Properties Of Ultrathin Co Films Research Articles

Effect of annealing on the magnetic properties of Cu-capped ultrathin Co films

We report the effect of intermixing of Cu on the magnetic properties of ultrathin Co films deposited on Si(1 0 0). Rutherford backscattering was employed to determine the extent of intermixing, which increased from 7% in an as-grown sample to nearly 23% when annealed at 400 °C. The as-grown sample showed a higher value of magnetization of 530 emu/cm 3, which dropped to 20 emu/cm 3 on annealing at 400 °C. The low temperature magnetization behavior of the as-grown Co films showed the presence of both positive and negative exchange bias due to the formation of antiferromagentic domains in parallel with ferromagnetic domains. This behavior is explained using the Malozemoff Random Field Model, which predicted values of exchange bias closely matching the present experimental findings.

Reversible enhancement of the magnetism of ultrathin Co films by H adsorption

By means of ab initio calculations, we have investigated the effect of H adsorption on the structural, electronic, and magnetic properties of ultrathin Co films on Ru(0001). Our calculations predict that H occupies hollow sites preserving the two-dimensional threefold symmetry. The formation of a complete H overlayer leads to a very stable surface with strong H-Co bonds. H tends to suppress surface features, in particular, the enhancement of the magnetic moments of the bare film. The H-induced effects are mostly confined to the Co atoms bonded to H, independent of the H coverage or of the thickness and the structure of the Co film. However, for partial H coverages a significant increase occurs in the magnetic moment for the surface Co atoms not bonded to H, leading to a net enhancement of surface magnetism.

Structure and electronic properties of ultrathin Co films on W(1 1 0)

The structure and electronic properties of ultrathin Co films on W(1 1 0) grown by molecular beam epitaxy in UHV were investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy and spectroscopy (STM and STS). For coverages above 0.7 ML the pseudomorphic (ps) monolayer is transformed gradually into close-packed (cp-) monolayer areas, showing up as separated islands that increase in size with coverage until the cp-monolayer is complete. Two different structures of the cp-monolayer were observed by atomically resolved STM, both leading to a 8 × 1 superstructure in the LEED pattern. Higher coverages continue to grow in the Stransky–Krastanov growth mode forming simultaneously double layer islands and triple layer islands in fcc(1 1 1) and hcp(0 0 0 1) stacking. STS reveals tunneling spectra that differ considerably depending on the thickness and on the structure. Two different classes of triple layer islands can be distinguished by a resonant peak at +0.3 eV appearing in only one of the two classes. We attributed this behavior to a different stacking according to a fcc or hcp structure.

Thickness dependent magnetization canting in Co on Cu (1 1 13)

The magnetic properties of ultrathin Co films grown on Cu (1 1 13) are investigated by means of the magneto-optic Kerr effect. Applying fields within the film-plane, different ellipticities are found parallel and perpendicular to the step edges. This disparity of the magneto-optical signals in the two perpendicular directions results from a canting of the magnetization that causes a mixing of longitudinal and polar Kerr effect. The canting angle of the magnetization with respect to the surface-plane is calculated. The dependence of the magnetization canting on film thickness is studied systematically.

Magnetic properties of ultrathin Co films on Si (111)

Ultrathin cobalt films on clean (7×7) and Au covered Si (111) substrates were prepared by molecular beam epitaxy. The structure was studied by using scanning tunnelling microscopy and low energy electron diffraction. Magnetic properties were determined with the magneto-optic Kerr effect. It was found that Co nucleates in grains that prefer to grow along the bunched step edges of the Si substrate ([112̄] direction), which induces a strong in-plane uniaxial anisotropy. By introducing Au buffer layers, the magnetic characteristics were improved by preventing the silicide reaction between Si and Co. Moreover, the tendency for step decoration disappears gradually results in the in-plane uniaxial anisotropy reduction.

Correlation of morphology and magnetic properties in ultrathin epitaxial Co films on Au( [formula omitted

Structural and magnetic properties of ultrathin Co films were studied by scanning tunneling microscopy and magneto-optics. Hundred nm Au(1 1 1) films grown epitaxially on mica were used as buffer layers. The initial Co nucleation at the elbows of the “herringbone” reconstruction determined the film morphology, which displayed long chains resulting from the coalescence of the self-organized Co nuclei. A height amplitude of the chains changed considerably upon film annealing. For the ex situ magnetic measurements, wedge samples were prepared, protected with a 5 nm Au layer. Using Kerr magnetometry it was possible to observe the spin reorientation transition. It turned out that the critical thickness decreases for annealed films. The magnetic surface anisotropy was determined and correlated with the film microstructure.

