I think Peter has beaten me too it but I was going to suggest using a thinner piece of steel, I dont know much about magnets but maybe a lower mass is required, as I said before the hot rails type of pick up use two thin strips, maybe this isnt a coincedence

Maybe you can create a sandwich of plastic and steel to make up the 3mm width?

Why do you need a blade anyway? is it cosmetic or are there dead spots between the poles?

Anyway good luck