Spintronics

The class of Heusler compounds, including the XYZ and the X ₂ YZ compounds, does not only have an endless number of members, but also a vast variety of properties can be found in this class of materials, ranging from semi-conductors, half-metallic ferromagnets, superconductors, and topological insulators to shape memory alloys. With this chapter, we would like to provide an overview of Heusler compounds, focusing on basis design principles, their properties and potential applications.

Spintronics is a multidisciplinary field and a new research area. New materials must be found for satisfying the different types of requirement. 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.

Heusler compounds are promising materials in many fields of contemporary research. The spectrum of their possible applications ranges from magnetic and magneto-mechanical materials over semiconductors and thermoelectrics to superconductors. An important feature of the Heusler compounds is the possibility of controlling the valence electron concentration by partial substitution of elements. On the other hand, the properties also depend on the degree of ordering of the crystal structure. In general, Heusler compounds crystallize in the Cu₂MnAl-type structure but in many cases certain types of disorder are observed. In this chapter, a detailed description of the crystal structure as well as different types of atomic disorder are given. Furthermore, the relationship of the chemical ordering and the spin polarization is discussed and useful experimental methods for the structural analysis of Heusler compounds are presented.

We systematically studied substituted Sr₂FeReO₆ with respect to experimental characterization and theoretical band structure calculations. In the framework of the tight-binding approach, hole- or electron-doping of Sr₂MM’O₆ were performed at the M or M’ positions either by transition or main group metals. Hole-doping, rather than electron-doping, has a favorable effect to improve the half-metallicity (Curie temperature and saturation magnetization) of the parent compound. When M is substituted by another metal, the original M’ metal will serve as a redox buffer (and vice versa). Substituting M by another metal with a size similar to that of the metal at M’ position causes disorder, which has high impact on the properties of the starting compound. Main group metals block the super-exchange pathways that underlie the half-metallic properties in Sr₂FeReO₆. Thus a Mott-insulating and spin-frustrated state is produced in an ordered phase due to the geometrical arrangement, e.g. in Sr₂InReO₆. However, M/M’ disorder is significant in the main group elements containing double perovskites, this triggers electronic conductivity arising from electron hopping from Re to adjacent Re ions as observed in Sr₂GaReO₆.

Half-metallic ferromagnetism and the related ferrimagnets are reviewed from a theoretical point of view. Concentrating on Heusler compounds as well as the double perovskites we discuss the electronic structure, the nature of the magnetic moments, and the physics of the energy-gap in one of the spin-channels. The role of spin–orbit coupling is examined and a brief account is given of a tetragonal Heusler compound. Exchange interactions are determined and used for calculations of the Curie temperatures and spin-wave spectra. In view of the large Curie temperatures of the Heusler compounds and the double perovskites we discuss double-exchange and super-exchange models.

The first part of this study deals with the effects of local electronic correlations and alloying on the properties of the Heusler compound Co₂Mn_1−x Fe _x Si. The analysis has been performed by means of first-principles band-structure calculations based on the local approximation to spin-density functional theory (LSDA) as well as photoemission calculations within the one-step model of photoemission. Correlation effects are treated using the Dynamical Mean-Field Theory (DMFT) and the LSDA+U approach. The formalism is implemented within the Korringa–Kohn–Rostoker (KKR) Green’s function method. In satisfactory agreement with available experimental data the magnetic and spectroscopic properties of Co₂Mn_1−x Fe _x Si are explained in terms of strong electronic correlations. In addition the correlation effects have been analyzed separately with respect to their static or dynamical origin. To achieve a quantitative description of the electronic structure of Co₂Mn_1−x Fe _x Si both static and dynamic correlations must be treated on equal footing. Furthermore, we report on our investigation of the spin-dependent electronic structure of ordered NiMnSb as well as of the disordered Ni _x Mn_1−x Sb alloy system. As a first point we studied the magneto-optical Kerr effect in ordered NiMnSb to extract information on the bulk-related electronic structure of this compound. In addition the influence of chemical disorder on the unoccupied electronic density of states was investigated by use of the ab-initio Coherent Potential Approximation method. These results are used for a detailed discussion of spin-resolved Appearance Potential Spectroscopy measurements. Our theoretical approach describes the spectra as the fully relativistic self-convolution of the matrix-element weighted, orbitally resolved density of states. The analysis is completed by one-step photoemission calculations focusing on the surface electronic structure of ordered NiMnSb(001).

