Interplay between localized and itinerant magnetism in Co-substituted FeGa3

Abstract

The evolution of the electronic structure and magnetic properties with Co substitution for Fe in the solid solution Fe1−xCoxGa3 was studied by means of electrical resistivity, magnetization, ab initio band structure calculations, and nuclear spin-lattice relaxation 1/T1 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 Fe1−xCoxGa3 system occurs at 0.025 < x < 0.075. The 69,71 (1/T1) 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 FeGa3, the temperature dependence of the 69 (1/T1) exhibits a huge maximum at about T ∼ 6 K indicating the existence of in-gap states. The opposite binary compound, CoGa3, demonstrates a metallic Korringa behavior with 1/T1 ∝ T . In Fe0.5Co0.5Ga3, the relaxation is strongly enhanced due to spin fluctuations and follows 1/T1 ∝ 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 µB/f.u., and ab initio band structure calculations, where a ferromagnetic state with an ordered moment of 0.5 µB/f.u. is predicted. The results are discussed in terms of the interplay between the localized and itinerant magnetism including in-gap states and spin fluctuations.

Magnetic order of intermetallicFeGa3−yGeystudied byμSRand<

“…Large Co doping induces substantial disorder as reflected by the line broadening of the 69,71 Ga Nuclear Quadrupole Resonance (NQR) spectra and by the deviation of the lattice parameters from Vegard's law 17 . Fe 1−x Co x Ga 3 remains paramagnetic for all Co concentrations investigated 11 , while showing a complex magnetic behavior including itinerant and localized moment character and strong antiferromagnetic (AFM) spin fluctuations for Co substitution close to 0.5 18 .…”

“…A recent example is the electron doping of the intermetallic FeGa 3 that leads to enhanced thermoelectric figures of merit [2][3][4][5][6][7][8][9][10] and to emergent magnetic behavior accompanied by the possible observation of a Ferromagnetic Quantum Critical Point (FMQCP) [11][12][13][14][15][16][17][18][19][20][21] .…”

“…A unit cell contains 4 formula units where each Fe has eight Ga neighbors at two distinct sites Ga1 (0.236 nm, 2 atoms) and Ga2 (0.239 nm, 2 atoms and 0.246 nm, 4 atoms, above the plane containing Fe) 2 . Whereas hole doping by Zn at the Ga site or Mn at the Fe site 14 does not induce an insulating-metal transition and introduces ingap states 16 , electron doping either at the Fe or the Ga site destroys the semiconducting behavior, and remarkably influences other physical properties [11][12][13][14][15][16][17][18][19][20][21][24][25][26] .…”

Electronic correlations inFeGa3and the effect of hole doping on its magnetic properties

“…Indeed, a recent nuclear spin-lattice relaxation study for FeGa 3 unambiguously proved the existence of in-gap states located just below the bottom of the conduction band [46]. The number of these states estimated from the Hall constant [32] is ∼1.6×10 16 cm −3 , which is much larger than typical impurity levels observed in intrinsic band semiconductors (e.g., Si, Ge) and even in correlated band insulators (e.g., FeSi, FeSb 2 ) [46]. Measurements of the Hall effect performed by Hadano et al [32] indicated that in region III mobility of carriers decreases strongly with increasing temperature (μ ∼ T −5/2 ), whereas the number of carriers remains nearly unchanged.…”

“…For FeGa 3 , electron-type doping induces a crossover to a correlated metallic state at x ≈ 0.05 and y ≈ 0.006 in Fe 1−x Co x Ga 3 [34] and FeGa 3−y Ge y [35], respectively. Interestingly, further doping with Ge in FeGa 3−y Ge y leads to a ferromagnetic quantum critical point at x ≈ 0.016-0.05 [35,37], whereas for Fe 0.5 Co 0.5 Ga 3 nuclear spin-lattice relaxation measurements revealed a very fast relaxation with temperature dependence 1/T 1 ∝ T 1/2 being a unique feature of weakly and nearly antiferromagnetic metals [46]. A recent computational study suggested the formation of an itinerant ferromagnetic state with half-metallic properties in FeGa 3 in case of doping with both electrons and holes [43].…”