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.

Decreasing Water Activity Using the Tetrahydrofuran Electrolyte Additive for Highly Reversible Aqueous Zinc Metal Batteries.

Aqueous zinc metal batteries show great promise in large-scale energy storage. However, the decomposition of water molecules leads to severe side reactions, resulting in the limited lifespan of Zn batteries. Here, the tetrahydrofuran (THF) additive was introduced into the zinc sulfate (ZnSO4) electrolyte to reduce water activity by modulating the solvation structure of the Zn hydration layer. The THF molecule can play as a proton acceptor to form hydrogen bonds with water molecules, which can prevent water-induced undesired reactions. Thus, in an optimal 2 M ZnSO4/THF (5% by volume) electrolyte, the hydrogen evolution reaction and byproduct precipitation can be suppressed, which greatly improves the cycling stability and Coulombic efficiency of reversible Zn plating/stripping. The Zn symmetrical cells exhibit ultralong working cycles with a wide range of current density and capacity. The THF additive also enables a high Coulombic efficiency in the Zn||Cu cell with an average value of 99.59% over 400 cycles and a high reversible capacity with a capacity retention of 97.56% after 250 cycles in the Zn||MnO2 full cells. This work offers an effective strategy with high scalability and low cost for the protection of the Zn metal electrodes in aqueous rechargeable batteries.

Achieving High Pseudocapacitance Anode by An In Situ Nanocrystallization Strategy for Ultrastable Sodium-Ion Batteries.

Conversion/alloying type anodes have shown great promise for sodium-ion batteries (SIBs) because of their high theoretical capacity. However, the poor structural stability derived from the large volume expansion and short lifetime impedes their further practical applications. Herein, we report a novel anode with a pomegranate-like nanostructure of SnP2O7 particles homogeneously dispersed in the robust N-doped carbon matrix. For the first time, we make use of in situ self-nanocrystallization to generate ultrafine SnP2O7 particles with a short pathway of ions and electrons to promote the reaction kinetics. Ex situ transmission electron microscope (TEM) shows that the average particle size of SnP2O7 decreases from 66 to 20 nm successfully based on this unique nanoscale-engineering method. Therefore, the nanoparticles together with the N-doped carbon contribute a high pseudocapacitance contribution. Moreover, the N-doped carbon matrix forms strong interaction with the self-nanocrystallization ultrafine SnP2O7 particles, leading to a stable nanostructure without any particle aggregation under a long-cycle operation. Benefiting from these synergistic merits, the SnP2O7@C anode shows a high specific capacity of 403 mAh g-1 at 200 mA g-1 and excellent cycling stability (185 mAh g-1 after 4000 cycles at 1000 mA g-1). This work presents a new route for the effective fabrication of advanced conversion/alloying anodes materials for SIBs.

Ab initio study of alloying of MnBi to enhance the energy product.

High energy density magnets are preferred over induction magnets for many applications, including electric motors used in flying rovers, electric vehicles, and wind turbines. However, several issues related to cost and supply with state-of-the-art rare-earth-based magnets necessitate development of high-flux magnets containing low-cost earth-abundant materials. Here, by using first-principles density functional theory, we demonstrate the possibility of tuning magnetization and magnetocrystalline anisotropy of one of the candidate materials, MnBi, by alloying it with foreign elements. By using density functional theory in the high-throughput fashion, we consider the possibility of various metal and non-metal elements in the periodic table occupying empty sites of MnBi and found that MnBi-based alloys with Rh, Pd, Li, and O are stable against decomposition to constituent elements and have larger magnetization energy product compared to MnBi. Combined with other favorable properties of MnBi, such as high Curie temperature and earth abundancy of constituent elements, we envision the possibility of MnBi-based high-energy-density magnets.

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.

Effect of boron doping on nanostructure and magnetism of rapidly quenched Zr2Co11-based alloys.

