D.c. magnetization study of fine-filamentary NbTi superconductors

Abstract

Measurements of d.c. magnetization M( H) and magnetic susceptibility χ( T) in perpendicular and parallel magnetic fields were performed by a SQUID magnetometer on four multifilamentary NbTi wires with a Cu matrix and a superconducting filament diameter ranging from 11.8 μm up to 0.54 μm (these samples belong to the VAMAS reference samples). Anomalies in the χ( T) and initial M( H) curves were observed in samples having lower filament diameters in the perpendicular magnetic field. The initial linear M(H) slope of these samples corresponds to perfect diamagnetism of their strands, indicating proximity effect coupling of the 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 cylinder in a perpendicular magnetic field, the London penetration depth λ was determined. Hysteretic effects were observed for samples having lower filament diameters in fields corresponding to the second linear M( H) slope, indicating a complex proximity effect pinning structure within the strands. Deviation from linearity of the two linear M( H) parts allows us to evaluate H c1 of the strands and NbTi filaments. Estimated H c1 values from the Ginzburg-Landau theory were lower than the measured values. Measurements in the parallel magnetic field did not indicate any proximity effect coupling within strands, with the exception of the sample with a filament diameter of 0.54 μm, where H c1 values were quite close to those calculated by the Ginzburg-Landau theory.