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James Cruz
James Cruz

Magnetic Structures Free WORK Download



Abstract:Materials composed of two dimensional layers bonded to one another through weak van der Waals interactions often exhibit strongly anisotropic behaviors and can be cleaved into very thin specimens and sometimes into monolayer crystals. Interest in such materials is driven by the study of low dimensional physics and the design of functional heterostructures. Binary compounds with the compositions MX2 and MX3 where M is a metal cation and X is a halogen anion often form such structures. Magnetism can be incorporated by choosing a transition metal with a partially filled d-shell for M, enabling ferroic responses for enhanced functionality. Here a brief overview of binary transition metal dihalides and trihalides is given, summarizing their crystallographic properties and long-range-ordered magnetic structures, focusing on those materials with layered crystal structures and partially filled d-shells required for combining low dimensionality and cleavability with magnetism.Keywords: layered materials; van der Waals; monolayer; transition metal compounds; halides; crystal structure; magnetism; magnetic structure




Magnetic Structures Free Download



The dramatic changes in electronic and magnetic properties are investigated using the first-principles calculations for halogen(X: Cl, Br, I, At)-adsorbed graphene nanoribbons. The rich and unique features are clearly revealed in the atoms-dominated electronic band structures, spin arrangement/magnetic moment, spatial charge distribution, and orbital- and spin-projected density of states. Halogen adsorptions can create the non-magnetic, ferromagnetic or anti-ferromagnetic metals, being mainly determined by concentrations and edge structures. The number of holes per unit cell increases with the adatom concentrations. Furthermore, magnetism becomes nonmagnetic when the adatom concentration is beyond 60% adsorption. There are many low-lying spin-dependent van Hove singularities. The diversified properties are attributed to the significant X-C bonds, the strong X-X bonds, and the adatom- and edge-carbon-induced spin states.


One-dimensional graphene nanoribbons (1D GNRs) have stirred many experimental1 and theoretical2 studies, mainly owing to the honeycomb lattice, nano-scaled thickness, finite-size confinement, edge structures, planar/non-planar structures, and stacking configurations. They are suitable for exploring the novel physical, chemical and material properties. The 1D GNRs are successfully synthesized by unzipping multi-walled carbon nanotubes3, cutting graphene layers4, and using other chemical methods5. Such systems are expected to have highly potential applications6 since the essential properties are greatly diversified by the external factors, such as the chemical doping7,8, electric field9, magnetic field10,11, and mechanical strain12. This work is mainly focused on the geometric, electronic and magnetic properties of chlorination-related GNRs. Besides, the halogenation effects are investigated thoroughly, in which a detailed comparison between chlorination and fluorination is also made.


The geometric, electronic and magnetic properties of halogen(X: Cl, Br, I, At)-adsorbed GNRs are investigated using the first-principles method. The binding energies, X-C and C-C bond lengths, adsorption positions, magnetic moments, atom- and spin-dominated energy bands, spatial charge distributions, free carrier densities, spin configurations, and orbital and spin-decomposed density of states (DOSs) are included in the calculations. The dependence on the concentrations, adsorption positions and edge structures are explored in detail. In this work, a theoretical framework is developed to fully investigate the orbital hybridizations and spin distributions, which play critical roles in creating the diverse electronic properties and magnetic configurations. The atom-dominated energy bands, spatial charge densities, and atom- and orbital-projected DOSs provide much information about the chemical bonding. They could be used to identify the multi- and single-orbital hybridizations in C-C, X-C and X-X bonds. Moreover, the edge-carbon- and adatom-dependent magnetic configurations are obtained from the net magnetic moment, the spin-split energy bands, the spin density distributions, and the spin-decomposed DOSs. The theoretical predictions could be verified by scanning tunneling microscopy (STM)25, transmission electron microscopy (TEM)26, angle-resolved photoemission spectroscopy (ARPES)27, scanning tunneling spectroscopy (STS)28,29, optical spectroscopy30, and transport spectroscopy31.


Whether there exists a simple relation between the free hole density (λ) and the adatom concentration is thoroughly examined for various halogen adsorptions. The former is associated with the partially unoccupied states in carbon- and halogen-dominated valence bands, in which λ is characterized the Fermi momentum under the linear relation \(\lambda =\frac2\pi k_F\). By a lot of numerical calculations (Table 1), the adatom concentration is deduced to play a critical role in determining the following relation: the number of holes per unit cell increases with the adatom concentrations. This is independent of the various distributions, the different kinds of adatoms, and the edge structures. The chlorination-related GNRs might be 1D p-type metals with very high hole density; therefore, they are expected to be good candidates in electronic devices and electrode materials.


