Cobalt Thickness Research Articles

Skyrmion bubbles are stable at room temperature and a zero magnetic field

Changing cobalt and ruthenium thickness in the magnetization process of a Pt/Co/Ru/MgO system proves to be a promising method for developing stable skyrmion bubbles with spintronics applications.

ДОСЛІДЖЕННЯ АМПЛІТУДНО-ЧАСТОТНИХ ЗАЛЕЖНОСТЕЙ ЗАХИСНИХ ВЛАСТИВОСТЕЙ МАГНІТНИХ ЕКРАНІВ НА ОСНОВІ АМОРФНИХ СПЛАВІВ

Сучасні методи екранування допускають використання найбільш придатних металевих матеріалів для одночасного вирішення декількох задач для захисту працюючих та електромагнітної сумісності технічного обладнання. Одним з таких ефективних металевих матеріалів вважають аморфні магнітом’які сплави. Предмет дослідження даної роботи – визначення захисних властивостей аморфного магнітом’якого сплаву при різних значеннях частотних та амплітудних характеристик в залежності від зміни товщини стрічки матеріалу. Досліджено магнітні властивості пермалою від різної частоти екранованого поля з різним відсотковим вмістом нікелю та різною товщиною захисного металевого матеріалу; залежності коефіцієнта екранування магнітом’яких аморфних сплавів від індукції зовнішнього магнітного поля; коефіцієнти екранування аморфного магнітом’якого сплаву різної товщини та різного відсоткового вмісту кобальту від змінних значень амплітуди; магнітооброблений стан та вихідний стан (недоліки та переваги для конкретних умов використання при захисті та економічних вимог); залежності магнітної проникності магнітом’якого аморфного сплаву від різних значень частоти та амплітуди. Мета роботи - визначення коефіцієнтів екранування захисних аморфних магнітом’яких сплавів від амплітудно-частотних характеристик екранованого магнітного поля та обґрунтування ефективності використання таких матеріалів та надання конкретних рекомендацій щодо захисту від електромагнітних полів та випромінювань працюючих людей та сумісності електричного та електронного технічного обладнання. Розроблено захисні властивості магнітних екранів на основі аморфних сплавів у різних амплітудно-частотних діапазонах. Запропоновано раціональні обґрунтовані рекомендації щодо вибору матеріалу для забезпечення найбільш придатних значень коефіцієнтів екранування Доведено експериментально що аморфні магнітом’які сплави мають сприятливі коефіцієнти екранування від частоти екранованого поля, що підвищує ефективність захисту від магнітних полів працюючих людей та спрощує, з економічної точки зору, обрання матеріалу на конкретному виробництві. Отримані графічні залежності надають можливість обирати необхідний захисний металевий матеріал та автоматизувати процеси розроблення засобів оптимального екранування магнітних полів. Багатосерійність досліджень та легка керованість властивостей матеріалів з різним вмістом металевої субстанції, різної товщини стрічки матеріалу, тощо надає змогу змінювати загальні захисті властивості металевого сплаву ( в залежності від поставленої умови на виробництві)

Measuring the Dzyaloshinskii-Moriya interaction of the epitaxial Co/Ir(111) interface

The in-plane orientation of the magnetization in the center of domain walls is measured in Co/Ir(111) as a function of Co thickness via scanning electron microscopy with polarization analysis. Uncapped, thermally evaporated cobalt on an Ir(111) single-crystal surface is imaged in situ in ultra-high vacuum. The initial pseudomorphic growth with an atomically flat interface of cobalt on iridium ensures comparability to theoretical calculations and provides a study of an interface that is as ideal as possible. Below a cobalt thickness of 8.8 monolayers, the magnetic domain walls are purely N\'eel oriented and show a clockwise sense of rotation. For larger thicknesses the plane of rotation changes and the domain walls show a significant Bloch-like contribution, allowing to calculate the strength of the Dzyaloshinskii-Moriya interaction (DMI) from energy minimization. From the angle between the plane of rotation and the domain-wall normal an interfacial DMI parameter $D_s = -(1.07 \pm 0.05)$ pJ/m is determined, which corresponds to a DMI energy per bond between two Co atoms at the interface of $d_{tot} = -(1.04 \pm 0.05)$ meV.

