Large Orbital Magnetic Moment Research Articles

Observation of size dependent orbital magnetism of single Fe impurity in nanocrystalline Pb hosts

Employing the time differential perturbed angular distribution (TDPAD) technique we have investigated the local magnetism of single Fe impurity in nanocrystalline Pb matrices, by measuring the local susceptibility and spin relaxation rate of 54Fe nuclei. Compared to the nonmagnetic behavior of Fe in bulk Pb host, characterized by independent local susceptibility χloc(T), the magnetic response of Fe in the nanocrystalline samples with particle size of 10 nm and below exhibit Curie-Weiss like χloc(T) with positive slope, indicating the presence of large orbital magnetic moment on the Fe impurity. The orbital moment of Fe has been estimated to be ∼ 1 µB for 6 nm nano-Pb. Further investigations are being carried out to understand the emergence of orbital magnetism of Fe in nano-Pb.

Interaction-Driven Giant Orbital Magnetic Moments in Carbon Nanotubes.

Carbon nanotubes continue to be model systems for studies of confinement and interactions. This is particularly true in the case of so-called "ultraclean" carbon nanotube devices offering the study of quantum dots with extremely low disorder. The quality of such systems, however, has increasingly revealed glaring discrepancies between experiment and theory. Here, we address the outstanding anomaly of exceptionally large orbital magnetic moments in carbon nanotube quantum dots. We perform low temperature magnetotransport measurements of the orbital magnetic moment and find it is up to 7 times larger than expected from the conventional semiclassical model. Moreover, the magnitude of the magnetic moment monotonically drops with the addition of each electron to the quantum dot directly contradicting the widely accepted shell filling picture of single-particle levels. We carry out quasiparticle calculations, both from first principles and within the effective-mass approximation, and find the giant magnetic moments can only be captured by considering a self-energy correction to the electronic band structure due to electron-electron interactions.

Magnetic Behavior of Superconductor 2H-NbSe2 Intercalated with Iron: First Principle Study

We investigated the structural, electronic and magnetic properties of iron-intercalated niobium selinium, $Fe_{x}NbSe_{2}$ for $x = 0$ and 0.25, from first principle calculations. The tendency towards the localization in superconductors criterion which has been extracted from our results placed the $NbSe_{2}$ compound in vicinity of Fe-based superconductors. The DFT plus on-site coulomb repulsion U (DFT+U) and the exact exchange for correlated electrons (DFT+EECE)-based hybrid functional were used including the spin-orbit coupling (SOC) to understand the role of correlation and SOC effects in this type of systems. The calculation predicts the correct antiferromagnetic (AF) ground state, from both the generalized gradient approximation (GGA) and GGA+U. A shift up in the Fermi energy was observed after the intercalation of 2H-NbSe2 with iron, indicating a charge transfer from Fe to the host compound. The obtained magnetic moment of Fe is enhanced by a correlation effect in both DFT+U and DFT+EECE over the small value from GGA-only. Moreover, a large unquenched orbital magnetic moment is saturated to $m_{orb} \sim 0.6\mu _{B}$ under a moderate correlation effect of $U_{eff}\sim 2.0eV$ . We also show that the correlation effect is important along side with SOC in order to get a true picture for the band filling consistent with that of the crystal field splitting and a large unquenched $m_{orb}$ which has been found experimentally in such systems. Therefore, the combination of correlation and SOC effects is a decisive choice for further studies of such systems.

Interdiffusion in epitaxial ultrathin Co2FeAl/MgO heterostructures with interface-induced perpendicular magnetic anisotropy

The interfacial atomic structure of epitaxial ultrathin Co2FeAl/MgO(001) heterostructures, which is related to the interface-induced perpendicular magnetic anisotropy (PMA), was investigated using scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, and X-ray magnetic circular dichroism. Al atoms from the Co2FeAl layer significantly interdiffused into MgO, forming an Al-deficient Co–Fe–Al/Mg–Al–O structure near the Co2FeAl/MgO interface. This atomic replacement may have enhanced the PMA, which is consistent with the observed large perpendicular orbital magnetic moments of Fe atoms at the interface. This work suggests that control of interdiffusion at ferromagnet/barrier interfaces is critical for designing an interface-induced PMA system.

