**Raman Spectroscopy in Zinc Ferrites Nanoparticles**

**Raman Spectroscopy in Zinc Ferrites Nanoparticles**

DOI: 10.5772/intechopen.72864

Pietro Galinetto, Benedetta Albini, Marcella Bini and Maria Cristina Mozzati Maria Cristina Mozzati Additional information is available at the end of the chapter

Pietro Galinetto, Benedetta Albini, Marcella Bini and

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72864

#### **Abstract**

ZnFe2 O4 ferrite nanoparticles are arousing a great interest in the biomedical field, thanks to their superparamagnetic behavior at room temperature. Functional properties depend on composition, size, nanoparticle architecture and, in turn, on the synthesis methods. Bulk ZnFe<sup>2</sup> O4 has the normal spinel structure (all Zn2+ ions in tetrahedral and all Fe3+ ions in octahedral positions), but at the nanometric size inversion takes place with a cationic mixing on divalent and trivalent sites. The sensitivity of the Raman probe to cation disorder favored the appearance of several works on a rich variety of nanosized zinc ferrites. An overview on these results is reported and discussed at variance with synthesis methods, grain dimensions, and dopants. We add to this landscape our results from new nanosized powder samples made by microwave-assisted combustion, with different dopants (Ca, Sr on Zn site and Al, Gd on Fe site). A detailed analysis of A1g, E<sup>g</sup> , 3F2g Raman modes has been performed and Raman band parameters have been derived from bestfitting procedures and carefully compared to literature data. The vibrational results are discussed taking into account the characterization from X-ray powder diffraction raction, SEM-EDS probe, EPR spectroscopy and, of course, the magnetic responses.

**Keywords:** zinc ferrites, nanostructures, Raman spectroscopy, cation disorder

#### **1. Introduction**

Zinc ferrite is a very popular material widely used at the nanometric size in different applicative fields [1–4]. ZnFe2 O4 (ZFO) has a spinel structure and similarly to the whole ensemble of materials with the general formula MFe<sup>2</sup> O4 displays unique physical and chemical properties. In particular, zinc ferrite exhibits a peculiar mixing of high-quality functional properties. Indeed its magnetic, thermal, electrical, and mechanical properties coupled to a high chemical stability allowed its usage in magnetic storage, ferrofluids, catalysis, and biomedical applications, as for instance, theranostics and hyperthermia.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

The spinel ferrite system can be written as (Me1−xFex )2+[Me<sup>x</sup> Fe2−x]3+O4 and contains two Fe3+ cations and a single divalent cation. The round and square brackets denote sites of tetrahedral (A) and octahedral [B] coordination, respectively, while x represents the degree of inversion defined as the fraction of the (A) sites occupied by Fe3+. A completely inverse spinel is determined when the trivalent cations are equally distributed on tetrahedral and octahedral sites, while the divalent cations fill up the remaining half of the octahedral positions.

interactions, characterized by a 90° angle. As a result, both spins on B sites are antiparallel to the A spin. The magnetic response of spinel is thus determined by the cations distribution

remnant Fe3+ is placed on the A site. The magnetic moment of Fe3+ on A site cancels with the magnetic moment of Fe3+ on B site, and the resulting magnetic moment is the spin value for nickel Ni2+ (2.3 Bohr magnetons). This is a strong interaction with a resulting high-temperature Curie transition. In the case of zinc ferrite, iron ions occupy the B sites and zinc is located on the A site. Since Zn2+ is a 3*d*10 ion, it has no magnetic moment, and the only interaction is B-O-B, leading to an antiparallel arrangement of spins. The spins are identical, and the material becomes antiferromagnetic. The low Néel temperature clearly illustrates the weakness of

The behavior of bulk spinel-type ferrites change markedly moving to the nanoscale regime. In particular, the insurgence of a superparamagnetic (SPM) behavior at room temperature (RT) has been observed [6]. In systems that are ferro- and ferrimagnetic in bulk, SPM state can appear when the grain size is reduced to 50 nm or less. In SPM phase, the thermal energy kBT is greater than the magnetic anisotropy energy, and therefore random fluctuations of the magnetization are possible. The magnetization of a SPM material, above the so-called blocking temperature (TB), is equal to zero in the absence of an external field, and it rapidly increases under application of an external field. This fact implies a closed sigmoidal shape of the M-H curve without appreciable hysteresis. Experimentally, the value of TB typically corresponds to the "merging point" of the zero-field cooled (ZFC) and field-cooled (FC) magnetization curves [5]. SPM is not the only magnetic phenomenon caused by the finite size effect of nanoparticles. The reduction in size and the increase of the surface/volume ratio can, for example, produce randomly oriented uncompensated surface spins, canted spins, and magnetically dead layer

Anyways, superparamagnetism is especially important in applications such as drug delivery or MRI, where the nanoparticles exhibit no magnetic properties upon removal of the external field and therefore have no attraction for each other, eliminating the major driving force for aggregation. More importantly, superparamagnetic nanoparticles allow better control over the application of their magnetic properties because they provide a strong response to an

For biomedical application, it is thus essential to finely control the functional parameters: satura-

by tailoring the material properties in terms of size, shape, composition (substitutions, doping, cation distribution in the crystal structure), and shell-core design, with possible different chemical and physical structures of internal-core and surface-shell parts of the nanoparticle [7].