Low temperature divergence in the AHE and AMR of ultra-thin Pt/Co/Pt trilayers

Abstract We study the anomalous Hall effect (AHE) and anisotropic magnetoresistance (AMR) in a series of ultra-thin Pt/Co(x)/Pt trilayers ( 0.1 nm ⩽ x ⩽ 0.5 nm ). We find that the AHE exhibits a significant increase at low temperatures (T). This increase cannot be attributed to a trivial increase in magnetization or resistivity. A similar T dependence is observed in the AMR measurements. Interestingly, these effects show almost no dependence on cobalt thickness. Our measurements indicate that the deviation from common behavior can be attributed to an induced magnetic proximity effect in the Pt layers that increases at low temperatures, in agreement with previous reports. An alternative explanation related to the evolution of a non-continuous magnetic layer is considered as well.

Modification of Magnetic Properties of Pt/Co/Pt Films by Ga+ Ion Irradiation: Focused versus Uniform Irradiation

30 keV Ga + irradiation-induced changes of magnetic and magneto-optical properties of sputtered Pt/Co/Pt ultrathin trilayers films have been studied as a function of the ion fluence. Out-of-plane magnetic anisotropy states with enhanced magneto-optical effects were evidenced for specific values of cobalt thickness and irradiation fluence. Results obtained after uniform or quasi-uniform focused ion beam irradiation on either out-of-plane or in-plane magnetized sputtered pristine trilayers are compared. Similar irradiation-induced magnetic changes are evidenced in quasi-uniformly focused ion beam or uniformly irradiated films, grown either by sputtering or molecular beam epitaxy. We discuss on plausible common mechanisms underlying the observed effects.

An investigation of magneto-optical Kerr effect signal by optical cavity in [Glass/Co/ZnS] structure

In this paper, using the magneto optical Kerr effect, the magnetic properties of [Glass/Co/ZnS] structure has been investigated. Initially, by co-precipitation method, zinc sulfide was prepared and then by X-ray diffraction pattern, its structure was confirmed. In the structure of [Glass/Co/ZnS], cobalt was used as a magnetic material and zinc sulfide acts as a capping layer. Before fabrication process, the optimums cobalt and zinc sulfide thicknesses were calculated using the MATLAB simulation software and the desired thicknesses were selected. To obtain layers with variable thicknesses, the oblique thermal evaporation technique was used and different thicknesses of about 5–90 nm for cobalt and 45 nm for zinc sulfide on the glass substrate were coated. The longitudinal Kerr signal (LKS) of the [Glass/Co/ZnS] samples were measured in different cobalt thicknesses. By using the magneto-optical Kerr effect (MOKE), the hysteresis loop of samples in the longitudinal geometry have been measured samples and it was observed that the sample has an easy axis in the plane of the film and in the direction of its large diameter. The results have shown that the capping layer with suitable thickness, by creation of an optical cavity, has been able to amplify the Kerr signal by more than 1.5 times, which is noticeable by considering the simplicity of the technique. This structure can be applied as a highly accurate magneto-optical sensor.

Structural and phase transformation of cobalt films grown on amorphous carbon

Structural evolution of the ultra-thin cobalt layers grown on amorphous carbon by DC magnetron sputtering were studied in detail by transmission electron microscopy and low-angle X-ray diffraction for a range of the cobalt thickness from 1.5nm to 4.6nm. It was shown that atomic structure of cobalt layers was amorphous at the layer thicknesses below 2nm, an amorphous matrix with embedded nuclei of the crystalline phase with in-plane size of 1–2nm in the thickness range from 3nm to 3.2nm, and polycrystalline with the randomly oriented HCP cobalt grains at thicknesses over 4nm. Increase of the cobalt thickness from 3.2nm to 4.6nm led to growth of the cobalt grains with in-plane average size up to ~70nm by the normal grain coarsening process. Transition of the atomic structure of cobalt from the isotropic amorphous state to the anisotropic crystalline state in the thickness range of ~2–4nm was accompanied by deterioration of the magnetization vector direction within the ferromagnetic domains due to high magnetic anisotropy of HCP lattice of cobalt.

