Abstract
Ph.D.
Spin—density—wave (SDW) effects are investigated in four dilute Cr—Ir alloy single crystals. The
Ir concentrations in these crystals were chosen to cover all four magnetic phases existing on the
magnetic phase diagram of the Cr—Ir system. Thermal expansion, electrical resistivity, elastic
constants and ultrasonic attenuation are studied as functions of temperature and alloy concentration.
The elastic constants are also studied as a function of applied hydrostatic pressure.
The SDW effects in the Cr—Ir system are compared with those in other Cr alloys. The full
temperature—concentration and temperature—pressure magnetic phase diagrams of the Cr—Ir
system are determined. Existing theories are used to discuss the observations.
The following major observations are made:
Magnetoelastic interactions in Cr—Ir alloys are very large, resulting in well defined magnetic
anomalies in the elastic constants and thermal expansion at all magnetic phase transition
temperatures. Elastic constant measurements as a function of temperature appear to be a very
sensitive tool to determine the magnetic phase transition temperatures of the Cr—Ir alloy system.
Below TN of Cr—Ir alloys, where TN is the transition temperature from the incommensurate
transverse spin—density-wave (TISDW) magnetic phase to the paramagnetic phase, elastic constant
and thermal expansion measurements show the existence of hysteresis effects, which are
probably due to a redistribution of antiferromagnetic domains. These hysteresis effects are the
first evidence of such effects in Cr alloys.
Spin fluctuation effects are shown to exist to temperatures well above the Neel temperatures
of the Cr—Ir alloys.
Analyses of the data for electrical resistivity measurements of Cr—Ir alloys show that the
fraction of the electron and hole Fermi surface sheets that nests is roughly the same in the
ISDW and CSDW (commensurate spin—density—wave) phases, making the resistivity anomaly near the ISDW—CSDW transition temperature very small or non—existing.
Measurements of elastic constants as a function of applied pressure at different constant
temperatures are shown to be a very powerful tool for an investigation of the interaction of the
SDW with the acoustic phonons in dilute Cr—Ir alloys.
It is found that the SDW in Cr—Ir alloys couples mainly with the longitudinal—mode acoustic
phonons. Coupling to the shear—mode phonons is relatively small.
Empirical correspondence is found between the temperature—concentration and temperature—
pressure magnetic phase diagrams of the dilute Cr—Ir alloy system by using a linear scaling
between pressure and concentration.
Existing thermodynamic models fit the experimental results for the elastic constants and
magnetovolume of the Cr—Ir alloys well.
The main features of the temperature—concentration and temperature—pressure magnetic
phase diagrams of the Cr—Ir alloy system are resonably well predicted by existing microscopic
theories.