D-electron Spin Research Articles

Local spin-state tuning enables high-efficiency nickel sulfide cathode for stable alkaline Zn batteries

Nickel sulfide (Ni3S2) has shown great promise as advanced cathode in alkaline nickel-zinc batteries (ANZBs) for its good electrochemical reversibility and electronic conductivity. However, due to the unfavorable OH− adsorption capability and low intrinsic activity of Ni sites originated from the unsuitable d-electron spin state, the overall performances of Ni3S2 are yet unsatisfactory for practical ANZBs. Herein, we demonstrate the engineering of local spit state of Ni sites via a facile electrochemical activation in Ni3S2 to significantly boost its comprehensive energy storage capability. After electrochemical treatment, the spit state of fractional Ni2+ (eg4t2g4) sites of Ni3S2 is successfully transformed into high-valence Ni3+ (eg4t2g3), which considerably enhances the OH− adsorption capability and redox reactivity. The optimal Ni3S2 electrode exhibits a high capacity of 1.11 mAh cm−2 at 1 mA cm−2 and no capacity decay after 3000 cycles, which are far greater than the pristine Ni3S2 (0.39 mAh cm−2) electrode and also comparable to the best ever-reported alkaline cathodes. Furthermore, the assembled full ANZB can afford a peak energy density of 1.59 mWh cm−2 and power density of 33.92 mW cm−2. This work affords valuable insights into local electronic structure modulation of electrode materials for aqueous energy storage devices.

Underlying mechanism of the driving force for generating spin currents thermally in a ferrimagnetic insulator due to a temperature gradient

We previously proposed the idea that the effective thermal-energy transfer from the phonon system to the spin system in liquid phase epitaxial (LPE) films of Bi-substituted yttrium iron garnet (Bi:YIG), a ferrimagnetic insulator, takes place through enhanced spin-orbit coupling (SOC) with local 3d-electron spins in Fe3+ ions, and that the spin-thermoelectric (STE) generation in a paramagnetic platinum (Pt) layer is raised by increased spin pumping to the Pt layer from the Bi:YIG film. However, we could not specifically relate the underlying mechanism to the driving force for the generation of spin currents due to nonequilibrium dynamics of magnons driven by the temperature gradient ∇T in a magnetic insulator. In this paper, we discuss the origin of the driving force for generating spin currents, which are increased through enhanced SOC in LPE yttrium iron garnet films.

Characteristics of spin-thermoelectric generators enhanced by incorporating liquid phase epitaxial Bi-substituted YIG films as a ferrimagnetic insulator

Liquid phase epitaxial (LPE) Bi-substituted YIG films exhibit a large growth-induced magnetic anisotropy (GIMA) perpendicular to the film surface. The GIMA depends on the melt composition and supercooling in the LPE growth. Our film shows a large Ku value of 0.1 × 104 J/m3 even at a Bi content, x, of 0. We assume that GIMA enhances the spin-orbital coupling (SOC) in the local d-electron spins of Fe3+, and that the SOC-enhanced thermal energy transfer from the phonon system to the spin system increases spin currents in spin-thermoelectric (STE) generation. We discuss the characteristics of STE generators on the basis of data obtained from ferromagnetic resonance measurements.

Interplay between the d - and π -electron systems in magnetic torque of the layered organic conductor κ -(BETS) 2Mn[N(CN)2]3

In the organic charge transfer salt k-(BETS)2Mn[N(CN)2]3 the metallic conductivity is provided by itinerant pi-electrons in the layers of BETS molecules, whereas magnetization is largely dominated by the localized d-electrons of the Mn2+ ions in the insulating anionic layers. We study magnetic properties of the compound in its low-temperature, Mott-insulating state by means of magnetic torque technique. The complex behavior of the torque can be qualitatively explained by the coexistence of two weakly interacting magnetic subsystems associated with paramagnetic d-electron spins and antiferromagnetically ordered pi-electron spins, respectively. Based on the experimental data, we determine the principal axes of magnetization of the Mn2+ sublattice and propose a qualitative model for the pi-electron spin arrangement, implying an important role of the Dzyaloshinskii-Moriya interaction.

Strong evidence for d-electron spin transport at room temperature at a LaAlO3/SrTiO3 interface.

