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A significant number of the intermetallic Heusler alloys have been predicted to be half-metals. In this contribution we present a study of the basic electronic and magnetic properties of both Heusler families; the so-called half-Heusler alloys like NiMnSb and the the full-Heusler alloys like CoMnGe. Based on results we discuss the origin of the gap which is fundamental for the understanding of their electronic and magnetic properties. We show that the total spin magnetic moment Mt scales linearly with the number of the valence electrons , such that t = t - 24 for the full Heuslers and t = t - 18 for the half Heuslers, thus opening the way to engineer new half-metallic alloys with the desired magnetic properties. Although at the surfaces and interfaces the half-metallic character is in general lost, we show that for compounds with Cr as the metallic element the large enhancement of the Cr surface moment can lead to a high polarization at the surface. Moreover we discuss the role of the spin-orbit coupling, which in principle destroys the half-metallic gap, but in practice only slightly reduces the 100% spin polarization at .

Heusler–alloys, such as CoMnGe and CoMnSi, predicted from firstprinciples to be half–metallic, have recently attracted great attention for spininjection purposes. However, spin polarizations of only 50%–60% were experimentally obtained for Heusler thin films – a decrease attributed to defects in the Mn and Co sublattices. We performed FLAPW calculations in order to determine the effects of several types of defects (Co and Mn antisites, atomic swaps, etc.) on the electronic and magnetic properties of the bulk Heusler compounds. Our findings, in general agreement with experiments, show that Mn antisites have the lowest formation energy and retain the half–metallic character. On the other hand, Co antisites have a slightly higher formation energy and a dramatic effe

Our Introduction starts with a short general review of the magnetic and structural properties of the Heusler compounds which are under discussion in this book. Then, more specifically, we come to the discussion of our experimental results on multilayers composed of the Heusler alloys CoMnGe and CoMnSn with V or Au as interlayers. The experimental methods we apply combine magnetization and magnetoresistivity measurements, x–ray diffraction and reflectivity, soft x-ray magnetic circular dichroism and spin polarized neutron reflectivity. We find that below a critical thickness of the Heusler layers at typically = 1.5 nm the ferromagnetic order is lost and spin glass order occurs instead. For very thin ferromagnetic Heusler layers there are pecularities in the magnetic order which are unusual when compared to conventional ferromagnetic transition metal multilayer systems. In [CoMnGe/Au] multilayers there is an exchange bias shift at the ferromagnetic hysteresis loops at low temperatures caused by spin glass ordering at the interface. In [CoMnGe/V] multilayers we observe an antiferromagnetic interlayer long range ordering below a well defined Néel temperature originating from the dipolar stray fields at the magnetically rough Heusler layer interfaces.

The classical concept of band structure tuning as used for semiconductors by partly replacing one atom by a chemical neighbor without altering the structure is applied examplarily to the half-metallic ferromagnetic Heusler compound CoCr-FeAl. Band structure calculations are presented for ordered and disordered compounds. We present experimental and theoretical results. The connection between specific site disorder and the band structure is shown explicitly with particular emphasis on the half-metallic properties. Experimentally observed deviations from the ideal Heusler structure and from the simple Slater-Pauling rule for the magnetization are discussed in close relation to theoretical models. It has been found that the orbital magnetic moment and hence the spin-orbit coupling is important for the understanding of the half-metallicity. Experimental techniques which can be used to determine the electronic properties are described.

The similarity in crystal structures allows for the epitaxial growth of the candidate half-metal NiMnSb on GaAs. We discuss the growth by molecular beam epitaxy using individual sources for Ni, Mn, and Sb on GaAs(001), (111)A and (111)B substrates. We focus the discussion on the aspects that are crucial for obtaining highly polarized films suitable for spin injection into a semiconductor: control of the interface quality and polarity; composition control to within 1% using a special Rutherford backscattering (RBS) calibration procedure; and reduction of point and extended defect density to the 1% level. We pay particular attention to the zero-field nuclear magnetic resonance technique (NMR) that was used to obtain this level of sensitivity to the local disorder.

CoMnGe and other related Heusler alloys are investigated in the context of magnetotransport properties in multilayer (ML) structures. The most important factors relevant to the problem of magnetotransport in these MLs are reviewed including (i) the growth of thin film Hesuler alloy structures by Molecular Beam Epitaxy and Magnetron Sputtering techniques; (ii) the effects of finite temperature on the half-metallicity, interface states and band structure matching (iii) the type of measured carrier transport and its relationship with the underlying band structure; and finally (iv) the magnetic interactions in an all Heusler ML structure. We have measured a room temperature current-in-plane (CIP) GMR effect in MLs and spin valves and attribute their small magnetoresistance to the materials high resistivity and low carrier mobility that is an inherent property of the Heusler alloys due to their complicated band structure for these p-type conductors. The main challenges for achieving an enhanced GMR effect in Heusler ML structures are discussed on the basis of our room temperature magneto-transport measurements and theoretical modelling results.