Room-temperature local ferromagnetism and its nanoscale expansion in the ferromagnetic semiconductor Ge(1-x)Fex.

We investigate the local electronic structure and magnetic properties of the group-IV-based ferromagnetic semiconductor, Ge1−xFex (GeFe), using soft X-ray magnetic circular dichroism. Our results show that the doped Fe 3d electrons are strongly hybridized with the Ge 4p states, and have a large orbital magnetic moment relative to the spin magnetic moment; i.e., morb/mspin ≈ 0.1. We find that nanoscale local ferromagnetic regions, which are formed through ferromagnetic exchange interactions in the high-Fe-content regions of the GeFe films, exist even at room temperature, well above the Curie temperature of 20–100 K. We observe the intriguing nanoscale expansion of the local ferromagnetic regions with decreasing temperature, followed by a transition of the entire film into a ferromagnetic state at the Curie temperature.

Specific features of the formation of atomic magnetic moments in amorphous films RE-Co (RE = La, Gd, Tb)

A systematic investigation of the magnetic properties of amorphous films in (RE) x Co100 − x binary systems in the ground state with rare-earth elements (RE) of different types has been performed. The concentration dependences of the average atomic magnetic moments of cobalt (m Co), gadolinium (m Gd), and terbium (m Tb) have been determined from the analysis of the spontaneous magnetization of the films with a nonmagnetic rare-earth element (La), a rare-earth element with a spherical electron shell (Gd), and a rareearth element with a large orbital magnetic moment (Tb). It has been shown that, in the range 0 < x < 50 at %, the magnetic moment m Co decreases from 1.7 μB to zero, the magnetic moment m Gd remains unchanged and almost coincides with the magnetic moment of the free atom (7 μB), and the value of m Tb decreases monotonically, but the rate of decrease depends on the method of the sample preparation. The revealed regularities are associated with the concentration change in the electronic structure of cobalt and with the specificity of the magnetic structure of the films, which has a ferromagnetic, ferrimagnetic, or sperimagnetic character for samples containing La, Gd, or Tb, respectively.

Trends in magnetism of free Rh clusters via relativistic ab-initio calculations

A fully relativistic ab-initio study on free Rh clusters of 13–135 atoms is performed to identify general trends concerning their magnetism and to check whether concepts which proved to be useful in interpreting magnetism of 3d metals are applicable to magnetism of 4d systems. We found that there is no systematic relation between local magnetic moments and coordination numbers. On the other hand, the Stoner model appears well-suited both as a criterion for the onset of magnetism and as a guide for the dependence of local magnetic moments on the site-resolved density of states at the Fermi level. Large orbital magnetic moments antiparallel to spin magnetic moments were found for some sites. The intra-atomic magnetic dipole Tz term can be quite large at certain sites but as a whole it is unlikely to affect the interpretation of x-ray magnetic circular dichroism experiments based on the sum rules.

Magnetic structures of FeTiO3-Fe2O3 solid solution thin films studied by soft X-ray magnetic circular dichroism and ab initio multiplet calculations

The solid solutions between ilmenite (FeTiO3) and hematite (α-Fe2O3) have recently attracted considerable attention as a spintronic material due to their interesting magnetic and electrical properties. In this study, the electronic and magnetic structures of epitaxially grown 0.6FeTiO3·0.4Fe2O3 solid solution thin films were investigated by combining x-ray absorption near-edge structure (XANES), x-ray magnetic circular dichroism (XMCD) for two different crystallographic projections, and first-principles theoretical calculations. The Fe L-edge XANES and XMCD spectra reveal that Fe is in the mixed-valent Fe2+–Fe3+ states while Fe2+ ions are mainly responsible for the magnetization. Moreover, the experimental Fe L-edge XANES and XMCD spectra change depending on the incident x-ray directions, and the theoretical spectra explain such spectral features. We also find a large orbital magnetic moment, which can originate the magnetic anisotropy of this system. On the other hand, although the valence state of Ti was interpreted to be 4+ from the Ti L-edge XANES, XMCD signals indicate that some electrons are present in the Ti-3d orbital, which are coupled antiparallel to the magnetic moment of Fe2+ ions.

