Effect of density, magnetic field modulation and magnetic field penetration depth on the microwave absorption properties of YBa2Cu3O7 –δ superconductors

Abstract

Magnetic field sweep hysteresis of the low-field microwave absorption (LFMA) in YBa₂Cu₃O_{7 –δ} is stronger for low magnetic field modulation, low temperatures and low microwave power. The field-sweep hysteresis is related to the pinning of magnetic fluxons. Under conditions favouring substantial fluxon pinning, noise-like structures are superimposed on the LFMA signal. The LFMA signal amplitude depends on magnetic flux pinning, the relative contributions of intergranular and intragranular currents, the distribution of Josephson junctions and the variation in the magnetic field penetration depth. The hysteresis behaviour is similar for samples with different densities at low and high modulation amplitudes. However, the LFMA amplitude decreases with increasing density. The effective magnetic field penetration depth is measured by electron paramagnetic resonance line broadening of spin probes adsorbed on the sample surface. The results are discussed in terms of several current models for the LFMA effect.