Composition dependent properties of quaternary full Heusler alloys exhibiting ferromagnetism, antiferromagnetism and ferrimagnetism

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Structural, magnetic and spintronic properties of (1) Co2FeGe1-xSix and Co2FeGa1-xSix, (2) Ru2FeSi1-xGex and (Ru1-xCox)2FeSi, and (3) (Mn1-xCox)2VAl and (Mn1-xCox)2VGa full Heusler alloys prepared by arc melting method followed by heat treatment. Ab initio calculations have been performed on the Co and Ru alloys using generalized gradient approximation (GGA) and GGA+U methods, where U is the Hubbard potential. Saturation magnetization (Ms) of For Co2FeGe1-xSix alloys with x > 0 do not Slater-Pauling (S-P) rule. The effective anisotropy constant of the alloys (~105 Jm-3) showed an increase with increase in x. Ab initio calculations have been performed for the alloy compositions with L21 structure and DO3 disorder in order to account for experimentally measured magnetic properties. These studies show that Co2FeGe1-xSix alloys can show half-metallic character even with small amounts of DO3 disorder. Ab initio calculations using GGA alone could describe Co2Fe(Ga1-xSix) alloys with high Ga content well. However, with increase in x, on-site Coulomb interactions increase significantly, demanding the need for GGA+U formalism. The Rhodes-Wohlfarth (R-W) ratio estimated for all the alloy compositions from experimental is less than unity. Density of states at the Fermi level clearly shows a gap in the spin-down band of all the Co alloys. These results confirm the half-metallic character in these alloys. In order to understand the transition from antiferromagnetism to ferromagnetism, Ru2FeSi1-xGex and (Ru1-xCox)2FeSi alloys have been studied. Ru2FeSi alloy is antiferromagnetic with a Néel temperature of 270 K. As Ge and Co concentrations are increased, magnetization value starts increasing due to development of ferromagnetic interaction. Ab initio calculations on Ru2FeSi1-xGex alloys reveals that the antiferromagnetism of Ru2FeSi alloy is due to its B2 disordered structure where the nearest neighbour inter-sublattice Fe atoms show a strong antiferromagnetic interaction. However, ferromagnetism in the rest of the alloys is due to improved L21 ordering which leads to ferromagnetic nearest neighbour interaction between Fe atoms. R-W ratio and ab initio studies reveal that the Ru alloys exhibit a pseudo-gap in the spin-down band and hence are not half-metallic. Co substitution for Mn in Mn2VZ (Z = Al, Ga) ferrimagnetic alloys have been studied to demonstrate the possibility of obtaining fully compensated ferrimagnet (FCF) in full Heusler alloys. Both (Mn1-xCox)2VZ (Z = Al, Ga) alloy series with high L21 order have been synthesized. Co substitution for Mn in Mn2VAl(Ga) decreases Ms and leads to a FCF for equal amounts of Co and Mn i.e., in MnCoVAl(Ga) alloy. Point contact Andreev reflection measurements have been performed on (Mn1-xCox)2VAl alloys and the intrinsic spin polarization was found to increase from 0.57 for x = 0 to 0.60 for x = 0.50. Thus, this thesis work has identified several alloys for different spintronic applications by effectively using experimental data and ab initio calculations.
Supervisor: A. Srinivasan