Abstract
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