Chapter 3: Co doped SnO 2 based DMS
3.1.2 Magnetic Properties
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Fig.3.6 Room temperature Raman spectra of Sn1-xCoxO2 for x= 0.0, 0.02, 0.10 sample.
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paramagnetic matrix in the system. The saturation magnetization Ms and coercivity Hc values for all the Co-doped samples are given in table-3.3.
The saturation moment of 0.036 µB/Co-ion (0.19emu/cm3) has been observed for x = 0.02 at 400 K and it enhances to 0.055 µB/Co-ion (0.28emu/cm3) when the temperature is reduced to 85 K.
As the temperature is reduced, there is a considerable increase in hysteresis loss due to the possible enhancement in magnetic anisotropy or other competing magnetic interaction. The M-H curve of N2 annealed x = 0.02 sample is shown in Fig.3.8 and we can see a large increase in magnitude of magnetization compared to air annealed material. The N2 annealing is expected to reduce the oxygen content in the sample and that leads to electron doping in the material. In other words, there would be an increase in electron concentration with N2 annealing. Thus enhanced carrier concentration plays an important role in ferromagnetic interaction.
Table-3.3 Coercive Field (Hc) and saturation magnetization (Ms) determined for Sn1-xCoxO2 samples.
Sample/Parameters x = 0.0 x = 0.02 x = 0.05 x = 0.07 x = 0.10
Hc Oe (85 K) diamagnetic 4100 3776 3434 2975
Hc Oe(400 K) --- 529 929 499 887
Ms emu/gm(85 K) --- 0.042 0.047 0.016 --- Ms emu/gm(400 K) --- 0.022 0.024 0.028 0.019
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Fig.3.7 M-H loops recorded for Sn1-xCoxO2 samples at (a) 85 K and (b) 400 K for x=0.02, 0.05, 0.07, 0.10.
-2 -1 0 1 2
-0.06 -0.04 -0.02 0.00 0.02 0.04 0.06
-0.05 -0.04 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 0.04 0.05
M ( µµµµ
B/C o -i o n ) M ( µµµµ
B/C o -i o n )
Field(Tesla)
x=0.02 x=0.05
x=0.07 x=0.10 (a)
T=85K
-1.0 -0.5 0.0 0.5 1.0
-0.04 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 0.04
-0.020 -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 0.020
M ( µµµµ
B/C o -i o n )
X=0.02 X=0.05
X=0.07 X=0.10
Field(Tesla) M ( µµµµ
B/C o -i o n )
(b)
T=400K
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Fig.3.8 Magnetic hysteresis loop of nitrogen annealed Sn0.98Co0.02O2 sample recorded at 400 K.
In order to further understand the magnetic properties, we have analyzed the measured initial M-H curves of the air annealed samples in terms of bound magnetic polaron (BMP) model by following refs [112, 113, 255] i.e.
M=M0L(x) + χm H ……… (3.2) Here the first term is from BMP contribution and the second term is due to paramagnetic matrix.
Here M0=Nms, N is the number of BMPs involved and ms is the effective spontaneous moment per BMP. L(x) = cothx-1/x is the Langevin function with x = meffH/(kBT), where meff is the true spontaneous moment per BMP. At relatively high temperature, where the interaction between BMPs can be ignored; ms = meff can be taken [112]. However at sufficiently high temperature, where there is a considerable mobility of charge carriers, so, eq.3.2 cannot be used due to the lack of BMP.χm is the susceptibility of the matrix. The M-H curves recorded at three different temperatures namely 85 K, 295 K and 400 K could be fitted to eq.3.2 for samples with x ≥ 0.05.
Intial M-H curves of different samples measured at 295 K along with BMP model fit are shown
-15000 0 15000
-0.4 -0.2 0.0 0.2 0.4
M ( µµµµ
B/C o -i o n )
Field(Gauss)
T=400K Sn
0.98Co
0.02O
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in Fig.3.9(a). BMP fit of magnetization measured at different temperatures for x = 0.05 and 0.07 samples are shown in Fig. 3.9(b) and (c) respectively. The fitted parameters M0, χm and meff are given in table-3.4.
