Atomic-scale defects in the two-dimensional ferromagnet CrI3 from first principles

The family of atomically thin magnets holds great promise for a number of prospective applications in magneto-optoelectronics, with CrI3 arguably being its most prototypical member. However, the formation of defects in this system remains unexplored to date. Here, we investigate native point defects in monolayer CrI3 by means of first-principles calculations. We consider a large set of intrinsic impurities and address the atomic structure, thermodynamic stability, diffusion and aggregation tendencies as well as local magnetic moments. Under thermodynamic equilibrium, the most stable defects are found to be either Cr or I atomic vacancies along with their complexes, depending on the chemical potential conditions. These defects are predicted to be quite mobile at room and growth temperatures, and to exhibit a strong tendency to agglomerate. In addition, our calculations indicate that the deviation from the nominal stoichiometry largely impacts the magnetic moments, and the defect-induced lattice distortions can drive local ferromagnetic-to-antiferromagnetic phase transitions. Overall, this work portrays a comprehensive picture of intrinsic point defects in monolayer CrI3 from a theoretical perspective.

Structural Disorder and Magnetic Order in Atomically Thin Crystals

Michele Pizzochero

This Thesis is mainly devoted to the exploration of the structural disorder and magnetic order in two-dimensional crystals by means of first-principles methodologies. Firstly, we present a comparative study of defect formation in the semiconducting 2H and semi-metallic 1T' phases of MoS2, the most representative member of transition metal dichalcogenides. We assess the thermodynamic stability of a broad set of point defects, along with their effect on the electronic structure and magnetic properties. We further investigate the response of 1T'-MoS2 to the electron beam irradiation. The energy range of the electron beam which is necessary to conduct sample imaging without inducing knock-on damage, as well as to intentionally create vacancy defects in 1T'-MoS2 is dicussed. Next, we show how native defects can be exploited to induce magnetism in otherwise non-magnetic ultrathin PtSe2 by providing a theoretical model which explains magneto-resistance measurements. We reveal an antiferro- and ferro-magnetic ordering in semiconducting mono- and bi-layers, respectively, together with a layer-dependent RKKY interaction in thicker metallic films. Secondly, we focus on magnetic interactions in monolayer CrI3 by determining the strength of both intra- and inter-site magnetic exchange interactions. We find that inter-site exchange interactions are primarily dominated by the ferromagnetic isotropic Heisenberg coupling, being inter-site anisotropic terms substantially weaker. Also, this system features a single-ion anisotropy pointing along the out-of-plane direction. Then, we show that Heisenberg exchange interactions can be effectively modulated through mild lattice deformations. In fact, depending on the magnitude and direction along which the strain is exerted, monolayer CrI3 undergoes a transition from the ferromagnetic phase to either Néel antiferromagnetic or ferrimagnetic phase. Additionally, we briefly examine the role point defects on monolayer CrI3, and point out an interplay between lattice disorder and local magnetic moments in this system. Thirdly, we revisit two fundamental yet poorly understood problems in physical chemistry. These include the hydration dynamics of the excess electrons in liquid water, a species relevant to many biological and chemical processes, and the nature of double bonds between higher main group elements, with a particular emphasis on the comparison between the properties of C=C vs Si=Si, an issue which is crucial to understand the structural dissimilarities between graphene and silicene.

EPFL2020

Investigation and modification of the magnetism of epitaxial Fe structures

Diego Repetto

In this thesis, the magnetic and structural properties of layered epitaxial film systems and clusters are presented. A main achievement is the investigation of the direct correlation between magnetism and structural or morphological details of ultrathin films. For this purpose, the film structure has been accessed directly by means of surface science methods. The effect of structural changes on the magnetism were observed in-situ by magneto-optical techniques. First, epitaxial Fe films grown at low temperature (∼ 120K) on Cu(100) have been investigated. Such layers assume an fcc-crystal structure due to their interaction with the underlying Cu substrate. Within this work it is shown that the morphology of these films can be altered by annealing treatments. The thermally-induced morphological changes have direct consequences for domain wall propagation and result in increased coercivity and in modified surface anisotropy. Flat fcc-Fe layers on Cu(100) have been used as templates to form fcc-Fe/Cu/Fe trilayers with coupled perpendicular magnetization. Such systems might be of relevance for data storage applications. The perpendicular coupling is ascribed to the realignment of the magnetization of the bottom Fe layer within the stray field produced by the top Fe layer magnetization. The magnetic coupling has been investigated as a function of the thickness of the top and the bottom Fe layer as well as the Cu spacer thickness. It was found that the magneto-static interaction is a direct consequence of the fcc-structure of both Fe layers and the interface roughness. It is demonstrated that Fe films with fcc-structure and perpendicular magnetization can also be prepared on other fcc-templates, such as Pt substrates. Monatomic steps on the substrate surface are exploited to steer the growth of the film and with it the magnetism. Despite significant differences in the morphology and the atomic structure of Fe films on flat Pt(111) and stepped Pt(997), the critical thickness for the spin reorientation transition is similar on the two surfaces. However, the local atomic arrangement above three monolayer film thickness determines in-plane easy magnetization axis parallel to the step edges. For all layered systems investigated in this thesis, epitaxial strain is a the driving force which influences the film structure and triggers structural transitions. The strain dependence of the morphology and the magnetism was probed by a comparative study of Fe deposited on Pt substrates with and without a noble gas buffer layer. The formation of relaxed Fe clusters during Xe layer desorption is favored in the absence of the overlayer-substrate interaction. Striking differences in the magnetic anisotropy between the systems are attributed to magneto-elastic contributions. Measurements of the blocking temperature of the Fe clusters allow estimating the cluster size. All samples have been investigated under ultra high vacuum conditions. The study of the sample magnetic properties study was done by in-situ magneto-optical Kerr effect measurements and Kerr microscopy. Structural characterization was done by low energy electron diffraction and, in collaboration with other scientists, by scanning tunnelling microscopy.

