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Optical Faraday rotation studies in praseodymium ethylsulphate Griffiths, David John

Abstract

The technique of detecting EPR spectra and measuring spin-bath relaxation times by the megneto-optical Faraday rotation has been applied to carefully grown crystals of praseodymium ethylsulphate (PrES). This method has been successfully applied to neodymium ethylsulphate by Rieckhoff and Griffiths. PrES was chosen because Pr³⁺ is an ion having an even number of unpaired hf electrons and thus possesses a non-degenerate ground state, the removal of any degeneracy in zero magnetic field being due to low symmetry components of the crystalline electric field. All experiments were done' at liquid helium temperatures (1.4 ≤ T ≤ 4.2°K) and in external magnetic fields up to 12 kilogauss. Successful measurements were made with the optic axis of the crystal aligned parallel to the polarized light beam and the external magnetic field. It was necessary to orient the magnetic field component of-the microwave power parallel to the external magnetic field in order to induce spin transitions. Theory shows that the magnitude of the Faraday rotation is proportional to the difference in population of the ground state doublet. The EPR spectrum and spin-bath relaxation times were measured in terms of saturation, the saturation being defined by s = [formula omitted], where Θ and Θ₀ are the rotation in the presence and absence of microwave power. The EPR spectrum at X band of undiluted PrES is a broad band Extending over several kilogauss with no evidence of hyperfine structure. The band decreases at 8400 φ and covers 800 φ from its maximum to half power point. Pr¹⁴¹ has nuclear spin 1 = [formula omitted] and the absence of any hyperfine structure indicates that crystal field distortion is the main source of line broadening. This result agrees with work done by Baker and Bleaney who in examining the low field side of this spectrum at K band found no trace of hyperfine structure. The empirical dependence of saturation on the relative microwave power is found to be s = [formula omitted]. This relationship cannot be explained theoretically. Since homogeneous line broadening predicts s = [formula omitted], we conclude that the resonance line is inhomogeneously broadened. The relaxation times measured as a function of temperature over the ranee 1.4 to 2.18°K and at 4.22°K and as a function of the external magnetic field, range from 0.1 to 10 msec. For 1.40 ≤ T ≤ 1.80°K, T decreases from 0.4 to 0.15 msec; is proportional to T[symbol omitted] and is consiaered to be the phonon-bath relaxation time. Values of T show a scatter within 0.20 ± 0.02 msec for 1.80 ≤ T ≤ 1.95°K. As the λ -point is approached (1.95 ≤ T ≤ 2.18°K), T exhibits a very rapid increase, having a value of 0.20 msec at 2.155°K and 0.85 msec at 2.165°K. This behaviour appears to be related to the energy exchange between the crystal and helium bath. Measurements of T at 2.18°K and 4.22°K indicate that the return to thermal equilibrium of the spin-phonon system is limited by the thermal conductivity of the liquid helium bath. The magnetic field dependence of T is interpreted in terms of phonon scattering by the paramagnetic Pr³⁺ ions. The effect is strongest at the lowest temperatures (i.e. ~ 1.4°K) where there is an increase in T from 0.5 to 2.0 msec for the magnetic field H increasing from 3.5 to 11.5 kilogauss. The effect of the magnetic field on T decreases with increasing temperature and at 2.15°K T appears to be independent of field. An unsuccessful attempt to detect electron spin resonance in neodymium ethylsulphate oriented with its optic axis .perpendicular to the light path (and external magnetic field) is described.

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