First principle study of the origin of the large orbital magnetic moment in α − CoV2O6

The electronic structure of the CoV2O6 magnetic oxide is investigated using first principle density functional theory calculations with the full potential linearized augmented plane wave method. The spin–orbit coupling (SOC) and the GGA+U functional are included in order to determine the orbital magnetic moment (OMM) for the monoclinic C2/m (or α phase) structure where, experimentally, the ground state is found insulating with a large magnetic moment perpendicular to the chains direction in an antiferromagnetic configuration. It is exhibited, that the perturbation approach of the SOC gives only small OMM values but large ones can be obtained when special initial charge densities are used. An OMM as large as 1.5 μB in the direction of the octahedron axis perpendicular to the chains is obtained in agreement with the experiment. The electronic structure analysis shows that this large OMM results from the occurrence of a (zx,yz) hybridized state.

Spectroscopic studies on the electronic and magnetic states of Co-doped perovskite manganite Pr0.8Ca0.2Mn1−yCoyO3thin films

We have investigated the electronic and magnetic properties of Co-doped Pr0.8Ca0.2MnO3 thin films using various spectroscopic techniques. X-ray absorption and hard x-ray photoemission spectroscopy revealed that the substituted Co ions are in the divalent state, resulting in hole doping on the Mn atoms. Studies of element-selective magnetic properties by x-ray magnetic circular dichroism found a large orbital magnetic moment for the Co ions. These spectroscopic studies reveal that the substituted Co ions play several roles of hole doping for Mn, ferromagnetic superexchange coupling between the Co2+ and Mn4+ ions, and orbital magnetism of the Co2+ ions. Competition among these complex interactions produces the unique electronic and magnetic behaviors including enhanced coercivity of the Co-doped Pr0.8Ca0.2MnO3.

Large anisotropic Fe orbital moments in perpendicularly magnetized Co2FeAl Heusler alloy thin films revealed by angular-dependent x-ray magnetic circular dichroism

Perpendicular magnetic anisotropy (PMA) in Heusler alloy Co2FeAl thin films sharing an interface with a MgO layer is investigated by angular-dependent x-ray magnetic circular dichroism. Orbital and spin magnetic moments are deduced separately for Fe and Co 3d electrons. In addition, the PMA energies are estimated using the orbital magnetic moments parallel and perpendicular to the film surfaces. We found that PMA in Co2FeAl is determined mainly by the contribution of Fe atoms with large orbital magnetic moments, which are enhanced at the interface between Co2FeAl and MgO. Furthermore, element specific magnetization curves of Fe and Co are found to be similar, suggesting the existence of ferromagnetic coupling between Fe and Co PMA directions.

Electrons in a Magnetic Field: Special Spin in de Haas-van Alphen effect

When the magnetic field is applied in a metal, the electrons behave like a harmonic oscillator. When the field is increased these harmonic oscillator type levels cross the Fermi energy at a particular point resulting into the discontinuities in the population of any particular level at that point. For a large orbital magnetic moment, different from L = 0 and both signs of spin in the total magnetic momentum quantum number, j = l ± s, the discontinuities in the population of the electrons in a particular level become double valued resulting into doubling of oscillations in the magnetization. There is a double valued change in the energy of the electrons when they transfer from the harmonic oscillator type level to the Fermi level. The magnetization depends on the value of j = l ± s so that there is a double valued period in the oscillations. The de Haas-van Alphen effect is usually described for the L = 0 electrons. Hence, we see that the de Haas-van Alphen effect is considerably modified in going from L = 0 to j = l ± s, with both signs in the spin.

