Local Spin Moments Research Articles

First principles calculations, neutron, and x-ray diffraction investigation of Y3Ni13B2, Y3Co13B2, and Y3Ni10Co3B2

Fully relativistic calculations within the local spin density approximation and the generalized gradient approximation were performed to determine the local spin and orbital magnetic moments, as well as the magnetocrystalline anisotropy energy of Y3Ni13B2, Y3Co13B2, and Y3Ni10Co3B2 compounds. A weak in-plane magnetic anisotropy is determined for Y3Ni13B2, under the assumption of a crystallographic-like magnetic unit cell and collinear magnetic moments. The calculations predict considerable c-axis anisotropy for Y3Co13B2 and Y3Ni10Co3B2, but smaller than that of YCo5. The values of the magnetocrystalline anisotropy energy correlate well with both the magnitude of the orbital magnetic moment and the orbital magnetic moment anisotropy. The mixing between Co or Ni 3d states and B 2p states, observable at the bottom of the valence band of the 3d metal having a boron atom nearest neighbor, decreases the 3d spin and especially, the 3d orbital magnetic moments. Y3Ni13B2 and Y3Ni10Co3B2 were also investigated by powder neutron diffraction experiments, at temperatures between 1.8 and 249 K. The Co and Ni site averaged magnetic moments calculated in the mixed compound are in fair agreement with the values obtained by the refinement of the magnetic contribution to the diffraction pattern.

Biquadratic magnetic interaction in parent ferropnictides

We discuss an effective spin Hamiltonian with biquadratic interaction for ferropnictide superconductors from the point of view of band structure theory and available experimental data. This model is consistent with electronic structure calculations and captures many observed magnetic properties, including the anisotropy of the exchange coupling, thin domain walls, and the crossover from first to second-order phase transition under doping. The parameters of the model are analyzed as a function of the local spin moment using first-principles calculations. Calculations show the biquadratic coupling is negative in stoichiometric KFe2Se2, and the phase diagram is extended into this region. We also consider magnetic short-range order and discuss the limitations of this model in comparison with experiment.

Temperature evolution of itinerant ferromagnetism in SrRuO3 probed by optical spectroscopy.

The temperature (T) dependence of the optical conductivity spectra σ(ω) of a single crystal SrRuO(3) thin film is studied over a T range from 5 to 450 K. We observed significant T dependence of the spectral weights of the charge transfer and interband d-d transitions across the ferromagnetic Curie temperature (T(c) ∼ 150 K). Such T dependence was attributed to the increase in the Ru spin moment, which is consistent with the results of density functional theory calculations. T scans of σ(Ω,T) at fixed frequencies Ω reveal a clear T(2) dependence below T(c), demonstrating that the Stoner mechanism is involved in the evolution of the electronic structure. In addition, σ(Ω,T) continues to evolve at temperatures above T(c), indicating that the local spin moment persists in the paramagnetic state. This suggests that SrRuO(3) is an intriguing oxide system with itinerant ferromagnetism.

Oxygen Control of Atomic Structure and Physical Properties of SrRuO3 Surfaces

Complex oxide thin films and heterostructures have become one of the foci for condensed matter physics research due to a broad variety of properties they exhibit. Similar to the bulk, properties of oxide surfaces can be expected to be strongly affected by oxygen stoichiometry. Here we explore the coupling between atomic structure and physical properties of SrRuO3 (SRO), one of the most well-studied oxide materials. We perform a detailed in situ and ex situ experimental investigation of the surfaces of SRO thin films using a combination of scanning tunneling microscopy (STM), X-ray and ultraviolet photoelectron spectroscopy, SQUID magnetometry, and magnetotransport measurements, as well as ab initio modeling. A number of remarkable linear surface reconstructions were observed by STM and interpreted as oxygen adatoms, favorably adsorbed in a regular rectangular or zigzag patterns. The degree of oxygen coverage and different surface patterns change the work function of the thin films, and modify local electronic and magnetic properties of the topmost atomic layer. The ab initio modeling reveals that oxygen adatoms possess frustrated local spin moments with possible spin-glass behavior of the surface covered by adsorbed oxygen. Additionally, the modeling indicates presence of a pseudo gap on the topmost SrO layer on pristine SrO-terminated surface, suggesting possibility for realization of a surface half-metallic film.

