Magnetoresistance in Thin Films of Silver Chalcogenides
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ABSTRACT Oriented and non-oriented thin films of silver(l) telluride, Ag 2Te, were prepared by ebeam evaporation, vapor transport technique and Chemical Vapor Deposition (CVD). Crystallinity and orientation of the films were studied by 0-20 XRD, rocking curve and pole figure measurements. The origin and conditions for the oriented growth are discussed. Special microdevice was prepared by photolitography from the oriented films of Ag 2Te in order to investigate magnetoresistance (MR) in this material. It was proved that the reported earlier negative MR in Ag 2Te films is a completely geometrical effect, which can be observed using non-linear arrangement of current and voltage contacts. INTRODUCTION A large variety of magnetoresistive materials have been intensively studied for the last decade because of the potential applications in magnetic recording and as magnetic field sensors. Both intrinsic (narrow bandgap semiconductors [1], manganites [2,3]) and artificial (magnetic multilayers [4], magnetic heterogeneous alloys [5,6]) magnetoresistive materials demonstrate high MR values. Non-magnetic narrow-gap semiconductors with high electron mobility, like InSb or InAs, are currently used for manufacturing of magnetoresistors [1]. Silver(I) telluride, Ag2Te, has two phases: low-temperature monoclinic aX-Ag 2Te and high-temperature cubic IP-Ag 2Te with transition temperature of 140'C. The high-temperature phase is metallic with high mobility of Ag atoms and superionic conductivity, while the lowtemperature phase is a semiconductor with a band gap of 0.1 eV and small effective mass of the electron carriers of 0.050-0.069m., with mo the free-electron mass [7]. Having resistivity in the order of milliohms-cm, a-Ag 2Te is a promising material for different MR devices. Peculiar MR properties were found recently in slightly non-stoichiometric silver telluride, Ag2sTe. High MR values were achieved in both bulk material [8] and thin films [9] of this compound. Unusual linear field dependence of MR down to several Oe was observed in the bulk Ag 2Te [8] and drew attention of some theoreticians [10]. Recently we reported another interesting feature of silver telluride: MR of oriented films has a clear peak at about 100K, whereas MR of non-oriented films is almost temperature independent below 120K [9]. This effect is similar to the one for GMR manganites [11-13]. Additionally, we observed unusual for non-magnetic materials negative MR at parallel field-current orientation in Ag 2Te films. At transverse field-current orientation MR turns positive [14]. All these effects still require elaborate experimental and theoretical study. EXPERIMENT Oriented thin films of silver telluride were grown by electron-beam evaporation from a mixture of Ag 2Te (Aldrich Chemicals, purity 99.999%) and Ag (Strem Chemicals, purity 99.99%) powders. Ag 2Te was ground, thoroughly mixed with Ag powder at a 16:1 ratio in weight and then pressed into a pellet. This pellet was used as the deposition source and was 47 Mat. Res. Soc. Symp. Proc. Vol. 602 0 2000 M
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