Geometric, electronic, and magnetic structure of ${\mathrm{Co}}_2\mathrm{FeSi}$: Curie temperature and magnetic moment measurements and calculations

In this work a simple concept was used for a systematic search for materials with high spin polarization. It is based on two semiempirical models. First, the Slater-Pauling rule was used for estimation of the magnetic moment. This model is well supported by electronic structure calculations. The second model was found particularly for ${\mathrm{Co}}_2$ based Heusler compounds when comparing their magnetic properties. It turned out that these compounds exhibit seemingly a linear dependence of the Curie temperature as function of the magnetic moment. Stimulated by these models, ${\mathrm{Co}}_2\mathrm{FeSi}$ was revisited. The compound was investigated in detail concerning its geometrical and magnetic structure by means of x-ray diffraction, x-ray absorption, and Mössbauer spectroscopies as well as high and low temperature magnetometry. The measurements revealed that it is, currently, the material with the highest magnetic moment $\left(6{\mu }_B\right)$ and Curie temperature (1100 K) in the classes of Heusler compounds as well as half-metallic ferromagnets. The experimental findings are supported by detailed electronic structure calculations.

Figure 1

Magnetic moments (a) and Curie temperature (b) of Heusler compounds. The heavy $3d$ transition metals are given for comparison. Full dots assign ${\mathrm{Co}}_2$ based and open circles assign other Heusler compounds. The lines in (a) assign the Slater-Pauling curve. The line in (b) is found from a linear fit for ${\mathrm{Co}}_2$ based compounds. The cross in (b) assigns ${\mathrm{Co}}_2\mathrm{FeSi}$ as measured in this work.

Figure 2

LDA band structure and DOS of ${\mathrm{Co}}_2\mathrm{FeSi}$. The calculation was performed by Wien2k using the experimental lattice parameter.

Figure 3

Minority gap in ${\mathrm{Co}}_2\mathrm{FeSi}$. Shown are the positions of the extremal energies of the upper and lower bands depending on the lattice parameter $(a_{\exp }=5.64\mathrm{\AA })$. Energies are given with respect to the Fermi energy. The gray shaded area marks the domain of HMF character (lines are drawn to guide the eye).

Figure 4

$\mathrm{LDA}+U$ band structure and DOS of ${\mathrm{Co}}_2\mathrm{FeSi}$. The calculation was performed by Wien2k using the experimental lattice parameter.

Figure 5

EXAFS results for ${\mathrm{Co}}_2\mathrm{FeSi}$. The x-ray absorption spectra (with constant background removed) were taken at the $K$-edges of Co and Fe. The radial distribution functions derived from the spectra (symbols) are compared to the ones calculated (lines) using the best fit data.

Figure 6

Mössbauer spectrum of ${\mathrm{Co}}_2\mathrm{FeSi}$. The spectrum was excited by ${}^{57}\mathrm{Co}$ and measured from a powder sample in transmission geometry at $T=85\mathrm{K}$.

Figure 7

Magnetic properties of ${\mathrm{Co}}_2\mathrm{FeSi}$. The field dependence of the magnetic moments was measured by SQUID magnetometry at different temperatures.

Figure 8

Site resolved magnetic properties of ${\mathrm{Co}}_2\mathrm{FeSi}$. Shown are the XAS $\left(I_0\right)$ and XMCD $\left(I_{\mathrm{MCD}}\right)$ spectra taken at the $L_{2,3}$ absorption edges of Fe (a) and Co (b) after subtracting a constant background.

Figure 9

Measurement of the specific magnetization as a function of temperature for ${\mathrm{Co}}_2\mathrm{FeSi}$. Lines are drawn to guide the eye.