Structure-magnetic property relations in FeNbO4 polymorphs: A spin glass perspective

Spin glass state originating from the magnetic frustration due to the geometric arrangement or cation disorder is an interesting topic of research. FeNbO₄, exhibiting multifarious applications, crystallizes mainly in three different polymorphic forms with cation ordered and disordered structures. Despite their antiferromagnetic nature, the orthorhombic (o-FeNbO₄) and monoclinic FeNbO₄ (m-FeNbO₄) polymorphs exhibit a difference in their magnetic properties at low temperatures. Here, we report our observation of spin glass behaviour of o-FeNbO₄ with a cation disordered structure. Our work is a combined experimental and theoretical study of structure-magnetic property relations of the antiferromagnetic o- and m-FeNbO₄ with the Néel temperatures of 30 and 46 K, respectively. o-FeNbO₄ contrasted itself from m-FeNbO₄ as a spin glass by exhibiting field-dependent bifurcation in ZFC and FC magnetization, frequency-dependent AC susceptibility, memory effect, thermoremanence, and anamoly in the heat capacity. The presence of antiphase domains and boundaries due to cation order/disorder in both the structural polymorphs was evidenced from the electron diffraction analyses that account for the observed low temperature magnetic interactions. Further, modeling the structures with varying amounts of cation disorder using first principles calculations revealed the structural stability and competing spin interactions that support our experimentally observed spin glass behaviour of o-FeNbO₄.