V. Mathet

Chemical order induced by ion irradiation in FePt (001) films

We demonstrate that the long-range order parameter S of sputtered FePt (001) films may be improved by using postgrowth He ion irradiation. This was demonstrated both on disordered (S∼0) and partially ordered (S∼0.4) films in which S was increased up to 0.3 and 0.6, respectively. X-ray diffraction analysis showed that these changes are due to irradiation-induced chemical ordering. The changes in the magnetic hysteresis loops correlate with the expected perpendicular magnetic anisotropy increase. This method may find applications in ultrahigh-density magnetic recording.

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.

Chemical order induced by He+ ion irradiation in FePt (001) films

We show that partial chemical ordering can be obtained at moderate processing temperatures by using postgrowth ion bombardment. This was demonstrated both on disordered (S∼0) and partially ordered (S∼0.4) sputtered FePt(001) films in which the order parameter S was increased up to 0.3 and 0.6, respectively. X-ray diffraction measurements indicate that the high crystalline quality of the films is not modified as irradiation proceeds. The changes on the magnetic hysteresis loops are compatible with the expected perpendicular magnetic anisotropy increase. This novel method could have a great impact on the current race toward high anisotropy materials to increase magnetic recording density.

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.

Influence of geometry on domain wall propagation in a mesoscopic wire

The propagation of a single magnetic Bloch domain wall (DW) in a mesoscopic wire is studied by use of the extraordinary Hall effect. Whereas a straight DW propagates freely in a wire with constant lateral width, the propagation in a Hall cross reveals that both DW shape and DW velocity vary strongly as a function of DW position. These results give the first experimental evidence that the dynamical properties of a magnetic DW in a patterned structure are strongly affected by the device geometry. This has to be taken into account for future applications to spin electronic devices.

Stripe domains morphology versus layers thickness in CoPt multilayers

The domain morphology in CoPt multilayers has been investigated by means of magnetic force microscopy for a broad range of thickness parameters. Following hard-axis saturation, complex closure domains, isolated reversed domains, and segmented and fully developed stripe patterns are clearly identified down to stripe widths of the order of 50 nm. Pending full numerical simulations, the stripe width behavior versus Co and Pt thicknesses has been confronted to the predictions of a purely magnetostatic model. The data are consistent with the existence of a single and physically realistic characteristic length, which, however, is viewed as mostly casual. Likely reasons are discussed.

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...

Spectroscopic polar magneto-optical Kerr rotation on Co/Pt interface

Spectroscopic magneto-optic (MO) Kerr rotation experiments have been performed on Pt/Co interfaces. This very sensitive experimental technique allows a precise chemical analysis of this interface. From these measurements, we deduced that the Co/Pt interface is formed of a disordered CoPt alloy layer and a spin polarized platinum atomic monolayer whose MO indices were calculated. In situ spectroscopic MO experiments have been performed during the Pt growth on the hcp Co (0001) layer. In situ postdeposition annealing were done on Pt/Co/Pt trilayers. In the 400–500 °C temperature range, the MO spectra show an important increase of the Kerr rotation, in particular around 370 nm, while the magnetic anisotropy goes through a maximum at 500 °C. These results are related to the formation of a CoPt alloy ordered along the (111) direction.