A study of magnetic liquids with the help of a paramagnetic sensor

The paramagnetic sensor method was used to study local magnetic fields in a magnetite aqueous suspension. The sensor was 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl, a stable nitroxide radical. The lines in the EPR spectrum of the sensor were demonstrated to be broadened due to the dipole-dipole interaction with magnetite nanoparticles. It was established that no spin exchange occurred between sensor molecules and magnetite nanoparticles. The g-factor was found to decrease with the concentration of magnetite nanoparticles in the suspension. The mean strengths of the local magnetic fields calculated from changes in the EPR spectrum of the sensor proved to be substantially lower than the values determined from magnetic measurements. This difference was accounted for by the formation of linear aggregates of magnetite nanoparticles under the action of a magnetic field.