1. 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: v1, con-mat.sup-con)

2. 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

3. 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

4. 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)
% aligned (intrinsic weak magnetic anisotropy or extrinsic?)

5. 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

6. -- 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

7. 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?

8. 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

9. 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?)

10. (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

11. (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

12. 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

13. 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 =
-- ill exponential or power law fitting in low temperature (s, p, d-wave?)

14. 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 =
-- magnetic fluctuation used for aligned at 300 K may
related with the pairing mechanism of superconductivity