  Superconductors with persistent zero-resistance currents serve as permanent
magnets for high-field applications requiring a strong and stable magnetic
field, such as magnetic resonance imaging (MRI). The recent global helium
shortage has quickened research into high-temperature superconductors (HTSs)
materials that can be used without conventional liquid-helium cooling to 4.2 K.
Herein, we demonstrate that 40-K-class metallic HTS magnesium diboride (MgB2)
makes an excellent permanent bulk magnet, maintaining 3 T at 20 K for 1 week
with an extremely high stability (<0.1 ppm/h). The magnetic field trapped in
this magnet is uniformly distributed, as for single-crystalline
neodymium-iron-boron. Magnetic hysteresis loop of the MgB2 permanent bulk
magnet was detrmined. Because MgB2 is a simple-binary-line compound that does
not contain rare-earth metals, polycrystalline bulk material can be
industrially fabricated at low cost and with high yield to serve as strong
magnets that are compatible with conventional compact cryocoolers, making MgB2
bulks promising for the next generation of Tesla-class permanent-magnet
applications.
