Moments Of Co Atoms Research Articles

Composition and microstructure engineering of Fe–Si–Co soft magnetic alloys with enhanced performance

The growing demand for high-efficiency and low-loss energy conversion and transportation techniques urges the development of advanced Fe–Si based soft magnet alloys. Simultaneous achievement of low coercivity (Hc) and large saturation magnetization (Ms) however, remains challenging. In this study, soft magnetic alloys with the composition Fe82–xSi18Cox (x = 0 at.%, 4 at.%, 8 at.%, 12 at.%, 16 at.%, and 20 at.%) have been designed followed by microstructural tuning. The Co incorporation results in initially decreased Hc followed by increment due to reduced magnetocrystalline anisotropy and increased saturation magnetostriction from negative to positive values of the alloys. Meanwhile, the Ms raises with subsequent reduction, which origins from competitive mechanisms of increased average moment of Fe atoms and decreased average moment of Co atoms according to first principles calculations. Microstructural evolution during annealing of the Fe70Si18Co12 with synergistically optimized Hc and Ms has been revealed that after elevated-temperature annealing, the DO3 phase is predominately transformed from the B2 phase accompanied by an increase in the degree of ordering. The growth of the DO3 phase deteriorates the Hc due to the aggravating pinning effect on the domain wall movement, which arises from the inhomogeneous magnetization distribution caused by increasing antiphase boundaries.

Fe-doping modulated critical behavior and magnetocaloric effect of magnetic Weyl Co3Sn2S2

${\mathrm{Co}}_{2.5}{\mathrm{Fe}}_{0.5}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ single crystal was grown by a flux method, and its magnetocaloric effect and critical behavior were systematically investigated. Significant changes in magnetic properties and critical behavior were observed in Fe-doped ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$, compared with undoped ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$. Specifically, the magnetization of ${\mathrm{Co}}_{2.5}{\mathrm{Fe}}_{0.5}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ measured at a low magnetic field ($H<500$ Oe, $H\ensuremath{\parallel}c$ axis) does not show the thermal hysteresis phenomenon, whereas it was observed in ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ near ${T}_{\mathrm{c}}$, which is characteristic of a typical second-order phase transition. Previous reports usually attribute the suppression of ferromagnetism by the substitution of Fe for Co to the decrease in the number of $3d$ electrons. Herein, according to the first-principles calculation, the magnetic moments of dopant Fe align antiparallelly to those of Co atoms, which is also the possible reason for the decrease in magnetization. The critical behavior analysis shows that the critical exponents of the Fe-doped sample around magnetic transition are very close to theoretical values of the mean-field model. Furthermore, the exchange interaction distance $J(r)\ensuremath{\approx}{r}^{\ensuremath{-}4.65}$ and the Rhodes-Wolfarth ratio (RWR) (RWR > 1) prove the long-range itinerant ferromagnetism. The hybridization between the $3d$ orbitals of Fe/Co and the orbitals of $\mathrm{Sn}\text{\ensuremath{-}}5p/\mathrm{S}\text{\ensuremath{-}}3p$ may be an important factor for long-range itinerant ferromagnetism. The zero-field cooling magnetization ($H=100$ Oe, $H\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}ab$ plane) undergoes the spin reorientation around 76 K, maybe because the magnetic moments of Fe, aligning antiparallelly to moments of Co atoms, reorient from out of plane to in plane. The critical behavior analysis shows that the first-order phase transition may occur below ${T}_{\mathrm{c}}$ in the case of the $H\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}ab$ plane.

