The thermal stability of the holmium single atom magnets in a magnetic field

The hallmark of a bar magnet is its stable magnetization: North Pole will stay North Pole, and South
Pole will remain South Pole. Once we split this magnet, both halves will also be stable magnets. As we
continue to break the shrinking magnets into ever smaller and smaller pieces, we will encounter a
critical size at which the poles start to spontaneously fluctuate owing to the onset of quantum
tunneling of their magnetization [1]; the magnets cease to be stable. The critical sizes range from only
a few atoms to hundreds. However, a recent report of magnetic remanence for ensembles of single
holmium atoms on magnesium oxide [2] provided us with a new perspective of how small a stable
magnet could effectively be and promised a new era of ultra‐dense magnetic data storage. At the
fundamental limit of a single atom, however, the question of how to read and write the magnetic
states remained. Here we show the control of the magnetization of selected Ho atoms by scanning
tunneling microscopy (STM) [3] using current pulses, tunnel magnetoresistance (TMR), and STM
enabled electron‐spin resonance (ESR) [4]. Below a tunneling voltage threshold, the individual Ho bits
remain stable for hours. We use this stability to build a prototypical two Ho bit atomic memory device
to which we write the four possible states and which we read out via TMR and ESR. We expand our
understanding of the reversal mechanism above the switching threshold voltage by studying the
magnetic field dependence of the switching rate in magnetic fields of up to 8T and at temperatures
exceeding 40 K [5].
[1] Gatteschi and Sessoli, Angew. Chem. Int. Ed. 42, 268 (2003)
[2] Donati et al., Science 352, 318 (2016)
[3] Natterer et al., Nature 543, 226 (2017)
[4] Baumann, Paul et al., Science 350, 417 (2015)
[5] Natterer et al., in preparation