Structural and magnetic properties of hydrothermally synthesized β-MnO2 and α-KxMnO2 nanorods

Hydrothermal synthesis was applied for obtaining different manganese dioxide polymorphs whose morphology, structure and magnetic properties were thoroughly investigated. Variations in the synthesis pathways lead to the formation of tetragonal rutile-type β-MnO2 and two tetragonal hollandite-type α-KxMnO2 samples with different concentration of potassium cations (x = 0.15 and 0.18). SEM images showed nanorod shape of all specimens, while microstructure characterization done by XRD measurements revealed that crystallites have an elongated rod-like shape and, hence, it confirmed the anisotropic morphology. Magnetic ordering of β-MnO2 is antiferromagnetic below 93 K, while both α-KxMnO2 samples possess more complicated low-temperature behaviour of the reentrant spin-glass type: the onset of the spin-glass like state occurs at temperatures below the weak ferromagnetic ordering.