Transition Temperature and Upper Critical Field in $ \hbox{SmFeAsO}_{1 - x}\hbox{F}_{x}$ Synthesized at Low Heating Temperatures

Low-temperature synthesis is a promising and potentially effective method for improving superconducting properties. We report on the fabrication of polycrystalline samples of SmFeAsO
_1-x
F
_x
with nominal
x
content varying in a wide range of
x
= 0-0.35 synthesized at 900
^°
C. This synthesis temperature is around 300
^°
C lower than the conventional synthesis temperature. The variation in the lattice parameters and transition temperature (
T
_c
) of various F-doped samples indicates that reduction of the unit cell volume (
V
) seems to be the main reason for the rise of
T
_c
up to 57.8 K. Magnetoresistance measurements showed that the upper critical field slope (
dH
_c2
/
dT
) increased with increasing F concentration up to
x
= 0.2, where it reached a maximum value of -8 T/K corresponding to a coherence length (ξ
_GL
) of 10 Å. At still higher F doping levels,
dH
_c2
/
dT
and the low field
J
_c
decreased; above 0.5 T, however,
J
_c
had almost the same value. Compared with previous reports, the present synthesis route with low synthesis temperatures and commonly available FeF
₂
as the source of F is more effective at introducing F into the SmFeAsO system and thereby resulting in improved superconducting properties for the system. In addition, this new sample preparation method also reduces unnecessary problems such as the evaporation of F and reaction between the crucible and superconductor during the solid-state reaction.