Magnetic Properties

Chapter 4: Ni-doped SnO 2 Series

4.1 Ni-doped SnO 2 by solid state route

4.1.2 Magnetic Properties

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Fig.4.6 FTIR spectrum for x = 0.02 and 0.07 samples along with parent SnO₂ and NiO compounds for comparison.

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magnetic anisotropy. The observed saturation magnetization is found to be comparable to those reported in literature for ZnO and SnO₂ based DMS materials [273-276]. The values of coercive field, H_c and saturation magnetization, M_s at 85 K and 400K for different samples are tabulated in table-4.3.

Fig.4.7 M-H loops recorded for x=0.02, 0.05, 0.07 and 0.10 samples at 300K and 80 K.

-1 0 1

-0.08 -0.04 0.00 0.04 0.08

x=0.02 x=0.05 x=0.07 x=0.10

M (e m u /g )

Field(Tesla)

T=300 K

-2 -1 0 1 2

-0.10 -0.05 0.00 0.05 0.10

x=0.02 x=0.05 x=0.07 x=0.10

M (e m u /g )

Field(Tesla)

T=80 K

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Table-4.3 Coercive Field (H_c) and saturation magnetization (M_s) values determined for various Ni-doped samples from the magnetization measurements.

Sample/Parameters x = 0.0 x = 0.02 x = 0.05 x = 0.07 x = 0.10

Hc Oe (85 K)

diamagnetic 643 650 1314 733

Hc Oe(400 K)

--- 450 490 555 547

Ms emu/gm(85 K)

--- 0.027 0.02 0.025 0.008

Ms emu/gm(400 K)

--- 0.021 0.014 0.023 0.006

The resistivity of these samples is found to be in the order of 10⁴ Ω-cm at room temperature and it enhances to 10⁷ Ω-cm at around 100 K. The rather large value of resistivity indicates the localized nature of charge carriers. The localized charge carriers are expected to promote the bound magnetic polaron mediated ferromagnetism. Here each trapped charge carrier polarizes the spin of the magnetic ions within its Bohr radius and it leads to ferromagnetic bubble or BMP embedded in a paramagnetic matrix. Ferromagnetism is observed, when these ferromagnetic bubbles start overlapping in such a way that all the magnetic spins are aligned in a particular orientation [112, 113, 255, 267].

In order to further understand the magnetic properties, we have fitted the measured initial M-H curve in terms of the bound magnetic polaron (BMP) model by following refs. [112, 113, 255]

and as per eq. 3.2. The parameters M₀, m_eff, χm were varied during the fit. Plots of magnetization curve for x = 0.02, 0.05, 0.07 and 0.10 samples at 80 K and 300 K along with fitted data to BMP model are shown in Fig.4.8. The theoretical fit closely follows the experimental data and the fitted parameters M0, meff and χm are given in table-4.4. For a given sample, M0 is found to increase marginally but m_eff is found to reduce with decrease in temperature. The increase in M₀ could be due to interaction between BMP and the paramagnetic matrix and however further analysis is required to verify such interactions. The enhanced hysteresis loop at low temperature can be explained on the basis of magnetic anisotropy. The possible reason for low meff at low temperature could be due to reduced size of BMP. The origin of such reduced BMP size can be due to the presence of competing magnetic interaction. The considerable increase in χm value at low temperature is consistent with smaller size of BMP. The above variation of χm cannot be simply explained based on the Curie law for the paramagnetic matrix and it supports the above

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argument of BMP size. With the increase in doping concentration; the M₀ and χm values are found to increase as a result of higher concentration of magnetic ions. In the present series of samples, the parameter M₀/m_eff is found to vary with temperature, so in such condition, one cannot assume m_s = m_eff. In view of the above restriction, we could not estimate N, the number of BMP per unit volume. The average radius of the BMP was estimated from the fitted value of m_eff and by assuming a spherical shape of BMP and, it is found to be 60 Å for x = 0.07 sample at 80 K. The BMP radius is found to be in the same order of magnitude as reported in other magnetic system such as, in CdMnSe by Dietl et al. [267]. The main uncertainty in the above calculation is from the estimation of number of transition element ions, Ni within each BMP, especially in weakly doped materials. On the other hand, in other magnetic system following BMP model such as Cu₂Mn_0.9Zn_0.1SnS₄ and Y_0.9Ce_0.1MnO₃ [112, 256] the number of magnetic ions per BMP is large by an order of magnitude. The meff values are found to increase with temperature and this could be mainly due to change in size of the BMP.

Table 4.4: List of parameters obtained from the analysis of M-H data by using BMP model. M0 is the spontaneous magnetization of the system, χ_m is the susceptibility of the matrix and m_eff is the effective magnetic moment for per BMP.

