Spin dynamics of hydrothermally synthesized δ-MnO₂ nanowhiskers

We have reported novel 2D monoclinic, P6₃/mnm, δ-MnO₂ nanowhiskers synthesized through a simple and facile hydrothermal route under optimized conditions without using any template. The X-ray diffraction pattern shows the formation of the δ phase of MnO₂, which is further confirmed by Fourier transform infrared (FT-IR) spectroscopy and Raman spectroscopy. The transmission electron micrograph revealed nanowhiskers having a diameter of ∼7 nm and the high-resolution TEM and SAED patterns demonstrated the interplanar spacing and distinguished diffraction rings corresponding to the monoclinic phase of δ-MnO₂. Fitting the temperature-dependent susceptibility with the Curie-Weiss law confirms the strong antiferromagnetic ordering and high effective magnetic moment of Mn⁴⁺ present in δ-MnO₂. The large effective magnetic moment is attributed to the presence of both Mn³⁺ and Mn⁴⁺, as confirmed by XPS. The reduced valency of Mn from 4 to 3 is accompanied with oxygen vacancies, affording the exact composition of MnO_1.58. The dynamic magnetic properties of the δ-MnO₂ nanowhiskers were investigated using the frequency-dependent AC susceptibility fitted with various phenomenological models like the Vogel-Fulcher law and power law, indicating the existence of interacting spin clusters, which could freeze at ∼11.2 K. The time dependence of thermoremanent magnetization fitted well with a stretched exponential function, supporting the existence of relaxing spin clusters. Thus, the spin glass relaxation in the δ-MnO₂ nanowhiskers is attributed to the interaction between Mn⁴⁺ and Mn³⁺, which results in intrinsic magnetic frustration and weak ferromagnetism with finite coercivity below T_f.