Interface engineering in order to exploit the possibilities of the interface electronic structure may be a route to a good spin injector. Nonetheless, for many potential half metallic systems, the stoichiometric surface is generally not in thermodynamic equilibrium with the bulk and the surface Debye temperature is often quite low. Several such classes of materials are currently under investigation as potential high spin polarization materials. These include the half metallic systems such as the semi-Heusler alloys, the manganese perovskites and the “simpler” oxides like chromium dioxide and magnetite. But all these materials suffer from a tendency of the surface to adopt a different composition than the bulk. Surface segregation and surface composition for three classes of potential half metallic systems: NiMnSb(100), La(M=Ca,Pb,Sr)MnO(100) and CrO will be explored. The surface compositional stability and different surface terminations will be compared with the results from recent electronic structure calculations.

As a consequence of the growing theoretically predictions of 100% spin polarized half– and full–Heusler compounds over the past 6 years, Heusler alloys are among the most promising materials class for future magnetoelectronic and spintronic applications. We have integrated CoMnSi as a representative of the full–Heusler compound family as one magnetic electrode into technological relevant magnetic tunnel junctions. The resulting tunnel magnetoresistance at 20 K was determined to be 95% corresponding to a CoMnSi spin polarization of 66% in combination with an AlO barrier thickness of 1.8 nm. For magnetic tunnel junctions prepared with an initially larger Al layer prior to oxidation the tunnel magnetoresistance at 20 K increases to about 108% associated with a CoMnSi spin polarization of 72% clearly proving that CoMnSi is already superior to 3-based magnetic elements or their alloys. The corresponding room temperature values of the tunnel magnetoresistance are 33% and 41%, respectively. Structural and magnetic properties of the CoMnSi AlO – barrier interface have been studied with X-ray diffraction, electron and X-ray absorption spectroscopy and X-ray magnetic circular dichroism and it is shown that the ferromagnetic order of Mn and Co spins at this interface is only induced in optimally annealed Co2MnSi layer. The underlying atomic ordering mechanism responsible for achieving about its theoretical magnetic moment could be assigned to the elimination of Co-Si antisite defects whereas the reduction of Co-Mn antisite defects results in large tunnel magnetoresistance. The presence of a step like tunnel barrier which is already created during plasma oxidation while preparing the AlO tunnel barrier has been identified as the current limitation to achieve larger tunnel magnetoresistance and hence larger spin polarization and is a direct consequence of the oxygen affinity of the CoMnSi - Heusler elements Mn and Si.

It is highly desirable to explore robust half-metallic ferromagnetic materials compatible with important semiconductors for spintronic applications. A state-of-the-art full potential augmented plane wave method within the densityfunctional theory is reliable enough for this purpose. In this chapter we review theoretical research on half-metallic ferromagnetism and structural stability of transition metal pnictides and chalcogenides. We show that some zincblende transition metal pnictides are half-metallic and the half-metallic gap can be fairly wide, which is consistent with experiment. Systematic calculations reveal that zincblende phases of CrTe, CrSe, and VTe are excellent half-metallic ferromagnets. These three materials have wide half-metallic gaps, are low in total energy with respect to the corresponding ground-state phases, and, importantly, are structurally stable. Halfmetallic ferromagnetism is also found in wurtzite transition metal pnictides and chalcogenides and in transition-metal doped semiconductors as well as deformed structures. Some of these half-metallic materials could be grown epitaxially in the form of ultrathin .lms or layers suitable for real spintronic applications.

Zinc-blende half-metallic ferromagnets are promising materials in order to open up a new world of semiconductor spintronics. We design a new class of half-metallic ferromagnets, the zinc-blende transition-metal mono-pnictides, using ab-initio calculations based on the density-functional theory. The calculations show that the total-energy difference between the ferromagnetic and the antiferromagnetic states for the zinc-blende CrAs is the largest among all the studied compounds and the highly spin-polarized electronic band structure is almost unaffected from the spin-orbit interaction.We further study the properties of zinc-blende CrAs/GaAs multilayers theoretically and show that they keep a high spin polarization. Experimental realization of the previously nonexistent zinc-blende CrAs thin film has been achieved by molecular-beam epitaxy. The crystallographic analysis is presented together with the magnetic properties of the epitaxial film. The morphological structure and the magnetic properties change sensitively depending on the substrate temperature during the growth. The room-temperature saturation magnetization of the film grown under optimum conditions reaches about 3 μB per formula unit, which agrees with the theoretically predicted value and reflects the half-metallic behavior. The epitaxial growth of multilayers consisting of 2–4 monolayers of CrAs and 2–4 monolayers GaAs with atomically flat surfaces and interfacess is demonstrated.