Paramagnetic Pd Research Articles

First Principles Investigation on Electrically Controlled Magnetic Moments of Ultrathin Palladium Film on MgO(001)

Electric field dependence of magnetic, electronic, and structural properties in a Pd bilayer film on MgO (001) is investigated by using the first-principles electronic structure calculations within density functional theory. We find that, due to the lattice mismatch of 6.7 %, a ferromagnetic ground state is stabilized at zero electric field on an otherwise paramagnetic Pd with the spin magnetic moments of m = 0.31 and 0.34 μB in surface and interface Pd, respectively. The application of an external electric field, E, causes the Pd magnetic moments to be modulated in a site- and electric field-dependent manner. As the external electric field pointing inward to the Pd surface is increased up to the critical value of 1.3 V/Å, the surface Pd moment is barely changed while the interfacial Pd moment increases linearly with E. Above this critical electric field, the moments of both the interfacial and surface Pd atoms exhibit nonlinear increase with E and reach up to 0.34 (surface) and 0.41 (interface) μB at E = 2.5 V/Å. When the electric field direction is inverted, a similar dependence on E is obtained for both Pd atoms at the critical electric field value of -0.5 V/Å. The peculiar behavior of the induced magnetic moments is explained by combining the linearized Stoner and itinerant conduction models. We reveal that this prominent magnetoelectric effect is due to the modulated charge occupancy of Pd 4d-states in the minority spin channel in which the dSUBzSUP2/SUP/SUB and dSUBxSUP2/SUP-ySUP2/SUP/SUB orbitals play crucial roles.

Analysis of Structural Features of Periodic Fe/Pd/Gd/Pd Multilayered Systems

The structure of a material promising for magnetic memory elements based on Fe/Pd/Gd/Pd superlattices has been investigated by X-ray analysis and electron microscopy. Microstructural transformations in the superlattices and the influence of microstructure modification on the magnetic properties of the system are determined. It is shown that intense diffusion of Pd atoms to Gd layers occurs in the superlattices under study. The presence of a paramagnetic Pd layer changes the character of exchange interaction between neighboring ferromagnetic layers due to the induced magnetic moment on Pd atoms near the interface because of the interface imperfection. Nanocrystalline inclusions are found in the Pd/Gd/Pd layers, which are suggested to affect the specificity of magnetic ordering in these systems, as confirmed by magnetic measurements.

Magnetic measurements of hydrogen desorption from palladium hydride PdH0.64 prepared by severe plastic deformation

Hydrogen absorption and desorption through diffusive processes and the accompanying phase transformations in hydrogen storage materials have been investigated by x-ray diffraction, thermogravimetry, pressure-composition-temperature isotherms, and electrical resistance measurements. We use time- and temperature-dependent magnetic measurements to gain a detailed understanding of these phenomena in palladium hydride PdH0.64 prepared by severe plastic deformation via the high-pressure torsion method. The concept of our method is pursuing an increase in the ratio of the paramagnetic α-phase Pd+H to the diamagnetic β-phase PdH. An antiferromagnetic correlation appears between the paramagnetic moments of Pd at a low temperature and a huge discrete change in magnetization appears due to collective H desorption above room temperature. It is also verified that H diffusion and its desorption can be changed with the accumulation of the severe lattice strain.

Paramagnetic Pd+ centers in the polymeric matrices of palladium(I) sorbates and 4-pentenate

The paramagnetic PdI centers with unpaired electrons have been first revealed by EPR spectroscopy in the polymeric matrices of PdI sorbates [Pd(C6H7O2)H2O]n, [Pd(C6H7O2)]n and 4-pentenate [Pd(C5H7O2)H2O]n. These compounds have been obtained by the reactions between Pd3(MeCO2)6 and sorbic (MeCH=CHCH=CHCOOH) and 4-pentenoic (CH2=CHCH2CH2COOH) acids in MeOH.

