A new tetragonal phase in CoFeCrGe Heusler alloy

CoFeCrGe ribbons were prepared by melt spinning and then annealed at 300 °C for 4 h and 500 °C for 4 h, respectively. The as-spun ribbons of CoFeCrGe have the B2 type structure with a = 0.287 nm. The CoFeCrGe ribbons annealed at 300 °C are composed mainly of the L2₁ type structure with a = 0.578 nm in partial disorder and the B2 type structure with a = 0.289 nm in further disorder case. The ribbons annealed at 500 °C are composed of three compounds with average compositions of Co_26.0Fe_26.5Cr_23.5Ge_24.0, Co_3.8Fe_6.4Cr_68.5Ge_21.3 and Co_53.4Fe_30.4Cr_8.6Ge_7.6, listed in a sequence of volume percentage from major to minor. The primary compound Co_26.0Fe_26.5Cr_23.5Ge_24.0 is the L2₁-type structure with a = 0.571 nm in partial disorder and the B2 type structure with a = 0.286 nm in further disorder case. The secondary compound Co_3.8Fe_6.4Cr_68.5Ge_21.3 has a cubic Cr₃Ge structure with a = 0.461 nm. The tertiary compound Co_53.4Fe_30.4Cr_8.6Ge_7.6 is a new tetragonal structure with lattice parameters a = 0.76 nm, c = 0.284 nm, 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.