Transfer of magnetic anisotropy in epitaxial Co/NiO/Fe trilayers.

The magnetic properties of Co(10 Å)/NiO(40 Å)/Fe trilayer epitaxially grown on W(110) substrate were investigated with use of x-ray magnetic linear dichroism (XMLD) and x-ray magnetic circular dichroism (XMCD). We showed that magnetic anisotropy of Fe film that can be controlled by a thickness-driven spin reorientation transition is transferred via interfacial exchange coupling not only to NiO layer but further to ferromagnetic Co overlayer as well. Similarly, a temperature driven spin reorientation of Fe sublayer induces a reorientation of NiO spin orientation and simultaneous switching of the Co magnetization direction. Finally, by element specific XMCD and XMLD magnetic hysteresis loop measurements we proved that external magnetic field driven reorientation of Fe and Co magnetizations as well as NiO Néel vector are strictly correlated and magnetic anisotropy fields of Fe and Co sublayers are identical despite the different crystal structures.

Resonant Auger spectromicroscopy in ultrathin Fe films on W(110).

L3M2,3M2,3Auger transition is measured near the L3resonance of ferromagnetic Fe films on W(110). The kinetic energies of the Auger peaks display the typical Raman behaviour for photon energies well below the absorption threshold, where the Auger energy follows the changes in the photon energy. Classical Auger behaviour with constant kinetic energy sets in at about 1.5 eV below the L3resonance independently from the number of Fe layers down to the monolayer thickness. Strong x-ray circular magnetic dichroism is observed at the L3edge in the entire L3M2,3M2,3Auger spectrum. Different Auger features originating from the final state with two 3p core holes show slight variations in the dichroic signal, which is attributed to the exchange interaction between the core holes and the valence band. Finally, XMCD-PEEM magnetic domain imaging using Auger electrons is demonstrated with a high level of contrast and lateral resolution approaching that of imaging with secondary photoelectrons.

Tunable magnetic anisotropy of antiferromagnetic NiO in (Fe)/NiO/MgO/Cr/MgO(001) epitaxial multilayers.

We report on the magnetic properties of antiferromagnetic NiO(001) thin films in epitaxially grown NiO/MgO(dMgO)/Cr/MgO(001) system for different thicknesses of MgO, dMgO. Results of X-ray Magnetic Linear Dichroism show that together with an increase of dMgO, rotation of NiO spins from in-plane towards out-of-plane direction occurs. Furthermore, we investigated how the proximity of Fe modifies the magnetic state of NiO in Fe/NiO/MgO(dMgO)/Cr/MgO(001). We proved the existence of a multidomain state in NiO as a result of competition between the ferromagnet/antiferromagnet exchange coupling and strain exerted on the NiO by the MgO buffer layer.

Correction: Fine tuning of ferromagnet/antiferromagnet interface magnetic anisotropy for field-free switching of antiferromagnetic spins.

Correction for 'Fine tuning of ferromagnet/antiferromagnet interface magnetic anisotropy for field-free switching of antiferromagnetic spins' by M. Slezak et al., Nanoscale, 2020, DOI: 10.1039/d0nr04193a.

Fine tuning of ferromagnet/antiferromagnet interface magnetic anisotropy for field-free switching of antiferromagnetic spins.

We show that in a uniform thickness NiO(111)/Fe(110) epitaxial bilayer system, at given temperature near 300 K, two magnetic states with orthogonal spin orientations can be stabilized in antiferromagnetic NiO. Field-free, reversible switching between these two antiferromagnetic states is demonstrated. The observed phenomena arise from the unique combination of precisely tuned interface magnetic anisotropy, thermal hysteresis of spin reorientation transition and interfacial ferromagnet/antiferromagnet exchange coupling. The possibility of field-free switching between two magnetic states in an antiferromagnet is fundamentally interesting and can lead to new ideas in heat assisted magnetic recording technology.

Coherent x-ray scattering in an XPEEM setup.

