Pt-SiO 2 Granular Metals for Cryogenic Thermometry in High Magnetic Fields
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Pt-SiO. GRANULAR METALS FOR CRYOGENIC THERMOMETRY IN HIGH MAGNETIC FIELDS Elida De Obaldia, T.D. Moustakas, Jeff Hettengier and J.S. Brooks. Boston University Department of Physics, Boston, Massachusetts 02215 ABSTRACT Pt-SiO. granular metal films were produced by RF diode sputtering and characterized as thermistors for cryogenic temperature measurements at high magnetic fields. The devices were found to have high sensitivity and small magnetoresistance. This, together with ease of fabrication and stability, makes such thermistor useful candidates for cryogenic thermometry. Introduction The determination of the physical properties of a solid at low temperatures and at high magnetic fields requires the measurement of the temperature of the sample with high accuracy. Currently, cryogenic thermometry at high magnetic fields is carried out by combining a number of resistance thermometers such as Pt, Rh-Fe, Ge, and Speer.['] An alternative resistance thermistor for cryogenic fields, based on Pt or Mo-Alg03 cermets, has recently been proposed by Gershenfeld, et. al.1] Such thermistors were found to be sensitive from 50mnK to 300miK and show a weak magnetoresistance. However, the observed variations of the residual resistivity by a factor of 5 between devices made under nominally identical conditions has cast doubt on their practical usefulness. Such variability may be attributed to the spatial deposition rates of the metal and the insulator, a phenomenon inherent to the electron beam evaporation process used for the thin film deposition. In this paper we report a preliminary study on the use of Pt-SiO. granular metals, produced by sputtering from a composite target, as thermistors for cryogenic thermometry at high magnetic fields. Experimental Methods Pt-SiO films were produced by sputtering from a composite target onto quartz substrates, following the techniques first proposed by Abeles, et. al.J] The target consists of an 8 inch Si0 2 disk with a 2 inch Pt disk superimposed over it, close to its periphery. Such a configuration leads to films with graded composition. The substrates were placed on a table 3 inches below the target and their positions were carefully located relative to the center of the Pt disk. The sputtering chamber was evacuated prior to film growth to a base pressure of 5 x 10-rtorr using a turbomolecular and a mechanical pump. Sputtering was carried out at 10mT of Argon and a frequency of 13.57MHz. Reproducibility from run to run was maintained by holding the DC component of the RF voltage supplied to the target at 1000 Volts. Film thickness as a function of sputtering geometry was determined by measuring the step height between the masked and unmasked area of each substrate with an accuracy of 5% using a Deetak IIA thickness monitor. The resistivity of the films was measured to be 4K to 300K in a magnetic field up to 8 Teslas using a cryostat with a Carbon thermistor as the temperature sensor.
Mat. Res. Soc. Symp. Proc. Vol. 195. 01990 Materials Research Society
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Experimental Results The comp
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