Magnetic properties of nanocrystalline KNbO₃

Newly synthesized undoped and iron-doped nanoscale powders of KNbO₃ are investigated using magnetic resonance and static magnetization methods in order to determine how the crystal size and doping affect the structure of magnetic defects and material properties. Although the bulk crystals of KNbO₃ are nonmagnetic, the undoped KNbO₃ powder with average particle size of 80 nm exhibits magnetic properties. The ferromagnetic resonance signal and the magnetization curve registered on the powder are thoroughly analyzed. It is concluded that the appearance of the defect driven ferromagnetism in the undoped powder is due to the nano-size of the particles. This effect disappears in the iron-doped KNbO₃ powder with particle sizes above 300 nm. In case of low doping (<1 mol. % Fe), a new electron paramagnetic resonance signal with g_eff = 4.21 is found out in the KNbO₃:Fe powder. Such a signal has not been observed in the bulk crystals of KNbO₃:Fe. We suppose that this signal corresponds to individual paramagnetic Fe³⁺ ions having rhombic symmetry.