Supercurrent transport inYBa2Cu3O7−δepitaxial thin films in a dc magnetic field

Abstract

Magnetic field and angle dependences of the critical current density J c ͑H , ͒ in epitaxial c-oriented YBa 2 Cu 3 O 7−␦ thin films are measured by the four-probe transport current technique, low-frequency ac magnetic susceptibility, and superconducting quantum interference device magnetometry. The films under study are deposited by off-axis dc magnetron sputtering onto r-cut sapphire substrates buffered with a CeO 2 layer. A consistent model of vortex pining and supercurrent limitation is developed and discussed. Rows of growthinduced out-of-plane edge dislocations forming low-angle boundaries ͑LAB's͒ are shown to play a key role in achievement of the highest critical current density J c ജ 2 ϫ 10 6 A/cm 2 at 77 K. The model takes into account the transparency of LAB's for supercurrent as well as the pinning of vortex lattice on a network of LAB's. Principal statistical parameters of the film defect structure, such as the domain size distribution and mean misorientation angle, are extracted from J c ͑H͒ curves measured in a magnetic field H applied parallel to the c axis and from x-ray diffraction data. An evolution of angle dependences J c ͑͒ with H is shown to be consistent with the model supposing dominant pinning on edge dislocations. Strongly pinned vortices parallel to the c axis appear to exist in tilted low magnetic fields up to a characteristic threshold field, below which the magnetic induction within the film obeys a simple relation B = H cos . This feature is shown to explain the absence of the expected maximum of J c ͑͒ at H ʈ c in a low applied field. A peak of J c ͑H͒ and an angular hysteresis of J c ͑͒, which have been observed in an intermediate-field range, are discussed in terms of film thickness, surface quality, and orientation of the applied field. The observed effects are found to be consistent with the developed model. DOI: 10.1103/PhysRevB.73.054508 PACS number͑s͒: 74.25.Sv, 74.72.Bk, 74.78.Bz lated disorder ͑e.g., by random pointlike pins͒, the J c ͑H , ͒ dependence is determined by a single parameter H͑cos 2 + 2 sin 2 ͒ 1/2 , 12,13 where = ͱ m ab / m c is the anisotropy parameter, =1/5-1/7 for YBCO. For the monotonic decreasing J c ͑H , =0͒ dependence the scaling naturally yields the J c ͑H = const, ͒ dependence with a minimum at = 0 and a maximum at = /2.Moreover, a maximum of the J c ͑H , ͒ dependence at H ʈ c was shown ͑e.g., Ref. 6͒ to be absent for YBCO films. Ironically, many researchers considered this observation to be strong evidence for the absence of out-of-plane linear pins or their ineffectiveness for vortex pinning in epitaxial films. In a contrast, recently Maiorov et al. have observed the J c ͑H , ͒ peak at H ʈ c. 14 The maximum does exist and tends to shift PHYSICAL REVIEW B 73, 054508 ͑2006͒

Modeling flux pinning in thin undoped and BaZrO3-doped YBCO films

“…We have presented a simple model modified from the low angle tilt boundary pinning theory of Pan et al 15 to describe the J c properties of undoped and BZO-doped YBCO thin films deposited on STO, where the pinning sites are twin boundaries and BZO nanorods. The model accurately describes the shape of J c ͑B , T͒ curves of the films, when the pinning site distributions were taken from distributions of twin spacings and BZO nanorods from TEM images.…”

“…Most of the pinning theories are based at least partly on the extensive treatment of vortices in high-temperature superconductors by Blatter et al 14 Recently Pan et al 15 published their theory, which links the straight "low angle tilt boundaries" ͑Laboratories͒ to J c ͑B , T͒. In this paper we present a modification of this theory to twin boundaries, dislocations, and nanorods.…”

“…It is clear from energy conservation 15 that there is a critical displacement ␦ c , below which the vortex is pinned to the twin plane. Thus, we get for the pinning radius of the twin boundary the following:…”

“…15 16 we can take the twin length, L, and spacing, W, to follow the gamma distribution as in 15 P͑x; ,…”

Dynamic magneto-optical imaging of superconducting thin films

“…This is because it has been established [1,34,41] that our high quality PLD films do not have grains but rather low-angle misoriented domains separated by domain walls consisting of edgedislocations providing strong vortex pinning and critical current, rather than grains connected by weak-links [1].…”