Chapter 3: Co doped SnO 2 based DMS
3.4 Co doped SnO 2 by Ball Milling
3.4.2 Magnetic Properties
132
133
However, the magnitude of Ms in the ball milled samples are found to be about two order of magnitude larger than those of solid state synthesized samples for the same concentrations of Co doping. Similar variation of Ms with increase in doping concentration has been reported in literature [198, 265]. The decrease in Ms value with increase in Co-concentration can be understood in terms of enhanced nearest-neighbor antiferromagnetic superexchange interaction.
The values of coercive field, Hc are given in Table-3.13. The Hc value is found to be maximum for x=0.02 sample. Similar kind of Hc variation was observed for the Co doped samples prepared by solid state route (section 3.1.2).
Table-3.13 Coercive Field (Hc) and saturation magnetization (Ms) determined for various Co doped SnO2 samples prepared by ball milling.
Sample/Parameters x = 0.0 x = 0.02 x = 0.07 x = 0.10 Ms(290K) emu/g 0.9 4.4 4.1 3.8 Ms(20K) emu/g 1 7.7 6.4 4.8 Hc(290K)Oe 440 879 786 470 Hc(20K)Oe 590 3735 3727 3245
The M-H loops recorded at 20 K for all the above samples are shown in Fig. 3.35(b). The Ms values were obtained after subtracting the high temperature linear region. The obtained Ms and Hc values are tabulated in Table-3.13. For x = 0 sample, no appreciable variation in Ms and Hc
values are observed with decrease in temperature. However for example, x = 0.02 sample exhibits a large increase in Ms and Hc values with decrease in temperature. Similar behavior was observed for other Co-doped samples. So basically, there is a clear difference between the magnetization behavior of x = 0 and other Co doped samples and it leads to the understanding that, the mechanism and the origin of FM in x = 0 sample is quite different from other doped samples. The large increase in Hc value at low temperature can be understood in terms of enhanced magnetic anisotropy due to domain growth. In order to quantitatively analyze the magnetization data due to Co substitution, the contribution from spinel related phase or other extrinsic effect need to be separated out. Since the concentration of spinel phase is almost constant for different samples and the FM behavior of x = 0 sample is found to be distinct compared to rest of the samples, it can be assumed that the observed FM signal of x = 0 sample TH-1126_07612102
134
is due to lattice defect such as oxygen vacancy, Fe3O4 type spinel impurity phase, or due to the small amount of Fe doping in Sn site. The initial M-H curve of x = 0 sample was fitted to the following expression [266],
M = Mso(1-a/H+b/H2)+χdH……… (3.6) Here Mso is the saturation magnetization, a and b are constants and χd is the paramagnetic susceptibility. The first term is to account for the FM behavior of x = 0 sample and the second term is to account for the observed paramagnetic contribution at higher field. The data were fitted by varying the parameter Mso, a, b and χd. The initial curve of x = 0 sample at 290 K along with the fitted data to eq.3.6 are shown in Fig. 3.36(a). The fitted data closely follow the experimental data. The fitted values of Mso, a, b and χd are found to be 0.915emu/g, 118emu.Oe/g, -1.7×105emu.Oe2/g, 1.4×10-7 emu/g.Oe respectively.
Fig. 3.36(a) Initial M-H curve for x = 0 sample along with fit to eq. 3.6. (b) BMP model fit for x
= 0.02, 0.07 and 0.10 sample.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0
0.2 0.4 0.6 0.8 1.0
T=290 K x=0.0
M(emu/g)
Magnetic Field(Tesla)
0.0 0.3 0.6 0.9 1.2 1.5 0.0
0.5 1.0 1.5 2.0 2.5 3.0
M(emu/g)
Magnetic Field(Tesla)
x=0.02 x=0.07 x=0.10
T=290K
Fit to eqn.[1]
(a)
BMP Fitting (b)
TH-1126_07612102
135
The intrinsic magnetization due to Co-doping was determined by subtracting the magnetization arising from spinel phase or other extrinsic effects. This can be done by estimating the background magnetization by using eq.3.6 and parameters Mso, a, b and χd corresponding to x = 0 sample. The background magnetization was subtracted from the measured magnetization of Co doped samples. The intrinsic initial magnetization data of x = 0.02, 0.07 and 0.10 samples are shown in Fig. 3.36(b). The intrinsic saturation magnetization, Msi was determined from the above plots and are found to be 2.9 emu/g, 2.6 emu/g, 2.2 emu/g for x = 0.02, 0.07 and 0.10 samples respectively at 290 K. Their corresponding values at 20 K are found to be 5.9 emu/g, 4.9 emu/g and 3.3 emu/g. The data were analyzed in terms of bound magnetic polaron model (BMP) [112, 113, 255]. The parameters Mo, meff, χm were varied during the fit to BMP model. The magnetization curves for x = 0.02, 0.07, 0.10 samples at 290 K along with the fitted data are shown in Fig.3.36(b). The fitted data closely follow the experimental data and the fitted parameters Mo, meff and χm are given in Table-3.14. The M0 value is found to decrease with increase in doping concentration and such a behavior has been reported in Mn-doped SnO2
compounds [198]. It is mainly due to the 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 increase with increase in doping concentration. The value of spontaneous moment per BMP, meff is found to be in the order of 10-17 emu and increases marginally with increase in Co concentrations. Similar kind of variation was observed for the solid state route prepared Co doped samples and with almost same order of magnitude as given in section 3.1.2. The average radius of the BMP was estimated from the fitted value of meff and by assuming spherical shape of BMP. The radius of BMP was found to be in the range of 30 Å to 39 Å and is comparable to that reported by Dietl et al. in CdMnSe based DMS [267].
