Abstract
We have synthesized the solid solutions of Fe 1−x Co x 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 (|k s |→0 , H c2 →0 ) at x c ≈0.6 , and, then increasing again at higher x . Thus, we observe a transformation of the helix structure to the ferromagnet at x→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
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