| dc.description.abstract | A Low Temperature Study of Electronic Transport Properties of Certain Highly Resistive Crystalline and Quasicrystalline Alloys
Low temperature electronic transport in metals and alloys has been and continues to be a topic of intense research. In the past, a number of studies have been done on low resistive crystalline alloys and highly resistive amorphous alloys such as metallic glasses. Crystalline alloys of transition metals can exhibit high resistivity [?(300K) ~ 100 ??·cm]. Recently a new class of materials, namely quasicrystalline alloys, has joined the list of very high resistive alloys having ?(300K) > 1000 ??·cm. A number of interesting physical phenomena have been observed in these highly resistive crystalline and quasicrystalline materials at low temperatures. It is now being realized that when conductivities are low, one can no longer explain the transport in a semiclassical framework. An example of this is the quantum interference effects which can lead to a rise in ? at low temperatures and to novel magnetoresistance. In metallic glasses one generally observes resistivity minima at T = Tmin due to these effects typically for T > 5K. This is a well-researched topic. In this thesis we made a systematic investigation of the resistivity minima in crystalline alloys with concentrated magnetic constituents. ?(T) was measured down to 0.4K for the crystalline alloys belonging to the pseudo-binary series Fe-Ni-Cr for 66 ? x ? 80 and for Fe-Cr. Magnetoresistivity was also measured for these samples from 1.7K ? T < 100K and fields up to 7 Tesla. In the present investigation some definite questions have been posed and we tried to seek answers to them. The questions are (a) What happens to the electrical transport properties in crystalline alloys with concentrated magnetic constituents when we are not in the Kondo regime
(b) What is the role of magnetic order, if any, in electronic transport in alloys which lie close to the critical composition for ferromagnetism, in particular when one has disordered magnetic phase close by (c) What is the difference in low temperature electronic transport in ?-phase and ?-phase iron alloys
(d) Can the quantum interference ideas be applied to explain low temperature resistivity
In case of quasicrystals, the problem of electronic transport is a new challenging area. The discovery of stable icosahedral quasicrystals in transition metal alloys Al-Cu-TM (TM = Fe, Ru, Os) has given a big boost to the study of electronic transport in these exotic materials. One observes extremely high electrical resistivity ? >> 1000 ??·cm and it seems that these alloys are close to a metal-insulator transition. It is believed that typical electron properties in these types of alloys are commonly associated with low carrier density due to decrease in the density of states (DOS) at Fermi energy originating from the Fermi surface–Brillouin zone (FS-BZ) interaction. This interaction becomes even stronger in the presence of sp-d hybridization as it happens in Al-Cu-TM alloys. It is also believed that the stability of these quasicrystalline phases is due to strong FS-BZ interaction. On annealing, the resistivity of these alloys increases. Also, we found that the rate of solidification and thermal treatment influences the observed resistivity value. In these alloys, quasicrystalline phase invariably coexists with a crystalline phase and ?(T) is a sensitive function of the resulting microstructure. We studied in detail the electronic transport (T ? 0.4K) and magnetotransport (T ? 1.7K, H ? 7T) in quasicrystalline Al??Cu??Cr?? on which detailed microstructural analysis was done using X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). We have also compared the limiting low temperature (T < 20K) resistivity ?(T) of Al??Cu??Cr?? and Al??.?Cu??.?Fe??. | |
