Resumé / Abstract :

Magnetometry using micron-size superconducting quantum interference devices (µ-SQUIDs) has been remarkably successful in probing classical as well as quantum regimes of magnetism in single nano-particles. This technique can be further improved for higher speed and sensitivity with hysteresis-free µ-SQUIDs. This is difficult, particularly, at low temperatures, which is essential for probing quantum-magnetism. The hysteresis in these devices arises from thermal instabilities in superconducting weak-links and neighboring region. The heat generated in resistive normal region gives rise to a self sustained hot-spot. This leads to two possible states, hot (normal) and cold (superconducting), and hence bistability. Such hot-spot and hysteresis has been modeled in the past by using steady state thermal heat balance equations.

However, as we approach the hysteresis-free regime by optimizing the relative heat evacuation, another regime of hysteresis is found in which the bistability results due to a phase dynamic steady state. We understand this dynamic regime using a thermal model that helps us quantitatively capture the behavior in both hysteretic and non-hysteretic regimes. Slow relaxation of quasi-particles, which are generated due to phase dynamics, is found to be a bottleneck, which is the case for several superconducting devices including SIN-coolers and superconducting qubits.

We solve the thermal model for different shunting conditions to find that an optimal shunt having resistance and inductance both can eliminate hysteresis at low temperatures and with a good sensitivity. A pure resistive shunt, which works well for hysteresis elimination in usual tunnel junction based SQUIDs, leads to a marked reduction in sensitivity of µ-SQUIDs. This new model also reveals an interesting non-linear dynamical system with various regimes. We successfully test this idea of inductive shunt eliminating hysteresis with good sensitivity. Finally, we present preliminary results on magnetization reversal in permalloy nano-needles by using these optimized non-hysteretic µ-SQUIDs.

Contact : equipe-seminaires-nano@fondation-nanosciences.fr