Enhancement Of Magnetic Moment Research Articles

Gram scale synthesis of high magnetic moment Fe100−xCox alloy nanoparticles: Reaction mechanism, structural and magnetic properties and its application on nanocomposite

Gram scale Fe100−xCox alloy nanoparticles with an average particle size of about 30 nm were synthesized in an inert atmosphere using a modified polyol process. The x-ray diffraction pattern clearly shows the formation of Fe100−xCox alloy nanoparticles. Electron microscopy studies depict the cubic morphology for the Fe57Co43 nanoparticles and nearly hexagonal shape for the Co66Fe34 alloy nanoparticles. The magnetic moment of Fe57Co43 nanoparticles that were synthesized in gram scale, were in the range of 21–21.5 kG (±0.5 kG) at room temperature and the Co66Fe34 nanoparticles were in the range 17.5–18 kG (±0.5 kG). Both samples had the intrinsic coercivity in the range of 150–165 Oe. The as-synthesized nanoparticles were used to fabricate the nanocomposite magnet by the hot press method. The composite demonstrated an exchange-coupled effect with a 15% enhancement of magnetic moment and remanence with a 2% addition of Fe57Co43 nanoparticles.

The magnetic phase transition in titanium oxide induced by proton irradiation

We have studied the magnetic properties of 57Fe-doped TiO2 compounds irradiated by proton with 0, 5 and 10pC/μm2. We have observed the enhancement of the magnetic moment, measured by superconducting quantum interference device magnetometer, with increasing proton irradiation ranging from 0 to 10pC/μm2. Mössbauer spectra of proton irradiated Ti0.9957Fe0.01O2 samples were taken at room temperature. Two sites of the wing (sextet) and the central (doublet) are shown in the spectra, which suggest the magnetically ordered phase and the paramagnetic phase, respectively. With increasing proton irradiation, the part of Fe3+ ions was converted to Fe2+ ions by compensation charge. This clearly suggests that the enhancement of magnetic moment after proton irradiation is contributed to the moment by the spin-orbit coupling of Fe2+ ions.

A-site ion-size effect on the transport and magnetic properties of Ce doping Pr0.3Ce0.2CaxSr0.5−xMnO3 (0≤x≤0.25)

The effects of A-site ion-size 〈rA〉 on the crystal structures, transport and magnetic properties of the perovskite manganese oxide Pr0.3Ce0.2CaxSr0.5-xMnO (0 ≤ x ≤ 0.25) have been investigated. In those compounds, when 0≤x≤0.125, the temperatures (Tmax) of the resistivity maximums below Curie Temperature TC are correlated with the Kondo-like scattering of Ce3+ and the onset of antiferromagnetic ordering of Ce3+ with respect to the Mn-sublattice moments. The decrease of 〈rA〉 causes the anomalous increase of lattice parameters b, c and unit cell volume, the decrease of the differences of TC and Tmax, the weakening of the Kondo-like scattering and magnetic order of Ce3+, and the enhancement of saturation magnetic moment, which give the evidences of the valence enhancing of Ce ions from + 3 toward + 4 with 〈rA〉 decreasing. Although the Ca doping is expected to drive the system toward the antiferromagnetic ground state, the increase of valence of Ce enhances the content of Mn3+ in the system, which drives the system to the ferromagnetic ground state. The changes of ion-size 〈rA〉 and the valence of Ce are co-operating on the transport and magnetic properties of the half-doped manganites.