EPFL2006

Magnetism of individual adatoms and of epitaxial monolayers

Anne Lehnert

This thesis reports on the magnetism of low coordinated Fe and Co atoms in different chemical and electronic environments. The objective of this research was to contribute to the fundamental understanding of the magnetic properties, such as magneto-crystalline anisotropy energy (MAE), orbital and spin magnetic moment, of surface supported transition metal (TM) atoms and one atomic layer thick epitaxial TM films. A detailed knowledge on the interplay of electronic hybridization and magnetic properties is mandatory to elaborate new materials with high perpendicular magnetic anisotropy and acceptable writing field for future use in magnetic recording devices. The magnetic properties of the different experimental systems were investigated by x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and magneto-optical Kerr effect (MOKE) and they were correlated to the morphology obtained by scanning tunneling microscopy (STM). The first part focuses on the effect of electronic hybridization between isolated 3d TM atoms and different kinds of substrates. First, we considered 4d and 5d TM substrates known for their high magnetic polarizability due to the close onset of ferromagnetism. Deposition of single Fe and Co atoms on Rh(111), Pd(111), and Pt(111) produces an increasing MAE going from Rh to Pd and finally to Pt that we directly correlate to the increasing spin-orbit constant of the substrate interlinking the induced magnetic moment and the MAE. Further we investigated single Fe and Co atoms on two different insulating layers, namely Al2O3/Ni3Al(111) and Cu2N/Cu(001), providing an efficient decoupling of the TM 3d states from the conduction electrons of the substrates. In case of the alumina film the absorption spectra of Fe and Co show a pronounced multiplet structure. On Cu2N the multiplet features of the adatoms are less defined due to the lower thickness of the screening layer. On both insulating layers we find a common out-of-plane easy axis, highly unquenched orbital moments, and giant MAE values of several meV/atom which are due to the formation of strong anisotropic bonds with the O and N atoms, respectively. The second part is dedicated to single atom thick layers of Fe and Co deposited on Pt(111) and Rh(111). We find the MAE to be strongly enhanced compared with bulk and we establish a direct proportionality between the induced magnetic moment of the topmost substrate layer and the MAE. The study of bimetallic Fe1-xCox alloys on platinum reveal a material with high MAE (0.5 meV at the equiatomic composition), large saturation magnetization (3μB to 2μB going from Fe to Co), and perpendicular magnetization easy axis. These characteristics hint at new possibilities for the further miniaturization of the grain size in recording devices. In the last part the theoretical description of the MOKE for ultra-thin films is extended to the transverse geometry and improved analytic formulae for the polar and longitudinal geometry are presented. This finding was used to optimize the Kerr signal for a new MOKE setup which is fully integrated in an ultrahigh vacuum chamber combining in situ MOKE and STM. The setup performance was demonstrated for a continuous film of 0.9 monolayers (ML) Co/Rh(111) with in-plane easy axis and for a superlattice of nanometric double layer Co islands on Au(11,12,12) with out-of-plane easy axis. The detection limit is 0.5 ML for transverse and 0.1 ML for polar Kerr geometry corresponding for island superlattices with the density of Co/Au(11,12,12) to islands composed of about 50 atoms. Moreover, the setup holds two independent ways to measure the MAE which is either by thermally induced magnetization reversal or by applying a torque onto the magnetization by turning the magnetic field. Both ways yield similar results for 1.1 ML Co/Au(11,12,12).

EPFL2009