Fig.3.9 (a) Magnetization versus field at 295K, for x= 0.02, 0.05, 0.07 and x=0.10 samples of Sn1-xCoxO2. The solid lines represent the fit to Bound magnetic polaron model (eq.3.2).
Temperature variation of M-H curves (b) for x=0.05 and (c) 0.07 samples along with the BMP fit.
0.0 0.2 0.4 0.6 0.8 1.0
0.00 0.01 0.02 0.03 0.04 0.05
T=85K T=300K T=400K
M(emu/g)
Field(Tesla) BMP Fit
x=0.05
(b)
0.0 0.5 1.0 1.5 2.0
0.00 0.01 0.02 0.03 0.04 0.05 0.06
0.07 x=0.02 x=0.05 x=0.07 x=0.10
M(emu/g)
Field(Tesla) (a)
0.0 0.2 0.4 0.6 0.8 1.0 0.00
0.01 0.02 0.03 0.04 0.05 0.06
T=85 K T=295 K T = 400 K
M(emu/g)
Field(Tesla) x=0.07
(c)
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Table 3.4: List of parameters obtained from the bound magnetic polaron model fit. M0 is the spontaneous magnetization, χm is the susceptibility of the matrix and meff is the effective spontaneous moment per bound magnetic polaron.
For a given doping concentration, the M0 values are found to decrease with increase in temperature and this can be understood as a result of the decrease in ferromagnetic interaction with increase in temperature. On the other hand, M0 value is found to decrease with increase in doping concentration and such a decrease in ferromagnetic interaction with increase in doping concentration has been reported in Mn-doped SnO2 based diluted magnetic semiconductor [198].
It is mainly due to a reduction in average Co-Co inter-atomic distance, which might contribute to nearest neighbor antiferromagnetic interaction at the expense of ferromagnetism. The paramagnetic susceptibility χm is found to decrease with increase in temperature as expected for any paramagnetic matrix and its value marginally increases with increase in doping concentrations. The spontaneous moment per BMP, meff is found to increase with temperature and such a variation of meff is in contradiction to the variation of M0 and it can be understood as a result of increase in size of BMP with temperature. At higher temperature, even though the magnetic moments are aligned parallel within each BMP, the overall alignment of BMPs along the applied field may not be complete due to the increase in thermal energy. The value of meff is found to be in the order of 10-17 emu (10-20 J/T) and it is comparable to that reported by Quintero et al. [114] in p-type Cu2FeGeTe4. However, the present meff value is found to be one order of magnitude larger than that reported in Cu2Mn0.9Zn0.1SnS4 and Y0.9Ce0.1MnO3 [112, 256]. In the present series of samples, the parameter M0/meff is found to vary with temperature, so in such condition, one cannot assume ms = meff. In view of above the restriction, we could not estimate N, the number of BMP per unit volume. The average radius of the BMP was estimated from the Sample/
parameter x=0.05 x=0.07 x=0.10
Temperature
(K) 85 295 400 85 295 85 295 400
M0(emu/g) 0.06 0.04 0.02 0.06 0.04 0.05 0.04 0.02
χχχ
χm(10-4 cgs) 0.03 0.03 0.002 0.09 0.03 0.12 0.11 0.003
meff(10-17emu) 0.42 3.8 5.2 0.41 3.3 0.22 2.7 4.5
BMP radius (Å) 38 72 80 31 61 22 51 80
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fitted value of meff and by assuming a spherical shape of BMP. The typical radius of BMP for x=0.05 sample is found to be 38 Å. Our result is comparable to that reported by Dietl et al. [6] in CdMnSe based DMS, where the radius was determined to be 40 Å. The BMP radius at different temperatures and doping concentrations are given in table-3.4
The magnetization data of N2 annealed sample could not be fitted to BMP model; on the other hand it could be fitted to the Brillouin function model by taking into account the ferromagnetic contribution.
