And what the heck's an Halbach Array?
Just put a lot of Magnets on a row, each one 90 degrees rotated in relation to the previous one, like this..
Tada! An Halbach Array.
In that picture, almost all of BOTH the magnet's poles strenght is on the bottom side!
On the Magnetic bands you find on the doors of discarded Refrigerators, that's where!