Transition Metal Dopants Essential for Producing Ferromagnetism in Metal Oxide Nanoparticles

Recent claims that ferromagnetism can be produced in nanoparticles of metal oxides without the presence of transition metal dopants have been challenged in this work by investigating 62 high quality well-characterized nanoparticle samples of both undoped and Fe doped (0-10% Fe) ZnO. The undoped ZnO nanoparticles showed zero or negligible magnetization, without any dependence on the nanoparticle size. However, chemically synthesized Zn _1-x Fe _x O nanoparticles showed clear ferromagnetism, varying systematically with Fe concentration. Furthermore, the magnetic properties of Zn _1-x Fe _x O nanoparticles showed strong dependence on the reaction media used to prepare the samples. The zeta potentials of the Zn _1-x Fe _x O nanoparticles prepared using different reaction media were significantly different, indicating strong differences in the surface structure. Electron paramagnetic resonance studies indicate that the difference in the ferromagnetic properties of Zn _1-x Fe _x O nanoparticles with different surface structures originates from differences in the fraction of the doped Fe ions that participate in ferromagnetic resonance.