Susanne Baumann

Bistability in Atomic-Scale Antiferromagnets

Structured Memories High-density magnetic memory is generally produced by using ferromagnetic materials. As the density increases and the memory elements become closer together, stray fields can result in cross-talk and a corruption of the stored information. Antiferromagetic structures, however, are expected to be relatively insensitive to magnetic fields and so should, in principle, allow the elements to be packed in even closer. Loth et al. (p. 196) carried out low-temperature experiments to construct antiferromagnetic structures atom by atom. Electrical switching of the magnetic states was observed, and data could be robustly stored on the structure for several hours, albeit at low temperature. Atomically engineered antiferromagnets consisting of a few atoms exhibit stable magnetic states at low temperature. Control of magnetism on the atomic scale is becoming essential as data storage devices are miniaturized. We show that antiferromagnetic nanostructures, composed of just a few Fe atoms on a surface, exhibit two magnetic states, the Néel states, that are stable for hours at low temperature. For the smallest structures, we observed transitions between Néel states due to quantum tunneling of magnetization. We sensed the magnetic states of the designed structures using spin-polarized tunneling and switched between them electrically with nanosecond speed. Tailoring the properties of neighboring antiferromagnetic nanostructures enables a low-temperature demonstration of dense nonvolatile storage of information.

Reaching the magnetic anisotropy limit of a 3d metal atom

Maximizing atomic magnetic memory A study of the magnetic response of cobalt atoms adsorbed on oxide surfaces may lead to much denser storage of data. In hard drives, data are stored as magnetic bits; the magnetic field pointing up or down corresponds to storing a zero or a one. The smallest bit possible would be a single atom, but the magnetism of a single atom —its spin—has to be stabilized by interactions with heavy elements or surfaces through an effect called spin-orbit coupling. Rau et al. (see the Perspective by Khajetoorians and Wiebe) built a model system in pursuit of single-atom bits—cobalt atoms adsorbed on magnesium oxide. At temperatures approaching absolute zero, the stabilization of the spins magnetic direction reached the maximum that is theoretically possible. Science, this issue p. 988; see also p. 976 A cobalt atom bound to a single oxygen site on magnesia has the maximum magnetic anisotropy allowed for a transition metal [Also see Perspective by Khajetoorians and Wiebe] Designing systems with large magnetic anisotropy is critical to realize nanoscopic magnets. Thus far, the magnetic anisotropy energy per atom in single-molecule magnets and ferromagnetic films remains typically one to two orders of magnitude below the theoretical limit imposed by the atomic spin-orbit interaction. We realized the maximum magnetic anisotropy for a 3d transition metal atom by coordinating a single Co atom to the O site of an MgO(100) surface. Scanning tunneling spectroscopy reveals a record-high zero-field splitting of 58 millielectron volts as well as slow relaxation of the Co atom’s magnetization. This striking behavior originates from the dominating axial ligand field at the O adsorption site, which leads to out-of-plane uniaxial anisotropy while preserving the gas-phase orbital moment of Co, as observed with x-ray magnetic circular dichroism.

Electron paramagnetic resonance of individual atoms on a surface

EPR, one atom at a time Electron paramagnetic resonance (EPR) usually detects atoms with unpaired electrons as ensemble averages. Baumann et al. used a spin-polarized scanning tunneling microscope tip to measure EPR spectra of single iron atoms adsorbed on a magnesium oxide surface at cryogenic temperatures. The measurement depends on the atomic orbital symmetry; no signal was observed for cobalt atoms under the same conditions Science, this issue p. 417 The electron paramagnetic signal of individual iron atoms on an oxide surface is probed with a scanning tunneling microscope. We combined the high-energy resolution of conventional spin resonance (here ~10 nano–electron volts) with scanning tunneling microscopy to measure electron paramagnetic resonance of individual iron (Fe) atoms placed on a magnesium oxide film. We drove the spin resonance with an oscillating electric field (20 to 30 gigahertz) between tip and sample. The readout of the Fe atom’s quantum state was performed by spin-polarized detection of the atomic-scale tunneling magnetoresistance. We determine an energy relaxation time of T1 ≈ 100 microseconds and a phase-coherence time of T2 ≈ 210 nanoseconds. The spin resonance signals of different Fe atoms differ by much more than their resonance linewidth; in a traditional ensemble measurement, this difference would appear as inhomogeneous broadening.

Origin of Perpendicular Magnetic Anisotropy and Large Orbital Moment in Fe Atoms on MgO

We report on the magnetic properties of individual Fe atoms deposited on MgO(100) thin films probed by x-ray magnetic circular dichroism and scanning tunneling spectroscopy. We show that the Fe atoms have strong perpendicular magnetic anisotropy with a zero-field splitting of 14.0±0.3  meV/atom. This is a factor of 10 larger than the interface anisotropy of epitaxial Fe layers on MgO and the largest value reported for Fe atoms adsorbed on surfaces. The interplay between the ligand field at the O adsorption sites and spin-orbit coupling is analyzed by density functional theory and multiplet calculations, providing a comprehensive model of the magnetic properties of Fe atoms in a low-symmetry bonding environment.

Measuring the Three-Dimensional Structure of Ultrathin Insulating Films at the Atomic Scale

The increasing technological importance of thin insulating layers calls for a thorough understanding of their structure. Here we apply scanning probe methods to investigate the structure of ultrathin magnesium oxide (MgO) which is the insulating material of choice in spintronic applications. A combination of force and current measurements gives high spatial resolution maps of the local three-dimensional insulator structure. When force measurements are not available, a lower spatial resolution can be obtained from tunneling images at different voltages. These broadly applicable techniques reveal a previously unknown complexity in the structure of MgO on Ag(001), such as steps in the insulator-metal interface.

Pasteur's Experiment Performed at the Nanoscale: Manual Separation of Chiral Molecules, One by One.

Understanding the principles of molecular recognition is a difficult task and calls for investigation of appropriate model systems. Using the manipulation capabilities of scanning tunneling microscopy (STM) we analyzed the chiral recognition in self-assembled dimers of helical hydrocarbons at the single molecule level. After manual separation of the two molecules of a dimer with a molecule-terminated STM tip on a Cu(111) surface, their handedness was subsequently determined with a metal atom-terminated tip. We find that these molecules strongly prefer to form heterochiral pairs. Our study shows that single molecule manipulation is a valuable tool to understand intermolecular recognition at surfaces.

Generation of constant-amplitude radio-frequency sweeps at a tunnel junction for spin resonance STM

We describe the measurement and successful compensation of the radio-frequency transfer function of a scanning tunneling microscope over a wide frequency range (15.5-35.5 GHz) and with high dynamic range (>50 dB). The precise compensation of cabling resonances and attenuations is critical for the production of constant-voltage frequency sweeps for electric-field driven electron spin resonance (ESR) experiments. We also demonstrate that a well-calibrated tunnel junction voltage is necessary to avoid spurious ESR peaks that can arise due to a non-flat transfer function.