Probing the magnetic exchange forces of iron on the atomic scale

First-principles study of the magnetic exchange forces between the RuO2 (110) surface and Fe tip

Magnetic exchange force microscopy (MExFM) is an important experimental technique for mapping the magnetic structure of surfaces with atomic resolution relying on the spin-dependent short-range exchange interaction between a magnetic tip and a magnetic surface. RuO₂ is a significant compound with applications in heterogeneous catalysis and electrocatalysis. It has been characterized recently as an antiferromagnetic (AFM) material, and its magnetism has been predicted somewhat surprisingly to play an important role in its catalytic properties. In the current study, we explore theoretically whether MExFM can visualize the magnetic surface structure of RuO₂ . We use density functional theory (DFT) calculations to extract the exchange interactions between a ferromagnetic Fe tip interacting with an AFM RuO₂ (110) surface, as a function of tip-surface distance and the position of the tip over the surface. Mimicking the MExFM experiment, these data are then used to calculate the normalized frequency shift of an oscillating cantilever tip versus the minimum tip-surface distance, and construct corrugation height line profiles. It is found that the exchange interaction between tip and surface is strongest for a parallel configuration of the spins of the tip and of the surface; it is weakest for an anti-parallel orientation. In a corrugation profile, this gives rise to a sizable height difference of 25 pm between the spin-up and spin-down Ru atoms in the RuO₂ (110) surface at a normalized frequency shift γ ${\gamma }$ =-10.12 fNm^1/2 . The O atoms in the surface are not or hardly visible in the corrugation profile.

Quantifying exchange forces of a spin spiral on the atomic scale

The large interest in chiral magnetic structures for realization of nanoscale magnetic storage or logic devices has necessitated methods which can quantify magnetic interactions at the atomic scale. To overcome the limitations of the typically used current-based sensing of atomic-scale exchange interactions, a force-based detection scheme is highly advantageous. Here, we quantify the atomic-scale exchange force field between a ferromagnetic tip and a cycloidal spin spiral using our developed combination of current and exchange force detection. Compared to the surprisingly weak spin polarization, the exchange force field is more sensitive to atomic-scale variations in the magnetization. First-principles calculations reveal that the measured atomic-scale variations in the exchange force originate from different contributions of direct and indirect (Zener type) exchange mechanisms, depending on the chemical tip termination. Our work opens the perspective of quantifying different exchange mechanisms of chiral magnetic structures with atomic-scale precision using 3D magnetic exchange force field measurements.

Tuning the Exchange Bias on a Single Atom from 1 mT to 10 T

Shrinking spintronic devices to the nanoscale ultimately requires localized control of individual atomic magnetic moments. At these length scales, the exchange interaction plays important roles, such as in the stabilization of spin-quantization axes, the production of spin frustration, and creation of magnetic ordering. Here, we demonstrate the precise control of the exchange bias experienced by a single atom on a surface, covering an energy range of 4 orders of magnitude. The exchange interaction is continuously tunable from milli-eV to micro-eV by adjusting the separation between a spin-1/2 atom on a surface and the magnetic tip of a scanning tunneling microscope. We seamlessly combine inelastic electron tunneling spectroscopy and electron spin resonance to map out the different energy scales. This control of exchange bias over a wide span of energies provides versatile control of spin states, with applications ranging from precise tuning of quantum state properties, to strong exchange bias for local spin doping. In addition, we show that a time-varying exchange interaction generates a localized ac magnetic field that resonantly drives the surface spin. The static and dynamic control of the exchange interaction at the atomic scale provides a new tool to tune the quantum states of coupled-spin systems.

Long Spin-Relaxation Times in a Transition-Metal Atom in Direct Contact to a Metal Substrate

Long spin-relaxation times are a prerequisite for the use of spins in data storage or nanospintronics technologies. An atomic-scale solid-state realization of such a system is the spin of a transition-metal atom adsorbed on a suitable substrate. For the case of a metallic substrate, which enables the direct addressing of the spin by conduction electrons, the experimentally measured lifetimes reported to date are on the order of only hundreds of femtoseconds. Here, we show that the spin states of iron atoms adsorbed directly on a conductive platinum substrate have a surprisingly long spin-relaxation time in the nanosecond regime, which is comparable to that of a transition metal atom decoupled from the substrate electrons by a thin decoupling layer. The combination of long spin-relaxation times and strong coupling to conduction electrons implies the possibility to use flexible coupling schemes to process the spin information.

Magnetic exchange force microscopy: theoretical analysis of induced magnetization reversals

In magnetic exchange force microscopy a magnetic tip is scanned over the surface of a solid and an image representing the exchange interaction recorded. Sudden changes in the image corresponding to magnetization switching can be monitored as a function of the tip-surface distance thereby giving important information about the lifetime of metastable magnetic states and how it is affected by the exchange interaction. Here, theoretical calculations are carried out to study the tip-surface interaction and determine the mechanism and rate of transitions in a magnetic exchange force microscopy experiment, and comparison made with reported experimental data on an Fe cluster interacting with an antiferromagnetic Fe overlayer on a W(001) surface. The activation energy was determined from calculations of minimum energy paths and the pre-exponential factor in the Arrhenius rate expression evaluated from harmonic transition state theory, extended to account for zero modes. A noncollinear extension of the Alexander-Anderson model was used to describe the magnetic properties of an atomic scale representation of the system. The calculations reveal how the tip size, tip-surface distance and magnetic field affect the lifetime of the magnetic states.

