Dynamic magneto-optical imaging of superconducting thin films

Abstract

We present a novel method for analysis of superconducting thin films using dynamic magneto-optical imaging, revealing hallmarks of flux penetration with temporal resolution around 1 ms (in the present work) or better. This method involves investigation of transient field and dynamic current distributions, which are calculated by an inversion procedure on the Biot-Savart Law, which we show to be valid under dynamic conditions. We compare and discuss the flux front penetration speed and evolution of current distribution in high quality YBa2Cu3O${}_{7-\delta }$ thin films with that of samples deliberately damaged in such a way as to reduce critical current density without causing macroscopic damage. We present a novel method for analysis of superconducting thin films using dynamic magnetooptical imaging, revealing hallmarks of flux penetration with temporal resolution around 1 ms (in the present work) or better. This method involves investigation of transient field and dynamic current distributions, which are calculated by an inversion procedure on the Biot-Savart Law, which we show to be valid under dynamic conditions. We compare and discuss the flux front penetration speed and evolution of current distribution in high quality YBa2Cu3O7−δ thin films with that of samples deliberately damaged in such a way as to reduce critical current density without causing macroscopic damage.

Observation of Transient Overcritical Currents in YBCO Thin Films using High-Speed Magneto-Optical Imaging and Dynamic Current Mapping

“…We have shown elsewhere that the Biot-Savart law is valid under dynamic conditions for our measurements, within a close approximation22.…”

“…3a of ref. 22) gives vortex velocities near the flux front in the range of ~10 −2  mm/s. Strikingly, this is a similar order of magnitude to the experimentally observed flux front velocity plotted in Fig.…”

“…A 125 nm thick Al reflective layer is deposited on top of the active layer, followed by a 120 nm capping layer which is placed face-down onto the sample leaving ~1  μ m gap. Similar indicator films have been shown to have a very fast linear response to changing magnetic fields, so the indicator does not limit measurement speed in this case2233. Eddy currents will be induced in the Al layer during ramping, but since this layer is thin and at a sufficiently large distance, these should not have any significant effect on the sample3435.…”

A new approach to the inverse problem for current mapping in thin-film superconductors

“…Magnetic imaging techniques are increasingly prevalent in many fields of science and medicine, as they can be used to determine the properties of materials and provide powerful medical diagnostic tools through non-contact, non-destructive measurements. Such techniques in physics and materials science research include but are not limited to Magnetic force microscopy; 1 SQUID (Superconducting Quantum Interference Device) microscopy, 2,3 as well as the more recent invention of SQUID-on-tip microscopy, whereby a nanoSQUID is fabricated on a quartz tip; 4,5 Magneto-optical imaging (MOI) for ferromagnetic and superconducting materials, 6,7 and its dynamic version for rapid imaging of transient current effects; 8,9 lHall-probe microscopy; 6,[11][12][13][14] bitter decoration; 15 scanning magnetoresistive microscopy; 16 scanning electron microscopy with polarization analysis (SEMPA); 17 and electron holography. 18 In medical and biological applications, such techniques include optical magnetic imaging of living cells, 19 magnetoencephalography (MEG) of brain activity, [20][21][22] magnetocardiography (MCG) for magnetic heart signals, 20,23 and Magnetic Resonance Imaging (MRI) for producing images of soft tissues, organs, body, and brain.…”

“…20 The similar inverse problem of the Biot-Savart law also allows the calculation of plane current components from magnetic field measurements of a sample. This is particularly useful for superconductors and thin films, [7][8][9][10]28 ferromagnetic and superconducting hybrids, 29,30 and even magnetic flux quanta (Abrikosov vortices). 3 In practice, methods of magnetic field detection via magnetometers often measure only the out-of-plane z-component of the magnetic field extending from a sample.…”