Keywords

1 Introduction

Spinel ferrites have received appreciable attention as an important magnetic material. They have been extensively explored for different applications in different domains of basic and applied sciences because of their certain desirable properties such as magnetic properties, electrical properties, optical properties, thermal and chemical stability. Their potential applications are also expanding to new areas like biomedical applications [1, 2], wastewater treatment [3], sensor technology [4], etc. with the progress in nanoscience and nanotechnology. Based on the Scopus database, research works on spinel ferrites were found to be reported since 1956 along with an exponential increase in the last 20 years which can be seen from Fig. 1.

Fig. 1
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Publications on spinel ferrites since 1956 based on Scopus database, searched by using keyword “spinel ferrites” on 13th October 2020

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Also according to the Scopus database, research works on spinel ferrites are found to be investigated in different subject areas which can be seen from Fig. 2. A major contribution comes from Material Sciences followed by Physics and Astronomy, Engineering, Chemistry, Chemical Engineering and a minor contribution from other fields. Although they possess certain desirable properties, most of their applications are found to have relied on their magnetic properties. Their properties are usually tuned by using different strategies such as composition variation, processing conditions, controlling morphologies, reducing size, adopting different preparation methods. This signifies a need for a clear understanding of factors affecting their properties and hence a precise control of their properties and for subjecting to certain applications. New preparation methods with the advancement of nanotechnologies lead toward the production of spinel ferrites into different morphologies with unique magnetic properties. Spinel ferrites of different morphologies such as nanospheres, microspheres, nanofibres, nanorods, nanowires, nanocomposites, core–shell structures were found to be reported by many researchers. Such a wide variety of spinel ferrites were found to be fabricated by different routes of synthesis methods such as co-precipitation, hydrothermal, sol–gel, electrospinning, thermal treatment method, etc. Also, emerging research on multifunctional materials consisting of a combination of magnetic spinel ferrites and other material like a luminescent material is found to be reported [5, 6]. The use of microstructural characterization techniques like SEM and TEM effectively provides their morphology which also helps in understanding their associated magnetic properties and hence further tailoring of their properties. This chapter gives a compilation report on research work conducted on nanostructured spinel ferrites, fundamental concepts of their magnetic properties, effects of processing conditions, composition, and morphologies thereby emphasizing their importance and emerging applications.

Fig. 2
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Percentage of published articles on spinel ferrites contributed from different branches of Science and Engineering based on Scopus database, extracted on 13th October 2020

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2 Magnetism in Spinel Ferrites

The unit cell of spinel ferrite (as shown in Fig. 3.) is made up of 8 formula units of AB2O4 formed by 32 anions and 24 cations. Nearly cubic close packing of anions offers 64 tetrahedral sites and 32 octahedral sites also called A-sites and B-sites respectively. Only 1/8th of tetrahedral sites and 1/2 of octahedral sites are occupied by cations. The spinel structure is categorized in the space group Fd3m.

Fig. 3
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Spinel structure generated using Vesta

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The Bravia lattice is a face-centered cubic with a basis formed by two formula units (2 AB2O4) [7]. According to the site occupancy of the cations, they are classified as normal, inverse, and mixed spinels. In a normal spinel, divalent ions take A-sites while trivalent ions take B-sites. In the case of inverse spinels, divalent ions take B-sites and trivalent ions are equally distributed between A and B-sites. Spinels having cation distribution between these two extremes are known as mixed spinels. Cation distribution in the spinels is dependent on factors such as temperature, cationic radii, cationic charge, crystal field effects, electrostatic contribution to lattice energy [7]. Cation distribution in spinels is not unique and for any composition of spinel, cation distribution has resulted from an equilibrium of oxygen positional parameter, inversion degree, and lattice constant.

Spinel ferrites have magnetic properties which are found to be greatly dependent on the constituent cations as well as their occupancy in A and B-sites. Using this fact or simply taking into account the magnetic moments associated with the cations and their preference for a specific site, the magnetic properties of spinel ferrites are usually tailored. A variety of spinel ferrites composed of transition metals and rare-earth elements of different compositions are found to be reported, some of which are listed in Table 1. The magnetism in spinel ferrites is found to be originated from the negative interaction or exchange force between the moments of the two cations on different interstitial sites which depends on the distances between the metal–oxygen-metal ions and the angle between them [8]. In general, the interaction is found to be greatest for an angle of 180° and the shortest inter-ionic distance. Three types of interactions are possible namely A-B, B-B, and A-A. Owing to preferable bond angles and lengths in the case of A-B interaction, it is the dominant exchange interaction among the three possible interactions. But, in the case of A-A and B-B interactions, the distances between the oxygen and metal ions are much large and the angle between the metal and oxygen ions are too small. Thus, the magnetic interaction in spinel ferrites, in general, comprises of strong A-B interaction with much weaker B-B interaction and most unfavorable A-A interaction. Neel first explained the ferrimagnetism of ferrites by considering the main negative interaction occurring between A and B-sites and thus developed the theory of ferrimagnetism [8].

