Structure and magnetic properties of Co-doped ZnO nanoparticles

Abstract

In this work we carefully analyze the role of the microstructure on the magnetic properties of Co-doped ZnO nanoparticles prepared by the vaporization-condensation method in a solar reactor. We show that a close correlation exists between microstructural features and the appearance of ferromagnetism. Both shape and size of the particles, as well as the microstructure, can be controlled by changing the pressure inside the evaporation chamber, as evidenced by transmission electron microscopy micrographs and high resolution electron microscopy ͑HREM͒. X-ray diffraction patterns and HREM make evident the absence of any significant Co segregation or any other phase different from würtzite type ZnO. On the other hand, electron energy loss spectroscopy analyses performed on several particles of würtzite type ZnO yielded an average Co concentration in good agreement with the nominal composition. Samples prepared in low pressure ͑Ϸ10 Torr͒ exhibit a very homogeneous microstructure and are ferromagnetic at low temperature but they have a very small saturation moment, well below that expected for a Co 2+ ion. Conversely, samples prepared at higher pressure conditions ͑Ϸ70-100 Torr͒ show a defective microstructure and are paramagnetic and increasing the Co content does not induce ferromagnetism.

Ferromagnetism in Fe-doped ZnO nanocrystals: Experiment and theory

“…The M vs T curves obtained in the present study have a basic difference in their nature from superparamagnetism-dominated curves in Ref. 43 for Co-doped ZnO nanoparticles and from those in Ref. 53 for highly reduced Co-doped TiO 2−␦ thin films, where the FC and ZFC curves diverge substantially at low temperatures.…”

“…With the gradual increase of the applied field value, the meeting point of the ZFC and FC curves shifts toward lower temperature and at a sufficiently high field, there will be absolutely no irreversibility. Figure 4͑a͒ indicates that the system will be weakly ferromagnetic even at room temperature, unlike the other two recent studies, 43,44 where the superparamagnetism is the pre- ͑c͒ Highresolution TEM micrograph for the same showing the singlecrystalline nature of each nanoparticle. ͑d͒ Nanometric particle size distribution of the same sample fitted with a log-normal distribution function, from which the estimated average particle size is ϳ7 nm.…”

“…Earlier studies on Co-doped ZnO nanoparticles prepared by the vaporization condensation method 43 and on Mn-doped ZnO nanoparticles prepared by the coprecipitation technique 44 have observed a ferromagnetic behavior at low temperature and superparamagnetic behavior at high temperature. For some of the samples, depending on the preparation procedure, Curie-Weiss behavior is observed, confirming an antiferromagnetic ground state.…”

“…For superparamagnetic systems, as is well known, the curves merge into a single one. 43,55 This rules out the possibility of thermal-energyinduced spontaneous magnetization reversal as happens in the case of an assembly of single-domain particles leading to superparamagnetic behavior. More convincingly, the superparamagnetic limit for Fe-doped systems can be calculated by the expression for the relaxation time = 0 exp͑E A V / k B T͒, where the anisotropy energy density E A has a value of 5 ϫ 10 4 J/m 3 for Fe, and V and k B are the particle volume and Boltzmann constant, respectively.…”

Superparamagnetic behavior of MOCVD grown ZnO:Co films

“…In particular ZnO and ZnO:Сo materials reveal weak ferromagnetic-like behavior persisting up to room temperatures [1][2]. This behavior remains the subject of discussion until now due to irregular dependence of magnetic properties on preparation method and on concentration of cobalt [1][2][3][4][5][6][7]. This irregularity points to the significant role of paramagnetic defects in the magnetic properties of undoped and Co-doped ZnO.…”