Magnetic Moment Of Co Research Articles

Observation of an electric field-induced interface redox reaction and magnetic modification in GdOx/Co thin film by means of depth-resolved X-ray absorption spectroscopy.

We study an electric field-induced redox reaction at the interface of GdOx/Co thin film, by means of soft X-ray absorption spectroscopy (XAS). The fluorescence-yield depth-resolved XAS analysis reveals that the interfacial Co layer at GdOx/Co is composed of ∼50% Co oxide when the negative field is applied, while metallic Co is dominant in the case of the positive field. We suppose that the interfacial layer is oxidized by oxygen migration from the GdOx layer with the negative field while the interfacial layer shows the metallic state with the positive field, which means that the redox reaction is induced by the electric field. In addition, it is found from the X-ray magnetic circular dichroism (XMCD) measurement that the orbital magnetic moment of Co is larger when the negative electric field is applied to the film, as compared to the positive field. Moreover, the depth-resolved XMCD analysis reveals that the interfacial Co layer shows no or little magnetization regardless of the electric field, while for the inner layer, an increase of the orbital magnetic moment is suggested with the negative field. The field-induced magnetic modification could be attributed to the change of the orbital moment in the inner Co layer due to interfacial modifications. We thus succeed in the direct observation of the redox reaction-induced change in the interface magnetic state.

Contribution of on-site Coulomb repulsion energy to structural, electronic and magnetic properties of SrCoO3 for different space groups: first-principles study

Abstract We report structural, electronic, and magnetic properties of SrCoO3 in Pm3̅m and P4/mbm space groups, which are calculated by using generalized gradient approximation corrected with on-site Coulomb repulsion U and exchange energies J. The cubic lattice parameter a and local magnetic moments of Co (μCo) are optimized by varying U at Co 3d site. Employing ultrasoft pseudopotential, the values of U = 8 eV and J = 0.75 eV are the best choice for Pm3̅m space group. We found the value of μCo = 2.56 μB, which is consistent with the previous results. It was also found that Co 3d, hybridized with O 2p, is the main contributor to ferromagnetic metallic properties. Besides, norm-conserving pseudopotential promotes a, which is in good agreement with experimental result. However, it is not suitable for P4/mbm space group. By using ultrasoft pseudopotential, the value of U = 3 eV (J = 0.75) is the most suitable for P4/mbm group. Ferromagnetic metallic properties, Jahn-Teller distortion, and reasonable lattice parameters have been obtained. This study shows that U has significant contribution to the calculated properties and also points out that P4/mbm space group with US-PP is suitable to describe experimental results.

Structural and magnetic properties of epitaxial thin films of the equiatomic quaternary CoFeMnSi Heusler alloy

We report the structural and magnetic properties of CoFeMnSi equiatomic quaternary Heusler alloy thin films. The epitaxial growth of the single-crystalline films with full $B2$ and partial $L{2}_{1}$ ordering on the Cr-buffered MgO(001) substrate was achieved using the in situ postannealing at temperature (${T}_{\mathrm{a}}$) of $500\ensuremath{-}600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. A saturation magnetization value of about $3.5\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ (where f.u. represents formula unit) was obtained at room temperature for the sample with ${T}_{\mathrm{a}}=600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, which is very close to the value of $3.7\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ reported previously for a bulk sample. Ferromagnetic resonance unveiled that negligible extrinsic relaxation due to magnetic softness as well as small intrinsic spin relaxation, i.e., the Gilbert damping constant of 0.005, which is smaller than that of permalloy. The spin magnetic moments for Co, Fe, and Mn atoms for the sample with ${T}_{\mathrm{a}}=600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ deduced from the x-ray magnetic circular dichroism were in agreement with those previously reported in the bulk sample, the effects of Fe-Mn swapping and Mn antisite disorder were discussed.