Table 1 Spinel ferrites were prepared using different methods under different processing conditions
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3 Materials and Method

Table 1 represents an overview of some of the research works reported in spinel ferrites prepared through different methods and processing conditions giving rise to different sizes and morphologies and their associated magnetic properties. From the table, it can be found that a wide variety of methods are found to be adopted for different types of spinel ferrites ranging from a low-temperature method such as simple co-precipitation methods to conventional methods including high-temperature heat treatments. Desirable magnetic properties are found to be tuned by using a particular synthesis method or processing conditions. Apart from these, composition variation is another factor used for tailoring their properties. Table 1 elucidates that for the varying composition of spinel ferrites and their processing conditions, the important magnetic parameters like saturation magnetization, coercivity, and retentivity values are different. Spinel ferrites of different morphologies ranging from simple spherical nanoparticles to complex structures are found to be reported. Yuan et al. [9] reported studies on CoFe2O4 microspheres which are composed of assemblies of nanoparticles of CoFe2O4 of average sizes of 8 nm shown in Fig. 4. The microspheres of CoFe2O4 were prepared using the solvothermal method at different reaction times resulting in the average size distribution of 200 to 330 nm along with transformation from spherical to octahedral shapes. In their studies, 220 nm-sized CoFe2O4 microspheres composed of 8 nm nanoparticles were found to exhibit superparamagnetism. Li et al. [10] described the preparation of monodispersed CoFe2O4 nanoparticles using the hydrothermal method (as shown in Fig. 5.). The prepared nanoparticles were nearly spherical shaped with a mean size of 5.5 nm and were found to exhibit superparamagnetism at room temperature. Yang et al. [11] fabricated nanorods of Fe3O4 which are shown in Fig. 6 and investigated for hyperthermia applications. Gao et al. [12] reported studies on the directional dependency of nanowire arrays of ZnFe2O4 having about 16 nm diameter ordered in anodic aluminum oxide (AAO) templates that were prepared using the electrodeposition method (shown in Fig. 7). Maensiri et al. [13] reported studies on nanofibres of MgFe2O4/polyvinyl pyrrolidone (PVP) composites fabricated by the method of electrospinning. Their study shows that the morphology of the nanofibres greatly depends on the calcination temperature. The structural transformation (which is shown in Figs. 8. and 9) from smooth and uniform cross-section nanofibres to a structure of packed crystallites of about 10–20 nm for 700 °C and 25–80 nm for 800 °C calcined samples. Along with the increase in crystallinity, saturation magnetization values were also found to be enhanced with increasing calcination temperature. In addition to these, different structures namely coaxial nanobelts, Janus nanofibres, hollow nanofibers, sandwiched structures, nanorattles, microspheres, core–shell structures formed with other kinds of materials are also found to be reported which are being discussed in Sect. 6.

Fig. 4
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SEM (left) and TEM (right) images of CoFe2O4 microspheres prepared using solvothermal method at different reaction time of a 12 h, b 24 h, c 36 h [9]

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Fig. 5
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TEM images of monodispersed CoFe2O4 nanoparticles prepared using hydrothermal method [10]

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Fig. 6
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Images of Fe3O4 nanorods prepared using hydrothermal method, TEM (left), SEM (middle), HRTEM (right), adapted from [11]

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Fig. 7
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ZnFe2O4 nanowires arrays in AAO templates prepared using electrodeposition method a SEM, b TEM, c HRTEM d SAED pattern [12]

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Fig. 8
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SEM images of as-spun (a), b MgFe2O4/PVP composites and calcined for 2 h in air at c 500 °C, d 600 °C, e 700 °C and f 800 °C, adapted from [13]

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Fig. 9
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TEM images and SAED pattern of MgFe2O4/PVP composites calcined at a 700 °C and b 800 °C for 2 h in air [13]