Multicomponent systems play a key role in materials design providing the multifunctionality and tunability of materials. At present, the ternary Heusler materials form probably the most thoroughly studied class of multicomponent systems. In present work, results of ab-initio band structure calculations for A ₂ BC Heusler compounds that consist of A and B sites occupied by transition metals and C by a main group element are presented. These systems cover an extremely wide range of properties from half-metallic ferromagnets to non-magnetic semiconductors. The calculations have been performed in order to understand the properties of the band gap in the minority spin channel, the peculiar transport properties and magnetic behaviour found in these materials. Among the interesting aspects of the electronic structure of these materials are the contributions from both A and B atoms to the total magnetic moment. In several classes of these compounds the magnitudes of the total magnetic moment show a trend consistent with the Slater–Pauling behaviour. The total magnetic moment depends as well on the kind of C atoms although they do not directly contribute to it. In Co₂ compounds, a change of the C element changes the contribution of the t _2g states to the moment at the Co sites. The localised moment in these magnetic compounds resides at the B site. Other than in the classical Cu₂-based Heusler compounds, the A atoms in Co₂-, Fe₂- and Mn₂-based compounds may contribute to the total magnetic moment significantly. It is shown that the inclusion of electron–electron correlation in the form of LDA+U/DMFT calculations helps to understand the magnetic properties of those compounds that exhibit already a minority gap in calculations where it is neglected. Beside the large group of Co₂ compounds, half-metallic ferromagnetism was here found only in such compounds that contain Mn. In parallel we consider the different types of chemical disorder often accompanying multicomponent systems and characterise its influence on spin-polarisation and magnetic properties.

This work discusses the electronic structure magnetic properties and metal–insulator transition in transition metal oxides (TMO). The unique feature of these compounds related to the fact that the spin, charge and orbital degrees of freedom plays an important role in all physical properties. While the local density approximation is quite reasonable for the electronic structure of a metallic oxide, the additional Hubbard-like correlation is important for the energy spectrum of insulating magnetic oxides. The LDA+U method was proven to be a very efficient and reliable tool in calculating the electronic structure of systems where the Coulomb interaction is strong enough to cause localization of the electrons. It works not only for nearly core-like 4f-orbitals of rare-earth ions, where the separation of the electronic states on the subspaces of the infinitely slow localized orbitals and infinitely fast itinerant ones is valid, but also for such systems as transition metal oxides (NiO). The main advantage of LDA+U method over model approaches is its “first principle” nature with a complete absence of adjustable parameters. At the same time, all the most subtle and interesting many-body effects (such as spectral weight transfer, Kondo resonances, and others) are beyond the LDA+U approach. The LDA+DMFT method seems to be effective and useful to describe the dynamical character using the self-energy instead of the effective exchange-correlation potential acting on the electrons. The results for metal–insulator transition in complex transition metal oxides demonstrate that the dynamical mean field theory gives an opportunity to unify the many-body theory with the practice of first-principle calculations of the electronic structure and properties for real materials.

A key tool in the rational design of spin polarised materials is the precise control of the relationships between structure and physical properties, such as between structure and magnetism or transport properties. Thus, a sophisticated and comprehensive characterisation is required in order to understand, tune and control the macroscopic properties of spin polarised materials towards optimised performance in spintronics devices. Nuclear magnetic resonance spectroscopy (NMR) probes the local environments of the active nuclei and is based on the interaction of the spin of a nucleus with the effective field present at the nucleus. The local character of NMR arises from local contributions to the hyperfine field, namely the transferred field which depends on the nearest neighbour atoms and their magnetic moments. This enables NMR to study structural properties of bulk samples as well as of thin films of spin polarised materials. Recent results confirmed that NMR is a very suitable tool to reveal structural contributions and foreign phases in spin polarised materials which are very difficult to detect with other methods like, e.g., conventional X-ray diffraction. In this chapter, recent NMR studies of the local structure of various Heusler compounds will be presented and the impact of the NMR results on their potential for spintronics will be discussed.

