Detection of ¹H NMR signal in a trapped magnetic field of a compact tubular MgB₂ superconductor bulk

We measured ¹H nuclear magnetic resonance (NMR) spectra from silicon rubber in a magnetic field trapped by a compact tubular MgB₂ superconductor bulk magnet (60 mm outer diameter, 40 mm inner diameter, and 60 mm height) which was magnetized by field-cooled magnetization from an applied field, $B_{app}$, of 0.4778 T at 20 K. The sharpest ¹H NMR spectrum of the resonant frequency, $f_R$, of 20.314 MHz, which corresponded to a field strength of 0.4771 T, was observed at 1 mm below the center of the bore of the MgB₂ tube. The homogeneity of the trapped field was within 100 ppm between z $=$ −2 and +2 mm along the central z-axis, which was evaluated by the position-dependent $f_R$. No change in the trapped field was observed for about 25 d, which corresponded to a decay rate of less than 0.1 ppb h⁻¹. The obtained results suggest that the MgB₂ bulk magnet is a promising candidate for a magnetic pole in an NMR apparatus. (letter)