The new generation of x-ray and gamma-ray detectors employ cryogenic detectors known as transition-edge sensors (TES) due to their high energy resolution and photon detection rates. These detectors require a refrigeration module that can operate at the transition temperature of the TES’s superconducting film—usually at mK temperatures. DR-TES consists of a novel mini-dilution refrigerator (DR) from Chase Research Cryogenics that can be used in balloon-borne missions to cool detectors to temperatures between 10 to 100mK. To test the viability of this DR module, we will be cooling down a SLEDGEHAMMER detector fabricated by the National Institute of Standards and Technology quantum sensor group. The SLEDGEHAMMER microcalorimeter uses TESs coupled to superconducting quantum interference devices which are in turn coupled to microwave resonators to detect x-rays and gamma-rays. We plan to fly the SLEDGEHAMMER detector cooled by the mini-DR on a stratospheric balloon flight in August of 2024 at Fort Sumner, NM. As a follow-up mission, 511-CAM will use a modified version of the detector to map the 511keV emission from the galactic center region.
We have measured a spin Hall conductivity in Ni60Cu40 that is comparable to the value we reported for Pt [1], using broadband FMR measurements of ferromagnet/nonmagnet bilayers that do not require patterning of the bilayer. In a series of samples with the layer structure Ta(3)/Py(3.5)/NixCu1-x(d)/Ta(3) (thickness in nm), we varied Ni fraction x for d = 10 nm and varied thickness d for x = 0.6, a composition with a critical temperature for magnetic ordering of 140 K. FMR measurements at room temperature showed a substantial SHE for all compositions we deposited, 0.3 ≤ x ≤ 0.75. For the thickness series at x = 0.6, we use the approach detailed in [1] to extract a spin diffusion length of (8.3 ± 0.4) nm and a spin Hall ratio of 1.05 ± 0.22.
[1] Berger, A. J., et al. "Determination of the spin Hall effect and the spin diffusion length of Pt from self-consistent fitting of damping enhancement and inverse spin-orbit torque measurements" Phys. Rev. B 98, 024402 (2018).
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