Isolated Co Atoms Anchored on Defective Nitrogen‐doped Carbon Graphene as Efficient Oxygen Reduction Reaction Electrocatalysts

Oxygen reduction reaction (ORR) is the heart of many new energy conversions and storage devices, such as metal‐air batteries and fuel cells. However, ORR is currently facing the dilemma of sluggish intrinsic kinetics and the noble electrocatalysts of high price and low reserves. In this work, isolated Co atoms anchored on defective nitrogen‐doped carbon graphene single‐atom catalyst (Co‐SAC/NC) are synthesized via the proposed movable type printing method. The prepared Co‐SAC/NC catalyst demonstrates admirable ORR performance, with a high half‐wave potential of 0.884 V in alkaline electrolytes and outstanding durability. In addition, an assembled zinc–air battery with prepared Co‐SAC/NC as air‐cathode catalyst displays a high‐peak power density of 179 mW cm−2 and a high‐specific capacity (757 mAh g−1). Density functional theory calculations confirm that the true active sites of the prepared catalyst are Co‐N4 moieties, and further reveal a significantly electronic structure evolution of Co sites in the ORR process, in which the project density of states and local magnetic moment of Co atom varies during its whole reaction process. This work not only paves a new avenue for synthesizing SACs as robust electrocatalysts, but also provides an electronic‐level insight into the evolution of the electronic structure of single‐atom catalysts.

Effect of Fe substitution with Co on the magnetic properties of Fe80P13C7 amorphous alloys: An abinitio molecular dynamics study

In this study, we analyze the effect of Fe substitution with Co on the magnetic properties of Fe80P13C7 amorphous alloy using experimental measurements and first-principle molecular dynamic calculations of the distribution functions, Voronoi polyhedra, and density of states of Fe80-xCoxP13C7 (x = 0, 5, 10, 15 at.%) alloys. The experimental results demonstrate that the saturation magnetization (Ms) of Fe80-xCoxP13C7 (x = 0, 5, 10, 15 at.%) increases and then decreases with increasing Co content, and that maximum Ms is achieved when the composition of Co is 5 at.%. This trend is consistent with the one determined using simulations. Compared to the other amorphous alloys investigated herein, Fe75Co5P13C7 exhibits the highest local symmetry, which explains why it has the largest exchange splitting energy and largest Ms. In addition, increasing substitution of Fe atoms with Co does not appreciably affect the magnetic moment of Co atoms, but it increases the magnetic moment of Fe atoms, particularly those adjacent to Co. This indicates that Co atoms can induce an increase in the magnetic moment of the Fe atoms surrounding them.

Half-metallic ferromagnetism in Ga1-xCoxP (x = 0.125, 0.25, 0.75, and 1) alloys: An ab-initio study

III-V compounds belong to one of the most used materials in the technological applications. The doping of transition metals can induce magnetic properties in III-V for the use in spintronics applications. Effect of Co doping with different concentrations (0.125, 0.25, 0.75, and 1) on the physical properties of GaP is investigated using the spin dependent density functional theory. The equilibrium structural parameters such as the lattice constant (a0), the bulk modulus (B0) and the first pressure-derivative of the bulk modulus (B’) are optimized for all these alloys. The calculations of the electronic properties for the Ga1-xCoxP alloys at their equilibrium lattice parameters disclose that all Ga0.875Co0.125P, Ga0.75Co0.25P, Ga0.25Co0.75P, and CoP alloys are complete half-metallic materials. The magnetic properties show that the total magnetic moment for all Ga0.875Co0.125P, Ga0.75Co0.25P, Ga0.25Co0.75P, and CoP alloys has integer value. The p-d hybridization reduces the atomic magnetic moment of Co atom from its free space charge value of 2μB and creates weak local magnetic moments on the nonmagnetic Ga and P sites. Furthermore, the s(p)-d exchange coupling constants N0α and N0β constants are computed to understand role of exchange splitting process in conduction and valence bands. The high spin polarization and originating the ferromagnetism in GaP by the doping of Co can be valuable systems for the use in spin based applications in various electronic devices.

