M.Sc. / Spin-density-wave (SDW) effects are investigated in a Cr + 1.9 at.% Fe alloy single crystal, where the Fe concentration lies just below the triple point found in the temperatureconcentration magnetic phase diagram of the Cr-Fe alloy system. The crystal is expected to undergo a commensurate (C) SDW to an incommensurate (I) SDW phase transition at a temperature Tc, and an ISDW-P (paramagnetic) phase transition at the Neel temperature, TN. The magnetoelastic properties and the anharmonic behaviour of this crystal were studied with the aid of velocity of sound measurements as function of temperature and pressure. Electrical transport measurements were carried out using the standard fourprobe method. In order to determine the various phases present in the crystal a preliminary neutron-diffraction study was also done. Fe belongs to the group-8 magnetic transition metals, possessing localized magnetic moments. The SDW effects in the Cr + 1.9 at.% Fe crystal are therefore compared with that of Cr-Ru and Cr-Ir alloys, as Ru and Ir also belong to the group-8 transition metals, however these impurities are nonmagnetic. The following observations were made: The longitudinal mode elastic constants and the bulk modulus show a prominent change in the slope at Tc, and a sharp, deep minimum at TN. For the c' shear propagation mode peaks were seen at Tcl as well as TN and the c4 4 propagation mode showed no anomalies at either phase transition temperatures. The longitudinal ultrasonic wave velocities for the cL propagation mode were measured as a function of temperature at different constant pressures. TN obtained from these measurements varies linearly with increasing pressure. High-pressure ultrasonic wave velocity measurements were taken at various constant temperatures in the range of 230 K to 350 K for the C L, c44 and c' propagation modes of the Cr + 1.9 at.% Fe alloy single crystal. This was used to determine the pressure derivatives of the second order adiabatic elastic constants (acu /ap). The pressure derivatives of the second order adiabatic elastic constant are shown to be a very powerful tool for investigating the interaction of the SDW with the acoustic phonons in the Cr-Fe crystal. II The long-wavelength acoustic-mode Gitmeisen parameters, calculated from (acu/ap), showed that the SDW in the Cr + 1.9 at.% Fe alloy single crystal couples mainly with the longitudinal acoustic phonons. Coupling to the shear modes is relatively small. The mean acoustic-mode GrOneisen parameter shows a small maximum between Tc, and TN. It increases on heating through TN, reaching a large maximum value above TN, and then decreases with further increase in the temperature. The electrical resistivity was measured_in the temperature region of 4 Kt() 900 K in order to obtain the nonmagnetic component of the resistivity at all temperatures. Only the Neel phase transition was observed in these measurements with no resistivity anomalies taking place at -Va. The experimental results on the resisitivity were analyzed according the model of Chui et al.. The magnetic component of the electrical resistivity was calculated from the model with and without the inclusion of the effects of resonant impurity scattering of the conduction electrons by the local impurity states lying in the SDW energy gap. The magnetic contributions were found to be appreciable above TN, even up to temperatures as high as 1.5TN. The neutron-diffraction experiments show that the Cr + 1.9 at.% Fe crystal remains in the ISDW phase at all temperatures below TN. This is an unexpected result as a CSDW-ISDW phase transition is expected at To, the temperature of the observed anomaly in elastic constant and thermal expansion measurements on the crystal
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:3447 |
| Date | 03 September 2012 |
| Creators | Derrett, Helen Anne |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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