Ferromagnetic instability in a doped band gap semiconductor FeGa${}_3$

We report the effects of electron doping on the ground state of a diamagnetic semiconductor FeGa${}_3$ with a band gap of $0.5$ eV. By means of electrical resistivity, magnetization, and specific heat measurements we have found that gradual substitution of Ge for Ga in FeGa${}_{3-y}$Ge${}_y$ yields metallic conduction at a very small level of $y=0.006$, then induces weak ferromagnetic (FM) order at $y=0.13$ with a spontaneous moment of 0.1 ${\mu }_B$/Fe and a Curie temperature $T_C=3.3$ K, which continues increasing to $T_C=75$ K as doping reaches $y=0.41$. The emergence of the FM state is accompanied by quantum critical behavior as observed in the specific heat, $C/T\propto -$ln $T$, and in the magnetic susceptibility, $M/B\propto T^{-4/3}$. At $y=0.09$, the specific heat divided by temperature $C/T$ reaches a large value of 70 mJ K${}^{-2}$ (mol Fe)${}^{-1}$, twice as large as that reported for FeSi${}_{1-x}$Ge${}_x$ with $x_c=0.37$ and Fe${}_{1-x}$Co${}_x$Sb${}_2$ with $x_c=0.3$ at their respective FM quantum critical points. The critical concentration $y_c=0.13$ in FeGa${}_{3-y}$Ge${}_y$ is quite small, despite the fact that its band gap is one order of magnitude larger than those in FeSi and FeSb${}_2$. In contrast, no FM state emerges by substituting Co for Fe in Fe${}_{1-x}$Co${}_x$Ga${}_3$ in the whole range $0\le x\le 1$, although both types of substitution should dope electrons into FeGa${}_3$. The FM instability found in FeGa${}_{3-y}$Ge${}_y$ indicates that strong electron correlations are induced by the disturbance of the Fe-$3d$–Ga-$4p$ hybridization.

Figure 1

Lattice parameters and unit cell volume of Fe${}_{1-x}$Co${}_x$Ga${}_3$ (a) and FeGa${}_{3-y}$Ge${}_y$ (b) as a function of concentrations $x$ and $y$.

Figure 2

Temperature dependence of electrical resistivity $\rho$ for Fe${}_{1-x}$Co${}_x$Ga${}_3$ (a) and FeGa${}_{3-y}$Ge${}_y$ (b). The resistivity of Fe${}_{1-x}$Co${}_x$Ga${}_3$ is normalized by the value at 380 K. The inset shows the resistivity for FeGa${}_3$ ($x=0$).[28]

Figure 3

Temperature dependence of magnetic susceptibility $M/B$ (b) and inverse magnetic susceptibility $B/M$ (a) of Fe${}_{1-x}$Co${}_x$Ga${}_3$

Figure 4

Isothermal magnetization curves of FeGa${}_{3-y}$Ge${}_y$ at 2 K.

Figure 5

Temperature dependence of magnetic susceptibility $M/B$ of FeGa${}_{3-y}$Ge${}_y$ for $y\ge 0.13$ where ferromagnetic transitions are observed. The inset shows the specific heat of FeGa${}_{3-y}$Ge${}_y$ for $y=0.41$ near $T_C$.

Figure 6

Temperature dependence of the inverse magnetic susceptibility $B/M$ of FeGa${}_{3-y}$Ge${}_y$. The upper and lower panels of the inset show the paramagnetic Curie temperature ${\theta }_P$ and ferromagnetic transition temperature $T_C$, and effective magnetic moments ${\mu }_{\text{eff}}$ and the Rhodes-Wohlfarth value ${\mu }_{\text{eff}}$/${\mu }_s$, respectively, as a function of $y$.

Figure 7

Temperature dependence of magnetic susceptibility $M/B$ of FeGa${}_{3-y}$Ge${}_y$ for $y=0.34$ under various pressures $P$. The inset shows $T_C$ as a function of $P^{3/4}$.

Figure 8

The specific heat divided by temperature $C/T$ for Fe${}_{1-x}$Co${}_x$Ga${}_3$ (a) and FeGa${}_{3-y}$Ge${}_y$ (b) as a function of $T^2$.

Figure 9

Ferromagnetic transition temperature $T_C$ (a) and electronic specific heat coefficient $\gamma$ (b) for Fe${}_{1-x}$Co${}_x$Ga${}_3$ and FeGa${}_{3-y}$Ge${}_y$ as a function of $x$ and $y$.

Figure 10

Logarithmic temperature dependence of $C/T$ (a) and $M/B$ (b) for FeGa${}_{3-y}$Ge${}_y$ near the FM instability.

Figure 11

Temperature dependence of electrical resistivity $\rho$ of $y=0.08$, 0.15, and 0.41 for FeGa${}_{3-y}$Ge${}_y$. The $\rho \left(T\right)$ data for $y=0.08$ and 0.15 were fit by $\rho -{\rho }_0\propto T^{1.9}$ (broken line) and $\rho -{\rho }_0\propto T$ (solid line), respectively.