Hi Mare Rosu,
Can you make up a story about the magnetized knife lying on top of a horseshoe magnet or two at some point (the legs of the horseshoe perpendicular to the blade). That would give the magnetic pattern you describe.
I suppose the other way you could magnetize a blade is to put it inside a big coil for a while with the power on. I think the problem there would be keeping the blade from acting like the center of a solenoid and getting shot out, possibly at high speed....
Neat stuff. My question is, why would anyone want to do it, unless it's the usual "sword mysticism" kicking in?