Co ultrathin films, of various thicknesses (0.8 nm ⩽ t _Co ⩽ 2.5 nm), have been grown by sputtering on Si substrates, using Pt buffer layers and different capping layers (Cu, Ir, MgO and Pt). The x-ray diffraction revealed that our films have a (1 1 1) out-of-plane texture with various degrees of strains. Their magnetic properties have been studied by vibrating sample magnetometry (VSM) and Brillouin light scattering (BLS) in the Damon–Eshbach geometry. VSM characterizations revealed that films with Co thickness below (above) the spin reorientation transition thickness, which is capping layer dependent, are perpendicularly (in-plane) magnetized, suggesting the existence of an interface anisotropy. The surface anisotropy constant was found to be 1.42 ± 0.02 erg cm ⁻² and of 1.33 ± 0.02 erg cm ⁻² for the Pt/Co/Cu and Pt/Co/Ir samples, respectively, suggesting that it is due to the Pt/Co interface and that the top Co/Cu, Co/Pt or Co/Ir interfaces have a minor contribution. A lower value of 1.07 ± 0.02 erg cm ⁻² has been obtained for Pt/Co/MgO most probably due to over-oxidation of Co at the Co/MgO interface. The BLS measurements revealed a pronounced nonreciprocal spin waves propagation, due to the interfacial Dzyaloshinskii–Moriya interaction (iDMI) induced by Pt interface with Co, which increases with decreasing Co thickness. The magnetic dead layer has been taken into account to precisely determine the surface iDMI constant D _s estimated at −0.8 pJ m ⁻¹ , −1.05 pJ m ⁻¹ and −0.95 pJ m ⁻¹ , respectively for Pt/Co/Ir, Pt/Co/Cu and Pt/Co/MgO for sample thicknesses where a linear thickness dependence of the effective iDMI constant has been observed.

[Pt(1.5 nm)/Co(t_Co)/W(1.5 nm)]_N multilayers of different Co thicknesses (t_Co) and number of repeats (N) have been grown by sputtering on Si substrates, and their magnetic properties have been studied. The x-ray reflectivity has been used to measure thicknesses of each layer as well as their roughness. The dependence of the magnetic moment on t_Co and N (as determined by vibrating sample magnetometry) indicates the existence of a magnetic dead layer, which increases with N and reaches its maximum values for N ≥ 3. A similar N dependence of the magnetization at saturation is found. Ferromagnetic resonance and Brillouin light scattering have been used to investigate perpendicular magnetic anisotropy, damping, and interfacial Dzyaloshinskii-Moriya interaction (iDMI) vs Co thickness and the number of Pt/Co/W sequence repeats. We show that these parameters result from interface contributions that vary in a similar way with N, confirming that the first two Pt/Co/W trilayers are of lower quality. We thus conclude that for these systems, the increase of N improves the quality of interfaces and the volume of the stack, leading to the enhancement of the magnetic properties. Moreover, the measured weak iDMI constant, even for the higher N values, suggests that most probably, this iDMI results mainly from the Pt/Co interfaces.

We present a comparative study of the magnetic Brillouin spectra of arrays of stripe-structured Co layers and of continuous unpatterned Co films. These spectra mainly differ from each other through the occurrence of an additional line on the low-frequency side for the patterned films. This feature is related to magnetic eigenmodes arising from the uniform bulk mode of the continuous film when taking into account the magnetic anisotropy in the dipolar approximation: the simultaneous presence of magnetic anisotropy and of structuration is needed to allow for light scattering by such pseudobulk modes. In the case of continuous films the spectral profiles that contain lines related to both the Damon-Eshbach and to standing spin waves are quantitatively interpreted. An approach to the magnetic eigenmodes observed in the stripes is developed; it is derived from a calculation that uses the dipolar approximation and includes anisotropy for cylinders with elliptical cross section. Finally, the influence of the demagnetizing field upon the spectra of the patterned layers is evidenced and discussed.

In this work, we present the magneto-ionic response to ionic liquid gating in Ta/CoFeB/MgO/HfO₂ stacks, where heavy metal dusting layers of Ta, W, and Pt are inserted at the Ta/CoFeB and CoFeB/MgO interfaces. Dusting layers of W inserted at the Ta/CoFeB interface increase perpendicular magnetic anisotropy (PMA) by more than 50%, while no significant changes are seen for Pt. In these samples, gating cannot break the PMA seeded at the CoFeB/MgO interface, only relatively small changes in the coercivity can be induced, about 20% for Ta and Pt and 6% for W. At the CoFeB/MgO interface, a significant quenching of the magnetization is seen when W and Ta dusting layers are inserted, which remains unchanged after gating, suggesting a critical deterioration of the CoFeB. In contrast, Pt dusting layers result in an in-plane anisotropy that can be reversibly converted to PMA through magneto-ionic gating while preserving the polycrystalline structure of the MgO layer. This shows that dusting layers can be effectively used not only to engineer magnetic properties in multilayers but also to strongly modify their magneto-ionic performance.

Interface Dzyaloshinskii–Moriya interaction (iDMI) is known to induce spinwaves non-reciprocity in ultrathin films. Indeed, Brillouin light scattering has been used to investigate how the lateral size reduction can affect the iDMI constant in Pt (6 nm)/Co (3 nm) based-nanostripe arrays. For this, 100 and 300 nm-width nanostripes have been fabricated using e-beam lithography and ion etching, and their behaviour has then been compared to the reference continuous film. The experimental data showed that the measured iDMI induced non-reciprocity is slightly different for the 100 nm-width nanostripes with respect to the other samples. This suggests that the width of the nanostripes can influence the strength of the apparent iDMI if this dimension is comparable to the spin waves attenuation length propagating within the nanostripes. Indeed, in contrast to the other samples, the linear frequency difference (non-reciprocity) behaviour versus wavenumber for the 100 nm-width nanostripes has been analysed and discussed through two approaches: either a different iDMI constant or an iDMI constant similar to one of the continuous films with a non-zero intercept for a zero wavenumber.