A d-orbital electron has an anisotropic electron orbital and is a source of magnetism. The realization of a two-dimensional electron gas (2DEG) embedded at a LaAlO3/SrTiO3 interface surprised researchers in materials and physical sciences because the 2DEG consists of 3d-electrons of Ti with extraordinarily large carrier mobility, even in the insulating oxide heterostructure. To date, a wide variety of physical phenomena, such as ferromagnetism and the quantum Hall effect, have been discovered in this 2DEG system, demonstrating the ability of d-electron 2DEG systems to provide a material platform for the study of interesting physics. However, because of both ferromagnetism and the Rashba field, long-range spin transport and the exploitation of spintronics functions have been believed difficult to implement in d-electron 2DEG systems. Here, we report the experimental demonstration of room-temperature spin transport in a d-electron-based 2DEG at a LaAlO3/SrTiO3 interface, where the spin relaxation length is about 300 nm. Our finding, which counters the conventional understandings of d-electron 2DEGs, highlights the spin-functionality of conductive oxide systems and opens the field of d-electron spintronics.

Stacked Nanosheets of Pr1–xCaxMnO3 (x = 0.3 and 0.49): A Ferromagnetic Two-Dimensional Material with Spontaneous Exchange Bias

Two-dimensional (2D) planar architectures of inorganic materials have attracted great attention owing to their fascinating physical properties. However, nanosheets of the technologically relevant doped rare-earth manganites are still elusive. Stacked 10–14 nm thick nanosheets of phase pure Pr1–xCaxMnO3 (PCMO; x = 0.3 and 0.49) were obtained by decomposition of carbon coated CaCO3/MnCO3 microsheets and Pr2O2CO3 aggregates, the latter synthesized under pressure at 500–800 °C. The PCMO nanosheets had flatter Mn–O–Mn tilt angles in the MnO6 octahedra and ferromagnetic (FM) moments at 5 K. Spontaneous exchange bias (SEB) coupling between the antiferromagnetic (AF)/FM spins was observed at 5 K under zero-field cooling. High FM moments and SEB were possible due to the long-range magnetic interactions in the stacked 2D arrangement of Mn3+/Mn4+ d-electron spins, where charge ordering was completely suppressed. The SEB behavior was dependent on the initial magnetization process and the direction of rotation of the random spins at the AF/FM interface.

Spin-Flop Switching and Memory in a Molecular Conductor

We report the first observation of spin-flop-induced sharp positive magnetoresistance as large as 100% and nonvolatile magnetoresistive memory in a π-d hybrid molecular conductor, (DIETSe)(2)FeCl(4) [DIETSe = diiode(ethylenedithio)tetraselenafulvalene]. The unprecedented magnetotransport phenomena originate from the coexistence of the spin density wave (SDW) of the quasi-one-dimensional (Q1D) π electrons and the antiferromagnetic order of d-electron spins, indicating the interplay between the electronic instability of Q1D π electrons and local moments of antiferromagnetic d-electron spins. These findings offer new possibilities in molecular electronics/spintronics.

Magnetic Properties versus Network Dimensionality of Cerium(III) Octacyanotungstate(V) Compounds

The reaction of cerium(III) nitrate, sodium octacyanotungstate(V), and 2,2'-bipyrimidine (bpm) under various conditions leads to the formation of three new compounds: ionic [Ce(2)(bpm)(dmf)(6)(H(2)O)(8)][W(CN)(8)](2).3H(2)O (1; dmf = N,N-dimethylformamide), tetranuclear molecules [Ce(2)(bpm)(dmso)(8)(H(2)O)(4)][W(CN)(8)](2).4H(2)O (2; dmso = dimethyl sulfoxide), and a two-dimensional inorganic-organic hybrid coordination polymer {[Ce(2)(bpm)(dmf)(8)(H(2)O)(2)][W(CN)(8)](2)}(n).2nH(2)O (3), all of which contain the subunit [Ce(2)(mu-bpm)] within the structure. These systems were characterized by single-crystal X-ray diffractometry, FTIR spectroscopy, and thermogravimetric analysis. Magnetic susceptibility measurements for 1-3 were performed on polycrystalline samples of the compounds. Magnetic behavior was interpreted in terms of the ligand-field splitting parameters and the exchange interaction between lanthanide centers and a d-electron spin carrier. The results confirmed the ferromagnetic cyano-mediated {Ce-NC-W} interaction J(CeW) = 1.7 (2) and 1.4(3) cm(-1) (3) compared to the antiferromagnetic {Ce-bpm-Ce} interaction J(CeCe) = -1.1 cm(-1) (1 and 2).

