Ru Impurities Research Articles

Theoretical study of doping effect on electronic and magnetic properties of BaZrO3

In this work, the (D = Mo, Ru)-doping effect on the electronic and magnetic properties of BaZrO3 is investigated by means of the Korringa-Kohn-Rostoker (KKR) method in combination with the coherent potential approximation (CPA). It is shown that substituting Ba by different concentrations x of Mo and Ru impurities deals with ferromagnetic half-metallic compounds 100% spin polarization at the Fermi level. The same behavior is observed for Zr-doped configurations, except BaZr1−xMoxO3 for x > 14 which exhibits metallic character. The stability of the ferromagnetic order is attributed to double exchange mechanism. Doping induces a net moment of 1 μB for the Ba substitution cases (Ba1−xDxZrO3) and 3 μB for BaZr1−xDxO3 conformers. Moreover, a high curie temperature, above ambient temperature is reached for the systems. Based on these results, we have designed half-metallic ferromagnetic compounds with high efficiency and great potential for spintronic applications.

Enhancing spin-Hall spin–orbit torque efficiency by bulk spin scattering modulation in ferromagnets with ruthenium impurities

We report that the efficiency of the spin-Hall spin–orbit torque (SOT) in perpendicularly magnetized Pt/Co/MgO films can be engineered by introducing ruthenium (Ru) impurities into the bulk of the Co layer. As the concentration of the Ru impurities increases, the effective field of the damping-like SOT is increased by a factor of 2.3, leading to SOT efficiency per unit current density enhancing from 0.069 to 0.155. However, the Ru incorporation shows less impact on the field-like SOT efficiency. First-principles calculations reveal that the Ru incorporation can significantly increase the density of states of the majority spin at the Fermi level but has less influence on the minority spin case. We suspect that such an electronic-structure modulation may reduce the scattering of the spin-Hall spin currents flowing across the Co-Ru layer, resulting in SOT efficiency enhancement. The SOT enhancement by Ru incorporation is also observed in Ta/CoFeB/MgO films, indicating that modulating the spin scattering inside ferromagnets with judiciously selected impurities is a promising strategy to construct low-power-dissipation SOT spintronic devices.

Synergetic effect in RuxMo(1-x)S2/SBA-15 hydrodesulfurization catalysts: Comparative experimental and DFT studies

The effect of introducing Ru impurities into the MoS2 crystalline structure of the sulfided RuxMo(1-x)S2/SBA-15 catalysts have been investigated using density functional theory (DFT) calculations and the catalyst characterization by different techniques (chemical analysis (ICP-AES), temperature-programmed reduction (TPR), X-ray diffraction (XRD), N2 physisorption, DRIFTS of adsorbed pyridine (DRIFTS-Py) and X-ray photoelectron spectroscopy (XPS)). The catalyst activity was tested in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) reaction carried out in a batch reactor, T = 320 °C and total H2 pressure of 5.5 MPa. From electronic structure DFT calculations is was concluded that the 4d orbitals of both Mo and Ru played an important role in the catalyst optimization being the processes of transport and charge transference the most important ones. It was found that the enrichment with Ru, promotes a greater electronic participation (DOS at the Fermi level) of the different atoms in the RuxMo(1-x)S2 phase leading to metallization of the Mo ions. The catalyst activity in HDS of DBT reaction demonstrated a similar behavior to that of theoretical density of states (DOS) calculated via DFT. All bimetallic systems presented the synergetic effect between Ru and Mo in the HDS of DBT reaction over RuxMo(1-x)S2/SBA-15 catalysts. The highest activity observed for Ru content of x = 0.4 was consistent with theoretical results predicting that the optimum DOS contributions should be around x = 0.44. The most active Ru0.4Mo0.6S2/SBA-15 exhibit the best hydrogenation properties linked with the Ru-induced metallization of Mo ions in the Ru0.4Mo0.6S2 phase. This catalyst showed two-fold higher hydrogenation properties than CoMoS/γ-Al2O3 reference catalyst. The linear dependencies of initial activity on Brønsted-to-Lewis acidities ratio (from DRIFT-Py) and total metal surface exposure (from XPS) were observed.

