Master thesis defense by Niels Momsen

In this thesis the critical Néel temperature, T_N , of the magnetic critical phase transition and the associated critical exponents of h- have been measured using elastic and inelastic neutron scattering. The thesis introduces relevant theoretical concepts related to crystal structures, magnetism, critical phase transitions and their associated critical exponents, as well as the theory of neutron scattering used to carry out the measurements in this work.

The Néel temperature, T_N = 71.48 ± 0.04 K, were computed by fitting the Magnetization vs temperature data from the elastic neutron scattering data measured at the q = (010) Bragg-peak by fitting the critical range to a power law relation. The critical range were estimated from plotting the data on double logarithmic plots, which should make the critical range appear linear. The power law fits to the magnetization in addition to the intensity and width of the critical scattering occurring close to T_N  yielded their corresponding critical exponents. These are β = 0.179 ± 0.002 associated with the magnetization, γ = 1.063 ± 0.002 and γ = 1.12 ± 0.02 from the intensity of the critical scattering above and below T_N respectively, and ν = 4.1 ± 1.0 and  ν = 6.3 ± 0.2associated with the width of the critical scattering again above and below T_N respectively.

The Néel temperature and the values of the critical exponents match those reported in the literature quite well, except for which is much too high. The critical exponents does not match any known universality class and as such support the notion that a new universality class of triangular frustrated systems is needed.

The inelastic neutron scattering data revealed two visually distinct magnon modes at hω = 2.3 meV and hω = 5.4 meV respectively at base temperature. As the temperature increases the excitation gab of the lower magnon mode decreases until it reaches 0 meV at T_N . The temperature dependence of this excitation gab were fitted to a power law and its critical exponent, suspected of being similar to ß from the elastic measurements, gave a value of ß  = 0.28 ± 0.08.These two ß's are within two standard deviations of each other and are assumed similar given the data.

The inelastic measurements also yielded two novel critical exponents, ζ = 0.72 ± 0.07 and ρ = 0.45 ± 0.02, associated with the temperature dependence of the intensity and width of the quasi-elastic scattering at hω = 0 meV. To the best of the authors knowledge these have not been measured anywhere before. Their significance, if any, is unknown.

Finally a new method of estimating the critical range of the elastic data is proposed in this thesis. This method is based on systemically varying the temperature range of the supposed critical region and then computing the values of T_N and/or ß for each range. Based on the analysis in this thesis the values one obtains becomes constant within the correct critical range and when plotted as done here seems to reach a plateau, indicating that the true critical range has been found.