Rational Design of New materials for Spintronics: Heusler compounds

Claudia Felser

Institute of Inorganic and Analytical Chemistry,

Staudingerweg 9, 55128 Mainz, Germany

The development of magnetic Heusler compounds specifically designed as materials for spintronic applications has made tremendous progress in the very recent past [1]. Heusler compounds can be made as half-metals, showing a high spin polarization of the conduction electrons of up to 100% in tunnel junctions [2]. High Curie temperatures were found in Co₂-Heusler compounds with values up to 1120 K in Co₂FeSi [3]. 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% [4]. Good interfaces and a well ordered compound is the precondition to realize the predicted half-metallic properties. Using XRD, it was shown conclusively that Co₂FeAl crystallizes in the B2

structure whereas Co₂FeSi crystallizes in the L2₁ structure [5].The series Co₂FeSi_1-xAl_x is found to exhibit half-metallic ferromagnetism over a broad range, and it is shown that electron doping stabilizes the gap in the minority states for x=0.5 [6]. This might be a reason for the exceptional temperature behaviour of Co₂FeSi_0.5Al_0.5-TMR devices [4]. For compounds Co₂FeGa or Co₂FeGe, with Curie temperatures expected higher than 1000 K, the XRD technique cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique and anomalous XRD was used to elucidate the structure of these two compounds. Analysis of the data indicated that both compounds crystallize in the L2₁ structure [7]. On the other hand we are looking for ferrimagnetic Heusler compounds for spin torque application. In general the Gilbert damping factor is low, with a small saturation magnetization despite a high Curie temperature Heusler are promising materials for Spin torque applications. Mn₃Ga with Heusler structure was predicted to be a half metallic compensated ferrimagnet [8]. However the synthesized material is tetragonal distorted, but still a suitable material for spin torque transfer applications. The material is hard magnetic and has a saturation magnetization in average about ¼ m_B / at. The Curie temperature is above the decomposition temperature of about 730 K. The electronic structure calculation indicates a ground state with ferrimagnetic order and 88% spin polarization at the Fermi Energy [9].

References

[1] C. Felser, G. H. Fecher, B. Balke, Angew. Chem. (int. ed.) 46, 668 (2007).

[2] Y. Sakuraba M. Hattori, M. Oogane, Y. Ando, H. Kato, A. Sakuma, T. Miyazaki, and H. Kubota,

Appl. Phys. Lett. 88, 192508 (2006).

[3] S. Wurmehl, G. H. Fecher, H. C. Kandpal, V. Ksenofontov, C. Felser, H.-J. Lin, and J.

Morais, Phys. Rev. B 72, 184434 (2005).

[4] N. Tezuka, N. Ikeda and S. Sugimoto, K. Inomata, Appl. Phys. Lett. 89, 252508 (2006).

[5] B. Balke, G. H. Fecher, C. Felser, Appl. Phys. Lett. 90, 242503 (2007).

[6] G. H. Fecher and C. Felser, J. Phys. D: Appl. Phys. 40, 1582 (2007).

[7] B. Balke, S. Wurmehl, G. H. Fecher, C. Felser, et al. Appl. Phys. Lett. 90, 172501 (2007)

[8] S. Wurmehl, G. H. Fecher, H. C. Kandpal, and C. Felser J.Phys. Cond. Matter. 18 (2006) 6171.

[9] B. Balke, G. H. Fecher, J. Winterlik, C. Felser, Appl. Phys. Lett. 90, 152504 (2007)