In this method difference in the NMR chemical shift in a
solvent caused by the presence of a paramagnetic species is
used.
It can be used to determine the magnetic moment of the
paramagnetic and the number of unpaired electrons present.
NMR shift reagents such as Eu complex exploit the
isotropic effect (difference between the chemical
shift of a given nucleus in a diamagnetic vs paramagnetic
environments).
NMR measurements are conducted on diamagnetic compounds.
Europium is a paramagnetic element.
t-butyl alcohol is a diamagnetic reference.
Hence, it is a poor reference.