New materials with high spin polarization: half-metallic Heusler compounds

The development of magnetic Heusler compounds, specifically designed as materials for spintronic applications, has made tremendous progress in the very recent past [1–21]. Heusler compounds can be made as half-metals, showing a high spin polarization of the conduction electrons of up to 100% [1]. These materials are exceptionally well suited for applications in magnetic tunnel junctions acting, for example, as sensors for magnetic fields. The tunnelling magneto-resistance (TMR) effect is the relative change in the electrical resistance upon application of a small magnetic field. Tunnel junctions with a TMR effect of 580% at 4 K were reported by the group of Miyazaki and Ando [1], consisting of two Co₂MnSi Heusler electrodes. High Curie temperatures were found in Co₂ Heusler compounds with values up to 1120 K in Co₂FeSi [2]. The latest results are for a TMR device made from the Co₂FeAl_0.5Si_0.5 Heusler compound and working at room temperature with a TMR effect of 174% [3].

The first significant magneto-resistance effect was discovered in Co₂Cr_0.6Fe_0.4Al (CCFA) in Mainz [4]. With the classical Heusler compound CCFA as one electrode, the record TMR effect at 4 K is 240% [5]. Positive and negative TMR values at room temperature utilizing magnetic tunnel junctions with one Heusler compound electrode render magnetic logic possible [6].

Research efforts exist, in particular, in Japan and in Germany. The status of research as of winter 2005 was compiled in a recent special volume of Journal of Physics D: Applied Physics [7–20]. Since then specific progress has been made on the issues of (i) new advanced Heusler materials, (ii) advanced characterization, and (iii) advanced devices using the new materials.

In Germany, the Mainz and Kaiserslautern based Research Unit 559 `New Materials with High Spin Polarization', funded since 2004 by the Deutsche Forschungsgemeinschaft, is a basic science approach to Heusler compounds, and it addresses the first two topics in particular, with emphasis on new rational design of Heusler compounds and advanced characterization tools. This volume of Journal of Physics D: Applied Physics summarizes the latest research results obtained in the Research Unit and presents it to the scientific public as a cluster of refereed papers.

Half-metallic ferromagnets are an impressive example for the rational design of new materials based on computational physics. The paper of Kandpal et al demonstrates how a detailed understanding of the electronic structure, especially from the viewpoint of the properties of the minority band gap and the peculiar magnetic behaviour, enables us to predict new half-metallic compounds. A high interface quality and a well ordered compound are the preconditions to realize the predicted half-metallic properties. Wurmehl et al have carefully studied the surface and bulk structure of the classical Heusler compound CCFA using a combination of characterization methods. A deposition process of epitaxial thin films of CCFA was described by Conca et al. Kallmayer et al have correlated the structural properties of thin magnetron sputtered films determined by x-ray diffraction with details of the x-ray magnetic circular dichroism (XMCD) spectra. From the value of the magnetic moment located at the Cr atom and features of the Co absorption spectra, they conclude that the buffer layers lead to an improvement of local atomic order. A highlight of this Cluster Issue is the spin resolved photoemission result of Cinchetti and co-workers. A careful in situ preparation of the sample surface of CCFA leads to values for the room temperature spin polarization up to 45% at the Fermi level, the highest value measured so far at the surface region of a full Heusler compound at room temperature.

Co₂FeSi (CFS) is the half-metal Heusler compound with the highest Curie temperature reported so far [4]. Schneider et al report the deposition of well ordered thin Co₂FeSi films by RF magnetron sputtering. The thickness dependence of these thin epitaxial Co₂FeSi (110) films was investigated using XMCD by Kallmayer et al. The magnetic moment as a function of film thickness demonstrates the presence of dead layers, reducing the magnetization and the spin polarization of these films at all interfaces. The influence of Ga⁺ ion irradiation was studied using the longitudinal (LMOKE) and quadratic (QMOKE) magneto-optical Kerr effect in a paper by Hamrle et al who, in a second paper, report an unusual huge quadratic magneto-optical Kerr effect in CFS films with L2₁ structure. The films exhibit a huge QMOKE signal, with its maxima of up to 30 mdeg, which is the largest QMOKE signal in reflection that has been measured thus far. Beside the half-metallicity and the high Curie temperature, an essential feature for such devices is the micro-magnetic domain structure. XMCD–PEEM has been used for a direct observation of the domain structure of single- and polycrystalline samples by Gloskowskii et al.

The spin polarization of Co₂FeSi films can be improved at room temperature, especially the temperature dependence of the magneto-resistance effect. For a TMR device with Co₂FeSi_0.5Al_0.5 Tezuka et al [3] have found a record TMR value for room temperature. Fecher et al have investigated the electronic structure of Co₂FeSi_{1 - x}Al_x. The series Co₂FeSi_{1 - x}Al_x is found to exhibit half-metallic ferromagnetism and it is shown that the electron-doping stabilizes the gap in the minority states for x = 0.5. This might be a reason for the exceptional temperature behavior of Co₂FeSi_0.5Al_0.5 TMR devices. Co₂Fe_0.5Mn_0.5Si is another candidate with Fermi energy in the middle of the minority states gap. Therefore Fecher et al have investigated the electronic structure of the series Co₂Fe_{1 - x}Mn_xSi by high energy, high resolution photoelectron spectroscopy. High energy photoemission is a new advanced method to study the electronic structure of bulk material, due to a large mean free path of the photo electrons. The high resolution measurements of the valence band close to the Fermi energy indicate the existence of the gap in the minority states for all investigated Co₂Fe_{1 - x}Mn_xSi compounds. Other Co₂ Heusler compounds are also possible candidates for magneto-electronic devices. Miura et al [21] have found that the disorder between Co and Y atoms correlates with the total valence electron charges around Y atom and have predicted that Ti-based compounds are better than Cr-, Mn- and Fe-based compounds in preventing the atomic disorder between Co and Y atoms. Kandpal et al have therefore investigated the electronic structure and disordering effects in Co₂TiSn using local probes, ¹¹⁹Sn Mössbauer spectroscopy and ⁵⁹Co nuclear magnetic resonance spectroscopy. They found that the sample possesses up to 10% of antisite (Co/Ti) disordering, a disorder that does not destroy the half-metallic character of this material.

We hope that this Cluster of papers will help to stimulate and push forward the research of materials with high spin polarization.

References

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[2] S Wurmehl, Fecher G H, Kandpal H C, Ksenofontov V, Felser C, and Lin H-J 2006 Investigation of Co₂FeSi: the Heusler compound with highest Curie temperature and magnetic moment  Appl. Phys. Lett. 88 032503

[3] Tezuka N, Ikeda N, Sugimoto S and Inomata K 2006 175% TMR at room temperature and high thermal stability using Co2FeAl_0.5Si_0.5 full-Heusler alloy electrodes Appl. Phys. Lett. 89 252508

[4] Block T, Felser C, Jakob G, Ensling J, Mühling B, Gütlich P, Cava R J 2003 Large negative magnetoresistance effects in Co₂Cr_0.6Fe_0.4Al  J. Solid State Chem. 176 646

[5] Marukame T, Ishikawa T, Matsuda K I, Uemura T and Yamamoto M 2006 High tunnel magnetoresistance in fully epitaxial magnetic tunnel junctions with a full-Heusler alloy Co₂Cr_0.6Fe_0.4Al thin film Appl. Phys. Lett. 88 262503

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[21] Miura Y, M Shirai and Nagao K 2006 Ab initio study on stability of half-metallic Co-based full-Heusler alloys J. Appl. Phys. 99 08J112