Vacancy and substitutional impurities in the spin-density wave state of Cr from first principles

Density functional theory (DFT) calculations are carried out to study magnetic and energetic properties of vacancy, and magnetic and nonmagnetic substitutional impurities (respectively Fe and Cu) in the ground state of bcc Cr, i.e., spin-density wave (SDW). We find the lowest energy site for all the defects to be around a magnetic node, as compared with the high-spin SDW sites and (100)-layered antiferromagnetic (AF) and nonmagnetic (NM) phases. The corresponding differences for vacancy formation energy are 0.29, 0.32, and 0.23 eV, respectively. The migration of a vacancy is revealed to be highly anisotropic in the SDW state, mainly confined in the nodal and adjacent planes. The energy barrier for such a quasi-bidimensional motion is indeed 0.52 eV lower than that for migration in perpendicular directions. Regarding magnetic modifications of the SDW introduced by point defects, they are confirmed to be weak and rather local at low defect concentrations ($0.27%$ and $0.55%$). Cu behaves similarly to a vacancy-inducing magnetic moment enhancement on neighboring Cr atoms. On the other side, the presence of Fe atoms leads to multiple energy minima with different local magnetic arrangements, particularly around a node, due to competition between neighboring Fe-Cr, Cr-Cr, and Fe-Fe magnetic coupling tendencies. The present results strongly suggest that simple AF and NM phases may not allow an accurate description of defect properties in the ground state of Cr. Instead, an explicit SDW representation is required. In addition, we point out that the presence of vacancy and both Cu and Fe may promote a migration of SDW nodes, which may activate the SDW-to-AF phase transition through a node-annihilation mechanism as proposed in previous works.

Magnetic moment enhancement at the Fe(100)/Co(bcc) interface: a nanometer-scale investigation from electron energy loss spectra

The electronic structure and the magnetic moment of metallic iron are strongly modified near the Fe(100)/Co( bcc ) interface. This effect has been evaluated from the iron L 2,3 energy loss near edge structure recorded for Fe/Co superlattices grown by molecular beam epitaxy with different periods. The interface-induced modification of the intensity ratio I ( L 3 )/ I ( L 2 ) which has been measured by electron energy loss spectroscopy is in good agreement with the enhancement of the magnetic moment calculated from first principles. This shows that the intensity ratio can be used to obtain information on magnetic moments at a nanometer scale in a transmission electron microscope.

Oxygen Deficiency and Room Temperature Ferromagnetism in Undoped and Cobalt-Doped $\hbox{TiO}_{\bm 2}$ Nanoparticles

The compositional analysis done on the synthesized undoped and cobalt (Co)-doped titanium dioxide ( TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) nanoparticles by X-ray photoelectron spectroscopy and energy dispersive X-ray analysis suggests oxygen deficiency in undoped and Co-doped TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> samples. Magnetic investigations by vibrating sample magnetometer (VSM) indicate that undoped TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles possess ferromagnetic ordering at room temperature due to the oxygen deficiency and in Co-doped TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , the enhancement in magnetic moment is due to Co doping.

Large Isotropic Volume Change due to Thermal-induced First-order Transition in La1−zPrz(Fe0.88Si0.12)13

NaZn13-type La0.5Pr0.5(Fe0.88Si0.12)13 exhibits an isotropic volume change associated with the thermal-induced first-order transition at the Curie temperature TC = 186 K. The magnitude of the isotropic volume change at TC of La0.5Pr0.5(Fe0.88Si0.12)13 is about 1.3 %, larger than that of La(Fe0.88Si0.12)13. As a result, the enhancement in the local magnetic moment of Fe at TC is caused by the partial substitution of Pr. The volume change around TC of La1−zPrz(Fe0.88Si0.12)13 is comparable in magnitude to that of La1−z,Ce2,(Fe0.88Si0.12)13 having the same TC. Accordingly, the enhancement in the local magnetic moment of Fe at TC for La1−zPrz(Fe0.88Si0.12)13 is closely related with the decrease of TC without the marked decrease of thermal stability of Fe moment.

