Evolution of bulk superconductivity in ${\text{SrFe}}_2{\text{As}}_2$ with Ni substitution

Single crystals of the Ni-doped FeAs-based superconductor ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$ were grown using a self-flux solution method and characterized via x-ray measurements and low-temperature transport, magnetization, and specific heat studies. A doping phase diagram has been established where the antiferromagnetic order associated with the magnetostructural transition of the parent compound ${\text{SrFe}}_2{\text{As}}_2$ is gradually suppressed with increasing Ni concentration, giving way to bulk-phase superconductivity with a maximum transition temperature of 9.8 K. The superconducting phase exists through a finite range of Ni concentrations centered at $x=0.15$ with full diamagnetic screening observed over a narrow range of $x$ coinciding with a sharpening of the superconducting transition and an absence of magnetic order. An enhancement of bulk superconducting transition temperatures of up to 20% was found to occur upon high-temperature annealing of samples.

Figure 1

(Color online) Digital image of a typical as-grown single crystal of ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$ harvested from flux growth. The arrow shows the large platelet dimension, indicative of crystals limited by crucible size.

Figure 2

(Color online) (a) Variation in the $a$- and $c$-axis lattice constants of ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$ with Ni content $x$, as determined from Rietfeld refinement of x-ray powder-diffraction spectra; (b) corresponding change in tetragonal $c/a$ ratio and unit-cell volume $V$; (c) actual Ni concentration of ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$ single-crystal samples as a function of nominal concentration $x$, as determined by wavelength-dispersive x-ray spectroscopy (data points represent average value of ten scanned points for each concentration, the dotted line is a linear fit with a slope of 1).

Figure 3

(Color online) (a) Temperature dependence of in-plane electrical resistivity of specimens of ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$, normalized to 300 K and offset for clarity (data sets placed above $x=0.30$ are successively offset vertically by 0.1, except for $x=0$ data, which are offset by 1.7). The direction of the broken arrow indicates the order of the resistivity curves with ascending $x$ as noted to the right. Short arrows indicate the position of the magnetic transition $T_0$, defined by the kink in $x=0$ data and the minima in $\rho \left(T\right)$ data for $0.08\le x\le 0.15$. (b) Expanded low-temperature view of resistivity normalized to 20 K for clarity showing the evolution of superconducting transitions with Ni concentration.

Figure 4

(Color online) Ni-substitution phase diagram of ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$ obtained from electrical resistivity data, showing the suppression of the magnetic/structural phase transition $T_0$ (blue squares) with increasing Ni concentration, and the appearance of a superconducting transition (red circles) with maximum $T_c$ of $\sim 10\text{K}$ centered around $x\simeq 0.15$.

Figure 5

(Color online) Temperature dependence of magnetic susceptibility $\chi$ of ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$, measured with 10 mT field applied parallel to the crystallographic basal plane from ZFC conditions, offset for clarity ($x=0$ data are vertically offset by $+0.0015{\text{cm}}^3/\text{mol}$ with other sets offset successively downward by $\sim 0.0002$). (b) Low-temperature zoom of the volume magnetic susceptibility in ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$ crystals under 1 mT ZFC and FC conditions after a $24\text{h}/700°\text{C}$ annealing treatment (see text for details).

Figure 6

(Color online) Temperature dependence of the electronic specific heat $C_e$ in ${\text{SrFe}}_{1.85}{\text{Ni}}_{0.15}{\text{As}}_2$ obtained by standard fitting and subtraction of a phonon contribution to the total specific heat (see text). Data show a small but distinct transition consistent with superconductivity below $T_c=8.5\text{K}$ determined from magnetic-susceptibility measurements.

Figure 7

(Color online) Evolution of normal and superconducting state parameters in ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$ with Ni concentration $x$: (a) absolute (circles—left scale) and normalized (triangles—right scale) residual resistivity; (b) variation in the width in temperature of the resistive superconducting transition $\Delta T_c$; (c) superconducting volume fraction determined from the magnetic susceptibility data; (d) expanded view of the superconducting phase determined by transition temperatures defined by 50% resistivity drop (circles) and 10% value of total diamagnetic screening (blue squares). All lines are guides.

Figure 8

(Color online) Effect of high-temperature annealing on an optimally doped $x=0.15$ sample of ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$, demonstrating typical results from before and after a 24 h, $700°\text{C}$ heat treatment performed on a sample sealed in a quartz tube with a pure argon environment. (a) Resistivity data of a $x=0.15$ sample measured before (blue circles) and after (red triangles) heat treatment. (b) Volume magnetic susceptibility of a $x=0.15$ sample at low fields measured before (blue circles) and after (red triangles) annealing. Arrows emphasize enhancement of $T_c$ by annealing, with good agreement in $T_c$ values for both cases.

Figure 9

(Color online) Comparison of the evolution of superconductivity as a function of Ni substitution in ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$ as compared to that previously observed in other transition-metal substitution series, with $\text{M}=\text{Co}$, Rh, and Pd (Refs. 6, 36, 26, respectively). Solid symbols denote $T_c$ values for $3d$-electron substituents Co (blue square) and Ni (red circle), and open symbols denote those of $4d$ substituents Rh (blue square) and Pd (red circle).