Investigation of the thermal stability of Mg/Co periodic multilayers for EUV applications

Applied Physics A - We present the results of the characterization of Mg/Co periodic multilayers and their thermal stability for the EUV range. The annealing study is performed up to a temperature...     

Proximity effect in Nb/Zr multilayers with variable Nb/Zr ratio

We have investigated the effect of varying the individual layer thickness on the superconducting transition temperature (T_C) of Nb/Zr multilayers. These thin film multilayer structures were deposited using UHV magnetron sputtering with layer thickness ranging from 0.5 to 8 nm. In conformity with the predictions of the de Gennes equations in the Cooper limit (layer thickness small compared to coherence length), we find that the T_C increases with increasing thickness of the Nb layer (when the Zr layer thickness is constant), and decreases with increasing thickness of the Zr layer (when the Nb layer thickness is constant). The possible effect of the existence of an interfacial Nb-Zr layer is discussed. We also point out the marked influence of the in-plane grain dimension on the T_C in these multilayers.     

Theoretical and experimental study of Fe/Cr nanometric quasiperiodic multilayers

An experimental and theoretical study of magnetization curves of Fe/Cr nanometric magnetic films grown with the structure of the quasiperiodic Fibonacci sequence is presented. Fe ferromagnetic films with interfilm exchange coupling provided by intervening Cr non-ferromagnetic layers were grown on MgO (100) by dc magnetron sputtering at 300 ^°C. The magnetization curves were investigated using the magneto-optical Kerr effect with the external field applied along the easy axis. The theoretical approach for this system is based on a realistic phenomenological model that includes the following contributions to free magnetic energy: Zeeman, cubic magneto-crystalline anisotropy, as well as bilinear and biquadratic exchange energies. Our numerical results are in very good agreement with the experimental data.     

Ion-beam-mixing induced amorphization of Fe/Zr multilayers

The Ar-ion-beam-mixing of the Fe/Zr multilayers is studied by conversion electron Mössbauer spectroscopy. The dependence of the amorphization process on ion dose is studied in detail for two sample thicknesses with an Fe to Zr ratio of 1 and modulation wavelength of 20 and 60 nm. Experimental results are compared with the predictions of the ballistic cascade and thermal spike models of mixing.     

Giant magnetoresistance of Co/ITO multilayers

The electrical and magnetic properties of Co/ITO multilayers with various ITO layer thickness are studied. Negative giant magnetoresistance with a maximum of −1.9% at room temperature and −2.57% at 15 K are observed. The magnitudes of GMR oscillate with a period of about 1 nm when varying the thickness of ITO layer.     

Magnetoresistive remanence curves in Co/Cu multilayers

We report measurements of magnetoresistive remanence curves and initial magnetoresistance curves on Co/Cu multilayers. The initial magnetoresistance curve shows an enhance magnetoresistance change over that of the hysteretic magnetoresistance curve. Using remanence curves we are able to separate out the irreversible and reversible components contributing to the magnetoresistance. We show that it is irreversible changes of resistance within the multilayers from an initial low energy demagnetised state that is responsible for this ‘extra’ magnetoresistance.     

Perpendicular exchange bias of Co/Pt multilayers

Exchange bias measurements of ferromagnetic/antiferromagnetic (F/AF) bilayers are typically performed with the magnetization of the F layer parallel to the AF interface. We describe measurements of Co/Pt multilayers with out-of-plane magnetic easy axis that are exchange biased with CoO. Field-cooling experiments with the applied field perpendicular and parallel to the sample plane exhibit loop shifts and enhanced coercivities. Modeling and comparison to biasing of samples with planar easy axis suggests such measurements provide a way to probe the spin projections at F/AF interfaces.     

Interfaces and solid state reactions of Zr/Fe multilayers

Metallic multilayers of Fe and Zr, prepared by electron-beam evaporation and sputter deposition, have been characterized by conversion electron Mössbauer spectroscopy and x-ray diffraction. Amorphous phase formation occurs at the interfaces during deposition for all samples and there is a critical layer thickness near 10 monolayers of each constituent where complete amorphization occurs. The amorphous phase formed upon annealing the 50 monolayer electron-beam sample consumes only about one-third of the Fe before the process saturates.     

Optical properties of hybrid periodic/quasiregular dielectric multilayers

Optical properties like transmission, omnidirectional reflection and localization of modes are theoretically investigated in hybrid periodic/quasiregular dielectric heterostructures based on porous silicon. It is shown that the fabrication of this class of multilayers would lead to an improvement of the formation of microcavities, the widening of the incidence angular interval for total reflection as well as the appearance of spatial localization of electric field intensity within specific regions of the structures.     

Structure and interlayer coupling in Co/V multilayers

Two series of sputtered Co/V multilayers were investigated by X-ray diffraction, ferromagnetic resonance (FMR) and magnetic measurements. The multilayers are found severely alloyed, resulting in two FMR peaks of uniform mode. Spin-wave resonance (SWR) spectra were observed when the thickness of vanadium is thin, indicating interlayer coupling. The data of SWR were treated approximately respectively by volume inhomogeneity (VI) and volume homogeneity and entire surface pinning (SP) models. The relatively small calculated effective exchange constants showed weak exchange coupling between Co layers across V spacers.     

Giant magnetoresistance and magnetothermopower in Co/Cu multilayers

The thermopower (Seebeck coefficient, S) has been measured on a series of Co/Cu multilayers that exhibit giant negative magnetoresistance (GMR). Negative in zero applied field, S(H) increases in magnitude as the field is increased, approaching that of bulk Co. The change in S is inversely proportional to the resistance of the sample which is, in turn, proportional to the square of the magnetization. A model is presented that yields both the GMR and S(H) from the spin-split density of states of the Co without the need of a spin-dependent scattering potential at the interfaces.     

