D. Renard

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.

Magnetization reversal in ultrathin ferromagnetic films with perpendicular anisotropy

Abstract This paper presents a detailed study of the magnetization reversal dynamics in ultrathin cobalt films ( t Co = 6–12 A ) sandwiched by gold (1 1 1) layers, their magnetic anisotropy being perpendicular to the film surface. The domain wall (DW) velocity and nucleation rate are determined from direct time-resolved domain structure imaging. Measurements as a function of the applied magnetic field and temperature enable us to estimate the magnetic parameters, such as activation volumes, coercive fields, Gilbert damping parameters, etc., controlling the magnetization reversal. Depending upon the applied magnetic field value three different regimes are evidenced and studied. At low fields the DW pinning by structural inhomogeneities controls the thermally activated magnetization reversal dynamics. Above the propagation field value the dynamics is due to viscous DW motion. In higher fields a DW velocity breakdown is observed. The DW jaggedness and its change with the applied field and temperature is examined through the DW fractal dimension. A simple DW motion simulation taking into account an activation volume distribution explains qualitatively the data.

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.

Magneto-optical effects in Au/Co/Au ultrathin film sandwiches

Abstract This paper deals with the study of magneto-optical effects exhibited in perfect Au/Co/Au ultrathin films structures as a function of magnetic field and photon energy. They stand as ideal basic structures to test theories describing the magneto-optical effects in multilayers. In both optical transmission and polar Kerr rotation dispersion spectra, the main feature is a prominent peak centered near the photon energy of 2.5 eV, which corresponds to the plasma edge and reflectivity minimum in gold. The observed trends in magneto-optical effects are explained using the electromagnetic wave theory starting from the published optical and magneto-optical constants values in gold and cobalt thick films. The polar Kerr effect in our samples is a sum of one independent plus one linearly dependent term on the Co film thickness. The nonzero value of the thickness-independent term indicates the presence of a contribution not accounted for by the employed electromagnetic wave model.

Magneto-optical anisometry of ultrathin cobalt films

The advantage of magneto‐optics to measure magnetic anisotropy constants in ultrathin films exhibiting perpendicular spin anisotropy is reported. This is illustrated for Au/Co/Au sandwiches with Co layer thickness ranging from 8 to 15 A. We stress the importance of preliminary measurements always necessary to define the (H∥,H⊥) region in which a homogeneous spin rotation occurs. These results confirm the existence of interface anisotropy and lead to its estimation Ks=0.56 erg/cm2 whereas the volumic term is shown to be Kv=7.5×106 erg/cm3.

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.

Domain wall dynamics in ultrathin Au/Co/Au films

Abstract We report on the first direct measurements of the domain wall speed and nucleation rate in ultrathin magnetic films with perpendicular anistropy. Nucleation process becomes dominating over the domain wall motion in higher fields, leading to changes in relaxation magnetization curve. The influence of the sample thickness is briefly discussed.

Magnetooptic depth sensitivity in a simple ultrathin film structure

Abstract Polar magnetooptics (MO) is able to probe the magnetization depth profile in ultrathin magnetic film structures on a nanometer scale. This information can be deduced by modelling the MO effects in the considered layered medium when performing MO experiments with a compensator or changing the wavelength of the incident light beam. Spectroscopic MO measurements are then helpful for analysing this phenomenon in detail. The in-depth selectivity of MO effects for magnetization is demonstrated for a simple ultrathin film structure consisting of two magnetic Co layers with perpendicular anisotropy separated by a non-magnetic Au spacer layer. The individual magnetic contributions of the two Co layers may be observed directly when performing MO Kerr measurements at selected compensator phase shifts or photon energies. The experimental data are interpreted by MO calculations in both cases.

Magnetization, coercive forces and magnetoresistance in simple and double Co films with perpendicular magnetization

Abstract Our Au(111)/Co/Au samples show perpendicular spontaneous magnetization for Co thicknesses t below t ∗ = 1.2 nm. Recent RHEED results are in favour of the existence of an important “strain-induced” anisotropy, helping to overcome the shape anisotropy. From the measured parameters of anisotropy and roughness, we propose a quantitative model that explains the fast variation of the perpendicular coercive force with t below t c . Finally, disymmetric Au/Co( t 1 )/Au/Co( t 2 )/Au bilayers ( t 1 and t 2 t c ) show enhanced magnetoresistance (5%) in the field range whe re the layers are magnetized antiparallel.