Keep in mind that the size of a magnetic grain (which stores 1 bit, roughly) is about 8nm. Compare that to the current cutting edge lithography scale of 14nm, which is the size of the semiconductor 'wires'.
Thus, a bit is smaller than the size even of the wires in any semiconductor circuit we can build, which means a system as you describe would have much lower storage density.
Keep in mind that the size of a magnetic grain (which stores 1 bit, roughly) is about 8nm. Compare that to the current cutting edge lithography scale of 14nm, which is the size of the semiconductor 'wires'.
Thus, a bit is smaller than the size even of the wires in any semiconductor circuit we can build, which means a system as you describe would have much lower storage density.