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RESEARCH PRODUCT

High-spin polarization of Heusler alloys

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Fermi surface of a half-metal compound with cubic Fm–3m symmetry: Heusler alloy or double perovskite.The inset shows the spin resolved density of state for a CCFA Heusler compound. ( This illustration appears on the cover of the print edition. ) This cluster issue of Journal of Physics D: Applied Physics is devoted to magnetic Heusler alloys. This class of materials is currently considered to contain the most attractive half-metallic ferromagnets due to their high Curie temperatures and their structural relation to conventional semiconductors. The first compound to be identified as a half-metallic ferromagnet, by de Groot et al back in 1983, was the half-Heusler alloy NiMnSb. After a period of rather slow research, magnetic Heusler alloys have received a strong boost of interest in the last few years owing to their potential application in magneto-electronic devices. Due to their half-metallic character, they may have a spin polarization of 100% at the Fermi level. The highest Curie temperature for Heusler alloys is 1100 K, found in the full-Heusler alloy Co2FeSi. Although there is no proof of their half-metallicity, Heusler alloys are, up to now, the only half-metallic materials that show a high magnetoresistance effect in tunnel junctions at room temperature. A breakthrough was achieved by using Co2MnSi (by Reiss's group in Bielefeld and Miyazaki's group in Sendai) and Co2Cr0.6Fe0.4Al (by Inomata in Sendai and Yamamoto in Sapporo) Heusler alloys as electrodes. This has resulted in a pronounced increase in magnetoresistance at room temperature. In this cluster issue, Miyazaki's group report their current world record, a magnetoresistance effect of 159% at 2 K with an Al-oxide tunnel barrier and Co2MnSi bottom electrode. The purpose of this cluster issue is to cover the various aspects (theory, sample preparation, characterization, devices) related to Heusler alloys with high-spin polarization. Galanakis et al present in their review article an overview of the basic electronic and magnetic properties of both Heusler families, the half- and full-Heusler compounds. In many Heusler compounds the total magnetic moment follows a simple electron-counting rule based on the Slater–Pauling behaviour. Kandpal et al demonstrate the relation between half-Heusler compounds with 8 or 18 valence electrons and classical semiconductors like silicon and ZnS. A half-metallic ferromagnet with a half-Heusler structure can be considered as being built up of a zinc blende framework filled with an electropositive magnetic ion. Besides the high Curie temperature, Co2FeSi exhibits the highest magnetic moment of 6 μB per formula unit. Kallmeyer et al have therefore investigated the magnetic properties of Co2Mn1-xFexAl Heusler alloys. Their experimental findings indicate that the magnetization determined near the Co2Mn1-xFexAl/Au interface is smaller than expected from the band structure for a defect-free Heusler structure, while the bulk properties agree well with prediction. It has been known for a long time that surface or interface states or surface segregation might be responsible for the reduced spin polarization measured by surface-sensitive methods like photoemission. The interface and the surface are the subjects of theoretical study in two papers. The continuing drama of the half-metal–semiconductor interface is addressed in the paper of Attema et al. They propose semiconductors based on transition metals rather than the main group metals as appropriate interfaces for Heusler compounds. The semiconducting nonmagnetic half-Heusler compound NiScSb has only a very small lattice mismatch with NiMnSb and shows a genuine half-metallic interface with NiMnSb for all interfaces of low index. Leciac et al have predicted the scanning tunnelling microscopy surfaces of NiMnSb(001) using ab initio methods. The quaternary Heusler compounds Co2Mn1-xFexAl were investigated theoretically and experimentally by Wurmehl et al. To avoid the problem of surface states, the authors investigated the electronic structure using higher photon energy for greater bulk sensitivity of excitation. The discrepancy between the calculated density of states and the measured density of states by resonant and high-energy photoemission suggests the presence of correlations. It is well established that well-ordered films are a precondition for large magnetoresistance effects in Heusler compounds. Inomata et al present here their recent results for tunnel junctions of Co2Mn1-xFexAl and Co2FeSi with an Al-oxide tunnelling barrier. A maximum TMR (tunnelling magnetoresistance) effect of 52% was achieved at room temperature for less-ordered Co2Mn1-xFexAl and of 41% for well-ordered Co2FeSi. The lattice mismatch of Co2FeSi with the MgO substrate probably accounts for the low effect in the well-ordered samples. Very recently, effects of about 300% at room temperature have been achieved with simple transition metals like Fe by using the spin filter effect of a MgO tunnelling barrier, and Yamamoto et al present here their first very promising results. However, the best electrode material is still well-ordered Co2MnSi in combination with Al-oxide with 159% at 2 K and 70% at room temperature as reported by Oogane et al in this issue. X-ray resonant magnetic scattering studies of multilayers of Co2MnGe have shown experimentally a nonferromagnetic interface layer of about 0.6 nm at the bottom of the Co2MnGe layer and 0.45 nm at the top. Bergmann et al interpret this as the natural explanation for the small GMR (giant magnetoresistance) amplitude in [Co2MnGe/V]n. In addition to the high-spin polarization, Heusler compounds are also promising candidates for magneto-optics. The article by Picozzi et al focuses on magneto-optical properties using first-principles calculations. Their results may pave the way to magneto-optical material design. Semiconducting full-Heusler compounds are candidates for heavy fermion behaviour. The review by Ślebarski shows that the disorder in Heusler compounds strongly influences the magnetic and transport properties and can also be responsible for unusual low-temperature behaviour. Another related topic is magneto-elastic effects in Heusler alloys, on which the review by Entel et al nicely completes the wide field of potential applications of Heusler alloys. We hope this cluster of papers will help to push forward a better understanding of this very interesting class of materials, which have the potential to revolutionize magnetism-based electronics.