Effects of Magnetic Fields on Highly Sensitive TiAu TES Bolometers

In the process to develop the transition edge sensor (TES) bolometer technology for the SAFARI FIR Imaging Spectrometer on the SPICA telescope and for future FIR space telescopes, we have recently demonstrated a TiAu TES with a Ta thin film absorber on a thin SiN membrane, which has a dark NEP as low as 4.2 × 10^-19 W/√Hz. It is known that superconductors are sensitive to magnetic fields. Therefore, it is important to know how these extremely sensitive TES detectors are affected by external magnetic fields. This is desirable from an application point of view since the design of the SAFARI instrument demands such knowledge and interesting from a fundamental point of view. In this paper, we will describe a detailed experimental study of the effects of magnetic fields, both parallel and perpendicular to the TiAu plane, on the performance of a TES. We investigated the effect on the current, responsivity, speed, and TES parameters of α (dependence of temperature) and β (dependence of current), around the operating points. In order to achieve this highly sensitive measurement, the temperature stability has been improved and a new coil assembly has been designed. The latter allows accurately applying a parallel magnetic field.