Nuclear magnetic resonance (NMR) measurements provide a detailed insight into the magnetic ordering of a material at the microscopic level, which can also be applied to the cobalt system BaCo$_2$(AsO$_4$)$_2$ (BCAO). In recent years, this system has attracted significant attention, as theoretical predictions suggested that it could realize the renowned Kitaev spin model. However, recent studies indicate that BCAO is more likely a realization of an anisotropic Heisenberg XXZ model with an additional antiferromagnetic interaction between the third-nearest-neighboring cobalt magnetic ions. The crystal field and strong spin-orbit coupling induce anisotropic exchange interactions among the effective Co$^{2+}$ spins, which form two-dimensional honeycomb layers. In the phase diagram, this system not only hosts various magnetically ordered states but also supports a quantum spin liquid. Magnetically ordered states in BCAO appear below 5.3 K and in external magnetic fields under 0.5 T, as confirmed experimentally. This master’s thesis focuses on the magnetic phase occuring at zero external magnetic field upon cooling. Using the NMR method, I observed transitions between nuclear energy states, which are split due to the presence of nuclear quadrupolar and hyperfine interactions.
For precise NMR measurements, correctly tuning the probe to the resonant frequency is crucial, as this allows for efficient excitation of transitions between nuclear energy levels in the material. The electrical circuit of the probe contains three capacitors whose capacitance can be adjusted by rotating their axes, thereby setting the resonant frequency of the entire resonant circuit, which, together with the capacitors, is formed by the coil containing the sample. To simplify and accelerate this procedure, I developed a dedicated unit that provides rapid and precise adjustment of the desired resonant frequency through a user interface. The capacitance adjustment is carried out using stepper motors controlled via a microcontroller board, which rotate the capacitors to the required values.
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