**3. Tuning via fuel-to-oxidizer ratio**

transformation, heterogeneous catalysis, magnetocaloric refrigeration, and medical diagnosis [1]. Nanoparticles are much more active than larger bulk particles because of their higher surface area, and they display unique physical and chemical properties [2]. Spinel oxides

Spinel oxides can be prepared by various techniques such as forced hydrolysis, microwave synthesis, sol-gel method, co-precipitation, polyol, self-combustion reaction, and sonochemical [3–7] methods. Among these methods, the self-combustion method, particularly solution self-combustion, is one easy method through which highly pure crystalline and homogenous material can be prepared with high yield [8]. In solution self-combustion method, it is easy to control the stoichiometry and crystallite size, through preparation conditions and post treatments, which have an important direct influence on the magnetic properties of the ferrite.

In solution self-combustion, there are two components, nitrate and fuel. The fuel helps in the combustion of nitrates. The powder characteristics such as crystallite size, surface area, and size distribution are governed by enthalpy or flame temperature generated during combustion which itself depends on the nature of the fuel and fuel-to-oxidizer ratio [9]. There are number of different fuels active in self-combustion such as glycine, citric acid, tartaric acid, urea, etc. Among these fuels, citric acid is a good one to initiate combustion reaction due to its

The Cr3+cations have strong preference to occupy in the B-site and have an affinity for antiferromagnetic coupling with Fe ions. The partial or total substitution of one of the Fe3+ cations with Cr3+ cations induces magnetic frustrations in the spinel ferrite system, which leads to interesting magnetic properties. Therefore, it is very interesting to study variation in magnetic

Highly pure and identical nanoparticles are essential for excellent performance of the materials. By tuning of structural and magnetic properties, we can have highly pure and identical nanoparticles with exact physical and chemical properties suitable for particular application. In application level, large-scale synthesis at low cost is needed. With these aims, this work presents low cost synthesis of highly pure nanospinel ferrites via solution self-combustion technique, and the tuning of structural and magnetic properties is undertaken in terms of fuel-to-nitrate ratio and Cr doping. This synthesizing process neither requires sophisticated

properties on doping of chromium ion in the place of Fe ion in spinel system [11].

, and Zn CrFeO<sup>4</sup>

H8 O7 H2

self-combustion technique using analytical grade corresponding to high purity metal nitrates

ratio of citric acid to metal nitrate (F/N ratio) was taken as 0.65 for fuel lean sample, 1 for stoichiometric sample, and 1.35 for fuel rich samples. The solutions of three nitrates (Fe, Cr, Co/Ni/Zn) and citric acid were prepared separately, mixed, and stirred completely to form a

fuel lean, stoichiometric and fuel rich sam-

O) as oxidizing agents and fuel, respectively. The

(with x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) were prepared by solution

, where D is a divalent cation like Co, Ni, Zn, Mn, Mg, Cu, etc.

have general formula DFe<sup>2</sup>

98 Recent Advances in Porous Ceramics

O4

negative combustion heat, −2.76 kcal/g [10].

instruments nor a high sintering temperature.

, CoCrFeO4

Fe2−xO4

**2. Experimental**

Nanosized NiCrFeO4

ples, and fuel rich CoCrx

(Merck-Germany) and citric acid (C6

XRD spectra of all CoCrFeO<sup>4</sup> , NiCrFeO4 , and ZnCrFeO<sup>4</sup> samples are shown in **Figures 1**–**3**, respectively. Well-defined characteristic reflections in powder X-ray diffraction patterns clearly indicate the formation of the spinel structure. XRD pattern of all the stoichiometric and fuel rich samples shows that they have single-phased cubic spinel structure, whereas fuel lean samples show a small percentage of impurity phases, α-Ni in fuel lean NiCrFeO<sup>4</sup> , α-Co in fuel lean CoCrFeO4 , and α-Fe<sup>2</sup> O3 in ZnCrFeO<sup>4</sup> . Average crystallite sizes (D) for the samples are calculated by substituting the FWHM values of the maximum intensity (311) peaks in the Scherrer formula:

$$\text{Crystalite sizes (D)} = \frac{0.9\lambda}{\beta \text{Cost}\theta} \tag{1}$$

where λ is the wavelength of X-rays and *β* is the width of the most intense peak in the XRD after correction for instrumental broadening. Actual density (ρ) of the samples is determined

**Figure 1.** Powder XRD of CoCrFeO<sup>4</sup> samples with Rietveld fitting, reprinted from Sijo [15], with permission from Elsevier.