Pseudogap state and strong scattering of current carriers by local spin moments as mechanisms for the semiconducting properties of near-stoichiometric iron-vanadium-aluminum alloys

New experimental evidence is obtained on the physical nature of the semiconducting properties of Fe2−xV1+xAl alloys with near-stoichiometric (x = 0) compositions. We examine the low-temperature thermal emf of two alloys: one is enriched in vanadium and exhibits a pseudogap in the electron density of states at the Fermi energy, while in the other, which is depleted in vanadium, the scattering of electrons by localized spins has a stronger influence. The experimental temperature dependences of the thermal emf at low temperatures display some characteristic anomalies that are observed here for the first time and are manifestations of these two effects. Existing theory is used to interpret the anomalous contributions of both effects (types), determine their characteristic parameters, and establish the conditions under which they appear in V-depleted and V-enriched alloys. Data on the thermal emf in magnetic fields show that that the thermal emf decreases substantially as the magnetic field is increased, confirming the key role of the mechanism of current carrier scattering by localized moments.

Significant room temperature ferromagnetism in PAA thin films

Room-temperature ferromagnetism has been observed in Porous Anodic Aluminum (PAA) thin films, with an out-of-plane saturation magnetization of 60 emu cm−3. The significant out-of-plane saturation magnetization is associated with the porous structure of the film. As there were no magnetic impurities present, the origin of the ferromagnetism is most likely due to exchange interactions between localized electron spin moments resulting from oxygen vacancies in the films. A similar hysteretic behavior is observed in porous thin films of nonmagnetic TiO2 and ZnO. This suggests that ferromagnetism may be a universal characteristic of porous thin films of nonmagnetic metal oxides. Our present results suggest a promising direction for research in spintronic devices.

Scattering at magnetic and nonmagnetic impurities on surfaces with strong spin-orbit coupling

Adsorption-induced reduction of surface-state conductivity in epitaxial Bi(111) films, a prototype system with large Rashba-induced surface-state splitting, by adsorbed atoms of Bi, Fe, and Co has been investigated by macroscopic surface magnetotransport measurements at a temperature of 10 K. A detailed analysis of magnetotransport, dc transport, and Hall data reveals that the scattering efficiencies for Co and Fe are larger by a factor of 2 than that for Bi. While for the latter charge transfer and change of band filling near the Fermi level are negligible, we find an increase of hole concentration upon Co and Fe adsorption. These atoms act as acceptors and immobilize on average about 0.5 electrons per adsorbed atom. Besides the dominant classical magnetoconductance signal the films show signatures of weak antilocalization, reflecting the strong spin-orbit coupling in Bi(111) surface states. This behavior can be changed to weak localization by the adsorption of high concentrations (0.1 monolayers) of magnetic impurities (Fe,Co), similarly to results found on the topological insulator Bi${}_{2}$Se${}_{3}$. Our results demonstrate that details of chemical bond formation for impurities are crucial for local spin moments and electronic scattering properties.

Origin of the anomalous magnetic behavior of the Fe13+cluster

By using density functional theory, we show that the exceptionally low value of the total magnetic moment of ${\mathrm{Fe}}_{13}{}^{+}$ observed in an experiment [Phys. Rev. Lett. 108, 057201 (2012)] is not due to antiferromagnetic coupling between the spins of the core and surface atoms as hypothesized but is due to the symmetry-driven quenching of the local spin moments of all atoms with the largest quenching observed for the central atom. Our study of ${\mathrm{Fe}}_{12}{}^{+}$, ${\mathrm{Fe}}_{13}{}^{+}$, ${\mathrm{Fe}}_{14}{}^{+}$, and their neutral parents reveals that the total magnetic moment of ${\mathrm{Fe}}_{13}{}^{+}$ decreases by 9${\ensuremath{\mu}}_{\mathrm{B}}$ with respect to that of neutral Fe${}_{13}$, whereas, the changes are 1${\ensuremath{\mu}}_{\mathrm{B}}$ and 3${\ensuremath{\mu}}_{\mathrm{B}}$ for ${\mathrm{Fe}}_{12}{}^{+}$ and ${\mathrm{Fe}}_{14}{}^{+}$, respectively.