Mössbauer spectroscopy and magnetic transition ofλ−(BETS)2FeCl4

The compound lambda-(BETS)2FeCl4 provides an effective demonstration of the interaction of pi-conduction electron and d-electron localized moment systems in molecular crystalline materials where antiferromagnetic insulating and magnetic field induced superconducting states can be realized. The metal-insulator transition has been thought to be cooperative, involving both the itinerant pi- electron and localized d-electron spins where antiferromagnetic order appears in both systems simultaneously. However, recent specific heat data has indicated otherwise [Akiba et al., J. Phys. Soc. Japan 78,033601(2009)]: although the pi-electron system orders antiferromagnetically and produces a metal-insulator transition, a "mysterious" paramagnetic d-electron state remains. We report 57Fe Mossbauer measurements that support the paramagnetic model, provided the d-electron spins remain in a fast relaxation state below the transition. From the measured hyperfine fields, we also determine the temperature dependence of the pi-d electron exchange field.

Crystal Structure and Physical Properties of Conducting Molecular Antiferromagnets with a Halogen-Substituted Donor: (EDO-TTFBr2)2FeX4(X = Cl, Br)

The crystal structure and physical properties of radical ion salts (EDO-TTFBr2)2FeX4 (X = Cl, Br) based on halogen-substituted organic donor and magnetic anions are investigated, including the comparison with the isomorphous compounds (EDO-TTFBr2)2GaX4 with nonmagnetic anions. The crystal structure of these four salts consists of uniformly stacked donor molecules and tetrahedral counter anions, and the Br substituents of the donor molecules are connected to halide ligands of anions with remarkably short intermolecular atomic distances. These salts show metallic behavior around room temperature and undergo a spin-density-wave transition in the low-temperature range, as confirmed with the divergence of the electron spin resonance (ESR) line width. Although close anion-anion contacts are absent in these salts, the FeCl4 salt undergoes an antiferromagnetic transition at TN = 4.2 K, and the FeBr4 salt shows successive magnetic transitions at TN = 13.5 K and TC2 = 8.5 K with a helical spin structure as a candidate for the ground state of the d-electron spins. The magnetoresistance of the FeCl4 salt shows stepwise anomalies, which are explained qualitatively using a pi-d interaction-based frustrated spin system model composed of the donor pi-electron and the anion d-electron spins. Although on the ESR spectra of the FeX4 salts signals from the pi- and d-electron spins are separately observed, the line width of the pi-electron spins broadens under the temperature where the susceptibility deviates from the Curie-Weiss behavior, showing the presence of the pi-d interaction.

Co-existence of long-range order and spin fluctuation in a new geometric frustration series M 2(OH) 3Cl

Recently, we observed the co-existence of a long-range magnetic order and spin fluctuation in a clean compound of clinoatacamite, Cu 2(OH) 3Cl (PRL95 (2005) 057201). The present work reports magnetic studies on other compounds of this transition metal series M 2(OH) 3Cl, where M represents three-dimensional (3D)-electron magnetic ions of Co 2+, Fe 2+, etc., respectively. The present study shows that this co-existence is a common feature of the M 2Cl(OH) 3 series, no matter whether it is anti-ferromagnetic, as in the case of Fe 2(OH) 3Cl ( T N=15 K), or ferromagnetic, as in the case of Co 2(OH) 3Cl ( T C=10.5 K). These compounds show a 3D network of corner-sharing tetrahedrons for the magnetic ions. The tetrahedron is slightly tilted with roughly 10% longer distance between the M–M bonded by Cl than those bonded by O and this distortion is suspected to be responsible for the partial order. This research suggests that the transition metal hydroxyhalide M 2Cl(OH) 3 series are new geometric frustration system on tetrahedral lattice for d-electron spins.

