Electronic, magnetic, and structural properties of NiFeMnAl.

Half-metallic Heusler compounds have been extensively studied in the recent years, both experimentally and theoretically, for potential applications in spin-based electronics. Here, we present the results of a combined theoretical and experimental study of the quaternary Heusler compound NiFeMnAl. Our calculations indicate that this material is half-metallic in the ground state and maintains its half-metallic electronic structure under a considerable range of external hydrostatic pressure and biaxial strain. NiFeMnAl crystallizes in the regular cubic Heusler structure, and exhibits ferromagnetic alignment. The practical feasibility of the proposed system is confirmed in the experimental section of this work. More specifically, a bulk ingot of NiFeMnAl was synthesized in A2 type disordered cubic structure using arc melting. It shows a high Curie temperature of about 468 K and a saturation magnetization of 2.3µB/f.u. The measured magnetization value is smaller than the one calculated for the ordered structure. This discrepancy is likely due to the A2 type atomic disorder, as demonstrated by our calculations. We hope that the presented results may be useful for researchers working on practical applications of spin-based electronics.

First principles study of nearly strain-free Ni/WSe2and Ni/MoS2interfaces.

Metal/transition metal dichalcogenide interfaces are the subject of active research, in part because they provide various possibilities for interplay of electronic and magnetic properties with potential device applications. Here, we present results of our first principles calculations of nearly strain-free Ni/WSe2and Ni/MoS2interfaces in thin-film geometry. It is shown that while both the WSe2and MoS2layers adjacent to Ni undergo metallic transition, the layers farther from the interface remain semiconducting. In addition, a moderate value of spin-polarization is induced on interfacial WSe2and MoS2layers. At the same time, the electronic and magnetic properties of Ni are nearly unaffected by the presence of WSe2and MoS2, except a small reduction of magnetic moment at the interfacial Ni atoms. These results can be used as a reference for experimental efforts on epitaxial metal/transition metal dichalcogenide heterostructures, with potential application in modern magnetic storage devices.

Perpendicular magnetic anisotropy in half-metallic thin-film Co2CrAl.

Magnetocrystalline anisotropy (MCA) is one of the key parameters investigated in spin-based electronics (spintronics), e.g. for memory applications. Here, we employ first-principles calculations to study MCA in thin film full Heusler alloy Co2CrAl. This material was studied in the past, and has been reported to exhibit half-metallic electronic structure in bulk geometry. In our recent work, we showed that it retains a 100% spin-polarization in thin-film geometry, at CrAl atomic surface termination. Here, we show that the same termination results in a perpendicular magnetic anisotropy, while Co surface termination not only destroys the half-metallicity, but also results in in-plane magnetization orientation. In addition, for films thicker than around 20 nm the contribution from magnetic shape anisotropy may become decisive, resulting in in-plane magnetization orientation. To the best of our knowledge, this is one of the first reports of half-metallic thin-film surfaces with perpendicular magnetic anisotropy. This result may be of interest for potential nano-device applications, and may stimulate a further experimental study of this and similar materials.

Half-metallicity in CrAl-terminated Co2CrAl thin film.

Half-metals with high Curie temperature are ideal candidates for applications in spin-based electronics-an emerging technology utilizing a spin degree of freedom in electronic devices. Many half-metallic materials have been predicted theoretically, and some have been confirmed experimentally. At the same time, in thin-film geometry the electronic structure of these materials may change due to the potential presence of surface/interface states. This could limit practical applications of these materials in nano-size devices, since typically these states result in reduced spin-polarization. Here, from first principles we study a full Heusler compound, Co2CrAl in thin film geometry. This material has been studied extensively, and it has been reported that it exhibits half-metallic properties in the bulk. We show contrary to the earlier reports that this material retains 100% spin polarization in CrAl-terminated thin film geometry (Co-termination results in destroyed half-metallicity). Moreover, we confirm that under biaxial strain Co2CrAl retains half-metallicity for a practically feasible range of considered pressure, i.e. in principle it may stay half-metallic if used in thin-film heterostructures, where lattice mismatch is a common scenario. The magnetic alignment of Co2CrAl is confirmed to be ferromagnetic, with the non-integer total magnetic moment of Co-terminated cell, and the integer total magnetic moment of CrAl-terminated cell, consistent with their corresponding non-half-metallic and half-metallic electronic structures. If confirmed experimentally, these results may have an important impact in spin-based electronics.

Half-metallic surfaces in thin-film Ti2MnAl0.5Sn0.5.

Materials exhibiting a high degree of spin polarization in electron transport are in demand for applications in spintronics-an emerging technology utilizing a spin degree of freedom in electronic devices. Room-temperature half-metals are considered ideal candidates, as they behave as an insulator for one spin channel and as a conductor for the other spin channel. In addition, for nano-size devices, one has to take into account possible modification of electronic structure in thin-film geometry, due to the potential presence of surface/interface states. It has been shown that typically these states have a detrimental impact on half-metallicity, i.e. their presence results in reduced spin-polarization. Here, we employ density functional calculations to explore an inverse Heusler compound, Ti2MnAl0.5Sn0.5, which exhibits half-metallic electronic structure in bulk geometry. In particular, this material behaves as a regular metal for majority-spin, and as a semiconductor for minority-spin states. We show that in thin-film geometry, the type of termination surface has a decisive effect on half-metallicity of this material. In particular, we analyze six possible termination configurations, and show that for four of them, energy states emerge in the minority-spin band gap, significantly reducing the spin polarization of Ti2MnAl0.5Sn0.5. At the same time, our calculations indicate that two termination surfaces preserve half-metallic properties of this material. This result is somewhat unexpected, as most of the available literature reports reduction of the spin-polarization due to the presence of surface states. Thus, our results show that a judicious choice of the termination surface may be a crucial factor in nano-device applications, where highly spin-polarized current is needed.

