Magnetic Properties Of Co Films Research Articles

Structure and Performance Optimization of Co Magnetic Thin Films Deposited by Vacuum Evaporation Coating.

Co magnetic films are widely used in high-frequency magnetic recording and vertical magnetic recording due to their high saturation magnetization and magnetocrystalline anisotropy. In this work, ferromagnetic Co magnetic films were prepared on copper substrate by vacuum evaporation combined with heat treatment (H2 atmosphere), to investigate the impact of film thickness and annealing temperature on microstructure and magnetic properties. The results show that with the increase in annealing temperature, the Co thin film physical phase does not change significantly, the crystallinity increases, and the grain size increases, which is consistent with the results obtained from the SEM morphology map of the sample surface, leading to an increase in coercivity. By annealing experiments (atmospheric atmosphere) on Co magnetic films with and without an Al protective layer, as shown by scanning electron microscopy microscopic characterization results, it was verified that the Al layer can protect the inner Co layer from oxidation. As the film thickness increases from 10 to 300 nm, the magnetic properties of Co films change significantly. The saturation magnetization gradually increases from 0.89 to 5.21 emu/g, and the coercivity increases from 124.3 to 363.8 Oe. The remanence ratio of the 10 nm magnetic film is 0.82, which is much higher than the film remanence ratio of 0.46 at 50 nm. This is because when the thickness of the film is between 10 and 50 nm, the magnetic moments partially deviate from the in-plane direction, and the out-of-plane component reduces the film remanence ratio. This study shows that optimizing annealing temperature and film thickness can effectively control the structure and magnetic properties of Co magnetic films, which is of great significance for the development of the magnetic recording field.

Thickness dependent structural and magnetic properties investigation of Co film interfaced with Hf

Thickness dependent structural and magnetic properties of Co films interfaced with Hf have been studied. X-ray reflectivity (XRR) and Grazing Incidence X-ray Diffraction (GIXRD) provides the structural properties of as-deposited samples. Film thicknesses as observed from XRR analysis are further implemented to study the magnetic properties of samples as function of Co film thickness, using magneto optical Kerr effect (MOKE) and polarized neutron reflectivity (PNR). MOKE results show the variation of magnetic coercivity with Co film thickness which exhibits a maximum value around 6 nm. Effective magnetic moment of samples as obtained from PNR fitting displays a linear variation with film thickness. Extrapolation of such variation on thickness axis show presence of magnetically dead layer (MDL) at interface, of thickness below 1 nm, in agreement with the reported values in literature for magnetic films interfaced with 5d heavy metals.

The accelerating nanoscale Kirkendall effect in Co films–native oxide Si (100) system induced by high magnetic fields

The morphology evolution and magnetic properties of Co films–native oxide Si (100) were investigated at 873, 973, and 1073 K in a high magnetic field of 11.5 T. Formation of Kirkendall voids in the Co films was found to cause morphology evolution due to the difference in diffusion flux of Co and Si atoms through the native oxide layer. The high magnetic fields had considerable effect on the morphology evolution by accelerating nanoscale Kirkendall effect. The diffusion mechanism in the presence of high magnetic fields was given to explain the increase of diffusion coefficient. The morphology evolution of Co films on native oxide Si (100) under high magnetic fields during annealing resulted in the magnetic properties variation.

Growth angle-dependent evolution of morphology and magnetic properties of Co films on highly ordered self-organized Ge substrates

This paper reports on the influence of growth angle on morphology and uniaxial magnetic anisotropy of 60 nm-thick sputter-deposited cobalt films on patterned-Ge substrates (having nanowire-like morphology with extreme regularity) prepared by keV-ion bombardment at room temperature. Morphological studies reveal that grain size in Co films increases with increasing growth angle, albeit lateral spacing among the ridges of conformally grown Co films (under growth angles in the range of 0–70°) on patterned-Ge substrates match quite well with that of a patterned-Ge substrate. While an increase in the Co grain size is expected to cause higher coercive fields, in the presence of magnetic pinning and slopes between ridges and grooves of nanowire-like patterned Co morphology, we observe an inverse trend. In other words, an increase in the grain size in conjunction with respective evolution of microstructures in Co films, at higher growth angles, lead to a decrease in the coercive field. Magnetic force microscopy is employed to probe the out-of-plane domains in ferromagnetic Co films. Tailored design of these novel nanostructured Co layers will be of interest for fabrication of high-density magnetic data storage devices, magneto-transport phenomena, and magneto-plasmonics.