M = MS BJ(x)……….. (3.3)
MS = ngµBS and
( )
1 1 coth 1 1coth2 2 2 2
J
B x S x S x
S
= + + −
Here n is the number of magnetic atoms per unit volume, S is the magnetic spin quantum number and g BB
x kT
= µ . The fitted data are shown as solid line in Fig.3.10, which closely follow the experimental data. The values of S and Ms are found to be 1.38 and 0.23emu/g respectively. The S value suggests the presence of Co2+ ions in high spin state and however, we cannot rule out the presence of Co3+ ions. The magnetization data of air annealed x=0. 02 sample could not be fitted to the Brillouin function model due to the weak signal.
Fig.3.10 The initial magnetization curve along with Brillouin function model fit for N2 annealed x=0.02 sample.
0.0 0.6 1.2
0.00 0.05 0.10 0.15 0.20 0.25
Sn0.98Co0.02O2
M(emu/g)
Field(Tesla) T=450K
--- experimental data ---fitted data
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In order to study the ferromagnetic transition, the temperature variation of magnetization was measured in the temperature range 400 to 1000 K for an applied field of H=0.2T. The M-T curves for all the Co-doped samples are shown in Fig.3.11, where a clear paramagnetic to ferromagnetic transition can be seen. The magnetization is found to increase gradually with decrease in temperature. The paramagnetic susceptibility was fitted to Curie-Weiss law,
( )
c
C x χ T
= θ
−
……… (3.4) where, C(x)= xC0= xnµeff2/3kB is the Curie constant, where x is the concentration of doped Co, θcis Curie temperature. The Curie temperature is found to be 692 K for x = 0.02 and it gradually decreases to 582 K for x = 0.10 sample as shown in Fig. 3.12. The effective magnetic moment µeff estimated from Curie constant is found to vary from 1.3µB/Co-ion to 1.9µB/Co ion which suggests that the Co-ions are mostly in the Co2+ lowspin state. The theoretical effective magnetic moment of Co2+ ion in the low spin state is
µ
effth= 1.73. The Curie temperature and the effective magnetic moment values for all the Co-doped samples are given in table-3.5
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Fig.3.11 M-T plots for Sn1-xCoxO2 (x=0.02, 0.05, 0.07 and 0.10).
300 400 500 600 700 800 0.000
0.005 0.010 0.015 0.020 0.025
x=0.02 x=0.05
M (e m u /g )
T(K)
0.2 Tesla
400 500 600 700 800 900 0.004
0.008 0.012 0.016 0.020 0.024
x=0.07 x=0.10
M (e m u /g )
0.2 Tesla
T(K)
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Fig.3.12 Plots of χ-1 vs. T of solid state synthesized Sn1-xCoxO2 (x = 0.02, 0.05, 0.07 and 0.10) samples along with the Curie-Weiss fit.
Table-3.5: Parameters obtained from Curie- Weiss fit. Here θc is the Curie temperature; µeff is the effective magnetic moment per Co-ion.
Sample/Parameters x=0.02 x=0.05 x=0.07 x=0.10 θθθθ
C(K) 696 670 595 582 µ µµ
µ
eff(µ µµ µ
B) 1.65 1.37 1.9 1.92
780 800 820 840 1.0
1.5 2.0
T(K) χχχχ−1−1−1−1 104 (emu.mol/Oe)-1
x=0.02
700 720 740 760
0.3 0.4 0.5 0.6 0.7
χχχχ−1−1−1−1 104 (emu.mol/Oe)-1
T(K) x=0.05
660 690 720 750
0.3 0.4 0.5
x=0.07
T(K) χχχχ−1−1−1−1 104 (emu.mol/Oe)-1
500 600 700 800
0.1 0.2 0.3 0.4
x=0.10
T(K) χχχχ−1−1−1−1 104 (emu.mol/Oe)-1
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