Experimental and ab initio studies on sub-lattice ordering and magnetism in Co₂Fe(Ge_1−xSi_x) alloys

Crystallographic and magnetic properties of bulk Co₂Fe(Ge_1−xSi_x) alloys with 0 ≤ x ≤ 1, synthesized by arc melting method, have been studied. Co₂FeSi alloy has been found to crystallize with L2₁ structure, but the super-lattice peaks are absent in the X-ray diffraction patterns of alloys containing high Ge concentration. Unit cell volume of this series of alloys decreased from 185.2 to 178.5 ų as Si content was increased from 0 to 1.00. All alloy compositions exhibit ferromagnetic behavior with a high Curie temperature (T_C). T_C showed a systematic variation with x. A comparison between the values of saturation magnetization (M_s) and effective moment per magnetic atom p_c estimated from the temperature dependent susceptibility data above T_C, shows that the alloys have half-metallic character. The alloy with x = 0 follows Slater-Pauling (S-P) rule with M_s of 5.99μ_B. However, M_s for the alloy with x = 1.00 was found to be 5.42μ_B, which is lower than the value of 6.0μ_B predicted by S-P rule. Since atomic disorder is known to affect the M_s and electronic structure of these alloys, ab initio calculations were carried out to explain the deviation in observed M_s from S-P rule prediction and the half-metallic character of the alloys. Ab initio calculations reveal that alloys with L2₁ structure have M_s value as predicted by S-P rule. However, introduction of 12.5% DO₃ disorder, which occurs due to swapping of Co and Fe atoms in the unit cell, decreases M_s of alloys with x > 0 from the S-P prediction to values obtained experimentally. The results analyzed from the view point of electronic structure of the alloys in different ordered states bring out the influence of disorder on the observed magnetic properties of these technologically important alloys