My application is EFNMR (Earth field nuclear magnetic resonance). In this application the earth filed, which is a DC field, has the main role and should not be altered. However, all other magnetic fields specially 50Hz are harmful and should be cancelled. The question is: what technique should be used.
I know the answer in brief. I should use an eddy current based structure. What I want to learn from your kind responses is the detail. What are the usual structures? What is the theory behind? etc.