3d 5d Hybridization Research Articles

Magnetic Field Enhanced Cobalt Iridium Alloy Catalyst for Acidic Oxygen Evolution Reaction.

Magnetic field mediated magnetic catalysts provide a powerful pathway for accelerating their sluggish kinetics toward the oxygen evolution reaction (OER) but remain great challenges in acidic media. The key obstacle comes from the production of an ordered magnetic domain catalyst in the harsh acidic OER. In this work, we form an induced local magnetic moment in the metallic Ir catalyst via the significant 3d-5d hybridization by introducing cobalt dopants. Interestingly, CoIr nanoclusters (NCs) exhibit an excellent magnetic field enhanced acidic OER activity, with the lowest overpotential of 220 mV at 10 mA cm-2 and s long-term stability of 120 h under a constant magnetic field (vs 260 mV/20 h without a magnetic field). The turnover frequency reaches 7.4 s-1 at 1.5 V (vs RHE), which is 3.0 times higher than that without magnetization. Density functional theory results show that CoIr NCs have a pronounced spin polarization intensity, which is preferable for OER enhancement.

Atomic Scale Control of Spin Current Transmission at Interfaces.

Ferromagnet/heavy metal bilayers represent a central building block for spintronic devices where the magnetization of the ferromagnet can be controlled by spin currents generated in the heavy metal. The efficiency of spin current generation is paramount. Equally important is the efficient transfer of this spin current across the ferromagnet/heavy metal interface. Here, we show theoretically and experimentally that for Ta as heavy metal the interface only partially transmits the spin current while this effect is absent when Pt is used as heavy metal. This is due to magnetic moment reduction at the interface caused by 3d–5d hybridization effects. We show that this effect can be avoided by atomically thin interlayers. On the basis of our theoretical model we conclude that this is a general effect and occurs for all 5d metals with less than half-filled 5d shell.

Near-Room-Temperature Ferrimagnetic Ordering in a B-Site-Disordered 3d-5d-Hybridized Quadruple Perovskite Oxide, CaCu3Mn2Os2O12.

A new 3d-5d hybridization oxide, CaCu3Mn2Os2O12 (CCMOO), was prepared by high-pressure and high-temperature synthesis methods. The compound crystallizes to an A-site-ordered but B-site-disordered quadruple perovskite structure with a space group of Im3̅ (No. 204). The charge states of the transition metals are determined to be Cu2+/Mn3.5+/Os4.5+ by X-ray absorption spectroscopy. Although most B-site-disordered perovskites possess lower spin-ordering temperatures or even nonmagnetic transitions, the current CCMOO displays a long-range ferrimagnetic phase transition with a critical temperature as high as ∼280 K. Moreover, a large saturated magnetic moment is found to occur [7.8 μB/formula units (f.u.) at 2 K]. X-ray magnetic circular dichroism shows a Cu2+(↑)Mn3.5+(↑)Os4.5+(↓) ferrimagnetic coupling. The corner-sharing Mn/OsO6 octahedra with mixed Mn and Os charge states make the compound metallic in electrical transport, in agreement with a specific heat fitting at low temperature. This work provides a rare example with high spin-ordering temperature and a large magnetic moment in B-site-disordered 3d-5d hybridization perovskite oxides.

Modulation of Magnetism and Magnetic Anistropy at the Heavy-Metal/FeRh Interface

Herein, using first-principles calculations, we explore the magnetic-phase order and magnetic anisotropy energy (MAE) of FeRh (001) films with different heavy transition-metal contact layers (HTM = Hf, Ta, and W). While the antiferromagnetic ground state of FeRh still persists at the W/FeRh interface with the thicker FeRh films, the earlier HTMs of Hf and Ta favor the robust ferromagnetism in the interfacial FeRh layers. Moreover, in contrast to an in-plane magnetization of W/FeRh bilayer, the magnetization direction of FeRh reorients from in-plane to perpendicular in the presence of Hf and Ta overlayers. In particular, a large perpendicular MAE of 2 meV is identified at the Hf/FeRh interface, which is associated with the interfacial magnetic-phase transition and the large spin-orbit coupled 3d-5d hybridization.

