SQUID magnetometer operating at 37K based on nanobridges in epitaxial MgB₂ thin films

Superconducting quantum interference devices SQUIDs and magnetometers are fabricated from nanoconstrictions in epitaxial MgB2 films. The nanobridges are contained within single-crystalline grains, resulting in clean transport, a large critical current density of 5107 A/cm2 at 4.2 K, and stable SQUID voltage modulation up to 38.8 K. The magnetometer is realized with an inductively coupled pickup loop, giving rise to a field sensitivity of 1 pT Hz¿1/2 down to 1 Hz. The device properties are governed by the two-band superconducting nature of MgB2, posing, however, no problems to a successful development of boride magnetic field sensing devices. The MgB2 zero-temperature London penetration depth is measured to be 62 nm, close to theoretical predictions.