Electronic structure of multi-walled carbon fullerenes.

Despite an enormous amount of research on carbon based nanostructures, relatively little is known about the electronic structure of multi-walled carbon fullerenes, also known as carbon onions. In part, this is due to the very high computational expense involved in estimating electronic structure of large molecules. At the same time, experimentally, the exact crystal structure of the carbon onion is usually unknown, and therefore one relies on qualitative arguments only. In this work we present the results of a computational study on a series of multi-walled fullerenes and compare their electronic structures to experimental data. Experimentally, the carbon onions were fabricated using ultrasonic agitation of isopropanol alcohol and deposited onto the surface of highly ordered pyrolytic graphite using a drop cast method. Scanning tunneling microscopy images indicate that the carbon onions produced using this technique are ellipsoidal with dimensions on the order of 10 nm. The majority of differential tunneling spectra acquired on individual carbon onions are similar to that of graphite with the addition of molecular-like peaks, indicating that these particles span the transition between molecules and bulk crystals. A smaller, yet sizable number exhibited a semiconducting gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels. These results are compared with the electronic structure of different carbon onion configurations calculated using first-principles. Similar to the experimental results, the majority of these configurations are metallic with a minority behaving as semiconductors. Analysis of the configurations investigated here reveals that each carbon onion exhibiting an energy band gap consisted only of non-metallic fullerene layers, indicating that the interlayer interaction is not significant enough to affect the total density of states in these structures.

Effects of pressure and strain on spin polarization of IrMnSb.

A high degree of spin polarization in electron transport is one of the most sought-after properties of a material which can be used in spintronics-an emerging technology utilizing a spin degree of freedom in electronic devices. An ideal candidate to exhibit highly spin-polarized current would be a room temperature half-metal, a material which behaves as an insulator for one spin channel and as a conductor for the other spin channel. In this paper, we explore a semi-Heusler compound, IrMnSb, which has been reported to exhibit pressure induced half-metallic transition. We confirm that the bulk IrMnSb is a spin-polarized metal, with dominant contribution to electronic states at the Fermi energy from majority-spin electrons. Application of a uniform pressure shifts the Fermi level into the minority-spin energy gap, thus demonstrating pressure induced half-metallic transition. This behavior is explained by the reduction of the exchange splitting of the spin bands consistent with the Stoner model for itinerant magnetism. We find that the half-metallic transition is suppressed when instead of uniform pressure the bulk IrMnSb is exposed to biaxial strain. This suppression of half-metallicity is driven by the epitaxial strain induced tetragonal distortion, which lifts the degeneracy of the Mn 3d t 2g and e g orbitals and reduces the minority-spin band gap under compressive strain, thus preventing half-metallic transition. Our calculations also indicate that in thin film geometry, surface states emerge in the minority-spin band gap, which has detrimental for practical applications impact on the spin polarization of IrMnSb.

Magnetic phases of cobalt atomic clusters on tungsten.

First-principle calculations are employed to show that the magnetic structure of small atomic clusters of Co, formed on a crystalline W(110) surface and containing 3-12 atoms, strongly deviates from the usual stable ferromagnetism of Co in other systems. The clusters are ferri-, ferro- or non-magnetic, depending on cluster size and geometry. We determine the atomic Co moments and their relative alignment, and show that antiferromagnetic spin alignment in the Co clusters is caused by hybridization with the tungsten substrate and band filling. This is in contrast with the typical strong ferromagnetism of bulk Co alloys, and ferromagnetic coupling in Fe/W(110) clusters.

Ferroelectric control of magnetocrystalline anisotropy at cobalt/poly(vinylidene fluoride) interfaces.

Electric field control of magnetization is one of the promising avenues for achieving high-density energy-efficient magnetic data storage. Ferroelectric materials can be especially useful for that purpose as a source of very large switchable electric fields when interfaced with a ferromagnet. Organic ferroelectrics, such as poly(vinylidene fluoride) (PVDF), have an additional advantage of being weakly bonded to the ferromagnet, thus minimizing undesirable effects such as interface chemical modification and/or strain coupling. In this work we use first-principles density functional calculations of Co/PVDF heterostructures to demonstrate the effect of ferroelectric polarization of PVDF on the interface magnetocrystalline anisotropy that controls the magnetization orientation. We show that switching of the polarization direction alters the magnetocrystalline anisotropy energy of the adjacent Co layer by about 50%, driven by the modification of the screening charge induced by ferroelectric polarization. The effect is reduced with Co oxidation at the interface due to quenching the interface magnetization. Our results provide a new insight into the mechanism of the magnetoelectric coupling at organic ferroelectric/ferromagnet interfaces and suggest ways to achieve the desired functionality in practice.

Ferroelectric control of the magnetocrystalline anisotropy of the Fe/BaTiO(3)(001) interface.

Density-functional calculations are employed to investigate the effect of ferroelectric polarization of BaTiO(3) on the magnetocrystalline anisotropy of the Fe /BaTiO(3)(001) interface. It is found that the interface magnetocrystalline anisotropy energy changes from 1.33 to 1.02 erg cm (-2) when the ferroelectric polarization is reversed. This strong magnetoelectric coupling is explained in terms of the changing population of the Fe 3d orbitals at the Fe/BaTiO(3) interface driven by polarization reversal. Our results indicate that the electronically assisted magnetoelectric effects at the ferromagnetic/ferroelectric interfaces may be a viable alternative to the strain mediated coupling in related heterostructures and the electric field-induced effects on the interface magnetic anisotropy in ferromagnet/dielectric structures.