The human brain is said to produce a magnetic field resulting from the action potentials released inside the brain. What's the nature of such a field in terms of size and strength, and what is the potential for manipulation of brain functions by interfering with it by means of electromagnetic radiation?
The human body produces a wide range of bioelectromagnetic signals from various electrical impulses in the brain. The origin of the magnetic field is the charge exchange in the muscular and neural tissues (i.e no magnetic material is usually present in the body with very rare exceptions).
The brain's magnetic field varies from 10s of fT to 100s of fT . The frequency varies from 0.1 Hz to <100 Hz. Measurement of brain magnetism is limited by the complexity of signal due to the overlap of the signals from various parts of the brain. In magentoencephelograpahy this is done using SQUIDS .
The second part of your question is about magnetic excitement of brain functions. This is in fact possible and some testing/use is reported with limited clinical success. The strength of the applied magnetic fields are 1-5T for trans-cranial excitation .
 http://www.bem.fi/book/12/12.htm#02  http://www.scholarpedia.org/article/Magnetoencephalogram  http://en.wikipedia.org/wiki/Transcranial_magnetic_stimulation  http://www.bem.fi/book/22/22.htm An excellent source for this topic is 
protected by Qmechanic♦ Jan 28 '13 at 14:41
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