Direct measurement of the magnetic anisotropy of thin sputtered MgB2 films

The simple hexagonal crystal structure and large London penetration depth make Magnesium-diboride an excellent candidate for spin polarization measurements using the Meservey-Tedrow method. In a step towards these measurements, we investigated the magnetic anisotropy of thin MgB2 films sputtered onto cubic (001)MgO and R-cut sapphire substrates. We present the high field cooling experiments with an applied magnetic field perpendicular and parallel to the film plane for those samples. These measurements were done in the high field laboratory in Nijmegen due to the required high upper critical field for thin sputtered films of up to 30 T. Our measurements show an upper critical field of 38.31 T with field parallel to the film plane and 27.7 T with a field perpendicular to the film plane for the samples with 120 nm MgB2. A linear fit shows a good approximation with the data points, although the parallel magnetic field could not destroy the superconductivity. We achieved a magnetic anisotropy ratio of 1.38. Literature values show an anisotropy ratio between 1.25 and 2 for thin films, which was fit with the Ginzburg Landau theory. Therefore our results agrees with the previously estimated anisotropy ratio for thin MgB2 films, where magnetic fields of up to 7 T were used.