J. Kübler

Rational design of new materials for spintronics: Co2FeZ (Z = Al, Ga, Si, Ge)

Abstract Spintronic is a multidisciplinary field and a new research area. New materials must be found for satisfying the different types of demands. The search for stable half-metallic ferromagnets and ferromagnetic semiconductors with Curie temperatures higher than room temperature is still a challenge for solid state scientists. A general understanding of how structures are related to properties is a necessary prerequisite for material design. Computational simulations are an important tool for a rational design of new materials. The new developments in this new field are reported from the point of view of material scientists. The development of magnetic Heusler compounds specifically designed as material for spintronic applications has made tremendous progress in the very recent past. Heusler compounds can be made as half-metals, showing a high spin polarization of the conduction electrons of up to 100% in magnetic tunnel junctions. High Curie temperatures were found in Co2-based Heusler compounds with values up to 1120 K in Co2FeSi. The latest results at the time of writing are a tunnelling magnet resistance (TMR) device made from the Co2FeAl0.5Si0.5 Heusler compound and working at room temperature with a (TMR) effect higher than 200%. Good interfaces and a well-ordered compound are the precondition to realize the predicted half-metallic properties. The series Co2FeAl1- xSix is found to exhibit half-metallic ferromagnetism over a broad range, and it is shown that electron doping stabilizes the gap in the minority states for x=0.5. This might be a reason for the exceptional temperature behaviour of Co2FeAl0.5Si0.5 TMR devices. Using x-ray diffraction (XRD), it was shown conclusively that Co2FeAl crystallizes in the B2 structure whereas Co2FeSi crystallizes in the L21 structure. For the compounds Co2FeGa or Co2FeGe, with Curie temperatures expected higher than 1000 K, the standard XRD technique using laboratory sources cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique was used to elucidate the structure of these two compounds. Analysis of the data indicated that both compounds crystallize in the L21 structure which makes these two compounds suitable new candidates as materials in magnetic tunnel junctions.

Realization of spin gapless semiconductors: the Heusler compound Mn2CoAl.

Recent studies have reported an interesting class of semiconductor materials that bridge the gap between semiconductors and half-metallic ferromagnets. These materials, called spin gapless semiconductors, exhibit a band gap in one of the spin channels and a zero band gap in the other and thus allow for tunable spin transport. Here, we report the first experimental verification of the spin gapless magnetic semiconductor Mn(2)CoAl, an inverse Heusler compound with a Curie temperature of 720 K and a magnetic moment of 2 μ(B). Below 300 K, the compound exhibits nearly temperature-independent conductivity, very low, temperature-independent carrier concentration, and a vanishing Seebeck coefficient. The anomalous Hall effect is comparatively low, which is explained by the symmetry properties of the Berry curvature. Mn(2) CoAl is not only suitable material for room temperature semiconductor spintronics, the robust spin polarization of the spin gapless semiconductors makes it very promising material for spintronics in general.

Design scheme of new tetragonal Heusler compounds for spin-transfer torque applications and its experimental realization.

Band Jahn-Teller type structural instabilities of cubic Mn(2)YZ Heusler compounds causing tetragonal distortions can be predicted by ab initio band-structure calculations. This allows for identification of new Heusler materials with tunable magnetic and structural properties that can satisfy the demands for spintronic applications, such as in spin-transfer torque-based devices.

First principle theory of metallic magnetism

Self-consistent spin-polarized energy-band calculations are used to explain the trends in the ferromagnetic moments oftransition-metal alloys. It is demonstrated that a large amount of magnetization data become interpretable by using the generalized Slater-Pauling curve recently introduced by Williams et al. The discussion includes Heusler alloys of both L21 and Clb structure for which exchange constants and hence the Curie temperatures can be estimated theoretically. CoMnSb is treated in detail and is shown to belong to the class of half-metallic ferromagnets first discovered by de Groot et al. Also included will be Fe3Cr and Au4V which represent interesting examples of itinerant ferromagnetism.

Calculated electronic band structure and magnetic moments of ferrites

Abstract A comparative study of the band structure on the basis of local spin-density functional theory is presented for the ferrites Fe 3 O 4 , CoFe 2 O 4 , NiFe 2 O 4 , MnFe 2 O 4 and ZnFe 2 O 4 . The saturation magnetization is obtained and the trend of the calculated magnetic moments is interpreted and compared with experimental values.

