Structural, optical, and magnetic properties of single-crystalline Mn3O4 nanowires

We report a simple and effective method to synthesize single-crystalline Mn3O4 nanowires using solvothermal technique. These nanowires with diameter in the range 15–40 nm grow uniformly along {2 0 0} direction and crystallize in tetragonal symmetry. A blue-shift in the absorption edge is observed when the synthesis temperature (TS) increases from 150 to 250 °C. The surface chemical analysis confirms the oxidation state of manganese as Mn3+. A relatively broad, intense, and symmetrical electron spin resonance (ESR) signal is detected at room temperature with the 'g' value 2. The relative intensity of ESR signal decreases and broadens with the rise of synthesis temperature due to the relaxation of surface anisotropy and induced magnetic field. The magnetization versus field measurements (M–H) at 300 and 5 K of Mn3O4 nanowires show hysteresis curves with large coercive field (HC) due to the large surface anisotropy. A step increase in the magnetization (ΔM = 1.7 emu/g) is noticed at H ∼±1.1 kOe in the M–H curves. Possible origin of such field-induced transition is discussed. The magnitude of magnetic moment, remanence magnetization, and HC values drastically reduces with increasing the TS due to the disassembly of nanowire arrays.