Microwave and dc pinning studies of BZO enhanced YBCO thin films.

Artificial pinning centers obtained as nanoinclusions of dielectric materials in YBa2Cu3O7-x (YBCO) films are of great interest in view of their great potential in applications such as coated conductors [1]. In particular, it has been observed [2] that BaZrO3 (BZO) inclusions in films grown by Pulsed Laser Deposition (PLD) assemble in the form of nanorods, 30-150 nm long, of typical cross section comparable to the vortex core size. Thus, they are optimal candidates for effective vortex core pinning. Previous studies [2] have already shown that the BZO nanorods, oriented approximately along the c-axis, determine a significant increase of the pinning strength both at dc, in terms of the critical current density Jc and of the maximum pinning force Fp=JcB, and at microwaves, in terms of the pinning constant kp and of overall consequent reduction of dissipation levels. Despite the promising experimental results, the physics underlying the enhanced artificial pinning in anisotropic superconductors is still unsettled. In particular, the interplay between the effective-mass anisotropy of YBCO and the preferential direction given by the BZO-induced nanocolumns is still an open question. In this framework, we performed a combined study of the pinning properties of YBCO thin films with BZO nanorods through the two complementary probes, dc measurements of Jc and microwave measurements of the vortex parameters, including the flux flow resistivity ρff and the pinning constant kp. The very different dynamic regimes explored through the mentioned techniques allow to fruitfully investigate the interplay among the various sources of pinning and their different anisotropic properties. Indeed, the overall anisotropy of pinning is expected to arise from the competition between the mass anisotropy of YBCO, the preferential direction introduced by the extended BZO structures and the intrinsic pinning occurring in the YBCO layers. The samples under study are grown by pulsed laser deposition (PLD) from targets with BZO powders at 5% mol. as extensively reported elsewhere [3]. They are 120 nm thick and c-axis oriented, with Tc ~ 90 K and a zero-field Jc as high as 3.7 MA cm-2 at 77 K. Dc measurements in tilted fields H (maximum Lorentz force configuration) yielded Jc(H) as a function of the tilting angle θ between the field direction and the c-axis at several field intensities μ0H≤12 T. The samples were patterned as strips, 30 μm wide and 1 mm long, in the standard four-contact configuration. The criterion of 1 μV cm-1 was used. The field-induced variation of the microwave complex resistivity was measured at fields μ0H≤0.8 T on unpatterned samples through a cylindrical sapphire loaded resonator operating at 48 GHz. Both field sweeps at selected angles and field rotations at selected fields were performed. Because of the small signal available at θ=90°, the temperature T≈80 K was chosen, being both sufficiently far from Tc to avoid thermal pair-breaking and compatible with a high enough signal-to-noise ratio. A careful analysis [4] of the microwave measurements shows that the flux flow resistivity ρff, which is dissipation-related, satisfies the well-known scaling law [5] for the mass anisotropy, i. e. it can be scaled as ρff(H, θ)= ρff(H/Hc2(θ)), provided that the scaling function is properly corrected to incorporate the geometrical contribution of the experimental setup [6]. By contrast, no scaling property can be observed on the pinning constant kp(H, θ), coherently with the presence of extrinsic anisotropy sources. Quantitative comparison of the angular pinning strength as measured in dc (large vortex displacement) and at microwaves (very short, ~1 Å, vortex oscillations around equilibrium positions), shows identical behavior in almosty the full angular range, apart when the field is aligned with the BZO defects (0°) or the a-b planes (90°): in such cases dc-measured pinning is stronger than microwave-measured pinning. The difference indicates the presence of a dynamic effect: the strongest, peaked pinning appears only when large vortex displacements, such as those determined at dc, are involved. The implication of these findings in terms of the operating pinning mechanisms are discussed in terms of a Mott-like phase for vortices. References: [1] J. L. Macmanus-Driscoll et al., Nat. Mater. 3, 439 (2004); J. Gutiérrez et al., Nat. Mater. 6, 367 (2007); B. Maiorov et al., Nat. Mater. 8 398 (2009) [2] N. Pompeo et al., Appl. Phys. Lett. 91, 182507 (2007); A. Augieri et al., J. Appl. Phys. 108, 063906 (2010) [3] V. Galluzzi et al., IEEE Trans. Appl. Supercond. 17, 3628 (2007) [4] N. Pompeo et al., Physica C 479, 160 (2012) [5] G. Blatter, V. B. Geshkenbein and A. I. Larkin, Phys. Rev. Lett. 68, 876(1992); Z. Hao and J. R. Clem, Phys. Rev. B 46,