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4 Characterization of Magnetic Properties

4.1 Fundamentals of Magnetization

The fascinating and versatile applications of magnetic materials are based on their magnetization curves or hysteresis loops. Applications of magnetic materials are determined using the knowledge of how they respond to a magnetic field which is represented by their hysteresis loops. It gives information about how magnetic materials take their path when they are subjected to a magnetic field. Magnetic materials are categorized based upon the characteristics of their magnetization curves or hysteresis loops. For instance, a paramagnetic material possesses a weak positive magnetization, showing a linear response when a field is applied. While ferromagnetic and ferrimagnetic types of materials show a non-linear S-shaped magnetization curve with a hysteresis loop. When a magnetic field is applied to ferromagnetic or ferrimagnetic materials, their magnetization increases until it reaches a maximum called saturation magnetization. But when the field is decreased from the saturation region it doesn’t return in the same path, rather takes up a new path retaining some magnetization even after complete removal of the applied field which is termed as retentivity. A field applied in opposite direction can remove the retentivity and is known as coercivity. With the further increase in the applied magnetic field along opposite direction, again saturation will be attained and after increasing the magnetic field along the original direction up to saturation, a complete hysteresis loop is formed. Based upon the coercivity and retentivity, they are categorized as hard and soft magnetic materials. A soft magnetic material refers to those having small values of coercivity and retentivity that are easy to magnetize and demagnetize while hard magnetic materials have the opposite case. Soft magnetic materials are thus found to be used in electromagnets, recording heads, transformer cores, etc. While hard magnetic materials are used in making permanent magnets, memory devices, loudspeakers, etc. [8, 14, 15].

4.2 Theoretical Models

The phenomenon of hysteresis is understood due to the existence of spontaneously magnetized small regions called magnetic domains in ferromagnetic and ferrimagnetic materials [8]. In a demagnetized state, domains are randomly oriented such that the specimen as a whole has zero magnetization but when a magnetic field is applied, they get aligned along the field direction and hence, a net magnetization. The process of magnetization varies in different regions consisting of three main different regions. Starting from the beginning, the 1st region with only reversible magnetization, the 2nd region with an additional non-reversible magnetization, and the 3rd region with reversible magnetization. The magnetization process takes place in two ways, one by magnetic domain wall motion that is the growth of favorably aligned domains at the cost of unfavorably aligned domains and the other by rotation of magnetic domains in the direction of the field applied. At the low field, magnetic domain wall motion dominates while at high field domain rotation dominates and the existence of both in between the two regions. Especially, the high field region of magnetization curves is studied by different forms of the law of approach to saturation. A model represents how basically the magnetization varies with the applied field which is usually expressed proportionally to magnetic field raised with different powers and their combinations [16, 17]. Their dependency and the constant associated with the different field terms give information on their magnetic microstructures, the directional dependency of magnetic properties or magnetic anisotropy, magnetic moments, etc. Increasing research work on tracing the magnetization curves using different models are found to be reported in different types of magnetic materials in different forms, shapes, dimensions for detailed information such as magnetic microstructures, anisotropies, magnetic moments which are intrinsically associated with them [17].

5 Magnetism in Spinel Ferrite Nanoparticles and Their Applications

Magnetic materials had become an indispensable one that covers a wide spectrum of applications like electronic devices, industrial applications, power supply, and storage devices, etc. [14, 15]. In addition, the emergence of nanoscience and nanotechnologies opens up entirely new scientific opportunities. Owing to its interdisciplinary nature, many applications are being extended to different areas like remedies to environmental problems and biomedical applications. It is found that magnetization curves or loops are also dependent on the size of the materials resulting in variation in the models in comparison with their bulk forms. It has been found that in the nano regime, the magnetic materials possess the characteristics of single domain nature which results in the phenomena of superparamagnetism and such materials are termed as superparamagnetic materials [8, 18]. Their behavior is similar to the paramagnetic material that is ideally no retentivity and no coercivity but possesses a much higher magnetization. These are reflected in their magnetization curves and detailed information on their magnetic microstructures can be traced by using suitable models like the Langevin function [19]. Their interesting and spectacular properties lead to new applications such as hyperthermia for cancer treatment, targeted drug delivery, biosensing, enhanced magnetic resonance imaging, etc. thereby widening up the applications of the magnetic materials. For instance, Yang et al. [11] demonstrated the applicability of Fe3O4 nanorods for use as magnetic hyperthermia agents. Nanorods of Fe3O4 were prepared by hydrothermal method using graphene oxide for avoiding aggregation at different reaction times and post-annealing. Spinel ferrites are also becoming an important candidate for a solution to environmental pollutions. Adsorption based on magnetic separation is desirable because of its effectiveness, low cost, and simple operation process, and hence nanosized spinel ferrites are becoming suitable candidates for adsorption.