**Figure 2.** Powder XRD of NiCrFeO<sup>4</sup> samples with Rietveld fitting, reprinted from Sijo [15], with permission from Elsevier.

using Archimedes principle. Porosity of the samples is calculated from the actual and x-ray densities. Crystallite size (D), density (ρ), and porosity of all samples are given in **Table 1**.

defects in the form of vacancies in the spinel ferrite phase and leads to an expansion of the lattice, which in turn makes the material porous and increases the crystallite size. In fuel lean samples, the amount of fuel is not adequate enough to react completely with metal nitrates and to release enough heat to form well-developed nanosized grains. Since the combustion reaction is incom-

**Density (gm/ cm3 )**

CoCrFeO4 0.65 29.5 4.0 22 5.9 28.5 352 CoCrFeO4 1;1 3.4 4.6 12 0.3 4.8 580 CoCrFeO4 1.35;1 5.7 4.7 10 0.5 5.5 645 NiCrFeO4 0.65 30.1 2.8 46 6.4 27.8 182 NiCrFeO4 1;1 3.2 3.6 32 0.25 2.3 192 NiCrFeO4 1.35;1 5.7 4.1 23 0.6 5.5 118 ZnCrFeO<sup>4</sup> 0.65 15 2.3 57 0.05 1.43 51 ZnCrFeO<sup>4</sup> 1;1 5 3.4 35 0.02 0.83 49 ZnCrFeO<sup>4</sup> 1.35;1 6 4.2 23 0.001 0.73 16

**Porosity (%)**

**MR (emu/g) (±0.1)**

Tailoring of the Magnetic and Structural Properties of Nanosized Ferrites

**MS (emu/g) (±0.1)**

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**HC (Oe) (±1)**

101

O3

samples prepared under the fuel lean condition. From these results, it is clear that stoichiometric

The variation of crystallite size with fuel content is plotted in **Figure 4**. From the figure, it is clear that the finest particles are obtained for stoichiometric fuel-to-nitrate ratio, and there is a small increase of crystallite size in the sample prepared under the fuel rich condition.

fuel-to-oxidizer ratio results finest single-phased nanoparticles [14–17].

**Figure 4.** Crystallite size with fuel-to-nitrate ratio: modified from Sijo [15] and Sijo et al. [14].

**Table 1.** Structural and magnetic parameters for the influence of fuel-to-nitrate ratio.

are also formed along with the spinel phase in

plete, impurity phases of α-Ni, α-Co and α-Fe<sup>2</sup>

**Sample name F/N ratio (φ)**

Source: Sijo [15] and Sijo et al. [14].

**Crystallite size (nm) (±1)**

It is seen from **Table 1** that the crystallite size of fuel lean sample is many times greater than that of the stoichiometric samples and fuel rich samples. Presence of the impurity phase creates lattice

**Figure 3.** Powder XRD of ZnCrFeO<sup>4</sup> samples with Rietveld fitting [14].


**Table 1.** Structural and magnetic parameters for the influence of fuel-to-nitrate ratio.

using Archimedes principle. Porosity of the samples is calculated from the actual and x-ray densities. Crystallite size (D), density (ρ), and porosity of all samples are given in **Table 1**.

samples with Rietveld fitting, reprinted from Sijo [15], with permission from Elsevier.

It is seen from **Table 1** that the crystallite size of fuel lean sample is many times greater than that of the stoichiometric samples and fuel rich samples. Presence of the impurity phase creates lattice

samples with Rietveld fitting [14].