Geometric frustration in a new material system M 2X(OH) 3

Muon spin relaxation/rotation (μSR) are carried out on a new magnetic system of M 2X(OH) 3 (M=transition metal, X=halogen), which we show to be a new geometric frustration system for d-electron spins. Contrasting behaviors were observed for clinoatacamite Cu 2Cl(OH) 3 and Co 2Cl(OH) 3. Clinoatacamite Cu 2Cl(OH) 3 undergoes antiferromagnetic transition at T N=18 K with distinct muon spin rotations, and then it transforms abruptly into a metastable state with nearly complete depolarization of muon spins which suggests strong fluctuation. The system then enters a state in which partial long-range order and spin fluctuation coexist down to the lowest experimentally attainable temperature of 20 mK. For Co 2Cl(OH) 3, although dynamic interactions exist at temperatures higher than 400 K, static order only forms at a much lower temperature of T C=10 K with coexistence of spin fluctuation. This work demonstrates the coexistence of long-range order and spin fluctuation in a novel system of d-electron transition metal oxides.

Transport property of π-d interaction system (EDTDM)2FeBr4 under pressure and magnetic field

Transport property under pressure and magnetic field is investigated for π-d interaction system (EDTDM) 2 FeBr 4 . At ambient pressure. the π-electrons undergo an SDW transition at T∼15K. The SDW state is suppressed with applying pressure, a metallic state being stabilized above P-9kbar down to liq. He temperature. At P∼9kbar, the resistivity shows an anomaly around Neel temperature of the Fe 3- d-electron spins, suggesting the magnetic ordering effect of the Fe 3+ spins on the π-electrons. A negative magnetoresistance having the largest value at P-9kbar comes from the reduction of the effective periodic potential of Fe 3+ spins stabilizing the SDW state upon the elevation of field.

Synthesis, redox chemistry, molecular and electronic structure of some cyclopentadienylcobalt pentafulvene complexes

The pentafulvene complexes [(C 5R 5)Co(C 6H 4R′R′′)] 3d (R=H, R′=Me, R′′=Ph), 3f (R=H, R′=Ph, R′′=C 6H 4NO 2-4), 3g (R=H, R′=R′′=SMe) and 9 (R=Me, R′=R′′=Ph) were prepared from the respective fulvenes and [(C 5R 5)Co(C 2H 4) 2] 8a, b. Protonation of 3 occurs at the fulvene C-α to give substituted cobaltocenium cations. The crystal and molecular structures of 3g, 9 and of the protonation product [ 3e+H] + (R=R′=Ph) were determined. In the neutral complexes the non-planar fulvene ligands are essentially η 4-coordinated with short uncoordinated exocyclic carbon–carbon double bonds. In contrast, the protonated species exhibit η 5-coordinated planar five-membered rings. Using cyclic voltammetry, the complexes 3e, f and 9 were shown to reversibly undergo one-electron oxidation and reduction reactions to give the cations [ 3e] +, [ 3f] +, [ 9] + and anions [ 3e] −, [ 3f] −, [ 9] −. Complex 3g also forms an anion [ 3g] −, but oxidation is irreversible. The X-band ESR spectra of [ 3e] −, [ 3e] + and of [ 9] −, [ 9] + were recorded. In marked contrast to the cations, the anion radicals exhibit considerable anisotropy of the g and A tensors. d-Electron spin densities ρ d=0.5 for the radical anions were derived from a detailed analysis and simulation of the ESR spectra. Using extended Hückel and Fenske–Hall MO calculations for the model complex [(C 5H 5)Co(C 5H 4CH 2)] 3h, the ESR and electrochemical properties were explained by the different metal participation in the HOMO (=SOMO of the cation radicals) and LUMO (=SOMO of the anion radicals).

Electronic structure of vanadium cluster anions as studied by photoelectron spectroscopy

Photoelectron spectra of vanadium cluster anions, Vn− (3⩽n⩽100), were measured at a photon energy of 3.49 eV (355 nm) by using a magnetic-bottle-type photoelectron spectrometer. The electronic density-of-state profiles and the photoelectron spectra of V4− and V3− were calculated by the spin-polarized DV (discrete variational)-Xα method for several plausible geometrical structures. The most plausible structure was determined so that the calculated photoelectron spectrum based on this structure well reproduces the observed one. The calculation showed that V4− has a square planar geometrical structure and a charge distribution of D4h symmetry. On the other hand, V3− was found to possess an equilateral triangle geometrical structure, but C2v symmetry for the charge distribution. The electronic states in the vicinity of the Fermi energy were found to consist of 3d atomic orbitals. The population differences between the minority and the majority spins per atom turned out to be 4.6 and 1.7 for V4− and V3−, respectively. The result implies that the d-electron spins are coupled ferromagnetically in these cluster anions. In addition, the size-dependence of the electron affinity of Vn was explained by a spherical conducting droplet model in the n≳9 range.

