S.-M. Chérif

Structural and magnetic properties of evaporated Co/Si(100) and Co/glass thin films

A series of Co thin films have been evaporated onto Si(100) and glass substrates. The Co thickness, tCo, ranges from 50 to 195 nm. The structural and magnetic properties have been investigated by x-ray diffraction, hysteresis curves, Brillouin light scattering and magnetic force microscopy (MFM) techniques. The Co thin films are found to be polycrystalline with (0001) texture. There is an increase of the grain size with increasing film thickness. The coercive fields range from values as low as 2 Oe in thinner films to the highest values, 2500 Oe, in 195 nm thick Co/Si films. The magnetocrystalline anisotropy field Ha decreases as the thickness increases; surface and stress induced anisotropies seem to contribute to the value of Ha. MFM images reveal a well-defined stripe pattern for thicker Co/Si samples. Such domains are not observed in the case of the thinner films. These so-called weak-stripe domains appear in magnetic films with a low or intermediate perpendicular anisotropy. Similar behaviour was observed in Co/glass samples, in addition, cross-tie walls were seen in thinner ones.

Shape anisotropy in arrays of micrometric magnetic stripes: a Brillouin study

The uniaxial in-plane anisotropy of patterned arrays of micrometric magnetic stripes is observed through their magnetic Brillouin spectra: the analysis is based on the frequency variations and on the Stokes/anti-Stokes dissymmetry versus an external applied magnetic field H. The saturation field Hs is measured for H along the hard axis (Hs = 200 Oe with stripes of thickness 29 nm and of width 1 µm). For H along the easy axis a hysteresis loop is observed, allowing us to derive a coercive field approximately equal to 150 Oe for this array of stripes. A simple model provides a satisfactory interpretation: it stipulates that the anisotropy energy simply consists of a demagnetizing term related to an average demagnetizing field. The experimental determination lies within the interval provided by a panel of different plausible approximations.

Non-conventional interaction contributions in permalloy/NiO composite thin films deduced from their static and dynamic magnetization behavior

A comparative study of thin permalloy films (10.4?nm) interfaced with NiO layers of various thicknesses (6 to 47?nm) is presented. The magnetic parameters deduced from different experimental techniques (Brillouin spectroscopy, ferromagnetic resonance and magnetometric (vibrating sample magnetometry and magneto-optical Kerr effect) investigations) show original characteristics which have not been previously evidenced: (i) for most of the samples studied, the exchange bias field is not parallel or perpendicular to the cooling field, in contrast with the in-plane anisotropy field; (ii) the magnetic properties under an out-of-plane applied magnetic field H do not match the calculated ones derived from the usual expression for the density of magnetic energy. We introduce a phenomenological additional term proportional to cos[?] where ? is the angle between H and the direction normal to the sample: this non-conventional interaction between NiO and permalloy layers allows us to fit all the experimental data.

Investigation of the interfacial Dzyaloshinskii-Moriya interaction sign in Ir/Co2FeAl systems by Brillouin light scattering

Co2FeAl (CFA) ultrathin films, of various thicknesses (0.9 nm<tCFA<1.8 nm), have been grown by sputtering on Si substrates, using Ir as a buffer layer. The magnetic properties of the structures have been studied by vibrating sample magnetometry (VSM), miscrostrip ferromagnetic resonance (MS-FMR) and Brillouin light scattering (BLS) in the Damon-Eshbach geometry. VSM characterizations show that films are mostly in-plane magnetized and the perpendicular saturating field increases with decreasing CFA thickness suggesting the existence of interface anisotropy. The presence of magnetic dead layers of 0.44 nm has been detected by VSM. The MS-FMR with perpendicular applied magnetic field has been used to determine the gyromagnetic factor. The BLS measurements reveal a pronounced nonreciprocal spin waves propagation, due to the interfacial Dzyaloshinskii-Moriya interaction (DMI) induced by Ir interface with CFA, which increases with decreasing CFA thickness. The DMI sign has been found to be the same (negative) as that of Pt/Co, in contrast to the ab-initio calculation on Ir/Co. The thickness dependence of the effective DMI constant shows the existence of two regimes similarly to that of the perpendicular anisotropy constant. The DMI constant Ds was estimated to be -0.37 pJ/m for the thickest samples where a linear thickness dependence of the effective DMI constant has been observed.