We have studied transport properties of a number of in situ epitaxial MgB2 thin films grown by hybrid physical-chemical vapor deposition. The films show single-crystal-like structure with a high Tc, a narrow ΔTc and a high zero-field jc. The best jc(H = 0) of the films at 5 K is comparable to the estimated depairing current density j0 of MgB2. The zero-field jc(T) data of different films can be scaled to j(t) = (1 − t2)α(1 + t2)1/2, where j(t) = jc(t)/jc(0) and t = T/Tc. We have found that α = 1.7 ± 0.1 fits the data measured with the 4-probe method and α ∼ 2 fits the data derived from the magnetization hysteresis M-H measurements. At low temperature the jc(H) of the films show a common feature that jc(H) first appears as a plateau when the external field is lower than H*, where μ0H* ∼ 10−2 T, and then jc(H) ∝ H−β when the field is lower than a crossover value μ0Hm ∼ 1 T, where β ∼ 0.7–1.0 resembling that of epitaxial high-Tc cuprate films. Above Hm, jc(H) decreases steeply. These results may be associated with the columnar growth mode of the films, where strong pinning centers are mainly located at the column boundaries or twin boundaries, and within a single-crystal-like column the pinning is weak. At low temperature, nearly temperature independent small peaks and flip-over in M-H loops are observed in the films, consistent with the high thermal activation energy U0(H, T) values calculated from the resistivity measurement of the films.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - Dec 5 2003|
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics