Explore the magnetism of a single atom

Magnetic devices such as hard drives, magnetic random access memories (MRAMs), molecular magnets, and quantum computers depend on the manipulation of magnetic properties. In an atom, magnetism results from the spin and orbital momentum of its electrons. “Magnetic anisotropy” describes how the magnetic properties of an atom depend on the orientation of the electron orbits relative to the structure of a material. It also provides directionality and stability to magnetization.

Publication in Science, researchers led by EPFL are combining various experimental and computer methods to measure for the first time the energy required to modify the magnetic anisotropy of a single atom of cobalt. Their methodology and findings may impact a range of fields from fundamental studies of single atom and single molecule magnetism to the design of spintronic device architectures.

Magnetism is widely used in technologies ranging from hard drives to magnetic resonance, and even in quantum computer designs. In theory, every atom or molecule has the potential to be magnetic, because it depends on the movement of its electrons. Electrons move in two ways: spin, which can be thought of as spinning around themselves, and orbit, which refers to the movement of an electron around the nucleus of its atom. The spin and orbital motion gives rise to magnetization, similar to an electric current flowing through a coil and producing a magnetic field. The direction of rotation of the electrons therefore defines the direction of magnetization in a material.

The magnetic properties of a material have a certain “preference” or “stubbornness” towards a specific direction. This phenomenon is called “magnetic anisotropy” and is described as the “directional dependence” of the magnetism of a material. Changing this “preference” requires a certain amount of energy. The total energy corresponding to the magnetic anisotropy of a material is a fundamental constraint in downscaling magnetic devices such as MRAMs, computer hard disks, and even quantum computers, which use different spin states of electrons. as separate units of information, or “qubits”.

Harald Brune’s team at EPFL, in collaboration with scientists from ETH Zurich, the Paul Scherrer Institute and the IBM Almaden research center, has developed a method to determine the maximum possible magnetic anisotropy for a single atom of cobalt. Cobalt, which is classified as a “transition metal”, is widely used in the manufacture of permanent magnets as well as in magnetic recording materials for data storage applications.

The researchers used a technique called inelastic electron tunneling spectroscopy to probe the quantum spin states of a single cobalt atom bound to a layer of MgO. The technique uses an atom-sized scanning tip that allows electrons to pass (or “tunnel”) into the bound cobalt atom. When electrons passed through a tunnel, they transferred energy to the cobalt atom, inducing changes in its spin properties.

The experiments showed the maximum magnetic anisotropy energy of a single atom (~ 60 millielectron volts) and the longest spin lifetime for a single transition metal atom. This large anisotropy leads to a remarkable magnetic moment, which was determined with measurements based on the synchrotron on the X-Treme beamline of the Swiss light source. Although fundamental, these findings pave the way for a better understanding of magnetic anisotropy and present a single atom model system that can potentially be used as a future “qubit”.

“Quantum computing uses quantum states of matter, and magnetic properties are such a quantum state,” says Harald Brune. “They have a lifetime and you can use the individual atoms adsorbed to the surface to create qubits. Our system is a model for such a state. It allows us to optimize quantum properties, and it’s easier than that. the previous ones because we know exactly where the cobalt atom is in relation to the MgO layer. ”