Research on Development of Thermometers for Cryogenic Temperature in Magnetic fields

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Zr-N sputtered films, which have been drawn attention as the best candidate for the thermometers at cryogenic temperature with magnetic fields, have been investigated by electrical and acoustical measurements. The results obtained in this research project are summarized as below.1. Classification of Zr-N films : Zr-N films fabricated with various sputtering conditions ; substrate temperature and nitrogen partial pressure, can be classified and divided into two groups by the resistance ratio. between room and helium temperatures.2. Origin of small magnetoresistance : Magnetoresistance(MR)of Zr-N films is investigated by the measurements in the field up to 30T. It is found that MR of Zr-N consists of various contributions from quantum interference, spin-orbit and interaction effects. The small MR in weak field arises from accidental cancellation of these contributions. The possibility of occurrence of ferromagnetism was also suggested.3. Universal behavior in temperature dependence of electrical conductivity : We present attempts for finding the universal function which describes T dependence of conductivity. The conductivity sigma is described as sigma=sigma_0+AAOT for major parts of samples at high temperatures, and as sigma=sigma_0+A(T-T_0)^<gamma> at low temperatures. The relations among gamma, A, etc. are found.4. Discovery of novel elastic anomalies : The velocity of surface acoustic wave shows remarkable anomalies as a function of temperature. The strongly localized sample with high resistance ratio shows a broad minimum at above 300 K and a softening toward low temperature. The weakly localized sample shows sole a broad minimum at 15 K. These elastic anomalies and the relation to the localization are discussed based on two-level-system.From the series of investigations, the physical properties of Zr-N films has been made clear. These results would lead the successful development of then-nometers for the use at cryogenic temperature in magnetic field.