X-ray photoemission electron microscopy, one of the most successful imaging tools at synchrotrons, is known to have limitations related to the application of external fields and to the short electron mean free path. In order to overcome such issues, we adapt an existing XPEEM instrument to simultaneously perform coherent x-ray scattering measurements in reflectivity mode, thus adding a complementary method to XPEEM. Photon-in photon-out x-ray scattering provides the sensitivity to buried interfaces as well as the possibility to work under external fields, which is challenging when using charged particles for imaging. XPEEM, in turn, greatly alleviates the difficulties associated with the reconstruction methods used in coherent diffraction imaging. The combination of the two methods is demonstrated for an artifical spin-ice lattice showing both chemical and magnetic contrast.

Erratum: Fast Domain Wall Motion Governed by Topology and Œrsted Fields in Cylindrical Magnetic Nanowires [Phys. Rev. Lett. 123, 217201 (2019)].

This corrects the article DOI: 10.1103/PhysRevLett.123.217201.

Fast Domain Wall Motion Governed by Topology and Œrsted Fields in Cylindrical Magnetic Nanowires.

While the usual approach to tailor the behavior of condensed matter and nanosized systems is the choice of material or finite-size or interfacial effects, topology alone may be the key. In the context of the motion of magnetic domain walls (DWs), known to suffer from dynamic instabilities with low mobilities, we report unprecedented velocities >600 m/s for DWs driven by spin-transfer torques in cylindrical nanowires made of a standard ferromagnetic material. The reason is the robust stabilization of a DW type with a specific topology by the Œrsted field associated with the current. This opens the route to the realization of predicted new physics, such as the strong coupling of DWs with spin waves above >600 m/s.

Spin structure and spin Hall magnetoresistance of epitaxial thin films of the insulating non-collinear antiferromagnet SmFeO3.

We report a combined study of imaging the antiferromagnetic (AFM) spin structure and measuring the spin Hall magnetoresistance (SMR) in epitaxial thin films of the insulating non-collinear antiferromagnet SmFeO3. X-ray magnetic linear dichroism photoemission electron microscopy measurements reveal that the AFM spins of the SmFeO3(1 1 0) align in the plane of the film. Angularly dependent magnetoresistance measurements show that SmFeO3/Ta bilayers exhibit a positive SMR, in contrast to the negative SMR expected in previously studied collinear AFMs. The SMR amplitude increases linearly with increasing external magnetic field at higher magnetic fields, suggesting that field-induced canting of the AFM spins plays an important role. In contrast, around the coercive field, no detectable SMR signal is observed, indicating that the SMR of the AFM and canting magnetization components cancel out. Below 50 K, the SMR amplitude increases sizably by a factor of two as compared to room temperature, which likely correlates with the long-range ordering of the Sm ions. Our results show that the SMR is a sensitive technique for non-equilibrium spin systems of non-collinear AFMs.

Synthesis of Antimonene on Germanium.

The lack of large-area synthesis processes on substrates compatible with industry requirements has been one of the major hurdles facing the integration of 2D materials in mainstream technologies. This is particularly the case for the recently discovered monoelemental group V 2D materials which can only be produced by exfoliation or growth on exotic substrates. Herein, to overcome this limitation, we demonstrate a scalable method to synthesize antimonene on germanium substrates using solid-source molecular beam epitaxy. This emerging 2D material has been attracting a great deal of attention due to its high environmental stability and its outstanding optical and electronic properties. In situ low energy electron microscopy allowed the real time investigation and optimization of the 2D growth. Theoretical calculations combined with atomic-scale microscopic and spectroscopic measurements demonstrated that the grown antimonene sheets are of high crystalline quality, interact weakly with germanium, exhibit semimetallic characteristics, and remain stable under ambient conditions. This achievement paves the way for the integration of antimonene in innovative nanoscale and quantum technologies compatible with the current semiconductor manufacturing.

Cathode lens spectromicroscopy: methodology and applications.

