P. Beauvillain

Hysteresis properties of ultrathin ferromagnetic films

We have investigated the hysteresis properties of Au/Co/Au films with ultralow Co thicknesses and perpendicular easy axis. At low temperature we observe a very strong thickness dependence of the coercivity, whereas striking dynamical effects are present at room temperature. We propose a model of wall motion which provides a consistent explanation of the overall observations. This interpretation emphasizes the crucial role played by the roughness with respect to the hysteresis properties of ferromagnetic ultrathin films.We have investigated the hysteresis properties of Au/Co/Au films with ultralow Co thicknesses and perpendicular easy axis. At low temperature we observe a very strong thickness dependence of the coercivity, whereas striking dynamical effects are present at room temperature. We propose a model of wall motion which provides a consistent explanation of the overall observations. This interpretation emphasizes the crucial role played by the roughness with respect to the hysteresis properties of ferromagnetic ultrathin films.

Very large magnetoresistance effects induced by antiparallel magnetization in two ultrathin cobalt films

Magnetoresistance (MR) and magnetization measurements by SQUID have been performed on Au/Co.Au/Co/Au multilayers in which the two ultrathin cobalt films have perpendicular magnetizations and different coercive fields Hc1 and Hc2 due to their different thicknesses. The two Co layers are magnetically uncoupled, as shown by their well separated square hysteresis loops. The MR exhibits a plateau for magnetic fields H such as Hc1<H<Hc2, giving antiparallel orientation of their magnetization. The δR/R value is 1.4% at 300 K and 6% at 4.2 K. These experiments confirm the mechanism of spin‐dependent diffusion at the interfaces leading to an increase of the resistance when the two magnetic layers have antiparallel magnetizations.

Magneto‐optical studies of Co/Au ultrathin metallic films

We report on static and dynamic magneto‐optical measurements of the magnetization in ultrathin cobalt films deposited on Au (111). Their large Faraday rotation associated to the squareness of their magnetic hysteresis loop makes these samples very promising as magneto‐optical storage media.

Effect of submonolayer coverage on magnetic anisotropy of ultrathin cobalt films M/Co/Au(111) with M=Au, Cu, Pd

Using in situ polar magneto‐optical Kerr‐effect measurements, the variation of the Kerr rotation and magnetic anisotropy of a (0001)Co ultrathin film on a (111)Au substrate has been precisely recorded, during the first stages of the growth of an overlayer of metal M=Au, Cu, and Pd. As reported earlier, a drastic increase of the magnetic anisotropy was observed, with a peak around 1 monolayer of overlayer thickness. From a careful study of the variation of the remanent Kerr rotation, it could be shown that only the interface contribution to the anisotropy changes with the overlayer thickness, while the bulk contribution remains mostly constant. The overall behavior can be interpreted in terms of electronic effects in the metal overlayer, acting on the interface anisotropy via band hybridization at the interface.

Large magnetoresistance effects in UHV grown fcc (111) Co/Cu multilayers

Abstract Co/Cu multilayers have been grown in ultrahigh vacuum on Au(111) buffer layers. As shown by X-ray diffraction and 59 Co NMR, their structure is fcc (111) with atomically flat interfaces. The studied samples with Cu thicknesses of 3, 5 and 10 atomic monolayers exhibit a large magnetoresistance (MR). This result and the large saturation field observed by MR and by magnetization measurements could be indicative of antiferromagnetic interlayer couplings, previously reported for sputtered Co/Cu multilayers with comparable Cu thickness.

Unidirectional band gaps in uniformly magnetized two-dimensional magnetophotonic crystals

By exploiting the concepts of magnetic group theory we show how unidirectional behavior can be obtained in two-dimensional magnetophotonic crystals (MOPhC) with uniform magnetization. This group theory approach generalizes all previous investigations of one-way MOPhCs including those based on the use of antiparallel magnetic domains in the elementary crystal cell. Here, the theoretical approach is illustrated for one MOPhC example where unidirectional behavior is obtained by appropriately lowering the geometrical symmetry of the elementary motifs. One-way transmission is numerically demonstrated for a photonic crystal slice.

CALIBRATION OF AN EYE-MOVEMENT SYSTEM FOR USE IN READING

A method for measuring horizontal eye movements in the msec range is described. Accurate measurement of horizontal eye movement over a linear range of 12° is achieved by processing the image of the eye illuminated with infrared light and with the head position fixed. The system has given very reliable results, and a resolution of 6 min of visual angle can be achieved with a character space of 45 min of arc. We also describe efficient numerical-data processing which allows the precise determination of the absolute position of the eye.

Magnetic and structural properties of rf‐sputtered Co/Fe and Co/Cr multilayers

Multilayers alternating cobalt with different spacers such as iron and chromium at nanometric scale have been deposited by rf diode sputtering. The structures have been characterized in situ by kinetic ellipsometry and ex situ by grazing x‐ray reflection, x‐ray diffraction, Auger profile analysis, and transmission electron microscopy. Nuclear magnetic resonance (NMR) and high‐field SQUID magnetometry have been used to determine the magnetic properties. In Co/Fe multilayers the structure strongly depends on the cobalt thickness tCo. For tCo below 2 nm, the cobalt layers exhibit a bcc crystalline structure, and the crystalline coherence extends over many periods. For tCo above 2 nm, for Co a mixed fcc‐hcp phase has been observed. In this case the crystalline coherence is destroyed from one iron layer to the other. The NMR frequency (198 MHz), and the magnetic moment (1.58 μB/atom) of bcc cobalt have been evaluated. A similar behavior is observed in Co/Cr multilayers, the bcc phase disappearing around tCo=1....

Light diffraction effects in the magneto-optical properties of 2D arrays of magnetic dots of Au/Co/Au(111) films with perpendicular magnetic anisotropy

Using X-ray lithography and ion beam etching we have patterned micrometer-sized square lattices of round dots, out of Au/Co/Au(111) films with perpendicular easy magnetization axes. The magneto-optical Kerr rotation shows strong variations versus the diffraction order, the light polarization direction and the ratio of film to background reflectance, particularly when the lattice spacing is high compared with the dot size. This reveals an important interplay between diffraction and magneto-optics.

Domains and magnetization reversal dynamics in Au/Co/Au (1 1 1) ultrathin films: Co and Au thickness dependence

Abstract The magnetic anisotropy of epitaxial ultrathin Co-films on Au(1 1 1) depends drastically on Co- or Au-overlayer thickness. We have used polar magneto-optical Kerr effect on stepped-wedge samples to perform ex situ domain imaging, together with in situ and ex situ magnetic aftereffect measurements. Domains configurations and magnetization reversal dynamics were thus investigated versus perpendicular magnetic anisotropy strength. Large changes with Co thickness in Au/Co/Au(1 1 1) sandwiches are observed; they can be interpreted using existing theories. The drastic evolution of the dynamics of the magnetization reversal process with submonolayer Au coverage is attributed to a structural reorganization of the Co-layer with Au deposition, due to surfactant effect.