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Okunev V.D.,Ukrainian Academy of Sciences | Samoilenko Z.A.,Ukrainian Academy of Sciences | Szewczyk A.,Polish Academy of Sciences | Szymczak R.,Polish Academy of Sciences | And 5 more authors.
Journal of Physics Condensed Matter | Year: 2010

Investigation of the conductivity mechanisms in ferromagnetic Fe 67Cr18B15 metallic glasses with clusterized structure reveals anomalies in the behaviour of resistance and magnetoresistance (MR) in a narrow temperature interval, T = 3.6-3.1 K. The anomalies are seen as a sharp decrease of the sample resistivity in this range, with a rate equal to 3.6% K-1, i.e. 200-500 times more than the rate 0.008-0.021% K -1 in the range of 300-4 K. MR in the same range increases with a rate 1000 times larger (4% K-1 at T ∼ 3.1-3.6 K) than in the 300-4 K range (<0.0015% K-1). We explain this result by the appearance of local superconductivity in the large-scale layered clusters of metallic Fe-Cr phase, 150-200 in size, with ferromagnetic Fe2Cr core and nonmagnetic FeCr2 superconducting shell. The superconducting phase, which occupies 0.4-0.5% of the sample volume, provides a resistance jump Δρ/ρ≈1.5% that corresponds to calculation. The superconducting state of the clusters collapses if the magnetic field exceeds 20 kOe. © 2010 IOP Publishing Ltd.


Okunev V.D.,Ukrainian Academy of Sciences | Samoilenko Z.A.,Ukrainian Academy of Sciences | Szewczyk A.,Polish Academy of Sciences | Szymczak R.,Polish Academy of Sciences | And 6 more authors.
Journal of Physics Condensed Matter | Year: 2011

We have reinforced local superconductivity in ferromagnetic Fe 67Cr18B15 metallic glasses by ion irradiation. Superconductivity in this medium appears due to the presence of large-scale layered clusters of metallic FeCr phase, 150230 in size, with a ferromagnetic (or superparamagnetic) Fe-rich core and nonmagnetic Cr-rich superconducting shell. Here we show that due to the intensification of concentration phase separation in the FeCr clusters under ion (Ar+) irradiation, the volume of the superconducting phase increases from the initial 0.40.5% up to 78%. After irradiation, the resistivity jump / in the temperature range T=3.13.6K increases 14 times, reaching 19%, as compared to 1.36% for the initial sample. In the interval of T=3.13.6K, the rate of resistance change reaches 79%K1 for the irradiated sample instead of 3.6%K1 for the initial sample. In the same temperature interval, the rate of magnetoresistance change increases from 3%K1 for the initial sample up to 70%K 1 after irradiation. © 2011 IOP Publishing Ltd.


Okunev V.D.,Ukrainian Academy of Sciences | Samoilenko Z.A.,Ukrainian Academy of Sciences | Szymczak H.,Polish Academy of Sciences | Szewczyk A.,Polish Academy of Sciences | And 10 more authors.
Journal of Magnetism and Magnetic Materials | Year: 2016

We show that cluster magnetism in ferromagnetic amorphous Fe67Cr18B15 alloy is related to the presence of large, D=150-250 Å, α-(Fe Cr) clusters responsible for basic changes in cluster magnetism, small, D=30-100 Å, α-(Fe, Cr) and Fe3B clusters and subcluster atomic α-(Fe, Cr, B) groupings, D=10-20 Å, in disordered intercluster medium. For initial sample and irradiated one (Φ=1.5×1018 ions/cm2) superconductivity exists in the cluster shells of metallic α-(Fe, Cr) phase where ferromagnetism of iron is counterbalanced by antiferromagnetism of chromium. At Φ=3×1018 ions/cm2, the internal stresses intensify and the process of iron and chromium phase separation, favorable for mesoscopic superconductivity, changes for inverse one promoting more homogeneous distribution of iron and chromium in the clusters as well as gigantic (twice as much) increase in density of the samples. As a result, in the cluster shells ferromagnetism is restored leading to the increase in magnetization of the sample and suppression of local superconductivity. For initial samples, the temperature dependence of resistivity ρ(T)~T2 is determined by the electron scattering on quantum defects. In strongly inhomogeneous samples, after irradiation by fluence Φ=1.5×1018 ions/cm2, the transition to a dependence ρ(T)~T1/2 is caused by the effects of weak localization. In more homogeneous samples, at Φ=3×1018 ions/cm2, a return to the dependence ρ(T)~T2 is observed. © 2015 Elsevier B.V. All rights reserved.

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