Study on the influence of magnesium doping on the magnetic properties of spinel Zn-Mg ferrite

Abstract

Polycrystalline spinel ferrite powders of Zn_1-xMg_xFe₂O₄, (x = 0.0, 0.4, 0.8, and 1.0) have been synthesized by solid-state reaction. An antiferromagnetic Néel temperature (T_N = 25 K) point is observed in ZnFe₂O_4, while MgFe₂O₄ shows a strong ferromagnetism. The magnetization value of Zn_1-xMg_xFe₂O₄ increases first and then decreases with the increase of x. When x = 0.8 (Zn_0.2Mg_0.8Fe₂O₄), the value of the saturation magnetization (M_s) reaches a maximum as 85.566 emu/g. The magnetization of Zn_1-xMg_xFe₂O₄ shows a very sensitive response to the Mg²⁺ concentration at the tetrahedral sites (A-sites) or the octahedral sites (B-sites). I suggest that the A–B super-exchange interaction is enhanced after Mg²⁺ ions substituting Zn²⁺ ions.

1. Introduction

As an important member of ferrite family, spinel ferrite has unique attraction. For a long time, spinel ferrites have attracted the extensive interests due to their remarkable properties reflected in various applications, such as data storage devices [1], microwave absorbing materials [2], ferrofluid and catalysis [3,4], gas sensing materials [[5], [6], [7]], anode materials of lithium-ion batteries [8], magnetic hyperthermia [[9], [10], [11]], optical and dielectric materials [12], fast frequency response in soft magnetic materials and biomedical fields [13], etc. In recent years, the research on ZnFe₂O₄ ferrite is fascinating. Due to its better chemical activity and thermal stability [14], ZnFe₂O₄ can be used as the electro-optical devices [15], magnetic hyperthermia materials in biomedical applications [16], photo-catalytic systems [17], contrast enhancers [18], and so on. Even the latest reports claim that it can be used as the potential detection of the corona virus disease (COVID-19) by S.B. Somvanshi et al. [19]. As known, the magnetic properties of such spinel ferrites strongly depend on their chemical compositions, element occupancies and substitutions. Even small amount of ions changing in the spinel ferrites can seriously impact the magnetic properties. Therefore, it is particularly important to select suitable metal cations to substitute. As a counterpart of ZnFe₂O₄, the MgFe₂O₄ is also applicable in the electronics and biomedical areas [20,21]. Meanwhile, these two ferrites have two variant lattice structures i.e. ZnFe₂O₄ (normal spinel) and MgFe₂O₄ (inverse spinel). Thus, a formation of mixed spinel lattice structure can give rise to superior properties than their individual parts. Furthermore, by controlling the stoichiometric ratio between zinc and magnesium, the magnetization parameters of zinc-magnesium ferrite can be fine-tuned to our expected values.

However, most of the synthesized magnetic nanoparticles are found to have mixed magnetic phases, so it is very important to understand the different magnetic phases in the synthesized materials for practical application [22]. In this paper, the microstructure, electronic valence state and magnetism of Zn-Mg ferrite are comprehensively explored and evaluated. These methods can be used to synthesize Zn-Mg ferrite such as ball milling [23], sol gel [24], solid state reaction [25], hydrothermal [26] and co-precipitation [27,28], etc. Considering the factors of low cost, no pollution and easy operation, the traditional solid sintering theory is used to synthesize it here. By measuring the low-temperature magnetic properties of the spinel ferrite powders, we found that the ion occupations of zinc ions and magnesium ions in the same crystal structure are very different, thus showing very different macroscopic physical properties. Therefore, further research and analysis of this kind of ferrite has attracted great interest and extensive attention of researchers.

2. Experimental

Zn-Mg spinel ferrite (Zn_{1- x}Mg_xFe₂O₄, x = 0.0, 0.4, 0.8, 1.0) particles were synthesized by the solid state reaction method. Fe₂O₃, ZnO and MgO (purity of 99.9 % for all) were used as the raw materials and mixed according to the calculated stoichiometric ratio. Then, the mixed raw materials were placed in a high-temperature sintering furnace to be gradually heated to 1373 K and sintered at this temperature for 2 h. The specific sample preparation process is shown in Fig. 1 . X-ray diffraction (XRD, Cu Kα radiation, λ =0.15406 nm, Philips) and scanning electronic microscopy (SEM) were used to analyze the crystal structure information and reveal the surface morphology of the samples at room temperature (300 K). The prepared Zn-Mg ferrite samples were authenticated by X-ray diffraction (XRD) patterns recorded within the angle scope of 20–90°. Sweep the samples at a scanning angle of 5 degrees per minute. The temperature dependent magnetization (M–T) curves were collected on a superconducting quantum interference device (SQUID) magnetometer from quantum design (MPMS). Both zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves were measured from a very low temperature 2 K to a high temperature 400 K.