A large number of synthesis methods were reported, including conventional ceramic solid state synthesis, high-energy ball milling, microwave-assisted combustion, sol-gel, hydrothermal, co-precipitation, ultrasonic cavitation, and thermal plasma, with the peculiar aim to tune electric, catalytic, and magnetic properties, these last ones particularly appealing for biomedical application [8]. In the following, the most widespread synthesis methods for ferrites will be

), and blocking temperature (TB). This control can be made

), coercivity (H<sup>c</sup>

this superexchange interaction, as compared with A-O-B of the nickel ferrite case [5].

, B sites are occupied by both Ni2+ and Fe3+ ions and the

Raman Spectroscopy in Zinc Ferrites Nanoparticles http://dx.doi.org/10.5772/intechopen.72864 225

O4

on A and B sites. In inverse NiFe<sup>2</sup>

at the surface [7].

external magnetic field.

tion magnetization (M<sup>s</sup>

briefly described.

A very important characteristic of the spinel system is that it admits an extremely large variety of *total solid solutions*. The divalent cations are usually Zn2+, Ni2+, Mn2+, Ba2+, Cu2+, but partial substitutions of divalent or trivalent Fe ions are possible, preserving the spinel crystal structure. When zinc enters to form bulk zinc ferrite the system assumes a normal spinel structure AB2 O4 : the oxygen atoms are arranged a network of close-packed face-centered cubic units; the divalent Zn cations (A) occupy tetrahedral sites and trivalent Fe cations (B) octahedral sites. Spinel unit cell is composed of 8 units builders. In the full unit cell, there are 8 Me+2, 16 Fe+3 cations, and 32 anions oxide O−2 [1–4]. In **Figure 1**, a scheme of the direct spinel structure is reported.

The magnetic behavior of bulk ferrite system is strongly influenced by the nature, type, amount, and distribution of cations. Let us consider a deeply studied system, the mixed Zn-Ni ferrites (Zn1−*<sup>x</sup>* Ni*<sup>x</sup>* Fe2 O4 , with 0 ≤ *x* ≤ 1). The end compositions are zinc ferrite, ZnFe<sup>2</sup> O4 (for *x* = 0), i.e., a direct spinel, and nickel ferrite, NiFe<sup>2</sup> O4 (*x* = 1), an inverse spinel. In direct spinel ZnFe<sup>2</sup> O4 , the behavior is antiferromagnetic with a Néel temperature about 9 K, while for nickel ferrite is ferrimagnetic with a Curie point about T = 860 K [5]. The possibility to control the chemical composition allows to finely tailor the magnetic features. These different behaviors can be understood looking at the magnetic interactions between B and A sites surrounding an oxygen [5].

In spinel ferrites, there are two sublattices: the tetrahedral one with one cation per formula and the octahedral one with two cations per formula. The magnetic behavior is determined by superexchange interactions between two transition cations separated by an oxygen. The A-O-B interactions have an axial symmetry, and thus are highly more efficient than the B-O-B

**Figure 1.** The direct spinel-type structure; the red circles represent oxygen ions while the ochre and cyan solids represent tetrahedral and octahedral units containing at the center Zn and Fe cations, respectively.

interactions, characterized by a 90° angle. As a result, both spins on B sites are antiparallel to the A spin. The magnetic response of spinel is thus determined by the cations distribution on A and B sites. In inverse NiFe<sup>2</sup> O4 , B sites are occupied by both Ni2+ and Fe3+ ions and the remnant Fe3+ is placed on the A site. The magnetic moment of Fe3+ on A site cancels with the magnetic moment of Fe3+ on B site, and the resulting magnetic moment is the spin value for nickel Ni2+ (2.3 Bohr magnetons). This is a strong interaction with a resulting high-temperature Curie transition. In the case of zinc ferrite, iron ions occupy the B sites and zinc is located on the A site. Since Zn2+ is a 3*d*10 ion, it has no magnetic moment, and the only interaction is B-O-B, leading to an antiparallel arrangement of spins. The spins are identical, and the material becomes antiferromagnetic. The low Néel temperature clearly illustrates the weakness of this superexchange interaction, as compared with A-O-B of the nickel ferrite case [5].

The spinel ferrite system can be written as (Me1−xFex

AB2 O4

is reported.

