Mario Žic, Karsten Rode, Naganivetha Thiyagarajah, Yong-Chang Lau, Davide Betto, J.M.D. Coey, Stefano Sanvito, Kerry J. O’Shea, Ciaran A. Ferguson, Donald A. MacLaren, and Thomas Archer ∗ CRANN and School of Physics, Trinity College Dublin, Dublin 2, Ireland CRANN, AMBER and School of Physics, Trinity College Dublin, Dublin 2, Ireland SUPA, School of Physics & Astronomy, University of Glasgow, Glasgow G12 8QQ, United Kingdom (Dated: October 14, 2018)

Recently magnetic tunnel junctions using two-dimensional MoS2 as nonmagnetic spacer have been fabricated, although their magnetoresistance has been reported to be quite low. This may be attributed to the use of permalloy electrodes, injecting current with a relatively small spin polarization. Here we evaluate the performance of MoS2-based tunnel junctions using Fe3Si Heusler alloy electrodes. Density functional theory and the non-equilibrium Green's function method are used to investigate the spin injection efficiency (SIE) and the magnetoresistance (MR) ratio as a function of the MoS2 thickness. We find a maximum MR of ~300% with a SIE of about 80% for spacers comprising between 3 and 5 MoS2 monolayers. Most importantly, both the SIE and the MR remain robust at finite bias, namely MR > 100% and SIE > 50% at 0.7 V. Our proposed materials stack thus demonstrates the possibility of developing a new generation of performing magnetic tunnel junctions with layered two-dimensional compounds as spacers.

By imposing the constraints of structural compatibility, stability and a large tunneling magneto-resistance, we have identified the Fe$_3$Al/BiF$_3$/Fe$_3$Al stack as a possible alternative to the well-established FeCoB/MgO/FeCoB in the search for a novel materials platform for high-performance magnetic tunnel junctions. Various geometries of the Fe$_3$Al/BiF$_3$/Fe$_3$Al structure have been analyzed, demonstrating that a barrier of less than 2~nm yields a tunneling magneto-resistance in excess of 25,000~\% at low bias, without the need for the electrodes to be half-metallic. Importantly, the presence of a significant spin gap in Fe$_3$Al for states with $\Delta_1$ symmetry along the stack direction makes the TMR very resilient to high voltages.

SiO2 and HfO2 are both high-k, wide-gap semiconductors, currently used in the microelectronic industry as gate barriers. Herewe investigate whether the samematerials can be employed to make magnetic tunnel junctions, which in principle can be amenable for integration in conventional Si technology. By using a combination of density functional theory and the nonequilibrium Green's functions method for quantum transport we have studied the transport properties of Co[0001]/SiO2[001]/Co[0001] and Fe[001]/HfO2[001]/Fe[001] junctions. In both cases we found a quite large magnetoresistance, which is explained through the analysis of the real band structure of the magnets and the complex one of the insulator. We find that there is no symmetry spin filtering for the Co-based junction since the high transmission Delta(2') band crosses the Fermi level, E-F, for both spin directions. However, the fact that Co is a strong ferromagnet makes the orbital contribution to the two Delta(2) spin subbands different, yielding magnetoresistance. In contrast for the Fe-based junction symmetry filtering is active for an energy window spanning between the Fermi level and 1 eV below E-F, with Delta 1 symmetry contributing to the transmission.

Magnetic materials underpin modern technologies, ranging from data storage to energy conversion to contactless sensing. However, the development of a new high-performance magnet is a long and often unpredictable process, and only about two dozen magnets are featured in mainstream applications. We describe a systematic pathway to the design of novel magnetic materials, which demonstrates a high throughput and discovery speed. On the basis of an extensive electronic structure library of Heusler alloys containing 236,115 prototypical compounds, we filtered those displaying magnetic order and established whether they can be fabricated at thermodynamic equilibrium. Specifically, we carried out a full stability analysis of intermetallic Heusler alloys made only of transition metals. Among the possible 36,540 prototypes, 248 were thermodynamically stable but only 20 were magnetic. The magnetic ordering temperature, TC, was estimated by a regression calibrated on the experimental TC of about 60 known compounds. As a final validation, we attempted the synthesis of a few of the predicted compounds and produced two new magnets: Co2MnTi, which displays a remarkably high TC in perfect agreement with the predictions, and Mn2PtPd, which is an antiferromagnet. Our work paves the way for large-scale design of novel magnetic materials at potentially high speed.

