The present invention provides the synthetic method and superparamagnetic iron oxide nanoparticles capable of targeting to the folic acid receptors existing on the cell membranes and with high relaxivity. The phasefrequency detector may include charge pump that charges a capacitor with a current representative of the phasefrequency difference between an input data signal and the clock signal produced by the loops clock divider. Further, the multi-loop oscillator further comprises a supply voltage detection circuit section for detecting variation of the supply voltage, and the supply voltage detection circuit section controls an operation of the loop selection section.