The implementation of imaging techniques with low-energy electrons at synchrotron laboratories allowed for significant advancement in the field of spectromicroscopy. The spectroscopic photoemission and low energy electron microscope, SPELEEM, is a notable example. We summarize the multitechnique capabilities of the SPELEEM instrument, reporting on the instrumental aspects and the latest developments on the technical side. We briefly review applications, which are grouped into two main scientific fields. The first one covers different aspects of graphene physics. In particular, we highlight the recent work on graphene/Ir(100). Here, SPELEEM was employed to monitor the changes in the electronic structure that occur for different film morphologies and during the intercalation of Au. The Au monolayer, which creeps under graphene from the film edges, efficiently decouples the graphene from the substrate lowering the Dirac energy from 0.42 eV to 0.1 eV. The second field combines magnetism studies at the mesoscopic length scale with self-organized systems featuring ordered nanostructures. This example highlights the possibility to monitor growth processes in real time and combine chemical characterization with X-ray magnetic circular dichroism-photoemission electron microscopy (XMCD-PEEM) magnetic imaging by using the variable photon polarization and energy available at the synchrotron source.

Reversible switching of in-plane polarized ferroelectric domains in BaTiO3(001) with very low energy electrons.

The switchable bipolar ground state is at the heart of research into ferroelectrics for future, low-energy electronics. Polarization switching by an applied field is a complex phenomenon which depends on the initial domain ordering, defect concentration, electrical boundary conditions and charge screening. Injected free charge may also to be used to reversibly switch in-plane polarized domains. We show that the interaction between the initial domain order and the bulk screening provided by very low energy electrons switches the polarization without the collateral radiation damage which occurs when employing a beam of high energy electrons. Polarization switching during charge injection adds a new dimension to the multifunctionality of ferroelectric oxides.

The bottom-up growth of edge specific graphene nanoribbons.

The discovery of ballistic transport in graphene grown on SiC(0001) sidewall trenches has sparked an intense effort to uncover the origin of this exceptional conductivity. How a ribbon's edge termination, width, and topography influence its transport is not yet understood. This work presents the first structural and electronic comparison of sidewall graphene grown with different edge terminations. We show that armchair and zigzag terminated ribbons, grown from SiC, have very different topographies and interact differently with the substrate, properties that are critical to device architecture in sidewall ribbon electronics.

Size distribution of magnetic charge domains in thermally activated but out-of-equilibrium artificial spin ice.

A crystal of emerging magnetic charges is expected in the phase diagram of the dipolar kagomé spin ice. An observation of charge crystallites in thermally demagnetized artificial spin ice arrays has been recently reported by S. Zhang and coworkers and explained through the thermodynamics of the system as it approaches a charge-ordered state. Following a similar approach, we have generated a partial order of magnetic charges in an artificial kagomé spin ice lattice made out of ferrimagnetic material having a Curie temperature of 475 K. A statistical study of the size of the charge domains reveals an unconventional sawtooth distribution. This distribution is in disagreement with the predictions of the thermodynamic model and is shown to be a signature of the kinetic process governing the remagnetization.

Desorption kinetics from a surface derived from direct imaging of the adsorbate layer.

There are numerous indications that adsorbed particles on a surface do not desorb statistically, but that their spatial distribution is important. Evidence almost exclusively comes from temperature-programmed desorption, the standard method for measuring desorption rates. However, this method, as a kinetics experiment, cannot uniquely prove an atomic mechanism. Here we report a low-energy electron microscopy investigation in which a surface is microscopically imaged while simultaneously temperature-programmed desorption is recorded. The data show that during desorption of oxygen molecules from a silver single crystal surface, islands of oxygen atoms are present. By correlating the microscopy and the kinetics data, a model is derived that includes the shapes of the islands and assumes that the oxygen molecules desorb from the island edges. The model quantitatively reproduces the complex desorption kinetics, confirming that desorption is affected by islands and that the often used mean-field treatment is inappropriate.

Coexistence of multiple silicene phases in silicon grown on Ag(1 1 1).