**Figure 3.** Powder XRD of ZnCrFeO<sup>4</sup>

**Figure 2.** Powder XRD of NiCrFeO<sup>4</sup>

100 Recent Advances in Porous Ceramics

defects in the form of vacancies in the spinel ferrite phase and leads to an expansion of the lattice, which in turn makes the material porous and increases the crystallite size. In fuel lean samples, the amount of fuel is not adequate enough to react completely with metal nitrates and to release enough heat to form well-developed nanosized grains. Since the combustion reaction is incomplete, impurity phases of α-Ni, α-Co and α-Fe<sup>2</sup> O3 are also formed along with the spinel phase in samples prepared under the fuel lean condition. From these results, it is clear that stoichiometric fuel-to-oxidizer ratio results finest single-phased nanoparticles [14–17].

The variation of crystallite size with fuel content is plotted in **Figure 4**. From the figure, it is clear that the finest particles are obtained for stoichiometric fuel-to-nitrate ratio, and there is a small increase of crystallite size in the sample prepared under the fuel rich condition.

**Figure 4.** Crystallite size with fuel-to-nitrate ratio: modified from Sijo [15] and Sijo et al. [14].

**Figure 5.** Room temperature magnetization curve of CoCrFeO4 samples.

Smaller crystallite sizes of samples prepared under stoichiometric ratio may be due to the fact that more gaseous products are formed under this condition, leading to the breaking up of particles on escaping and resulting in finer particles. The superior powder properties present in CoCrFeO4 , ZnCrFeO<sup>4</sup> , and NiCrFeO4 samples prepared on stoichiometric ratio is due to the dominant effect of the number of gas molecules over the flame temperature [14–17].

magnetization of these samples is estimated by plotting M vs. 1/H for 1/H tending to zero. All the DC magnetization parameters are listed in **Table 1**. As seen from structural characteriza-

fuel lean condition have the presence of a small amount of impurity phases α-Co, α-Ni, and

uted to the presence of impurity phases. The saturation magnetization depends on the average crystallite size. As the size increases, the saturation magnetization is also increased. From

and remenance (MR) are highly dependent upon the fuel-to-nitrate ratio, and we can tune the structural and magnetic properties of spinel ferrites by changing fuel-to-oxidizer ratio.

We can tune the structural and magnetic properties of spinel ferrites by suitable substitution

combined X-ray diffraction spectra for Cr doped powder are shown in the **Figure 8(a)**. These diffraction spectra provide clear evidence of the formation of ferrite spinel phase in all the samples. The broad XRD line indicates that the ferrite particles are of nanosize. The crystallite size for each composition was calculated from Rietveld fitting of spectra and given in **Table 2**. Room temperature magnetization measurements are taken for all samples and shown in **Figure 8(b)**. The magnetic parameters of all the samples are obtained and tabulated in **Table 2**, the value of remanence and coercivity obtained directly from individual M-H loops, while saturation

From **Table 2**, the crystallite size, remanence, saturation magnetization, and coercivity were

**Table 1**, we can conclude that the behavior of saturation magnetization (MS

magnetization obtained by plotting 1/H vs. M and extrapolating to zero.

Fe2−xO4

, NiCrFeO4

samples [14].

, respectively. The higher magnetic moments observed in these samples can be attrib-

, and ZnCrFeO<sup>4</sup>

Tailoring of the Magnetic and Structural Properties of Nanosized Ferrites

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103

(with x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0). The

samples and are shown in **Figure 9**. From this figure, we

samples prepared at

), coercivity (HC),

tion using Rietveld fitted XRD, the CoCrFeO<sup>4</sup>

**Figure 7.** Room temperature magnetization curve of ZnCrFeO<sup>4</sup>

**4. Tuning via Cr3+ substitution**

plotted vs. Cr content of CoCr<sup>x</sup>

also. For this, consider the samples CoCrxFe2−xO4

α-Fe<sup>2</sup> O3

Room temperature (RT) M-H loops of CoCrFeO<sup>4</sup> , NiCrFeO4 , and ZnCrFeO<sup>4</sup> samples are shown in **Figures 5**–**7**, respectively. The coercivity and remenance are obtained directly from the M-H loops. Some of the samples show nonsaturating behavior, and hence, the value of saturation

**Figure 6.** RT magnetization curve of NiCrFeO4 samples. **Figures 5** and **6** are reprinted from Sijo [15], with permission from Elsevier.

