TiO2 nanowires grown from nanoparticles: structure and charge density study

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.

Role of Ni substitution on structural, magnetic and electronic properties of epitaxial CoCr2O4 spinel thin films

Cubic spinel CoCr2O4 has recently attained attention due to its multiferroic properties. However, the Co site substitution effect on the structural and magnetic properties has rarely been studied in thin film form. In this work, the structural and magnetic properties of Co1-x Ni x Cr2O4 (x= 0, 0.5) epitaxial thin films deposited on MgAl2O4 (100) and MgO (100) substrates to manipulate the nature of strain in the films using pulsed laser deposition (PLD) technique are presented. The epitaxial nature of the films was manifested through x-ray diffraction (XRD), reciprocal space mapping (RSM) and Rutherford backscattering spectrometry (RBS) measurements. Raman measurements revealed a disappearance of characteristic A 1 g and F 2 g modes of the CoCr2O4 with increase in the Ni content. Atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM) studies show a modification of the surface morphology upon Ni substitution. Magnetic measurements disclose that the ferrimagnetic Curie temperature (T C) of the CoCr2O4 in thin film grown on MgAl2O4 (100) and MgO (100) substrates were found to be 100.6 ± 0.5 K and 93.8 ± 0.2 K, respectively. With Ni substitution the T C values were found to be enhanced to 104.5 ± 0.4 K for MgAl2O4 (100) and 108.5 ± 0.6 K for MgO (100) substrates. X-ray photoelectron spectroscopy (XPS) suggests Cr3+ oxidation states in the films, while Co ions are present in a mixed Co2+/Co3+ oxidation state. The substitution of Ni at Co site significantly modifies the line shape of the core level as well as the valence band. Ni ions are also found to be in a mixed 2+/3+ oxidation state. O 1s core level display asymmetry related to possible defects like oxygen vacancies in the films.