Electron paramagnetic resonance studies of 3d3 ions in magnesium oxide

Abstract

This thesis reports some electron spin resonance spectra
of v2+ and cr3+ ions in magnesium oxide.
Chapters I and II describe the basic features of electron paramagnetic resonance and its application to transition metal ions in insulating crystalline environments. A review of previous work on 3d3 ions in magnesium oxide is given in Chapter III, together with the theory underlying the resonances observed.
In Chapter V this theory is adapted to a previously unreported spectrum of v2+ in orthorhombic sites in magnesium oxide, an unusual and unexpected situation in this cubic oxide. With the help of a digital computer we find the following Hamiltonian parameters for the observed resonances at X-band and Q-band:- gx = gy = gz = 1.991, |D| = 0.182cm-1, |?| = 0.042cm-1, |A| = 0.0072cm-1. The orthorhombic distortion is attributed to a nearest neighbour cation vacancy.
A "forbidden" hyperfine pattern is also observed, which becomes allowed off-axis due to the mixing of zero field splitting and hyperfine terms. The observed and computed intensities of these lines were comparable.
Chapter VI deals with the resonances observed from the S=2 state of next nearest neighbour pars of super- exchange-coupled V 2+ ions. The spectra may be fitted to the following spin Hamiltonian: = Bg.H.S + D(Sz 2 - %s(S+l)) + A.I.s, with the following parameters: gx = gy = gz = 1.980, D= -0.072cm-1, |A| = 0.004cm-l. From the temperature variation of line intensity the exchange energy J is determined to be -60cm-1. A temperature dependent linewidth of the pair spectra was explained as due to spin-lattice relaxation processes via phonons of energy comparable to the level splittings.
Finally, we describe measurements on Cr3+ ions in orthorhombic sites at a frequency higher than those previously used.
In contra-distinction to measurements at X- and K-bands by other authors, all possible transitions were observed and their orientation dependence determined. The Hamiltonian parameters obtained were found to be consistent with those of other workers.