Magnetooptical, optical, and magnetotransport properties of Co/Cu superlattices with ultrathin cobalt layers

We investigated the field dependences of the magnetization and magnetoresistance of superlattices [Co($t_{x}$, $\mathring{\mathrm{A}}$)/Cu(9.6 $\mathring{\mathrm{A}}$)]$_{30}$ prepared by magnetron sputtering, differing in the thickness of cobalt layers (0.3 $\mathring{\mathrm{A}}\le t_{\operatorname{Co}}\le$ 15 $\mathring{\mathrm{A}}$). The optical and magnetooptical properties of these objects were studied by ellipsometry in the spectral region of $\hbar\omega$ = 0.09–6.2 eV and with the help of the transverse Kerr effect ($\hbar\omega$ = 0.5–6.2 eV). In the curves of an off-diagonal component of the tensor of the optical conductivity of superlattices with $t_{\operatorname{Co}}$ = 3–15 $\mathring{\mathrm{A}}$, a structure of oscillatory type (“loop”) was detected in the ultraviolet region, resulting from the exchange splitting of the 3$d$ band in the energy spectrum of the face-centered cubic structure of cobalt ($fcc$ Co). Based on magnetic experiments and measurements of the transverse Kerr effect, we found the presence of a superparamagnetic phase in Co/Cu superlattices with a thickness of the cobalt layers of 3 and 2 $\mathring{\mathrm{A}}$. The transition from superlattices with solid ferromagnetic layers to superparamagnetic cluster-layered nanostructures and further to the structures based on Co and Cu ($t_{\operatorname{Co}}$ = 0.3–1 $\mathring{\mathrm{A}}$) with a Kondo-like characteristics of the electrical resistivity at low temperatures is analyzed.