Unusual magnetic properties induced by local structure in a quasi-one-dimensional Ising chain system:α-CoV2O6

We have investigated the origins of large orbital magnetic moment (OM) and unique magnetic anisotropy in the quasi-1D magnetic cobaltate, $\ensuremath{\alpha}$-CoV${}_{2}$O${}_{6}$, employing both the ab initio band structure method and the microscopic cluster model calculations. We have found that the peculiar crystal electric field effect in $\ensuremath{\alpha}$-CoV${}_{2}$O${}_{6}$ combined with the strong spin-orbit coupling induces the unusually large OM and other intriguing magnetic properties of $\ensuremath{\alpha}$-CoV${}_{2}$O${}_{6}$. The observed 1/3 magnetization plateau in the $M$-$H$ curve is explained by the spin-flip mechanism based on the MC simulation. Anomalous magnetic entropy behavior is attributed to the strong uniaxial magnetic anisotropy in the quasi-1D Ising chain system.

RKKY Ferromagnetism with Ising-Like Spin States in IntercalatedFe1/4TaS2

We investigated the magnetic nature of Fe(1/4)TaS2 using x-ray absorption spectroscopy, photoemission spectroscopy, and first principles band calculations. The results show a large unquenched orbital magnetic moment (∼1.0 μ(B)/Fe) at intercalated Fe sites, resulting in a gigantic magnetic anisotropy (H(A)≃60 T). The magnetic coupling is well understood in terms of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, suggesting a novel RKKY ferromagnet with Ising-type spin states. We also found that this indirect exchange coupling between the neighboring Fe spins is ferromagnetic and maximized at the Fe-Fe distance of 2×2 superstructure.

Iron oxide nanoparticles coated with gold: Enhanced magnetic moment due to interfacial effects

In this paper, we show that when nanoparticles of Fe3O4 are coated with gold there is a distinct enhancement of magnetization by a factor of six. This increase of magnetization has been attributed to large orbital magnetic moment formation at the magnetic particle/Au (core/shell) interface. Our theoretical analysis indicates that the enhanced magnetism observed in Fe3O4-Au (core-shell) nanoparticles is an interfacial effect. The origin of magnetism in Au as an interfacial phenomenon is supported by the observation of positive magnetization in citrate coated gold nanoparticles. In citrate coated gold nanoparticles, we observe a crossover from positive magnetization value to negative magnetization value upon increasing magnetic field indicating cancellation of interfacial magnetization by the diamagnetic contribution from the bulk. We propose a theoretical formalism which semi-quantitatively explains our experimental results and supports the origin of magnetization in Au as an interfacial effect.

Strong spin–orbit effects in small Pt clusters: Geometric structure,magnetic isomers and anisotropy

Ab initio density functional calculations including spin-orbit coupling (SOC) have been performed for Pt(n), n = 2-6 clusters. The strong SOC tends to stabilize planar structures for n = 2-5, whereas for clusters consisting of six atoms, three-dimensional structures remain preferred. SOC leads to the formation of large orbital magnetic moments and to a mixing of different spin states. Due to the spin-mixing the total magnetic moment may be larger or smaller than the spin moment in the absence of SOC. Both spin and orbital moments are found to be anisotropic. Because of the strong SOC the energy differences between coexisting magnetic isomers can be comparable to or even smaller than their magnetic anisotropy energies. In this case the lowest barrier for magnetization reversal can be determined by a magnetic isomer which is different from the ground state configuration.

Magnetic properties of Fe 2 n and (FePt) n[formula omitted] clusters and magnetic anisotropy of transition metal dimers

Structures and magnetic properties of (FePt) n clusters (with n ≤ 5 ) have been studied within density functional theory (DFT). Specific attention has been paid to the influence of the spin–orbit interaction on the geometry, binding energy and magnetic moments. It is found that for (FePt) n clusters, the ground states usually display collinear magnetic behavior, although small deviations from the collinear direction are noticeable when the spin–orbit interaction (SOI) is included. The spin magnetic moment and the binding energy increase with the size of the cluster, although the spin magnetic moment per (FePt) unit remains unaltered. The spin magnetic moment of the Pt atoms slightly increases with the cluster size. The orbital and spin magnetic moments of Fe atoms increases when Fe is alloyed to Pt atoms, larger than that is obtained in pure iron clusters and shrinks with cluster size. For the dimers studied, a large magnetic anisotropy energy (MAE) is observed for Pt 2, Co 2 and CoPt with a large orbital magnetic moment in the ground state(easy axis). In other way, we note the dependence of MAE on the orbital magnetic anisotropy. In contrast for Fe 2 and FePt, the MAE is relatively small.

