As far as I know, the story
goes like this:
The first time Magica came to see Scrooge (in the story 'The Midas touch') she told him she collected coins that had been touched by the world's richest persons. From these, she would make an amulet by lowering them into the mount Vesuvio volcano. (The amulet was to give her the Midas touch, i.e. everything you touch turns into gold.) Scrooge laughed at this, but gave her a coin anyway. Later, he discovered that he'd given her his number one dime by mistake. He ran after her and asked if he could get number one back and give her another coin instead. She complied, and he then told her how much the number one dime meant to him. Magica quickly figured Scrooge would have touched it a great number of times, and this is when (and why) her ever-ongoing pursuit of this particular coin began.
As for the raven, I believe he is actually her brother.
Magica is quite a name by the way, is anyone called this in real life?