Effect of Impurities on the Formation Energy of Point Defects in the γ-TiAl Alloy

The projector augmented-wave method within density functional theory is used to study the effect of 4d elements and Group IIIA and IVA elements on the energetics of point defect formation and elastic moduli of the γ-TiAl alloy. To calculate the point defect formation energy, the grand canonical formalism was used. It is shown that the formation energy of aluminum vacancies decreases by approximately 1.3 eV with increasing its content in the series of alloys Ti3Al–TiAl–TiAl3, whereas the formation energy of titanium vacancies changes insignificantly. In general, the formation of titanium vacancies in these alloys is preferred to the formation of aluminum vacancies. It has been found that Nb, Mo, Tc, Ru, Rh, and Pd impurities on the aluminum sublattice contribute to an increase in the formation energy of an aluminum vacancies, and Mo and Tc also lead to an increase in the formation energy of titanium vacancies. All 4d impurities, if they substitute for titanium, reduce the formation energy of aluminum vacancies, and Nb and Mo are favorable for increasing the formation energy of titanium vacancies. The influence of impurities on the chemical bond in the γ-TiAl alloy and its elastic moduli and characteristics based on them is discussed.

Half-metallic ferromagnetism and metal–insulator transition in Sn-doped SrRuO3 perovskite oxides

We investigate the electronic and magnetic properties of SrRu1-xSnxO3 by carrying out density-functional-theory calculations to show that a half-metallic ferromagnetic ground state emerges for the Sn doping of x≳0.5. To examine the effect of on-site Coulomb interactions for the Ru d orbitals, which was suggested to enhance the half-metallicity in SrRuO3, we employed both the local spin-density approximation (LSDA) as well as the LSDA + U method. For all the possible configurations of Sn doping for x=1/8,1/4,1/2,5/8,3/4, and 7/8 within the 2×2×2 unit cell, we monitor the Ru t2g bandwidth as well as the valence band maximum in the majority-spin channel and demonstrate that the Ru d electron hopping is blocked by the Sn-substituted sites so that the Ru t2g bandwidth becomes reduced as the doping x increases. For x<0.5, the valence band maximum still touches the Fermi level so that SrRu1-xSnxO3 remains as a usual ferromagnetic metal. A further reduction of the Ru t2g bandwidth for the range of 0.5≲x≲0.7 turns it into a half-metallic ferromagnet. As for x>0.7, the Ru t2g bandwidth gets so narrow that even a small on-site Coulomb interaction, e.g., Ueff=1.0 eV induces a band-gap, which indeed corresponds to a gap of the Ru impurity bands in the SrSnO3 oxide semiconductor.

Doping Li-rich cathode material Li2MnO3 : Interplay between lattice site preference, electronic structure, and delithiation mechanism

We report a detailed first-principles study of doping in Li$_2$MnO$_3$, in both the dilute doping limit and heavy doping, using hybrid density-functional calculations. We find that Al, Fe, Mo, and Ru impurities are energetically most favorable when incorporated into Li$_2$MnO$_3$ at the Mn site, whereas Mg is most favorable when doped at the Li sites. Ni, on the other hand, can be incorporated at the Li site and/or the Mn site, and the distribution of Ni over the lattice sites can be tuned by tuning the materials preparation conditions. There is a strong interplay between the lattice site preference and charge and spin states of the dopant, the electronic structure of the doped material, and the delithiation mechanism. The calculated electronic structure and voltage profile indicate that, in Ni-, Mo-, or Ru-doped Li$_2$MnO$_3$, oxidation occurs on the electrochemically active transition-metal ion(s) before it does on oxygen during the delithiation process. The role of the dopants is to provide charge-compensation and bulk electronic conduction mechanisms in the initial stages of delithiation, hence enabling the oxidation of the lattice oxygen in the later stages. This work thus illustrates how the oxygen-oxidation mechanism can be used in combination with the conventional mechanism involving transition-metal cations in design of high-capacity battery cathode materials.