Pressure-induced ferromagnetism in cubic perovskite SrFeO3and BaFeO3

The spin order in cubic perovskite SrFeO3 and BaFeO3 under high pressure is studied by density functional theory (DFT) calculation with local spin density approximation plus Hubbard U (LSDA+U). At ambient pressure, A-type and G-type helical spin orders are almost degenerate in BaFeO3 whose lattice constant is 3.97{\AA} . When the lattice constant is reduced to 3.85 {\AA} which is same as the lattice constant of SrFeO3 at ambient pressure, G-type helical spin order becomes stable, being consistent with SrFeO3. This is because superexchange interaction is enhanced as compared with double exchange interaction. Phase transition from helical spin state to ferromagnetic state in both SrFeO3 and BaFeO3 takes place if the lattice constant is further reduced to 3.70 {\AA}. This is because reduced local spin moment weakens the contribution from superexchange interaction. Our result agrees with recent experimental result of BaFeO3 under high pressure. Additionally, our calculation predicts that half-metal BaFeO3 at ambient pressure will become a good metal under high pressure

Local spin density in the Cr7Ni antiferromagnetic molecular ring and 53Cr-NMR

We present 53Cr-NMR spectra collected at low temperature in a single crystal of the heterometallic antiferromagnetic (AF) ring Cr7Ni in the S = 1/2 ground state with the aim of establishing the distribution of the local electronic moment in the ring. Due to the poor S/N we observed only one signal which is ascribed to three almost equivalent 53Cr nuclei in the ring. The calculated spin density in Cr7Ni in the ground state, with the applied magnetic field both parallel and perpendicular to the plane of the ring, turns out to be AF staggered with the greatest component of the local spin 〈s〉 for the Cr3+ ions next to the Ni2+ ion. The 53Cr-NMR frequency was found to be in good agreement with the local spin density calculated theoretically by assuming a core polarization field of Hcp =− 11 T/μB for both orientations, close to the value found previously in Cr7Cd. The observed orientation dependence of the local spin moments is well reproduced by the theoretical calculation and evidences the importance of single-ion and dipolar anisotropies.

Electronic and optical spectra in a diluted magnetic semiconductor multilayer

The effects of random distribution of magnetic impurities with concentration $x$ in a semiconductor alloy multilayer at a paramagnetic temperature are investigated by means of coherent potential approximation and tight-binding model. The change in the electronic states and the optical absorption spectrum with $x$ is calculated for weak and strong exchange interactions between carrier spins and localized spin moments on magnetic ions. We find that the density of states and optical absorption are strongly layer-dependent due to the quantum size effects. The electronic and optical spectra are broadened due to the spin fluctuations of magnetic ions and in the case of strong exchange interaction, an energy gap appears in both spectra. Furthermore, the interior layers show higher contribution in the optical absorption of the system. The results can be helpful for magneto-optical devices at a paramagnetic temperature.

Itinerant electrons, local moments, and magnetic correlations in the pnictide superconductors CeFeAsO1−xFxand Sr(Fe1−xCox)2As2

A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3$s$ core level photoemission spectra in the parent and optimally doped CeFeAsO${}_{1\ensuremath{-}x}$F${}_{x}$ ($x$ $=$ 0, 0.11) and Sr(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ ($x$ $=$ 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10${}^{\ensuremath{-}15}$ s time scale in the paramagnetic, antiferromagnetic, and superconducting phases, indicative of the occurrence of ubiquitous strong Hund's magnetic correlations. The magnitude of the spin moment is found to vary significantly among different families, 1.3${\ensuremath{\mu}}_{B}$ in CeFeAsO and 2.1${\ensuremath{\mu}}_{B}$ in SrFe${}_{2}$As${}_{2}$. Surprisingly, the spin moment is found to decrease considerably in the optimally doped samples, 0.9${\ensuremath{\mu}}_{B}$ in CeFeAsO${}_{0.89}$F${}_{0.11}$ and 1.3${\ensuremath{\mu}}_{B}$ in Sr(Fe${}_{0.9}$Co${}_{0.1}$)${}_{2}$As${}_{2}$. The strong variation of the spin moment against doping and material type indicates that the spin moments and the motion of itinerant electrons are influenced reciprocally in a self-consistent fashion, reflecting the strong competition between the antiferromagnetic superexchange interaction among the spin moments and the kinetic energy gain of the itinerant electrons in the presence of a strong Hund's coupling. By describing the evolution of the magnetic correlations concomitant with the appearance of superconductivity, these results constitute a fundamental step toward attaining a correct description of the microscopic mechanisms shaping the electronic properties in the pnictides, including magnetism and high-temperature superconductivity.