Sensing Noncollinear Magnetism at the Atomic Scale Combining Magnetic Exchange and Spin-Polarized Imaging

Storing and accessing information in atomic-scale magnets requires magnetic imaging techniques with single-atom resolution. Here, we show simultaneous detection of the spin-polarization and exchange force with or without the flow of current with a new method, which combines scanning tunneling microscopy and noncontact atomic force microscopy. To demonstrate the application of this new method, we characterize the prototypical nanoskyrmion lattice formed on a monolayer of Fe/Ir(111). We resolve the square magnetic lattice by employing magnetic exchange force microscopy, demonstrating its applicability to noncollinear magnetic structures for the first time. Utilizing distance-dependent force and current spectroscopy, we quantify the exchange forces in comparison to the spin-polarization. For strongly spin-polarized tips, we distinguish different signs of the exchange force that we suggest arises from a change in exchange mechanisms between the probe and a skyrmion. This new approach may enable both nonperturbative readout combined with writing by current-driven reversal of atomic-scale magnets.

Probing the Nano-Skyrmion Lattice on Fe/Ir(111) with Magnetic Exchange Force Microscopy

We demonstrate that the magnetic nano-Skyrmion lattice on the Fe monolayer on Ir(111) and the positions of the Fe atoms can be resolved simultaneously using magnetic exchange force microscopy. Thus, the relation between magnetic and atomic structure can be determined straightforwardly by evaluating the Fourier transformation of the real space image data. We further show that the magnetic contrast can be mapped on a Heisenberg-like magnetic interaction between tip and sample spins. Since our imaging technique is based on measuring forces, our observation paves the way to study Skyrmions or other complex spin textures on insulating sample systems with atomic resolution.

Quantitative measurement of the magnetic exchange interaction across a vacuum gap

We demonstrate that magnetic exchange force spectroscopy allows for a quantitative determination of the distance-dependent magnetic exchange interaction across a vacuum gap. Experiments were performed on the antiferromagnetic Fe monolayer on W(001) with magnetically sensitive tips and compared to first-principles calculations performed for different cluster tip models. For stable tips, which can be distinguished from unstable tips by analyzing the dissipation signal, very good agreement with theory is observed.

Unambiguous determination of the adsorption geometry of a metal--organic complex on a bulk insulator

Individual molecules of Co-Salen, a small chiral paramagnetic metal--organic Schiff base complex, were deposited on NaCl(001) and subsequently imaged with noncontact atomic force microscopy employing Cr coated tips in a cryogenic ultrahigh vacuum environment. Images were obtained in which both the position and orientation of the adsorbed molecules and the atomic structure of the surface are resolved simultaneously, enabling the determination of the exact adsorption site. Density functional theory calculations were used to identify the ionic sublattice resolved with the Cr tip and also to confirm the adsorption site and orientation of the molecule on the surface. These calculations show that the central Co atom of the molecule physisorbs on top of a Cl ion and is aligned along 110-directions in its lowest energy configuration. In addition, a local energy minimum exists along 100-directions. Due to the chirality of the molecule, two mirror symmetric configurations rotated by approximately +/-5 degrees away from these directions are energetically equivalent. The resulting 16 low energy configurations are observed in the experimental images.

Atomic force microscopy as a tool for atom manipulation

During the past 20 years, the manipulation of atoms and molecules at surfaces has allowed the construction and characterization of model systems that could, potentially, act as building blocks for future nanoscale devices. The majority of these experiments were performed with scanning tunnelling microscopy at cryogenic temperatures. Recently, it has been shown that another scanning probe technique, the atomic force microscope, is capable of positioning single atoms even at room temperature. Here, we review progress in the manipulation of atoms and molecules with the atomic force microscope, and discuss the new opportunities presented by this technique.

Towards an understanding of the atomic scale magnetic contrast formation in NC-AFM: a tip material dependent MExFM study on NiO(001)

Recently, magnetic exchange force microscopy (MExFM) was established as a new force microscopy based technique capable of imaging arrangements of magnetic moments with atomic resolution on NiO(001). However, before this final achievement many unsuccessful experiments were performed on this particular sample system, and it is still not entirely clear which factors are important to attain an atomic scale magnetic contrast. Varying the tip's magnetic properties, we investigate the contrast formation on NiO(001). Fe-, Ni- and Gd-coated tips yielded a chemical contrast between Ni and O atoms, but a magnetic signal between Ni atoms with magnetic moments pointing in the opposite direction could only be observed with out-of-plane sensitive Fe-coated tips. Our observations suggest that three factors are crucial to obtain a sufficiently large magnetic signal: (i) a large overlap between the spin-carrying electronic states, (ii) a collinear orientation between the magnetic moments and (iii) a large magnetic moment.