The importance of the magnetic dipole term in magneto-circular x-ray absorption dichroism for 3d transition metal compounds

The application of the magneto-circular x-ray absorption dichroism (MCXD) spin sum rule for 3d transition metal compounds faces two problems: the unknown value of the magnetic dipole operator and the division between the and edges. A systematic study of the order of magnitude of the -operator for ions is presented. The variation of the -values with temperature is described and analysed, for all cases from to cations in two different situations. Firstly the perfect octahedral case is considered. It is shown that is non-zero for low temperature; but, as it originates only from d-electron spin - orbit splitting, it is washed out at room temperature. Secondly, a model of the surface situation is considered. In this case originates mainly from the crystal-field splitting. It then exhibits quite large values at any temperature and can by no means be neglected when applying the sum rule. The error introduced in the sum rule due to the mixing of and edges has been estimated.

Electronic states of high-Tc cuprate superconductors as studied by the use of the composite-operator approach.

The doping dependence of the electronic states in cuprate superconductors is studied by use of the p-d-mixing model. The p-d hopping under strong correlation is treated by introducing electronic excitations associated with Cu-O bonds, which are described by use of composite operators composed of p electrons and neighboring d-electron spin and charge fluctuations. It is shown that changes of electronic states with carrier doping are understood by mixing and decays among those composite electronic excitations. By hole doping, transfer of the density of states to the Fermi level is induced from both the upper Hubbard band and the bottom of the valence band. It is shown that the crossover from a highly correlated electron band to a simple mixing band may be understood by allowing an interchange of energy positions between the upper Hubbard and composite excitation levels. The doping dependence of intrasite charge and spin fluctuations at the Cu site is also discussed. Specially, a prominent intensity transfer to a low-energy region with carrier doping is found to result in the charge fluctuation.

NMR Study of Nearly Itinerant Ferromagnetic LuCo2

The Knight shift and the nuclear spin-lattice relaxation rate, 1/ T 1 , of 59 Co in LuCo 2 have been measured. 1/ T 1 T due to the d-electron spins was found to be proportional to the d-spin susceptibility in good agreement with the SCR theory. From the quantitative analysis, the characteristic temperature, T 0 , implying the energy width of the dynaical spin-fluctuation spectrum is estimated for LuCo 2 together with those for the typical nearly ferromagnetic metals. The value T 0 may be related to the degree of the ferromagnetic tendency as welll as the temperature of χ maximum in the nearly itinerant ferromagnet.

Resistivity, susceptibility, and superconductivity in mixed valent Ce(Rh1−xRux)2

Resistivity, susceptibility, and superconducting transition temperatures Tc have been measured for the Ce(Rh1−xRux)2 system. Tc is depressed from ∼6 K for CeRu2 to zero at ∼Ce(Rh0.5Ru0.5)2. Striking violations of Nordheim’s and Matthiessen’s rules are observed for the resistivity in not only Ce(Rh1−xRux)2 but also La(Rh1−xRux)2 suggesting that large changes in d-electron spin fluctuations may be occurring. Striking variations in the susceptibility behavior are also observed across the Ce(Rh1−xRux)2 system.

Nuclear Magnetic Relaxation in Palladium

The Knight shift and the nuclear spin-lattice relaxation rate (1/ T 1 ) of 105 Pd nuclei has been measured in an exchange-enhanced paramagnet Pd metal, in the temperature range between 4.2 and 300 K. Frorn the Knight shift measurement the core polarization hyperfine field is obtained to be -(3.39±0.03)×10 5 Oe/µ B . 1/ T 1 is proportional to temperature with the value, 1/( T 1 T )=1.35±0.03 sec -1 K -1 , below about 150 K, while 1/ T 1 begins to deviate from this relation to smaller values at higher temperature. Relaxation rate due to d-electron spin, (1/ T 1 ) d , was obtained by subtracting the estimated values of other contributions from measured 1/ T 1 . Frorn the value and the temperature dependence of (1/ T 1 ) d , we conclude that (1/ T 1 ) is mostly determined by the long wavelength component of the spin fluctuation in the whole temperature range.