Combining material informatics and high-throughput electronic structure calculations offers the possibility of a rapid characterization of complex magnetic materials. Here we demonstrate that datasets of electronic properties calculated at the ab initio level can be effectively used to identify and understand physical trends in magnetic materials, thus opening new avenues for accelerated materials discovery. Following a data-centric approach, we utilize a database of Heusler alloys calculated at the density functional theory level to identify the ideal ions neighbouring Fe in the $X_2$Fe$Z$ Heusler prototype. The hybridization of Fe with the nearest neighbour $X$ ion is found to cause redistribution of the on-site Fe charge and a net increase of its magnetic moment proportional to the valence of $X$. Thus, late transition metals are ideal Fe neighbours for producing high-moment Fe-based Heusler magnets. At the same time a thermodynamic stability analysis is found to restrict $Z$ to main group elements. Machine learning regressors, trained to predict magnetic moment and volume of Heusler alloys, are used to determine the magnetization for all materials belonging to the proposed prototype. We find that Co$_2$Fe$Z$ alloys, and in particular Co$_2$FeSi, maximize the magnetization, which reaches values up to 1.2T. This is in good agreement with both ab initio and experimental data. Furthermore, we identify the Cu$_2$Fe$Z$ family to be a cost-effective materials class, offering a magnetization of approximately 0.65T.

The Heusler compound Mn2RuxGa (MRG) may well be the first compensated half metal. Here, the structural, magnetic and transport properties of thin films of MRG are discussed. There is evidence of half-metallicity up to x = 0.7, and compensation of the two Mn sublattice moments is observed at specific compositions and temperatures, leading to a zero-moment half metal. There are potential benefits for using such films with perpendicular anisotropy for spin-torque magnetic tunnel junctions and oscillators, such as low critical current, high tunnel magnetoresistance ratio, insensitivity to external fields and resonance frequency in the THz range.

Recent experimental work on Mn2RuxGa demonstrates its potential as a compensated ferrimagnetic half-metal (CFHM).Here we present a set of high-throughput ab initio density functional theory calculations and detailed experimental characterisation, that enable us to correctly describe the nominal Mn2RuxGa thin films, in particular with regard to site-disorder and defects. We then construct models that accurately capture all the key features of the Mn-Ru-Ga system, including magnetic compensation and the spin gap at the Fermi level. We find that electronic doping is neccessary, which is achieved with a Mn/Ga ratio smaller than two. Our study shows how composition and substrate-induced biaxial strain can be combined to design the first room-temperature CFHM.

The massive tunnel magnetoresistane (TMR) is expected when Fe3Al Heusler alloy are used for magnetic electrodes in magnetic tunnel junction (MTJ). We demonstrate the Fe3Al/MgO/Fe3Al MTJ, which have good lattice matching interfaces. The electronic and transport properties of these system are systematically investigated by first principle calculation. The electronic band structure of Fe3Al Heusler alloy and MgO are represented for discussion spin filtering effect. Besides, the transmission conductance is calculated to study the spin tunneling effect. We found that the massive TMR is clearly achieved at zero-bias condition, and the TMR is decreased when the finite bias are applied. Thus, among ferromagnetic materials, the Fe3Al Heusler alloy will be good alternative to bcc-Fe based electrodes with MgO barrier for the MTJs beyond.

Spin-filtering efficiency of ferrimagnetic spinels CoFe2O4 and NiFe2O4 Nuala M. Caffrey,1,* Daniel Fritsch,2 Thomas Archer,1 Stefano Sanvito,1 and Claude Ederer3,† 1School of Physics and CRANN, Trinity College, Dublin 2, Ireland 2H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, United Kingdom 3Materials Theory, ETH Zürich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland (Received 11 January 2013; published 25 January 2013)

We assess the potential of the ferrimagnetic spinel ferrites CoFe2O4 and NiFe2O4 to act as spin filtering barriers in magnetic tunnel junctions. Our study is based on the electronic structure calculated by means of first-principles density functional theory within different approximations for the exchange correlation energy. We show that, in agreement with previous calculations, the densities of states suggest a lower tunneling barrier for minority spin electrons, and thus a negative spin-filter effect. However, a more detailed analysis based on the complex band-structure reveals that both signs for the spin-filtering efficiency are possible, depending on the band alignment between the electrode and the barrier materials and depending on the specific wave-function symmetry of the relevant bands within the electrode.

We propose, by performing advanced ab initio electron transport calculations, an all-oxide composite magnetic tunnel junction, within which both large tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects can coexist. The TMR originates from the symmetry-driven spin filtering provided by an insulating BaTiO(3) barrier to the electrons injected from the SrRuO(3) electrodes. Following recent theoretical suggestions, the TER effect is achieved by intercalating a thin insulating layer, here SrTiO(3), at one of the SrRuO(3)/BaTiO(3) interfaces. As the complex band structure of SrTiO(3) has the same symmetry as that of BaTiO(3), the inclusion of such an intercalated layer does not negatively alter the TMR and in fact increases it. Crucially, the magnitude of the TER also scales with the thickness of the SrTiO(3) layer. The SrTiO(3) thickness becomes then a single control parameter for both the TMR and the TER effect. This protocol offers a practical way to the fabrication of four-state memory cells.

