We describe a method of measuring thermal conductivity of films as thin as 15 nm from 2-300 K and in magnetic fields up to at least 8 T using a silicon-nitride membrane based microcalorimeter. The thermal transport in the membrane is measured before and after a sample film is deposited on the membrane. Accurate knowledge of the geometry of the microcalorimeter allows the thermal conductivity of the sample film to be determined from the difference of these measurements. We demonstrate the method for two thin film samples, a 16 nm thick Au film and a 200 nm Pb film. Results are in good agreement with the expected thermal conductivity. Below 10 K, surface scattering effects in the nitride membrane become important and limit the usefulness of this technique in some cases. Above 100 K radiative loss becomes important; we describe a method for correcting for this, taking advantage of its temperature dependence.
The introduction of magnetic moments such as Gd into amorphous Si produces dramatic effects in electrical transport below a characteristic temperature T * . Below T * , the conductivity of the magnetically doped systems is strongly suppressed compared to equivalent nonmagnetic Y doped samples, and displays enormous negative magnetoresistance. T * occurs at relatively high temperatures ͑ϳ10-100 K͒ and decreases sharply with increasing Gd concentration, passing smoothly through the metal-insulator transition. In ternary samples with both Gd and nonmagnetic Y, T * decreases strongly with increasing metallization, whether due to the addition of Gd alone or a mixture of Gd and Y. These results cannot be explained by simple magnetic interaction models, suggest the crucial role of electron screening and are reminiscent of mass enhancement behavior.Amorphous metal-semiconductor alloys ͑a-M-Si͒ offer unique insight into the metal insulator transition ͑MIT͒ as a comparison to doped crystalline materials. It has been widely documented that amorphous alloys undergo a MIT with identical low temperature behavior but at much greater dopant concentration compared to their crystalline counterparts due to significant additional disorder. The doping of local magnetic moments into semiconductors near the MIT causes dramatic effects in the magnetic and transport properties, including enormous negative magnetoresistance, fielddependent anomalous ͑nonspectral weight conserving͒ optical conductivity, and a magnetic susceptibility with a near-Curie law temperature dependence but a nonmonotonic dependence on composition, including a large peak at the MIT. 1-3 The enormous magnetic field dependence has allowed measurements of scaling behavior continuously through the 3D MIT on a single sample, including tunneling determination of the electron density of states. 4,5 Strong similarities exist between the a-M-Si systems studied here and both dilute magnetic semiconductor systems ͑DMS͒, such as ͑Ga,Mn͒As and the perovskite manganites. In all these systems, there are indications of strong coupling of electrical conductivity, magnetic properties, and even the structural or lattice system, suggesting the possibility of an all-encompassing theoretical description. Distinct differences in our system, e.g., the strong disorder, and magnetic moments from f rather than d-shell electrons, offer unique insights into the underlying physics. In these systems the separate control of electron and moment concentrations is critical to understanding the underlying physics.While there has been some success in describing the low temperature properties of amorphous doped semiconductors on both the metallic and insulating sides of the MIT, including the magnetically doped semiconductors, 6-8 a strikingly unresolved question is the nature of the higher temperature behavior where the effects upon the charge carriers due to the magnetic dopants "turns on." This temperature, which we call T * , is clearly seen in a sharp decrease of dc conductivity dc ͑T͒ for the magn...
A high-temperature superconducting magnet system for investigations of physical properties of bulk, powder, and thin-film samples is presented. This system provides a capability for a commercial vibrating-sample magnetometer, as well as thermal and electric characterization techniques to be employed in an environment with reduced refrigeration demands. These measurements can be performed over a wide range of temperatures down to 77K and in applied magnetic fields to 1T. In this report, we outline important elements of the cryogenic design, as well as measurements of the magnetic properties of a high-quality CoMn ferromagnetic thin film.
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