High-purity cobalt thin films with perpendicular magnetic anisotropy prepared by chemical vapor deposition

A study of the chemical vapor deposition (CVD) of high-purity cobalt thin films is described. The Co layer prepared by a thermal CVD technique with a Pt/Ta underlayer and a Pt cap layer shows a saturation magnetization (Ms) of ∼1.8 T and perpendicular magnetic anisotropy (PMA) with an anisotropy energy (Ku) of ∼105 J/m3. The cobalt thickness dependence of Ku reveals that the interfacial anisotropy at the Pt/Co interface is most likely the origin of the obtained PMA.

Selective Area Band Engineering of Graphene using Cobalt-Mediated Oxidation

This study reports a scalable and economical method to open a band gap in single layer graphene by deposition of cobalt metal on its surface using physical vapor deposition in high vacuum. At low cobalt thickness, clusters form at impurity sites on the graphene without etching or damaging the graphene. When exposed to oxygen at room temperature, oxygen functional groups form in proportion to the cobalt thickness that modify the graphene band structure. Cobalt/Graphene resulting from this treatment can support a band gap of 0.30 eV, while remaining largely undamaged to preserve its structural and electrical properties. A mechanism of cobalt-mediated band opening is proposed as a two-step process starting with charge transfer from metal to graphene, followed by formation of oxides where cobalt has been deposited. Contributions from the formation of both CoO and oxygen functional groups on graphene affect the electronic structure to open a band gap. This study demonstrates that cobalt-mediated oxidation is a viable method to introduce a band gap into graphene at room temperature that could be applicable in electronics applications.

Interfacial Structure Dependent Spin Mixing Conductance in Cobalt Thin Films.

Enhancement of Gilbert damping in polycrystalline cobalt thin-film multilayers of various thicknesses, overlayered with copper or iridium, was studied in order to understand the role of local interface structure in spin pumping. X-ray diffraction indicates that cobalt films less than 6nm thick have strong fcc(111) texture while thicker films are dominated by hcp(0001) structure. The intrinsic damping for cobalt thicknesses above 6nm is weakly dependent on cobalt thickness for both overlayer materials, and below 6nm the iridium overlayers show higher damping enhancement compared to copper overlayers, as expected due to spin pumping. The interfacial spin mixing conductance is significantly enhanced in structures where both cobalt and iridium have fcc(111) structure in comparison to those where the cobalt layer has subtly different hcp(0001) texture at the interface.

XAS and XMCD studies of magnetic properties modifications of Pt/Co/Au and Pt/Co/Pt trilayers induced by Ga⁺ ions irradiation.

Magnetic and magneto-optical properties of Pt/Co/Au and Pt/Co/Pt trilayers subjected to 30 keV Ga(+) ion irradiation are compared. In two-dimensional maps of these properties as a function of cobalt thickness and ion fluence, two branches with perpendicular magnetic anisotropy (PMA) for Pt/Co/Pt trilayers are well distinguished. The replacement of the Pt capping layer with Au results in the two branches still being visible but the in-plane anisotropy for the low-fluence branch is suppressed whereas the high-fluence branch displays PMA. The X-ray absorption spectra and X-ray magnetic circular dichroism (XMCD) spectra are discussed and compared with non-irradiated reference samples. The changes of their shapes and peak amplitude, particularly for the high-fluence branch, are related to the modifications of the local environment of Co(Pt) atoms and the etching effects induced by ion irradiation. Additionally, in irradiated trilayers the XMCD measurements at the Pt L2,3-edge reveal an increase of the magnetic moment induced in Pt atoms.