Spontaneous distortion via the appearance of ferromagnetism in Pd ultrathin films: Observation of an inverse mechanism for the Stoner criterion

We study the crystal structure of Pd(100) ultrathin films, which show ferromagnetism induced by the quantum confinement effect, using in-situ X-ray crystal truncation rod measurement and density functional calculation. The energy gain due to the appearance of ferromagnetism in Pd results in flatter and uniform film growth of ferromagnetic Pd films compared with paramagnetic Pd. In addition, ferromagnetic Pd films expand the lattice constant in order to suppress the increase in kinetic energy of electrons accompanied by the occurrence of exchange splitting. Although the traditional theory of magnetism in metals indicates that the increase in density of states that induces ferromagnetism (Stoner criterion), our present finding reveals a mechanism of modulation in the density of states via the appearance of ferromagnetism, i.e., the inverse mechanism of Stoner's theory.

Localized Mixed-Valence and Redox Activity within a Triazole-Bridged Dinucleating Ligand upon Coordination to Palladium.

The new dinucleating redox-active ligand (LH4 ), bearing two redox-active NNO-binding pockets linked by a 1,2,3-triazole unit, is synthetically readily accessible. Coordination to two equivalents of PdII resulted in the formation of paramagnetic (S=1/2 ) dinuclear Pd complexes with a κ2 -N,N'-bridging triazole and a single bridging chlorido or azido ligand. A combined spectroscopic, spectroelectrochemical, and computational study confirmed Robin-Day Class II mixed-valence within the redox-active ligand, with little influence of the secondary bridging anionic ligand. Intervalence charge transfer was observed between the two ligand binding pockets. Selective one-electron oxidation allowed for isolation of the corresponding cationic ligand-based diradical species. SQUID (super-conducting quantum interference device) measurements of these compounds revealed weak anti-ferromagnetic spin coupling between the two ligand-centered radicals and an overall singlet ground state in the solid state, which is supported by DFT calculations. The rigid and conjugated dinucleating redox-active ligand framework thus allows for efficient electronic communication between the two binding pockets.

Tuning magnetotransport in PdPt/Y3Fe5O12: Effects of magnetic proximity and spin-orbit coupling

We report that anisotropic magnetoresistance (AMR) and anomalous Hall conductivity (AHC) in the Pd1−xPtx/Y3Fe5O12 (YIG) bilayers could be tuned by varying the Pt concentration (x) and also temperature (T). In particular, the AHC at low T changes its sign when x increases from 0 to 1, agreeing with the negative and positive AHC predicted by our ab initio calculations for the magnetic proximity (MP)-induced ferromagnetic Pd and Pt, respectively. The AMR ratio is enhanced by ten times when x increases from 0 to 1. Furthermore, the AMR of PdPt/YIG bilayers shows similar T-dependence as the magnetic susceptibility of the corresponding bulk Pd/Pt, also indicating the MP effect as the origin of the AMR. The present work demonstrates that the alloying of Pt and Pd not only offers tunable spin-orbit coupling but also is useful to reveal the nature of the AMR and AHC in Pt/YIG bilayers, which are useful for spintronics applications.

Electrically driven magnetism on a Pd thin film

Using first-principles density functional calculations we demonstrate that ferromagnetism can be induced and modulated on an otherwise paramagnetic Pd metal thin-film surface through application of an external electric field. As free charges are either accumulated or depleted at the Pd surface to screen the applied electric field there is a corresponding change in the surface density of states. This change can be made sufficient for the Fermi-level density of states to satisfy the Stoner criterion, driving a transition locally at the surface from a paramagnetic state to an itinerant ferromagnetic state above a critical applied electric field, Ec. Furthermore, due to the second-order nature of this transition, the surface magnetization of the ferromagnetic state just above the transition exhibits a substantial dependence on electric field, as the result of an enhanced magnetoelectric susceptibility. Using a linearized Stoner model we explain the occurrence of the itinerant ferromagnetism and demonstrate that the magnetic moment on the Pd surface follows a square-root variation with electric field consistent with our first-principles calculations.

