Epitaxy and magnetic properties of surfactant-mediated growth of bcc cobalt

High resolution core level photoemission spectroscopy, photoelectron diffraction, and x-ray magnetic circular dicroism (XMCD) have been used to characterize the structural and magnetic properties of bcc-cobalt films grown on GaAs(110) substrates by using Sb as a surfactant. We have unambiguously disentangled the surfactant role played by the Sb which improves the crystallinity and reduces the lattice distortion of the metallic films as well as changes the interdiffusion process at the interface compared to the Co/GaAs(110) system. As a consequence of these combined effects, an improvement on the magnetic response of the grown Co thin films has been observed by XMCD measurements.     

Thin films of molecule-based charge transfer complex cobalt tetracyanoethylene: In situ X-ray photoemission study

Thin films of molecule-based charge transfer magnet, cobalt tetracyanoethylene [Co(TCNE)_x, x ~ 2] consisting of the transition metal Co, and an organic molecule viz. tetracyanoethylene (TCNE) have been deposited by using physical vapor deposition method under ultra-high vacuum conditions at room temperature. X-ray photoelectron spectroscopy (XPS) technique has been used extensively to investigate the electronic properties of the Co(TCNE)_x thin films. The XPS measurements show that the prepared Co(TCNE)_x films are clean, and oxygen free. The stoichiometries of the films, based on atomic sensitive factors, are obtained, and yields a ~ 1:2 ratio between metal Co and TCNE for all films. Interestingly, the positive shift of binding energy position for Co(2p), and negative shifts for C(1s) and N(1s) peaks suggest a charge-transfer from Co to TCNE, and cobalt is assigned to its Co(II) valence state. In the valence band investigation, the highest occupied molecular orbital (HOMO) of Co(TCNE)_x is found to be at ~ 2.4 eV with respect to the Fermi level, and it is derived either from the TCNE^? singly occupied molecular orbital (SOMO) or Co(3d) states. The peaks located at ~ 6.8 eV and ~ 8.8 eV are due to TCNE derived electronic states. The obtained core level and valence band results of Co(TCNE)_x, films are compared with those of V(TCNE)_x thin film magnet: a well known system of M(TCNE)_x type of organic magnet, and important points regarding their electronic properties have been brought out.     

Exchange bias and negative magnetoresistance in [Co/Bi/Co]/IrMn thin films]

The artificial control of grain-boundary resistance and its contribution to magnetic and magneto-transport properties in [Co(3 nm)/Bi(2.5 nm)/Co(3 nm)]Ir₂₀Mn₈₀(12 nm) thin films that exhibit exchange bias is studied. Transverse magnetoresistance (MR) loops exhibit a negative MR in thin films grown by magnetron sputtering on Si/SiN_x(100 nm) substrates. This negative MR effect is of the giant-MR (GMR) type, although its magnitude is less than 1%. A considerable exchange bias (EB) effect is observed only at lower temperatures, where both, GMR and isothermal magnetization loops exhibit a shift of −600 Oe at 5 K.     

Co-doped In2O3 thin films: Room temperature ferromagnets

Laser-ablated Co-doped In₂O₃ thin films were fabricated under various growth conditions on R-cut Al₂O₃ and MgO substrates. All Co:In₂O₃ films are well-crystallized, single phase, and room temperature ferromagnetic. Co atoms were well substituted for In atoms, and their distribution is greatly uniform over the whole thickness of the films. Films grown at 550 °C showed the largest magnetic moment of about 0.5 μ_B/Co, while films grown at higher temperatures have magnetic moments of one order smaller. The observed ferromagnetism above room temperature in Co:In₂O₃ thin films has confirmed that doping few percent of magnetic elements such as Co into In₂O₃ could result in a promising magnetic material.     

