Critical magnetic-field angle for high-field current transport in YBa 2Cu 3O 7 at 76 K

Abstract

High magnetic field (0.5 T to 9 T) measurements of the in-plane transport critical current density J c of YBa 2Cu 3O 7 at liquid nitrogen temperature are reported as a function of the orientation of magnetic field B with respect to the a,b-plane. In contrast to earlier results at lower fields (<3 T), the measurements reported here at high fields reveal a J c versus angle curve with a head-and-shoulders shape with two angular regions having distinctly different behaviour. For magnetic field oriented close to the a,b-plane, a sharp, narrow intrinsic-pinning peak in J c about the B ⊥ c-axis is observed; on either side of the peak, J c is still relatively high (∼ 10 4 A cm −1) and nearly independent of magnetic field magnitude and angle. However, beyond a critical angle θ c, the J c versus θ dependence crosses over to a markedly different behaviour, characterized by a decrease in the sharpness of the voltage-current characteristic and a rapid decrease in J c. The critical angle θ c where this change occurs is determined experimentally from the sharp break in the J c versus θ curve at the edge of the shoulders, defined by the angle where the minimum in d 2/ J c/d θ 2 occurs. Characteristic values of θ c for oriented-grained YBa 2Cu 3O 7 at 76 K are ±39° at 5 T, ±24° at 7 T and ±17° at 9 T (where θ ≡ 0 for B in the a,b-plane). To take advantage of the high, nearly field-independent J c on either side of the intrinsic pinning peak, magnet design will need to allow an adequate angular margin to avoid the pinning transition at θ c.