Silicene, the silicon equivalent of graphene, is attracting increasing scientific and technological attention in view of the exploitation of its exotic electronic properties. This novel material has been theoretically predicted to exist as a free-standing layer in a low-buckled, stable form, and can be synthesized by the deposition of Si on appropriate crystalline substrates. By employing low-energy electron diffraction and microscopy, we have studied the growth of Si on Ag(1 1 1) and observed a rich variety of rotationally non-equivalent silicene structures. Our results highlight a very complex formation diagram, reflecting the coexistence of different and nearly degenerate silicene phases, whose relative abundance can be controlled by varying the Si coverage and growth temperature. At variance with other studies, we find that the formation of single-phase silicene monolayers cannot be achieved on Ag(1 1 1).

Fine tuning of graphene-metal adhesion by surface alloying.

We show that bimetallic surface alloying provides a viable route for governing the interaction between graphene and metal through the selective choice of the elemental composition of the surface alloy. This concept is illustrated by an experimental and theoretical characterization of the properties of graphene on a model PtRu surface alloy on Ru(0001), with a concentration of Pt atoms in the first layer between 0 and 50%. The progressive increase of the Pt content determines the gradual detachment of graphene from the substrate, which results from the modification of the carbon orbital hybridization promoted by Pt. Alloying is also found to affect the morphology of graphene, which is strongly corrugated on bare Ru, but becomes flat at a Pt coverage of 50%. The method here proposed can be readily extended to several supports, thus opening the way to the conformal growth of graphene on metals and to a full tunability of the graphene-substrate interaction.

Self-organization in Pd/W(110): interplay between surface structure and stress.

It has recently been shown that submonolayer Pd on W(110) forms highly ordered linear mesoscopic stripes at high temperatures. The stripes display an internal Pd superstructure with a nano-scale periodicity along the direction perpendicular to the periodicity of the stripes. The same type of superstructure is also observed in a wide range of temperatures below the stripe formation temperature. We present a combined experimental and theoretical study of this superstructure of Pd on W(110) and investigate its influence on the appearance of the linear mesoscopic stripes. By means of low-energy electron diffraction and low-energy electron microscopy we show that it has a far more peculiar dependence on temperature and coverage than expected from a regular surface reconstruction. Using density-functional theory, we model the Pd superstructures as periodic vacancy-line type configurations and investigate their energetics and elastic properties. From our calculated surface stresses and anisotropies for the vacancy-line configurations, and based on the continuum elasticity theory, we demonstrate quantitatively that the vacancy-line type of structure is a prerequisite for the formation of the linear mesoscopic stripes. Moreover, we show that the physics driving the formation of the internal superstructure is very similar to the one at play in forming the mesoscopic stripes themselves.

Spectromicroscopy of pulses transporting alkali metal in a surface reaction.

The NO + H2 reaction on a potassium promoted Rh(110) surface is shown to sustain the formation of spatio-temporal periodic patterns leading to mass transport phenomena. The excitation of pulses and the mass transport mechanism are studied in the 10(-7) and 10(-6) mbar pressure range, with the potassium coverage varying between θK = 0.05 and θK = 0.12 ML. Using spectroscopic photoemission and spectroscopic low energy electron microscopy (SPELEEM) as well as related microprobe diffraction techniques, we show that the excitation mechanism comprises a cyclic structural transformation: K + O-coadsorbate → (2 × 1)-N → c(2 × 4)-2O,N → K + O coadsorbate. Laterally resolved spectroscopy demonstrates that potassium is accumulated in front of the nitrogen pulses, suggesting that adsorbed nitrogen acts as a diffusion barrier for potassium.

Growth of magnetic nanowires on self-organized stripe templates: Fe on Pd-O/W(110).

The structural and magnetic properties of Fe-Pd nanostructures grown on regular alternating Pd and oxygen stripes on the W(110) surface are studied using SPELEEM methods. The self-organized Pd-O stripe template, formed at about 1000 °C and aligned with the [001] direction, has an average period of only few tens nm and is preserved upon quenching the sample temperature to ambient conditions. Fe is shown to preferentially bond to the Pd stripes, forming an Fe-Pd surface alloy within the stripe phase. The magnetic easy axis is found to be aligned perpendicular to the Fe-Pd stripe axis.