  The highest-density magnetic storage media will code data in single-atom
bits. To date, the smallest individually addressable bistable magnetic bits on
surfaces consist of 5-12 atoms. Long magnetic relaxation times were
demonstrated in molecular magnets containing one lanthanide atom, and recently
in ensembles of single holmium (Ho) atoms supported on magnesium oxide (MgO).
Those experiments indicated the possibility for data storage at the fundamental
limit, but it remained unclear how to access the individual magnetic centers.
Here we demonstrate the reading and writing of individual Ho atoms on MgO, and
show that they independently retain their magnetic information over many hours.
We read the Ho states by tunnel magnetoresistance and write with current pulses
using a scanning tunneling microscope. The magnetic origin of the long-lived
states is confirmed by single-atom electron paramagnetic resonance (EPR) on a
nearby Fe sensor atom, which shows that Ho has a large out-of-plane moment of
$(10.1 \pm 0.1)$ $\mu_{\rm B}$ on this surface. In order to demonstrate
independent reading and writing, we built an atomic scale structure with two Ho
bits to which we write the four possible states and which we read out remotely
by EPR. The high magnetic stability combined with electrical reading and
writing shows that single-atom magnetic memory is possible.
