A comparative study of spinel ZnFe 2 O 4 ferrites obtained via a hydrothermal and a ceramic route: structural and magnetic properties

The spinel ZnFe 2 O 4 specimens were obtained via a hydrothermal and a ceramic method, respectively, and their structural and magnetic properties were comparatively studied. It was found that all the specimens exhibited a single-phase and mixed spinel structure. The magnetism of specimens synthesized via the hydrothermal method is obviously greater than that of specimen prepared via the ceramic method. This can be ascribed to the occupancy of Fe ions resulted from the loss of Zn during the hydrothermal process.


Introduction
The spinel structure with a formula of AB 2 O 4 is widely studied and used in the fields of electronics, magnetism, catalyst, energy storage and so on [1][2][3][4][5][6][7][8]. Generally, A denotes the divalent metal ions, such as Mg 2+ , Fe 2+ , Co 2+ , Ni 2+ , Mn 2+ and Zn 2+ , and B denotes the trivalent metal ions, such as Al 3+ , Fe 3+ , Co 3+ and Cr 3+ . Spinel ferrites (AFe 2 O 4 ) are usually used as the magnetically soft materials, such as MgFe 2 O 4 [6], [7], NiFe 2 O 4 [8] and so on. Spinel ZnFe 2 O 4 (ZFO) ferrite is widely studied as a kind of potential catalytic or anodic material [10][11][12][13]. Theoretically, ZFO is a typical normal spinel ferrite with a cation distribution in lattice sites as the following: Where Zn 2+ in A site is nonmagnetic, and the magnetic moment of two Fe 3+ ions in B sites presents an antiparallel arrangement as described by the Yafet-Kittle configuration [14,15], which results in the zero magnetic moment in ZFO. Therefore, if partial Fe 3+ ions in B sites are forced to locate in A sites, the magnetism in ZFO could transform from antiferromagnetism to ferrimagnetism. This is instructive to the study of magnetism in spinel structures, and consequently the magnetism in ZFO also aroused the interest of some researchers [16][17][18]. It has been proved that the sintering  temperature should be a key factor for the site occupancy of Fe 3+ [16,17]. In this study, we prepared the ZFO specimens via a hydrothermal method and a traditional ceramic method, respectively, and proved that the Zn-deficiency can also cause the transition of magnetism from antiferromagnetism to ferrimagnetism in ZFO. hours to be used as the CZFO-750 specimen in this study.

Characterization
The phase composition of specimens were identified and confirmed by using an X-ray diffractometer (XRD, Rigaku D/max-2550V/PC) using Cu K radiation and a

HZFO synthesized via a hydrothermal route
In this study, ZFO specimens were prepared using the Fe/Zn atomic ratio of 2:1, a stoichiometric composition, in the starting materials. Fig. 1 shows the XRD patterns of as-synthesized HZFO specimens with different T h . Typical peak information from ZFO without any other impurity peaks is revealed in the three XRD patterns, indicating their single-phase spinel ZFO structure (PDF#22-1012) in the space group The results of FTIR can help to confirm the formation of spinel structure and give some information of chemical bond. Fig. 2 gives the FTIR spectra of HZFO specimens. The characteristic absorption bands at around 450 cm -1 and 575 cm -1 can be ascribed to the bending or stretching vibrations of A-O 4 and B-O 6 in spinels, which is consistent with the results in previous reports [10,20], suggesting the formation of spinel structure.
Using the XRD patterns in Fig Obviously, the count from both Zn and Fe are uniformly distributed in the selected area just like that in Fig. 5, also indicating the uniformity of chemical composition in CZFO-750.