DFT calculations of Anatase TiO2 (1 0 1) Surface Doped with Ruthenium for Application in Dye Sensitised Solar Cell

Titanium dioxide (TiO2) is considered to be an ideal semiconductor for photocatalysis because of its high stability, low cost and safety towards both humans and the environment. Doping TiO2 with different elements has attracted much attention as the most important way of enhancing the energy band gap and the adsorption on different wavelength region, in order to improve the efficiency of the dye sensitized solar cells (DSSCs). In this study, first principle density function theory was used to investigate the reaction mechanisms between anatase TiO2 (1 0 1) surface doped with ruthenium (Ru) atom as a dopant. Ru-doped anatase TiO2 (1 0 1) surface was geometrically optimized using the generalized gradient approximation implemented within the CASTEP CODE. The calculated density of states and band structures for Ru-doped systems shows that the synergistic effects of Ru impurities at lattice titanium sites lead to a sharp band gap narrowed to 0.345 eV.

Controlling n-Type Doping in MoO3

We study the electronic properties of native defects and intentional dopant impurities in MoO3, a widely used transparent conductor. Using first-principles hybrid functional calculations, we show that electron polarons can be self-trapped, but they can also bind to defects; thus, they play an important role in understanding the properties of doped MoO3. Our calculations show that oxygen vacancies can cause unintentional n-type doping in MoO3. Mo vacancies are unlikely to form. Tc and Re impurities on the Mo site and halogens (F, Cl, and Br) on the O site all act as shallow donors but trap electron polarons. Fe, Ru, and Os impurities are amphoteric and will compensate n-type MoO3. Mn dopants are also amphoteric, and they show interesting magnetic properties. These results support the design of doping approaches that optimally exploit functionality.

Exchange Bias Tuning for Magnetoresistive Sensors by Inclusion of Non-Magnetic Impurities.

The fine control of the exchange coupling strength and blocking temperature ofexchange bias systems is an important requirement for the development of magnetoresistive sensors with two pinned electrodes. In this paper, we successfully tune these parameters in top- and bottom-pinned systems, comprising 5 nm thick Co40Fe40B20 and 6.5 nm thick Ir22Mn78 films. By inserting Ru impurities at different concentrations in the Ir22Mn78 layer, blocking temperatures ranging from 220 °C to 100 °C and exchange bias fields from 200 Oe to 60 Oe are obtained. This method is then applied to the fabrication of sensors based on magnetic tunneling junctions consisting of a pinned synthetic antiferromagnet reference layer and a top-pinned sensing layer. This work paves the way towards the development of new sensors with finely tuned magnetic anisotropies.

Preparation and characterisation of Ru doped MgB2

Samples with Mg1−xRuxB2 nominal stoichiometry were prepared by sintering at 800°C. The critical transition temperature decreases up to a substitution level of x≈0.015. A maximum solubility limit slightly in excess of 1at.% Ru for Mg is confirmed by energy dispersive spectroscopy measurements. From and beyond this limit, an unidentified phase appears in the X-ray diffraction patterns. Interestingly, the lattice parameters of the MgB2 phase are constant up to x=0.015, but start to decrease for higher Ru contents. This feature could be related to the fact that a Mg–Ru impurity phase results in a Mg-deficient (Mg,Ru)B2 matrix.

Influence of natural impurity on electronic structure and catalytic activity of pyrite

The electronic structures and the optical properties of pyrite containing twenty natural impurities, Co, Ni, As, Se, Te, Cu, Au, Ag, Mo, Zn, Tl, Sn, Ru, Pd, Pt, Hg, Cd, Bi, Pb and Sb, are investigated using the density functional theory and the plane wave pseudopotential method, and the catalytic activity of pyrite is discussed. For the transition metal-bearing pyrite, there are introduced impurity energy levels in the band contributing from impurity d orbital, while for the other metal and non-metal-bearing pyrite, the impurity energy levels are contributed from impurities s or p orbital. The presences of Cu, Mo, As, Au, Ag or Ni can enhance the electrocatalytic ability of pyrite to the oxygen reduction. All the impurities, except Zn, Mo, Ru, As, Sb, Se and Te, can enhance the ability of pyrite surface to capture electrons and hence the recombination rate of photoinduced electrons and holes wile be reduced. Calculations of optical properties indicate that Cd, Hg, Ru, Se, Te and Zn impurities each have small influence on the absorption band edge, while the presence of other impurity makes a red shift of absorption band edge of pyrite. Especially, the presences of Au and Ag impuritie increase the adsorption coefficient of pyrite by two orders of magnitude.