Magnetic properties and surface morphology of layered In2Se3 crystals intercalated with cobalt

The magnetic properties of layered Co _x In₂Se₃ crystals electrochemically intercalated with cobalt in an external magnetic field and without a magnetic field and the morphology of the van der Waals surfaces of layers of these crystals have been investigated. It has been found that the ferromagnetic ordering at room temperature is observed only for Co _x In₂Se₃ crystals intercalated in an external magnetic field. These crystals are nanocomposite materials that consist of a layered matrix and arrays of nanorings and nanowires formed from Co nanocrystals on the van der Waals surfaces of the In₂Se₃ layers. Cobalt nanocrystals in Co _x In₂Se₃ crystals have a pyramidal equilibrium shape, which is characteristic of the face-centered cubic crystal structure, and their geometrical sizes are of the order of a few nanometers. The specific features of self-organization of cobalt magnetic nanostructures on the van der Waals surfaces of layered semiconductor crystals during their electrolytic intercalation in a magnetic field and the magnetic properties of these structures have been considered.     

Modulation of the ferromagnetic insulating phase in Pr0.8Ca0.2MnO3 by Co substitution

Ferromagnetic insulator Pr_0.8Ca_0.2Mn_1–yCo_y O₃ (0 ≤ y ≤ 0.7) thin films were epitaxially grown by pulsed laser deposition on substrates of (LaAlO₃)_0.3(SrAl_0.5Ta_0.5O₃)_0.7 (100). To probe the ferromagnetic insulator state, the Co content dependences of the structural, magnetic, and transport properties were studied. Variation of lattice constant by the Co substitution is well reproduced considering that divalent and trivalent Co ions substitute for Mn ions at the perovskite B‐sites. For 0 ≤ y ≤ 0.3, the Curie temperature, saturation magnetization, and magnetoresistance increase with increasing Co content, retaining the insulating properties. Detailed analyses of transport and magnetic properties indicate the contribution of both double exchange and superexchange interactions to the appearance of the ferromagnetic insulating phase. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Geometric and magnetic properties of Co/Pd system

We measured geometric and magnetic properties of Co films on the Pd(1 1 1) surface by X-ray photoelectron diffraction (XPD), X-ray magnetic circular dichroism (MCD) at the Co L_2,3 edge, and the surface magneto-optical Kerr effect (SMOKE) measurements. Co thin films are found to grow incoherently with fcc island structure on the smooth Pd(1 1 1) substrate. Comparison of MCD and SMOKE measurements of Co thin films grown on rough and smooth Pd(1 1 1) surfaces suggests that perpendicular remnant magnetization and Co orbital moment are enhanced by the rough interface. Pd capping layer also induces perpendicular orbital moment enhancement. These observations indicate the influence of hybridization between Co 3d and Pd 4d at the interface on the magnetic anisotropy.     

Magnetic properties of cobalt nanowires: Study by polarized SANS

Structural and magnetic properties of two-dimensional spatially ordered system of ferromagnetic cobalt nanowires embedded into Al₂O₃ matrix have been studied using polarized small-angle neutron scattering. A comprehensive analysis of contributions to the scattering intensity was carried out, including nonmagnetic (nuclear) contribution, magnetic contribution depending on the magnetic field, and nuclear-magnetic interference indicating the correlation between the magnetic and nuclear structures. Experiments have revealed an anomalously low value of the magnetic contribution as compared to the nuclear one. This behavior is interpreted in terms of low coherence of the magnetic structure caused by the anisotropy of Co crystallites as compared with the large coherency of nuclear structure of nanowires.     

Room temperature ferromagnetism in Zn1−xCoxS thin films with wurtzite structure

The magnetic properties of Zn_1−xCo_xS (x=0.025 and 0.05) thin films grown on α-quartz substrates at different temperatures (T_S) of 200, 400 and 600 °C by means of pulsed laser deposition are presented. The films are crystallized with wurtzite structure. Optical absorption and transmission electron microscopy measurements indicate that Co ions are substituted to Zn on tetrahedral sites. Their magnetic response is composed of ferromagnetic and paramagnetic components of which respective strengths depend on T_S and Co concentration. This behavior is interpreted as due to fluctuations in the magnetic ordering, depending on grain size and site location in grain boundaries or in crystal cores.     