Magnetic properties
The magnetism of spinels is associated with the difference of magnetic moments between A and B sites [9]. According to the cation distribution in lattice sites described above for a normal spinel ferrite, the magnetic moment in a ZFO molecular formula can be calculated as: Where M A and M B present the magnetic moment in A and B sites, respectively.
Therefore, generally, the normal spinel ferrite should exhibit zero magnetic moment.
The RT magnetic hysteresis loops of HZFO specimens are illustrated in Fig.7.
According to the Ref. [18] combined with D obtained from the results of XRD and TEM, the negligible coercivity (H c ) and remanent magnetism (M r ) together with unsaturated magnetization at relatively high magnetic field could help to confirm the superparamagnetism of our HZFO nanoparticles. However, the superparamagnetism of HZFO nanoparticles is not the emphasis in this study, and will be discussed in another report systematically. Therefore, the dependence of magnetization (M) on the temperature (T) under the zero field cooling (ZFC) and field cooling (FC) that can confirm the existence of superparamagnetism is not given here.
Seen from Fig. 7 exhibiting a value much higher than zero. As is well-known, Zn(OH) 2 is amphoteric, which can also be dissolved in the alkaline solution [21]. Therefore, for a hydrothermal method, Zn 2+ is easy to lose in the aqueous solution during the synthesis.
As described above, theoretically, the molecular magnetic moment of normal ZFO should be zero. However, the Zn-deficiency in A sites could result in the occupancy of partial Fe ions in A sites, and consequently form the mixed spinel ZFO. According to the Yafet-Kittle configuration for spinels [15], this was sure to result in that the magnetic moment in A and B sites could not cancel each other out, which would enhance the molecular magnetic moment. As for the increasing trend of M s with the increasing T h , it should deal with the occupancy of Fe as well as the better crystallinity of specimens at higher T h [22].
In order to prove the Zn-deficiency in HZFO specimens, the ICP results of both HZFO and CZFO-750 specimens are listed in Table 1. It can be seen that for HZFO specimens, the R a is obviously higher than 2, the nominal composition in the starting materials. This is obviously related with the loss of Zn during the hydrothermal synthesis process as described above. However, the R a of CZFO-750 is 1.984, which is very close to 2. As is well-known, Zn 2+ ions are not easy to lose during a ceramic process.
The RT magnetic hysteresis loop of CZFO-750 is also given in Fig. 7. It can be seen that the M was not saturated at the maximum external field of about 2388 kA·m −1 (30,000 Oe) and was nearly proportional to the external magnetic field.
Moreover, the M at the maximum external field of ±2388 kA·m −1 is lower than 4 emu/g, a very small value compared with that of the HZFO specimens.
In order to further interpret the difference in magnetism between HZFO and CZFO, the XPS spectra of two typical specimens HZFO-200 and CZFO-750 were presented in Fig. 8 to show more information of ion occupancy in A and B sites, and the corresponding binding energies (BEs) are listed in Table 2. Seen from the Fig. 8 ( sites, respectively. Generally, Zn 2+ ions strongly prefer to occupy A sites, while they were also reported to occupy B sites [16,17]. The similar results can be seen in Fig. 8 (b) of CZFO-750. Fig. 8 (c-f) shows the XPS spectra of Fe 2p, 3p and the corresponding satellite (sat) peaks. For the HZFO and CZFO specimens, all the Fe 2p and 3p peaks can be fitted to 2 peaks resulted from the Fe 3+ ions in tetrahedral sites and octahedral sites (as shown in Table 2), respectively, which implies that Fe 3+ ions occupy A sites as well as B sites. This is similar to the results in the previous reports [16,17,23]. . Similar to equation (1), theoretically, the molecular magnetic moment of the former is markedly larger than the latter.
From the discussion of magnetism, it is obvious that the Zn-deficiency affected the occupancy of Fe and the magnetism in ZFO.

Conclusions
Single-phase ZnFe 2 O 4 specimens were obtained via a hydrothermal and a traditional ceramic method, respectively. The magnetization at assigned magnetic fields of the former specimens is obviously greater than that of the latter specimen.
This can be ascribed to the loss of Zn ions in the aqueous solution during the hydrothermal process, which should result in the occupancy of some Fe ions in A sites of ZnFe 2 O 4 to form the mixed spinel structure. Consequently, the magnetism could be tuned by the Zn-deficiency in the specimens synthesized via a hydrothermal route.