Probing The Local Properties of Co-Honeycomb Magnets as an Avenue to Understanding Their Complex Magnetic Behaviour

In the field of frustrated magnetism, the relationship between the crystal structure of a material and its magnetic properties has been a fruitful area of research for decades. A problem that has tormented materials scientists for an equal length of time, is the effect of small structural perturbations on the overall magnetic behaviour of a sample. An example of the interconnectedness of disorder sensitivity with rich magnetic phase spaces can be seen in well-studied examples of honeycomb magnets like ��-RuCl₃, where large differences in its physical characteristics emerge from subtle variations in stacking. A less well-understood example of this type of phenomena can be seen in the material BaCo₂(PO₄)₂ where the Co²⁺ ions form a highly frustrated, undistorted honeycomb lattice of Co²⁺ that are well separated by Ba and PO₄ spacer layers, hence this material is highly 2-dimensional. Neutron diffraction of BaCo₂(PO₄)₂ reveals not one, but two distinct short range ordered magnetic states that develop over two successive transition temperatures. In contrast, BaCo₂(AsO₄)₂, which is isostructural to the phosphate, long range orders into a single well-defined magnetic ordered state at low temperatures. Muon spin relaxation (μSR) is a local probe that is uniquely suited to the investigation of multiple coexisting, competing magnetic phases. A comprehensive μSR study has been performed on both the phosphate, and the arsenate for comparison, to determine how these competing magnetic phases develop with changing temperature across the temperature range of interest. The local structure of the material has also been investigated using total x-ray scattering techniques in an attempt to elucidate the origin of the unique magnetic behaviour of BaCo₂(PO₄)₂. The combined evidence from these local proves leads us to conclude that this phase coexistence in BaCo₂(PO₄)₂is likely a result of synthesis-sensitive polymorphism.