The Smallest Magnet

Single holmium atoms on magnesium oxide. (left) Scanning tunneling microscope image and (right) magnetic hysteresis from x-ray magnetic circular dichroism.

Single holmium atoms adsorbed on few monolayers of magnesium oxide are extraordinarily stable magnets. They exhibit significant remanence, i.e., they retain a large fraction of their saturation magnetization when the external magnetic field is switched off. The present study clearly demonstrates for the first time magnetic blocking and remanence in single atoms up to temperatures of 30 K. While most transition metal or rare earth atoms adsorbed on surfaces possess rather short magnetization lifetimes, the relaxation of the holmium atoms on magnesium oxide is blocked because of the proper combination of the magnetic ground state and the crystal field exerted by the oxide film. To obtain these results, the authors used a combination of x-ray magnetic circular dichroism performed at the Swiss Light Source (SLS) and at the European Synchrotron Radiation Facility (ESRF) as well as scanning tunneling microscopy. The study opens perspectives of storing and processing information at ultrahigh density.