Transport and magnetic properties of a magnetic-superconductor Co70.4Fe4.6Si15B10-Nb bulk core-shell structured material

The soft-ferromagnetic Co_70.4Fe_4.6Si₁₅B₁₀ amorphous alloys gained special attention in the last two decades for presenting, among other interesting magnetic properties, the giant magnetoimpedance effect (GMI). Moreover, bulk amorphous and nanocrystalline alloys were also found to be good candidates for magnetic sensor for presenting soft magnetic properties as well. The GMI is strongly influenced by the sample composition and by the amplitude (I_AC) and the frequency (f) of the ac current. The temperature (T), and both the magnitude and direction of the applied magnetic field (H) do also influences the GMI. In the present work, bulk samples of core-shell structured samples with a magnetic shell of Co_70.4Fe_4.6Si₁₅B₁₀ and a superconducting core of niobium were prepared. The results reported in this work were obtained for a sample with an approximately volume of 3 mm×3 mm×30 mm and a core diameter of 1 mm. The Co_70.4Fe_4.6Si₁₅B₁₀ alloy was prepared mixing the powder elements and melting the mixture in an argon atmosphere for five times in an arc furnace. The precursor pellet was then melted inside a quartz crucible and RF-heated until it melted. The liquid was then rapid solidified around an Nb wire vertically placed inside a liquid nitrogen cooled copper mound forming the core-shell structure. X-ray diffraction was used for identifying the crystalline structure of the phases present in the composite sample. The dc magnetization (M) and the ac magnetic susceptibility (χ_ac) were measured by using a Physical Properties Measurement System (PPMS) in the temperature range 2-300 K, 0.1 ≤ f ≤ 10 kHz and for magnetic fields up to 85 kOe. The PPMS was also used for measuring the dc electrical resistivity while a lock-in system was used for measuring the high-frequency magnetoimpedance. Th- four electrical contacts were attached to the shell and H was applied either parallel or longitudinal to the electrical current. Even though the magnetic properties are dominated by the ferromagnetic magnetic phase a diamagnetic contribution was observed below the Nb superconducting transition temperature (T_c), e.g., below about 8.8 K in the χ_ac data. The T-dependence of M measured by using a zero-field-cooled procedure and recording the data while warming up the sample in a magnetic field of 10 Oe did also show the diamagnetic contribution below T_c. Hysteresis loops were measured at room temperature and at 9 K yielding values for M and for the coersivity of 55 emu/g and 82 emu/g and of 20 Oe and 50 Oe, respectively. The remanence (M = 1.7 emu/g) did not show a significant variation with T. The real part of χ_ac showed a minimum near 120 K while the out-of-phase component showed a peak around 65 K. The peak become broader and shift to high values of T with increasing values of f. Finally, the maximum value for the transversal magneto-resistance below T was found to be about 260% while for the GMI this value is 3,500% for an applied magnetic field of 4.0 kOe, yielding an overall sensitivity of about 1 %/Oe at room temperature. The results are analyzed by taking into consideration the parallel resistances of the two metallic phases and the skin-depth parameter associated to the Co_70.4Fe_4.6Si₁₅B₁₀ phase. This work was partially supported by CNPq, CAPES, FACEPE and FINEP (Brazilian Agencies).