J.-P. Jamet

Creep and flow regimes of magnetic domain-wall motion in ultrathin Pt/Co/Pt films with perpendicular anisotropy.

We report on magnetic domain-wall velocity measurements in ultrathin Pt/Co(0.5-0.8 nm)/Pt films with perpendicular anisotropy over a large range of applied magnetic fields. The complete velocity-field characteristics are obtained, enabling an examination of the transition between thermally activated creep and viscous flow: motion regimes predicted from general theories for driven elastic interfaces in weakly disordered media. The dissipation limited flow regime is found to be consistent with precessional domain-wall motion, analysis of which yields values for the damping parameter, alpha.

Sub-50 nm planar magnetic nanostructures fabricated by ion irradiation

He+ ion irradiation of Co–Pt multilayers through a silica mask obtained by a combination of high resolution lithography and reactive ion etching can produce an optical contrast-free, entirely planar, sub-50 nm magnetically patterned array. Furthermore, the specificity of magnetization reversal in such arrays leads to a weak dispersion of coercive forces. The technique holds promise for both present hard disk technology and future near field magneto-optical recording.

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.

Modification of Co/Pt multilayers by gallium irradiation - Part 1: The effect on structural and magnetic properties

Atomic force microscopy, transmission electron microscopy, optical, and magneto-optical microscopy have been used to study how structural and magnetic properties are changed when a Co/Pt multilayer is quasihomogeneously irradiated with Ga ions. Under low irradiation fluence, both grain size and texture in the multilayer increase. These effects continue for fluences in excess of 1×1015 Ga ions/cm2, but beyond this dose significant thinning of the multilayer is also observed. Three distinct irradiation-induced magnetic regimes with sharp transitions between each were identified. For Ga fluences less than 5×1012 ions/cm2, the irradiated region retains perpendicular uniaxial anisotropy but with coercivity lower than that of the as-grown film. For fluences between 5×1012 and 1×1013 Ga ions/cm2, a transition from perpendicular to in-plane magnetization was experienced. Very little change of the in-plane magnetic properties of irradiated multilayers is then observed until the sample experiences a ferromagnetic t...

Irradiation induced effects on magnetic properties of Pt/Co/Pt ultrathin films

Abstract He+ ion irradiation-induced interface mixing modifies the magnetic properties of Pt/Co/Pt sandwiches. A strong decrease in magnetic anisotrophy (hence of the coercivity and Curie temperature) can be controlled without significant changes in sample roughness or optical properties. This opens exciting possibilities to develop a planar technology to pattern the magnetic properties of multilayers by irradiation through a lithographically defined mask.

Magnetically textured γ-Fe2O3 nanoparticles in a silica gel matrix: Structural and magnetic properties

This paper is devoted to magnetic and structural properties of anisotropic γ-Fe2O3 superparamagnetic particles dispersed in a transparent xerogel matrix. The effect of frozen anisotropy axes and magnetic texture, induced by a magnetic field applied during the solidification of the matrix on the in-field magnetization process, is studied by alternating gradient force magnetometry and first and second order magneto-optical effects. The changes of magnetization curves with respect to the ferrofluid solution at the same particle concentration are interpreted on the basis of an existing statistical approach extended to systems with particle size distribution, which has to be taken into account for real samples. A very good agreement between the experiment and theory was achieved for a log-normal distribution of diameters which well resembles that deduced from electron microscopy observations in different imaging modes. This structural analysis states the parameter values used in calculations and confirms the r...

Modifications of magnetic properties of Pt/Co/Pt thin layers by focused gallium ion beam irradiation

We show how the magnetic properties of the Pt/Co ultrathin film structure can be modified and even controlled under uniform irradiation by Ga+ ions at low fluence in the 20–100 keV range. A systematic magneto-optical study is presented for the Pt/Co(1.4 nm)/Pt(111) ultrathin-film structure. At ion fluences below D=1014 Ga+/cm2, the coercive field is steadily reduced when increasing the fluence. At large fluences, in the range D=(5–10)×1014 Ga+ ions/cm2, the magnetization of the Co layer drops rapidly and the film finally becomes paramagnetic at room temperature for D>2×1015 Ga+ ions/cm2. We demonstrate that these magnetic changes are related to the effect of ion-induced collisional intermixing of the Co/Pt interfaces, leading to the formation of stable Co–Pt alloys with varying composition across the interfaces. A simple model is derived to relate the ion beam-induced mixing to the changes in magnetic properties. The present work allows us to gain a quantitative understanding of previous experiments using...

Modification of Co/Pt multilayers by gallium irradiation—Part 2: The effect of patterning using a highly focused ion beam

The local and collective behavior of magnetic arrays fabricated by focused ion beam (FIB) patterning of a Co/Pt multilayer is described. The arrays comprised 1 μm nonirradiated square elements separated by narrow lines which were written using the FIB. While the square elements supported perpendicular magnetization, the ion fluence used to write the lines was chosen to make the local magnetization there lie in-plane. Lorentz microscopy showed that lines were approximately 60 nm wide and that the magnetization had the expected orientation. Application of fields perpendicular and parallel to the array showed that the magnetization in the square elements and in the lines could be controlled essentially independently of each other. Magneto-optic microscopy was used to study the behavior of the arrays as a whole. Frustrated checkerboard patterns were observed, whose detailed properties depended to an extent on the fluence used to write the lines.

Magnetization reversal in irradiation-fabricated nanostructures

We study the magnetization reversal mechanisms of buried submicron magnetic nanostructures fabricated by ion irradiation through a patterned mask. By means of 30 keV He+ ion irradiation of Co–Pt multilayers, we have produced a nearly optical contrast-free, planar array of magnetically hard lines embedded in a softer matrix. Magnetization reversal in these nanostructures exhibits specific features: Low field nucleation centers and preferred domain wall propagation paths are located at the borders between irradiated and nonirradiated areas. The magnetization reversal dynamics is limited everywhere by domain wall motion, ensuring a relatively weak spread of coercive forces. In contrast with usual magnetic nanostructures, the coercivity of irradiation-fabricated lines decreases when reducing their size. Modeling of the irradiation process when performed through a mask indicates that in such a geometry, the irradiation effects are not homogeneous. It arises from collateral damage around the mask. An irradiatio...

MAGNETIZATION REVERSAL IN PATTERNED CO(0001) ULTRATHIN FILMS WITH PERPENDICULAR MAGNETIC ANISOTROPY

Using x‐ray lithography we have patterned dot arrays in Au/Co/Au(111) sandwiches based on ultrathin Co layers with perpendicular anisotropy. Large area arrays of dots with diameters of 1 and 2 μm have been obtained, keeping mostly undamaged the ultrathin Co layer. Hysteresis loops of the arrays depend drastically on the dot diameter. Magneto‐optical domain visualization experiments confirm a magnetization reversal mechanism based on a large distribution of nucleation fields in the film, with complete reversal of the magnetization of one dot through the domain wall propagation after a local nucleation process. This could give information on the magnetization reversal processes in Au/Co/Au(111) continuous films.