Synthesis of manganese oxide nanocrystal by ultrasonic bath: effect of external magnetic field

A novel technique was used for the synthesis of manganese oxide nanocrystal by applying an external magnetic field (EMF) on the precursor solution before sonication with ultrasonic bath. The results were compared in the presence and absence of EMF. Manganese acetate solution as precursor was circulated by a pump at constant speed (7 rpm, equal to flow rate of 51.5 mL/min) in an EMF with intensity of 0.38 T in two exposure times (t_MF, 2 h and 24 h). Then, the magnetized solution was irradiated indirectly by ultrasonic bath in basic and neutral media. One experiment was designed for the effect of oxygen atmosphere in the case of magnetic treated solution in neutral medium. The as prepared samples were characterized with X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (HRTEM, TEM), energy-dispersive spectrum (EDS), and superconducting quantum interference device (SQUID) analysis. In neutral medium, the sonication of magnetized solution (t_MF, 24 h) led mainly to a mixture of Mn₃O₄ (hausmannite) and γ-MnOOH (manganite) and sonication of unmagnetized solution led to a pure Mn₃O₄. In point of particle size, the larger and smaller size of nanoparticles was obtained with and without magnetic treatment, respectively. In addition, the EMF was retarded the nucleation process, accelerated the growth of the crystal, and increased the amount of rod-like structure especially in oxygen atmosphere. In basic medium, a difference was observed on the composition of the products between magnetic treated and untreated solution. For these samples, the magnetic measurements as a function of temperature were exhibited a reduction in ferrimagnetic temperature to T_c = 39 K, and 40 K with and without magnetic treatment, respectively. The ferrimagnetic temperature was reported for the bulk at T_c = 43 K. A superparamagnetic behavior was observed at room temperature without any saturation magnetization and hysteresis in the measured field strength. The effect of EMF on the sample prepared in the basic medium was negligible but, in the case of neutral medium, the EMF affected the slope of the magnetization curves. The magnetization at room temperature was higher for the samples obtained in neutral medium without magnetic treatment. In addition, a horizontal shift loop was observed in neutral medium at low temperature.