Cadmium (C4H6MnO4.4H2O) salts had been blended and completely dissolved

Cadmium oxide with nanostructure has attracted an
extensive attention due to the potential applications in chemical sensors and optoelectronic
devices 1-5. Although the highly toxicity of cadmium and its compounds, it is
used in battery manufactures, corrosion protection in steels, and barriers to
control neutrons in nuclear fission processes. CdO is n-type transparent
conducting oxide(TCO) with a conductivity of 102–104 S/cm
and good transparency especially in NIR spectral region with a direct band gap
of nearly 2.2–2.7eV 1–3. Experimentally it was observed that one can control
the optoelectronic properties of CdO by doping with different types of metallic
ions. So, doping with ions like Y,In,Sm,Cu 6–9 improves its n-type conduction
. While doping with Cd2+ ions may enhance the low dimensional size
to reach radius of 9.5×10-2 nm 10. CdO doped  with transition metals (3d elements)   or  
magnetic  ions    such as 
Ni ,Mn, Co  and  Fe  
11,12   could lead   magnetic 
properties  resulting  in dilute 
magnetic semiconductors (DMS). In general, doped semiconductors with 3d elements,
known as dilute magnetic semiconductor (DMS), where a fraction of the magnetic
ions substitute the host cations.  The Ferromagnetic
behavior of DMS materials made them useful in spintronic applications. Metal
nanoparticles have attracted a great attention, resulting in novel opticl,
magnetic and electronic properties.

 Calculation
methods based on density functional theory (DFT) also predict ferromagnetism in
most 3d transition metals (TM) based CdO 5–7. The structural defects and
impurities serves as traps or recombination centers for migrating charge
carriers generated by ionizing radiation. Electron Spin Resonance (ESR) can be
used to evaluate such defects.

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The aim of this study is to give more insight
into the effect of Mn on the structure, thermal and magnetic properties of CdO.

2- Experimental  Procedure :

DMS samples have been prepared using the co-precipitation
method. The required amounts of cadmium nitrate 
Cd(No3)2.4H2O and Manganese Acetate (C4H6MnO4.4H2O)
salts had been blended and completely dissolved in absolute ethanol.  Also a required amount of NaOH was completed
dissolved in absolute ethanol. The two solutions were completely dissolved
using a magnetic stirring for 1 hour at 60 oC. NaOH was added drop
wise to the salts solution till complete precipitation. The precipitate was
separated by decantation process, washed with absolute ethanol and separated by
centrifuge with 5000 rpm for 15 minutes. The final ingot has been dried at 110 oC
for 1 hr and calcinied at 450 oC for 3 hrs. The dried powders were
grained in an agate mortar.

    Shimadzu X-ray diffraction (XRD), with Cu
(K?) tube, was used to determine the structure of  the as-prepared samples using. The range of
scanning angle ranging from 10 to 900. The surface morphology of the investigated samples were
investigated using  Joel 6400 scanning electron
microscope. Thermal analysis
was carried out using Shimadzu instrument (type-50), differential scanning
calorimeter (DSC-50) under nitrogen atmosphere (20 cm3 /min). Electron
spin Resonance  (ESR) at room temperature
using  Bruker   EMX spectrometer (X-band) German  were used to investigate the magnetic
properties of the prepared samples. The magnetic hysteresis loops were
determined using Lakeshore 7410  
magnetometer (VSM) with the applied field   20 K Oe.

 

3-Results
and Discussion:

3-1-Structural Determination:

Figure
(1)  shows XRD of  the as- prepared Cd1-xMnxO
Powder samples (x = 0.0,0.01, 0.05, 0.1, 0.15, 0.2, and 0.3). All the
diffraction peaks, which correspond to the planes (111), (200), (220),
(311),(222) and (400), are perfectly indexed to the Cubic CdO structure (JCPDS
65-2908), revealing that doping of Mn does not afect the structure of the CdO.
The Diffract ograms reveal that there is no additional peak corresponding to
secondary phase of Mn in CdO, which confirms the formation of the Cd1-xMnxO
solid solution. Hence Mn2+ substitutes Cd2+ site into the crystal lattice. Figure (1)
illustrates that increasing Mn content leads to 
increase the  intensity  of diffraction peaks. This means the slightly
decrease of  the  nanocrystalline nature of the samples which
confirmed by the crystallite sizes increasing as shown in table1. Table (1)
shows the refined structural parameters obtained from Rietveld refinement using
the PCW program. The obtained data reveal that the unit cell volume is almost a
composition independent. This is due to the small difference between ionic
radius of Mn and Cd. The shifting of diffraction peaks towards lower angles
with the increase of Mn content suggested 
the incorporation of Mn+2 ions in CdO lattice at the site of
Cd+2 ions 13. Figure (2) shows the refinement of CdO as an
example.

