio calculations are performed to investigate the structural, electronic and magnetic properties of spin-polarized diluted magnetic alloys in zinc blende structure. The first-principles study is carried out on Mn doped III–V semiconductors. The calculated band structures, electronic properties and magnetic properties of Ga1−xMnxX (X=P, As) compounds reveal that Ga0.75Mn0.25P is half metallic turned to be metallic with increasing x to 0.5 and 0.75, whereas substitute P by As cause to maintain the half-metallicity nature in both of Ga0.75Mn0.25As and Ga0.5Mn0.5As and tune Ga0.25Mn0.75As to be metallic.Calculated total magnetic moments and the robustness of half-metallicity of Ga0.75Mn0.25P, Ga0.75Mn0.25As and Ga0.5Mn0.5As with respect to the variation in lattice parameters are also discussed. The predicted theoretical evidence shows that some Mn-doped III–V semiconductors can be effectively used in spintronic devices.

Analytic theory of Hund's metals: A renormalization group perspective

We study the emergence of quasiparticles in Hund's metals with an SU($M$)$\times$SU($N$)-symmetric Kondo impurity model carrying both spin and orbital degrees of freedom. We show that the coupling of the impurity spin to the conduction electrons can be \emph{ferromagnetic}, notably for hole-doped iron pnictides. We derive the weak-coupling renormalization group (RG) equations for arbitrary representations of SU($M$)$\times$SU($N$). A ferromagnetic spin coupling results in a protracted RG flow, accounting for the surprising particle-hole asymmetry that is observed in the iron-pnictide systems. We establish the low coherence scale $T_{\rm K}$ which depends on the filling through the impurity representation. We also discuss the temperature dependence of the spin and orbital susceptibilities. Finally, we argue that this mechanism explains the strong valence dependence of the coherence scale observed in dilute transition-metal magnetic alloys.

Influence of iron impurity interaction in copper-iron dilute magnetic alloy at low temperatures

In the paper the coupling impurities theory is used, and both s-d interaction effect and phonon effect in dilute magnetic alloys are discussed . The Green’s function is used to analyse the Hamiltonian of the system. In copper-iron dilute magnetic alloy the magnetic impurities interaction has a huge impact on thermoelectric power in the condition of high concentration of iron. Theoretic value of thermoelectric power of dilute magnetic copper-iron alloy with high concentration of iron changing with temperature is given. We have chosen three typical copper-iron dilute magnetic alloys and calculated the thermoelectric power under the effect of impurities and the effect of impurities interaction. Their atomic percentage concentrations are 0.1%, 0.13% and 0.15% respectively. Theoretical value of the thermoelectric power under the effect of impurities interaction in copper-iron alloy complies with experimental value. This paper provides the basic theoretical analysis for promoting the application of low-temperature copper-iron dilute magnetic thermocouple.

Universal Memory Effects in Heisenberg Spin Glasses and Spontaneous Restoration of the Spin Configuration

Reversion of the reduction of thermoremanent magnetization in a zero magnetic field is commonly observed for two different kinds of Heisenberg spin glasses, a dilute magnetic alloy Cu97Mn3 and a frustrated spinel oxide ZnFe2O4, when the temperature is reheated/recooled back to the aging temperature in modified memory-dip experiments. This unexpected phenomenon indicates that the spin configurations altered at the other temperatures are spontaneously restored to the previously stabilized ones when the temperature returns to the original. Whereas such a restoration does not occur in a multi-valley structure of energy landscape, it is possible temperature-sensitive funnel-like structure. This finding thus gives a fresh insight into the nature of spin glasses.

Slow dynamics in the geometrically frustrated magnetZnFe2O4: Universal features of aging phenomena in spin glasses

To clarify the universal features of spin glasses, we carefully studied slow dynamics in a geometrically frustrated magnet ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$ with slight disorders, regarded as an ``unconventional'' Heisenberg spin glass, using time-resolved neutron diffractometry and magnetometry. The results indicate that ``aging'' can be attributed not to growth of the short-range order detected by a diffuse scattering but to aging of a hidden aperiodic correlation, as expected from theories for spin glasses. Concerning aging, peculiar behavior was found; the decay of thermoremanent magnetization is extremely accelerated if the sample is heated/cooled briefly midway through the isothermal slow relaxation. Conversely, magnetization surprisingly increases despite the absence of a magnetic field when the temperature returns after the brief heating/cooling. The behavior can be explained as a destabilization of the aged spin configuration due to the thermal perturbations and subsequent spontaneous restoration of the original spin configuration after the destabilization. Whereas such destabilization and restoration do not occur during freezing into numerous metastable states in a fixed energy landscape, these are possible in an energy landscape with a temperature-sensitive funnel-like structure. These features, consistent with the ghost domain scenario of the droplet picture, are the same as for conventional Heisenberg spin glasses such as dilute magnetic alloys and dilute magnetic semiconductors. In other words, they are universal features in Heisenberg spin glasses including unconventional ones.

Origin of the volume collapse under pressure in elemental Dy

Most lanthanide metals exhibit a volume collapse at a critical pressure Pc; for Dy this pressure is ~73 GPa. The primary mechanism responsible for the volume collapse is a matter of debate and may involve the 4/ electrons themselves or be the result of simple pressure-induced s – d transfer in the conduction electrons. Possible mechanisms involving the 4/ electron system include: (i) valence increase, (ii) 4/ band formation, and (iii) increased 4/-conduction electron hybridization leading to a Kondo volume collapse. The present high pressure resistivity experiments on the dilute magnetic alloy Y(Dy) to 114 GPa give evidence for the validity of the Kondo volume collapse model for elemental Dy.

Determinação da temperatura Kondo a partir de curvas de susceptibilidade magnética em regime não líquido de Fermi

Em algumas ligas metálicas existem átomos magnéticos cercados por vários outros não magnéticos. Os primeiros são tão poucos que não interagem entre si. Os segundos são maioria e formam uma banda de condução. Nestas ligas é comum a ocorrência de um fenômeno conhecido como Efeito Kondo. Trata-se do acoplamento entre o spin do átomo magnético com os spins dos elétrons da banda de condução. Este fenômeno, em baixas temperaturas, é perceptível em propriedades como suscetibilidade magnética, calor específico eletrônico, resistividade elétrica, entre outros; e é caracterizado por um parâmetro denominado Temperatura Kondo. O valor deste parâmetro indica a faixa de temperatura em que o Efeito Kondo passa a ser significativo. Sua determinação é importante para a compreensão de compostos metálicos em baixa temperatura, como Ce0.5La0.5Ni9Ge4,URu2Si2 entre outros. Krishna-murthy e colaboradores, através de cálculos perturbativos do modelo de Anderson para ligas magnéticas diluídas, propuseram uma tabela de dados de suscetibilidade magnética a partir da qual é possível a determinação da temperatura Kondo para sistemas físicos no regime líquido de Fermi. Neste artigo foi reproduzido este procedimento utilizando regressão numérica para ajustar um tipo de equação não-linearizável aos dados desta tabela. Em seguida, determinou-se a temperatura Kondo de um modelo de Anderson de dois canais no regime não líquido de Fermi. Confirmou-se que o método numérico proposto por Krishna-murthy e colaboradores é válido para os dois regimes.