  The role of Cr incorporation into the ZnO were probed through investigations
into the structural, optical and magnetic properties. Zn1-xCrxO with x = 0,
0.01, 0.03 and 0.05, nanoparticles were prepared by solution combustion method.
Powder x-ray diffraction (XRD) results reveal, all the synthesized samples are
in single hexagonal wurtzite crystal structures, indicating that Cr3+ ions
substitute the Zn2+ ions without altering the structure. The crystallite size
and microstrain were calculated using the Willamson-Hall method and found to be
36 +- 2 nm for ZnO and it reduced with the increase of Cr dopant concentration
to 20 +- 2 nm for Zn0.95Cr0.05O. Transmission electron microscopy (TEM)
revealed that the particle size were 48 +- 2 nm, 29 +- 2 nm and 25 +- 2 nm for
the Zn1-xCrxO with x = 0, 0.03 and 0.05, respectively. TEM morphology indicated
particles are agglomerated in the doped samples. The band-gap decreases
slightly from 3.305 +- 0.003 eV to 3.292 +- 0.003 eV with increase of Cr
content from x = 0 to 0.05, respectively. Photoluminescence measurements
revealed the presence of defects in the samples, associated with zinc vacancies
and singly ionized oxygen vacancy. The field-dependent magnetization
measurements of ZnO and Cr-doped ZnO were carried out using a vibrating sample
magnetometer (VSM) at 300 K. All the samples exhibits ferromagnetic behavior.
This long-range ferromagnetism ordering observed in ZnO is explained based on
bound magnetic polaron (BMP) mechanism. The singly ionized oxygen vacancies
playing a crucial role in observed room temperature ferromagnetism (RTFM) in
ZnO. There is a sufficient amount of BMPs formed in Cr doped ZnO because of the
defects present in these samples. Therefore, the overlapping of BMPs results in
the RTFM. However, the antiferromagnetic coupling at a higher doping
concentration of Cr, weakens the observed RTFM.
