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Magnetic imaging of unconventional superconductors

Local imaging of the magnetization can be a very useful tool for understanding the physics of superconductors at the micron scale. A magnetic field can penetrate into a superconductor in the form of vortices, which are microscopic tubes of magnetic flux each carrying one flux quantum Φ0 = h/2e. The vortex core is not superconducting, so vortices constitute defects in the superconductor whose behaviour gives information about the material’s superconducting mechanisms.

We have developed a unique instrument : a magnetic microscope working at very low temperature (0.1 K< T< 10 K) which can draw a map of magnetic field at the micron scale. The detector is a one micron diameter superconducting loop (a microSQUID) which we move over the sample surface to map the surface magnetic field.


SQUID micrograph
Fig. 1 : Magnetic probe (µSQUID) made in aluminium. (b) The silicon beam used for imaging, with the mSQUID (magenta colour) near its end.

We have demonstrated the power of this «microSQUID microscopy» technique in studies of magnetic flux structures at the surface of the superconductor Sr2Ru04 (Tc 1.5 K). The monocrystal was grown in our institute by a molten zone method in an imaging furnace.

Sr2Ru04 is a material with an “unconventional” superconducting state. We use a mSQUID to probe this material under a magnetic field. We expect to find domains where the Cooper pairs have antiparallel orbital moments (opposite chilarity); the presence of fractional vortices on the domain walls is predicted, and also preferential nucleation of vortices in one of the two kinds of domains.
We have indeed observed vortices in this material, for the first time. Individual vortices, each carrying a quantum of flux, appear in very weak field (H<10-4 Tesla). At higher fields, the vortices merge together to form magnetic domains. We deduce that there is a mechanism in this compound that alters the usual repulsion between vortices.
Vortex images in Sr<sub style=2RuO4' />
[fr]images par microscopie à µSQUID du flux magnétique de Sr2RuO4 à T= 0.38 K, H = 10 G, appliqué (field cooled) sous un angle par rapport à l’axe c de (a) 70° , (b) 60° , (c) 50°. Taille : 17µm x 31 µm. (d) montre le profil à travers un vortex et une ligne de vortex à l’endroit du trait en a).[en]Microsquid microscopy image (17 x 31 mm2) of magnetic flux distribution on Sr2Ru04 at T= 0.38 K after “field cooling” in H = 10 Gauss at angles (a) 70°, (b) 60° and (c) 50° to the c axis. Graph (d) shows the flux profile through both a vortex and a “vortex line”, along the black trace shown in (a).

We also observe the anisotropy of this lamellar superconductor, which coexists with the fusion of the vortices. The vortex domains are stretched out and lengthened when the magnetic field is tilted towards the crystal planes.

Our novel mSQUID-based instrument has thus enabled us to probe the nature of the superconductivity of Sr2Ru04 via the behaviour and interactions of vortices in the material. This technique could be applicable to many other magnetic materials.

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