A novel technique was used for the synthesis of manganese oxide nanocrystal by applying an externalmagnetic field (EMF) on the precursor solution before sonicationwith ultrasonic bath. The resultswere 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 (tMF, 2 h and 24 h). Then, the magnetized solution was irradiated indirectly by ultrasonic bath in basic and neutral media. One experimentwas designed for the effect of oxygen atmosphere in the case ofmagnetic 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 (tMF, 24 h) led mainly to a mixture of Mn3O4 (hausmannite) and c-MnOOH (manganite) and sonication of unmagnetized solution led to a pureMn3O4. Inpoint 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 differencewas observed on the composition of the products between magnetic treated and untreated solution. For these samples, themagneticmeasurements as a function of temperature were exhibited a reduction in ferrimagnetic temperature to Tc = 39 K, and 40 K with and without magnetic treatment, respectively. The ferrimagnetic temperature was reported for the bulk at Tc = 43 K. A superparamagnetic behavior was observed at room temperature without any saturation magnetization and hysteresis in the measured field strength. The effect ofEMF on the sample prepared in the basicmediumwas negligible but, in the case of neutralmedium, the EMF affected the slope of themagnetization curves. Themagnetization at roomtemperature washigher for the samples obtained in neutralmediumwithoutmagnetic treatment. In addition, a horizontal shift loop was observed in neutral medium at low temperature.

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