Scanning SQUID microscopy on polycrystalline SmFeAsO_{0.85} and NdFeAsO_{0.94}F_{0.06} Page: 2 of 4
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88 J. Phys. Soc. Jpn. 77 (2008) Supplement C
(a)
(e) 2 6
4--
0 2-
t 0 5 10 15
vortex FWHM (pm)
(f) a(b)
(c)
(d)
Fig. 1. (a) Image of the front end of the SQUID; the pick-up coil is the inner loop and the field coil the outer loop. (b-d) Scans of a the
same area of polycrystalline sample of SmFeAsO0.85 in different fields. Each image is on the same length and color scales. T = 4.4 K.
Applied field during cooling and scanning: (b) 33 mG, (c) 17.4 mG, and (d) 8.4 mG. (There appears to be a ~ -17 mG background
field.) (e) Histogram of the observed vortex FWHMs for the vortices in panel (d); two FWHMs are plotted per vortex. (f) A section
of panel (c) on an expanded color scale; the grayscale spans 50 m4D%. The area indicated is used to analyze the background.diameter pick-up coil (the inner coil), the leads to which
are shielded; a signal of 1 4o hc/2e 20.7 G-pm2 cor-
responds to a mean Bz in the pick-up coil of , 1.25 G.
The larger loop around the pick-up coil is a field coil;
a measure of the local susceptibility can be obtained by
applying a local field with this coil and measuring the
response in the pick-up coil.
Figure 1(b-d) show scans of SmFeAsO0.85 at 4.4 K,
cooled in different fields. Individual vortices are clearly
resolved. They can be cancelled by cooling in an applied
field, and most appear in different places after thermal
cycling in different fields. All the vortices in Fig. 1(d) in-
tegrate to within 20% of <D0, a level of error accounted for
by uncertainty in the background. These observations in-
dicate that these are 4m vortices, rather than frustration-
related spontaneous moments.
In addition to the vortices a few prominent magnetic
dipoles and a widespread irregular background are vis-
ible, shown in Fig. 1(f). By lifting the SQUID to just
above the sample, the sample temperature can be raised
while maintaining the SQUID below its T,. The dipoles
persist above T, while the irregular background disap-
pears at Tc. In the area indicated in Fig. 1(f) the root-
mean-square signal, after subtraction of a second-order
polynomial background, is 4.3 m4%. On eight different
thermal cycles, with the cooling field varied between 8.7
and 33 mG, this background was identical to the extent
visible between vortices. Therefore it is not due to spon-
taneous orbital currents, which result in moments polar-
izable by an external field.15, 19) The most likely source
of the background is an uncancelled in-plane field: we
must apply ~ 17 mG to cancel the local z-axis field, so
an in-plane field of similar magnitude can be expected.
This would result in in-plane vortices which would leak
out near the surface, and also in-plane field lines above
the sample that would deflect upward and downward onpassing over the inhomogeneous surface, resulting in a
mottled background signal.
Figure 1(e) shows the distribution of the observed full-
width half-maxima of the vortices in Fig. 1(d). The min-
imum observed FWHMs are , 6 pm, indicating the res-
olution limit. Far above an isotropic, homogeneous su-
perconductor the field of a vortex approaches that of a
monopole source placed one penetration depth, A, be-
neath the surface. In this model, observation of a FWHM
of 6 pm with a 4.6 pm pick-up coil indicates an effective
scan height, the actual scan height plus A, of ~ 3 pm.
Most of the vortices have irregular shapes, and many
of the observed FWHMs exceed 6 pm by a significant
margin, indicating that the vortices in the sample are
spread out on the range of microns. Many of the vortices
are elongated, resolution-limited along one axis and ~ 10
pm wide along the other, suggesting Josephson vortices
trapped in junctions between grains.
A susceptibility scan (Fig. 2(a)) shows clearly the gran-
ular nature of the sample. Over strongly superconducting
areas the field coil is partially shielded by the Meissner
screening of the sample, and the coupling between the
pick-up coil flux and the field coil current is reduced. For
this SQUID the coupling in vacuum is 0.83 4%/mA. Over
this sample the minimum observed coupling was 0.44
4%/mA, and the peak of the distribution 0.57 4%/mA.
The field coil can be approximated as a thin wire whose
diameter is the inner diameter of the actual coil (11.7
pm); a coupling of 0.44 0.57 4%/mA indicates an effec-
tive scan height of 3 4.5 pm (that is, 3 4.5 pm above a
theoretical A 0 plane).
An electron backscatter diffraction (EBSD) image, a
technique which reveals grain orientation, of a sample
from the same batch is shown in Fig. 2(b). In a circle
approximation the grains average 8 9 pm in diameter.
A d- or p-wave OP would give well-defined spontaneous0.2
0.1
0
-0.1
F/F0C. W. HICKs et al.
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Hicks, Clifford W.; Lippman, Thomas M.; Moler, Kathryn A.; Huber, Martin E.; Ren, Zhi-An & Zhao, Zhong-Xian. Scanning SQUID microscopy on polycrystalline SmFeAsO_{0.85} and NdFeAsO_{0.94}F_{0.06}, article, January 8, 2009; [Menlo Park, California]. (https://digital.library.unt.edu/ark:/67531/metadc899897/m1/2/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.