L. R. Tagirov

Experimental Observation of the Spin Screening Effect in Superconductor/Ferromagnet Thin Film Heterostructures

We have studied the nuclear magnetic resonance (NMR) of 51V nuclei in the superconductor/ferromagnet thin film heterostructures Pd_{1-x}Fe_{x}/V/Pd_{1-x}Fe_{x} and Ni/V/Ni in the normal and superconducting state. Whereas the position and shape of the NMR line in the normal state for the trilayers is identical to that observed in a single V layer, in the superconducting state the line shape definitely changes, developing a systematic distortion of the high-field wing of the resonance line. We consider this as the first experimental evidence for the penetration of ferromagnetism into the superconducting layer, a phenomenon which has been theoretically predicted recently and dubbed the spin screening effect.

Oscillations of superconducting transition temperature in strong ferromagnet-superconductor bilayers

The superconducting transition temperature Tc of a “clean ferromagnet-dirty superconductor” bilayer is calculated using boundary conditions derived for the quasiclassical Green’s function. This combination corresponds to the majority of experiments, in which Fe, Ni, Co, or Gd are used as a material for the ferromagnetic layer. It is shown that Tc oscillates upon changing thickness of the ferromagnetic layer, in accordance with the experimental observations.

Superconducting and magnetic properties of epitaxial high-quality Fe/Nb bilayers

Single crystal Fe/Nb (110) bilayers with an Nb thickness dNb in the range from 140 to 650 A and with an Fe thickness dFe in the range from 5 to 100 A were prepared using molecular beam epitaxy (MBE) techniques. For dFe⩾20 A a decrease of the superconducting transition temperature Tc with decreasing dNb was observed. For dFe=20 A the FMR data revealed a decrease of the effective magnetization of the Fe layer below the superconducting transition. This magnetization behavior is attributed to a spatial modulation of ferromagnetic order due to a modification of the RKKY interaction in the superconducting state.

Experimental observation of the spin screening effect in superconductor/ferromagnet thin film heterostructures

When studying the nuclear magnetic resonance (NMR) of 51V nuclei in superconductor/ferromagnet thin film heterostructures, anomalous behavior of the NMR signal was observed. This effect was manifested as a systematic distortion of the high-field wing of the resonance line. We consider this to be the first experimental evidence for penetration of ferromagnetism into a superconductor.

Knight shift in superconducting vanadium

The Knight shift in metallic vanadium in the normal and superconducting states has been measured. In contrast to the previously obtained results, this shift appears to change after the transition to the superconducting state. The behavior of the Knight shift in the superconducting state in vanadium samples doped with iron impurities has been found to be different from that in the “pure” samples. As a possible explanation of the effect, the broadening of the peak of the density of states near the Fermi level due to the scattering of conduction electrons on the iron impurities and the earlier predicted impurity polarization shift of the NMR line are discussed.

The proximity effect in an Fe-Cr-V-Cr-Fe system

Previously suggested correlations of BL Lac type objects with the arrival directions of the ultra-high-energy cosmic ray primaries are tested by making use of the HiRes stereoscopic data. The results of the study support the conclusion that BL Lacs may be the cosmic ray sources and suggest the presence of a small (a few percent) fraction of neutral primaries at E>10^{19} eV.The proximity effect was studied in a thin-film Fe-Cr-V-Cr-Fe layered system. As the chromium layer thickness (dCr) increases at a fixed thickness of iron layers (dFe), the dependence of the superconducting transition temperature (Tc) on dCr exhibits a maximum at dCr ≃ 40 Å followed by a sharp decrease. Investigation of the dependence of Tc on dFe at a fixed dCr showed that the depth of penetration of the Cooper pairs into a chromium layer does not exceed 40 Å. Analysis of the results obtained suggests that, at dCr ≃ 40 Å, chromium layers exhibit the transition from a nonmagnetic state to an incommensurate spin density wave state.

Superconductivity and magnetic properties of Fe/Nb multilayers (abstract)

The interplay between superconductivity (SC) and ferromagnetism (FM) in dilute magnetic alloys and intermetallic compounds attracted considerable attention during the last 30 years. Usually the ferromagnetic state is more stable and therefore tends to suppress SC. On the other hand, the magnetic state can be changed due to a modification of the RKKY interaction in the SC state. Mutual influence of SC and FM may acquire new peculiarities in such artificial systems as FM/SC multilayers. This work is focused on the study of Fe/Nb multilayered system, prepared on Al2O3(1120) substrates by rf‐sputtering and by molecular beam epitaxy(MBE) techniques. The Nb thickness t Nb was varied from 150 to 500 A and the Fe thickness t Fe was changed from 5 to 100 A. The sputtered samples were highly layered with sharp interfaces as revealed by x‐ray reflectivity and were textured in the [110] direction. The epitaxialMBE samples showed (110) growth of Nb and Fe with a coherence lengths comprising the total film thickness. Surface and interface roughnesses were very small. The dependence of the ferromagnetic resonance (FMR) spectra parameters on the direction of the dc magnetic field rotating in the plane of the samples show sixfold anisotropic behavior indicating the well known three‐domain in‐plane structure of Fe layers on sapphire substrates. FMR and SQUID measurements showed that FM of Fe layers survived down to t Fe=10 A. The out‐of‐plane FMR measurements also showed that the easy axis of magnetization lies in the plane of the samples down to this thickness. The superconducting transition temperature T c was determined by measurements of the electrical resistivity and by SQUID measurements. It was established that there is a critical thickness of the Fe layer t Fe crit above which SC was not detected for temperatures down to 1.5 K. This value was dependent on t Nb. Thus, for example, it was found that for t Nb=300 A the value of t Fe crit=30 A and for t Nb=350 A—t Fe crit=50 A. For constant t Fe with increasing t Nb up to a certain thickness, SC was not detected; then T c started to increase, and, finally, it approached the constant value of the order of 6 K above a certain t Nb which depended on t Fe. The critical magnetic fieldH C2 obtained from the resistivity measurements, strongly decreased with increasing t Fe or decreasing t Nb. The temperature dependences of H C2 were typical for two‐dimensional superconductors. For the interpretation of the data obtained, various theories were employed.