**Figure 7.** Room temperature magnetization curve of ZnCrFeO<sup>4</sup> samples [14].

magnetization of these samples is estimated by plotting M vs. 1/H for 1/H tending to zero. All the DC magnetization parameters are listed in **Table 1**. As seen from structural characterization using Rietveld fitted XRD, the CoCrFeO<sup>4</sup> , NiCrFeO4 , and ZnCrFeO<sup>4</sup> samples prepared at fuel lean condition have the presence of a small amount of impurity phases α-Co, α-Ni, and α-Fe<sup>2</sup> O3 , respectively. The higher magnetic moments observed in these samples can be attributed to the presence of impurity phases. The saturation magnetization depends on the average crystallite size. As the size increases, the saturation magnetization is also increased. From **Table 1**, we can conclude that the behavior of saturation magnetization (MS ), coercivity (HC), and remenance (MR) are highly dependent upon the fuel-to-nitrate ratio, and we can tune the structural and magnetic properties of spinel ferrites by changing fuel-to-oxidizer ratio.

#### **4. Tuning via Cr3+ substitution**

Smaller crystallite sizes of samples prepared under stoichiometric ratio may be due to the fact that more gaseous products are formed under this condition, leading to the breaking up of particles on escaping and resulting in finer particles. The superior powder properties present

in **Figures 5**–**7**, respectively. The coercivity and remenance are obtained directly from the M-H loops. Some of the samples show nonsaturating behavior, and hence, the value of saturation

, NiCrFeO4

samples.

dominant effect of the number of gas molecules over the flame temperature [14–17].

samples prepared on stoichiometric ratio is due to the

, and ZnCrFeO<sup>4</sup>

samples. **Figures 5** and **6** are reprinted from Sijo [15], with permission

samples are shown

in CoCrFeO4

102 Recent Advances in Porous Ceramics

, ZnCrFeO<sup>4</sup>

**Figure 6.** RT magnetization curve of NiCrFeO4

from Elsevier.

Room temperature (RT) M-H loops of CoCrFeO<sup>4</sup>

**Figure 5.** Room temperature magnetization curve of CoCrFeO4

, and NiCrFeO4

We can tune the structural and magnetic properties of spinel ferrites by suitable substitution also. For this, consider the samples CoCrxFe2−xO4 (with x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0). The combined X-ray diffraction spectra for Cr doped powder are shown in the **Figure 8(a)**. These diffraction spectra provide clear evidence of the formation of ferrite spinel phase in all the samples. The broad XRD line indicates that the ferrite particles are of nanosize. The crystallite size for each composition was calculated from Rietveld fitting of spectra and given in **Table 2**. Room temperature magnetization measurements are taken for all samples and shown in **Figure 8(b)**. The magnetic parameters of all the samples are obtained and tabulated in **Table 2**, the value of remanence and coercivity obtained directly from individual M-H loops, while saturation magnetization obtained by plotting 1/H vs. M and extrapolating to zero.

From **Table 2**, the crystallite size, remanence, saturation magnetization, and coercivity were plotted vs. Cr content of CoCr<sup>x</sup> Fe2−xO4 samples and are shown in **Figure 9**. From this figure, we

**Figure 8.** X-ray diffractograms (a) and RT magnetization curves (b) of Cr-doped powder samples. Reprinted from Sijo [13], with permission from Elsevier.

can clearly observe that crystallite size, remanence, saturation magnetization, and coercivity decreases, as chromium content increases. The area of hysteresis curve linearly decreases with increase in Cr content. That is, CoFe2 O4 has the largest area, and CoCrFeO4 has the smallest area inside the hysteresis loop. This indicates that the increased Cr substitution has made the material magnetically soft [14–17].