Four-fold magnetic anisotropy in a Co film on MgO(0 0 1)

The development of devices based on magnetic tunnel junctions has raised new interests on the structural and magnetic properties of the interface Co/MgO. In this context, we have grown ultrathin Co films (≤30 Å) by molecular-beam epitaxy on MgO(0 0 1) substrates kept at different temperatures (T_S). Their structural and magnetic properties were correlated and discussed in the context of distinct magnetic anisotropies for Co phases reported in the literature. The sample characterization has been done by reflection high energy electron diffraction, magneto-optical Kerr effect and ferromagnetic resonance. The main focus of the work is on a sample deposited at T_S=25 °C, as its particular way of growth has enabled a bct Co structure to settle on the substrate, where it is not normally obtained without specific seed layers. This sample presented the best crystallinity, softer magnetic properties and a four-fold in-plane magnetic anisotropy with Co〈1 1 0〉 easy directions. Concerning the samples prepared at T_S=200 and 500° C, they show fcc and polycrystalline structures, respectively and more intricate magnetic anisotropy patterns.     

Synthesis of heavy fermion CeCoIn5 thin film via pulsed laser deposition

CeCoIn₅ (Co115) thin films have been grown on Al₂O₃ (000l) substrates through the pulsed laser deposition (PLD). The films are grown mainly along the c-axis, with CeIn₃ and In-related alloys. The rock-salt type grains are nucleated, where Co115 grains mixed with excess indium are evenly distributed over the substrate. The electrical resistivity of the films shows a Kondo coherence peak near 47 K and the zero-resistance superconducting state at 1.8 K, which is the first observation in the PLD grown thin films of Co115. The Rietveld refinement of the thin films shows that the c/a ratio (tetragonality) is suppressed to 1.6312 from 1.6374 of single crystals, which is consistent with the linear relationship between the superconducting transition temperature and tetragonality. The good agreement indicates that the PLD could provide an alternative route to tune the 2D character of the critical spin fluctuations to understand the superconducting pairing mechanism of Co115.     

Structural, optical and magnetic properties of Nd-doped BiFeO3 thin films prepared by pulsed laser deposition

Nd-doped BiFeO₃ thin films were grown by pulsed laser deposition on quartz substrate and their structural, optical and magnetic properties have been studied. X-ray diffraction analysis revealed that Nd addition caused structural distortion even with 5% of Nd concentration, additional secondary phase appeared in all samples but its intensity was greatly reduced with Nd addition. Doping-induced variations in texture and structure modifying both magnetic and optical properties of BiFeO₃ thin films. The energy band gap decreases while the refractive index increases with addition of Nd³⁺ in BiFeO₃ for Bi³⁺. These variations in energy band gap and refractive index have been explained on the basis of density of states and increase in disorders in the system. All the samples were found to exhibit ferromagnetism at room temperature and the saturation magnetization increases with the increase in structural distortion with addition of Nd. Finally, Nd-doping modifies the physical properties of BiFeO₃ in comparison to undoped BiFeO₃ thin films.     

Effect of crystallinity of Co layer on perpendicular exchange bias in Au-capped ultrathin Co film on Cr2O3(0 0 0 1) thin film

The effect of the crystalline quality of ultrathin Co films on perpendicular exchange bias (PEB) has been investigated using a Au/Co/Au/α-Cr₂O₃ thin film grown on a Ag-buffered Si(1 1 1) substrate. Our investigation is based on the effect of the Au spacer layer on the crystalline quality of the Co layer and the resultant changes in PEB. An α-Cr₂O₃(0 0 0 1)layer is fabricated by the thermal oxidization of a Cr(1 1 0) thin film. The structural properties of the α-Cr₂O₃(0 0 0 1) layer including the cross-sectional structure, lattice parameters, and valence state have been investigated. The fabricated α-Cr₂O₃(0 0 0 1) layer contains twin domains and has slightly smaller lattice parametersthan those of bulk-Cr₂O₃. The valence state of the Cr₂O₃(0 0 0 1) layer is similar to that of bulk Cr₂O₃. The ultrathin Co film directly grown on the α-Cr₂O₃(0 0 0 1) deposited by an e-beam evaporator is polycrystalline. The insertion of a Au spacer layer with a thickness below 0.5 nm improves the crystalline quality of Co, probably resulting in hcp-Co(0 0 0 1). Perpendicular magnetic anisotropy (PMA) appears below the Néel temperature of Cr₂O₃ for all the investigated films. Although the PMA appears independently of the crystallinequality of Co, PEB is affected by the crystalline quality of Co. For the polycrystalline Co film, PEB is low, however, a high PEB is observed for the Co films whose in-plane atom arrangement is identical to that of Cr³⁺ in Cr₂O₃(0 0 0 1). The results are qualitatively discussed on the basis of the direct exchange coupling between Cr and Co at the interface as the dominant coupling mechanism.     

