Studies On Flux Pinning Mechanism And Ciritical Current Density Of MgB₂ Thin Film

Following the discovery of superconductivity in MgB₂ with a critical temperatureTc of 39K, Alarge number of studies have been carried out to understand the mechanism of superconductivity.Less well explored especially in films, however, are aspects of vortex pinning and associatedvoltage-current density relation. In this work, the critical current density, pinning barrier (thermalactivation energy) and flux pinning of c-axis oriented MgB₂ thin film, fabricated by pulsed laserdeposition, are widely studied by the measurement of transport with the 4-prode method andmagnetization hysteresis M-H measurements. The major contents are summarized as following:1. From the resistance data versus inverse temperature on a logarithmic scale, the dependence offlux creep activation energy on the magnetic field has been determined to beU₀(B)∝b^p (1-b)² . Upper critical field and irreversibility field are obtained, theanisotropy of upper critical field has been analyzed.2. The current-voltage (I-V) characteristics of MgB₂ thin film at different temperature and field,measured with field applied parallel to c-axis and parallel to ab-plane, showed the character offlux motion in different regimes. Critical current density and depairing current density can beestimated. The current-voltage (I-V) characteristics at low voltage revealed a staircaseresponse, which was attributed to the presence of weak links, creating phase slip centers.3. In magnetic studies, a SQUID magnetometer was used conventionally to determine theinduced current density by the Bean model. The magnetic field and temperature dependence ofcritical current density is determined to fit fully the form of Kramer model, Parameter valuealso are obtained. Comparing parameters of theoretic with of the experiment data, theconclusion is coming: When applied field is less than peak filed , the dominant pinning ofMgB₂ thin film is surface pinning; .When applied field is more than peak filed, the dominantpinning of MgB₂ thin film is surface pinning and core pinning.4. Magnetization relaxation studies have been made on MgB₂ thin film as a function of magneticfield and temperature. It is found that m(t) is logarithmic and follows the Anderson-Kim...