Gao et al. [20] investigated the experiment on hollow α-Fe2O3 nanofibers for use as a dye adsorbent. In their studies, hollow α-Fe2O3 nanofibers were synthesized using a nanofiber template of polyvinyl alcohol by hydrothermal method with calcination. Their studies reported that the α-Fe2O3 nanofibers exhibited efficient adsorption of methyl orange present in water along with good magnetic response performance. Zhao et al. [21] studied the adsorption capability of Gd doped Co-ferrites for Congo- red for varying composition of Gd3+. Due to the great surface-to-volume ratio, nano-sized particles exhibit enhanced photocatalytic activity. Ali and Mostafa [22] investigated the photocatalytic activity of Mn-ferrites, Mn-Zn-ferrites, and Mn-Cd-ferrites in reduction of Cr(VI) to Cr(III). Madhubala et al. [23] studied the photocatalytic degradation of dye like methylene blue using Mn-Ni ferrites of 15–20 nm crystallites. Ahalya et al. [24] reported the applicability of Mn-Co-ferrites as adsorbents of Cr(VI). Their results showed the effective adsorption of the heavy metal using spinel ferrites as well as the possibility of reuse of the adsorbent and the heavy metal. Moreover, by combining their properties with other materials in desirable morphologies, new applications in different areas have been reported which are being discussed in Sect. 6. Better understanding leads to the development of new tricks in expanding their applications. Thus, it can be said that the scenario for the application of magnetic materials is expanding because of more understanding of their behavior.

6 Hybrid-Structured with Other Materials and Their Applications

Apart from the above-discussed applications, magnetic materials are also becoming an important component in multifunctional materials. As magnetic materials can be simply controlled with a magnetic field applied externally, they are also used as an important component in multifunctional materials, vehicles for drug delivery in specific sites inside the human body, and many others. Bifunctional magnetic and luminescent materials are reported to be investigated in biomedical applications such as transporters for drug delivery, agents for MRI, hyperthermia, etc. using simultaneously the magnetic and the luminescent property as the control and tracking respectively. A core–shell structured material comprising of a magnetic core with a luminescent shell occupies an important place in such a class of hybrid-structured materials. For instance, Sun et al. [80] reported multifunctional properties of CdTe quantum dots linked with silica-coated Fe3O4 nanoparticles having superparamagnetic properties and their potential applications as immune-labeling with fluorescent imaging of tumor cells. By integrating the magnetic and fluorescent properties into single nanostructured composites their potential applicability as simultaneous targeted drug delivery and bioimaging were investigated. Their studies found that the prepared nanocomposites can be magnetically guided for the delivery of drugs and at the same time their fluorescent property enabled the optical imaging of the nanocomposites and hence with the feasibility of their optical tracing. Atabaev et al. [81] reported fabrication and characterization of bifunctional composites comprising of magnetic Fe3O4 particles coated with luminescent Y2O3:Tb3+ shell. The average size of about 306 to 330 nm magnetic core covered with a phosphor shell of a thickness of about 25 nm was found to exhibit desirable magnetic and luminescent properties suggesting ease for magnetic targeting and separation as well which may find applications in biomedical and bioanalytical applications. Liu et al. [5] investigated bifunctional properties of core–shell Fe3O4-CdSe nanoparticles prepared by a polyol process. Magnetic Fe3O4 core of about 10 nm diameter covered with CdSe luminescent shell of about 2 nm thicknesses. Sun et al. [82] reported the fabrication of hybrid materials consisting of Fe3O4 nanoparticles encapsulated with SiO2 and functionalized by YVO4:Eu3+ phosphors. The prepared nanocomposites were found to exhibit good ferrimagnetic behavior with a strong red emission. Shi et al. [6] prepared and investigated bifunctional properties of Fe3O4@C/YVO4:Sm3+ microspheres synthesized by hydrothermal combined with the sol–gel method. In their work, the carbon layer was used to protect Fe3O4 particles from oxidation and protect the lanthanide-based luminescent shell from quenching of luminescence due to Fe3O4. Strong red–orange emission and good ferrimagnetic behavior were observed in their composites. Zhang et al. [83] conducted the studies on nanorattles composed of SiO2 coated Fe3O4 covered with luminescent shells of α-NaYF/Yb, Er which was fabricated using an ion-exchange route for application in targeted chemotherapy. The mesoporous composites were found to possess both upconversion luminescent and magnetic properties along with a high capacity for loading drugs, low cytotoxicity, and excellent cell imaging properties. Yang et al. [84] also reported similar sandwich structured materials having magnetic, mesoporous and luminescent properties. Microspheres of Fe3O4 encapsulated with silica and functionalized through YVO4:Eu3+ phosphor deposition which was prepared using a combination of hydrothermal and sol–gel method with heat treatment. The resulted composites were found to possess ordered hexagonal mesoporous, good luminescent properties and high magnetization values and were proposed for using as potential candidates for drug delivery system.

