Master thesis defense by Rasmus Tang

In this work, I study a series of Al(OH)₃-based layered double hydroxides (LDHs) with Cu²⁺, Ni²⁺ and Co²⁺ as cations. They respectively carry S = 1/2, S = 1 and S = 3/2, which allows me to study the influence of different spin quantum number on otherwise isomorph compounds. Furthermore, the cations are organised in chains, which makes them inherently interesting to study from a quantum mechanical point of view, as quantum effects are more pronounced in the low-dimensional case.

I studied their magnetic properties, which were previously unexplored, through bulk susceptibility and heat capacity measurements. They clearly revealed that none of the three variants order magnetically at T ≥ 2 K. Modelling of the experimental data predicted that Cu-LDH is a ferromagnetic S = 1/2 spin chain with J = 1.4(3) K, and that Co-LDH is mainly governed by a strong zero-field splitting of size D = 138⁺⁵₋₃ K with a weak antiferromagnetic exchange of J ≈ −0.1 K. The predictions on Ni-LDH were less clear cut. I was able to reproduce the data both with pure ZFS and pure exchange models, which respectively yielded D = −24 to −8 K and J = 1 to 4 K depending on the model.

To study Ni-LDH more in-depth, neutron diffraction and spectroscopy experiments were carried out. Diffraction confirmed the absence of magnetic order down to T = 1.6 K. In addition, I found no signs of emerging order in the form of magnetic critical scattering, which tells than an eventual ordering temperature will be much lower than T = 1.6 K. Spectroscopy found a substantial zero-field splitting of D = 9.59(2) K; a value somewhat different from the modelling prediction. Thus, further modelling of Ni-LDHs properties and more experiments are necessary for unravelling it’s true nature.