The effect of varying Co layer thickness on the time-temperature characteristics of Co/Sb semimetal embedded magnetic nanoparticles

We report the effect of varying cobalt thickness on the temperature-dependent time decay of the electrical resistance of Co/Sb multilayer samples. We find that for a given temperature, a five fold change in the Co thickness produces a 100 fold change in the characteristic decay time of the resistance. We find that the characteristic decay time, as a function of temperature, follows an Arrhenius law. During deposition, the Co evolves single domain magnetic nanoparticles, on the Sb, in either a Volmer-Weber or Stranski-Krastanov growth mode. This metastable state is then encased in 2.5 nm of Sb producing an embedded nanoparticle system. Scanning tunneling microscopy measurements taken before sample aging (annealing at a given temperature for enough time to complete the resistance decay) and after aging show that these nanoparticles undergo morphological transformations during aging. These transformations lead to well defined time dependent decays in both the magnetization and the electrical resistance, making this material an excellent candidate for an electronic time-temperature sensor.

Three-Dimensional Micromagnetic Simulation of Spatial Distribution of Magnetization in Thick Cobalt Layers

Spatial magnetization distribution of cobalt layer is studied by means of three-dimensional micromagnetic simulations in the range of cobalt thickness d from 21 to 249 nm. In this range, a spin-reorientation phase transition occurs, while the cobalt thickness increases, from a state with in-plane magnetization, to a state with out-of-plane components of magnetization. An in nite cobalt layer is modelled by the 750 nm×750 nm×d structure consisting of cubic cells of size of 3 nm and the periodic boundary conditions. For larger thicknesses, a labyrinth, partially closed, stripe structure has been found.

Interplay of interfacial compounds, catalyst thickness and carbon precursor supply in the selectivity of single-walled carbon nanotube growth

This study is devoted to elucidate the interplay of catalyst thickness and growth conditions in the activation and selectivity of single-walled carbon nanotube growth using cobalt deposited on Si/SiO2 as a model system. In situ Raman studies reveal that thin catalyst layers require a higher pressure of carbon precursor to initiate nanotube growth. However, if the catalysts are pre-reduced, all catalyst thicknesses display the same low threshold pressure and a higher yield of single-walled carbon nanotubes. To explain these results, catalysts formed from a gradient of cobalt thickness are studied. Surface analyses show that during the catalyst preparation, catalyst atoms at the interface with silica form small and hard-to-reduce silicate nanoparticles while the catalyst in excess leads to the formation of large oxide particles. Weakly-reducing conditions of pretreatment or synthesis are sufficient to reduce the large oxide particles and to lead to the growth of large-diameter multi-walled carbon nanostructures. However, highly-reducing conditions are required to reduce the small silicate domains into small cobalt particles able to grow single-walled carbon nanotubes. These results show that reaction of the catalyst with the support to form more refractory compounds greatly impact the nucleation yield and the growth selectivity of single-walled carbon nanotubes.

Magneto-Optical Kerr Effect Enhanced by Surface Plasmon Resonance and Its Application on Biological Detection

We investigated the magneto-optical Kerr signals enhanced by surface plasmon resonance (SPR) and its application of the biomolecular detection. We measure the SPR curve obtained in multilayered metallic structures (t nm Co/45 nm Au) with varying cobalt thickness t. The thicker cobalt layer causes the weaker SPR. The magneto-optical signals also receive different magnitude of amplification due to measuring the hysteresis loop on different segments of the SPR curve via magneto-optical Kerr effect (MOKE). It also worth noticing that the magneto-optical signals from the reflectance maximum or reflectance minimum of SPR would be enhanced significantly, if the thickness of Co is within the range of 3 to 28 nm. By measuring and analyzing the magneto-optical signals from samples with different thicknesses of Co, we determined an optimal thickness of 8 nm where the signal amplification is the largest. In biomedical applications, this combined system is able to lead as a sensitive biomolecule sensing. We have verified the feasibility via fetal bovine serum (FBS) attached.

