Hysteretic loss vs. filament width in thin YBCO films near the penetration field

Magnetization vs. applied field measurements (M-H loops) were taken on YBCO thin films with filaments patterned into them. The YBCO was deposited onto LaAlO₃ substrates using PLD, and the filaments were formed by laser ablation. M-H loops were taken at 4.2 K in fields up to ±9 T using a vibrating sample magnetometer technique, the field applied perpendicular to the film width, d. The losses were seen to be greatly reduced by filament width reductions following the standard expression Q_h/H_m ∝ dJ_c/10. The penetration field was also well described by the standard "high field" expression H_p = (0.4J_ct)ln(d/t + 1), where t is the film thickness. The regimes of applicability of the loss expression were investigated, in particular near H_m ≈ H_p. A more general form of the loss equation was obtained and compared with the high-field approximation. The result was that although Q_h/H_m still increased in proportion to d, the rate of increase decreased as H_m approached H_p.