Can a mgnet ruin the battery of an iPhone 4s?

I had my phone in my pocket last night and forgetting it was there I stuck my name tag in the pocket, when I pulled my phone out the name tag was stuck to it, I pulled it off and put it somewhere else.

I charged my phone last night to 96% and when I woke up this morning it was completely dead, did I ruin my phone?

You could possibly have done just that yes, because it will suck all the life right out of it; it's the same if you take a CD and a magnet, it will wipe the CD clean. If your battery doesn't start working again or doesn't seem as good as before I would call up apple and pretend like it wasn't your fault and don't mention the magnet, you might be able to stay under the warranty, just say that the battery is messed up and you don't know why