Growth and characterization of transition-metal and rare-earth doped III-nitride semiconductors for spintronics

ABSTRACTTransition metal (Cr) and rare-earth (Dd, Dy) doped III-nitride semiconductor bulk layers and superlattice (SL) structures are grown on sapphire (0001) substrates and GaN (0001) templates by plasma-assisted molecular-beam epitaxy. For the GaGdN/GaN and InGaGdN/GaN SL and Si co-doped samples, enhancement of magnetization and magnetic moment are observed, suggesting the carrier-mediated ferromagnetism. Low temperature growth of GaGdN can increase the Gd concentration and magnetization. Results for the Dy-doped GaN as well as the GaCrN/AlN/GaCrN tunnel magneto-resistance (TMR) diodes are also described.

Neutrino magnetic moment in a magnetized plasma

The magnetized-plasma contribution to the neutrino anomalous magnetic moment is calculated. It is shown that, in a magnetized plasma, only part of the neutrino additional energy associated with the neutrino spin and with the magnetic-field strength contributes to the neutrino magnetic moment. It is found that, in contrast to results presented previously in the literature, the presence of a magnetized plasma does not lead to a substantial enhancement of the neutrino magnetic moment.

Influence of Ni doping on magnetic behavior of Mn doped ZnO

Enhancement in room temperature ferromagnetism has been observed in Mn doped ZnO, when co-doped with Ni. In the co-doped system, Ni–O–Ni and Ni–O–Mn carrier mediated exchange interaction dominates the Mn–O–Mn exchange interaction which causes a fair increase in the value of saturation magnetization at room temperature. It is believed that the simultaneous doping of Ni and Mn onto ZnO increases the carrier concentration in form of clusters of oxygen vacancies and hence favors the carrier mediated exchange interaction for the enhancement of magnetic moments in the system.

Giant magnetic moment enhancement of nickel nanoparticles embedded in multiwalled carbon nanotubes

We report a giant magnetic moment enhancement of ferromagnetic nickel nanoparticles (11 nm) embedded in multiwalled carbon nanotubes (MWCNTs). High-energy synchrotron x-ray diffraction experiment and chemical analysis are used to accurately determine the ferromagnetic nickel concentration. Magnetic measurements show that the room-temperature saturation magnetization of the nickel nanoparticles embedded in the MWCNTs is enhanced by a factor of about $3.4\ifmmode\pm\else\textpm\fi{}1.0$ as compared with what they would be expected to have for free nanoparticles. The giant moment enhancement is unlikely to be explained by a magnetic proximity effect but possibly arise from the interplay between ferromagnetism in nickel nanoparticles and strong diamagnetism in multiwalled carbon nanotubes.

Electronic and Magnetic Structures in O/Cr(001) Surface from Angle-Resolved Photoemission Spectroscopy

We measured the angle-resolved photoemission spectra of Cr(001) surface with and without oxygen adsorption to study the surface and interfacial electronic structures in relation to its magnetism. The experimental results show that the surface states and Cr 3 d –O 2 p interfacial state are found near the Fermi level, and they are responsible to the surface ferromagnetism of Cr(001). The energy band dispersion of this interfacial state indicates the odd symmetry with respect to the mirror plane (010), and originates from the 3 d x y –2 p y antibonding state. It was found that the fully occupied antibonding state leading to the negative surface relaxation results in the enhancement of magnetic moments in O/Cr(001). This magneto-volume effect also realized that the energy band width of O/Cr(001) is at most 50% less than that of clean Cr(001) surface.

Experimental and theoretical analysis of magnetic moment enhancement in oxygen-deficient EuO

We have carried out both experimental and theoretical studies of the magnetic properties of thin films of oxygen-deficient EuO. Accurate control of the oxygen vacancy concentration in these films was achieved by sputter codeposition of Eu and ${\text{Eu}}_{2}{\text{O}}_{3}$. The films were characterized by superconducting quantum interference device, x-ray reflectometry and polarized neutron reflectometry and the magnetic moment was found to increase monotonically with oxygen vacancy concentration. The electronic structure of ${\text{EuO}}_{1\ensuremath{-}x}$ was calculated using density-functional theory $(\text{DFT}+U)$. In agreement with previous studies, these calculations show that oxygen vacancies act as $n$-type dopants in EuO and that the excess electrons preferentially populate the majority spin branch of the spin-polarized conduction band. The observed increase in the magnetic moment originating from these excess electrons was accurately determined experimentally and found to be in good agreement with our quantitative predictions.

