About those GCRs?

I've often heard of radiation given as an "insurmountable" problem to getting men to mars. Or even sending them back to the moon. And yet, people spend months at a time on the ISS, orbiting the Earth above the atmosphere. In terms of uncharged radiation, they should be getting about half of all the stuff that Mars or moon astronauts would get (The earth shields the sun about half of the time). The only type of radiation that space station astronauts aren't getting in equivalent quantities are the charged radiation due to the van Allen belts sheilding them.
The main radiation culprit fretted over are galactic cosmic rays - very energetic free nuclei that have the energy to punch through most shielding, generating secondary particles as they go. The thing that gets me though, is that nuclei are all charged. If a planetary magnetic field is enough to shield you from GCRs while in low earth orbit, why can't you just set up your own magnetic field to deflect charged radiation?

A little back of the envelope non-reletavistic highschool physics says that a magnetic field of strength B would deflect a particle in a loop of radius r = mv/qB. You would have to make the field stronger to cause tighter deflection, but the particles should loop around the field lines until they impact the magnet.

Doesn't sound so insurmountable to me. Furthermore, it sounds a whole lot more reasonable than lining your crew cabin with 10 feet of lead. You're going to have to have a reactor on your mars spacecraft to power propulsion anyway. If you take more payload weight (all the shielding), you'll have to spend a lot more energy on propulsion to accelerate. If you can save on payload weight by having lighter shielding and a magnetic shield, I wonder if it makes more sense to spend some energy on that instead?

Just an idea. Correct me if I'm wrong. (The magnet would have to be too energetic/ other serious radiation threats/ ect)