Spin-polarized low-energy electron microscopy (SPLEEM) is a technique for imaging magnetic microstructures at surfaces and in thin films. In this article, principles, advantages and limitations of SPLEEM are reviewed. Several recent studies illustrate how SPLEEM can be used to investigate spin reorientation transition phenomena, to determine magnetic domain configurations in low-dimensional structures, or to explore physics of magnetic couplings in layered systems. The work highlights the capability of the technique to reveal in situ and in real time quantitative information on micromagnetic configurations and structure-property relationships. In addition, spectroscopic reflectivity measurements with spin-polarized low-energy electron beams can be a useful tool to probe spin-dependent unoccupied band structure of magnetic materials and electronic properties of buried magnetic interfaces.

https://hal.archives-ouvertes.fr/hal-00505025/document

Magnetic imaging with spin-polarized low-energy electron microscopy

Graphene is an attractive candidate in spintronics for a number of reasons, among which are its electric-field-controlled conductivity, its expected long spin lifetime and its two-dimensional nature. A number of recent proposals call for the development of high-quality ferromagnetic thin films in contact with graphene, whereas only thick polycrystalline or three-dimensional (nanoclusters) morphologies have been demonstrated so far. We report on the growth of flat, epitaxial ultrathin Co films on graphene using pulsed laser deposition. These display perpendicular magnetic anisotropy (PMA) in the thickness range 0.5–1 nm, in agreement with our first-principles calculations. PMA, epitaxy and ultra-small thickness bring new perspectives on graphene-based spintronic devices making use of the zero-field control of an arbitrary magnetization direction, band matching between electrodes and graphene, and interface phenomena such as the Rashba effect and electric field control of magnetism.

https://iopscience.iop.org/article/10.1088/1367-2630/12/10/103040/pdf

Ultrathin epitaxial cobalt films on graphene for spintronic investigations and applications

Recent trends in the emerging field of surface-supported magnetic nanostructures are reviewed. Current strategies for nanostructure synthesis are summarized, followed by a predominantly theoretical description of magnetic phenomena in surface magnetic structures and a review of experimental research in this field. Emphasis is on Fe- or Co-based nanostructures in various low-dimensional geometries, which are studied as model systems to explore the effects of dimensionality, atomic coordination, chemical bonds, alloying and, most importantly, interactions with the supporting substrate on the magnetism. This review also includes a discussion of closely related systems, such as 3d element impurities integrated into organic networks, surface-supported Fe-based molecular magnets, Kondo systems or 4d element nanostructures that exhibit emergent magnetism, thereby bridging the traditional areas of surface science, molecular physics and nanomagnetism.

/pdf/magnetic-surface-nanostructures-7iyv2zoe5b.pdf

Magnetic surface nanostructures

Background: Magnetic nanostructures and nanoparticles often show novel magnetic phenomena not known from the respective bulk materials. In the past, several methods to prepare such structures have been developed – ranging from wet chemistry-based to physical-based methods such as self-organization or cluster growth. The preparation method has a significant influence on the resulting properties of the generated nanostructures. Taking chemical approaches, this influence may arise from the chemical environment, reaction kinetics and the preparation route. Taking physical approaches, the thermodynamics and the kinetics of the growth mode or – when depositing preformed clusters/nanoparticles on a surface – the landing kinetics and subsequent relaxation processes have a strong impact and thus need to be considered when attempting to control magnetic and structural properties of supported clusters or nanoparticles.