We investigate the spin transport properties of molecules belonging to the acenes series by using density functional theory combined with the non-equilibrium Green's function approach to electronic transport. While short acenes are found to be non-magnetic, Molecules comprising more than nine acene rings have a spin-polarized ground state. Once the molecule is attached asymmetrically to Au electrodes via thiol linkers, a net fractional magnetic moment is produced. In this situation the system Au/n-acene/Au can act as an efficient spin-filter with interesting applications in the emerging field of organic spintronics.

All-oxide magnetic tunnel junctions (MTJs) incorporating functional materials as insulating barriers have the potential of becoming the founding technology for novel multifunctional devices. We investigate, by first-principles density functional theory, the bias-dependent transport properties of an all-oxide SrRuO${}_{3}$/BaTiO${}_{3}$/SrRuO${}_{3}$ MTJ. This incorporates a BaTiO${}_{3}$ barrier which can be found either in a nonferroic or in a ferroelectric state. In such an MTJ not only can the tunneling magnetoresistance reach enormous values, but also, for certain voltages, its sign can be changed by altering the barrier electric state. These findings pave the way for a new generation of electrically controlled magnetic sensors.

We investigate the spin-transport properties of molecules belonging to the acenes series by using density functional theory combined with the nonequilibrium Green's function approach to electronic transport. While short acenes are found to be nonmagnetic, molecules comprising more than nine acene rings have a spin-polarized ground state. In their gas phase, these have a singlet total spin configuration, where the two unpaired electrons occupy the doubly degenerate highest molecular orbital. Such an orbital degeneracy is however lifted once the molecule is attached asymmetrically to Au electrodes via thiol linkers, leading to a net fractional magnetic moment. In this situation, the system Au/n-acene/Au can act as an efficient spin filter with interesting applications in the emerging field of organic spintronics.

The magnetic properties of the transition metal monoxides MnO and NiO are investigated at equilibrium and under pressure via several advanced first-principles methods coupled with Heisenberg Hamiltonian MonteCarlo. The comparative first-principles analysis involves two promising beyond-local density functionals approaches, namely the hybrid density functional theory and the recently developed variational pseudo-self-interaction correction method, implemented with both plane-wave and atomic-orbital basis sets. The advanced functionals deliver a very satisfying rendition, curing the main drawbacks of the local functionals and improving over many other previous theoretical predictions. Furthermore, and most importantly, they convincingly demonstrate a degree of internal consistency, despite differences emerging due to methodological details (e.g. plane waves vs. atomic orbitals)

The evolution of the magnetic ordering temperature of the 4d(3) perovskites RTcO3 (R = Ca, Sr, Ba) and its relation with its electronic and structural properties has been studied by means of hybrid density functional theory and Monte Carlo simulations. When compared to the most widely studied 3d perovskites the large spatial extent of the 4d shells and their relatively strong hybridization with oxygen weaken the tendency to form Jahn-Teller like orbital ordering. This strengthens the superexchange interaction. The resulting insulating G-type antiferromagnetic ground state is characterized by large superexchange coupling constants (26-35 meV) and Neel temperatures (750-1200 K). These monotonically increase as a function of the R ionic radius due to the progressive enhancement of the volume and the associated decrease of the cooperative rotation of the TcO6 octahedra.

Co-doped ZnO is the prototypical dilute magnetic oxide, showing many of the characteristics of ferromagnetism. The microscopic origin of the long-range order, however, remains elusive, since the conventional mechanisms for magnetic interaction, such as super-exchange and double exchange, fail either at the fundamental or at a quantitative level. Intriguingly, there is growing evidence that defects both in point-like and in extended form play a fundamental role in driving the magnetic order. Here, we explore one such possibility by performing ab initio density functional theory calculations for the magnetic interaction of Co ions at or near a ZnO (10 (1) over bar0) surface. We find that extended surface states can hybridize with the e-levels of Co and efficiently mediate the magnetic order, although such a mechanism is effective only for ions placed in the first few atomic planes near the surface. We also find that the magnetic anisotropy changes at the surface from a hard-axis easy plane to an easy axis, with an associated increase in its magnitude. We then conclude that clusters with high densities of surfacial Co ions may display blocking temperatures much higher than in the bulk.