Technological innovations in the nanoscience field now allow physicists to ascertain the magnetic interactions and properties of single atoms, a paramount discovery in the quest to increase novel device memory and computing capacity and efficiency.

A recent report in Science magazine by a group of world-class researchers demonstrates “the ability to measure magnetization curves of individual magnetic atoms absorbed on a nonmagnetic substrate with use of a scanning tunneling microscope with a spin-polarized tip.” Their findings challenge the fundamental theoretical understanding generally accepted on the polarization of atoms in response to a magnetic field; suggesting that further theory work may be necessary to explain the empirical phenomena of increased oscillatory spacing discovered in the polarization switching of single atoms due to their proximity to the monolayer, according to Arthur R. Smith, director of the Nanoscale and Quantum Phenomena Institute at Ohio University.

The findings reported in “Revealing Magnetic Interactions from Single-Atom Magnetization Curves,” by Focko Meier, Lihui Zhou, Jens Wiebe and Roland Wiesendanger of the Institute of Applied Physics and Microstructure Research Center at the University of Hamburg in Germany, a partner institution in the PIRE Spin Triangle with NQPI, demonstrate the ability to study the individual magnetic properties of single atoms using the unique SP-STS technology.

Since the dawn of magnetism research, scientists have explored the magnetization response of samples to an external magnetic field, known as a magnetization curve. Modern technological advancements are allowing researchers to gather magnetism information on smaller and smaller samples, now extending to the atomic level.

Their investigations of “magnetic nanostructures consisting of a few atoms on nonmagnetic substrates (adatoms) are explored as model systems for miniaturized data storage and spintronic devices for the implementation of quantum computing.”

By using magnetization curves, a sample's magnetic moment – the strength and direction of the dipole – and magnetic anisotropy energy is deduced. Ever-increasing sensitivity and detection methods are essential to studying the magnetic properties and individual spins of smaller and smaller samples.

In their experiments, the Hamburg researchers used the spin-polarized scanning tunneling spectroscopy (SP-STS) method, which is ideal for detecting “single spins stabilized by direct exchange to (anti)ferromagnetic layers.” In the past, the experimental challenge posed by spin instability has prevented the detection of individual spins in a nonmagnetic environment, they reported.

By using the SP-STS method, the researchers were able to “demonstrate the direct detection of the magnetization of single adatoms on a nonmagnetic metallic substrate as a function of an external magnetic field.” To do so, Cobalt (Co) adatoms were placed on a strongly polarizable platinum (111) substrate, to form significant and measurable magnetic moments, which have a strong out-of-plane anisotropy.

In their initial experiments, the Hamburg researchers “intent was to measure the magnetic interaction between stripes of one atomic layer of Co grown at room temperature and the individual adatoms deposited at about 25K on the bare Pt(111).” Their results are crucial to understanding why variations in magnetic properties exist between evidently identical adatoms due to different local environments induced by temperature change. By creating monolayer stripes that are magnetized perpendicular to the surface, they were able to develop a calibration standard for the magnetic properties of the SP-STM tip.

Subsequent experimental results “imply the dominance of a temperature-independent switching process, for example, quantum tunneling of the magnetization or current-induced magnetization switching by inelastic processes,” they reported. To determine the magnetic moment of the individual Co atoms, the researchers fitted the measured magnetization curves to the data gained through prior experimental procedures, revealing a corresponding fit superbly reproducing single-atom magnetization curves.

They determined the variation in the magnetic moments observed was induced by a magnetic interaction between the adatoms at a specific energy scale. “This is consistent with an increase variance only at very low temperatures,” according to the report. “Direct exchange and dipolar interactions can be neglected because of the large separation of the adatoms. Therefore, we assume that indirect exchange via the Pt substrate is responsible for the variance.”

In assuming the validity of this hypothesis, the Hamburg researchers expected a long-range coupling between the adatoms and the monolayer stripes. Using SP-STS, the behavior of adatoms observed varies significantly between those close to and those distant from the monolayer. The closer adatoms' behavior corresponds to typical ferromagnetic behavior, switching its magnetization to align with the direction of the magnetic field.

When the magnetization is in the downward direction, the atoms produce a greater response, while the same atom has a smaller response to a magnetic field in the upward direction. In addition, when the SP-STS tip magnetization is parallel to that of the atom, a greater response is measured in comparison to when the tip is antiparallel to the Co atom. The parallel alignment of the atom with the tip occurs when the magnetic field is introduced.

The magnetic field is applied using a circular superconductive electromagnet, which carries a large current while refraining from dissipating energy and creating heat. When a magnetic field is not present, the spin and magnetic orientation of the spins on the surface are haphazard; the application of the magnetic field therefore aligns the atoms with the direction of the magnetic field.

These experiments reveal that single-atoms of Co can be magnetized, and that when the magnetic field is turned off, the magnetization of atoms is the same for those closer to and further from the monolayer stripe. In studying the interaction of the monolayer stripe and the spin and magnetization curves of individual atoms, the Hamburg researchers discovered that some atoms are polarized even when they're close to the stripe without the application of a magnetic field, and that those closer to the stripe have the propensity to polarize in a certain direction.

An oscillating effect is created due to the proximity of the atom to the monolayer stripes; the further away the atom is, its polarization changes from up to down. The spacing observed between the polarization switches of the atoms is larger than predicted by the Ruderman-Kittel-Kasuya-Yosida (RKKY) theory. This raises the question of whether this long-standing theory is appropriate, or if these experiments are an indication that the fundamental theoretical understanding we have about the polarization of atoms needs to be reassessed.

The study of the magnetic interactions from single-atom magnetization curves suggests that in the future single atoms may be utilized for storing information in practical devices. This will be possible because when the magnetic field is turned off, the magnetization of the single-atoms close to the monolayer stripe is fixed. If the magnetization of the atoms was not fixed in the absence of a magnetic field, the ferromagnets used as a storage medium would lose their information.

The development of novel devices implementing single-atom technology would increase storage capacity dramatically to that of our current disc drive capabilities, said Smith. “As a ball park estimate, you could store roughly thousands of times more information per square inch, due to the increased capacity and the smaller storage space necessary.”

The Hamburg group's pioneering research on single-atom magnetization curves and magnetic interactions is crucial to the development of novel devices with increased storage capacity and efficiency. By demonstrating a single-atom's magnetization can be fixed in the absence of a magnetic field, when it is in close proximity to the monolayer, single-atoms are now viable candidates for storing information – the beginning of the technological revolution at the atomic scale is upon us.