 The surface morphology, distribution of the particles and particle size
of the undoped and doped samples were investigated by SEM. Figure (3) shows the
SEM images of the investigated samples. The figure illustrates that the
investigated samples  consists of
numerous  irregular flowers like shaped
structure. The photomicrographs shows that addition of Mn did not change the
main features of the growth process.

 

3-2-Transition
Temperatures and Crystallization Process:

 

DSC thermograms for the investigated
compounds measured at a constant heating rate of 10deg/min (Fig. 4) show the
three phenomena of  interest: the glass
transition (Tg ), The crystallization exothermic (Tc )
and the melting endothermic (Tm ). The transition temperatures were
listed in table (2).

The
effect of Mn doping on the kinetics of crystallization process of CdO was
studied through determination of the kinetic parameters n (reflects the
nucleation rate or the growth morphology) and the activation energy of the
process.

Avrami  phenomenological equation for solid-to-solid
transformation was used to describe the kinetics of isothermal crystallization
14,15,

a = 1- exp(Ktn)                         (1)

Where
a  is the time
dependent crystalline fraction at time t, n is Avrami exponent and K is the
reaction rate,

                                  K=K0(-E/kT)                                   (2)   

 

Where T is the temperature in degrees
Kelvin, K0 is the rate constant and k is Bolzman constant. E is the
apparent activation energy including the activation energy for nucleation En
and for growth EG and can be written as

E= (En
+ EGm )/n         (3)

Where n and m are numerical factors
that depend on the mechanism of crystallization

mechanism

n

3-dimension
2-dimension
1-dimension
Surface nucleation

4
3
2
1

For prepared compositions containing
non-nuclei n=m+1, while n=m for compositions containing a large number of
nuclei.

From equations ( 1&2 ) the following
equation could be derived:

ln(-ln(1-a)) = ln K0 – E/kT + nlnt    (4)

 

At constant (t ) plot of ln(-ln(1-?)
vs. 1/T gives E.

Non-isothermal single scan technique
was applied to determine the crystallization kinetic parameters of the
investigated compositions. Slow constant scan-rate of 2 deg/min was used.  On this assumption a plot of log g(?) versus 1/T
yields a straight line when appropriate mathematical description of the
reaction is used. Such a description can be written as

log g(?) = log (E Ko1/n / R) – E/nRT  (5)

Ko
is considered constant with respect to temperature. The function  g() has been calculated by Savara and Stava
16 for different reaction kinetic equations, details of the theoretical
approach of the applied model are reported in a previous study 17 . The value
of E/n can obtained from the slope of the straight line from the plot of  g(a) 
vs. 1/T. Logg(a) versus 1/T was plotted for the
investigated compositions for different kinetic equations. The best fit for the
increasing part of DSC was obtained for the function A3 {A3
= ?ln(1??)1/3
= KT}. A3
indicates the presence of random nuclei in the as-prepared  Cd1-xMnxO compositions
and that the growth of these nuclei is being diffusion controlled.  Figure (5) shows the plot of log g (?) versus 1/T for Cd1-xMnxO system for
the appropriate choice A3. From the figure, it can be noticed that
logg(?) has two distinct slopes indicating
that the crystallization process proceeds through two different rates. From
the slopes of the straight portions one can determine the values of the
effective activation energies E/n. The values of E were calculated from the slope of ln?ln(1 ??) versus 1/T
given in Figure (6). Table (3) gives the values of E and n. The values of n reveal that the growth of the random
nuclei, formed in the as prepared samples, the crystallization process of CdO
can be carried out by surface nucleation while the doped compositions
crystallization takes place in two dimensions. The table illustrates that
adding 0.01 of   Mn to CdO leads to
decrease the activation energy of crystallization from 34.24 to 23.1 eV.  The table shows that the crystallization
energy increasing with increasing Mn content which means that the addition of
Mn impedes the crystallization process.

