Abstract
M.Sc.
Impurity resonance scattering effects are investigated in the Cr-Ga alloy system. This
system has a triple point on its magnetic phase diagram where the paramagnetic (P),
incommensurate (I) and commensurate (C) spin-density-wave (SDW) states co-exist.
Alloying Cr with the nonmagnetic nontransitional element Ga affects the magnetic
properties of Cr in a very unique way.
In order to investigate the presence of resonant impurity scattering effects in binary Cr-Ga
alloys, electrical resistivity measurements were carried out in the temperature range
between 6 K and 85 K. The results of the investigation show:
• A nonmonotonic increase in the residual resistivity of the Cr-Ga system
with an increase in the Ga content, due to the presence of resonant impurity
scattering of conduction electrons.
• A low-temperature resistivity minimum observed in some of the Cr-Ga
alloys, taken as further evidence for the presence of resonant impurity
scattering effects on the conduction electrons.
The impurity resonance scattering effects on the electrical resistivity of a Cr + 1.2 at.% Ga
alloy, doped with V and Mn to tune the Fermi level through the impurity level, are also
investigated. The investigation was complemented by thermal expansion and velocity of
sound measurements in the temperature range 77 K to 450 K for the Cr + 1.2 at.% Ga alloy
only. This specific Ga concentration was chosen to allow for studying resonant scattering
effects in both the ISDW and CSDW phases of the system. This is possible because
concentration of 1.2 at.% Ga is just above the triple point concentration. Doping with Mn
to increase the electron concentration (eA) drives the alloy deeper into the CSDW phase
region of the phase diagram, while doping with V, on the other hand, will drive the alloy
towards the ISDW phase region. The results of the study are summarized as follows:
• Two relatively sharp peaks, attributed to resonant impurity scattering
effects, are observed in the curve of the residual resisitivity as a function of
dopant concentration in the ISDW phase of the ternary (Cr0.988Ga0.012)1-xVx
and (Cr0.988Ga0.012)1-yMny alloy systems.
v
• At 0 K the (Cr0.988Ga0.012)1-yMny alloy system transforms from the ISDW to
the CSDW phase at y ≅ 0.0032, giving a CSDW phase for y > 0.0032. A
peak is observed in the residual resistivity at about this Mn content. This
peak can then either be ascribed to a jump occurring in the residual
resistivity when the CSDW phase is entered from the ISDW phase or to
resonant scattering effects. The conclusion is that the peak is rather related
to the latter effect.
• The resistivity as a function of temperature of the above two ternary alloy
series show well-developed or weak minima at low temperatures for some
of the samples. This is taken as further evidence of the influence of impurity
resonant scattering effects on the resistivity of these alloys.
• The resistivity and thermal expansion coefficient of the polycrystalline
Cr0.988Ga0.012 alloy of the present study behaves anomalously close to the
ISDW-CSDW phase transition temperature and warrant further
investigation.
The concentration-temperature magnetic phase diagram of the (Cr0.988Ga0.012)(Mn,V) alloy
system was constructed from the magnetic transition temperatures obtained from electrical
resistivity measurements. Theoretical analysis of the phase diagram was done using the
two-band imperfect nesting model of Machida and Fujita. The results show:
• A triple point at (0.21 at.% V, 225 K) where the ISDW, CSDW and P
phases coexist on the magnetic phase diagram.
• The curvature of all three theoretically calculated phase transition lines in
the region of the triple point is of the same sign as that observed
experimentally.
• The theoretical fit is very good for the ISDW-P and ISDW-CSDW phase
transition boundaries, while there is some discrepancy for the CSDW-P
phase transition line. This may be attributed to the fact that the theory is one
dimensional and that it does not include electron-hole pair breaking effects
due to impurity scattering and also not effects of changes in the density of
states due to alloying.
Dr. A.R.E Prinsloo
Prof. H.L. Alberts