Study of Spin Dynamics by Means of On-Chip SQUIDs

We developed an on-chip SQUID (Superconducting Quantum Interference Device) measurement technique to study spin dynamics in Single Molecular Magnets (SMMs) and other spin systems. SMMs systems are thought as potential qubit candidates, due to their quantum nature and the possibility to construct superposition of molecular spin states. If spins are enough diluted to minimize dipolar interactions and therefore increase decoherence time, the information carried by each molecular qubit can be preserved for sufficiently long times and measured in a single crystal sample. The SQUID, as the most sensitive magnetometer, is used to investigate the magnetization of macroscopic samples containing the spins under study. Traditional SQUID equipment performs magnetization measurements by shielding the SQUID from external magnetic fields, since a high field would destroy its superconductivity. The SQUID-sample coupling in such arrangement is done by a pick-up coil, at the expense of sensitivity. In contrast, the setup implemented by us at NHMFL, uses an ultra thin (~ 3 nm in thickness) niobium SQUID with nano-bridge junctions. Such a SQUID can preserve its superconductivity up to a very high field, as long as the field is perfectly aligned in the film's plane. We conducted experiments at low temperature, in a 3He/4He dilution refrigerator. The measured SQUID switching current is periodically modulated by the magnetic flux, generated only by the studied sample if the field is perfectly aligned with the SQUID plane. Through an initial calibration process, the vectorial magnetic field is aligned with the SQUID plane, for fields up to the maximum field achievable in this setup (7~Tesla). A superconducting feedback coil, placed above the SQUID, is used to compensate the magnetic flux variation caused by the sample. The feedback coil (and its current) can therefore relay information about the sample's magnetization. The control of the experimental setup is accomplished through Labview programs and is ready to incorporate spin excitation done by microwave or laser pulses. We have investigated the molecular paddle-wheel complex of Ru2+5. This molecule shows an enhanced magnetic hysteresis, with a valley of negative differential susceptibility, due to an abrupt spin reversal followed by a phonon avalanche. We simulated the process using the phonon bottleneck effect. Another theoretical model is developed to describe a blending of non-adiabatic spin rotation and the phonon bottleneck effect. Also, using our on-chip SQUID technique, we have observed a spin transition at 1.2K in Gd3N@C80, which is caused by the charge transfer between one of the Gd ions and the carbon cage. In another molecular magnet, Mn12-tBuAc, the influence of the transverse field on the tunneling probability is studied in detail and compared to numerical calculations which we performed using a diagonalization technique.