Anisotropic superconducting properties LT25 romp session (8/8/2008) Anisotropic superconducting properties of aligned Sm0.95La0.05FeAsO0.85F0.15 microcrystalline powder H. C. Ku, B. C. Chang, and C. H. Hsu National Tsing Hua University, Hsinchu, Taiwan Y. Y. Hsu National Taiwan Normal University, Taipei, Taiwan Z. Wei, K. Q. Ruan, and X. G. Li University of Science and Technology of China, Hefei, China (arXiv:0807.2833v1, con-mat.sup-con)

Question: What is the anisotropic superconducting properties of quasi-2D iron-based superconductor RFeAsO1-xFx? I. Good single crystal: for in-depth study (not yet) II. Magnetic alignment of micro-crystalline powder (~1-10 m powder) in magnetic field Ba at 300 K -- through spin-orbital related anisotropic exchange coupling in paramagnetic susceptibility cab, cc -- c-axis or ab-plane parallel to Ba -- if ab-plane aligned along Ba (spin-orbital related cab > cc), field-rotational method is needed to aligned c-axis along the rotation axis

Rotating Motor nonmagnetic s.s. holder w = 10 rpm c-axis  field Ba = 0.9 T c-axis quartz tube f = 8 mm cap ab-plane // field N S no rotation c-axis powder : epoxy = 1 : 5 after rotation c-axis aligned along the rotation axis Ba = 0.9 T Electromagnet T = 300 K rotation Block diagram for field-rotation powder alignment method with c-axis  aligned magnetic field and // rotation axis

Powder XRD patterns for Sm0.95La0.05FeAsO0.85F0.15 (a) random powder, (b) ab-plane // Ba (hk0) [h + k = 2n] (c) c-axis // rotation axis (00l) (perpendicular to Ba) -- 80-90% aligned (intrinsic weak magnetic anisotropy or extrinsic?)

Mechanism of powder alignment -- magnetic anisotropy due to spin-orbital related anisotropic exchange coupling: asymmetric orbital charge distribution is tied to the spin direction, different orbital overlap for different directions -- two major contributions of magnetic moment at 300 K c ~ c(Fe) + c(Sm) -- Sm3+ (4f55s25p6, S = 5/2, L = 5, J = L – S = 5/2) large localized moment in the (Sm,La)(O,F) insulating layer weak anisotropic exchange coupling

-- Fe2+ (3d6) itinerant moment in the FeAs metallic layer small moment from powder neutron diffraction data Fe2+ in distorted FeAs4 tetragonal crystal field with 3 low-lying manifold & 2 up-lying manifold (dxz, dyz) 3d6 in 3 hole-like bands & 2 electron-like bands with Fermi surface eF from five 3d orbitals strong anisotropic exchange coupling (magnetic fluctuation): I. strong Fe-As 3dxz,yz-4p hybridization II. Fe-Fe direct hopping in ab-plane

Anisotropic magnetization Mab(Ba) and Mc(Ba) at 300 K for aligned Sm0.95La0.05FeAsO0.85F0.15 -- weak magnetic anisotropy ration ab /c ~ 1.2, ab-plane // Ba -- crossover and non linear in low field, magnetic impurity or intrinsic?

Anisotropic low-field FC and ZFC susceptibility cab(T) and cc(T) for aligned Sm0.95La0.05FeAsO0.85F0.15 -- Tc = 52 K, pure intragrain shielding, diamagnetic anisotropic parameter g = c/ab = 2.4 ~ lc/lab (penetration depth) at 5 K/10 G -- T > Tc, paramagnetic anisotropy c/ab ~ 1.3 at 300 K and 10 G

Anisotropic high-field superconducting hysteresis loop Mab(Ba) and Mc(Ba) at 5 K for aligned Sm0.95La0.05FeAsO0.85F0.15 -- anisotropic upper critical field m0Hc2 > 7 T -- large paramagnetic background (magnetic/impurity?)

(a) Temperature variation of c-axis superconducting initial magnetization Mc(Ba) for aligned Sm0.95La0.05FeAsO0.85F0.15 -- at 5 K, Mc peak field m0Hc(peak) = 850 G, minimum lower critical field m0Hc1c ~ 200 G with linear cc ~ -2.7 cm3/mol

(b) Temperature variation of ab-plane superconducting initial magnetization Mab(Ba) for aligned Sm0.95La0.05FeAsO0.85F0.15 -- at 5 K, Mab peak field m0Hab(peak) = 1250 G, minimum lower critical field m0Hc1ab ~ 85 G with linear cab ~ -1.1 cm3/mol

Low field anisotropic phase diagram m0H(T) for aligned Sm0. 95La0 Low field anisotropic phase diagram m0H(T) for aligned Sm0.95La0.05FeAsO0.85F0.15 -- zero-temperature anisotropic lower critical field m0Hc1c(0) ~ 230 G, m0Hc1ab(0) ~ 95 G

Temperature dependence of anisotropic penetration depth lab(T) and lc(T) for aligned Sm0.95La0.05FeAsO0.85F0.15 -- anisotropy ratio g = lc(0)/lab(0) = 280 nm/120 nm = 2.4 + 0.6 -- ill exponential or power law fitting in low temperature (s, p, d-wave?)

Conclusions -- due to Fe spin-orbital related short-range anisotropic exchange coupling (AF or FM fluctuation), the tetragonal Sm0.95La0.05FeAsO0.85F0.15 microcrystalline powder can be aligned at 300 K using field-rotational method where ab-plane // aligned field Ba & c-axis // rotation axis -- transition temperature Tc = 52 K, anisotropic diamagnetic ratio c/ab = 2.4 at 5 K and 10 G -- zero-temperature anisotropic penetration depth lc(0) ~ 280 nm, lab(0) ~ 120 nm, g = 2.4 + 0.6 -- magnetic fluctuation used for aligned at 300 K may related with the pairing mechanism of superconductivity