The initial interesting resultssuch as ferromagnetism with a critical temperaturebelow or above room temperature,118 antiferromagnetism, 119 or paramagnetism120 have been reported in cobalt-dopedZnO nanorods. The defects in ZnO nanorods such as oxygen vacancies, zinc vacancies, zincinterstitials and defect complexes played an important role in influencingmagnetic behavior. However, the underlying mechanism for ferromagnetism in ZnOnanostructures is still in debate and various mechanisms have been proposed.121–124 The first probability is theformation of a secondary phase such as cobalt oxide (CoO), but this can beeasily ruled out, because bulk CoO is antiferromagnetic in nature with a Neeltemperature of 291 K,125 and in the nanocrystallineform, CoO shows weak ferromagnetic or superparamagnetic behavior at lowtemperatures.126 Another possibility is thepresence of cobalt metal because cobaltis a well known ferromagnetic material.
However, no cobalt metal phase wasobserved in the XRD patterns and HRTEM images. According to the Ruderman-Kittel-Kasuya-Yosida(RKKY) theory,127,128 the magnetism is due to anexchange interaction between local spin-polarized electrons such as theelectrons of Co++ ions and conductive electrons. This interaction gives rise to spin polarization of conductiveelectrons. Consequently, the spin-polarizedconductive electrons take part in an exchange interaction with other local spin-polarizedelectrons of Co++ ions, since the conductive electrons serve as amedium to make contact with all Co++ ions. Thus, after thelong-range exchange interaction, almost all Co++ ions exhibit thesame spin direction, and as a result, the material exhibits ferromagnetism.18 Field-dependent magnetization(M–H) curve analysis by Chanda et al.
58 exhibits the diamagneticbehavior of un-doped ZnO at 2 K and room temperature while cobalt-doped ZnOsamples show coexistence of superparamagnetic and ferromagnetic behavior atroom temperature and 2 K temperature. They explained the observedferromagnetism may have originated due to the exchange interaction between thelocalized d-electrons in Co++ atoms and free charge carriersgenerated due to cobalt-doping as well as due to cobalt clustering in the dopedsamples. Pal et al.129 found the magnetic moment of1.83 emu/g for the 2T field, the coercivity of 53 G and retentivity of 160 memu/g for the 7% cobalt-doped ZnOnanorods, measured at room temperature.
Itslow coercive field indicates the soft ferromagnetic nature of the ZnO nanorods.Azam et al.18 observed magnetization valueincreased with an increase of cobalt concentration, which showed that cobalt-dopedZnO nanorods exhibit a ferromagnetic behavior. Hao et al.40 observed that when the cobaltconcentration in ZnO nanostructures increased up to 8 mol%, the doped samplesshow linear magnetization curves with no hysteresis visible within the appliedfield, which can be identified as paramagnetism. Wang et al.
130 argued that hidden secondaryphases such as ZnyCo3-yO4 (0?y?1) were alsoclearly detected by the micro-Raman spectroscopic technique in cobalt-doped ZnO nanorods. They proposed thatthe predominant diffusion-limited Ostwald ripening crystal growth mechanismunder the hydrothermal process yielded such hidden phase segregation. Therefore,the origin of the ferromagnetism is probably due to the presence of the mixedcation valence of Co via a d–d double-exchange mechanism rather than the real dopingeffect of cobalt.