 

3-3-Magnetic
Properties:

 

Magnetization
versus magnetic field (M-H) hysteresis loop characterizes the magnetic
properties of materials. Magnetization measurements on CdO and Mn doped CdO
samples were performed at room temperature in the range of applied magnetic
field (0-2000G) as declared in figure (5). Figure (7) illustrates the presence
of magnetic order in the structure of the as-prepared samples. The figure shows
small loops passing through the origin for the investigated compositions. The
prepared Cd1-xMnxO nanostructures show a weak
ferromagnetic nature at room temperature, this may be due to the intrinsic
defect. Metal doped CdO magnetic properties are mediated by the ferromagnetic
exchange between the available defect states in CdO and the dopant ion. The
different magnetic parameters such as remnant magnetization (Mr), saturation
magnetization (Ms), reduced magnetization (Ms/Mr),
coercivity (Hc) and hysteresis loop area are listed in table ( 4).
It is obvious from this table that Ms decreases with increasing Mn
content upto 0.2.  The decrease of  Ms with increasing Mn content upto
0.2 can be assigned  to the presence of
magnetic disorder at the surface of the nanoparticles.  The relation between Mn concentration and Ms
is shown in figure (8). The behavior of Mr with Mn concentration is
the same as that of Ms . Coercivity is an important parameter of
magnetic materials, it reflects the degree of permanent magnetism. Table (4)
shows that Cd0.80Mn0.2O  
has the largest Coercivity value (885.9 Oe ) this might open up possible
applications in novel computer logic systems.

 The observed  ferromagnetic nature for the Mn-doped CdO
film  may be due to the magnetic
interactions between the dopant ions or might have originated from
substitutional spin polarized Mn atoms present in the host lattice. In metal
oxide S.C., RTFM occurs due to the oxygen vacancies present in the lattice. The
increased oxygen vacancies might also be responsible for the RTFM noted for the
Mn-doped CdO film.

3-4-ESR Study:                                                                                                             

ESR spectrum of the
investigated samples are given in figure (9). The figure shows six splitting
lines (hyperfine splitting) of undoped and doped CdO. The hyperfine splitting
is due to electron spin-nucleus spin interaction. Line width data can be
extracted from any of the lines of the spectra. The chosen line at3200G because
it has the highest amplitude. The calculated values of g and hyperfine
splitting constants are given in table(5). The electronic configuration of Mn2+
ion is 3d5, and the electronic ground state is 6s5/2,
which splits into three Kramers doublets (5/2, 
3/2 and  1/2).Undpoed electrons of
Mn2+ interact with neighboring Cd nuclei to give superfine lines.

The hyperfine constant (A) is calculated by:

A (cm-1)
= (?B/ hc)*g* A (G)                    (6)

Where (?B) is magneton Bohr (9.27400949*10-24
J.T-1), (hc) is Plank’s constant (6.6260693*10-34
J.S)

The g-value has been obtained using the following equation:

                             
g=h /BB0                                                 
(7)

where ?B is microwave frequency (9.45GHz)

The obtained g of the investigated samples are
around 1.9856. The g-factor depending on the electronic configuration of the
radical or ion also represent spectroscopic splitting. Unpaired electron can
lose or gain angular momentum, which may change the value of g-factor, causing
it to differ from ge . This is significant to systems with metal
ions. As g-values show a negative shift with respect to the free electron value
(2.0023),  the bonding is ionic in nature
18.

One  can
calculate the number of defects N from the following  relation:

              
N= K H0( H)2(A/2) /Ge?PHHm                           (8)

Where K is
constant of spectrum (1013), H0 is the magnetic field
corresponding to zero point (3508.797G), 
Hm is the width of the magnetic field from peak to peak (G), A is  the height of the signal (cm),Hm is modulation field (5), PH is Puissance (1.418)  and Geis gain detection (7960).

The value of hyperfine constant (A), g- factor (g), the width of the
magnetic field from peak to peak (? H), the height of the signal (A), and
Number of defects (N) are illustrated in table (5).

From this table, it can be deduced that the width of the magnetic
field from peak to peak increases with increasing the Mn concentration in Cd1-xMnxO
system. The number of defects increases with increasing the Mn concentration,
but the higher value of the number of defects in the  sample Mn5%.

4- Conclusion

The prepared nanostructure samples Cd1-xMnxO (x = 0.0,0.01, 0.05, 0.1, 0.15, 0.2, and
0.3 ) are having a cubic  structure and
their  cell volume are almost composition-independent. The absence of  Mn or Mn related impurity phases confirms the
formation of Cd1-xMnxO
solid solution.  The non-isothermal
kinetic analysis indicates that the crystallization of Cd1-xMnxO
occurs in two dimensional processes but ,that of CdO takes place through  surface nucleation process .ESR spectrum of
Cd1-xMnxO showed Hyperfine lines. Interpretation of these
lines showed that Mn2+ ions occupy substitutional sites in cubic
CdO.