We investigate element-specific spin and orbital magnetic moments of polycrystalline bulk Heusler alloys that are predicted to be half-metallic with composition Co₂YZ (Y = Ti, Cr, Mn, Fe and Z = Al, Ga, Si, Ge, Sn, Sb) using magnetic circular dichroism in X-ray absorption spectroscopy (XAS/XMCD). In addition to stoichiometric compounds we also investigate composition series with partly replaced elements on the Y-site (Co₂Fe _x Cr_1−x Si, Co₂Mn _x Ti_1−x Si and Co₂Mn _x Ti_1−x Ge) and on the Z-site (Co₂MnGa_1−x Ge _x ) promising a tailoring of the Fermi level with respect to the minority band gap. We compare experimental results with theoretical predictions elucidating the influence of local disorder in the experimental samples. Moreover, we demonstrate that a consideration of electron correlation in local density approximation theories is necessary for reproducing experimental results. Increased orbital magnetic moments in respect to theoretical predictions put forward the role of spin–orbit coupling for half-metallic properties. For the case of single crystalline thin films we developed a method of simultaneous measurement of bulk and surface sensitive magnetic properties including those at the crucial interface to a tunneling barrier. Exploiting the comparison of bulk and interface information, film growth can be improved for specific applications. In order to directly determine the spin-resolved density-of-states function we present a calculation scheme for extracting this information from the XMCD spectra considering final-state electron correlations. We investigate the electronic properties of epitaxial Co₂(Fe _x Mn_1−x )Si, Co₂Fe(Al_1−x Si _x ), and Co₂(Cr_0.6Fe_0.4)Al films on MgO(100) substrates and for the case of several bulk samples including Co₂TiSb as a reference sample for normal metallic ferromagnetism. The experimental results, revealing the distribution of magnetic moments and the relative position of the Fermi energy as a function of the number of valence electrons, confirm the predicted possibility of tailoring the minority band gap using substitutional quaternary Heusler compounds. These findings may be of general importance for the understanding of the electronic structures in complex intermetallic compounds.

In this work, results of hard X-ray photoelectron spectroscopy (HAXPES) of Heusler compounds and new materials for spintronics are presented. The class of Heusler materials includes some interesting half-metallic and ferromagnetic properties that were predicted by theory. HAXPES allows a direct comparison of the measured and the calculated electronic structure. Valence band spectroscopy of bulk materials by HAXPES is illustrated for the case of the half-metallic ferromagnet Co₂MnGe. The feasibility of HAXPES to explore the valence band electronic structure in deeply buried metallic layers is demonstrated for buried Co₂MnSi films. The films exhibit the same valence density of states as bulk samples and confirm the promise of an epitaxial, single-crystalline Co₂-based Heusler compound film as a ferromagnetic electrode for spintronics devices. The study of complete CoFe(B)/MgO/CoFe(B) tunneling junctions demonstrates the capability of HAXPES to explore the electronic structure in deeply buried layers in a non-destructive way. The improvement of the TMR by annealing of the junction is explained by an improvement of the structure together with a change of the composition in the CoFeB layers.

Due to their half metallic properties and high Curie temperatures Heusler alloys are potential key materials for future application in spin-based devices. One crucial aspect for device performance is the electron spin polarization at surfaces and interfaces, which in general deviates from bulk values due to the different bonding environment. Here, we present spin-resolved photoemission data on surfaces of Co₂Cr_1−xFe_xAl thin films. The influence of bulk and surface contributions on the spin-resolved spectra is discussed for x=0, x=0.4 and x=1, yielding information on disorder, surface states and calculation schemes that are consistent with our spin-resolved photoemission experiments.

