From strong to weak interaction: reconciling SQUID and μSQUID-EPR data in anomalous Co(ii) dimers

Magnetic molecules have been proposed as scaffolds for novel quantum technologies, ranging from quantum sensing and quantum memory to multilevel quantum bits (qudits) and fault-tolerant quantum computation. Integration of magnetic molecules into cutting-edge applications hinges on a deep understanding and tunability of their spin states. To date, the strategic manipulation of the local environment of the ion and careful selection of the magnetic core have enabled the desired tunability and scalability of the spin states. For such goals, however, extracting the anisotropic parameters that dictate the characteristics of the Spin Hamiltonian is challenging, especially for molecules consisting of multiple magnetic cores. We address these challenges by studying two cobalt(II) dinuclear systems, complicated by inherent spin–orbit coupling. We explore the magnetic properties of these systems in two temperature regimes: (i) at sub-Kelvin temperatures employing single crystals at 30 mK using a unique μSQUID-EPR technique that examines the microwave absorption peaks in the magnetisation data and their variation with field angle and frequency; and (ii) in bulk employing convectional SQUID magnetometry above 2 K i.e., χ_MT(T) and M(H). Unexpectedly, sub-Kelvin temperature investigations reveal a negligible interaction, whereas the SQUID data reveal a much stronger interaction between the Co(II) ions. An understanding of these data is developed based on a strong coupling model and the coupling of two moieties with a spin-effective ground state.