First-principles study of structural, electronic and magnetic properties of Co 13−nM n ( n = 1, 2, M = Mn, V and Al) clusters

The geometries, stabilities, electronic and magnetic properties of Co 13− n M n ( n = 1, 2, M = Mn, V and Al) clusters have been systematically studied by means of a density functional theory with generalized gradient approximation. The calculated results show that the most stable structures of Co 12M and Co 11M 2 (M = Mn, V and Al) clusters all prefer icosahedral configurations with high symmetry, in which the Mn and Al atoms prefer to occupy surface positions of the icosahedron, while the V atom favors the central position. In these clusters, the Al atoms have nearly zero local magnetic moments, while Mn and V atoms are coupled with the remaining Co atoms ferromagnetically and antiferromagnetically, respectively. These several kinds of different magnetic behaviors result in the total magnetic moment enhancement with doped Mn and reduction with doped V/Al as compared to the optimal icosahedral structure of Co 13 cluster, in good agreement with recent Stern–Gerlach experiment results [26,31]. Analysis based on the binding energy and density of state has been made to clarify different magnetic behaviors of the Co 12M and Co 11M 2 (M = Mn, V and Al) clusters. In addition, our calculations suggest that in the Stern–Gerlach molecular beam the Co 12M and Co 11M 2 (M = Mn, V and Al) alloy clusters mainly exist as icosahedral structures.

Symmetry‐related motional enhancement of exciton magnetic moment

AbstractA dramatic motional enhancement of heavy‐ and light‐hole exciton magnetic moment in zinc‐blende semiconductors under a magnetic field applied parallel to the [001] direction has been put into evidence when the exciton moves along the same direction [J. J. Davies et al., Phys. Rev. Lett. 97, 187403 (2006)]. The authors of the paper assigned the effect to a mixing between the 1S and 2P exciton states arising from the cubic term in the Luttinger Hamiltonian expansion in momentum. Such exciton states do not take into account the exact crystal structure since they are just eigenstates of the angular momentum. In addition, the Luttinger Hamiltonian does not take into account the full magnetic‐field effect since it does not include the gauge transformations under the symmetry operations of the structure under the field. By determining the exact symmetry of the exciton states, it is shown here that, under a field parallel to the [001] direction, the Zeeman Splitting value vanishes at the Γ point and, due to accidental quasi‐degeneracy in energy between dark and bright exciton states, becomes finite when the exciton moves parallel to the field. A perturbation model allows fitting experimental data and explains the exciton magnetic‐moment enhancement with kinetic energy. On the contrary, under a field parallel to the [110] direction with the exciton moving parallel to the field, no accidental degeneracy probably takes place between exciton states. As a consequence, the concept of Zeeman Splitting is not relevant since no energy level is degenerate. In addition, a possible quasi‐degeneracy between the excitons recombining with the σ+ and σ− polarizations, respectively, would not allow coupling their two states, hence would not change notably the experimental results.

Enhanced ferromagnetism in nano-sized Zn 0.95Mn 0.05O grains

The present work reports ferromagnetism by doping magnetic Mn atoms in the diamagnetic ZnO matrix and the ferromagnetism has been extended up to 640 K in nano-grained Zn 0.95Mn 0.05O samples. The bulk and nano-grained samples were stabilized in hexagonal crystal structure with space group p63mc. The grain size and lattice strain of the samples were estimated from room temperature XRD spectrum. Surface morphology of the samples was examined at room temperature using SEM picture and EDX spectrum. The ferromagnetism of the bulk material shows enhancement in nano-grained samples, which was mainly due to the solution of Mn atoms into the lattice sites of ZnO by mechanical milling. The enhancement of magnetic moment and ferromagnetic ordering temperature with reduction in grain size has been understood in terms of the core–shell structure and existing theoretical models. The present work also demonstrated the role of surface spin disorder on the enhancement of ferromagnetism in Zn 0.95Mn 0.05O nanograins.