The role of B on the microstructure and magnetism of Zr16Co82.5-x Mo1.5B x ribbons prepared by arc melting and melt spinning is investigated. Microstructure analysis show that the ribbons consist of a hard-magnetic rhombohedral Zr2Co11 phase and a minor amount of soft-magnetic Co. We show that the addition of B increases the amount of hard-magnetic phase, reduces the amount of soft-magnetic Co and coarsens the grain size from about 35 nm to 110 nm. There is a monotonic increase in the volume of the rhombohedral Zr2Co11 unit cell with increasing B concentration. This is consistent with a previous theoretical prediction that B may occupy a special type of large interstitial sites, called interruption sites. The optimum magnetic properties, obtained for x = 1, are a saturation magnetization of 7.8 kG, a coercivity of 5.4 kOe, and a maximum energy product of 4.1 MGOe.

Control of phase in phosphide nanoparticles produced by metal nanoparticle transformation: Fe2P and FeP.

The transformation of Fe nanoparticles by trioctylphosphine (TOP) to phase-pure samples of either Fe(2)P or FeP is reported. Fe nanoparticles were synthesized by the decomposition of Fe(CO)(5) in a mixture of octadecene and oleylamine at 200 degrees C and were subsequently reacted with TOP at temperatures in the region of 350-385 degrees C to yield iron phosphide nanoparticles. Shorter reaction times favored an iron-rich product (Fe(2)P), and longer reaction times favored a phosphorus-rich product (FeP). The reaction temperature was also a crucial factor in determining the phase of the final product, with higher temperatures favoring FeP and lower temperatures Fe(2)P. We also observe the formation of hollow structures in both FeP spherical nanoparticles and Fe(2)P nanorods, which can be attributed to the nanoscale Kirkendall effect. Magnetic measurements conducted on phase-pure samples suggest that approximately 8 x 70 nm Fe(2)P rods are ferromagnetic with a Curie temperature between 215 and 220 K and exhibit a blocking temperature of 179 K, whereas FeP is metamagnetic with a Neel temperature of approximately 120 K. These data agree with the inherent properties of bulk-phase samples and attest to the phase purity that can be achieved by this method.

Discrete, dispersible MnAs nanocrystals from solution methods: phase control on the nanoscale and magnetic consequences.

Nanocrystals of thermodynamically stable alpha-MnAs (hexagonal NiAs-type) and metastable beta-MnAs (orthorhombic MnP-type) have been synthesized by the reaction of triphenylarsine oxide (Ph(3)AsO) and dimanganesedecacarbonyl (Mn(2)CO(10)) at temperatures ranging from 250 to 330 degrees C in the presence of the coordinating solvent trioctylphosphine oxide (TOPO). Morphologically, both alpha- and beta-MnAs nanoparticles adopt a core-shell type structure with a crystalline core and low-contrast noncrystalline shell. In contrast to prior studies on MnAs particles, disks, and films, the present bottom-up synthesis yields discrete, dispersible MnAs nanoparticles without a structural support. Even in the absence of epitaxial strain, the lattice parameters of the nanocrystals are decreased relative to bulk MnAs, resulting in a volume decrease of 0.35% in alpha-MnAs and 0.38% in beta-MnAs nanoparticles. In contrast to bulk MnAs, where the ferromagnetic phase transition upon warming through 313-317 K is concomitant with a structure change from ferromagnetic alpha- to paramagnetic beta-MnAs, powder X-ray diffraction studies suggest there is no conversion of alpha-MnAs to beta over the temperature range 298-343 K. Moreover, magnetic measurements suggest that both alpha- and beta-MnAs are ferromagnetic with T(C) approximately 315 K. Partial phase transformation of beta-MnAs nanoparticles into thermodynamically stable alpha-MnAs occurs slowly over time (i.e., months) at room temperature. However, there is no associated change in magnetization, suggesting the ferromagnetism observed in beta-MnAs is intrinsic and cannot be attributed to alpha-MnAs impurities.