What Size Magnet Can ERASE a Credit Card?

Even more secure than chip cards are pin numbers. I had my cards stolen and the person was able to try to make charges using my cards. (Try because I was able to cancel them quickly) The only card they couldn't was my company gas card because it has a 4 digit pin number. If credit card companies added pin numbers to all credit cards it would be extremely difficult for the thief to use stolen cards.  Debit cards have them, why not credit cards?