A new tetragonal phase in CoFeCrGe Heusler alloy

Abstract

CoFeCrGe ribbons were prepared by melt spinning and then annealed at 300°C for 4h and 500°C for 4h, respectively. The as-spun ribbons of CoFeCrGe have the B2 type structure with a=0.287nm. The CoFeCrGe ribbons annealed at 300°C are composed mainly of the L21 type structure with a=0.578nm in partial disorder and the B2 type structure with a=0.289nm in further disorder case. The ribbons annealed at 500°C are composed of three compounds with average compositions of Co26.0Fe26.5Cr23.5Ge24.0, Co3.8Fe6.4Cr68.5Ge21.3 and Co53.4Fe30.4Cr8.6Ge7.6, listed in a sequence of volume percentage from major to minor. The primary compound Co26.0Fe26.5Cr23.5Ge24.0 is the L21-type structure with a=0.571nm in partial disorder and the B2 type structure with a=0.286nm in further disorder case. The secondary compound Co3.8Fe6.4Cr68.5Ge21.3 has a cubic Cr3Ge structure with a=0.461nm. The tertiary compound Co53.4Fe30.4Cr8.6Ge7.6 is a new tetragonal structure with lattice parameters a=0.76nm, c=0.284nm, which were determined by using tilt-series electron diffraction technique in this work. The tertiary (Co, Fe)-rich phase is usually embedded in the matrix of the secondary Cr-rich phase, but there is no special orientation between the two phases. The difference in the magnetic hysteresis loops of the samples annealed at 300°C and 500°C has been interpreted as the appearance of the new tetragonal crystalline phase. Element doping is the possible way for further developing the single tetragonal phase or increasing the amount of this phase.