Many Heusler compounds are predicted to be half-metallic ferromagnets and find extensive interest as materials for spintronic applications, an example of which is the Co₂Cr_1-xFe_xAl  series. So far, bulk samples of that series, in particular Cr-rich samples, do not verify those predictions and various studies yield results which are fraught with anomalies. Thin films as well do not meet the expectations in magnetoresistance ratio nor in degree of spin polarization and magnetic moments. In the present work, crystal growth of Co₂Cr_1-xFe_xAl  Heusler compounds has been done, as a step towards understanding the intrinsic material properties of this series. To begin with, polycrystalline samples of Co₂CrAl, Co₂FeAl and Co₂Cr_0.6Fe_0.4Al were prepared and found to be phase segregated, a hurdle already reported and easily tackled by using the Optical Floating Zone technique (FZ). Our results demonstrate that, although this phase segregation is eliminated in the FZ grown samples, an unexpected phase transformation via spinodal decomposition is taking place in samples containing Cr. This phase decomposition is found to strongly affect the magnetic properties and hence also other properties as e.g. the spin polarization. Our finding presents new insight into the phase dynamics in this system and provides answers to many of the issues in this series of half-metallic ferromagnets.

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