Interplay between localized and itinerant magnetism in Co substituted FeGa${}_3$

Abstract

The evolution of the electronic structure and magnetic properties with Co substitution for Fe in the solid solution Fe${}_{1-x}$Co${}_x$Ga${}_3$ was studied by means of electrical resistivity, magnetization, ab-initio band structure calculations, and nuclear spin-lattice relaxation $1/T_1$ of the ${}^{69,71}$Ga nuclei. Temperature dependencies of the electrical resistivity reveal that the evolution from the semiconducting to the metallic state in the Fe${}_{1-x}$Co${}_x$Ga${}_3$ system occurs at $0.025<x<0.075$. The ${}^{69,71}(1/T_1)$ was studied as a function of temperature in a wide temperature range of $2-300$ K for the concentrations $x=0.0,$ $0.5,$ and $1.0$. In the parent semiconducting compound FeGa${}_3$, the temperature dependence of the ${}^{69}(1/T_1)$ exhibits a huge maximum at about $T\sim 6$ K indicating the existence of in-gap states. The opposite binary compound, CoGa${}_3$, demonstrates a metallic Korringa behavior with $1/T_1$ $\propto T$. In Fe${}_{0.5}$Co${}_{0.5}$Ga${}_3$, the relaxation is strongly enhanced due to spin fluctuations and follows $1/T_1\propto T^{1/2}$, which is a unique feature of weakly and nearly antiferromagnetic metals. This itinerant antiferromagnetic behavior contrasts with both magnetization measurements, showing localized magnetism with a relatively low effective moment of about 0.7 $\hat{I}{¼}_B$/f.u., and ab initio band structure calculations, where a ferromagnetic state with an ordered moment of 0.5 $\hat{I}{¼}_B$/f.u. is predicted. The results are discussed in terms of the interplay betwen the localized and itinerant magnetizm including in-gap states and spin fluctuations.

10 pages, 11 figures