Structural and magnetic properties of Co films on highly textured and randomly oriented C60 layers

The structural and magnetic properties of Co/C60/pentacene and Co/C60 thin film structures were investigated. Atomic force microscopy and x-ray reflectivity analysis show that the presence or absence of a pentacene buffer layer leads to a highly textured or randomly oriented C60 layer, respectively. A Co film deposited on a randomly oriented C60 layer penetrates into the C60 layer when it is deposited at a slow deposition rate. The Co penetration can be minimized, regardless of the Co deposition rate, by growth on a highly textured and nanostructured C60/pentacene layer. Vibrating sample magnetometry measurements show that the saturation magnetization of Co/C60/pentacene is significantly reduced compared to that of Co/C60. On the other hand, the Co penetration does not seem to have an effect on the magnetic properties, suggesting that the structural properties of the Co and C60 layer, rather than the Co penetration into the organic C60 layer, are critical to the magnetic properties of the Co/C60.

Effect of CoSi2 buffer layer on structure and magnetic properties of Co films grown on Si (001) substrate**Project supported by the National Basic Research Program of China (Grant Nos. 2011CB921801 and 2012CB933102), the National Natural Science Foundation of China (Grant Nos. 11374350, 11034004, 11274361, and 11274033), and the Research Fund for the Doctoral Program of Higher Education of China (Grant

Buffer layer provides an opportunity to enhance the quality of ultrathin magnetic films. In this paper, Co films with different thickness of CoSi2 buffer layers were grown on Si (001) substrates. In order to investigate morphology, structure, and magnetic properties of films, scanning tunneling microscope (STM), low energy electron diffraction (LEED), high resolution transmission electron microscopy (HRTEM), and surface magneto-optical Kerr effect (SMOKE) were used. The results show that the crystal quality and magnetic anisotropies of the Co films are strongly affected by the thickness of CoSi2 buffer layers. Few CoSi2 monolayers can prevent the interdiffusion of Si substrate and Co film and enhance the Co film quality. Furthermore, the in-plane magnetic anisotropy of Co film with optimal buffer layer shows four-fold symmetry and exhibits the two-jumps of magnetization reversal process, which is the typical phenomenon in cubic (001) films.

Modification of the magnetic properties of Co films grown on MgO (100) by treatment with NaOH solution

We investigated the surface morphology and magnetic properties of Co films grown on MgO (100) substrates before and after treatment with a NaOH solution. The surface morphology evolves from a continuous film into isolated hexagonal nanodisks resulting from the formation of cobalt hydroxide. In the as-grown Co films the magnetic anisotropy behaves as a superposition of fourfold magnetocrystalline anisotropy and uniaxial magnetic anisotropy (UMA), and the magnetization reversal proceeds by two-step domain-wall motion. After the NaOH treatment the UMA is reduced and the magnetization reversal appears to be dominated by magnetization rotation. In addition, the exchange bias effect, which in the as-deposited films results from the presence of a cobalt oxide layer, disappears after the NaOH treatment, and can be reinduced by annealing the sample. The observed significant changes in the magnetic properties can be related to the disappearance and reappearance of a cobalt oxide top layer.

Magnetic properties of Co films grown on the modified Si(111) surface

The magnetic properties and domain structure of epitaxial Co films grown on a modified Si(111) surface were studied. First, the processes of growth of copper silicide nanostructures on the Si(111) surface were investigated. Copper silicide clusters were formed on the Si(111)-5.55 × 5.55-Cu surface at a substrate temperature of ∼550°C. It was established that the nanostructures formed have a perfect faceting, and the lateral edges and long wire side are oriented along the Si〈110〉 crystallographic directions. Then, Co films were deposited on the formed structures. The investigation of the coercive force and reduced remanent magnetization showed that the Co(111) films have the sixth-order crystalline anisotropy. It was found that the coercive force of the Co films deposited on the Cu buffer layer is approximately six times less than that of the Co films deposited on the Si(111)-5.55 × 5.55-Cu surface and Si(111)−5.55 × 5.55-Cu/(Cu-Si) cluster surface.

