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Harmonic components of magnetic signals from ferrofluids excited by alternating magnetic fields are studied in view of applications to liquid sensing. The harmonic signals from ferrofluids of iron-oxide magnetic nanoparticles excited at the field strength of 100 mTp-p and the frequency range from 1510−6500 Hz are detected by a pickup coil and are processed with a lock-in amplifier. The harmonic signal exhibits a minimum in the frequency dependence at a frequency, which is correlated to the magnetic relaxation. The minimum frequency is decreased and the minimum value is increased with the increase of the ion concentration; the frequency dependence around the minimum frequency exhibits characteristic features depending on the ion concentration. The features are originated from polydispersity in the aggregation formed in the ferrofluids at higher ion concentration, and are different from the frequency characteristics depending on the viscosity. This magnetic relaxometry using the harmonic signals is useful for ion sensing in liquids without the influence of viscosity

Relationship between ion concentration of ferrofluid and response signals of magnetic nanoparticles against ac magnetic fields