Dec 2, 2011
From single-atom magnetometry to tailor-made magnets
The development of novel magnetic materials as well as spin-based electronics ('spintronics') are hot topics in materials science and solid state physics. Both research fields could profit tremendously from atomic-scale insight into magnetic properties and spin-dependent interactions at the atomic level. Based on the development of spin-polarized scanning tunneling microscopy (SP-STM), researchers have recently established the novel method of single-atom magnetometry which allows the measurement of magnetization curves and the determination of magnetic moments on an atom-by-atom basis. While the sensitivity level of single-atom magnetometry is below one Bohr magneton, it can easily be combined with the atomic-resolution imaging and manipulation capabilities of conventional STM, thereby offering a novel approach towards a rational material design based on the knowledge of the atomic-level properties and interactions within the solid state. Moreover, an atom-by-atom design and realization of all-spin logic devices has recently been demonstrated by researchers at Hamburg University based on the combined knowledge derived from surface physics, nanoscience, and magnetism.
The development of the scanning tunneling microscope by Gerd Binnig, Heinrich Rohrer, Christoph Gerber, and coworkers 30 years ago has revolutionized our perception about atomic-scale structures and processes, and has been one of the most important pillars for the broad field of nanoscience and nanotechnology. Adding spin resolution to the STM has been a great challenge since the early days of STM, requiring the precise control of the spin state of the front atom of the probe tip.
While vacuum tunneling of spin-polarized electrons was first observed at magnetic surfaces of bulk crystals more than 20 years ago, it has recently become possible to measure spin-polarized currents between a magnetic single-atom tip and a single magnetic adatom on an otherwise non-magnetic substrate, thereby demonstrating that tunnelling magnetoresistance (TMR) is detectable even for atomic-sized magnetic tunnel junctions. Moreover, by measuring the spin-resolved tunnelling conductance as a function of an externally applied magnetic field, the Germany-based team has recently demonstrated the first measurements of single-atom magnetization curves, allowing the detection of individual magnetic moments with a sensitivity level as small as a fraction of a Bohr magneton.
The novel method of single-atom magnetometry allowed the researchers to study spin-dependent interactions between individual magnetic atoms and magnetic nanostructures as well as between individual magnetic adatoms at an energy resolution down to 0.05 meV. Moreover, by combining the new method of single-atom magnetometry with spin-polarized STM imaging and manipulation, the group has recently been able to investigate artificially constructed magnets on an atom-by-atom basis, leading us to the intriguing possibility of developing novel magnets based on a rational material design.
The direct combination of single-atom magnetometry, single-atom manipulation, and spin-resolved STM imaging recently culminated in the first demonstration of an all-spin logic device at the atomic level. Since the individual spins of that device switch on an intrinsic time scale of only 200 fs, this type of device concept promises to be extremely fast and energy-efficient, while simultaneously reaching the ultimate limit of miniaturization.
While the method of single-atom magnetometry has so far been applied to metallic and semiconducting as well as molecular systems, it also offers great potential for superconductivity research.
Additional information can be found in J. Phys. D. Appl. Phys. 44 464009.
• For more on this theme, check out the special issue of Journal of Physics D: Applied Physics celebrating the 30th anniversary of the invention of the scanning tunelling microscope - three decades of scanning tunnelling microscopy that changed the course of surface science.
About the author
Professor Roland Wiesendanger pioneered the technique of spin-polarized scanning tunnelling microscopy (SP-STM) in the late eighties. At the beginning, magnetic surfaces of bulk crystals as well as ultra-thin magnetic films and nanostructures were in the focus of his SP-STM research activities. Thanks to the development of a 300 mK / 14 T ultrahigh-vacuum STM system by Dr Jens Wiebe and coworkers at the University of Hamburg, single magnetic atoms on non-magnetic substrates became accessible by the SP-STM technique, leading to the novel method of single-atom magnetometry combined with atomic-scale spatial resolution. During his PhD study, Lihui Zhou demonstrated the application of single-atom magnetometry towards the investigation of indirect-exchange mediated interactions between individual magnetic atoms with an energy resolution down to about 0.05 meV. His PhD work at Hamburg University was recently awarded by the Chinese Government.