#### **5. Conclusion**

High-purity magnetic spinel ferrites can easily prepare by solution self-combustion method. It provides energy and cost-saving advantages over other methods. The finest nanoparticles obtained for stoichiometric fuel-to-nitrate ratio in solution self-combustion method. Increased Cr substitution in ferrites has made the material magnetically soft. That is, the structural and magnetic properties of ferrites are function fuel content and doping content, and therefore, they are highly tuneable via selection of proper preparation method, proper fuel content,


and via proper substitution. Therefore, we can desirably modify properties of spinel ferrites

Fe2−xO4

Tailoring of the Magnetic and Structural Properties of Nanosized Ferrites

http://dx.doi.org/10.5772/intechopen.72382

105

powder

I acknowledge DAE-BRNS, Department of Physics, Mohanlal Sukhadia University Udaipur and my PhD supervisor Dr. K. Venugopalan (Late) Mohanlal Sukhadia University Udaipur. The contents of chapters three and four of my PhD thesis titled—*Studies of Magnetic and* 

according to the requirement and can use in wide range of applications.

samples. Reprinted from Sijo [13], with permission from Elsevier.

**Figure 9.** Crystallite size, remanence, saturation magnetization and coercivity vs. Cr content of CoCrx

**Acknowledgements**

**Table 2.** Structural and magnetic parameters of CoCrx Fe2−xO4 samples [13].

**Figure 9.** Crystallite size, remanence, saturation magnetization and coercivity vs. Cr content of CoCrx Fe2−xO4 powder samples. Reprinted from Sijo [13], with permission from Elsevier.

and via proper substitution. Therefore, we can desirably modify properties of spinel ferrites according to the requirement and can use in wide range of applications.

#### **Acknowledgements**

can clearly observe that crystallite size, remanence, saturation magnetization, and coercivity decreases, as chromium content increases. The area of hysteresis curve linearly decreases with

**Figure 8.** X-ray diffractograms (a) and RT magnetization curves (b) of Cr-doped powder samples. Reprinted from Sijo

area inside the hysteresis loop. This indicates that the increased Cr substitution has made the

High-purity magnetic spinel ferrites can easily prepare by solution self-combustion method. It provides energy and cost-saving advantages over other methods. The finest nanoparticles obtained for stoichiometric fuel-to-nitrate ratio in solution self-combustion method. Increased Cr substitution in ferrites has made the material magnetically soft. That is, the structural and magnetic properties of ferrites are function fuel content and doping content, and therefore, they are highly tuneable via selection of proper preparation method, proper fuel content,

**Sample name Crystallite size (nm) (±1) MR (emu/g)(±0.1) MS (emu/g) (±0.1) HC (Oe) (±1)** CoCr0.0Fe2.0O4 21 17.9 50.8 1080 CoCr0.2Fe1.8O4 12 5.7 35.8 377 CoCr0.4Fe1.6O4 11 1.9 25.3 192 CoCr0.6Fe1.4O4 09 2.1 16.9 291 CoCr0.8Fe1.2O4 07 1.1 14.6 227 CoCr1.0Fe1.0O4 05 0.4 2.6 378

Fe2−xO4

samples [13].

has the largest area, and CoCrFeO4

has the smallest

O4

increase in Cr content. That is, CoFe2

**Table 2.** Structural and magnetic parameters of CoCrx

material magnetically soft [14–17].

[13], with permission from Elsevier.

104 Recent Advances in Porous Ceramics

**5. Conclusion**

I acknowledge DAE-BRNS, Department of Physics, Mohanlal Sukhadia University Udaipur and my PhD supervisor Dr. K. Venugopalan (Late) Mohanlal Sukhadia University Udaipur. The contents of chapters three and four of my PhD thesis titled—*Studies of Magnetic and*  *Transport Properties of Some Nanosized Fe and Cr Based Spinels* have used for writing this chapter. I also acknowledge Nirmalagiri College, Kannur, Kerala for providing additional facilities for my research.

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