Evolution of non-collinear magnetic state of exchange biased ferromagnet/normal metal/ferromagnet/superconductor heterostructure in magnetic field studied by polarized neutron reflectometry

By using waveguide enhanced polarized neutron reflectometry we have characterized the magnetic state of exchange biased CoOx(20 nm)/Co(4 nm)/Nb(5 nm)/Co(2 nm)/Nb(25 nm)/Al2O3 system. Measurement allowed to determine the dependence of the inclination angles of magnetic moment of the both Co layers as a function of applied field. According to the measurement the soft Co(2 nm) layer magnetization turns towards external field in magnetic fields as small as 20 Oe. In contrast direction of magnetic moment of Co(4 nm) layer cannot be altered in magnetic fields as high as 2 kOe.

Half-Metallic Ferrimagnetic Characteristics of Co2YZ (Z = P, As, Sb, and Bi) New Full-Heusler Alloys: a DFT Study

The full-Heusler alloys Co2YZ (Z = P, As, Sb, and Bi) were investigated by using the state-of-the-art full-potential linearized augmented plane wave plus local orbital (FP-LAPW + lo) method within the framework of density functional theory (DFT), to understand the structural, electronic, and magnetic properties of these new Heusler alloys. The generalized gradient approximation (GGA) was chosen to define the exchange-correlation potential for studying the structural properties, whereas electronic and magnetic properties were developed by using the GGA + U scheme, where U is the Hubbard term, which explains the Coulomb repulsion and was applied to describe the “d” electrons. The structural properties indicated that the ferromagnetic states of these compounds are energetically more favorable than the paramagnetic states. Through results on the electronic structures (band structures and densities of states), it is found that all these alloys exhibit a complete half-metallic nature, with half-metallic gaps (EHM) of 0.632, 0.605, 1.223, and 0.922 eV for Co2YP, Co2YAs, Co2YSb, and Co2YBi alloys, respectively. The magnetic properties indicated that the total spin magnetic moments of these Heusler alloys obey the Slater-Pauling rule around value 2 µ B approximately. The local magnetic moments of Co and Y transition elements have opposite signs with different intensities, thus revealing the ferromagnetic character. Our study of both electronic and magnetic properties demonstrates in detail that Co2YZ (Z = P, As, Sb, and Bi) full-Heusler alloys are complete half-metallic ferrimagnetic materials.

Structure and magnetization of [formula omitted] thin film

In this work, we studied the local structure and the magnetization of Co4N thin films deposited by a reactive dc magnetron sputtering process. The interstitial incorporation of N atoms in a fcc Co lattice is expected to expand the structure. This expansion yields interesting magnetic properties e.g. a larger magnetic moment (than Co) and a very high value of spin polarization ratio in Co4N. By optimizing the growth conditions, we prepared Co4N film having lattice parameter close to its theoretically predicted value. The N concentration was measured using secondary ion mass spectroscopy. Detailed magnetization measurements using bulk magnetization method and polarized neutron reflectivity confirm that the magnetic moment of Co in Co4N is higher than that of Co.

Synthesis and study of electronic state of Sr2CrO2Co2As2 with CoAs conduction layers

We successfully synthesized a new member of compounds with the CoAs layer, Sr2CrO2Co2As2, and its partially substituted systems Sr2CrO2(TmxCo1−x)2As2 (Tm = Fe, Ni), and measured magnetization and electric resistivity of these polycrystalline compounds. As a result of magnetic measurement for Sr2CrO2Co2As2, magnetic moments of Co do not construct an itinerant electronic ferromagnetism unlike other compounds with the CoPn (Pn=P and As) layers. Both Sr2CrO2(TmxCo1−x)2As2 with Tm = Fe and Ni also do not show an itinerant electronic ferromagnetism down to 2 K. For each solid solution of Sr2CrO2(FexCo1−x)2As2 with x > 0.0, ρ weakly increases with the decrease of T at low temperature region, indicating that the mixed occupancy of Cr and Fe within the conducting layers occurs in Sr2CrO2(FexCo1−x)2As2. We conclude that the absence of ferromagnetism in Sr2CrO2Co2As2 is due to the self-electron-doping from Cr to the conduction bands and the attempt to recover the ferromagnetism by the hole-doping effect is prevented by the mixed occupancy of Cr and Fe in Sr2CrO2 (FexCo1−x)2As2 with x > 0.0. The result of our structural analysis supports that the disappearance of itinerant electronic ferromagnetism in Sr2CrO2Co2As2 is due to the self-electron-doping from Cr.