Comparative studies of magnetic properties of Co films on annealed and unannealed rubrene/Si(100)

Abstract Structures and magnetic properties of Co films on rubrene/Si(1 0 0) have been investigated by employing atomic force microscopy, X-ray diffraction and magneto-optic Kerr effect techniques. For Co/rubrene/Si(1 0 0) films, separated Co clusters are found as the Co thickness increases. For 20 nm Co overlayer, the Co clusters are uniform with an average diameter around 90 nm. After annealing treatments at 550 K for rubrene/Si(1 0 0), rubrene molecules nucleate to form clusters and pinholes develop in the rubrene layer. For Co/annealed rubrene/Si(1 0 0) films, the formation of a Co/Si(1 0 0) interface is important for the occurrence of the pyramid-like nanostructure with an hcp stacking of the Co layer. Comparative studies of thickness dependencies of the magnetic properties for x nm Co on rubrene/Si(1 0 0) prepared at 300 and 550 K have been performed. With increasing the cobalt thickness, the Kerr intensities approach a saturation value for thicker Co due to an attenuation of the laser intensity in a condensed material. For Co deposited on annealed rubrene/Si(1 0 0), the enhancement of the coercive force is attributed to the imperfection introduced by rough interface to impede the magnetization reversal. As the Co thickness increases from 10 to 40 nm on rubrene/Si(1 0 0), the squareness increases from 0.9 to near unity. For Co deposited on annealed rubrene/Si(1 0 0), the squareness of magnetization curve is close to unity for all the Co thicknesses. The formation of Co islands in triangular shapes plays an important role on the enhancement of the squareness of magnetization curve for Co overlayers.

The structure and magnetic properties of Co films on Si(111) and Si(001) substrates

Epitaxial Co films have been grown on single-crystal Si(111) and Si(001) substrates. The structure and the lattice parameter of Co films have been determined using the method of reflection high-energy electron diffraction. The domain structure of the epitaxial Co films has been studied, and the presence of a magnetocrystalline anisotropy and an anisotropy induced by substrate steps have been established. It has been shown that the coercive force of epitaxial Co films is determined by the roughness of the surface and by misfit dislocations.

Magneto-optical properties of Co∕Ge(100) with ultrathin Ag buffer layers

Magnetic properties of Co films (<2nm) with Ag buffer layers (<0.7nm) grown on Ge(100) at room temperature and 200K were studied by surface magneto-optical Kerr effect. Without the buffer, the films reveal in-plane magnetic anisotropy even Co and Ge forms nonmagnetic interfacial alloys. The hysteresis due to intercalation of Ag can be detected at thinner Co thicknesses. The buffer can effectively cutoff the intermixing of Co and Ge. As the thickness of Ag is reduced, out-of-plane magnetic anisotropy due to the interface interactions between Co∕Ag and Co∕Ge was discovered and was only at 200K.

Ag buffer layer effect on magnetization reversal of epitaxial Co films

Nano-sized Ag(111) islands were first prepared by using molecular beam epitaxy technique on diluted-hydrofluoric acid etched Si(111) substrate. Epitaxial Co films were then grown onto the Ag films at 100 °C to decrease interdiffusion. The Ag buffer layer designed to form isolated islands with {111} sidewalls on the Si(111) substrate, and provided Co films (111) texture growth to study the correlation between magnetic properties of Co films and Ag buffer layer effect. It reveals that the Ag rough surface acts as a pinning source and Ag {111} sidewalls also plays an important role on the magnetoresistance transition of Co films.

The structure and magnetic properties of Co films on Ni3Al(001)

The structure and magnetic properties of Co films on Ni3Al(001)

The structure and magnetic properties of Co films on Ni3Al(0 0 1)

The structure and magnetic properties of thin Co films on Ni3Al(0 0 1) were studied by scanning tunneling microscopy and magneto optical Kerr effect. At 300 K, for 3.5 ML of Co, 2D islands are observed with the diameter distribution centered at ∼45 Å. Annealing at 700 K leads to a coalescence in larger and flat FCC Co(0 0 1) 2D islands. The hysteresis loops show a higher coercivity field for the annealed samples. The samples show a four-fold magnetic anisotropy axis in the (0 0 1) plane. Magnetization out of plane was not found.

