  This paper presents some results concerning the size of magnetic fields that
support zero modes for the three dimensional Dirac equation and related
problems for spinor equations. It is a well known fact that for the
Schr\"odinger in three dimensions to have a negative energy bound state, the
3/2- norm of the potential has to be greater than the Sobolev constant. We
prove an analogous result for the existence of zero modes, namely that the 3/2
- norm of the magnetic field has to greater than twice the Sobolev constant.
The novel point here is that the spinorial nature of the wave function is
crucial. It leads to an improved diamagnetic inequality from which the bound is
derived. While the results are probably not sharp, other equations are analyzed
where the results are indeed optimal.
