  We are developing X-ray microcalorimeters as a backup option for the baseline
detectors in the X-IFU instrument on board the ATHENA space mission led by ESA
and to be launched in the early 2030s.5$\times$5 mixed arrays with TiAu
transition-edge sensor (TES), which have different high aspect ratios and thus
high resistances, have been designed and fabricated to meet the energy
resolution requirement of the X-IFU instrument. Such arrays can also be used to
optimise the performance of the Frequency Domain Multiplexing (FDM) readout and
lead to the final steps for the fabrication of a large detector array. In this
work we present the experimental results from tens of the devices with an
aspect ratio (length-to-width) ranging from 1-to-1 up to 6-to-1, measured in a
single-pixel mode with a FDM readout system developed at SRON/VTT. We observed
a nominal energy resolution of about 2.5 eV at 5.9 keV at bias frequencies
ranging from 1 to 5 MHz. These detectors are proving to be the best TES
microcalorimeters ever reported in Europe, being able to meet not only the
requirements of the X-IFU instrument, but also those of other future
challenging X-ray space missions, fundamental physics experiments, plasma
characterization and material analysis.