Induction of an atomically thin ferromagnetic semiconductor in 1T′ phase ReS2 by doping with transition metals

Two-dimensional 1T′ phase ReS₂, a transition metal dichalcogenide, has a unique structure and electronic properties that are independent of thickness. The pure phase is a nonmagnetic semiconductor. Using density functional theory calculations, we show that ReS₂ can be tuned to a magnetic semiconductor by doping with transition metal atoms. The magnetism mainly comes from the dopant transition metal and neighboring Re and S atoms as a result of competition between exchange splitting and crystal field splitting. ReS₂ doped with Co can be considered as a promising diluted magnetic semiconductor owing to its strong ferromagnetism with long-range ferromagnetic interaction, high Curie temperature (above room temperature) and good stability. These findings may stimulate experimental validation and facilitate the development of new atomically thin diluted magnetic semiconductors based on transition metal dichalcogenides.     

Investigations of cobalt and carbon codoping in gallium nitride for spintronic applications

Extensive theoretical investigations have been carried out to study the ferromagnetic properties of transition metal doped wurtzite GaN using the Tight Binding Linear Muffin-tin Orbital (TBLMTO) method within the density functional theory. The present calculation reveals ferromagnetism in cobalt doped GaN when one gallium is replaced by cobalt in a 3×3×2 supercell of GaN, which gives rise to a cobalt concentration of 2.77%. The system is half-metallic with a magnetic moment of 4.0 μ_B. When Co is bonded with one carbon, there is a drastic decrease in magnetic moment and the system becomes metallic. When Co dimer is introduced via nitrogen which corresponds to the Co concentration of 5.5% the magnetic moment is 3.99 μ_B and the system is half-metallic. Same trend is observed when Co is bonded via nitrogen with unequal distance. When cobalt dimer is formed via carbon, the moment becomes 2.95 μ_B and it shows metallic character. For dimer via carbon with unequal distance, the moment is 3.0 μ_B and the system becomes semiconductor. For higher percentage of cobalt dopant the system shows metallic character. C and Co doped GaN samples have been synthesized experimentally and characterized with X-ray diffraction, transmission electron microscopy, micro-Raman and superconducting quantum interface device measurements. The observed results are correlated with the theoretical studies.     

Silicide formation in bilayer ultrathin iron and cobalt films on silicon

The processes that occur in ultrathin (up to 1 nm) Fe and Co layers during deposition onto the Si(100)2 × 1 surface in various sequences and during annealing of the formed structures to a temperature of 400°C are studied. The elemental and chemical compositions of the films are analyzed by in situ high-resolution X-ray photoelectron spectroscopy using synchrotron radiation, and their magnetic properties are determined using the magnetic linear dichroism effect in the angular distribution of Fe 3p and Co 3p electrons. It is shown that, when iron is first deposited, the formed structure consists of the layers of FeSi, Fe₃Si, Co-Si solid solution, and metallic cobalt with segregated silicon. The structure formed in the alternative case consists of the layers of CoSi, Co-Si solid solution, Co, Fe-Si solid solution, and Fe partly covered by silicon. All layers (apart from FeSi, CoSi) form general magnetic systems characterized by ferromagnetic ordering. Annealing of the structures at temperatures above 130dgC (for the Co/Fe/Si system) and ～200°C (for Fe/Co/Si) leads to the formation of nonmagnetic binary and ternary silicides (Fe _x Co_{1 ? x} Si, Fe _x Co_{2 ? x} Si).     

