The 2T field, the coercivity of 53 G

The initial interesting results
such as ferromagnetism with a critical temperature
below or above room temperature,118 antiferromagnetism, 119 or paramagnetism120 have been reported in cobalt-doped
ZnO nanorods. The defects in ZnO nanorods such as oxygen vacancies, zinc vacancies, zinc
interstitials and defect complexes played an important role in influencing
magnetic behavior. However, the underlying mechanism for ferromagnetism in ZnO
nanostructures is still in debate and various mechanisms have been proposed.121–124 The first probability is the
formation of a secondary phase such as cobalt oxide (CoO), but this can be
easily ruled out, because bulk CoO is antiferromagnetic in nature with a Neel
temperature of 291 K,125 and in the nanocrystalline
form, CoO shows weak ferromagnetic or superparamagnetic behavior at low
temperatures.126 Another possibility is the
presence of cobalt metal because cobalt
is a well known ferromagnetic material. However, no cobalt metal phase was
observed in the XRD patterns and HRTEM images. According to the Ruderman-Kittel-Kasuya-Yosida
(RKKY) theory,127,128 the magnetism is due to an
exchange interaction between local spin-polarized electrons such as the
electrons of Co++ ions and conductive electrons. This interaction gives rise to spin polarization of conductive
electrons. Consequently, the spin-polarized
conductive electrons take part in an exchange interaction with other local spin-polarized
electrons of Co++ ions, since the conductive electrons serve as a
medium to make contact with all Co++ ions. Thus, after the
long-range exchange interaction, almost all Co++ ions exhibit the
same 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 diamagnetic
behavior of un-doped ZnO at 2 K and room temperature while cobalt-doped ZnO
samples show coexistence of superparamagnetic and ferromagnetic behavior at
room temperature and 2 K temperature. They explained the observed
ferromagnetism may have originated due to the exchange interaction between the
localized d-electrons in Co++ atoms and free charge carriers
generated due to cobalt-doping as well as due to cobalt clustering in the doped
samples. Pal et al.129 found the magnetic moment of
1.83 emu/g for the 2T field, the coercivity of 53 G and retentivity of 160 m
emu/g for the 7% cobalt-doped ZnO
nanorods, measured at room temperature. Its
low coercive field indicates the soft ferromagnetic nature of the ZnO nanorods.
Azam et al.18 observed magnetization value
increased with an increase of cobalt concentration, which showed that cobalt-doped
ZnO nanorods exhibit a ferromagnetic behavior.


Hao et al.40 observed that when the cobalt
concentration in ZnO nanostructures increased up to 8 mol%, the doped samples
show linear magnetization curves with no hysteresis visible within the applied
field, which can be identified as paramagnetism. Wang et al.130 argued that hidden secondary
phases such as ZnyCo3-yO4 (0?y?1) were also
clearly detected by the micro-Raman spectroscopic technique in cobalt-doped ZnO nanorods. They proposed that
the predominant diffusion-limited Ostwald ripening crystal growth mechanism
under the hydrothermal process yielded such hidden phase segregation. Therefore,
the origin of the ferromagnetism is probably due to the presence of the mixed
cation valence of Co via a d–d double-exchange mechanism rather than the real doping
effect of cobalt.