Oxidization states of metal atoms in linear bimetallic multi-sandwich molecules Vn(FeCp2)(n+1) and magnetic moment enhancement mechanism of its 1D wire

Using density functional theory calculations, we demonstrate that one-dimensional bimetallic molecular ferromagnets (FeCpMCp)(infinity) (M = Sc, Ti, V, Cr and Mn, Cp = cyclopentadienyl) exhibit significant enhancement in local and global atomic magnetic moments as well as relatively large spin-polarization energy as compared to their monometallic counterparts. These yield an unusual charge configuration for one of the wires: (Fe(0)Cp(-1)V(+2)Cp(-1))(infinity). Hückel's rule and double-exchange interaction model are used to illustrate the details of local charge transfer and long-range ferromagnetic order. We then propose a growth mechanism for V(n)(FeCp(2))(n+1) (n = 1-4) clusters, which is supported unambiguously with time-of-flight mass spectroscopy data.

Surface structures of [formula omitted] Heusler alloys: A first-principles study

We have studied the stable surface structures of the full-Heusler alloys Co 2 FeAl 0.5 Si 0.5 in the [ 0 0 1 ] direction by first-principles calculations. Only the pure Al-terminated surface is found to be the most stable surface in the whole allowed chemical potential range of the Co 2 FeAl 0.5 Si 0.5 bulk. The bulk truncated FeAlSi and pure Co surface structures are unstable. For the eight calculated surface terminations, the missing neighbor effects in surfaces result in: (1) the oscillatory relaxation behaviors of the surfaces, (2) the enhancement of magnetic moments of surface Co, Fe, and Si atoms, (3) the reduction of the hybridization of 3d-orbitals for surface transition metals or the weakening of the bonding–antibonding splitting. Surface states appear at the Fermi level and the half-metallic properties are lost at surfaces for the eight surface terminations. For the pure Al-terminated surface, the surface and subsurface layers keep spin polarizations of 98% and 17% at the Fermi level, respectively.

Calculated magnetic and transport properties of Fe/Nacl/Fe (0 0 1) magnetic tunnel junction

The electronic structure and magnetic properties of Fe/NaCl/Fe (0 0 1) magnetic tunnel junction have been studied by means of a first-principles Green's function technique based on the tight-binding linear muffin-tin orbital method in the atomic sphere approximation. The results show the formation of sharp Fe/NaCl interfaces and the enhancement of the interface iron magnetic moment ( M Fe=2.96 μ B). The spin-dependent transport properties in current-perpendicular-to-plane configuration were calculated by means of the transmission matrix formulation of Kubo–Landauer formalism. The main contribution to the conductance in the ferromagnetic state is given by minority-spin electrons. The tunneling magnetoresistance rapidly increases with increasing NaCl barrier width.

The Electronic and Magnetic Properties of a Few Transition-Metal Clusters

Using density functional theory we present a systematic study of the electronic and magnetic properties of various nickel clusters and two small bimetallic clusters, Ni n Co m and Ni n Fe m (n + m ≤ 6). A detail study of binding energy, magnetic moment and stability function of pure nickel clusters of nuclearity (N) 40–60 have been performed. We observe that the magic numbers occur at N = 43, 46, 49, 53, 55, and 58, which correspond to the most stable clusters. We find that, with increase in substitution of Co and Fe atoms in Ni cluster, while Ni n Co m becomes more stable, the Ni n Fe m clusters become less stable. The significant enhancement of average magnetic moment and suppression of local magnetic moment of nickel atoms are found in both clusters with increase in Co and Fe concentration.