Since neutrons can not be diverted using electromagnetic forces, I do not see how fission based bombs could be improved using any kind of magnet - nuclear fusion is another story, but then one could always go with the "sufficiently advanced technology argument".
You misunderstood me; I meant using magnets instead of high explosives to implode the fissile material.
Here's my design for a Sufficiently Advanced Technology Corp. Dial-A-Yield Nuclear Grenade:
The S.A.T.C.D.A.Y.N.G. consists of a series of concentric spherical shells surrounding a vacuum.
1. The innermost shell is made of a highly fissile alloy of U-235 and certain synthetic top-secret transuranic elements.
2-3. The second and third shells are spherical arrays of S.A.T.C. Ultra Electro Magnets, each with its own internal power source (also used in S.A.T.C. Micro Fusion Pack), facing each other. Shell 3 also includes a mesh of U.E.M's configured to hold together by magnetic attraction when the array is triggered.
4. The fourth shell is a lead-bismuth alloy.
5. The fifth shell is a spherical array of Nano Proton Accelerators, also each with its own internal power source.
When the device is triggered, the U.E.M. arrays in shells 2-3 energize at opposing polarities. This causes shell 1 to implode into the vacuum, forming a superhot sphere of molten fissile material. The U.E.M's forming the mesh in shell 3 prevent the outer shell from exploding as the inner one implodes. Simultaneously, the N.P.A. in shell 5 floods shell 4 with accelerated protons. This releases a flood of slow neutrons from the lead-bismuth shell (by spallation). When the flood of neutrons reaches the fissile material in the center of the sphere, it causes it to undergo fission. This also results in a phase change to plasma. The U.E.M array in shell 2 will contain the plasma for a nanosecond, allowing the chain reaction to complete, before being vaporized by the released energy.
To control the yield, the amount of protons released by the N.P.A. shell is user-controllable: fewer protons mean fewer neutrons, which means that a smaller amount of the fissile material undergoes fission.
Note that S.A.T.C.'s product catalog also includes the Dial-A-Yield fusion grenade, which, although being slightly bulkier and more expensive, has the advantage of producing less radioactive residue.