Microscopie à microsquid

La microscopie à microsquid est une technique d’imagerie magnétique à haute résolution magnétique (2 10^-7 Tesla) et spatiale, de l’ordre de 10^-6 m. Elle est employée à l’étude de supraconducteurs et de structures ferromagnétiques. Le microscope est placé au centre d’un refrigérateur à dilution. La gamme de température de fonctionnement est entre 10 K et 100 mK. Des extensions à plus hautes températures sont possibles.

The µSQUID Force Microscopy is a unique instrument to observe locally magnetic flux with very high sensitivity (2mG/Hz-0.5 in an area of 1 micrometer).

The high sensitivity allows us to reveal the local state of magnetization in artificial nanostructures, superconductors, ferromagnets and electronic circuits. With the acquired images we can elucidate the mechanisms responsible for the magnetic pattern observed. This will lead eventually to superconductors with higher critical currents, with less vortex motion, to magnetic circuits with well controlled domain structures, optimized detectors ... .

The µSQUID (Superconducting quantum interference device) is a microfluxmeter, based on the quantification of magnetic flux in a superconducting ring and the Josephson effect. The critical current of such a two junction device is a h/2e periodic function of the applied flux, h quantum of Planck, e charge of one electron. The use of 20 nm wide nano-constrictions (Dayem Bridges) as Josephson Junction elements allows us to obtain this very small sensor. The detection head of the µSQUID Force Microscopy consists of a 1 micrometer diameter SQUID loop positioned at the edge of the silicon chip. The SQUID loop is fabricated by high resolution electron beam lithography. It is important to scan the surface at close proximity (SQUID height < 1 micrometer). A commercially fabricated piezoelectric tuning fork resonators carries the SQUID chip.

When the tip of the SQUID chip comes into proximity of the sample surface, the resonance frequency and the amplitude change. These signals are used to control height and to obtain topographic information. The combination of commercially available elements and the nanofabricated circuits allowed a rapid development. The scan range is 60 micrometers times 60 micrometers. A range very important for scanning probe microscopy. The height control via the tuning fork is essential in order to maintain the scanning height above the surface.

The complete control of the instrument is possible by the support and the input of the technical staff at Néel enabling us to pursue further improvements. We have ongoing projects for the increase of the sensibility of the SQUID by a factor of hundred allowing us to scan faster, the microfabrication of µSQUID-tips using Silicon machining leading to 5 times higher spatial resolution.

FIG 1 : The critical current of a DC SQUID is a h/2e periodic function of the applied flux. The period is about 16 gauss in 1,2 µm diameter µSQUID.
FIG 2 : DC-µSQUID of 1µm inner diameter, 200 nm wide ring and 20 nm wide Dayem Bridges, patterned at LPN by e-beam lithography (D. Mailly).
FIG 3 : The piezoelectric tuning fork carries the Si chip, at whose apex the 1 µm Squid is located. C. Veauvy et al., RSI 73, 3825(2002)
FIG 4 : A micromachined NanoSQUID allows for precise alignement and scanning at close proximity < 1µm to the sample. ( Neel, IRAM collaboration)
FIG 5 : NanoSQUID Force microscope image of a superconducting Nb chessboard pattern at 0.2 K. Image size is 70µm x 85µm. Vortices and field screening by Nb squares are seen, imaged by Danny Hykel.
FIG 6 : NanoSQUID Force microscope built by Zhao Sheng Wang, working at 0.2 kelvin

We are planning to increase the scan range to 2 mm times 2 mm. This will allow us to reveal long range correlations in the magnetic pattern and the µSQUID Force microscope images are to be correlated with other chemically sensitive probes, allowing us to discover new materials, magnetically characterized on the micrometer scale. The use of nanofabricated Hall sensors for imaging at variable temperatures is in preparation. In the past the instrument has been built and used during the thesis of C. Veauvy and V. O. Dolocan. The present instrument and its software (a DSP based PLL and PI regulation) has been developped by Danny Hykel, in collab. with J. Minet from the electronics lab of Néel. Danny defended his PhD thesis on magnetic imaging of the ferromagentic superconductor UCoGe in february 2011.

Zhao Sheng Wang joined our group in 2009 in the framework of a IOP CNRS/UJF Collaboration to work on the magnetic properties of iron arsenide superconductors. In 2011 as a Post Doc Hazra Dibyendu joined us from Indian Institute of Technology Kampur to work on high resolution Nb nanoSQUIDs destined for microscopy.

The Research is supported by the French National Research Agency : SINUS and TetraFer program (-> 2012). The Nanoscience Foundation has alloted a chair of excellency to J. R. Kirtley (-> 2012).

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