  We demonstrate a simple method for the determination of the magnetic field in
an ion trap using laser-cooled Be+ ions. The method is not based on magnetic
resonance and thus does not require delivering radiofrequency (RF) radiation to
the trap. Instead, stimulated Raman spectroscopy is used, and only an easily
generated optical sideband of the laser cooling wave is required. The d.c.
magnetic vector, averaged over the Be+ ion ensemble, is determined.
Furthermore, the field strength can be minimized and an upper limit for the
field gradient can be determined. The resolution of the method is 0.04 G at
present. The relevance for precision rovibrational spectroscopy of molecular
hydrogen ions is briefly discussed.