Universal magnetic properties of sp3-type defects in covalently functionalized graphene

Using density-functional calculations, we study the effect of sp3-type defects created by different covalent functionalizations on the electronic and magnetic properties of graphene. We find that the induced magnetic properties are universal, in the sense that they are largely independent of the particular adsorbates considered. When a weakly polar single covalent bond is established with the layer, a local spin moment of 1.0 μB always appears in graphene. This effect is similar to that of H adsorption, which saturates one pz orbital in the carbon layer. The magnetic couplings between the adsorbates show a strong dependence on the graphene sublattice of chemisorption. Molecules adsorbed at the same sublattice couple ferromagnetically, with an exchange interaction that decays very slowly with distance, while no magnetism is found for adsorbates at opposite sublattices. Similar magnetic properties are obtained if several pz orbitals are saturated simultaneously by the adsorption of a large molecule. These results might open new routes to engineering the magnetic properties of graphene derivatives by chemical means.

Ab initiostudy of2pcore-level x-ray photoemission spectra in ferromagnetic transition metals

We study the 2p-core level x-ray photoemission spectra in ferromagnetic transition metals, Fe, Co, and Ni using a recently developed ab initio method.The excited final states are set up by distributing electrons on the one-electron states calculated under the fully screened potential in the presence of the core hole. We evaluate the overlap between these excited states and the ground state by using one-electron wave functions, and obtain the spectral curves as a function of binding energy. The calculated spectra reproduce well the observed spectra displaying interesting dependence on the element and on the spin of the removed core electron. The origin of the spectral shapes is elucidated in terms of the one-electron states screening the core hole. The magnetic splitting of the threshold energy is also estimated by using the coherent potential approximation within the fully screened potential approximation. It decreases more rapidly than the local spin moment with moving from Fe to Ni. It is estimated to be almost zero for Ni despite the definite local moment about 0.6\mu_B, in agreement with the experiment.

Nonmagnetic and magnetic thiolate-protected Au25superatoms on Cu(111), Ag(111), and Au(111) surfaces

Geometry, electronic structure, and magnetic properties of methylthiolate-stabilized Au${}_{25}$L${}_{18}$ and MnAu${}_{24}$L${}_{18}$ (L = SCH${}_{3}$) clusters adsorbed on noble-metal (111) surfaces have been investigated by using spin-polarized density functional theory computations. The interaction between the cluster and the surface is found to be mediated by charge transfer mainly from or into the ligand monolayer. The electronic properties of the 13-atom metal core remain in all cases rather undisturbed as compared to the isolated clusters in gas phase. The Au${}_{25}$L${}_{18}$ cluster retains a clear highest occupied molecular orbital/lowest unoccupied molecular orbital energy gap in the range of 0.7 to 1.0 eV depending on the surface. The ligand layer is able to decouple the electronic structure of the magnetic MnAu${}_{24}$L${}_{18}$ cluster from the Au(111) surface, retaining a high local spin moment of close to 5 ${\ensuremath{\mu}}_{B}$ arising from the spin-polarized Mn($3d$) electrons. These computations imply that the thiolate monolayer-protected gold clusters may be used as promising building blocks with tunable energy gaps, tunneling barriers, and magnetic moments for applications in the area of electron and/or spin transport.

Synthesis and Characterization of CaPd<sub>3</sub>O<sub>4</sub> Crystals

A new method for the crystal growth of alkaline-earth palladate CaPd3O4 was developed. The crystals were synthesized on a voltage-applied electrode in a molten chloride solvent. The maximum length of the crystal was about 1.5 mm. The X-ray diffraction data were refined well by assuming a cubic structure of the space group Pm n, and the lattice constant a was 5.7471 (10) ?. The temperature dependence of the resistivity showed semiconductor-like characteristics with a very small activation energy Ea of 0.45 meV at low temperatures, and the resistivity at 300 K was 0.1 cm. The temperature dependence of the molar magnetic susceptibility showed the Curie-Weiss paramagnetic behavior with a small molar Curie constant Cmol of 5.0(1) × 10-3 emu K/(mol Oe), indicating the existence of localized spin moments.