Effects of swift heavy ion-irradiation on magnetic properties of Co-doped TiO2

Ferromagnetic Co-doped TiO2 thin films at room temperature have been synthesized by RF sputtering and ion-irradiated with 100 MeV Au ions. The surface roughness of the film increases with increasing Au-ion fluence. Kerr microscopy measurements at room temperature give isotropic hysteresis loop with negligible coercivity, which gets modified into in-plane uniaxial magnetic anisotropy after anion fluence of 5×1011 ions/cm2. Further ion-irradiation reduced the coercivity of the film. X-ray magnetic circular dichroism (XMCD) measurements have been performed in total electron yield mode, to investigate the effects of ion-irradiation on magnetic properties of the Co-doped TiO2 thin films. From the XMCD analysis, we observed that the total magnetic moment of Co atoms in case of as-deposited film is 1.771±0.016μB. After the ion-irradiation it became 0.3818±0.014μB at room temperature.

Investigations of the Co-Pt alloy phase diagram with neutron diffuse scattering, inverse cluster variation method, and Monte Carlo simulations

The short-range order in a CoPt alloy was determined at 1203 and 1423 K using neutron diffuse scattering measurements. The effective pair interactions provided by data analysis reproduce well the experimental order-disorder transition temperature in Monte Carlo simulations. They complete previous results reported for the Co-Pt system and are compared to those obtained within tight-binding and ab initio formalisms. Our results show that the important dependence of the nearest-neighbor pair interactions with composition is not related to the sample magnetic state at the measured temperatures. Interactions measured in the paramagnetic domain for the CoPt alloy behave like those in the ferromagnetic domain for the ${\mathrm{Co}}_{3}\mathrm{Pt}$ and ${\mathrm{Co}}_{0.65}{\mathrm{Pt}}_{0.35}$ alloys. The effective pair interactions related to the tight-binding Ising model provide a relatively good description of the CoPt alloy thermodynamics close to the ordering temperature (short-range order and temperature of phase transformation), even if they strongly differ from those measured in this study. The average magnetic moment of Co atoms at high temperatures was determined from the analysis of the intensity contribution that is not dependent on the scattering vector. The obtained value is very close to the moment measured at room temperature or determined from ab initio calculations. This confirms the Curie-Weiss behavior of the CoPt alloy. Finally, transmission electron microscope observations carried out on samples annealed for about 30 days confirmed that the order-disorder transition takes place in the 830--843 K temperature interval at the ${\mathrm{Co}}_{3}\mathrm{Pt}$ composition.

Element-specific spin and orbital moments and perpendicular magnetic anisotropy in Ta/CoFeB/MgO structures

Perpendicular magnetic anisotropy (PMA) in the Ta/CoFeB/MgO system has been studied using x-ray magnetic circular dichroism and vibrating sample magnetometry. The ratios of the orbital to spin magnetic moments of Co atoms in the Ta/CoFeB/MgO structures with PMA have been found to be enhanced by 100%, compared with the Ta/CoFeB/Ta structure without PMA. The orbital moments of Co are as large as 0.30μB, more than half of their spin moments in the perpendicularly magnetized Ta/CoFeB/MgO structures. The results indicate that the PMA observed in the CoFeB/MgO structures is related to the increased spin–orbital coupling of the Co atoms. This work offers experimental evidence of the correlation between PMA and the element-specific spin and orbital moments in the Ta/CoFeB/MgO systems.

Probing the structural, bonding, electronic and magnetic properties of transition–metal borazine systems: Co m (borazine) n (m = 1, 2; n = 1–3)

ABSTRACT The geometrical structures of neutral and anionic Co m (borazine) n (m = 1, 2; n = 1–3) complexes have been determined by using density functional theory. The results indicate that most of the ground state structures for the complexes are similar to those of Co m (benzene) n , which might because borazine is isoelectronic and isostructural to benzene. The frontier molecular orbitals (FMOs) analyses show that their FMOs mainly arise from the 3d/4s electrons of cobalt atoms and the weak π-cloud of borazine molecule. Furthermore, the magnetic moments of complexes were studied and the results revealed that the Co atoms carry most of the magnetic moments. Comparing with the magnetic moment of a free Co atom (3.0μB), the magnetic moments of Co atom in most of Co m (borazine) n 0/- complexes are significantly reduced and even quenched except that the Co(borazine) remains unchanged. More importantly, there is a transition FM-to-AFM between neutral and anionic Co2(borazine)2. Finally, natural population analyses were performed to insightfully explore the reliable electronic structure properties.