Growth of SrTiO3 on Si(001) by hybrid molecular beam epitaxy

We report the heteroepitaxial growth of SrTiO₃ thin films on Si(001) by hybrid molecular beam epitaxy (hMBE). Here, elemental strontium and the metal‐organic precursor titanium tetraisopropoxide (TTIP) were co‐supplied in the absence of additional oxygen. The carbonization of pristine Si surfaces during native oxide removal was avoided by freshly evaporating Sr into the hMBE reactor prior to loading samples. Nucleation, growth and crystallization behavior as well a structural properties and film surfaces were characterized for a series of 46‐nm‐thick SrTiO₃ films grown with varying Sr to TTIP fluxes to study the effect of non‐stoichiometric growth conditions on film lattice parameter and surface morphology. High quality SrTiO₃ thin films with epitaxial relationship (001)SrTiO₃ || (001)Si and [110]SrTiO₃ || [100]Si were demonstrated with an amorphous layer of around 4 nm thickness formed at the SrTiO₃/Si interface. The successful growth of high quality SrTiO₃ thin films with atomically smooth surfaces using a thin film technique with scalable growth rates provides a promising route towards heterogeneous integration of functional oxides on Si. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Valence control of cobalt oxide thin films by annealing atmosphere

The cobalt oxide (CoO and Co₃O₄) thin films were successfully prepared using a spin-coating technique by a chemical solution method with CH₃OCH₂CH₂OH and Co(NO₃)₂·6H₂O as starting materials. The grayish cobalt oxide films had uniform crystalline grains with less than 50 nm in diameter. The phase structure is able to tailor by controlling the annealing atmosphere and temperature, in which Co₃O₄ thin film was obtained by annealing in air at 300-600, and N₂ at 300, and transferred to CoO thin film by raising annealing temperature in N₂. The fitted X-ray photoelectron spectroscopy (XPS) spectra of the Co2p electrons are distinguishable from different valence states of cobalt oxide especially for their satellite structure. The valence control of cobalt oxide thin films by annealing atmosphere contributes to the tailored optical absorption property.     

Reactive sputtering of (Co,Fe) nitride thin films on TiN-bufferd Si

Epitaxial (Co,Fe) nitride films were prepared on TiN buffered Si(001) substrates by dual-target reactive co-sputtering method. With lower Co content, thin films mainly consist of (Co _x Fe_1?x )₄N phase. With higher Co content, STEM EELS found no N signal in the thin film, and, combined with XRD results, shows that fcc Co is the main phase of the thin films instead of Co₄N. The N₂ atmosphere is helpful to induce the fcc Co phase formation during dual-target reactive co-sputtering deposition. For the films with less Co content, the RT magnetization measurements show similar magnetic properties as epitaxial Fe₄N(001) films, while increasing the Co content, the resulting fcc Co thin films show biaxial anisotropy with the [110] in-plane easy axis.     

Magnetic properties for cobalt nanorings: Monte Carlo simulation

In this paper, two structure models of cobalt nanoring cells (double-nanorings and four-nanorings, named as D-rings and F-rings, respectively) have been considered. Base on Monte Carlo simulation, the magnetic properties of the D-rings and F-rings, such as hysteresis loops, spin configuration, coercivity, etc., have been studied. The simulated results indicate that both D-rings and F-rings with different inner radius (r) and separation of ring centers (d) display interesting magnetization behavior and spin configurations (onion-, vortex- and crescent shape vortex-type states) in magnetization process. Moreover, it is found that the overlap between the nearest single nanorings connect can result in the deviation of the vortex-type states in the connected regions. Therefore, the appropriate d should be well considered in the design of nanoring device. The simulated results can be explained by the competition between exchange energy and dipolar energy in Co nanorings system. Furthermore, it is found that the simulated temperature dependence of the coercivity for the D-rings with different d can be well described by H_c=H₀ exp[−(T/T₀)^p].