TH-1126_07612102
136
Table 3.14: List of parameters obtained from the analysis of M-H data by using BMP model. Mo
is the spontaneous magnetization of the system, χm is the susceptibility of the matrix and meff is the effective magnetic moment per bound magnetic polaron.
Sample Parameters obtained from BMP Fitting
Mo(emu/g) χχχχm(10-4emu/g.Oe) meff(10-17emu) BMP radius(Å) x=0.02 3.34 1.04 5.7 39 x=0.07 2.8 2.1 5.8 31 x=0.10 2.21 5.1 7.8 33
In order to completely understand the magnetic transitions, we have carried out the temperature variation of magnetization for an applied magnetic field of H = 0.2 T and they are shown in Fig. 3.37. M versus T plot of x = 0 sample (sample S2) is shown in Fig. 3.27(c) and for comparison; it is reproduced in Fig. 3.37(a) where we can see the FM transition. The temperature derivative of magnetization as a function of temperature in the expanded scale is shown in Fig.3.37(b), where we can see a sharp negative peak at 690 K and a minor negative peak at 750 K. So, basically the x = 0 sample exhibits two magnetic transitions. The transition observed at 750 K can be attributed to ferrite based structure or extrinsic effect such as oxygen vacancy.
However, the observed Tc at 690K could be attributed to the doping of some of Fe-ions into the Sn site. The M-T curves of x = 0.02, 0.07 and 0.10 samples are shown in Fig. 3.37(a) and (c) and they all exhibit single Paramagnetic (PM) to FM transition. The FM Tc is found to decrease with increase in Co-concentration and it is in correlation with a decrease in Ms value observed from M-H data. Unlike x = 0 sample, no secondary transition is observed, and it could be mainly due to the strong FM signal of Co-doped samples.
The paramagnetic susceptibility was fitted to the modified Curie-Weiss law,
0
( )
c
C x χ χ T
= + θ
−
……… (3.7)where, χ0 is temperature independent susceptibility, C(x) = xC0= xNµeff2/3kB is the Curie constant, where x is the concentration of doped Co-ions, / is Curie temperature. Fig. 3.37(d) shows a typical plot of temperature variation of susceptibility along with fit to eq.3.7 for x = 0.07
TH-1126_07612102
137
sample. The Curie temperature (FM Tc) of x = 0.0, 0.02, 0.07 and x=0.10 samples are found to be 692±0.2K (690 K), 653±0.5K (Tc=640K), 519±0.7 K (Tc=501 K) and 446±0.4 K (Tc=396K) respectively. The observed values suggest that the transition temperature is not due to any Fe or Co related clustering effects or spinel related structures. The µeff values are found to be 0.94 µB/TM-ion, 2.34 µB/TM-ion, 3.5 µB/TM-ion and 2.15 µB/TM-ion respectively for x = 0.0, 0.02, 0.07 and 0.10 samples. The above results suggest that Co doping gives rise to room temperature FM. Strong room temperature FM was observed with a maximum Msi value of 2.9 emu/g (3.8 µB/Co-ion) and it increases to 5.9emu/g (7.9 µB/Co-ion) at 20 K for x = 0.02 sample. Thus the observed giant magnetic moment value is comparable to that reported by Ogale et al. [39] for 5 % Co-doped SnO2 thin film. Such a high Msi value can be explained in terms of unquenched d-orbital of the doped Co-ion and due to oxygen vacancy as explained in ref. [45].
TH-1126_07612102
138
Fig. 3.37 Temperature variations of magnetization M, (a, b and c) and dM/dT (d) for samples T1, S1 and S2.
300 400 500 600 700 800 900 0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5
M(emu/g)
x=0.0
T(K) x=0.02
0.2 Tesla (a)
600 700 800
0.00 0.01 0.02 0.03
T(K) χχχχ mol(emu/mol.Oe)
Curie-Weiss Fit x=0.07
0 200 400 600 800
0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2
x=0.10 x=0.07
0.2 Tesla
M(emu/g)
T(K)
(c) (d)
600 700 800
-0.008 -0.006 -0.004 -0.002 0.000
(Tc=750K)
(dM/dT)
T(K)
(Tc=690K)
x=0.0
(b)
[e m u /g K ]
TH-1126_07612102
139