In work that could open the door to atomic-scale spintronic devices, researchers have created a single-atom magnet that is surprisingly stable. By placing rare earth atoms on an ultrathin magnesium oxide (MgO) film, the researchers were able to overcome factors that traditionally limit the performance of magnets on this scale, achieving stability at 30 K for half an hour or more.

The quest to shrink magnets down to single-molecules or single-atoms size is challenging. At small scales, a magnet’s permanent magnetization, described by its magnetic remanence, can be easily destroyed by thermal vibrations and quantum tunneling. Until now, the smallest magnets were made of single molecules and could retain information for only a small fraction of a second at temperatures above a few K.

The new single-atom magnets, reported in a recent issue of Science, were created and characterized by a research team led by Pietro Gambardella from the Swiss Federal Institute of Technology in Zürich and Harald Brune from the Swiss Federal Institute of Technology in Lausanne. The magnets were created by depositing individual holmium (Ho) atoms on an MgO substrate. Ho is a rare earth element with the largest magnetic moment of all known elements. Grown on a silver surface, the MgO acts as a stiff buffer layer that insulates the Ho atoms from electrons and thermal fluctuations. However, the insulation is not enough to ensure the stability of a quantum magnet.

Previous work on single-molecule magnets showed that magnetic remanence is lost through quantum tunneling at such a small scale. In order to store information for a reasonable period of a time, the magnet needs some mechanism to protect against this. Here again, the Ho-MgO pair turns out to be a winning combination, and one that warrants more exploration. Their electronic structures interact in such a way that the Ho becomes immune to quantum tunneling effects.

“Until recently it was considered very unlikely to achieve magnetic stability of this kind in a solid-state environment,” Gambardella says. “The same Ho atoms embedded in an insulating crystal display magnetic remanence only up to 150 mK. So what is special about Ho-MgO? This is the real question, which will eventually lead us to better understand how to control and enhance the magnetic lifetime of atoms and molecules, in a regime where quantum effects are all important.”

To characterize their system, the team exposed it to an external magnetic field to the point of saturation. Then they traced the magnetization loop for the Ho atoms, a measurement that provides information on how well a material retains its magnetization after the external field is turned off. Not only did they find high levels of magnetic remanence, they also saw exceptional stability even at very high magnetic fields that typically reduce the lifetime of single-molecule magnets.

Single-atom magnets with high magnetic remanence and long lifetimes could make it possible to probe and manipulate individual atomic spins in a controlled way, according to Gambardella. He calls this “paramount for the development of model classical and quantum memory devices based on single atom magnet arrays.”

“These scientists have been able to reach an extraordinary control of fundamental parameters that govern the magnetization dynamics at the atomic scale,” says Roberta Sessoli, an expert in molecular magnetism at the University of Florence. “The isolated Ho atoms on the MgO surface experience unprecedented magnetic anisotropy and protection from quantum fluctuations showing us a possible strategy to realize atomic magnetic memory units. Transferring these exceptional properties to objects that can be connected to the external world or inserted in devices is, however, an open challenge.”

Read the abstract in Science.