Abstract

1
Abstract
We present here a new high-sensitivity
magneto-optical imaging system. It consists of a
custom microscope built to optimize polarization
contrast, a Hi-res cryostat (Oxford) and a
high-sensitivity digital camera. Two samples have
been studied A NbSe2 bulk sample mounted with a
BiYiG indicator film clamped on top, and a Nb
thin film directly deposited on to the MO-film.
In the NbSe2 sample we observe both the static
Abrikosov lattice and single vortex jumps at low
fields. By using subtraction of subsequent frames
we can observe single vortex jumps in Nb thin
film. This is the first technique suitable for
real-time observation of single vortex motion in
bulk samples.
2
Typical MO setup(A. Polyanskii et al., ISSP
Chernogolovka)
YBCO thin film
3
Experimental considerations

• Modulation of magnetic field.
• Dependence on ? and B.
• Importance of
• - Proximity to surface.
• - High sensitivity MO films.
• Sample with short ?.
• Signal loss through optical
• system. Importance of optics
• optimized for polarization
• contrast.
• Vibrations/movements of sample.

1 ?m
?
4
Custom Microscope

• Special features
• No window between objective lens and sample.
• Glan-Taylor polarizers.
• Smith beamsplitter.
• Hi-res Cryostat (Oxford).

5
Practicals
5 mm
Sample mounted with vacuum grease and MO-film
clamped on top using tape.
The Microscope
6
Ferrite garnet films(with in-plane
magnetization)- sensitivity -
- dispersion -
at ? 633 nm
(measurements by L. E. Helseth)
7
Imaging the Static Abrikosov Lattice

• Sample Field Cooled NbSe2 .

H 0.5 Oe (earth field)
H 5 Oe
MO indicator placed on top of bulk sample. T
4.3 K
8
Static imaging
Rotating the analyzer in different directions
gives inversion of contrast creating increase in
SNR and removal of background
5 ?m
2.5
-2.5
Subtraction
Sample Fieldcooled NbSe2 T 4.3 K, H 0.5 Oe
9
Statics and dynamics
H 5 Oe

• Overlap of field from neighbouring vortices can
  make direct observation difficult.
• Differential imaging can still be used to reveal
  dynamic behaviour.

5 ?m
4.3 K
11 K
Subtracted
t 0 s
Sample Nb thin film
t 1 s
Subtracted
H ? 10 Oe, ?H ? -0.001 Oe/s
10
Imaging Dynamics
Differential detection
Niobium thin film directly deposited on MO
indicator. Field cooled in 20 Gauss, then slowly
decreasing field. Subtraction of subsequent
frames to obtain difference in B. Dark spot One
vortex has left Bright spot One vortex has
arrived. ?t 1 sec.
11
Dynamics I
2
3
1
2-1
3-1
Sample NbSe2 ?H ? 0.05 Oe/sec. ?t 1 sec.
5 ?m
12
Dynamics II
1
2
3
20 ?m
4
5
Sample Nb thin film T 3.7 K ?H ? 0.001
Oe/sec. ?t 1 sec.
13
Conclusions

• Real-time MO-imaging of SC with single vortex
  resolution.
• Both static picture and dynamic behaviour.
• Field range lt 20 Gauss (direct observation)
• lt ? Differential scheme.
• Time resolution Dependent on Camera system.
  Currently 10 frames/sec.
• Raw video of flux motion available (on laptop
  for those interested).