  Digital magnetic recording is based on the storage of a bit of information in
the orientation of a magnetic system with two stable ground states. Here we
address two fundamental problems that arise when this is done on a quantized
spin: quantum spin tunneling and back-action of the readout process. We show
that fundamental differences exist between integer and semi-integer spins when
it comes to both, read and record classical information in a quantized spin.
Our findings imply fundamental limits to the miniaturization of magnetic bits
and are relevant to recent experiments where spin polarized scanning tunneling
microscope reads and records a classical bit in the spin orientation of a
single magnetic atom.
