  The study particularly focuses on the effect of the Cr co-doping on the
structural, optical and magnetic properties of Zn0.99Fe0.01O.
Zn0.99-xFe0.01CrxO (x =0, 0.01, 0.03 and 0.05) nanoparticles. Powder x-ray
diffraction (XRD) analysis confirms all the samples have hexagonal wurtzite
structures. The average crystallite size (D) and microstrain of the samples
were calculated using the Williamson-Hall relation and D was found to be 13 +-
1 nm for Zn0.99Fe0.01O. In Cr co-doped samples, D increase slightly with Cr
content and are given by 9 +- 1, 10 +- 1, and 11 +- 1 nm for the samples with x
= 0.01, 0.03 and 0.05, respectively; while the strain decreased with increase
in Cr co-doping. Transmission electron microscopy (TEM) images of the samples
indicate that particles are in the nano-regime and agglomerated. Particle sizes
are found to be 14 +- 1, 11 +- 1 and 12 +- 1 nm for Zn0.99-xFe0.01CrxO with x =
0, 0.03 and 0.05, respectively. The substitution of Fe3+ and Cr3+ at Zn2+ sites
has an impact on the optical properties. The band-gap decreases from 3.296 +-
0.002 eV to 3.258 +- 0.002 eV with increase of Cr concentration.
Photoluminescence (PL) of Zn0.96Fe0.01Cr0.03O revealed the presence of defects,
the emission peaks at 410 nm and 513 nm are attributed to Zn vacancies (VZn)
and singly ionized oxygen vacancies, respectively. The M(-H) curves of
Zn0.99-xFe0.01CrxO (x = 0.03 and 0.05) measured at room temperature using a
vibrating sample magnetometer (VSM) are found to be hysteretic, signifying room
temperature ferromagnetism (RTFM). Maximum saturation magnetization, 0.67 +-
0.01 emu.g-1, is observed in Zn0.94Fe0.01Cr0.05O. The observed RTFM in Fe+Cr
co-doped ZnO is explained by of bound magnetic polaron (BMP) mechanism, the
BMPs are formed by VZn and Vo^+ defects. This paper enhances the understanding
of the origin of RTFM in Fe+Cr co-doped ZnO nanoparticles.