224 Raman Spectroscopy

direct spinel, and nickel ferrite, NiFe<sup>2</sup>

(Zn1−*<sup>x</sup>* Ni*<sup>x</sup>* Fe2 O4 )2+[Me<sup>x</sup>

cations and a single divalent cation. The round and square brackets denote sites of tetrahedral (A) and octahedral [B] coordination, respectively, while x represents the degree of inversion defined as the fraction of the (A) sites occupied by Fe3+. A completely inverse spinel is determined when the trivalent cations are equally distributed on tetrahedral and octahedral sites,

A very important characteristic of the spinel system is that it admits an extremely large variety of *total solid solutions*. The divalent cations are usually Zn2+, Ni2+, Mn2+, Ba2+, Cu2+, but partial substitutions of divalent or trivalent Fe ions are possible, preserving the spinel crystal structure. When zinc enters to form bulk zinc ferrite the system assumes a normal spinel structure

The magnetic behavior of bulk ferrite system is strongly influenced by the nature, type, amount, and distribution of cations. Let us consider a deeply studied system, the mixed Zn-Ni ferrites

behavior is antiferromagnetic with a Néel temperature about 9 K, while for nickel ferrite is ferrimagnetic with a Curie point about T = 860 K [5]. The possibility to control the chemical composition allows to finely tailor the magnetic features. These different behaviors can be understood

In spinel ferrites, there are two sublattices: the tetrahedral one with one cation per formula and the octahedral one with two cations per formula. The magnetic behavior is determined by superexchange interactions between two transition cations separated by an oxygen. The A-O-B interactions have an axial symmetry, and thus are highly more efficient than the B-O-B

**Figure 1.** The direct spinel-type structure; the red circles represent oxygen ions while the ochre and cyan solids represent

tetrahedral and octahedral units containing at the center Zn and Fe cations, respectively.

, with 0 ≤ *x* ≤ 1). The end compositions are zinc ferrite, ZnFe<sup>2</sup>

looking at the magnetic interactions between B and A sites surrounding an oxygen [5].

O4

: the oxygen atoms are arranged a network of close-packed face-centered cubic units; the divalent Zn cations (A) occupy tetrahedral sites and trivalent Fe cations (B) octahedral sites. Spinel unit cell is composed of 8 units builders. In the full unit cell, there are 8 Me+2, 16 Fe+3 cations, and 32 anions oxide O−2 [1–4]. In **Figure 1**, a scheme of the direct spinel structure

while the divalent cations fill up the remaining half of the octahedral positions.

Fe2−x]3+O4

and contains two Fe3+

O4

(*x* = 1), an inverse spinel. In direct spinel ZnFe<sup>2</sup>

(for *x* = 0), i.e., a

O4 , the The behavior of bulk spinel-type ferrites change markedly moving to the nanoscale regime. In particular, the insurgence of a superparamagnetic (SPM) behavior at room temperature (RT) has been observed [6]. In systems that are ferro- and ferrimagnetic in bulk, SPM state can appear when the grain size is reduced to 50 nm or less. In SPM phase, the thermal energy kBT is greater than the magnetic anisotropy energy, and therefore random fluctuations of the magnetization are possible. The magnetization of a SPM material, above the so-called blocking temperature (TB), is equal to zero in the absence of an external field, and it rapidly increases under application of an external field. This fact implies a closed sigmoidal shape of the M-H curve without appreciable hysteresis. Experimentally, the value of TB typically corresponds to the "merging point" of the zero-field cooled (ZFC) and field-cooled (FC) magnetization curves [5].

SPM is not the only magnetic phenomenon caused by the finite size effect of nanoparticles. The reduction in size and the increase of the surface/volume ratio can, for example, produce randomly oriented uncompensated surface spins, canted spins, and magnetically dead layer at the surface [7].

Anyways, superparamagnetism is especially important in applications such as drug delivery or MRI, where the nanoparticles exhibit no magnetic properties upon removal of the external field and therefore have no attraction for each other, eliminating the major driving force for aggregation. More importantly, superparamagnetic nanoparticles allow better control over the application of their magnetic properties because they provide a strong response to an external magnetic field.

For biomedical application, it is thus essential to finely control the functional parameters: saturation magnetization (M<sup>s</sup> ), coercivity (H<sup>c</sup> ), and blocking temperature (TB). This control can be made by tailoring the material properties in terms of size, shape, composition (substitutions, doping, cation distribution in the crystal structure), and shell-core design, with possible different chemical and physical structures of internal-core and surface-shell parts of the nanoparticle [7].

A large number of synthesis methods were reported, including conventional ceramic solid state synthesis, high-energy ball milling, microwave-assisted combustion, sol-gel, hydrothermal, co-precipitation, ultrasonic cavitation, and thermal plasma, with the peculiar aim to tune electric, catalytic, and magnetic properties, these last ones particularly appealing for biomedical application [8]. In the following, the most widespread synthesis methods for ferrites will be briefly described.