Also, core–shell structures with Fe3O4 as cores and other luminescent shells such as YVO4:Eu3+ [85] and Gd2O3:Eu3+ composites [86] having good magnetic and luminescent properties are also reported. Apart from the core–shell structures, other forms of composites are also found. For instance, Huarac et al. [87] prepared the composites of magnetic Fe2O3 and luminescent ZnS: Mn nanoparticles prepared by the co-precipitation method. Highly crystalline two phases were found to co-exist in XRD studies and the clusters of nanoparticles of Fe3O4 and ZnS: Mn existed side by side from HRTEM studies. In addition to these, nanofibre composites of different types such as core–shell nanofibers, nanobelts, Janus nanofibres are also reported. For instance, using the electrospinning method, flexible hollow nanofibers of Fe3O4/Eu(BA)3 phen/PVP [88] and core–shell nanofibres composites of Fe3O4/PVP@NaYF4:Yb3+, Er3+/PVP [89] having bifunctional properties were fabricated. Xue et al. [90] reported the fabrication of coaxial nanobelts with tunable bifunctional properties of magnetic and luminescent properties. The composites were composed of the magnetic core of CoFe2O4/polymethyl methacrylate (PMMA) and photoluminescent shells of [Tb(BA)3(phen) + Eu(BA)3 (phen)]/PMMA synthesized by the electrospinning method. Zhou et al. [91] investigated bifunctional magnetic and luminescent properties of double-stranded nanofibers called Janus nanofibers fabricated by the electrospinning method. A Janus nanofiber is composed of side-by-side assembled two strands of nanofibers, one possessing magnetic properties and the other one having luminescent properties. It was found that they have superior luminescent and magnetic properties owing to their special nanostructures and tunable colors based on their composition. Their work demonstrated an approach for the preparation of composites of controlled and tunable luminescent properties, expected to find applications in magnetic nano-bio-label and anti-counterfeit materials, etc. Thus, nanocomposites having combined properties such as magnetically responsive and luminescent properties with different morphologies were proposed to find applications in biomedical applications like drug delivery, targeting on specific sites, bio-separation, and diagnostic applications.

7 Conclusion

To conclude, it can be summarized that the ever-increasing research work on spinel ferrites and their applications in different fields can be observed with the exposure of nanoscience. A variety of compositions of spinel ferrites in different morphologies were fabricated and investigated through different experimental approaches of nanotechnology. Spinel ferrites of different types, compositions, and their composites of different morphologies were found to be fabricated by different routes of synthesis such as co-precipitation, auto combustion, sol–gel, thermal decomposition, microemulsion, hydrothermal, solvothermal method, and electrospinning method. In addition to their composition, their properties also depend on a particular method, specific processing conditions, and morphologies. Spinel ferrites of desirable nanostructured morphologies have been able to realize using advanced techniques in fabrication and microscopy at nano levels. These advanced nanostructured materials on the other hand lead toward new emerging applications which extend to many multidisciplinary areas such as dye degradation, adsorption of heavy metals, drug delivery, hyperthermia applications. Nanoscience and nanotechnology also lead toward the development of hybrid-structured materials possessing bifunctional properties. As a magnetic component in bifunctional materials, Fe3O4 has been reported to be successfully used by many researchers. So, it is desirable to extend the investigation also to other varieties of spinel ferrites. Moreover, the development of advanced bifunctional materials could lead toward device miniaturization, designing of cost-effective and energy-efficient devices, etc. The combination of magnetic materials with other different properties will open up new exciting applications in addition to the improvement in the existing applications.