Variation of structural and hyperfine parameters in nanoparticles
of Cr-substituted Co-Zn ferrites
Ram Kripal Sharma,
1
Varkey Sebastian,
1,2
N. Lakshmi,
1
K. Venugopalan,
1
V. Raghavendra Reddy,
3
and Ajay Gupta
3
1
Department of Physics, Mohanlal Sukhadia University, Udaipur 313001, India
2
Department of Physics, Nirmalagiri College, Kerala 670701, India
3
UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452017, India
Received 28 November 2006; published 19 April 2007
The effect of Cr substitution into nanocrystalline Co-Zn ferrite prepared by the chemical coprecipitation
method has been studied. Mössbauer studies at a temperature 20 K well below the blocking temperatures of
the samples show that Cr goes preferentially into the octahedral B site and that the hyperfine fields at both A
and B sites decrease with increase in Cr concentration. Based on the cation distribution obtained from fitting
Mössbauer spectra, structural parameters such as lattice parameters, site bond and edge lengths, and the oxygen
parameter u have been calculated. The trend of theoretically calculated lattice parameter with Cr content
matches well with the experimentally obtained values.
DOI: 10.1103/PhysRevB.75.144419 PACS numbers: 75.75.+a, 81.16.Be
I. INTRODUCTION
Nanocrystalline ferrite systems have been the topic of in-
tense research in recent years due to the wide possibilities of
use in technological applications that require miniaturization
as in high-density data storage. In ferrites, the magnetic
properties are mainly dependent on the type of metal ions
and their distribution between the tetrahedral A and octahe-
dral B sites.
1
Hence, the tuning of magnetic properties re-
quires a thorough understanding of the influence of prepara-
tion methods, substitution, and thermal treatments on the
cation distribution. In ferrites of the same composition, the
physical properties are also size dependent, since the proper-
ties of nanosystems are largely determined by interparticle
spacing and surface-to-volume ratio.
2
The mixed spinel Co
0.5
Zn
0.5
Fe
2
O
4
is well suited for the
study of size effects, since Co and Zn have strong prefer-
ences for the octahedral and tetrahedral sites, respectively.
According to these preferences, the cation distribution
should be Zn
0.5
Fe
0.5
Co
0.5
Fe
1.5
O
4
. However, in nanopar-
ticles, various effects arising due to small particle sizes
3
may
result in a different distribution. The spinel structure also
allows introduction of different metallic ions, which can
change the magnetic and electrical properties considerably.
4,5
Magnetic properties in these materials such as magnetic an-
isotropy, coercivity, appearance of superparamagnetism, and
variation in hyperfine fields can be tailored by simple substi-
tution of other magnetic and/or nonmagnetic ions.
6
A study of the hyperfine fields gives valuable information
about the Fe site occupancy and strength of interactions,
which can be used to estimate the cation distribution, thus
enabling a comparative study of the differences between bulk
system and nanosystem of the same nominal composition.
Recently, we had reported the magnetic properties of a bulk
Cr
x
Co
0.5-x
Zn
0.5
Fe
2
O
4
system prepared by the chemical co-
precipitation method and studied using Mössbauer effect.
7
In
the present study, we report the effect of particle size on the
properties of a nanosystem of the same composition prepared
under identical conditions. At room temperature, this nano-
system is superparamagnetic.
8
To compare the magnetic hy-
perfine fields at Fe sites in the nanosized particles to that in
the bulk, Mössbauer measurements have been made at a tem-
perature much less than the blocking temperature.
II. EXPERIMENT
Nanoparticles of Cr
x
Co
0.5-x
Zn
0.5
Fe
2
O
4
were synthesized
by a chemical coprecipitation method.
8
The precipitate was
dried at 373 K for 24 h and subsequently annealed at 573 K
for 24 h to obtain the nanosamples. To confirm the stoichi-
ometry of the samples, energy dispersive x-ray EDX analy-
sis was done on all the samples. EDX spectra show that the
stoichiometry of the samples is within ±2% error of the de-
sired ratio. X-ray diffractograms XRD were obtained using
a Rigaku Miniflex x-ray diffractometer 30 kV, 15 mA, Ni
filter with Cu K radiation. The average crystallite size d
was calculated from the broadening of the most intense 311
peak in the XRD spectrum. Correction for instrumental
broadening was done using the expression
= B - b . 1
Here, B is the full width at half maximum FWHM obtained
by fitting a Lorentzian Cauchy profile to individual XRD
peak and b is the instrumental broadening determined from
an x-ray pattern of standard silicon sample used for instru-
mental calibration. The average crystallite size was obtained
using the Scherrer equation t = 0.9 / cos where t is the
crystallite size, is the x-ray wavelength, and is the posi-
tion of the 311 peak in the XRD. Lattice parameter a
0
was
obtained using the powder–diffraction package
9
PDP 1.1
program. Using the INDEX subroutine available in the pro-
gram, the spectrum was checked to confirm the cubic struc-
ture. Appropriate value of s = h
2
+ k
2
+ l
2
was determined, and
the lattice parameter calculated using the equation
sin
2
s
=
2
4a
0
2
. 2
Mössbauer measurements were made with a 25 mCi Co
57
Rh source at 20 K. Velocity calibration was done using
PHYSICAL REVIEW B 75, 144419 2007
1098-0121/2007/7514/1444196 ©2007 The American Physical Society 144419-1