Characterization of EUV periodic multilayers

Nanometric Co/Mg, Co/Mg/B₄C, Al/SiC and Al/Mo/SiC periodic multilayers deposited by magnetron sputtering are studied in order to correlate their optical performances in the extreme ultraviolet (EUV) range to their structural quality. To that purpose, our recently developed methodology based on high‐resolution X‐ray emission spectroscopy (XES) and X‐ray and EUV reflectometry is now extended to nuclear magnetic resonance (NMR) spectroscopy and time‐of‐flight secondary ions mass spectrometry (ToF‐SIMS). The analysis of the Co Lαβ and Mg Kβ emission spectra shows that the Co and Mg atoms within the multilayers are in a chemical state equivalent to that of the atoms in the pure Co and Mg references, respectively. But NMR spectra give evidence for a reaction between Co atoms and B and/or C atoms from B₄C. The Al and Si Kβ emission spectra do not reveal the formation of an interfacial compound in Al/SiC and Al/Mo/SiC. Only the roughness limits the optical quality of Al/SiC. The comparative analysis of the ToF‐SIMS spectra of Al/SiC and Al/Mo/SiC indicates that the structural quality is enhanced when Mo is introduced within the stack. Copyright © 2011 John Wiley & Sons, Ltd.     

NMR spin-echo study on Co/Pd multilayers

Hyperfine field spectra of thin Co/Pd multilayers have been determined by spin echo NMR. The main contribution to the spectra seems to arise from Co ions in a polyerystalline FCC phase. For the layers with thinnest Co sublayer thickness the centre of the spectrum shifts to lower hyperfine fields upon decreasing Co sublayer thickness. This is attributed to a decrease of the magnetic moment caused by stretching of the Co “lattice” due to neighbouring Pd atmos. An analysis of the integrated intensity in the hyperfine field region 19 T to 22 T shows a loss of signal equivalent to about one layer of Co per sublayer.     

Theory of periodic ferromagnetic multilayers

We investigate the statics and dynamics of periodic multilayers consisting of two ferromagnetic materials. Our theory is based on a general Ginzburg-Landau functional for inhomogeneous systems and the appropriate equations of motion. We compute the transition temperature of the composite material, the temperature dependence of the magnetization and the magnon dispersion relation. The statics apply also to other multilayer systems such as ferroelectrics.     

Magnetization reversal and domains structures in (Co/Ni/Co/Pt) multilayers

Multilayers of [Co/Ni(t_Ni)/Co/Pt]_×4 are investigated for different Ni insertion layer thicknesses. The resulting magnetic properties and magnetic domain structures are compared with [Co/Ni]_×8 multilayers. As determined by magneto-optical Kerr effect microscopy and a vibrating sample magnetometer measurements, all multilayers exhibited a perpendicular magnetic anisotropy. It is found that the nucleation field and magnetic coercivity of [Co/Ni(t)/Co/Pt]_×4 multilayers are lower than (Co/Ni)_×8 and decreased with Ni thickness. Magnetization decay measurements reveal that these multilayers did not show an exponential decay behavior as was observed in rare earth transition metal alloys. Very narrow wires will remain stables for several hours even with an applied magnetic field closer to the coercivity. Insertion of very thin Ni in (Co/Pt) multilayers offers a good way to optimize the magnetic properties of the material and adjust the domain size for nanowire-based devices.     

On the perpendicular anisotropy of Co/Pd multilayers

The two principal mechanisms for explaining the perpendicular anisotropy in Co–Pd multilayers were investigated. These mechanisms are the broken symmetry at sharp Co surfaces, and the magnetostriction of interfacial Co–Pd alloys under stress due to epitaxy with Pd. The approach was to examine multilayers in which the Co component was intentionally alloyed with different amounts of the Pd, keeping the total amount of Co and Pd constant. These multilayers were deposited both at room temperature, at which thermally activated interfacial intermixing augmented the intentional alloying, and at 77 K where sharper interfaces reflecting the intentional alloying were expected. Comparison of the properties of these two sets of multilayers clearly indicated that stressed interfacial alloying is the dominant mechanism. Low-temperature measurements supported the alloying model, and the presence of polarized Pd. Furthermore, the hard-axis magnetization was modeled with a distribution of local perpendicular anisotropies which reflect local composition variations.     

Structural and magnetic properties of Pt in Co/Pt multilayers

[Co(30 Å)/Pt(x Å)]₂₀ multilayers with the Pt layer thicknesses varying from 5 Å to 20 Å were characterized structurally by high angle X-ray diffraction, X-ray reflectivity, X-ray absorption spectroscopy and magnetically by X-ray magnetic circular dichroism. It is found that the structure and magnetic properties of Pt have a strong correlation with the Pt layer thickness. The 20 Å thickness Pt layer is not almost influenced by the adjacent Co layer and the nearest neighbors are dominated by Pt-Pt shells. With decreasing Pt layer thickness, the nearest neighbors are gradually dominated by Pt-Co shells and the Pt-Co intermixing regions also remarkable increase at the interfaces, especially for the 5 Å thickness Pt layer. The orbital and spin magnetic moments as well as the ratio m_orb/m_spin all decrease systematically with increasing Pt layer thickness, indicating that the interface atoms are polarized by direct Pt-Co hybridization, but that the adjacent layers are polarized by Pt-Pt interactions.