Results: In this contribution we focus on mass-filtered Fe nanoparticles in a size range from 4 nm to 10 nm that are generated in a cluster source and subsequently deposited onto two single crystalline substrates: fcc Ni(111)/W(110) and bcc W(110). We use a combined approach of X-ray magnetic circular dichroism (XMCD), reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) to shed light on the complex and size-dependent relation between magnetic properties, crystallographic structure, orientation and morphology. In particular XMCD reveals that Fe particles on Ni(111)/W(110) have a significantly lower (higher) magnetic spin (orbital) moment compared to bulk iron. The reduced spin moments are attributed to the random particle orientation being confirmed by RHEED together with a competition of magnetic exchange energy at the interface and magnetic anisotropy energy in the particles. The RHEED data also show that the Fe particles on W(110) – despite of the large lattice mismatch between iron and tungsten – are not strained. Thus, strain is most likely not the origin of the enhanced orbital moments as supposed before. Moreover, RHEED uncovers the existence of a spontaneous process for epitaxial alignment of particles below a critical size of about 4 nm. STM basically confirms the shape conservation of the larger particles but shows first indications for an unexpected reshaping occurring at the onset of self-alignment.

Conclusion: The magnetic and structural properties of nanoparticles are strongly affected by the deposition kinetics even when soft landing conditions are provided. The orientation of the deposited particles and thus their interface with the substrate strongly depend on the particle size with consequences regarding particularly the magnetic behavior. Spontaneous and epitaxial self-alignment can occur below a certain critical size. This may enable the obtainment of samples with controlled, uniform interfaces and crystallographic orientations even in a random deposition process. However, such a reorientation process might be accompanied by a complex reshaping of the particles.

/pdf/structure-morphology-and-magnetic-properties-of-fe-2mqtl4h67q.pdf

Structure, morphology, and magnetic properties of Fe nanoparticles deposited onto single-crystalline surfaces

We study how single-crystal chromium films of uniform thickness on W(110) substrates are converted to arrays of three-dimensional (3D) Cr islands during annealing. We use low-energy electron microscopy (LEEM) to directly observe a kinetic pathway that produces trenches that expose the wetting layer. Adjacent film steps move simultaneously uphill and downhill relative to the staircase of atomic steps on the substrate. This step motion thickens the film regions where steps advance. Where film steps retract, the film thins, eventually exposing the stable wetting layer. Since our analysis shows that thick Cr films have a lattice constant close to bulk Cr, we propose that surface and interface stress provide a possible driving force for the observed morphological instability. Atomistic simulations and analytic elastic models show that surface and interface stress can cause a dependence of film energy on thickness that leads to an instability to simultaneous thinning and thickening. We observe that de-wetting is also initiated at bunches of substrate steps in two other systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are converted into patterns of unidirectional stripes as the trenches that expose the wetting layer lengthen along the W[001] direction. Finally, we observe how 3D Cr islands form directly during film growth at elevated temperature. The Cr mesas (wedges) form as Cr film steps advance down the staircase of substrate steps, another example of the critical role that substrate steps play in 3D island formation.

/pdf/how-metal-films-de-wet-substrates-identifying-the-kinetic-4dvulrn1fz.pdf

How metal films de-wet substrates—identifying the kinetic pathways and energetic driving forces

We have revisited the epitaxial growth modes of Fe on W(110) and Mo(110), and propose an overview or our contribution to the field. We show that the Stranski?Krastanov growth mode, acknowledged for a long time in these systems, is in fact characterized by a bimodal distribution of islands for a growth temperature in the range ~250?700??C. We observe firstly compact islands whose shape is determined by Wulff?Kaischev's theorem, and secondly thin and flat islands that display a preferred height, i.e.?are independent of nominal thickness and deposition procedures?(1.4?nm for Mo, and 5.5?nm for W on the average). We used this effect to fabricate self-organized arrays of nanometres-thick stripes by step decoration. Self-assembled nanoties are also obtained for nucleation of the flat islands on Mo at fairly high temperature, i.e.?~800??C. Finally, using interfacial layers and solid solutions we separate two effects on the preferred height, first that of the interfacial energy, and second that of the continuously varying lattice parameter of the growth surface.

https://hal.archives-ouvertes.fr/hal-00089174/document

Pierre-Olivier Jubert

Papers

Growth modes of Fe(110) revisited: a contribution of self-assembly to magnetic materials

Growth Modes of Fe(110) Revisited: A Contribution of Self-Assembly to Magnetic Materials