The effect of Fe-coverage on the structure, morphology and magnetic properties of α-FeSi₂nanoislands

Abstract

Self-assembled α-FeSi(2) nanoislands were formed using solid-phase epitaxy of low (~1.2 ML) and high (~21 ML) Fe coverages onto vicinal Si(111) surfaces followed by thermal annealing. At a resulting low Fe-covered Si(111) surface, we observed in situ, by real-time scanning tunneling microscopy and surface electron diffraction, the entire sequence of Fe-silicide formation and transformation from the initially two-dimensional (2 × 2)-reconstructed layer at 300 °C into (2 × 2)-reconstructed nanoislands decorating the vicinal step-bunch edges in a self-ordered fashion at higher temperatures. In contrast, the silicide nanoislands at a high Fe-covered surface were noticeably larger, more three-dimensional, and randomly distributed all over the surface. Ex situ x-ray photoelectron spectroscopy and high-resolution transmission electron microscopy indicated the formation of an α-FeSi(2) island phase, in an α-FeSi(2){112} // Si{111} orientation. Superconducting quantum interference device magnetometry showed considerable superparamagnetism, with ~1.9 μ(B)/Fe atom at 4 K for the low Fe-coverage, indicating stronger ferromagnetic coupling of individual magnetic moments, as compared to high Fe-coverage, where the calculated moments were only ~0.8 μ(B)/Fe atom. Such anomalous magnetic behavior, particularly for the low Fe-coverage case, is radically different from the non-magnetic bulk α-FeSi(2) phase, and may open new pathways to high-density magnetic memory storage devices.

Study on the electronic structures and optical properties of -FeSi2 using the pseudopotential method

“…Therefore, β-FeSi 2 shows a quasi direct band gap. This calculated results is consistent with other theoretical calculated results [13,14]. range of -5eV～0eV, the density of states of β-FeSi 2 is mainly composed of 3d electronic states of Fe and some contributions of 3p electronic states.…”

“…First-principles simulation based on density functional theory is one of the most promising methods for predicting properties of materials [9][10][11][12][13][14][15][16][17][18]. This work uses the pseudo potential energy to calculate the band structure, the dielectric constant, absorption coefficient, refractive index, reflectivity and energy loss function of theβ-FeSi 2 .…”

Self-consistent mapping: Effect of local environment on formation of magnetic moment in α−FeSi2

“…However, as was mentioned in Introduction, several recent studies [13,19,20] discovered that a ferromagnetic state arises in the films of α − F eSi 2 . The explanation of the emergence of the magnetic structure, suggested in these works, is within the commonly accepted opinion, that the magnetism arises due to an increase of Fe concentration in the material.…”

“…At the Fe-rich side of the binary phase diagram, metallic as well as ferromagnetic F e 5 Si 3 and F e 3 Si (DO 3 structure) [7,8]have already been established as key materials for spintronics [8,9]. The Si -rich side of the phase diagram contains several variants of a disilicide stoichiometric compound, such as the high-temperature tetragonal metallic α − F eSi 2 phase [11], with applications as an electrode or an interconnect material [12,13], and the orthorhombic semiconducting β − F eSi 2 phase [14], which due to its direct band gap is an interesting candidate for thermoelectric, photovoltaic and optoelectronic devices [15]. While room-temperature stable β-phase is wellstudied, tetragonal α-phase do not attract great interest until recently.…”

“…The authors of Refs. [13], [20] reported that the magnetic moments on Fe atoms µ = 1.8µ B [13] and µ = 3.3µ B [20] in α − F eSi 2 nanoislands and nano-stripes on Si (111) substrate arise. These experimental achievements have good perspective for the integration of the F eSi -based magnetic devices into silicon technology, and, therefore, demands for the detailed understanding of the physics of the magnetic moment formation in these compounds.…”