Spin-polarized vacuum tunneling in field emission from Co-coated W(111) tips

Spin-polarized vacuum tunneling has been achieved in field emission from spontaneously and remanently magnetized ultrathin film Co-coated W(111) tips. The onset thickness of polarization at room temperature, equivalent to 3 Co (0001) layers, and the transverse spin-polarization directions are consistent with magnetic properties of Co films on macroscopic W(111) single-crystal surfaces. The orientation of the polarization vector in any of 12 transverse directions is due to crystallographic symmetry and the influence of tip morphology. Polarization of 10%–25% was obtained typically, while the highest recorded value was 48%. Manipulation of the polarization direction and magnitude were also achieved by application of transverse magnetic field pulses and demagnetization.

Radio frequency substrate bias effect on properties of Co thin film and multilayer structures

This paper reports the surface roughness effect on magnetic properties of Co thin films and the switching properties of the multilayer structures. Surface roughness is varied by using Ni 80Fe 20 thin film as an underlayer. The surface roughness of Ni 80Fe 20 thin films was controlled by applying different radio frequency (RF) biases to the substrate during the sputtering of Ni 80Fe 20 underlayer. Surface roughness effect on the magnetic properties of Co thin films and the switching properties of the multilayer structures were studied with Si/Ni 80Fe 20/Al/Co/Al and Si/Ni 80Fe 20/Al/Co/Al 2O 3/Ni 80Fe 20/Al multilayer films. Results indicate that the RF-substrate-bias-induced surface roughness has a great influence on the magnetic properties of Co films and the switching properties of the multilayer structures.

Magnetic properties of Co films deposited onto obliquely sputtered Ta underlayers

Gold-capped Co films deposited on obliquely sputtered Ta underlayers have produced coercivities in excess of 400 Oe, and anisotropy fields of 800 Oe. The coercivity increases with Ta deposition angle and thickness. The coercivity and anisotropy can be increased further to values >600 and 900 Oe, respectively, by replacing the Au capping layer with Cu. Films with an obliquely deposited underlayer exhibit a reduced density and an increased interface roughness compared to normally deposited films. The trilayers fabricated in this study show promising results for use in giant magnetoresistance sensors.

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 Co films on Cu/Si(110) and Cu(111)

The magnetic anisotropy of strained epitaxial Co films deposited on strained Cu(111) on Si(110) and miscut Cu(111) are reported. First, a Cu(111) buffer layer with an in-plane strain is formed by epitaxial growth on Si(110). Co films deposited on the buffer layer develop a uniaxial anisotropy axis originating from the strain along the twofold axis of the Si(110) surface. The magnetic anisotropy of this film is compared to that of a Co film deposited on a miscut Cu(111) single crystal with uniaxial step-induced anisotropy in the Co layer. The measured hysteresis loops are compared to hysteresis loops simulated within a Stoner-Wolfarth model with a varying fraction of uniaxial and triaxial anisotropies. The measurements and the model are in qualitative agreement and reveal uniaxial and triaxial anisotropies of the same order of magnitude for both systems.

Effect of surface roughness on magnetic properties of Co films on plasma-etched Si(100) substrates

Magnetic and morphological properties of ∼970-Å-thick Co films, deposited simultaneously on ten plasma-etched Si(100) substrates, were measured through the magneto-optic Kerr effect (MOKE) technique, ferromagnetic resonance (FMR), magnetic force microscopy (MFM), and atomic force microscopy. As the etch time t increased from 0 to 100 min, the vertical interface width w of Co films increased from ∼5 to ∼1400 Å; the lateral correlation length ξ, from ∼300 to ∼10 500 Å. The MOKE and FMR measurements gave the in-plane azimuthal angular dependence of the hysteresis loops and the ferromagnetic resonance absorption spectra, respectively. From MOKE and FMR, the smoother films showed uniaxial magnetic anisotropy (t⩽40 min). The uniaxial anisotropy decreased with the increase of the surface roughness and disappeared for the roughest films (t=60 and 100 min). The MOKE hysteresis loop measurements suggested that, with the increasing surface roughness, the magnetization reversal changed gradually from magnetization rotation dominated for the smoothest films to domain-wall motion dominated for the roughest films. The MFM images of the films showed anisotropic magnetic domain contrasts for the smooth film, and the subsequent fragmentation of these domains as the roughness parameters increased. This supports the results of the MOKE and FMR measurements.