First principles study of the electronic and magnetic properties of Zn1-xCoxSe alloys

The spin-polarized density functional theory as implemented within self-consistent plane wave pseudopotential method is used to investigate the electronic and magnetic properties of ternary alloys Zn1−xCoxSe (for x = 0.125, 0.25 and 0.375) in zinc blende structure. The spin polarised energy band structures reveals that the Zn1-xCoxSe is semiconducting ferromagnet for x = 0.125 and 0.25, it becomes HM ferromagnet for x = 0.375. The band gaps for x = 0.125 and 0.25 configurations are 1.09 and 1.07 eV for spin up channel and 0.65 and 0.39 for spin down channel respectively. The band gaps for x = 0.375 configuration is 0.18 eV for spin up channel. The magnetic moments for x = 0.125 and 0.25 configurations are 3.09 and 3.0 μB and for x = 0.375 configuration is 2.97 μB. The p-d hybridization reduces the local magnetic moment of Co atom from its free space charge value and induced the small atomic magnetic moment on Zn and Se atoms.

Stability, electronic and magnetic properties of small M-doped rhodium clusters

The geometries, relative stabilities, electronic and magnetic properties of small RhnM (M = Rh, Al, Co, V) (n ≤ 8) clusters were investigated by employing the density functional theory (DFT). CALYPSO method (structure analysis by a particle swarm optimization algorithm) was used to search the structures. The doping of V atom in Rh clusters can enhance the stability of host clusters due to the strong Rh-V interaction. The magnetic moment of Rh clusters can be enhanced by doping Co atom because of the large local magnetic moment of Co atom and the ferromagnetic coupling of Co and Rh atoms. Magnetic moments of the studied clusters are dominantly contributed by d electrons of metal atoms.

First principle study of weak Dzyaloshinsky-Moriya interaction in Co/BN surface

Based on density functional theory calculations, we elucidate the atomic and electronic structures of Co atom of hexgonal BN (Co/h-BN). The interaction between magnetic moments of Co atoms is realized through Co-N_-B_ grid, which is indicated by the analysis of spin charge contour plot and partial density of states of each atom, where and denote the site of B or N atom close to and away from Co atom, respectively. Then the dispersion relations E(q) and E(-q) (q denotes the direction vector of spin spiral) between energy and wave vector of spin spiral in the opposite directions are calculated with generalized Bloch equations. In the incommensurate spin spiral calculations, all the magnetic moments of Co atom are arranged in the same plane that is perpendicular to the Co/h-BN film. The difference between E(q) and E(-q) is caused by the interface of Co/h-BN, where the symmetry of space perpendicular to the film is broken. Moreover, the effective Heisenberg exchange interaction (HBI) and Dzyaloshinsky-Moriya interaction (DMI) parameters between different neighbors (Ji and di) are derived by well fitting the ab initio magnon dispersion E(q) to HBI with DMI model and E(q)-E(-q) to DMI model, respectively. The J1 has a negative value and plays a major role, J3 is one order of magnitude smaller than J1, and other parameters are close to zero. Hence, Co/h-BN is triangular antiferromagnetic material with the q at k point in the first Brillouin zone. However, the spin spiral with the q at M point is only 2 meV larger than the basic state with the only negative J1 and smaller positive J2. The DMI is not shown in this interface with d1 and d2 close to zero. Based on the non DMI character and its stability in air, h-BN can be capped on other DMI interfaces. The reason that the DMI in Co/h-BN is much smaller than in Co/Gra is much larger height between Co and h-BN. It is 0.192 nm for h-BN but it is 0.156 nm for Co/Gra. We may reduce the height to enhance the DMI by other ways, such as adding electrical and magnetic fields in the future.