Theoretical investigation of the structural, electronic, magnetic and elastic properties of binary cubic C15-Laves phases TbX2 (X = Co and Fe)

We report on a comprehensive theoretical investigation of the physical properties of the cubic MgCu2-type binary Laves phases TbCo2 and TbFe2 compounds. The density functional full-potential linearized augmented planewave (FP-LAPW) method was used. We adopted the generalized gradient approximation (GGA) to estimate the exchange correlation potential and the GGA+U (i.e. Hubbard correction) calculations in accurately characterizing the correlation effects. The lattice parameter a0, bulk modulus B and magnetic moment M at the equilibrium state were found to well corroborate the experimental data. We have calculated the magnetic moments of Co and Fe in the TbCo2 and TbFe2, respectively, by using GGA and GGA+U methods, where the magnetic moments value of Fe was found to be higher than that of Co. The GGA+U gave higher value than that obtained by GGA. To obtain further insight into the type of states associated with each orbital, the projected density of states of the Co-3d and Fe-3d orbitals were calculated using GGA and GGA+U, respectively. This work highlights the role of the correlated electrons processing for an accurate description of these binary Laves phases compounds.

Tailoring the interfacial exchange coupling of perpendicularly magnetized Co/L10-Mn1.5Ga bilayers

We have studied the magnetic properties of Co (2–12 MLs)/L10-Mn1.5Ga (15 nm) bilayers without and with annealing at 300 °C by a combination of superconducting quantum interference device (SQUID) magnetometry and x-ray magnetic circular dichroism (XMCD). We find that the Co layer can remain perpendicularly magnetized when its thickness is less than six monolayers due to the coupling between Co and L10-Mn1.5Ga layers, which is doubly confirmed by both SQUID and XMCD measurements. Such an exchange coupling between L10-Mn1.5Ga and Co layers changes from ferromagnetic coupling to antiferromagnetic coupling after the annealing process. Furthermore, the magnetic coupling can also be tailored from ferromagnetic to antiferromagnetic by changing the L10-Mn1.5Ga surface from Mn-rich to Ga-rich. The first-principles calculations show that the interfacial coupling type is ferromagnetic for a Mn-terminated L10-Mn1.5Ga bilayer, while antiferromagnetic for a Ga-terminated bilayer. The spin and orbital moments of Co in the Co/L10-Mn1.5Ga bilayers are calculated according to the sum rules and the ratio of the orbital to spin magnetic moments for Co is not enhanced like other perpendicularly magnetized Co-based multilayers such as Co/Pd and Co/Pt.

Co K-edge magnetic circular dichroism across the spin state transition in LaCoO3 single crystal

We report on Co K-edge x-ray magnetic circular dichroism (XMCD) measurements of LaCoO3 single crystal in temperature range from 5 to 300 K and external magnetic field of 17 T. The response consists of pre-edge (at 7712 eV) and bi-polar peak (up at 7727, down at 7731 eV) with amplitudes, respectively, less than 10-3 and 10-2 of the Co K-edge jump. Using the sum rule the orbital magnetic moment of 4p Co is evaluated. Its temperature dependence reaches a maximum of (2.7 ± 0.9) x10-3 μB at 120 K, following the trend for the total magnetic moment on the Co obtained from the superconducting quantum interference device measurements. However, on warming from 25 to 120 K, the orbital magnetic moment of the 4p Co doubles while total magnetic moment of Co increases 10 times. First principle calculations are in order to relate the Co K-edge XMCD results to the orbital and spin moment of 3d Co.

