Vortex pinning and slow creep in high-J_c MgB₂thin films: a magnetic and transport study

Abstract

We have investigated the pinning of vortices in high-J c films of polycrystalline MgB 2 , by studying the dependence of current density J on electric field E using both magnetic and transport methods. Precursor films of amorphous boron, deposited on sapphire substrates, were converted to 0.6 µm thick MgB 2 by post-annealing in the presence of Mg vapour at 890 • C for 1 h. In magnetic studies, a SQUID magnetometer was used conventionally to determine the induced current density by the Bean model. The decay of J with time t was determined unconventionally with the sample fixed in position, by monitoring the SQUID feedback voltage ∝J versus time. The logarithmic decay rate S = −d ln(J )/d ln(t) was found to be very low in the H-T phase space away from the irreversibility line. Complementary four-probe transport studies of E(J ) were analysed as a power law dependence of the form E ∝ J n and used to obtain the corresponding creep rate S = 1/(n − 1). Effective values for n approach and often significantly exceed 100. From these results, we estimate the effective energy U 0 for vortex pinning, as a function of magnetizing field H.

Universal lower limit on vortex creep in superconductors

“…By compiling results from the literature for different superconductors, we reveal a cogent picture of this limitation and find few materials, including our Ba122:P film, MgB2 38,39 , and YBCO, with measured creeps rates near our proposed limit (Fig. 3b).…”

Flux dendrites of opposite polarity in superconductingMgB2rings observed with magneto-optical imaging

“…In this thesis equation (3.1) is exclusively used as a tool which simplifies computations. In fact, for MgB 2 thin films n can be very large, Thompson et al [55] found n = 78 at T = 25 K and B = 1 T. At lower temperature and field the value would be even larger. Along with the fact that the experiments described here typically takes only a few minutes to complete, it should be expected that creep effects can be safely ignored.…”

Magnetic field dependence of vortex activation energy: A comparison between MgB2, NbSe2 and Bi2Sr2Ca2Cu3O10 superconductors

“…While the large coherence length and the size of the as grown point defects in MgB 2 enables strong grain boundary pinning, the larger anisotropy leads to greater U 0 at low fields. The magnitude of U 0 at low fields is given by U 0 ~ φ 0 2 ξ/48 π 2 λ 2 [17]. For MgB 2 using the value ξ ∼ 5 nm and λ ~ 100 nm [22] U o is estimated to be to be ~ 7400 K where as for the case of NbSe 2 with ξ ∼ 2.3 nm and λ ~ 230 nm, U o is calculated to be ~ 1000K.…”

“…In Figure 4 we plot the activation energy U o , determined from the magnetoresistance study versus applied magnetic field. Following Thompson et al [17], we fit the data using the equation…”