Co 2 Fe 1.25 Ge 0.75 : A single-phase full Heusler alloy with highest magnetic moment and Curie temperature

Elemental substitution by a different element is a well utilized technique of stabilizing a single-phase compound, in case the parent alloy is multi-phase, and this has been demonstrated specifically in a number of Heusler systems. This can, however, give an increased propensity for chemical disorder as well as adding complexity to synthesis and characterization. In this paper, we present the successful synthesis of a single-phase compound, Co 2 Fe 1.25 Ge 0.75 , by tuning the parent Co 2 FeGe stoichiometry (which exhibits multi-phase structure) rather than introducing a fourth element. The compound is found to crystallize in L2 1 structure (space group # 225). Magnetization measurements reveal Co 2 Fe 1.25 Ge 0.75 has a saturation magnetization as high as 6.7 ± 0.1 μ B /f.u. at 5 K and a Curie temperature of 1135 ± 5 K – both being the highest reported to date for cubic full Heusler alloys to our knowledge. Thin films of Co 2 Fe 1.25 Ge 0.75 deposited on Al 2 O 3 (110) and MgAl 2 O 4 (100) substrates show excellent expitaxial quality, among the reported for Heusler films to date, and exhibit magnetic properties comparable to bulk samples. First principle calculations suggest the system exhibits total energy minimum at the experimentally-observed lattice parameter. Furthermore, the calculations corroborate the observed enhancement in magnetization and point to the importance of on-site Coulomb interactions. While our novel approach of substitution led to the discovery of stable Co 2 Fe 1.25 Ge 0.75 alloy with very high moment and Curie temperature that can be readily grown as a high-quality epitaxial thin film, making it a candidate for device applications, this approach can be taken as a new paradigm for the discovery of novel single-phase Heusler compounds with enhanced magnetic properties.