Thickness dependence of magnetic properties of thin amorphous ferrimagnetic rare earth–transition metal multilayers

Gd-Co/Ti multilayers were prepared by magnetron sputtering deposition onto glass substrates. The temperature dependence of magnetization of the multilayered structures depends on the Gd-Co layer thickness. It was shown that the obtained experimental results can be satisfactorily described in the framework of the joint application of models of the “size factor” and the “molecular field”. In addition, the possible effect of the electronic structure of titanium on the average magnetic moment of Co atoms, the efficiency of which depends on the thickness of the magnetic layers was also taken into account.

Magnetic and magnetocaloric properties of Gd6(Mn1−xCox)23 compounds (x ≤ 0.3)

The magnetic and magnetocaloric properties of the Th6Mn23-type structure Gd6(Mn1−xCox)23 (x ≤ 0.3) compounds are investigated from DC magnetization measurements. The Curie temperature of these ferrimagnetic materials evolves non-continuously with the Co content from TC = 489 K for x = 0 to TC = 118 K for x = 0.3. In relation with the lower magnetic moment of Co atoms compared with that of Mn atoms, the Co to Mn substitution leads to a decrease of the maximal magnetization, recorded at 5 K in an applied magnetic field of 9 T, from 54.7 μB/f.u. for x = 0 to 36.3 μB/f.u. for x = 0.3. These materials present an anomalous magnetocaloric effect, characterized by two broad –ΔSM maxima of moderate magnitude at TC and around 100 K, yielding to wide magnetocaloric response from TC down to the lowest temperature, and consequently, a relatively large cooling capacity q10–350K (μ0ΔH = 5 T) reaching ∼ 3.7 J cm−3 for Gd6(Mn0.875Co0.125)23. The results are analyzed and discussed in connection with previously published data.

Correlation between valence electronic structure and magnetic properties in RCo5 (R = rare earth) intermetallic compound**Project supported by the National Natural Science Foundation of China (Grant No. 11274110).

The magnetisms of RCo5 (R = rare earth) intermetallics are systematically studied with the empirical electron theory of solids and molecules (EET). The theoretical moments and Curie temperatures agree well with experimental ones. The calculated results show strong correlations between the valence electronic structure and the magnetic properties in RCo5 intermetallic compounds. The moments of RCo5 intermetallics originate mainly from the 3d electrons of Co atoms and 4f electrons of rare earth, and the s electrons also affect the magnetic moments by the hybridization of d and s electrons. It is found that moment of Co atom at 2c site is higher than that at 3g site due to the fact that the bonding effect between R and Co is associated with an electron transformation from 3d electrons into covalence electrons. In the heavy rare-earth-based RCo5 intermetallics, the contribution to magnetic moment originates from the 3d and 4f electrons. The covalence electrons and lattice electrons also affect the Curie temperature, which is proportional to the average moment along the various bonds.

Unexpected magnetic behavior in amorphous Co90Sc10 alloy

An amorphous alloy Co90Sc10 has been prepared by rapid quenching from the melt. The results of magnetization measurements show that this alloy has the highest Curie temperature reported for any amorphous transition metal based alloys. Furthermore, for a Co based amorphous alloy, the magnetic moment is remarkably high. Moreover, the alloy exhibits soft magnetic properties. Based on the findings, amorphous Co90Sc10 appears to be an attractive candidate for applications as a soft magnetic material. The temperature dependence of the reduced magnetization can be described by the Bloch power law. The results show that the B coefficient of the amorphous Co90Sc10 alloy, which is a measure of the rigidity of spin waves, exhibits the lowest value observed until now for any amorphous alloy and is comparable to crystalline alloys. It is found that the Sc atoms in the Co90Sc10 alloy lead to an increase of the itinerant spin moment of Co atoms, and, in contrast to this behaviour, to a decrease of the local 3d-electrons of Co.

