Anisotropy Of Co Films Research Articles

Strong uniaxial magnetic anisotropy in Co films on highly ordered grating-like nanopatterned Ge surfaces

We present a systematic investigation on uniaxial magnetic anisotropy (UMA) in Co thin films induced by high aspect ratio nanopatterned anisotropic substrates. Self-organized long grating-like nanostructures, with extreme regularities, are fabricated on Ge surfaces using Au-ion implantation at room temperature. Subsequently deposition of Co films are carried out on the same at two different angles. Magneto-optical Kerr effect measurements show strong UMA in Co films grown on ion-patterned Ge substrates, fabricated under different ion fluences, along and perpendicular to the direction of the patterns (long grating-like nanostructures). Magnetic force microscopy measurements under different externally applied magnetic fields reveal an easy domain wall motion when the field is applied along the grating-like nanostructures. On the other hand, high amplitude grating-like nanostructures hinder the spin rotation when the field is applied along the hard axis. The present study will be useful for magnetic recording media and ultra-small magnetic field sensors.

Anatomy and Giant Enhancement of the Perpendicular Magnetic Anisotropy of Cobalt-Graphene Heterostructures.

We report strongly enhanced perpendicular magnetic anisotropy (PMA) of Co films by graphene coating from both first-principles and experiments. Our calculations show that graphene can dramatically boost the surface anisotropy of Co films up to twice the value of its pristine counterpart and can extend the out-of-plane effective anisotropy up to unprecedented thickness of 25 Å. These findings are supported by our experiments on graphene coating on Co films grown on Ir substrate. Furthermore, we report layer-resolved and orbital-hybridization-resolved anisotropy analysis, which help understanding of the physical mechanisms of PMA and more practically can help design structures with giant PMA. As an example, we propose superexchange stabilized Co-graphene heterostructures with a robust constant effective PMA and linearly increasing interfacial anisotropy as a function of film thickness. These findings point toward possibilities to engineer graphene/ferromagnetic metal heterostructures with giant magnetic anisotropy more than 20-times larger compared to conventional multilayers, which constitutes a hallmark for future graphene and traditional spintronic technologies.

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.

Experimental confirmation of quantum oscillations of magnetic anisotropy in Co/Cu(001)

The effect of quantum well states on the magnetic anisotropy of Co films grown on vicinal Cu(001) substrates was studied by in situ magneto-optic Kerr effect in a temperature range of 5 K to 365 K. The uniaxial magnetic anisotropy is found to oscillate as a function of Co thickness with a period of 2.3 atomic layers, modulated exactly as theoretically predicted by L. Szunyogh et al. [Phys. Rev. B 56, 14036 (1997)] and independently by M. Cinal [J. Phys. Condens. Matter 15, 29 (2003)] and M. Cinal and A. Umerski [Phys. Rev. B 73, 184423 (2006)]. Sub-monolayer Au coverage was used to fine-tune magnetic anisotropy and to provide convenient experimental access to anisotropy oscillations. The anisotropy oscillations, which are present up to room temperature, are attributed to quantum well states in the minority $d$ band at the Fermi level of Co.

Surface morphology and magnetic anisotropy of obliquely deposited Co/Si(111) films

We report an investigation on magnetic anisotropy of Co/Si(111) films deposited at oblique incidence. An in-plane uniaxial magnetic anisotropy (UMA) with the easy axis perpendicular to the incident flux plane was observed to superimpose on sixfold magnetocrystalline anisotropy of Co films. We built a total energy model to investigate the magnetization reversal mechanism around hard axis. The simulated value of UMA is Ku=1.7×105 erg/cm3⁠, which is consistent with Kshape=1.1×105 erg/cm3 calculated from scanning tunneling microscope image. This good agreement suggests the in-plane UMA is mainly originated from the shape of the oblique deposited Co stripes.

Magnetic Anisotropy of Co Films Annealed by Laser Pulses

The magnetic properties of an ultrathin cobalt film were modified by a focused femtosecond pulsed laser beam. The Co wedge, with a thickness ranging from 0 to 2 nm, sandwiched by Au films was prepared using ultra-high vacuum magnetron sputtering on a mica substrate. The modifications of the laser induced magnetic anisotropy were investigated using magneto-optic Kerr microscopy and MFM/AFM techniques. The laser induces: (i) local reorientation of magnetization from an in-plane to a perpendicular state and (ii) an increase of the coercivity field. A corresponding increase of the perpendicular magnetic anisotropy is discussed considering an improvement of the Co/Au interfaces.

Spin-resolved photoemission and electronic structures of magnetic thin films: Perpendicular magnetic anisotropy of Co films

We have investigated the electronic and magnetic properties of Co thin films epitaxially grown on Au(111) substrate by spin- and angle-resolved photoelectron spectroscopy. At Co/Au(111) interface, we observed strong mixing between Au 5d and Co 3d states resulting in the spin polarization of Au 5d states and the increase in Co 3d orbital parallel to the surface, which causes the perpendicular magnetic anisotropy of Co films. The electronic structure of Co thin film in low coverage region is different from that of bulk hcp Co and it becomes closer to those of the bulk as film thickness increases. Reorientation of the magnetization direction was observed at about 6 ML, the origin of which is qualitatively explained by the increasing contribution of Co 3d orbitals perpendicular to the surface.

Electronic structure and magnetic anisotropy of Co/Au(111): A spin-resolved photoelectron spectroscopy study

We have measured spin- and angle-resolved photoemission spectra of Co thin films epitaxially grown on the Au(111) substrate and investigated their electronic and magnetic properties. At the Co/Au(111) interface, we observed strong mixing between $\mathrm{Au}5d$ and $\mathrm{Co}3d$ states resulting in the spin polarization of $\mathrm{Au}5d$ states and the increase of $\mathrm{Co}3d$ orbitals parallel to the surface, which causes the perpendicular magnetic anisotropy of Co films. The electronic structure of Co thin films in a low-coverage region is different from that of bulk hcp Co and it becomes closer to those of the bulk as film thickness increases. The reorientation of the magnetization direction was observed at about 6 ML, the origin of which is qualitatively explained by the relaxation of the atomic distance of Co perpendicular to the surface.

Magnetic anisotropy oscillations (invited)

The magnetic anisotropy in Co films epitaxially grown on Cu(001) is investigated by measuring magneto-optical Kerr hysteresis loops during film growth. As a function of Co thickness the magnetic anisotropy oscillates with a period of 1 monolayer. These oscillations are attributed to the periodic variations of the film morphology alternating between filled and incompletely filled atomic layers. Upon coverage of the Co films with Cu overlayers the anisotropy of the magnetic film is found to oscillate with increasing Cu layer thickness. We relate these oscillations to quantum interferences due to the confinement of electrons in ultrathin films.

Oscillatory magnetic anisotropy and quantum well states in Cu/Co/Cu(100) films.

The magnetic anisotropy of Co films grown on stepped Cu(100) substrates is found to oscillate with varying Cu overlayer thickness. We relate these oscillations to the occurrence of quantum well states in the Cu overlayer.

Morphology-induced oscillations of the magnetic anisotropy in ultrathin Co films.

The magnetic anisotropy in Co films epitaxially grown on Cu(100) is found to oscillate as a function of the Co thickness. The oscillation period corresponds to one monolayer, as revealed by measuring magneto-optical Kerr hysteresis loops during film growth. These oscillations are attributed to the periodic variations of the film morphology alternating between filled and incompletely filled atomic layers.