Effects of Ru Doping on the Transport Behavior and Superconducting Transition Temperature of NdFeAsO0.89F0.11

The transport behavior and superconducting transition temperature Tc of NdFe1-yRuyAsO0.89F0.11 have been studied for various y values. Because Ru impurities are isoelectronic to host Fe atoms, we basically expect that the number of electrons does not change with y, at least in the region of small y values. The results indicate that the rate of Tc suppression by Ru atoms is too small to be explained by the pair breaking effect of nonmagnetic impurities expected for the S+- symmetry, confirming our previous results for Co doping.

Absence of local magnetic moments in Ru and Rh impurities and clusters on Ag(100) and Pt(997)

The magnetism of quench-condensed Ru and Rh impurities and metal films on Ag(100) and Pt(997) has been studied using x-ray magnetic circular dichroism. In the coverage range between 0.22 and 2.0 ML, no dichroic signal was detected at the ${M}_{3,2}$ absorption edges of Ru on Ag(100) at a temperature of $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the presence of an applied magnetic field. The same was found for coverages between 0.12 and 0.5 ML of Rh on Ag(100) and Pt(997). It is concluded that the magnetic moments of single impurities, small clusters of various shape, and monolayers of the $4d$ metals are below the detection limit of $0.04\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ per atom. These results provide an unambiguous determination of the local magnetic moment of Ru and Rh deposited on nonmagnetic transition-metal surfaces, which are in contrast with theoretical predictions.

Electronic structure of Ru impurities in iron

The molecular cluster discrete variational method has been employed to study the magnetic properties of Ru impurities and Ru monolayers in iron. With the introduction of Ru atoms in the neighborhood of an Fe atom the local moment changes appreciably and the magnetization decreases steadily. By increasing Ru concentration both ferromagnetic and antiferromagnetic Fe–Ru couplings can be expected in dilute alloys. The calculations indicate that the contact hyperfine fields of Fe atoms are strongly dependent on the chemical environment as well as on the local symmetry.

Magnetic properties of Al, V, Mn, and Ru impurities in Fe–Co alloys

Theoretical studies on the magnetic properties of impurities in Fe–Co alloys have been carried out using a molecular-orbital approach within a gradient corrected density functional formalism. The defected alloy is modeled by a large cluster and the calculations on the ordered alloy are used to show that a cluster containing 67 atoms can provide quantitative information on the local magnetic moment. It is found that although bulk Al, V, and Ru are nonmagnetic, all the impurities carry finite moments. While Al and V impurities couple antiferromagnetically, Ru impurities couple ferromagnetically to the host sites. It is shown that the observed composition dependence of the rate of increase of magnetic moment of FexCo1−x alloys upon addition of Mn impurities is due to the change in the magnetic moment of Mn impurities with composition. The reasons for this change and the possibility of stabilizing a higher Mn moment at all concentrations are discussed.

Inverse magnetoresistance in Fe/Si ion beam sputtered sandwiches

A series of Fe/Si sandwiches have been prepared by ion beam sputtering at room temperature onto glass substrate with the following nomenclature: Glass/Si20 nm/Fe5/6 nm/Fe5 nm/Ru2 nm. Magnetization measurements have been performed at 300 K and show no evidence of antiferromagnetic exchange coupling. However, the magnetoresistance curves recorded at 300 K are very interesting and show an inverse magnetoresistance for sandwiches with Si spacer layer thicknesses between 1.2 and 1.5 nm. The resistance is smaller at zero field than at saturation. This inverse magnetoresistance is due to the superparamagnetic interfaces and finds its origin in the difference of the electronic nature of the Fe/Si interfaces and Fe/Ru interfaces. Fe1−xSix iron silicide at Fe/Si interfaces has a scattering spin asymmetry ratio (α=ρ↓/ρ↑) larger than one, whereas, Fe with Ru impurities at the Fe/Ru interfaces presents a scattering spin asymmetry ratio smaller than one.

