Is it Kosher to have a mezuzah hang by a magnet that can easily be removed ? [bracha]
Yes, this is in principle okay, because at the end of the day the mezuzah is hung on the doorpost by means of the magnet.
However, if the mezuzah is liable to fall off the method is of course not recommended.
It is best to hang the mezuzah with nails so that it is firmly attached to the doorpost.
Sources: See Ashrei Ish, Vol. 2, Chap. 64, no. 3.