Observation of large anisotropy in c-axis oriented thin films of MgB₂ prepared by an in situ pulsed laser deposition process

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A large anisotropy in the upper critical field (HC₂) is observed for MgB₂ films grown in situ by a pulsed laser deposition process involving growth and annealing of Mg and B multilayers. Measurements of resistivity as a function of temperature and magnetic field yield the estimated zero temperature values of HC₂ in the range 100 to 130 T and about 10 T, for the field in ab plane and along c axis, respectively, depending on the criterion chosen for the transition temperature. The corresponding anisotropy parameter is thus in the range 9 to 13. The estimated coherence length in ab plane is about 50 Å, whereas that in the c direction is much smaller (≈ 5 Å). No significant magnetoresistance is observed in the normal state.