Using an alpha, why am I getting black edges/halos around my selections??

I've rendered out an image with an alpha channel. In PS I'd now like to be able to cut out, paste into another image and then resize the selection taking advantage of it alpha channel.

I tried this by going to load selection then selecting the alpha then I cut and paste into another image. However when I do this I get a horrible dark/black halo around my selection? Presumably this is because the alpha is against black. But how do I do this then? I thought by using an alpha edges would merge perfectly?

Any help greatly appreciated.