Upper critical fields well above 100 T for the superconductor ${\text{SmFeAsO}}_{0.85}{\text{F}}_{0.15}$ with $T_c=46\text{K}$

We report specific-heat measurements at magnetic fields up to 20 T on the recently discovered superconductor ${\text{SmFeAsO}}_{0.85}{\text{F}}_{0.15}$. The $B\text{−}T$ diagram of a polycrystalline ${\text{SmFeAsO}}_{0.85}{\text{F}}_{0.15}$ sample with $T_c=46\text{K}$ was investigated. The temperature dependence of $B_{c2}$ was extracted from the specific-heat curves; the corresponding $B_{c2}\left(T=0\right)$ value derived from the Werthamer-Helfand-Hohenberg formula being 150 T. Based on magnetization measurements up to 9 T, a first estimation of the field dependence of the inductive critical current $J_c$ is given. Evidence for granularity is found. The presence of a peak effect is reported, suggesting a crossover in the vortex dynamics, in analogy to the behavior observed in high $T_c$ cuprates.

Figure 1

(a) X-ray diffraction pattern for the sample with nominal composition ${\text{SmFeAsO}}_{0.85}{\text{F}}_{0.15}$; SmOF impurity phase is indicated with an asterisk. (b) Scanning electron microscope images for the ${\text{SmFeAsO}}_{0.85}{\text{F}}_{0.15}$ polycrystalline sample showing conglomerate particle sizes varying between 5 and $30\mu \text{m}$.

Figure 2

$C/T$ versus $T$ at zero field (black squares) and 20 T (gray circles). Inset (a): superconducting transition from $AC$ susceptibility. Inset (b): low-temperature specific-heat anomaly, related to the magnetic ordering of the ${\text{Sm}}^{3+}$ ions (Ref. 17).

Figure 3

Superconducting contribution to the specific heat for $B=0$, 6, 10, 14, and 20 T. The temperatures corresponding to $T_{\text{mid}}$, the midpoint of the calorimetric transition and to $T_{\text{max}}$, are the maximum of $C/T$ at the superconducting jump are indicated for the curve at zero field.

Figure 4

Distribution of $T_c$ at zero field and at $B=10\text{T}$, obtained by the deconvolution of the calorimetric data.

Figure 5

Upper critical field $B_{c2}$ versus $T/T_c$ for the ${\text{SmFeAsO}}_{0.85}{\text{F}}_{0.15}$ sample as determined from the midpoint $T_{\text{mid}}$ (solid diamonds) and the maximum $T_{\text{max}}$ (open diamonds) of the superconducting transition in the specific-heat curves. The $B_{c2}$ values for ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ in Refs. 24, 25 are also reported for comparison.

Figure 6

Magnetic moment as a function of the applied magnetic field $B$ at $T=5\text{K}$ (solid squares) and 55 K (open circles). The $M\left(B\right)$ curve at 5 K is the superposition of a superconducting contribution and a ferromagnetic background.

Figure 7

Superconducting contribution to the magnetic moment as a function of $B$ at (a) $T=5\text{K}$ and (b) 25 K. The superconducting signal was isolated from the ferromagnetic background determined above $T_c$.

Figure 8

Real part of the $AC$ susceptibility ${\chi }^{\prime }$ versus $T$: the double transition in the curve is determined by the crossover from intragrain to intergrain superconductivity, as a consequence of the granularity of the sample.

Figure 9

(a) FE-SEM image showing grain sizes varying between less than 0.1 and $2–3\mu \text{m}$. (b) Magnification of a submicrometric grain.

Figure 10

Inductive critical current density $J_c$ versus $B$ from 5 to 40 K. $J_c$ has been determined for $⟨R⟩=10\mu \text{m}$ (left axis) and $⟨R⟩=0.1\mu \text{m}$ (right axis), the current carrying length scale within the sample being not measured yet.

Figure 11

$B\text{−}T$ phase diagram for ${\text{SmFeAsO}}_{0.85}{\text{F}}_{0.15}$. Data points in the $B_{c2}\left(T\right)$ curve were determined from specific-heat measurements. The irreversibility line and the $B_{\text{peak}}\left(T\right)$ line were extracted from magnetization measurements. The solid line is a fit $B_{\text{irr}}\propto {\left[1-\left(T/T_c\right)\right]}^2$.