Introduction

The development of magnetic Heusler compounds, specifically designed as materials 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 tunnel junctions [SHO06]. These materials are exceptionally well suited for applications in magnetic tunnel junctions acting as, for example, sensors for magnetic fields. The tunneling magneto-resistance (TMR) effect is the relative change in the electrical resistance of a tunnel structure upon application of a small magnetic field. Tunnel junctions with the highest tunneling magneto-resistance (TMR) effect achieved so far were reported by the group of Miyazaki and Ando [SHO06] with a TMR effect of 580% at 4 K, consisting of two Co₂MnSi Heusler electrodes. High Curie temperatures were found in Co₂- Heusler compounds with values up to 1120 K in Co₂FeSi [WFK06]. The latest results at the time of writing are a TMR device made from the Co₂FeAl_0.5Si_0.5 Heusler compound and working at room temperature with a TMR effect higher than 200% [TIS07].

The first significant magneto-resistance effect was discovered in Co₂Cr_0.6Fe_0.4Al in Mainz [BFJ03], reported on the INTERMAG-Conference in Amsterdam 2002. With the classical Heusler compound Co₂Cr_0.6Fe_0.4Al as one electrode the record TMR effect at 4 K is 317% [MIM07]. Positive and negative TMR values obtained at room temperature by utilising magnetic tunnel junctions with one Heusler compound electrode render magnetic logic possible [TME06]. The two important materials Co₂Cr_0.6Fe_0.4Al and Co₂FeSi were discovered by the Research Unit 559 and these materials have inspired the design of TMR devices in particular in Japan. Both countries Germany and Japan are therefore leading in the field of Heusler compounds.

Figure 1: Shown is the number of papers per year related to one of the subjects of the Research Unit: X₂YZ Heusler compounds. The red bars are the number of papers published by other groups, the green bars are the number of papers published by members of the Research Unit 559.