  We perform a thorough study of the effect of an external magnetic field on a
spin-1 ultracold Bose gas via mean field approach corresponding to the both
signs of the spin dependent interaction. In contrast to some of the earlier
studies, the magnetic field in our work is included through both the hopping
frequencies (via Peierls coupling) and the zeeman interaction, thereby
facilitating an explo- ration for competition between the two. The phase
diagrams in the antiferromagnetic case shows that the Mott insulating (MI)
phase with even particle occupancies is stable at low magnetic fields. At
higher magnetic fields, due to a competition between the hopping and the zeeman
interaction terms, the latter tries to destabilize the MI phase by suppressing
the formation of singlet pairs, while the former tends to stabilize the MI
phase. In the ferromagnetic case, the MI lobes become more stable with
increasing flux strengths. Further inclusion of a three body interaction
potential in order to ascertain its role on the phase diagram, we found that in
absence of the magnetic field, the MI lobes become more stable compared to the
superfluid (SF) phase and the location of the transition point for the MI-SF
phase increases with increasing the three body interaction strength. A strong
coupling perturbative calculation has also been done to provide a comparison
with our mean field phase diagrams. Lastly, with inclusion of the external
field, the insulating phases are found to be further stabilized by the three
body interaction potential.
