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Title: Some electron spin resonance experiments on exchange interactions
Author: Harris, E. A.
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 1963
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In the work described in this thesis, the exchange interactions between neighbouring magnetic ions in semidilute crystals have been measured from their paramagnetic resonance spectra. The results of these measurements are compared with the magnetic bulk properties of the concentrated crystals. The relationships between these bulk properties and the exchange interactions are considered in chapter 1, where a brief account of the origin of antiferromagnetic exchange is also given. The experimental measurements have mainly been made using a Q band (λ = 0.85 cm) spectrometer, which is described in chapter 2. In chapter 3, experiments are described on n.n. pairs of Mn++ ions in the face centred cubic MgO crystal lattice. The interaction between two n.n. spins s1 and s2 can be divided into two parts, an isotropic and an anisotropic part. The isotropic exchange interaction is taken to be, H = J(s1s2) - j(s1s2)2 + … The spins are strongly coupled together by this interaction and act as a single unit with spin S = s1 + s2, which can take the values 0, 1, 2, 3, 4, and 5. For purely first order exchange (j, etc = 0), the energy levels for the different S states are spaced according to a Lande interval rule, with S = 0 lowest. In earlier measurements on these pairs, Coles (Thesis 1959) measured the energy splitting between the S = 0 and S = 1 levels, and interpreted it in terms of a first order exchange term, Jk = 28°K, which could not be reconciled with the bulk magnetic properties. In the present work the intervals up to S = 4 have been measured by comparing the temperature variations of intensity for lines from different S states. The measurements of Coles have been confirmed, but it is found that the intervals to not follow a Lande rule, and the present results are interpreted in terms of a first order exchange term, Jk ≈ 15°K, and a second order term such that j ≈ .05 J. This is now in good agreement with bulk magnetic properties which suggest Jk ≈ 11°K. The difference is consistent with the difference in lattice spacings for the dilute and concentrated crystals. Further evidence for this second order biquadratic exchange in the concentrated crystals is provided by the anomalous sublattice magnetization below the transition temperature (Sievers and Tinkham 1963) and by the low temperature transverse susceptibility measured by Jacobs (private communication). The anisotropy of the exchange interaction is reflected in the anisotropy of the pair resonance lines. The anisotropic exchange is given by, H = De(3s1zs2z - s1s2) + Ee(s1xs2x - s1ys2y) + higher order terms.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available