Sample/

parameter

x=0.02 x=0.05 x=0.07 x=0.10

T (K) 80 300 80 300 80 300 80 300

M

₀

(emu/g) 0.02 0.02 0.04 0.02 0.07 0.06 0.03 0.02 χχχχ

_m

(10

^-4

cgs) 0.04 0.002 0.03 0.002 0.1 0.004 0.02 0.003

m

_eff

(10

^-17

emu) 2.17 5.9 2.3 7.4 1.25 3.6 1.6 7.4 BMP

radius (Å) 70 98 52 78 38 54 37 61

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Fig. 4.8 Magnetic hysteresis loops for x = 0.02, 0.05, 0.07 and 0.10 samples at 80 K and 300 K along with fit to BMP model.

0.0 0.5 1.0 1.5 2.0

0.010 0.015 0.020 0.025 0.030

Field(Tesla)

M(emu/g) x=0.02

T=300 K T=80 K

-2 -1 0 1 2

-0.028 -0.014 0.000 0.014 0.028

x=0.02 T=300 K

M(emu/g)

Field(Tesla) T=80 K

-2 -1 0 1 2

-0.04 -0.02 0.00 0.02 0.04

M(emu/g)

Field(Tesla) 80 K

300 K

x=0.05

0.0 0.5 1.0 1.5

0.000 0.009 0.018 0.027 0.036 0.045

300 K

Field(Tesla)

M(emu/g)

80 K

x =0.05

0.0 0.5 1.0 1.5 2.0

0.02 0.04 0.06 0.08 0.10

Field(Tesla)

M(emu/g)

x=0.07 T=300 K T=80 K

-2 -1 0 1 2

-0.10 -0.05 0.00 0.05 0.10

x=0.07 T=300 K T=80 K

Field(Tesla)

M(emu/g)

0.5 1.0 1.5 2.0

0.01 0.02 0.03 0.04

x=0.10 T=80 K

T=300 K

Field(Tesla)

M(emu/g)

-2 -1 0 1 2

-0.04 -0.02 0.00 0.02 0.04

x=0.10 T=300 K T=80 K

Field(Tesla)

M(emu/g)

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In order to study the ferromagnetic transition, the temperature variation of magnetization was measured for a wide temperature range i.e. 300 K to 1000 K. The M-T curves for all the Ni doped samples are shown in Fig.4.9, where a clear PM to FM transition can be seen. The magnetization is found to increase gradually with decrease in temperature. The paramagnetic susceptibility was fitted to modified Curie-Weiss law, i.e. by fitting to eq. 3.7. The paramagnetic susceptibility along with the fit to eq.3.7 are shown in Fig.4.10. The typical values of Curie temperature are found to be 770 K and 730 K respectively for x = 0.02 and x = 0.07 samples.

The θ_c and effective magnetic moment µ_eff values for all the Ni doped samples are given in table-4.5. The typical value of µeff estimated from Curie constant is found to be 1.4µB/Ni ion for x=0.07 sample and it suggests that the doped Ni ions are expected to be mostly in Ni²⁺ state. If the doped Ni ions are in the Ni²⁺ state, one would expect the µ_eff value to be of the order of 2.8µB/Ni ion, the observed difference between experimental and theoretical value could be due to only a fraction of Ni ions entering the Sn site.

Fig.4.9. Temperature variation of magnetization for x=002, 0.05, 0.07 and 0.10 samples.

400 600 800 1000

0.00 0.01 0.02 0.03

0.04 x=0.02

x=0.05 x=0.07 x=0.10

M (e m u /g )

T(K)

0.2 Tesla

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Fig.4.10 paramagnetic susceptibility along with Curie-Weiss law fit for x = 0.02, 0.07 and 0.10 Ni doped SnO₂.

Table-4.5 Parameters obtained from Curie-Weiss fit. Where θc is the Curie temperature, µeff is the effective magnetic moment per Ni-ion.

800 880 960

-4 -2 0 2

T(K) χ (10χ (10χ (10χ (10−5−5−5−5 emu.mol/Oe)

x=0.02

800 850 900 950 1000 -6

0 6 12 18 24

x=0.07

Modified Curie Weiss Fit

T(K) χ (10χ (10χ (10χ (10−5−5−5−5 emu.mol/Oe)

800 900 1000

0 20

T(K)

x=0.10

Modified Curie Weiss Fit

χ (10χ (10χ (10χ (10−5−5−5−5 emu.mol/Oe)

840 860 880 900 920 940 -2.0

-1.5 -1.0

T(K)

χ (10χ (10χ (10χ (10−5−5−5−5 emu.mol/Oe) x=0.05

Sample/Parameters x=0.02 x=0.05 x=0.07 x=0.10 θθθθ

_C

(K) 770 750 730 690 µ

µµ

µ

eff

(µ µµ µ

B

) 1.3 1.4 1.43 1.54

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