Preparation and characterisation of Co–Fe–Ni–M-Si–B (M = Zr, Ti) amorphous powders by wet mechanical alloying

Co-based amorphous alloys were prepared via wet mechanical alloying process starting from elemental powders. The reference alloy Co70Fe4Ni2Si15B9 (at. %) as well as the alloys derived from this composition by the substitution of 5 at.% of Zr or Ti for Si or B (Co70Fe4Ni2Si15B4Zr5, Co70Fe4Ni2Si15B4Ti5, Co70Fe4Ni2Si10B9Zr5 and Co70Fe4Ni2Si10B9Ti5) are obtained in amorphous state, according to X-ray diffraction (XRD) investigation, after 40 h of milling. The calculated amount of amorphous fraction reaches 99% after 40 h of milling. The largest increase of the crystallisation temperature was induced by the substitution of Zr or Ti for Si while, regardless of the type of substitution, an important increase of the Curie temperature of the alloy was obtained. A Co-based solid solution, with Co2Si and Co2B phases, result after crystallisation of the amorphous alloys as proved by XRD investigations. Saturation magnetisation of the alloys decreases upon increasing milling time, however it remains larger than the saturation magnetisation of the reference alloy. This was discussed in correlation with the specificity of the wet mechanical alloying process and the influence of the chemical bonding between Co and metalloids atoms over the magnetic moment of Co.

Reentrant spin-glass behavior and bipolar exchange-bias effect in “Sn” substituted cobalt-orthotitanate

We report the co-existence of longitudinal ferrimagnetic behavior with Néel temperature TN ∼ 46.1 K and reentrant transverse spin-glass state at 44.05 K in Tin (Sn) doped cobalt-orthotitanate (Co2TiO4). The ferrimagnetic ordering is resulting from different magnetic moments of Co2+ on the A-sites (3.87 μB) and B-sites (5.069 μB). The magnetic compensation temperature (TCOMP) shifts from 31.74 K to 27.1 K when 40 at. % of “Sn4+” substitutes “Ti4+” at B-sites where the bulk-magnetization of two-sublattices balance each other. For T > TN, the dc-magnetic susceptibility (χdc = M/Hdc) fits well with the Néel's expression for the two-sublattice model with antiferromagnetic molecular field constants NBB ∼ 15.44, NAB ∼ 32.01, and NAA ∼ 20.88. The frequency dependence of ac-magnetic susceptibility χac data follows the Vogel-Fulcher law, and the power-law of critical slowing-down with “zν” = 6.01 suggests the existence of spin-clusters (where “z” and “ν” being dynamic critical-exponent and correlation length of critical-exponent, respectively). This system exhibits unusual hysteresis loops with large bipolar exchange-bias effect (HEB ∼ 13.6 kOe at 7 K) after zero-field cooling process from an un-magnetized state, and a dramatic collapse of remanence (MR) and coercive field (HC) across TCOMP. The possible origins of such anomalous characteristics were discussed.

Naturally Produced Co/CoO Nanocrystalline Magnetic Multilayers: Structure and Inverted Hysteresis.

Cobalt-based multilayers with excellent sequencing are grown via radiofrequency magnetron sputtering with the use of one Co target and natural oxidation. The Co layers are continuous, fully textured {111} and have the face centered cubic structure. At the end of deposition of each Co layer air is let to flow into the vacuum chamber via a fine (leak) valve. The top of Co is oxidized. The oxidized layers consist of cubic CoO crystallites. Near the film surface hexagonal Co(OH)2 is also detected. Magneto-optical Kerr effect hysteresis loops show in-plane magnetized films. The magnetic saturation field in the out-of-plane measurements is large exceeding 12 kOe. This observation supports indirectly the fact that Co is face centered cubic; if it was c-axis textured hexagonal the magnetocrystalline anisotropy would be large resulting in smaller values of the saturation field. As the Co-layer thickness decreases the in-plane loops show reduced remanence, slow approach to magnetic saturation and the out-of-plane loops show inverted hysteresis and/or crossing loop features with sizeable remanence. The effects are discussed with respect to the enhanced orbital magnetic moment of Co and the antiferromagnetic coupling between Co spins at the Co/CoO interface.