Six-fold Oxygen-Coordinated Triplet (S= 1) Palladium(II) Moieties Templated by Tris(bipyridine)ruthenium(II) Ions

Tris(bipyridine)ruthenium(II) is used as a templating agent to insert palladium(II) into three-dimensional oxalate-based networks. The templated-assembly of [Ru(bpy)(3)][Pd(2)(ox)(3)] (Pd(2)) and [Ru(bpy)(3)][PdMn(ox)(3)] (PdMn) is described. The latter compound is structurally characterized by powder X-ray diffraction and X-ray absorption spectroscopy. These techniques reveal an unusual 6-fold oxygen environment around the Pd(II) atoms with two short (2.02 Angstrom) and four long (2.17 Angstrom) Pd-O distances. As stated by magnetometry, this environment is associated with a triplet ground state (S = 1) of the palladium(II) ion: when the temperature is decreased, the chiMT product shows a monotonous decrease from 5.54 cm(3) K mol(-1) at 300 K, a value which is slightly lower than the one expected for independent paramagnetic Pd(II) (S = 1, g = 2) and Mn(II) (S = 5/2, g = 2) ions. This thermal variation is due to antiferromagnetic exchange interactions between the two spin bearers. Nevertheless, no long-range magnetic order is detected down to 2 K. These results are confirmed by an analysis of the [MII(C(2)O(4))(3)](4-) (M = Ni, Pd, Pt) complex and of a [Pd(II){mu-(C(2)O(4))Mn(II)(OH(2))(4)}(3)](2+) tetranuclear model using density functional theory.

Interface transparency of Nb/Pd layered systems

We have investigated, in the framework of proximity effect theory, the interface transparency ${\cal T}$ of superconducting/normal metal layered systems which consist of Nb and high paramagnetic Pd deposited by dc magnetron sputtering. The obtained ${\cal T}$ value is relatively high, as expected by theoretical arguments. This leads to a large value of the ratio $d_{s}^{cr}/ \xi_{s}$ although Pd does not exhibit any magnetic ordering.

Thermal and electrochemical C-X activation (X = Cl, Br, I) by the strong Lewis acid Pd3(dppm)3(CO)2+ cluster and its catalytic applications.

The stoichiometric and catalytic activations of alkyl halides and acid chlorides by the unsatured Pd(3)(dppm)(3)(CO)(2+) cluster (Pd(3)(2+)) are investigated in detail. A series of alkyl halides (R-X; R = t-Bu, Et, Pr, Bu, allyl; X = Cl, Br, I) react slowly with Pd(3)(2+) to form the corresponding Pd(3)(X)(+) adduct and "R(+)". This activation can proceed much faster if it is electrochemically induced via the formation of the paramagnetic species Pd(3)(+). The latter is the first confidently identified paramagnetic Pd cluster. The kinetic constants extracted from the evolution of the UV-vis spectra for the thermal activation, as well as the amount of electricity to bring the activation to completion for the electrochemically induced reactions, correlate the relative C-X bond strength and the steric factors. The highly reactive "R(+)" species has been trapped using phenol to afford the corresponding ether. On the other hand, the acid chlorides react rapidly with Pd(3)(2+) where no induction is necessary. The analysis of the cyclic voltammograms (CV) establishes that a dissociative mechanism operates (RCOCl --> RCO(+) + Cl(-); R = t-Bu, Ph) prior to Cl(-) scavenging by the Pd(3)(2+) species. For the other acid chlorides (R = n-C(6)H(13), Me(2)CH, Et, Me, Pr), a second associative process (Pd(3)(2+) + RCOCl --> Pd(3)(2+.....)Cl(CO)(R)) is seen. Addition of Cu(NCMe)(4)(+) or Ag(+) leads to the abstraction of Cl(-) from Pd(3)(Cl)(+) to form Pd(3)(2+) and the insoluble MCl materials (M = Cu, Ag) allowing to regenerate the starting unsaturated cluster, where the precipitation of MX drives the reaction. By using a copper anode, the quasi-quantitative catalytic generation of the acylium ion ("RCO(+)") operates cleanly and rapidly. The trapping of "RCO(+)" with PF(6)(-) or BF(4)(-) leads to the corresponding acid fluorides and, with an alcohol (R'OH), to the corresponding ester catalytically, under mild conditions. Attempts were made to trap the key intermediates "Pd(3)(Cl)(+)...M(+)" (M(+) = Cu(+), Ag(+)), which was successfully performed for Pd(3)(ClAg)(2+), as characterized by (31)P NMR, IR, and FAB mass spectrometry. During the course of this investigation, the rare case of PF(6)(-) hydrolysis has been observed, where the product PF(2)O(2)(-) anion is observed in the complex Pd(3)(PF(2)O(2))(+), where the substrate is well-located inside the cavity formed by the dppm-Ph groups above the unsatured face of the Pd(3)(2+) center. This work shows that Pd(3)(2+) is a stronger Lewis acid in CH(2)Cl(2) and THF than AlCl(3), Ag(+), Cu(+), and Tl(+).