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.

Enhanced inverse spin-Hall effect in ultrathin ferromagnetic/normal metal bilayers

Electrically detected ferromagnetic resonance is measured in microdevices patterned from ultra-thin Co/Pt bilayers. Spin pumping and rectification voltages are observed and distinguished via their angular dependence. The spin-pumping voltage shows an unexpected increase as the cobalt thickness is reduced below 2 nm. This enhancement allows more efficient conversion of spin to charge current.

CoSi2 ultra-thin layer formation kinetics and texture from X-ray diffraction

In this study we investigated the influence of cobalt thickness (from 50nm to 10nm) on the kinetics and texture of CoSi2 layers. In-situ X-ray diffraction measurements were performed during isothermal annealing to determine CoSi2 kinetics as explained in a previous publication. Decreasing the initial cobalt thickness (50nm, 30nm, and 10nm) induces a slowdown of the formation rate, especially for the 10nm Co layer. A model based on CoSi2 nucleation is used to explain this phenomenon.

Spin waves in perpendicularly magnetized Co/Ni(111) multilayers in the presence of magnetic domains

Co/Ni(111) multilayers with variable cobalt thickness ${t}_{\mathrm{Co}}$ between 0.15 and 0.35 nm and fixed nickel thickness ${t}_{\mathrm{Ni}}=0.6$ nm were grown on a Pt(1.6 nm)/Ta(3 nm) substrate by dc magnetron sputtering. A strong perpendicular magnetic anisotropy was found both using vibrating sample magnetometry and Brillouin light scattering from thermally excited spin waves. The simultaneous presence of two spin-wave modes, for a range of magnetic fields $H$ applied in plane between $\ensuremath{\approx}$$2.5$ and 5 kOe, was connected with the presence of bubble domains, revealed by polar Kerr microscopy. The lower frequency mode ($\ensuremath{\nu}\ensuremath{\approx}3$ GHz), which exhibits a smooth dependence on the strength of $H$, was attributed to harmonic oscillations of the domain walls. The higher frequency mode, which displayed the typical field behavior of a film with a perpendicular anisotropy, was interpreted as the superposition of two nearly degenerate modes, associated, respectively, with the in-phase and out-of-phase precession of the spins in the bubble array. The higher frequency mode also displayed an unprecedented, nonmonotonic dependence on cobalt thickness, reflecting the nonmonotonic ${t}_{\mathrm{Co}}$ dependence of the effective anisotropy field of the multilayer.

Influence of thin platinum layer on the magnetic properties of multiple layers of CVD cobalt thin films

This paper presents the results obtained on the multiple layers of cobalt (Co)/platinum (Pt) and cobalt (Co)/platinum (Pt)/cobalt (Co) on the oxidised silicon substrate. The cobalt layers were deposited by metal-organic chemical vapour deposition on oxidised-silicon substrates at 450 °C, in H2 ambient with 2-torr processing pressure. The platinum layers were deposited by E-beam evaporation in a separate vacuum system. The magnetic properties of Co/Pt/Co and Co/Pt multilayer were compared with the single cobalt layers of similar thicknesses on the oxidised silicon substrate. From the hysteresis loops it was observed that thin cobalt layers on oxidised-silicon substrate shows hard magnetic property with coercivity H c values of 360 Oe and 500 Oe respectively for the 30- and 15-nm cobalt layers. The multiple layers of Co/Pt/Co and Co/Pt of cobalt thickness 15- and 30-nm with platinum 1.5-nm spacer-layer show significant change in magnetic properties (i.e. coercivity H c and magnetisation M s ) and, gave soft magnetic properties with H c values 51 and 49 Oe respectively, which are significantly less than the H c values of single cobalt layers on oxidised silicon. Also, single and multiple layers cobalt with platinum were annealed and compared with the as-deposited layer structures. From the microstructure analysis by SEM, and AFM it was found that the single and multilayer had similar roughness. Magnetic images were observed by MFM and analyzed in terms of domain structure.