2
R. Padam et al. / Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
using commercial Netzsch DSC/TG machine (Model: STA449-
F3A00). Magnetization measurements as a function of tempera-
ture in the range of 2–320 K, and as a function of magnetic field up
to 50 kOe are performed using commercial SQUID-VSM (Model
5 Quantum design).
bonded to the Coþ2, Crþ3 and Alþ3 ions. We can see that above
600 °C the structure of the samples is stabilized. From the DSC
curve it can be seen that the evaporation of moisture and the or-
ganic solvents are exothermic in nature. But after the decom-
position of the citrates samples undergo endothermic process that
could be due to the formation of new bonds in the oxide network.
Temperature dependent magnetization (M–T) is measured
under the small applied field ( ≃ 10 Oe) to evaluate the actual
magnetic ordering temperature of the samples. Further, M–T curve
under low applied field is also useful to investigate magnetization
reversal in ferrimagnetic systems. Fig. 4 presents the M–T curves
of Co(Cr1−xAlx)2O4 samples measured in field cooled cooling mode
with the applied field of H ≃ 10 Oe. Here field is applied at 300 K
and the magnetization is measured while lowering the tempera-
ture to 3 K. We can see that parent compound CoCr2O4 undergoes
3. Results and discussions
Structural characterization of all the samples is conducted by
recording the X-ray diffraction patters at room temperature. Main
panel of Fig. 1 shows the measured and fitted X-ray diffraction
patterns of the Co(Cr0.8Al0.2)2O4 (20% Al substituted) sample by
employing Rietveld refinement technique with the help of FULL
PROF utility software. The non-scattered difference curve between
the experimental and fitted XRD patterns, along with the χ2 value
of 1.4, shows the quality of the fitting the data. It can be seen that
the patterns are well fitted and all the peaks can be indexed to
ferrimagnetic ordering at T ≃ 96 K, and the peculiarity at TS ≃ 26 K
c
that is in agreement with previous reports [1,2], may be related to
the temperature, TS, of non-collinear spin-spiral ordering. Here, it
is understood that the Al substituted samples do not show any
magnetization reversal phenomenon as noticed in the Fe sub-
stituted samples [17]. We can see a clear increment in the mag-
nitude of magnetization up to 50% Al substitution, beyond this
substitution level it decreases. Further, a decrease in the ferri-
magnetic transition can be also noticed on increasing the Al sub-
stitution. Temperature dependence of zero field cooled (ZFC) and
field cooled (FC) magnetization curves of various Al substituted
samples measured in the applied field of 1 kOe are presented in
Fig. 5(a). It can be seen that CoCr2O4 exhibits large bifurcation
between ZFC and FC curves. From the neutron scattering study this
bifurcation was ascribed to reentrant-spin-glass-like behavior
caused due to freezing and fluctuation of the spiral component
[11]. Decrement in bifurcation between ZFC and FC curves with
increase in Al substitution may indicate significant decrease in the
freezing and fluctuation of the spiral component. It can be also
seen that after reaching maximum value below Tc, magnetization
of CoCr2O4 in 10 Oe (Fig. 4) and as well as in 1 kOe (Fig. 5(a))
decreases with temperature up to TS. As it was discussed in Refs.
[9–12], this decrement may be connected with the growth of short
range spin-spirals in the frustrated B-site such that the net B-site
moment is antiparallel to that of A-site moment. However, owing
to the formation of long range conical spin-spirals below TS, again
magnetization increases down to 3 K. With an increase in the Al
substitution it can be seen that this behavior changes, the mag-
netization does not decrease with lowering temperature. One can
see 50% Al sample rather shows increasing trend. 80% Al sample
exhibits very broad transition similar to typical glassy systems and
CoAl2O4 orders at very low temperature.
̄
spinel structure with Fd3m space group (No. 227). From the
Rietveld refinement, it is found that all the Al substituted samples
crystallize in spinel structure similar to that of parent compound
CoCr2O4. Unit cell length of parent CoCr2O4 is found to be 8.332(4),
which is in better agreement with the previous reports [14,15].
Variation of unit cell length ‘a’ as a function of Al substitution
obtained from the Rietveld refinement is shown in the inset of
Fig. 1. We can see a linear decrement of unit cell length with in-
creasing the Al substitution. From the refinement it is also noticed
that the substituted Al ions occupy the octahedral(B) site. Left
panel of Fig. 2 shows the SEM images of x¼0.1 and x¼0.8 samples,
where as the right panel shows EDAX spectra of the same samples.
Average particle size is found to be around 1 m and the EDAX
μ
spectra showed that composition of elements of the samples is
similar to the calculated values.
To investigate the phase stability of the samples Scanning Ca-
lorimetric (DSC) and Thermogravimetric (TG) analyses have been
carried out on the black powders obtained after self-combustion.
Fig. 3(a) and (b) show the temperature dependent DSC/TG curves
of x¼0.0 and x¼0.5 samples, respectively. The weight loss around
100 °C in both the samples could be due to the evaporation of
moisture. Weight loss at around 360–390 °C may be due to the
decomposition of organic compounds such as citrates covalently
To see how the behavior of M–T curve changes with Al sub-
stitution we plotted the magnetizations ratios of Mmax/Mmin and
M3K/Mmin as a function of x as shown in Fig. 5(b) and (c), respec-
tively. Here Mmax, Mmin and M3K are taken from the FC curves as
shown in the inset of Fig. 5(b). Here we can see that Mmax/Mmin
decreases with increase in Al substitution and becomes one for
20% Al sample. This may indicate that due to the substituted Al
ions, the net moment of B-site does not grow faster like the parent
compound. Further, M3K/Mmin also shows the similar trend as that
of Mmax/Mmin indicates that the conical spin-spirals are also af-
fected by Al substitution. As the conical spin-spirals become weak
the amount of canting of moments decrease as a result the mag-
nitude of moments along the magnetization axis increases. This
also causes the enhancement in net magnetization of the samples
up to 50% Al substitution. Further, the decrement of magnitude of
magnetization and the broad transition in 80% Al sample could be
due to the dilution of Cr ions in the B-site.
Fig. 1. Rietveld refinement of powder XRD patterns of Co(Cr0.8Al0.2)2O4 sample by
using FULLPROF program. χ2 value is found to be 1.4. The difference curve is shifted
downward for better clarity of the Bragg positions. Inset is the variation of lattice
parameter ‘a’ as a function of Al substitution x.
Main panel of Fig. 6 shows the temperature dependence of the
first derivative susceptibility curves of Co(Cr1−xAlx)2O4 samples. We