Flip of spin helix chirality and ferromagnetic state in ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Ge}$ compounds

We have synthesized the solid solutions of ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Ge}$ compounds with $x$ running from 0.0 to 0.9. Small-angle neutron scattering and magnetization measurements have shown that these compounds are ordered into the spin helix structure below the critical temperature $T_c$. The helix is transformed into the ferromagnet by application of the magnetic field above the critical value $H_{c2}$. It is shown that $T_c$ decreases smoothly with concentration $x$ from 280 K for FeGe to 0 for CoGe. The values of the helix wave vector $k_s$ and the critical field $H_{c2}$ depend strongly on concentration $x$, firstly, decreasing from pure FeGe to its minimum ($|{\mathbf{k}}_s|\to 0$, $H_{c2}\to 0$) at $x_c\approx 0.6$, and, then increasing again at higher $x$. Thus, we observe a transformation of the helix structure to the ferromagnet at $x\to x_c$ at zero field. We concluded that this transformation is caused by different signs of the spin helicity for the compounds with $x>x_c$ and $x<x_c$. We believe that the mechanism of the transformation is caused by the competition between the Dzyaloshinskii-Moriya interaction and the cubic anisotropy.

Figure 1

(a) The temperature dependence of the magnetization $M$ for ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Ge}$ compounds with $x=0.0÷0.8$ at $H=10$ mT. (b) The first derivative of the magnetization on the temperature $dM/dT$.

Figure 2

The magnetic field dependence of the magnetization $M$ for $x=0.0÷0.8$ at $T=5$ K.

Figure 3

Dependence of the critical temperature $T_c$ and the ordered spin value $S$ on the concentration $x$ of ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Ge}$ compounds.

Figure 4

The small angle neutron scattering maps for the compounds with $x=0.1$ (a) and 0.5 (b) at $T\approx 10$ K.

Figure 5

Momentum transfer dependence of the SANS intensity (normalized) at $T\approx 10$ K for compounds with $x=0.0÷0.8$. The lines are the Gaussian fits.

Figure 6

Dependence of the helix wave vector $k_s$ on the concentration $x$.

Figure 7

The small angle neutron scattering maps for the compound with $x=0.0$ and (a) $H=0.01$ T, (b) $H=0.1$ T, (c) $H=0.25$ T, (d) $H=0.5$ T at $T\approx 10$ K.

Figure 8

Magnetic field dependence of the integral intensity of the peak at $T=10$ K for the compounds with $x=0.0,0.1,0.3,0.8$. The arrows correspond to the critical fields $H_{c1}$ and $H_{c2}$ for $x=0.1$.

Figure 9

Dependence of the critical field $H_{c1}$ and $H_{c2}$ on the concentration $x$.