  We use spin-polarized scanning tunneling microscopy to demonstrate that Ho
atoms on magnesium oxide exhibit a coercive field of more than 8 T and magnetic
bistability for many minutes, both at 35 K. The first spontaneous magnetization
reversal events are recorded at 45 K for which the metastable state relaxes in
an external field of 8 T. The transverse magnetic anisotropy energy is
estimated from magnetic field and bias voltage dependent switching rates at 4.3
K. Our measurements constrain the possible ground state of Ho single atom
magnets to either Jz = 7 or 8, both compatible with magnetic bistability at
fields larger than 10 mT.

Mesoscale and Nanoscale Physics