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We present magnetic characterization, charge resistivity, and optical photoluminescence measurements on amorphous yttrium iron oxide thin films (a -Y–Fe–O), with supporting comparisons to amorphous germanium (a -Ge) films. We measured magnetic properties with both SQUID magnetometry and polarized neutron reflectometry. These results not only confirm that a -Y–Fe–O is a disordered magnetic material with strong predominantly antiferromagnetic exchange interactions and a high degree of frustration, but also that it is best understood electrically as a disordered semiconductor. As with amorphous germanium, a -Y–Fe–O obeys expectations for variable-range hopping through localized electron states over a wide range of temperature. We also clarify the consequences of charge transport through such a semiconducting medium for non-local voltage measurements intended to probe spin transport in nominally insulating magnetic materials. We further compare non-local resistance measurements made with "quasi-dc" automated current reversal to ac measurements made with a lock-in amplifier. These show that the "quasi-dc" measurement has an effective ac current excitation with frequency up to approximately 22 Hz, and that this effective ac excitation can cause artifacts in these measurements including incorrect sign of the non-local resistance. This comprehensive investigation of non-local resistance measurements in a -Y–Fe–O shows no evidence of spin transport on micrometer length scales, which is contrary to our original work, and in line with more recent investigations by other groups. [ABSTRACT FROM AUTHOR]

Spin-to-charge conversion and the reverse process are now critically important physical processes for a wide range of fundamental and applied studies in spintronics. Here, we experimentally demonstrate effective spin-to-charge conversion in thermally evaporated chromium thin films using the longitudinal spin Seebeck effect (LSSE). We present LSSE results measured near room temperature for Cr films with thicknesses from 2 to 11 nm, deposited at room temperature on bulk polycrystalline yttrium-iron-garnet (YIG) substrates. Comparison of the measured LSSE voltage, V LSSE , in Cr to a sputtered Pt film at the same nominal thickness grown on a matched YIG substrate shows that both films show comparably large spin-to-charge conversion. As previously shown for other forms of Cr, the LSSE signal for evaporated Cr/YIG shows the opposite sign compared to Pt, indicating that Cr has a negative spin Hall angle, θ sh . We also present measured charge resistivity, ρ , of the same evaporated Cr films on YIG. These values are large compared to Pt and comparable to β -W at a similar thickness. Non-monotonic behavior of both ρ and V LSSE with film thickness suggests that spin-to-charge conversion in evaporated Cr, which we expect has a different strain state than previously investigated sputtered films, could be modified by spin density wave antiferromagnetism in Cr. [ABSTRACT FROM AUTHOR]

Measurements of thermoelectric effects such as the Seebeck effect, the generation of electric field in response to an applied thermal gradient, are important for a range of thin films and nanostructures used in nanoscale devices subject to heating. In many cases, a clear understanding of the fundamental physics of these devices requires knowledge of the intrinsic thermoelectric properties of the material, rather than the so-called "relative" quantity that comes directly from measurements and always includes contributions from the voltage leads. However, for a thin film or nanostructure, determining the absolute Seebeck coefficient, α abs , is challenging. Here, we first overview the challenges for measuring α abs and then present an approach for determining α abs for thin films from relative measurements made with a micromachined thermal isolation platform at temperatures between 77 and 350 K. This relies on a relatively simple theoretical description based on the Mott relation for a thin film sample as a function of thickness. We demonstrate this technique for a range of metal thin films, which show that α abs almost never matches expectations from tabulated bulk values, and that for some metals (most notably gold) even the sign of α abs can be reversed. We also comment on the role of phonon and magnon drag for some metal films. [ABSTRACT FROM AUTHOR]

We present thermal conductivity measurements of micromachined 500 nm thick silicon-nitride (Si–N) beams suspended between two Si–N islands, in the temperature range from 77 to 325 K. The measured thermal conductivity, k, of Si–N at high temperatures is in good agreement with previously measured values for Si–N grown by low-pressure chemical vapor deposition, but behaves much differently as temperature is lowered, showing a dependence more similar to polycrystalline materials. Preliminary structural characterization by x-ray diffraction suggests that the material is likely nano- or polycrystalline. The micromachined suspended platform structure is designed to allow highly accurate measurements of the thermal conductivity of deposited metallic, semiconducting, or insulating thin films. As a demonstration, we present measurements of a 200 nm thick sputtered molybdenum film. In the entire temperature range the measured thermal conductivity matches the prediction of the Wiedemann–Franz thermal conductivity determined from measured electrical conductivity. [ABSTRACT FROM AUTHOR]

We briefly review the principles of a magnetic microcalorimeter x-ray detector and the requirements for fabricating an array of such sensors, which include a paramagnetic sensor fabricated with thin-film techniques. We discuss two methods for depositing an Er-doped Au sensor film and present magnetization measurements from 4 to 300 K in 5 T and near the detector’s operating temperature and field. The properties of the deposited films that we have studied to date deviate from the expected behavior of bulk materials. [ABSTRACT FROM AUTHOR]

The authors present a prototype for a high-energy-resolution, high-count-rate, gamma-ray spectrometer intended for nuclear forensics and international nuclear safeguards. The prototype spectrometer is an array of 14 transition-edge-sensor microcalorimeters with an average energy resolution of 47 eV (full width at half maximum) at 103 keV. The resolution of the best pixel is 25 eV. A cryogenic, time-division multiplexer reads out the array. Several important topics related to microcalorimeter arrays are discussed, including cross-talk, the uniformity of detector bias conditions, fabrication of the arrays, and the multiplexed readout. The measurements and calculations demonstrate that a kilopixel array of high-resolution microcalorimeters is feasible. [ABSTRACT FROM AUTHOR]

The authors describe a microcalorimeter γ-ray detector with measured energy resolution of 42 eV full width at half maximum for 103 keV photons. This detector consists of a thermally isolated superconducting transition-edge thermometer and a superconducting bulk tin photon absorber. The absorber is attached with a technique compatible with producing arrays of high-resolution γ-ray detectors. The results of a detailed characterization of the detector, which includes measurements of the complex impedance, detector noise, and time-domain pulse response, suggest that a deeper understanding and optimization of the thermal transport between the absorber and thermometer could significantly improve the energy resolution of future detectors. [ABSTRACT FROM AUTHOR]