Weak ferromagnetism of $\mathrm{Cu}{}_x\mathrm{Fe}{}_{1+y}\text{As}$ and its evolution with Co doping

We have investigated electronic and magnetic properties of iron pnictide $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$, which is isostructural to iron pnictide superconductors $\mathrm{LiFeAs}$ and $\mathrm{NaFeAs}$, using polycrystalline and single-crystal samples. We found that $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ is characterized by strong electron correlations and shows weak ferromagnetic behavior with the Curie temperature $T_{\mathrm{c}}\sim 42\mathrm{K}$, in contrast with the superconductivity in $\mathrm{LiFeAs}$ and $\mathrm{NaFeAs}$. Its electronic transport properties exhibit quasi-two-dimensional characteristics: While the in-plane resistivity follows quadratic temperature dependence for $T<10\mathrm{K}$, the $c$-axis resistivity shows nonmetal-to-metal crossover near 20 K. In addition, we have studied the Co-doping effect on $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ and found that Co doping enhances the ferromagnetism with $T_{\mathrm{c}}$ increasing up to 83 K for $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{As}$. From these results, we discussed the origin of the ferromagnetism of $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ and its relevance with the magnetism of iron pnictide superconductor parent compounds.

Figure 1

(a) Powder XRD of $\mathrm{C}{\mathrm{u}}_{1+x}\mathrm{F}{\mathrm{e}}_y\mathrm{As}$. (b) Crystal structure of $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$; α refers to the As-Fe-As bonding angle formed by two neighboring As ions lying above the Fe plane, while β denotes the As-Fe-As angle formed by one As ion lying above and one As below the Fe plane.

Figure 2

$\left(00l\right)$ XRD pattern of a $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ single crystal. Inset: an image of a typical $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ single crystal.

Figure 3

(a) Temperature dependence of magnetization for $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ polycrystalline and single-crystal samples, measured under a magnetic field of $1000\mathrm{Oe}$ and zero-field-cooling (ZFC) and field-cooling histories (FC). Inset: temperature dependence of magnetization for $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ polycrystalline measured under a field of $100\mathrm{Oe}$. (b) Isothermal magnetization at 2 K for $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ polycrystalline and single-crystal samples; the field was applied to the $ab$ plane and $c$ axis, respectively, for the single-crystal sample. The inset in (b) shows the magnetization as a function of field in a small field range.

Figure 4

Temperature dependences of in-plane and out-of-plane resistivity for $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ single crystals. Inset shows the in-plane resistivity on the scale of $T^2$.

Figure 5

Specific heat divided by temperature $C/T$ as a function of temperature for a $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ single crystal. Inset, $C/T$ vs $T^2$.

Figure 6

Powder XRD patterns of $\mathrm{Cu}(\mathrm{F}{\mathrm{e}}_{1–z}\mathrm{C}{\mathrm{o}}_z)\mathrm{As}$ polycrystalline samples.

Figure 7

Lattice parameters as a function of Co concentration at room temperature for $\mathrm{Cu}(\mathrm{F}{\mathrm{e}}_{1–z}\mathrm{C}{\mathrm{o}}_z)\mathrm{As}$.

Figure 8

(a) Temperature dependence of magnetization measured under a field of $100\mathrm{Oe}$ with ZFC and FC histories for $\mathrm{Cu}(\mathrm{F}{\mathrm{e}}_{1–z}\mathrm{C}{\mathrm{o}}_z)\mathrm{As}$ polycrystalline samples. (b) Isothermal magnetization at 2 K for $\mathrm{Cu}(\mathrm{F}{\mathrm{e}}_{1–z}\mathrm{C}{\mathrm{o}}_z)\mathrm{As}$ polycrystalline samples.

Figure 9

Temperature dependence of normalized resistivity for $\mathrm{Cu}(\mathrm{F}{\mathrm{e}}_{1–z}\mathrm{C}{\mathrm{o}}_z)\mathrm{As}$ polycrystalline samples.