Magnetic properties of Sr2FeTaO6 double perovskite epitaxially grown by pulsed-laser deposition

We have investigated the magnetic properties of B-site-ordered Sr2FeTaO6 double perovskite thin films grown on (111) SrTiO3 substrates by using pulsed-laser deposition. High degree of ordering of Fe and Ta ions has been confirmed using x-ray diffraction and atomically resolved high-angle annular dark-field scanning transmission electron microscopy. We have found clear signatures of magnetic frustration in {111} planes of local spin moment arising from next-nearest-neighbor interaction in triangular FeO6 sublattice below 50 K. It is also found that this behavior is suppressed by increasing disorder and eventually leads to antiferromagnetic state.

Boron and nitrogen substitutional impurities inducing magnetic and half‐metallic behavior in zigzag silicon carbon nanoribbons

AbstractZigzag silicon carbon nanoribbons (ZSiC NRs) containing one substitutional nitrogen (N) or boron (B) atom in each super cell that includes four units are investigated by first‐principles calculations. It is found that the system where one of the edge Si atoms was substituted by a B atom presents ferromagnetic half‐metallic behavior, while the system where one of the edge C atoms was substituted by an N atom shows ferrimagnetic half‐metallic behavior. Moreover, in contrast to the pristine system, where the edge magnetism appears at both edges, the edge magnetism of the B‐doped system only appears at the C edge, while the edge magnetism of the N‐doped system only appears at the Si edge. These are because a B atom substituting one of the edge Si atoms destroys the local spin moment at the edge Si atoms, but an N atom substituting one of the edge C atoms destroys the local spin moment at the edge C atoms. When the size of the super cell increases to six units, the dilute doping case, the local spin moment at some edge atoms, where the substitution impurity resides at the edge, remains. This means that the dilute doping fails to completely destroy the local spin moment at the edge atoms, where the impurity resides at the edge. However, the half‐metallic band structure remains in the dilute doping case. Thus, a B atom substituting one of the edge Si atoms, as well as an N atom substituting one of the edge C atoms, induces a semiconductor–half‐metallic transition in the ZSiC NRs.

Ferrimagnetism and spontaneous ordering of transition metals in double perovskite La2CrFeO6films

We report on the atomic ordering of B-site transition metals and magnetic properties of epitaxial La${}_{2}$CrFeO${}_{6}$ double-perovskite films grown by pulsed-laser deposition under various conditions. The highest-ordered sample exhibited a fraction of antisite disorder of only 0.05 and a saturation magnetization of \ensuremath{\sim}2 ${\ensuremath{\mu}}_{B}$ per formula unit at 5 K. The result is consistent with the antiferromagnetic ordering of local spin moment (3${d}^{3}$${}_{\ensuremath{\downarrow}}$3${d}^{5}$${}_{\ensuremath{\uparrow}}$; $S$ = \ensuremath{-}3/2 + 5/2 = 1). Therefore, the magnetic ground state of La${}_{2}$CrFeO${}_{6}$ double perovskite that has been long debated is unambiguously revealed to be ferrimagnetic. Our results present a wide opportunity to explore novel magnetic properties of binary transition-metal perovskites upon epitaxial stabilization of the ordered phase.

First principles study of the electronic and magnetic properties of semi-Heusler alloys NiXSb (X = Ti, V, Cr and Mn)

The electronic density of states (DOS), magnetic moments and band structure of semi-Heusler alloy NiXSb (where X = Ti, V, Cr and Mn) has been studied by using the first principles full-potential linearized augmented plane wave method (FP-LAPW) based on density functional theory (DFT). For the exchange and correlation potential generalized-gradient approximation (GGA) is used. From the observations, NiTiSb shows the possibility of half-metallic ferromagnet (HMF) behavior with a band gap of 0.53 eV and an effective moment of 0.35 μ B. The alloys like NiVSb, NiCrSb and NiMnSb are HMF with band gap of 0.49 eV, 0.38 eV and 0.48 eV and an effective moment of 1.995 μ B, 3.01 μ B and 3.99 μ B respectively. DOS and band structure result shows the 3d states of Ni overlap with 3d states of X atoms suggesting hybridization between them. The exchange-splitting of Ni-3d and X-3d state electrons lead to localized spin moment which determines the HMF behavior of NiXSb. The results obtained are compared and found to be in close agreement with the available data.