Spin dynamics in a halogen-bridged 1-D complex, [PdBr(chxn)2]Br2; (chxn = (1R,2R)-1,2-diaminocyclohexane)

[PdBr(chxn) 2 ]Br 2 with a halogen bridged 1-D structure forms a mixed-valence structure and contains a small amount of paramagnetic Pd 3+ sites which were reported to migrate along the chain by forming neutral solitons. In this study, we prepared highly pure crystals by the electrochemical oxidation technique and measured electric conductivity, spin susceptibility and 1 H NMR relaxation. Obtained results were compared with the data of crystals prepared by the previous Br 2 gas diffusion method.

Magnetocapacitance: probe of spin-dependent potentials.

The magnetic field dependence of the capacitance of Pd-AlOx-Al thin-film structures has been measured. The observed quadratic dependence of capacitance on the magnetic field is consistent with a theoretical model that includes the effect of a spin-dependent electrochemical potential on electron screening in the paramagnetic Pd. This spin-dependent electrochemical potential is related to the Zeeman splitting of the narrow d bands in Pd. The quantitative details depend on the electronic band structure at the surface of Pd.

Bromine-adsorption behavior of a one-dimensional bromine-bridged mixed-valence palladium (II), (IV) complex, [Pd(chxn) 2][PdBr 2(chxn) 2]Br 4

The bromine-adsorption behavior of [Pd(chxn) 2][PdBr 2(chxn) 2]Br 4 (chxn: 1 R,2 R-cyclohexanediamine) has been investigated by elemental analysis, X-ray powder diffraction (XRD), 1H NMR and ESR. From XRD, the reaction of the palladium complex with bromine was revealed to be reversible depending on the partial pressure of bromine around the complex. Elemental analysis suggested the adsorption of two Br 2 molecules per the formula unit. Temperature dependence of the ESR intensity showed the existence of Curie and thermally activated spins in both compounds with and without bromine-adsorption. The spin density, as well as the fraction of Curie spins, decreased after bromine-adsorption suggesting the oxidation of paramagnetic Pd(III) into Pd(IV). 1H NMR spin-lattice relaxation of these compounds is determined by the coupling with the thermally activated spins as well as with the Curie spins.

Spectroscopic and quantum chemical study of the structure of a new paramagnetic dimeric palladium(II,III) complex with creatine

The structure and coordination mode of the newly synthesized dimeric paramagnetic Pd(II,III) complex are studied using magneto-chemical, EPR and IR spectroscopic methods. In order to perform reliable assignment of the IR bands, the structure and IR spectrum of the free creatine were calculated using ab initio method. For calculation of the configuration of its deprotonated and doubly deprotonated forms the semiempirical AM1 method was used.

Electrochemically induced C-Br and C-I bond activation by the Pd3(dppm)3CO(2+) cluster, and characterization of the reactive Pd3(dppm)3CO(+) intermediate: the first confidently identified paramagnetic Pd cluster.

Electrochemically induced C-Br and C-I bond activation by the Pd3(dppm)3CO(2+) cluster, and characterization of the reactive Pd3(dppm)3CO(+) intermediate: the first confidently identified paramagnetic Pd cluster.

