Dependence of Epitaxial ${\rm Ba}{({\rm Fe}_{1-{\rm x}}{\rm Co}_{\rm x})}_{2}{\rm As}_{2}$ Thin Films Properties on ${\rm SrTiO}_{3}$ Template Thickness

In our previous report, we demonstrated the growth of high-quality epitaxial Co-doped Ba(Fe
_1-x
Co
_x
)
₂
As
₂
(Ba-122) thin films using single crystalline SrTiO
₃
(STO) templates on (La,Sr)(Al,Ta)O
₃
(LSAT) substrates with pulsed laser deposition (PLD). Here, we report the dependence of the structural and superconducting properties of Ba-122 thin films on STO templates that were 2-150 unit cell (u.c.) thick. We obtained genuine epitaxial thin films with high crystalline quality and excellent superconducting properties for templates above a critical thickness, which was 50 u.c. for our growth condition. Although the quality of Ba-122 is excellent above the critical thickness, the best crystalline quality and superconducting properties were obtained on 100 u.c.-thick STO templates. The best-quality Ba-122 thin films show an on-set transition temperature (T
_c
,
_on-set
) as high as 22.8 K, a transition width (ΔT
_c
) as narrow as 1.3 K, and critical current density (J
_c
) as high as 3 MA/cm
²
.