Title : Hydrothermally synthesized novel delta-MnO2 nanowhiskers: Microstructure and magnetic properties
Abstract:
We will discuss about one of the novel 2D monoclinic, P63/mnm, δ-MnO2 phase which were synthesized in the form of nanowhiskers through a simple and facile hydrothermal route under the optimized condition without using any template. The X-ray diffraction pattern showed the formation of the δ phase of MnO2 which was further confirmed from Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. Transmission electron micrograph revealed nanowhiskers of diameter ~7 nm and high- resolution TEM and SAED pattern demonstrated the interplanar spacing and distinguished diffraction rings corresponding to the monoclinic phase of δ-MnO2. Temperature dependent magnetization showed magnetic transition to paramagnetic phase at 13.2K which decreased to lower temperature with increasing the applied field. Temperature dependent susceptibility after fitted with Curie-Weiss law confirmed the strong antiferromagnetic ordering and high effective magnetic moment than that of Mn4+ present in δ-MnO2. A large effective magnetic moment is attributed to the presence of both Mn3+ and Mn4+ as confirmed from XPS. Reduced valency of Mn from 4+ to 3+ is accompanied with oxygen vacancies giving the exact composition as MnO1.58. The dynamic magnetic properties of δ-MnO2 nanowhiskers investigated using frequencydependent ac susceptibility fitted with various phenomenological models like Vogel-Fulcher law and power law clearly indicates the existence of the interacting spin clusters which freezes at ~ 11.2 K. Time dependence of thermoremanent magnetization, fitted well with stretched exponential function, supports the existence of relaxing spin clusters. Thus, spin glass relaxation in δ-MnO2 nanowhiskers was attributed to the interaction between Mn4+ and Mn3+ which results in an intrinsic magnetic frustration and weak ferromagnetism with finite coercivity below Tf.
Audience take-away:
- Can use the unique synthesis technique.
- Synthesize different dimension materials like nanowires of various aspect ratio, nanoparticles of various sizes and quantum dots.
- Vary the shapes from spherical to non-spherical.
- Magnetic properties can vary from super-paramagnetic to ferromagnetic depending on its size and shape.
- In job requirement if anything required for varying the size of particle from nanosize to micron size then this method can be used.
- Certainly, they can use this unique technique for other materials as well as we do in our laboratory.
- Many unusual properties one could obtain through this technique for example we observe coercivity of ~4-6 kOe in an antiferromagnetic pseudocubic shape hematitite particles of different sizes.
