  Cryogenic microcalorimeters are outstanding tools for X-ray spectroscopy due
to their unique combination of excellent energy resolution and close to 100%
detection efficiency. While well-established microcalorimeter concepts have
already proven impressive performance, their energy resolution has yet to
improve to be competitive with cutting-edge wavelength-dispersive grating or
crystal spectrometers. We hence present an innovative SQUID-based
superconducting microcalorimeter with an in-situ tunable gain as alternative
concept that is based on the strong temperature dependence of the magnetic
penetration depth of a superconductor operated close to its critical
temperature. Measurements using a prototype device show no sign for any
hysteresis effects that often spoil the performance of superconducting
microcalorimeters. Moreover, our predictions of the achievable energy
resolution show that a competitive energy resolution O(300meV) with a suitable
combination of absorber and sensor material should be easily possible.