Metal valence structures in 1-D mixed-metal complexes [Ni 1−xPd xX(chxn) 2]X 2 studied by solid NMR

13C magic-angle-spinning NMR spectra in solid were measured in mixed-metal 1-D complexes [Ni 1-x Pd x X(chxn) 2 ]X 2 (X: Cl, Br; 0.0≤x ≤1.0) where the antiferromagnetically coupled paramagnetic -X-Ni 3+ -X-Ni 3+ - chains are formed at x=0.0, while the mixed-valence -X-Pd 2+ -X-Pd 4+ - state is made at x=1.0. Upon increasing x from 0.0, 13 C spectra in the C α position in the chxn ligand was broadened and shifted from original Pd 2+ and Pd 4+ positions indicating the fluctuation of Pd valency in mixed states. A marked spectrum broadening observed in C β is also attributable to the effect from the partial mixing of paramagnetic Pd 3+ sites in consistent with reported ESR results.

An in situ ESR study of Pd/H-ZSM-5 interaction with different adsorbents

In situ ESR at 120–473 K permits to monitor formation of transient paramagnetic ions/complexes (isolated Pd+ sites; Pd+/H2O; Pd+/C6H6) upon interaction of isolated Pd2+ cations stabilized by the H-ZSM-5 matrix with different organic compounds and gas mixtures (NO, O2, H2O, H2, propene, benzene). The in situ study provides insight into the elementary steps of redox processes on isolated Pd species in H-ZSM-5 zeolite under realistic conditions. Adsorbed water stabilizes the transient paramagnetic complex and decreases the rate of Pd2+ to Pd0 reduction by H2. Strong bonding of NOx ligands to Pd2+ species suppresses the reduction of Pd(II) ions. Sorption of benzene on preoxidized Pd2+/HZSM-5 is accompanied by an easy formation of organic cation-radicals and of a Pd+/benzene complex, the paramagnetic Pd+/benzene structure indicating a surprisingly high resistance to further reduction to Pd0. Illumination of the Pd/HZSM-5 by UV-visible light causes no measurable change in the redox properties of the catalyst.

Formation and Adsorbate Interactions of Paramagnetic Pd(I) Species in Pd(II)-Exchanged NaK- and H-Clinoptilolite

The formation of monovalent palladium in PdNaK-clinoptilolite where Pd(II) is introduced into extraframework sites as [Pd(NH3)4]2+ by liquid-state ion exchange at 298 K, is compared to that observed in PdH-clinoptilolite where Pd(II) is incorporated by solid-state ion exchange at 823 K, using electron spin resonance (ESR) and electron spin−echo modulation (ESEM) spectroscopies. Dehydration at 473 K produces one Pd(I) species in PdH-clinoptilolite but no ESR signal in PdNaK-clinoptilolite. This indicates that the stability of Pd(I) between PdH-clinoptilolite and PdNaK-clinoptilolite is different, probably due to the different locations and environments of Pd in these systems. Hydrogen reduction of Pd(II) in these two materials after activation reveals that Pd(II) ions in PdNaK-clinoptilolite occupy relatively accessible sites in comparison to those in PdH-clinoptilolite. The interactions of Pd(I) formed by thermal reduction of PdH-clinoptilolite with various adsorbates are also studied. The ESR studies cou...

First-principles studies on magnetism of Ni clusters coated and alloyed with Pd

First-principles electronic-structure calculations are performed for the Ni clusters coated and alloyed with Pd. For coated clusters, the magnetic moments of the Ni core are reduced as compared to the bared one. Especially, Ni 1Ni 12Pd 6Pd 24Pd 12 cluster with O h symmetry consists of the ferromagnetic Ni core, the ferromagnetic Pd and paramagnetic Pd shells, the induced magnetic moment of Pd in the interface is 0.1 μ B, qualitatively in agreement with experimental results found in Ni particles coated with Pd. The dependence of magnetic moment on Pd thickness displays an oscillatory behavior, similar to that found in Co clusters coated with Cu or Ag and in Ni/Pd multilayers. For the alloyed clusters, the enhanced magnetism is confirmed, and the mechanism is clarified: which is resulted not only due to the large atomic distance of Pd but also due to electron transfers. The explanations are given for the different magnetic behaviors between coated and alloyed clusters.