X-ray magnetic circular dichroism temperature dependent study of RCo2 compounds

We have performed a temperature-dependent study of the rare earth L2,3 and Co K edges x-ray magnetic circular dichroism (XMCD) in HoCo2 and PrCo2. We have found in both samples that the L2/L3 ratio is temperature independent. This result contradicts previous findings that indicated a possible different behavior for L2 and L3 edges owing to modifications in the degree of 3d–5d hybridization. A further interesting result is that the temperature dependence of ratio E1/E2 for the L3 edge — where E1 and E2 correspond to the two area under the dichroic signal separated in two peaks, positive dipolar dominant, negative with significant contributions of both dipolar and quadrupolar channels — is also constant, suggesting that the dipolar and quadrupolar channels follow the same temperature dependence. This effect might be interpreted as the dominant 4f contribution to the 5d polarization (much larger than the 3d-induced contribution) to create the L-edge XMCD signal. The microscopic origin of the metamagnetic Co subsystem is also addressed.

Analysis of the magnetic behaviour of Ce 1− xGd xCo 2 compounds

Experimental data on the spontaneous magnetization m s( T) of Ce 1− x Gd x Co 2 compounds were analysed using a mean-field phenomenological model which takes into account the exchange interactions between atoms of the three magnetic species (Gd, Ce, Co) as well as the crystal electric field interaction on the Ce ions. This model allowed us to obtain, through a numerical fitting procedure to the m s( T) curves, the composition dependence of the Gd–Co and Co–Co exchange interaction strengths ( A Gd–Co and A Co–Co, respectively) and of the Co magnetic moment m Co. Both A Gd–Co and A Co–Co increase considerably with the Ce content, while m Co decreases linearly from x=1 down to the composition x=0.15; below the latter value of x, the magnetic ordering is suppressed. The intermediate valence of Ce, as suggested by the composition dependence of the room temperature lattice parameter, being also compatible with the obtained thermal variation of the Ce contribution to the total magnetic moment, is expected to play an important role in the enhancement of the A Gd–Co and A Co–Co interaction strengths, through a net reinforcement of the 3d–5d hybridization.

Effects of Spin-Dependent Spectral Weight on Magnetic Circular X-Ray Dichroism: Application to R(NixCo1-x)5Intermetallic Compounds

Systematic study of the RE(Ni x Co 1-x ) 5 compounds (RE=La, Gd and Tb) has allowed to investigate the combined effects of the 3d-5d hybridization and 4f-5d exchange on the RE L 2,3 -edge MCXD signal. In La(Ni x Co 1-x )5 compounds, where the 5d polarization results only from the hybridization between the 5d and 3d bands, the MCXD signal scales with the 5d moment and can be explained by a one-particle model for the absorption process [1-3]. When only the 4f-5d exchange intervenes, like in the GdNi 5 and TbNi 5 compounds, its effects on the MCXD sign and amplitude are very important, they are understood in term of spin-dependence of the radial matrix elements. When both the 3d-5d hybridization and the 4f-5d exchange are present like in the Tb(Ni x Co 1-x ) 5 compounds, the amplitude variations of the L 2,3 -edge MCXD spectra may be qualitatively descripted by a balanced contribution of both effects.

ELECTRONIC STRUCTURE OF STRAINED VANADIUM OVERLAYERS ON W(100) AND Ta(100)

We study the role of hybridization and overlayer–substrate lattice mismatch in determining the surface electronic structure of strained V monolayers and bilayers on W(100) and Ta(100). The local density of states is calculated in the tight-binding approximation within the surface-Green-function-matching formalism. For one monolayer of V on W(100) and Ta(100), the strong monolayer–substrate 3d–5d hybridization determines the features of the surface local density of states, with essentially no differences between 1V/W(100) and 1V/Ta(100). For the bilayer we find that the electronic structure of the topmost layer depends strongly on the lattice mismatch between overlayer and substrate. In particular, we find that the surface local density of states at the Fermi level in 2V/Ta(100) is 69% higher than in 1V/Ta(100); the lattice mismatch between bulk constants of V and Ta is 9.0%. These results indicate that strain induces strong band narrowing in vanadium overlayers on transition metals, despite the large overlayer–substrate hybridization, but depends critically on the film thickness.

Structural and magnetic properties of La/Fe multilayers

The structural and magnetic properties of La/Fe multilayers were investigated by X-ray diffraction, RHEED, magnetometry and57Fe Mossbauer spectroscopy. Comparison is made with previous results obtained for Ce/Fe multilayers. Remarkably sharp interfaces are found, with roughness between 2 and 2.5 A. The magnetic interface in the Fe sublayers resulting from the distribution of magnetic hyperfine fields distinctly exceeds the extension of the structural interface and points to a magnetic proximity effect. This is discussed in relation to a strong 3d-5d hybridization recently found in measurements of magnetic circular X-ray dichroism. Both the structural and magnetic La/Fe interface is less extended than the interface in Ce/Fe multilayers. Below a thickness of about 25 A, the individual Fe layers grow in an amorphous structure on the La layers. In this case, Curie temperatures are below 200 K and the Fe-layer saturation magnetization is reduced up to 50%, and there is evidence of a non-collinear spin structure. It is argued that this mainly reflects the properties of pure amorphous Fe.

