Abstract
Ph.D.
A comprehensive experimental study of the effects of the spin-density-wave
(SDW) on the physical properties of antiferromagnetic , doped with Mn
and V, and Cr-Re alloys is reported. The purpose of the study is twofold:
c c Al Cr − 1
(i) To gain insight in the anomalous behaviour of the magnetic phase
diagram reported for the binary Cr-Al system.
(ii) To investigate SDW effects on the anharmonic behaviour of the lattice
vibrations of Cr alloys with a member of the transition metals of group-7
in the periodic table.
The investigation entails the following measurements: thermal expansion in the
temperature range 77 – 450 K for all the specimens, velocity of sound in the
temperature range 4 – 300 K for the Cr-Al-V alloys, ultrasonic wave velocity for
the Cr-Re alloys as a function of applied pressure (up to 0.242 GPa) at different
temperatures and electrical resistivity in the temperature range 77 – 450 K for
the Cr-Re alloys.
Concentration-temperature magnetic phase diagrams of the (Mn, V)
alloy systems were constructed from the measurements. Alloying with Mn, to
increase the electron concentration, is observed to drive an incommensurate (I)
SDW alloy towards a commensurate (C) SDW state. This results in a
triple point, where the ISDW, CSDW and paramagnetic (P) phases coexist on
the magnetic phase diagram. A hysteretic first-order ISDW-CSDW/CSDWISDW
phase transition line is then observed on the phase diagram for Mn
concentrations above the triple point concentration. Adding V, in order to
decrease the electron concentration, to an ISDW alloy is found to have
the opposite effect. It drives such a system deeper into the region of the ISDW
c c Al Cr − 1
c c Al Cr − 1
phase. A CSDW alloy is, on the other hand, driven towards the triple
point by addition of V, instead of Mn.
c c Al Cr − 1
Theoretical analysis of the magnetic phase diagrams of the (Mn, V)
systems confirms a previous suggestion that the Al impurity acts as an electron
acceptor in the Cr matrix for
c c Al Cr − 1
2 < c at.% Al, as opposed to an electron donor for
at.% Al. 2 > c
The high-pressure ultrasonic studies on the Cr-Re alloys were used to construct
their pressure-temperature ( T p − ) magnetic phase diagrams. Applying
hydrostatic pressure to a CSDW Cr-Re alloy induces a hysteretic first-order
CSDW-ISDW phase transition at a certain critical pressure, resulting in a triple
point on the phase diagram. An interesting aspect of the observations on
the Cr-Re alloys is the suggestion of a new phase line, separating pressureinduced
and temperature-induced ISDW phases, on the
T p −
T p − phase diagram.
Acoustic-mode Grüneisen parameters, which quantify the lattice anharmonicity,
were calculated for the Cr-Re alloys from the high-pressure ultrasonic
measurements. The results indicate exceptionally large interactions between
the SDW and the long-wavelength longitudinal phonons in Cr-Re alloys. These
effects are particularly large in the vicinity of the Néel phase transition
temperature. Interactions of the SDW with the shear mode phonons are on the
other hand relatively much smaller. The work on the Cr-Re alloys is considered
to finally complete studies of the lattice anharmonicity of Cr alloy systems with
elements of all the important groups of the periodic table. It now paves the way
for developing microscopic theories to explain the unique behaviour of the
magneto-elasticity of dilute Cr alloys.
Prof. H.L. Alberts
Dr. A.R.E. Prinsloo