Band theory for electronic and magnetic properties of

We report results of calculations that explain in the itinerant-electron picture magnetic and electronic properties of haematite, . For this we use the local approximation to spin-density functional theory and the ASW method incorporating spin - orbit coupling and noncollinear moment arrangements. The insulating character of the compound is obtained correctly and features in the density of states connected with Fe - O hybridization correlate well with experimental features seen in direct and inverse photoemission intensities. The total energy correctly predicts the experimentally observed magnetic order of the ground state, and, using total energies of different magnetic configurations, we can give a rough estimate of the Neel temperature. We also obtain a state showing weak ferromagnetism. The rate of change is calculated for the decrease of the insulating gap when an external magnetic field is applied.

Large anomalous Hall effect driven by a nonvanishing Berry curvature in the noncolinear antiferromagnet Mn3Ge

A large anomalous Hall effect is observed in the triangular antiferromagnet Mn3Ge arising from an intrinsic electronic Berry phase. It is well established that the anomalous Hall effect displayed by a ferromagnet scales with its magnetization. Therefore, an antiferromagnet that has no net magnetization should exhibit no anomalous Hall effect. We show that the noncolinear triangular antiferromagnet Mn3Ge exhibits a large anomalous Hall effect comparable to that of ferromagnetic metals; the magnitude of the anomalous conductivity is ~500 (ohm·cm)−1 at 2 K and ~50 (ohm·cm)−1 at room temperature. The angular dependence of the anomalous Hall effect measurements confirms that the small residual in-plane magnetic moment has no role in the observed effect except to control the chirality of the spin triangular structure. Our theoretical calculations demonstrate that the large anomalous Hall effect in Mn3Ge originates from a nonvanishing Berry curvature that arises from the chiral spin structure, and that also results in a large spin Hall effect of 1100 (ħ/e) (ohm·cm)−1, comparable to that of platinum. The present results pave the way toward the realization of room temperature antiferromagnetic spintronics and spin Hall effect–based data storage devices.

Electronic and magnetic states of γ-Fe

Abstract The electronic and magnetic structure of fcc (γ-)Fe is determined using the local spin-density functional approximation and the ASW method to solve the band-structure problem self-consistently. The calculations reveal that the detailed nature of the ground state of γ-Fe depends sensitively on the volume but quite unambigously consists of a spiral magnetic structure that is incommensurate with the lattice, in agreement with recent neutron-diffraction experiments. We give, in some detail, the theory necessary to implement the calculations. The noncollinear spin arrangement is described using for the spiral vector q a wide range of values that continuously cover ferromagnetic and antiferromagnetic configurations. The stability of the structure having minimum total energy is related to band-structure features and we emphasize the occurence of multiple magnetic states, which we study using a “constrained-moments” method.

The Fermi surface of

The de Haas - van Alphen spectrum of is calculated and compared with the experimental spectrum for continuously varying directions of the magnetic field. The local density approximation to spin-density functional theory is used for the self-consistent calculations treating the U 5f electrons as itinerant and including spin - orbit coupling. The amount of f angular momentum character is obtained and exhibited graphically for each sheet of the Fermi surface. The band-decomposed spin susceptibility, , is calculated for the states at the Fermi surface and the anisotropy of is discussed.

Ab-initio molecular dynamics in the full-potential LMTO method: Derivation of a practical force theorem

A major advance in electronic structure calculations was the combination of local-density techniques with molecular dynamics by Car and Parrinello seven years ago. Unfortunately, application of the Car-Parrinello scheme has been limited essentially to sp materials because only in the plane-wave pseudopotential method forces are trivial to calculate. We present a systematic approach to derive force theorems with desired characteristics within complicated basis sets, which are applicable to all elements of the periodic table equally well. Application to the LMTO basis set yields an accurate force theorem, quite distinct from the Hellman-Feynman form, which is exceptionally insensitive to errors in the trial density. The forces were implemented in a new full-potential LMTO method which is suited to arbitrary geometries. First results for ab-initio molecular dynamics and simulated annealing runs are shown for some random small molecules and small clusters of silver atoms.