Rational design of new materials for spintronics: Co2Fe Z (Z=Al, Ga, Si, Ge)

Abstract

Spintronic is a multidisciplinary field and a new research area. New materials must be found for satisfying the different types of demands. 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. The development of magnetic Heusler compounds specifically designed as material 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 magnetic tunnel junctions. High Curie temperatures were found in Co₂-based Heusler compounds with values up to 1120 K in Co₂FeSi. The latest results at the time of writing are a tunnelling magnet resistance (TMR) device made from the Co₂FeAl_0.5Si_0.5 Heusler compound and working at room temperature with a (TMR) effect higher than 200%. Good interfaces and a well-ordered compound are the precondition to realize the predicted half-metallic properties. The series Co₂FeAl_{1- x} Si _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. This might be a reason for the exceptional temperature behaviour of Co₂FeAl_0.5Si_0.5 TMR devices. Using x-ray diffraction (XRD), it was shown conclusively that Co₂FeAl crystallizes in the B2 structure whereas Co₂FeSi crystallizes in the L2₁ structure. For the compounds Co₂FeGa or Co₂FeGe, with Curie temperatures expected higher than 1000 K, the standard XRD technique using laboratory sources cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique was used to elucidate the structure of these two compounds. Analysis of the data indicated that both compounds crystallize in the L2₁ structure which makes these two compounds suitable new candidates as materials in magnetic tunnel junctions.

Keywords: Heusler compounds; electronic structure; half-metallic ferromagnets; intermetallics.

Figure 1

Slater–Pauling curve for 3d transition metals and their alloys. Experimental values for selected Co₂-based Heusler compounds are given for comparison. (Note: the A_{1- x}B_x alloys are given as AB in the legend, for short.)

Figure 2

Curie-temperatures of X₂YZ Heusler compounds. The line is found from a linear fit of the measured T_C for Co₂-based compounds (red cross). The elemental metals Fe, Co and Ni are given for comparison (left panel). Calculated versus measured Curie temperatures for Co₂-based compounds. The calculation done with GGA are denoted with a plus sign (right panel).

Figure 3

The ordered L2₁ Heusler structure (left) and the simplest types of disorder: The B2 (middle) and the A2 (right) structure. Note that all positions are shifted by with respect to the standard cell to make the CsCl superstructure visible.

Figure 4

LDA+U band structure and DOS of Co₂FeSi. The calculation was performed by Wien2k using the experimental lattice parameter.

Figure 5

Site resolved magnetic properties of Co₂FeSi. Shown are the XAS and XMCD (I_MCD) spectra taken at the L_{2, 3} absorption edges of Fe (a) and Co (b) after subtracting a constant background.

Figure 6

Dependence of the minority gap on the effective Coulomb exchange parameter. The extremal energies of the gap involving states for Co₂MnSi (a) and Co₂FeSi (b) are shown. The shaded areas indicate the region of half-metallic ferromagnetism. Lines are drawn for clarity. The atomic values for the neutral atoms are 22.71, 24.13 and 25.53 eV for Mn, Fe and Co, respectively.

Figure 7

Dependence of the minority gap on the Fe concentration. The extremal energies of the gap involving states are shown. The shaded areas indicate the region of half-metallic ferromagnetism. Lines are drawn for clarity.

Figure 8

Concentration dependence of the magnetic moment in Co₂Mn_{1- x}Fe_xSi. All measurements were performed at T=5 K.

Figure 9

Valence density of Co₂Mn_{1- x}Fe_xSi (x=0, 1/2, 1). (a)–(c) Compare the calculated total DOS with photoelectron spectra excited by hν=7.939 keV. The calculated DOS is convoluted by a Fermi–Dirac distribution using T=20 K. (e)–(g) Show high resolution spectra close to the Fermi energy. The range of the calculated minority gap is marked by areas.

Figure 10

Spin resolved DOS of Co₂FeAl_{1- x}Si_x. The panels (a, … , e) show—from top to button—the DOS with increasing amount of Si for x=0, 0.25, 0.5, 0.75 and 1. The DOS is calculated using LDA+U.

Figure 11

Order–disorder phase transitions in Co₂FeAl_{1- x}Si_x. Shown is the composition dependence of the phase transition temperature. The length of the vertical bars corresponds to the experimental hysteresis.

Figure 12

LDA+U band structure and DOS of Co₂FeGa. The calculation was performed by Wien2k using the experimental lattice parameter.

Figure 13

LDA+U band structure and DOS of Co₂FeGe. The calculation was performed by Wien2k using the experimental lattice parameter.

Figure 14

EXAFS at the Fe K-edges (left panel) and Co K-edges (right panel) of Co₂FeZ with Z=Al, Si, Ga, Ge. EXAFS oscillations extracted from the x-ray absorption measurements at the Fe K-edge (a) and Co K-edge (c). (b) and (d) Corresponding Fourier transforms (symbols) and best fitting results (grey line). The imaginary part of the Fourier transform is displayed for the Co₂FeGe compound (open circles).