Controllable [formula omitted] transition in the superconductor/carbon nanotube quantum dot/superconductor junction by an axial magnetic field

The supercurrent flowing through the superconductor/carbon nanotube quantum dot/superconductor junction has been investigated. It shows that the direction of the supercurrent can be controlled by a small axial magnetic field, leading to the 0 – π transition, due to the electron's large orbital magnetic moment in the carbon nanotubes.

Magnetic moment and magnetic anisotropy of linear and zigzag4dand5dtransition metal nanowires: First-principles calculations

An extensive ab initio study of the physical properties of both linear and zigzag atomic chains of all $4d$ and $5d$ transition metals (TMs) within the generalized gradient approximation by using the accurate projector-augmented wave method, has been carried out. The atomic structures of equilibrium and metastable states were theoretically determined. All the TM linear chains are found to be unstable against the corresponding zigzag structures. All the TM chains, except Nb, Ag, and La, have a stable (or metastable) magnetic state in either the linear or zigzag or both structures. Magnetic states appear also in the sufficiently stretched Nb and La linear chains and in the largely compressed Y and La chains. The spin magnetic moments in the Mo, Tc, Ru, Rh, W, Re chains could be large $(\ensuremath{\ge}1.0\text{ }{\ensuremath{\mu}}_{B}/\text{atom})$. Structural transformation from the linear to zigzag chains could suppress the magnetism already in the linear chain, induce the magnetism in the zigzag structure, and also cause a change in the magnetic state (ferromagnetic to antiferromagnetic or vice verse). The calculations including the spin-orbit coupling reveal that the orbital moments in the Zr, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir, and Pt chains could be rather large $(\ensuremath{\ge}0.1\text{ }{\ensuremath{\mu}}_{B}/\text{atom})$. Importantly, large magnetic anisotropy energy $(\ensuremath{\ge}1.0\text{ }\text{meV}/\text{atom})$ is found in most of the magnetic TM chains, suggesting that these nanowires could have fascinating applications in ultrahigh-density magnetic memories and hard disks. In particular, giant magnetic anisotropy energy $(\ensuremath{\ge}10.0\text{ }\text{meV}/\text{atom})$ could appear in the Ru, Re, Rh, and Ir chains. Furthermore, the magnetic anisotropy energy in several elongated linear chains could be as large as 40.0 meV/atom. A spin-reorientation transition occurs in the Ru, Ir, Ta, Zr, La, Ta, and Ir linear chains when they are elongated. Remarkably, all the $5d$ as well as Tc and Pd chains show the colossal magnetic anisotropy (i.e., it is impossible to rotate magnetization into certain directions). Finally, the electronic band structure and density of states of the nanowires have also been calculated in order to understand the electronic origin of the large magnetic anisotropy and orbital magnetic moment as well as to estimate the conduction electron spin polarization.

Electronic Origin of Giant Magnetic Anisotropy in MultiferroicLuFe2O4

We investigated the orbital anisotropy of LuFe(2)O(4) using the Fe L(2,3)- and O K-edge x-ray absorption spectroscopy (XAS) and cluster model calculations. X-ray magnetic circular dichroism reveals a surprisingly large orbital magnetic moment (m(o) approximately 0.8 micro(B)/f.u.), which originates the giant magnetic anisotropy. The polarization dependent XAS enables us to identify the orbital states and occupations, different from the band calculation predictions. These findings were examined by using the cluster model analysis, which also explains the orbital magnetic moment as well as the total moment (2.9 micro(B)/f.u.). Taking into account the charge order, we also determined the spin structure.