First-principles Investigation of Half-metallicity and Ferrimagnet Properties of Co2ScZ (Z = As, Sb, and Bi)

Based on the first-principles methods, the half-metallic ferrimagnet properties of Co2ScZ (Z = As, Sb, and Bi) full-Heusler alloys have been predicted by employing the full-potential linearized plane wave plus local orbital method (FP-LAPW + lo) within the framework of density functional theory (DFT) and generalized gradient approximation plus Coulomb repulsion (GGA + U) parameterization. The GGA scheme is served in this work to obtain only the equilibrium structural parameters in both paramagnetic and ferromagnetic phases, whereas the GGA + U scheme is exploited to treat the “d” electrons through electronic and magnetic properties of these Heusler alloys. The electronic structure investigation shows that all Co2ScAs, Co2ScSb, and Co2ScBi Heusler alloys are stable half metals with half-metallic gap energies (EHM) of 0.351, 1.080, and 0.836 eV, respectively. The total magnetic moment follows the generalized Slater-Pauling rule, where its value is around 2 μB for all these compounds. We have also found opposite signs between atomic magnetic moments of Co and Sc transition elements, demonstrating the ferrimagnetic behavior.

Energetics, diffusion, and magnetic properties of cobalt atom in a monolayer graphene: An ab initio study

We use ab initio methods to study the binding, diffusion, and magnetic properties of cobalt atom embedded in graphene vacancies. We investigate the diffusion of Co-monovacancy (Co-MV) and Co-divacancy (Co-DV) defect complexes, and determine the minimum energy path (MEP), as well as the activation energy barrier of migration. We obtained similar activation energy barriers, of ∼5.8 eV, for Co-MV and Co-DV diffusion, respectively. Our calculations also suggest that, at electron–irradiation energy of 200 keV as used in a related experiment, the maximum energy transfer to the Co atom, of approximately 9.0 eV is sufficiently high to break metal-carbon bonding. The incident electron energy is also high enough to displace graphene's carbon atoms from their lattice positions. The breaking of metal-carbon bonding and the displacement of graphene atoms may act to facilitate the migration of Co. We conclude therefore that the detrapping and diffusion of cobalt as observed experimentally is likely to be radiation-induced, similar to what has been observed for Au and Fe in electron-irradiated graphene. Furthermore, we show that Co migration in graphene is such that its magnetic moment varies along the diffusion path. The magnetic moment of Co is consistently higher in Co-DV diffusion when compared to that of Co-MV diffusion.

Structural, electronic and magnetic properties of Cd1–xTMxS (TM=Co and V) by ab-initio calculations

The structural, electronic and ferromagnetic properties of Cd1−xTMxS (TM=Co and V) compounds at x=0.25, 0.50 and 0.75 in zinc blende (B3) phase, have been investigated using all-electron full-potential linear muffin tin orbital (FP-LMTO) calculations within the frame work of the density functional theory and the generalized gradient approximation. The electronic properties exhibit half-metallic behavior at x=0.25, 0.50, and 0.75 for Cd1–xVxS and x=0.25 and 0.50 for Cd1−xCoxS, while Cd1−xCoxS with x=0.75 is nearly half-metallic. The calculated magnetic moment per substituted transition metal (TM) atom for half-metallic compounds is found to be 3µB, whereas that of a nearly half-metallic compound is 2.29µB. The analysis of band structure and density of states shows that the TM-3d states play a key role in generating spin-polarization and magnetic moment in these compounds. Furthermore, we establish that the p–d hybridization reduces the local magnetic moment of Co and enhances that of V from their free space charge value of 3µB and creates small local magnetic moments on nonmagnetic Cd and S sites. The exchange constant N0α and N0β have been calculated to validate the effects resulting from exchange splitting process.