Interfacial origin of inverse magnetoresistance in uncoupled Fe/Si/Fe/Ru sandwiches

Experimental evidence of inverse magnetoresistance for ferromagnetic layers separated by a Si layer is reported. A series of Fe/Si sandwiches have been prepared by ion-beam sputtering at room temperature onto a glass substrate with the following nomenclature: glass/Si20 nm/Fe5 nm/Six nm/Fe5 nm/Ru2 nm. Magnetization measurements have been performed at 300 K and show no evidence of antiferromagnetic exchange coupling. However, the magnetoresistance curves recorded at 300 K are very interesting and show a reversed magnetoresistance for sandwiches with Si spacer layer thicknesses between 1.2 and 1.5 nm. Indeed, the resistivity is smaller at zero field than at saturation. This reversed magnetoresistance is due to the superparamagnetic interfaces and finds its origin in the difference of the electronic nature of the Fe/Si interfaces and Fe/Ru interfaces. Indeed, iron silicide Fe1−ySiy at Fe/Si interfaces have scattering spin asymmetry ratios (α=ρ↓/ρ↑) larger than 1, whereas, Fe with Ru impurities at the Fe/Ru interfaces present scattering spin asymmetry ratios lower than 1.

Effect of Ru Impurities on Magnetoresistance in Co/Ru/Co(Ru) Sandwiches

ABSTRACTMagnetoresistance (MR) measurements have been performed on two series of Co(3.2 nm)/Ru(x=0.5 to 3 nm)/Co(3.2 nm) and Co(3.2 nm)/Ru(x)/Co.92Ru.08 (3.2 nm) sandwiches prepared by ultra high vacuum evaporation onto mica substrates. At 300 K, all samples of the first series present giant magnetoresistance (GMR) whereas the samples of the second series, and with the same Ru spacer thickness, show an inverse GMR. This inverse GMR is induced by alloying the second Co layer with 8 at% Ru and can be attributed to the change of the scattering coefficients ratio (α) between the two magnetic layers. Indeed, for the first Co layer the scattering coefficient ratio a1 = p↓/↑&gt; 1. However, for the second magnetic layer, alloying Co and Ru may induce an increase of the density of states at the Fermi level for majority spin electrons and as a consequence 0C2 = p↓/↑&lt; 1

Electronic structure of 4d impurities in Rb: a local-spin-density approximation +U density-functional study

The electronic structure of the 4d substitutional impurities in Rb is studied by means of self-consistent density functional calculations. Exchange and correlation corrections for localized orbitals, as included in the mean-field solution of Anderson's model, are superimposed on the spin-dependent potential of the traditional local-spin-density approximation. We find ionic-like dn configurations and a quite important spin polarization of the extended sp states. For Nb and Ru impurities we obtain two stable configurations in each case and their energetic stability is studied by means of constrained density-functional calculations. Our results are compared with the results of other calculations and with the available experimental data.

Magnetic properties of isolated Rh and Ru impurities in Pd and Pd-Fe: A theoretical study.

We perform self-consistent electronic-structure calculations to investigate the possible existence and formation mechanism of local magnetism in isolated late-4d transition-metal impurities in Pd and dilute Pd-Fe alloys, using the local-spin-density approximation and the cluster model for simulating the bulk. We find that Ru in Pd exhibits a giant moment of 4μ B , but reduced in magnitude compared to its 3d counterpart Fe in Pd. Tc and Rh in Pd turn out to be nonmagnetic, the latter in disagreement with time-differential perturbed-angular-correlation (TDPAC) experiments, which suggest a reduced giant moment per Rh atom also