Half-metallic Co-based Heusler compounds are attracting attention due to their anticipated use as high-performance materials for spintronics applications, such as spin-source or spin-detector. In order to use these materials in applications, their structural and magnetic properties must be well understood. The important phenomena in those materials are, amongst others, exchange and spin–orbit coupling, the latter one giving rise to effect such as magnetic anisotropy and magneto-optical Kerr effect. In this chapter, we present our investigations of magnetic exchange stiffness, magnetic anisotropy, magnetization reversal, and magneto-optical Kerr effect in Co-based Heusler compound thin films. Furthermore, we have also investigated the modification of the compounds under He⁺ and Ga⁺ ion beam irradiation with the aim of improving and tailoring structural properties.

This work reports on the structural and magnetic properties of the Heusler Co₂FeAl _x Si_1−x epitaxial thin films and their applications to magnetic tunnel junctions (MTJs), giant magnetoresistive (GMR) devices and spin transfer magnetization switching. It is shown for Co₂FeAl _x Si_1−x that the Fermi level position can be tuned by the composition. The temperature dependence of the tunneling magneto resistance (TMR) in epitaxial MTJs using B2-Co₂FeAl_0.5Si_0.5 (CFAS) electrode is fitted well by a spin wave excitation model for tunneling spin polarization. Half-metallic B2-CFAS provided a large GMR up to 34 % at room temperature. Magnetization switching was observed in the resistance-current curves and exhibited a two-step switching process originating from the interplay between the magnetocrystalline anisotropy of the CFAS layers and the spin-transfer torque. A small average intrinsic switching current density is obtained by analyzing the data using the thermal activation model. The results show that the use of the Heusler alloy CFAS with high spin polarization is an effective way to reduce the switching current density even in MTJs.

Thin Heusler films with the composition Co₂Mn_1−x Fe _x Si were grown by both sputter and pulsed laser deposition. The samples show a high degree of structural order and very good magnetic properties. The availability of thin film samples on dielectric substrates allowed the systematic investigation of their electronic properties by transport experiments. The normal Hall effect shows a transition from a hole-like charge transport in Co₂MnSi to an electron-like transport in Co₂FeSi. This is in agreement with calculations, which predict that the substitution of Mn by Fe leads to a band filling and a shift of the Fermi energy. Furthermore, the behavior of the anomalous Hall effect was studied. It is the sum of two opposing mechanisms: an intrinsic contribution, caused by the topology of the Fermi surface and a temperature dependent impurity scattering.

In the framework of spin polarization investigations of Heusler compounds by the measurement of the magnetoresistance (TMR) of tunneling junctions with AlO _x barrier special emphasis is put on the role of the interfaces.

It is demonstrated how an unsuitable morphology can limit the TMR. The barrier morphology could be improved by inserting a Mg layer at the Heusler/barrier interface. Evidence is given that this very thin Mg layer is acting as a seed layer for improving the Al morphology and not as a barrier for coherent MgO tunneling. Thus the Jullière model can be used for evaluating a relatively large spin polarization of 67 % for the B2 ordered Heusler compound Co₂Cr_0.6Fe_0.4Al, which is close to theoretical predictions.

The magnetic surface and bulk moments of the ferromagnetic Heusler compounds Co₂Cr_0.6Fe_0.4Al and Co₂CrAl were comparatively investigated by X-ray magnetic circular dichroism measurements (collaboration with Prof. H.-J. Elmers). We provide evidence that the magnetism of the film interface region is fully developed with an interface magnetic ordering temperature which can be higher than in the bulk.

We demonstrated that a large TMR ratio of 753 % has been observed at 2 K in a MTJ using a Co₂MnSi Heusler alloy electrode and a crystalline MgO tunnel barrier. At room temperature (RT), we also have observed a large TMR ratio of 217 %, which value at RT is much larger than that of MTJs using an amorphous Al-oxide tunnel barrier. However, the temperature dependence of the TMR ratio was still large. In order to improve the interface, we investigated the TMR effect in Co₂MnSi/CoFeB(0–2 nm)/MgO/CoFe MTJs. TMR ratio was enhanced by inserting a thin CoFeB layer at the Co₂MnSi/MgO interface. The MTJ with CoFeB thickness of 0.5 nm exhibited the highest TMR ratio. From the conductance–voltage measurements for the fabricated MTJs, we inferred that the highly spin polarized electron created in Co₂MnSi can conserve the polarization through the 0.5 nm thick FeB layer.