Modifying Spin Injection Characteristics in the Co/Alq3 System by Using a Molecular Self-Assembled Monolayer

We present the results of experiments that explore the influence of molecular self-assembled monolayers (SAMs) on characteristics of spin injection into an organic semiconductor, Alq3 [tris(8-hydroxyquinoline)aluminum], from a ferromagnetic metal, Co. Two different SAMs, MHA (16-mercaptohexadeconic acid, HS(CH2)15CO2H) and ODT (1-octadecanethiol, CH3(CH2)17SH), are inserted between Alq3 and Co layers, and their effects on electronic structure hybridization and related changes in energy levels and spin-dependent properties at the interface are investigated. X-ray photoelectron measurements of the surface structure for the MHA, ODT, and Alq3 organic layers provide bonding and chemical conformational information. Ultraviolet photoelectron spectroscopy (UPS) measurements reveal that both MHA and ODT treatments lower the work function of Co. X-ray magnetic circular dichroism (XMCD) spectra imply that SAMs reduce the hybridization between Co and Alq3, and furthermore, they enhance the spin magnetic moment of Co.

Electronic and magnetic properties of off-stoichiometric Co2MnβSi/MgO interfaces studied by x-ray magnetic circular dichroism

We have studied the electronic and magnetic states of Co and Mn atoms at the interface of the Co2MnβSi (CMS)/MgO (β = 0.69, 0.99, 1.15, and 1.29) magnetic tunnel junction (MTJ) by means of x-ray magnetic circular dichroism. In particular, the Mn composition (β) dependences of the Mn and Co magnetic moments were investigated. The experimental spin magnetic moments of Mn, mspin(Mn), derived from x-ray magnetic circular dichroism weakly decreased with increasing Mn composition β in going from Mn-deficient to Mn-rich CMS films. This behavior was explained by first-principles calculations based on the antisite-based site-specific formula unit (SSFU) composition model, which assumes the formation of only antisite defect, not vacancies, to accommodate off-stoichiometry. Furthermore, the experimental spin magnetic moments of Co, mspin(Co), also weakly decreased with increasing Mn composition. This behavior was consistently explained by the antisite-based SSFU model, in particular, by the decrease in the concentration of CoMn antisites detrimental to the half-metallicity of CMS with increasing β. This finding is consistent with the higher tunnel magnetoresistance ratios which have been observed for CMS/MgO/CMS MTJs with Mn-rich CMS electrodes.

Restoring the Co magnetic moments at interfacial Co-porphyrin arrays by site-selective uptake of iron.

Magnetochemistry recently emerged as a promising approach to control addressable spin arrays on surfaces. Here we report on the binding, spatial ordering, and magnetic properties of Fe on a highly regular Co-tetraphenylporphyrin (Co-TPP) template and highlight how the Fe controls the magnetism of the Co centers. As evidenced by scanning tunneling microscopy (STM) single Fe atoms attach to the saddle-shape conformers site-selectively in a unique coordination environment offered through a heptamer defined by the Co-N-C-C-C-N cyclic subunit. While the magnetic moment of Co is quenched for bare Co-TPP/Ag(111), the Fe presence revives it. Our X-ray magnetic circular dichroism (XMCD) experiments, complemented by density functional theory (DFT) calculations, evidence a ferromagnetic coupling between the Fe and the Co center concomitant with a complex charge redistribution involving the porphyrin ligand. Thus, we demonstrate an unusual metalloporphyrin coordination geometry that opens pathways to spatially order and engineer magnetic moments in surface-based nanostructures.

Relativistic first-principles study on spin and orbital magnetism of mattagamite (CoTe2)

We have demonstrated electronic and magnetic properties of CoTe2 compound in the framework of relativistic density functional theory using generalized gradient approximation (GGA). The spin and orbital magnetic moments of Co and Te atoms were obtained using full-potential local orbitals band structure scheme. We have found a partially quenched orbital moment of 0.096μB for Co atom in the local spin density approximation. In order to improve the intrinsic deficiency of local density approximation for describing orbital magnetism, an orbital polarization correction was applied to CoTe2 and a significant orbital moment of 0.153μB was found for Co atom in this compound.