Oxygen Effect on Magnetic Anisotropy Energy of Co Nanowires on Cu(210) Surface - An <i>Ab Initio</i> Study

An ab initio study of Co monatomic nanowires (NW) magnetic properties on pure and oxygen reconstructed Cu (210) surface was performed. Strong influence of oxygen surface impurity on MAE in Co nanowire was found. High value of magnetic moment and small energy of magnetic anisotropy (MAE) were obtained in Co nanowires on pure Cu (210) surface. After oxygen reconstruction of Cu (210) surface (Cu (210)-(2×1)O) the significant enhancement of MAE was found in Co nanowires, while the magnetic moment of Co atoms is practically unchanged. Oxygen on Cu (210) surface rotates easy magnetization axis in Co NW with respect to the pure surface.

First Principles Study of Structural Stability and Magnetic Property of Non-equilibrium Co–Mo Alloys

First principles calculations are carried out to investigate the structural stability of several non-equilibrium intermetallic phases in the cobalt (Co)–Mo system using spin polarized projected augmented-wave potentials. It is revealed that the Co3Mo, CoMo, and CoMo3 alloys are energetically favored to be in D019, B11, and A15 structures, respectively, and that the magnetic moments of Co atoms would decrease rapidly with an increasing percentage of Mo content and would most probably disappear when the content of Mo is no less than 50 at%. Generally, the calculated results in the present work match well with the available experimental observations.

Geometrical, electronic, and magnetic properties of small ConAu (n = 1–9) clusters

The configurations, stabilities, electronic, and magnetic properties of ConAu (n = 1–9) clusters have been systematically investigated by using relativistic all-electron density functional theory with generalized gradient approximation. The growth way for ConAu (n = 1, 3–7) clusters is Au-substituted Con+1 clusters. And the growth way for ConAu (n = 2, 8, 9) clusters is that the Au atom occupies a peripheral position of Con clusters. The fragmentation energy and the second-order difference energy of the ground-state ConAu clusters show a pronounced odd-even oscillation with the number of Co atoms, and the clusters exhibit higher stability at n = 5. Compared with corresponding pure Con+1 clusters, the total magnetic moment is reduced by 2μB for most of ConAu clusters except n = 3 and 8. The magnetic moment contribution of Au atom is very small, and the doping-Au atom almost has no effect on the average magnetic moment of Co atom which still keeps about 2μB of magnetism of pure Con+1 clusters, resulting in the decreasing magnetism of the doping-Au cobalt clusters.

Co/Ni(111) superlattices studied by microscopy, x-ray absorption, andab initiocalculations

We explore the origins of perpendicular magnetic anisotropy in epitaxial and textured Co/Ni(111) superlattices using a combination of thin-film growth, structural characterization, x-ray magnetic circular dichroism (XMCD), and ab initio calculations. Transmission electron microscopy and x-ray diffraction experiments allow us to show that the bulk magnetoelastic contribution to the total magnetic energy is small compared to the interface anisotropy. The magnetic properties are studied by using XMCD at the Co and Ni L 2,3 edges. Hysteresis loops performed at the Co L 3 edge confirm the perpendicular magnetization for Co thicknesses up to four monolayers. The spin and orbital moments were deduced using the XMCD sum rules. The results are explained by considering two kinds of magnetic moments for Co, distinguishing the interfaces from the rest of the layers. Both effective spin and orbital moments of Co atoms are found to be enhanced at the Co-Ni interfaces, whereas the magnetic moment of Co surrounded by Co is similar to the bulk value. Ab initio calculations allow us to show a strong enhancement of the dipole operator contribution on Co atoms at the interface that is partly responsible for this high effective spin moment at the interface. Such a moment enhancement is not observed for Ni, with the dipole operator contribution being close to zero. Finally, we observed a very surprising proportionality between the effective spin and orbital moments, independent of the absorption edge or deposition technique used. We assign this peculiar behavior to the fact that the magnetic dipole operator involved in the sum rules is closely linked to the increase of the Co orbital moment at the interface. Based on XMCD results obtained on both molecular beam epitaxy and sputter-deposited samples, this link allows us to show the extreme sensitivity of the perpendicular anisotropy with the chemical ordering at the interface.