The detailed first-principles calculations show that the diverse electronic and magnetic properties arise from the critical orbital hybridizations in X-C and X-X bonds and the spin configurations due to Cl adatoms and zigzag-edge C atoms. Such mechanisms are examined/identified from the atom-dominated band structures, free hole densities, spatial charge densities, net magnetic moments, spin distributions, and orbital- and spin-projected DOSs. The predicted chlorination effects on the optimal geometries, energy bands, and van Hove singularities in DOS could be verified using STM/TEM, ARPES and STS measurements, respectively. The further development of the theoretical framework is expected to be suitable for studying emergent layered materials, such as, silicene39,40, germanene41,42 and MoS243 nanoribbons with the buckled structures, the multi-orbital hybridizations, and the spin-orbital couplings.


(a) Lattice and magnetic structures of BiFeO3 (adapted from reference 47). Red lines show the perovskite pseudo-cubic cell. (b) Top. Applied electric field E switches ferroelectric domain A into domain B. The magnetic propagation vectors τi rotate together with the electric polarisation vector P. Bottom. Volume fractions of the domains A and B in an applied field. Both the magnetic signal (τi wave vectors) and the structural signal (due to the elongated [111] diagonal) give essentially the same dependence. (Adapted from reference 43) (c) Within a single-ferroelectric domain, applied electric field or uniaxial pressure convert one equivalent magnetic domain (τi) into another. Dashed arrow shows corresponding rotation of the magnetic easy plane. Bottom. Volume fractions of the domains τ1 and τ2 in an electric field. (No domain τ3 was detected in this sample; adapted from references 46,50.)


In some systems, strain can mediate or enhance magnetoelectric coupling. For example, epitaxial strain from a DyScO3 (space group Pbnm) substrate stabilizes ferroelectricity and ferromagnetism simultaneously in a thin film of EuTiO3 when neither property is present in the bulk.65 Recently, Singh et al.73 have been successful in inferring the magnetisation depth profile across interfaces using PNR measured as simultaneous functions of applied magnetic field, stress and temperature. Elastic bending stress was applied to the thin film using a four-point mechanical jig. Use of the new capability for studies of strain mediated magnetoelectric multiferroic heterostructures is an obvious next step, as is development of pressure cells for neutron reflectometry.


Even with some of the promising developments recently realized, the magnetoelectric coupling of (planar) composite multiferroics remains small. Recognising that the interface is responsible for the coupling, some researchers have proposed to increase the interfacial component. This can be accomplished through lithography or by self-assembly of one component, or by mixing immiscible solutions so the resulting composite is chemically and magnetically non-uniform at length scales of tens of nm. Probing magnetic structures on such length scales with nm resolution, and especially in systems in which the interfaces are buried, is extraordinarily challenging. Fortunately, the techniques of grazing incidence SANS (GISANS, i.e., SANS in grazing incident angle reflection geometry), conventional SANS, and the especially powerful technique of SANS with polarised neutron beams and polarisation analysis of the scattering are ideal characterisation tools for these problems. SANS has been effective for identifying the correlation of magnetic lengths in materials exhibiting magnetic and electronic phase separation,4,74 magnetisation reversal mechanisms of nm-sized domains and domain walls in exchange bias systems,75 and the existence of uncompensated magnetisation in single crystals of HoMnO3.76 Notably, in the HoMnO3 study, control of the magnetisation via AFM domain walls as opposed to bulk magnetic order was demonstrated by application of an electric field during the SANS experiment. SANS has also identified differences of magnetic structure for nanoparticles and their interfaces in bulk materials.13 An important question SANS can answer is whether the induced magnetisation seen in the planar heterostructures (as depicted in Figure 9b, and reported in references 61,70) is also present in three-dimensional composite multiferroics (as depicted in Figure 9a,d).


Studies of multiferroic phenomena have been driven by novel materials. For potential device applications, there is a continued push to develop materials with magnetic transitions above room temperature. Presently, type II multiferroics with strong coupling between ferroelectric and magnetic order parameters appear very promising. Theory has been playing an increasing role in predicting new materials89 with strong coupling between ferroelectric and magnetic order parameters. Neutron scattering will continue to play a critical role in determining the magnetic structures of these materials and the strength of their exchange interactions, and how they couple to the electric polarisation. Furthermore, as more materials are studied in thin films and composites, neutron scattering will play an increasing role in determining the magnetic structure of these materials. In some devices where ferromagnetic layers are coupled to underlying multiferroic layers, neutron reflectivity measurements will be critical in determining the nature of the magnetic interactions between the multiferroic and ferromagnetic layers or composites of ferroelectric and ferromagnetic layers that exhibit multiferroic behaviour. 041b061a72


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