DC MAGNETIZATION STUDY OF FINE-FILAMENTARY NBTI SUPERCONDUCTORS

Measurements of d.c. magnetization M(H) and magnetic susceptibility ch i(T) in perpendicular and parallel magnetic fields were performed by a SQUID magnetometer on four multifilamentary NbTi wires with a Cu matr ix and A superconducting filament diameter ranging from 11.8 mum up to 0.54 mum (these samples belong to the VAMAS reference samples). Anoma lies in the chi(T) and initial M(H) curves were observed in samples ha ving lower filament diameters in the perpendicular magnetic field. The initial linear M(H) slope of these samples corresponds to perfect dia magnetism of their strands, indicating proximity effect coupling of th e NbTi filaments. At higher fields, a second linear slope of M(H) was observed, corresponding to diamagnetism of individual NbTi filaments. From this slope, using the M(H) relation for the Meissner state of a c ylinder in a perpendicular magnetic field, the London penetration dept h A was determined. Hysteretic effects were observed for samples havin g lower filament diameters in fields corresponding to the second linea r M(H) slope, indicating a complex proximity effect pinning structure within the strands. Deviation from linearity of the two linear M(H) pa rts allows us to evaluate H(c1) of the strands and NbTi filaments. Est imated H(c1) values from the Ginzburg-Landau theory were lower than th e measured values. Measurements in the parallel magnetic field did not indicate any proximity effect coupling within strands, with the excep tion of the sample with a filament diameter of 0.54 mum, where H(c1) v alues were quite close to those calculated by the Ginzburg-Landau theo ry.