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Title: Studies of magnetic solids at low temperatures
Author: Cashion, John D.
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 1969
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The series of compounds which crystallize in the orthorhombic Pbnm distortion of the perovskite structure has proved a lucrative source of experiments in solid state physics. In the general formula ABO3, either or both the metallic ions may be magnetic and since the two site symmetries are also different this provides several different degrees of complication. The main study in this thesis is of the two members GdAlO3 and DyAlO3 in which the B site is occupied by the diamagnetic aluminium ion. We therefore only have to consider the interactions of the rare earth ions with each other and with their surroundings. Three types of experimental measurements have been carried out. Specific heat measurements in the range 0.5°K to 20°K were performed in a conventionally designed He3 cryostat using a germanium resistance thermometer. Measurements of the magnetic moment and most of the magnetic susceptibility measurements were carried out by a ballistic method and the induced signal measured on an integrating digital voltmeter. Applied magnetic fields up to 90kOe were available for use in the moment experiments. The temperature range 0.4°K to 4.2°K could be covered using pumped He3 or He4. Susceptibility measurements in the pumped liquid hydrogen range and some measurements in the liquid He4 range were carried out using a mutual inductance technique. The most prominent feature of the specific heat measurement on GdAlO3, was a lambda anomaly peaked at 3.870°K which we take to be the ordering temperature. Around 1°K there is another smaller bump which is a Shottky anomaly due to the crystal field splitting. The total entropy under the magnetic part of the specific heat was measured to be R ln8 showing that the Gd3+ ion is in its 8S7/2 ground state. Measurement of the magnetic susceptibility of GdAlO3, along the three orthorhombic axes showed results similar to that expected for a simple two sublattice antiferromagnet with the b-axis as the direction of magnetic alignment. Above the Néel temperature there was very little anisotropy. As expected for an antiferromagnet with small anisotropy, the phenomenon of "spin flop" was observed in the magnetic moment measurement along the b-axis. The critical field was found to be 11.58kOe. Magnetic saturation was achieved along the a, b and c-axes at 45.6kOe, 39.5kOe and 44.4kOe respectively with an isotropic saturation moment within 1% of 7 Bohr magnetons per atom. Prom a simple molecular field theory analysis of these experiments we obtain the values 21.0kOe for the exchange field and 2.7kOe for the anisotropy field. Closer analysis reveals that the anisotropy is mainly due to the crystal field interaction and the principal axis is canted at ±38° to the b-axis in the a-b plane for the two different magnetic sites. This means that the ordering mode is really divided into four sublattices with hidden canting in an AxGy configuration following the notation of Bertaut. At low temperatures the Gy component, which corresponds to antiparallel nearest neighbours, is dominant showing why many of the properties could be explained on the basis of a simple two sublattice antiferromagnet. The sublattice magnetization was calculated in several different ways and helped show that the internal consistency of molecular field theory is much better than its accuracy in ab initio calculations. Using results from the moment experiments along the b-axis the magnetic phase diagram on the H-T plane was plotted. The value of the nearest neighbour exchange constant was evaluated from results of several different experiments using the Heisenberg model and a best value of J/k = -0.067°K obtained. Prom the consistency of the results it was concluded that second nearest neighbour exchange is negligible. Specific heat measurements on DyAlC3 below 8°K showed only a lambda anomaly peaked at the Néel point of 3.53°K. The entropy under the specific heat was measured to be almost R ln2 showing that the Dy3+ ion is in the lowest lying Kramers' doublet of the 6H15/2 ground state. Thus the Dy3+ is well described by an effective spin of andfrac12; and this gives rise to very anisotropic interactions with its surroundings. Optical measurements have shown that one g-value is very much larger than the other two so we expect it to behave to a good approximation as an Ising system. The susceptibility measurements below the Néel temperature confirm this belief, with the susceptibility decreasing with decreasing temperature along both the a- and b-axes. The ratio of the two measurements remains constant at a value corresponding to tan233° showing that the canting angle is ±33° to the b-axis. The susceptibility along the c-axis is very small and almost temperature independent. The moment measurements in the a-b plane all showed the phenomenon of "spin flip" or reversal of the direction of a sublattice, expected in an anisotropic antiferromagnet. Measurements at an arbitrary angle showed two flips due to the magnetically inequivalent sublattices and the angular dependence could be explained well using dipole interactions alone. Although the magnetic ordering is due mainly to dipole interactions it is shown that there is a significant antiferromagnetic exchange interaction with J/k~-1°K between nearest neighbours along the c-axis. Susceptibility measurements were carried out on four series of rare earth compounds of the general formulae RNbO4, R3NbO7, R2TiO5 and R2Ti2O7 as well as several other members of the RAlO3 family. Half of the compounds showed ordering transitions but further detailed work could not be carried out because of the difficulty in obtaining suitable specimens.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available