Magnetic studies of ultrathin cobalt layers on a Ge(1 0 0) surface

We present the magnetic properties of ultrathin Co films deposited at 120 and 300 K on a Ge(1 0 0) surface. As deposited at 120 K, coercive force increases monotonically from 30 to 70 Oe as the coverage of the cobalt film increases from 8 to 28 monolayers (ML). No Kerr intensity has been observed for samples with Co thickness below 5 ML. However, this nonmagnetic Co layer increases to 12 ML as the substrate temperature increases to 300 K, and the coercive force is roughly 15±5 Oe lower than the sample with the same thickness deposited at 120 K. This could be attributed to the thermal annealing effect. From the linear extrapolation of the Kerr intensity below 28 ML, the nonmagnetic behavior of the initial deposit (∼5 ML) is attributed to the formation of nonmagnetic Co–Ge compound at the interface.

Growth and magnetic properties of ultrathin Co films on Pd(111) investigated by ultrahigh vacuum in situ surface magneto-optical Kerr effect and scanning tunneling microscope

Growth and magnetic properties of Co ultrathin films on Pd(111) surface are studied with a ultrahigh vacuum scanning tunneling microscope (STM) and in situ surface magneto-optical Kerr effect measurement as a function of Co thickness. STM images of Co-covered surface reveal that the growth proceeds as a good layer-by-layer epitaxy up to a thickness of 2 ML. Image at a coverage of 1 ML shows almost complete monolayer coverage instead of disconnected islands. At higher coverages, the surface shows uniformly distributed three-dimensional (3D) clusters of ∼5 nm size manifesting 3D-like growth mode. Co films begin to show ferromagnetism at a coverage of 1.5 ML with a strong perpendicular magnetic anisotropy, which persists up to 5 ML. For the Co films thicker than 5 ML, reorientation transition from perpendicular to in-plane magnetic anisotropy is observed to occur over a thickness range of ∼1.5 ML. Interestingly, in this thickness range, Pd overlayer is found to drastically change the magnetic behavior.

Surfactant action in heteroepitaxy: Growth of Co on(4×4)Pb/Cu(111)studied by LEED and STM

The magnetic properties of ultrathin Co films and Co/Cu heterostructures grown on Cu(111) have been shown to improve when Pb is used as a surfactant: the thickness range of Co films displaying perpendicular magnetic anisotropy is extended and a complete antiferromagnetic coupling between them is made possible. In this paper, we aim to understand the origin of these magnetic properties by illuminating the crystallographic structure and morphology of the films. We apply low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to ultrathin epitaxial Co films grown on the Cu(111) surface precovered by 1 monolayer (ML) of Pb in the $(4\ifmmode\times\else\texttimes\fi{}4)$ superstructure. The Pb layer is found to segregate to the surface of the growing films and STM images show that the growth of Co proceeds in the layer-by-layer mode for coverages higher than 2 ML. For lower coverages, a coexistence of 1 ML and 2 ML high islands is observed on the terraces. A quantitative comparison of experimental LEED spectra shows that the structural transition from fcc to hcp stacking with increasing thickness of the Co films deposited on the Cu(111) substrate is not significantly affected by the surfactant. Instead, the latter induces the formation of sharp interfaces that favor the interface contribution to the magnetic anisotropy and allow the growth of rather smooth films with well-defined thicknesses.

Magnetic properties of ultrathin Co films on Si(111) and CoSi2 surfaces

The orientation of the magnetization and the occurrence of interfacial ferromagnetic dead layers for ultrathin Co films on Si(111) and CoSi2 surfaces have been systematically studied using in situ surface magnetic-optic Kerr effect. We have experimentally demonstrated that an in-plane magnetization can be obtained by using CoSi2 as a buffer layer for ultrathin Co films between 2.8 and 10.5 monolayers (MLs) deposited on Si(111) at 300 K. The ferromagnetic dead layers at the interface are most likely due to the formation of a Co–Si alloy. This region can be reduced from 2.1 to 1.4 ML by lowering the substrate temperature from 300 to 120 K. From a dynamic study of the silicide formation in Co/Si(111), a two-step diffusion mechanism is suggested with two different diffusion activation energies of the Co atoms resulting from different chemical environments.

Variations in the magnetic properties of ultrathin Co films due to the adsorption of non-magnetic metal atoms at the Co/vacuum interface

The magnetic properties of ultrathin Co/Cu films are strongly dependent on the nature of the interfaces. Using the magneto-optical Kerr effect, we report drastic effects due to the adsorption of sub-monolayer quantities of non-magnetic metals.

In situ observation of epitaxial growth of Co thin films on Cu(100)

The magnetic and electronic properties of ultrathin Co films have been found to depend sensitively on their growth and structure. Therefore, we studied the evolution of Co films during their deposition onto Cu(100) substrates under UHV conditions by STM. Our STM-design offers a simple way to retrieve particular positions on a sample even after preparations like sputtering or annealing, which require multiple sample transfers to a preparation stage. We have observed the nucleation and growth of individual Co islands during the growth of films from the sub-monolayer regime up to several monolayers. At room temperature, the system exhibits epitaxial growth in a nearly ideal layer-by-layer mode. The density of island nucleation is significantly higher in the growth of the first monolayer, compared with additional layers. The onset of growth of the second layer is well before the completion of the first layer, contrasting the more perfect growth of subsequent layers.