High critical current density and vortex pinning of epitaxial Mg ₂ thin films

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 T_c, a narrow ΔT_c and a high zero-field j_c. The best j _c(H = 0) of the films at 5 K is comparable to the estimated depairing current density J₀ of MgB₂. The zero-field j_c(T) data of different films can be scaled to j(t)= (1-t ²)α(1+t²)^1/2, where j(t) = j _c(t)/j_c(0) and t = T/T_c. 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 j_c(H) of the films show a common feature that j_c(H) first appears as a plateau when the external field is lower than H*, where μ₀H* ∼ 10^-2 T, and then j_c(H)∝H-^β when the field is lower than a crossover value μ₀H_m ∼ 1 T, where β ∼ 0.7-1.0 resembling that of epitaxial high-T_c cuprate films. Above H_m, j_c(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.