Exchange interactions in amorphous GdCo alloys

The intersublattice R-T exchange coupling parameter (ART) in amorphous Gd1 − xCox has been evaluated from the values of the Curie temperatures or of the compensation temperatures. As the Co composition increases, AGdCo is found to decrease. Such a variation is similar to that observed for crystalline alloys. However, AGdCo is smaller in the amorphous than in the crystalline state. The results are discussed in terms of the influence of amorphicity on the 3d–5d hybridization.

Appearance potential study of PrMn2 and SmMn2 intermetallics

Soft X-ray appearance potential spectroscopy (SXAPS) has been employed to study the changes in the electronic structure of PrMn2 and SmMn2 intermetallics as a result of aloying. In this technique the total X-ray emission associated with the thresholds for the excitations of core levels of the atoms in the surface region of the materials is measured. It sensitively provides information regarding the density of unoccupied states at the Fermi level. The SXAPS spectra of the M4,5 levels of Pr and Sm and the L2,3 levels of Mn in these intermetallics are compared with the corresponding elemental spectra. The observed shift in the core levels represents contribution from the shifts due to charge transfer and that due to the hybridization between the Mn 3d and the rare earth 5d bands. The charge transfer shifts have been estimated from the differences in the Fermi levels of the intermetallic and the element using the free electron model. These values have been utilized to obtain the hybridization shifts. The hybridization shifts show a decreasing trend in these intermetallics and is attributed to the decrease in the number of electrons in the 3d–5d hybridized bands. Our results correlating the Mn magnetic moment with the number of electrons are consistent with the reported experimental data. It is concluded that the 3d–5d hybridization plays a significant role in determining the magnetic properties of these intermetallics.

Effects of spin-dependent spectral weight on magnetic circular x-ray dichroism: Applications to R(NixCo1-x)5 intermetallic compounds.

This systematic study of R(${\mathrm{Ni}}_{\mathit{x}}$${\mathrm{Co}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${)}_{5}$ compounds (R=La, Gd, and Tb) has allowed us to investigate the combined effects of the 3d-5d hybridization and 4f-5d exchange on the rare-earth ${\mathit{L}}_{2,3}$-edge magnetic circular x-ray dichroism (MCXD) signal. In La compounds, where the 5d polarization results only from the magnetic transition metals via the hybridization of 5d and 3d bands, the MCXD signal scales with the 5d moment and can be explained by a one-particle model for the absorption process. When the 4f-5d exchange intervenes, like in the Gd and Tb compounds, its effects on the MCXD sign, amplitude, and the amplitude variations are very important. A description of the ${\mathit{L}}_{2,3}$-edge MCXD spectra taking into account the spin-dependence of the radial matrix elements explains qualitatively the results of this study.

A study of the electronic structure of GdMn 2 by appearance potential spectroscopy (SXAPS)

The GdMn 2 intermetallic has been studied using soft X-ray appearance potential spectroscopy (SXAPS) which exploits the abrupt changes in the total X-ray emission associated with the thresholds for the excitations of core levels of the atoms in the surface region of the materials. This technique is highly sensitive to the density of states at the Fermi level and is therefore particularly suitable for investigating the changes in the unoccupied density of states as a result of alloying rare earths with transition metals. The SXAPS spectra of the M 4,5 levels of Gd and the L 2,3 levels of Mn in the intermetallic are compared with the corresponding elemental spectra. These core levels exhibit a positive chemical shift. The experimentally observed core level shift is a sum of the shifts resulting from charge transfer and hybridization. The charge transfer shift is approximated by the Fermi energy shift. The Fermi energies in the pure elements and in the intermetallic have been estimated from the free electron theory model. Utilizing these Fermi level and core level shifts, the hybridization effects between 3d and 5d bands are obtained in this intermetallic. The 3d–5d hybridization plays a significant role in the magnetism observed in the intermetallic.

Exchange interactions in rare earth—transition metal compounds

Using the strenght of R—R interactions deduced from the ordering temperatures of RNi 2 compounds, the R—T exchange-coupling parameter ( A RT) in R m T n ( R = rare earth, T = Co or Fe, m / n = 1 2 ; 1 3 ; 6 23 ; 1 5 ; 2 17 ; 1 12 ) and R mT nB k ( m / n / k = 1/4/1; 2/14/1; 1/12/6) systems has been evaluated from analyses of Curie temperatures. Whereas A RR , as deduced from the RNi 2 compounds, increases by almost an order of magnitude in a given series, A RT is enhanced by a factor of only about three from the heavy to the light rare earth compounds. These phenomena, already reported in literature, are based on 4f–5d exchange interactions at R sites. In addition, for a given rare earth element, going from T-poor to T-rich compounds, a tendency to decrease is found for both A RCo and A RFe; this variation is discussed in terms of 3d–5d hybridization.

Effects of the 3d—5d Hybridization on the 4f—3d Coupling in the Rare Earth—Transition Metal Compounds

Effects of the 3d—5d Hybridization on the 4f—3d Coupling in the Rare Earth—Transition Metal Compounds

XAS Investigation of the Structural and Electronic Changes Induced by Hydrogen Absorption in CeFe11Ti

We present an extensive study using X-ray absorption spectroscopy on CeFe11 TiHx at variable hydrogen concentration. Cerium L3-edge, iron and titanium K-edge EXAFS and XANES spectra have been measured. We have investigated the influence of hydrogen absorption in electronic localization and local structure around the metal atoms as well as on the electronic state of cerium. The structural and magnetic changes in the hydrides are discussed in terms of the modification of the 4f-3d coupling strength via 3d-5d hybridization.

Correlation between mixed valence behaviour of cerium and the magnetic and superconducting phenomena of CeFe 2 and CeRu 2

This contribution concerns an X-ray absorption investigation of the cubic Laves phase rare earth compounds CeFe 2, CeRu 2 and their related hybrides. These compounds have been investigated by Extended X-ray Absorption Fine Structure (EXAFS) both at cerium and iron sites and by X-ray Absorption Near Edge Spectroscopy (XANES) at different edges of both ruthenium and iron species. The hydrogen absorption process is correlated with the suppression of the cerium fluctuating valence state in the case of CeFe 2, while the intermediate valence character is detected in CeRu 2 also upon the absorption process. These results and the change of 3d–5d hybridization determined by structural disorder effects in the hydridate phase of these systems can explain the anomalous magnetic behaviour of the hydridate phases when compared with those of the whole series REFe 2 and RERu 2.

Ab initio calculation of molecular field interactions in rare-earth transition-metal intermetallics (invited)

The interaction, KRM, between the rare-earth 4f moment and the transition-metal 3d moments in rare-earth transition-metal intermetallics is shown to depend upon the R-5d moment, which is due to 3d–5d hybridization, and local 4f–5d exchange integrals. Both the R-5d moment and KRM may be calculated ab initio from the local spin-density approximation to density functional theory in self-consistent energy-band calculations with the localized 4f-moments fixed at their Russel–Saunders values. Detailed examples are given for the RFe2 (R=Gd−Yb) series. The exchange integrals are similar to those entering into the density functional version of Stoner theory and their energy dependence must be treated carefully. The calculated local exchange integrals are shown to be related to the molecular fields derived from spin Hamiltonians, hence to the spin-wave spectra. Reasonable agreement with values of the molecular fields extracted from inelastic neutron scattering and high field susceptibility measurements is obtained.

Rare-earth transition-metal intermetallics

The combination of itinerant transition metal (M = Fe) and localized rare-earth (R = Gd-Y) magnetism in RFe 2 compounds has been investigated in self-consistent energy band calculations. The computed and measured total moments are in good agreement for all cases where single crystal data are available. We find, however, that there is a significant contribution to the moment from the R-5d partial moments coupled antiparallel to the Fe-3d moment which results from 3d–5d hybridization. The R-4f moments interact with the conduction band system solely by local exchange interactions, which are calculated ab initio from density functional theory. A sum rule for the total 3d + 5d moment is shown to be obeyed and the effective ferrimagnetic exchange interaction between rare-earth and transition-metal moments is discussed. Finally, the spin wave spectra of these materials are evaluated in terms of a model arising from these calculations.

Origin and ab initio evaluation of magnetic interactions in rare earth intermetallics

The interaction, KRM, between the rare earth 4f moment and the transition metal 3d moments in the series RFe2 (R=Gd-Yb) is shown to depend upon the R-5d moment, which is due to 3d-5d hybridization and local 4f-5d exchange integrals. KRM is calculated ab initio from density functional theory and is in excellent agreement with values extracted from inelastic neutron scattering experiments.