Fig. 1.

Schematical version of the synthesis route employed for preparation of Zn_1-xMg_xFe₂O₄ ferrites.

3. Results and discussion

3.1. Structural analysis

The Rietveld refined XRD patterns for all the samples of Zn_{1- x}Mg_xFe₂O₄, (x = 0.0, 0.4, 0.8, 1.0) spinel ferrite are shown in Fig. 2 . All samples show the single phase of the standard face centered cubic structure (space group: Fd-3 m, 227). In order to fully investigate the structural information of Zn-Mg spinel ferrite, the values of the credible parameters (R _p, R _wp, and χ ²) are obtained from the Rietveld refined XRD patterns of Zn_{1- x}Mg_xFe₂O₄, (x = 0.0, 0.4, 0.8, 1.0) at room temperature. The detailed refinement parameters are shown in Table 1 . As mentioned above, the samples obtained from our experiment are in accordance with the calculated stoichiometric ratio and is intrinsically effective.

Fig. 2.

The refined XRD patterns of Zn_1-xMg_xFe₂O₄ at room temperature.

Table 1.

The achieved XRD Refinement Parameters at Room Temperature and Magnetic Parameters at 5 K of Zn_1-xMg_xFe₂O₄ (x = 0.0, 0.4, 0.8, 1.0).

Composition	Rp (%)	Rwp (%)	χ2	Ms (emu/g,5 K)	Hc (Oe,5 K)	Cation distribution
ZnFe2O4	15.274	19.317	1.012	‒	17.5	Zn0.91Fe0.09[Zn0.09Fe1.91]O4
Zn0.6Mg0.4Fe2O4	17.265	24.803	1.060	‒	100	Zn0.50Mg0.05Fe0.45[Zn0.10Mg0.35Fe1.55]O4
Zn0.2Mg0.8Fe2O4	18.552	25.814	1.061	85.566	20	Zn0.14Mg0.06Fe0.80[Zn0.06Mg0.74Fe1.20]O4
MgFe2O4	18.984	24.534	1.064	60.452	15	Mg0.10Fe0.90[Mg0.90Fe1.1]O4

In order to facilitate the understanding of their magnetic properties, the crystal structure of the samples and the occupancy of different ions therein need to be further analyzed and discussed. As shown as Fig. 3 , the positions occupied by all ions in the unit cell and the positions occupied by substituted ions (metal cations) are clearly displayed. As we known, the formula unit of a cubic spinel ferrite can be expressed as (A²⁺ _xFe³⁺ _{1- x})[A²⁺ _{1- x}Fe³⁺ _{1+ x}]O₄, representing that the tetrahedral sites (A-sites) are occupied by the metal cations in parentheses and the octahedral sites (B-sites) are occupied by the metal cations in square brackets [13]. When x = 1, the divalent metal cations only occupy tetrahedral sites, resulting in spinel ferrite being a normal spinel ferrite. However, the divalent metal cations only occupy octahedral sites when x = 0, then spinel ferrite is an inverse spinel ferrite. If 0 < x < 1, divalent metal cations occupy both tetrahedral and octahedral sites. At this time, spinel ferrite is in a state between normal spinel ferrite and inverse spinel ferrite [29,30]. The distribution law of metal cations is generally related to factors such as ion radius, electron layer structure, ion valence bond balance and ion ordering, etc. Besides, it also depends on the heat treatment of ferrite [31,32]. For example, metal cations such as Zn²⁺ and Ni²⁺ have exclusive A-sites and B-sites preferences [[33], [34], [35]], respectively. Thus, in a typical nickel ferrite, Ni²⁺ cations only occupy the B-sites, forming an inverse spinel. While in a zinc ferrite, Zn²⁺ cations only occupies the A-sites, forming a normal spinel. However, Mg²⁺ ions occupy both the A-sites and B-sites in a spinel ferrite [1,13,29,32]. The variational proportion of cations distribution in A-sites and B-sites will greatly affect the structure, electricity and magnetism of spinel ferrite [29,32].

Fig. 3.

The crystal structure diagram affected by Magnesium doping.

Fig. 4 shows the scanning electron microscope of the surface morphology of Zn_{1- x}Mg_xFe₂O₄ (x = 0.0, 0.4, 0.8 and 1.0). The pictures of (a), (b), (c) and (d) represent the samples of x = 0.0, 0.4, 0.8 and 1.0, respectively. It can be seen from the Fig. 4 that the particle sizes of the obtained samples do not change significantly with the increase of the substitution amount of Mg²⁺ ions for Zn²⁺ ions. Meanwhile, the average particle size is mainly concentrated between 200 nm and 400 nm. This fully shows that the samples obtained from the solid-phase reaction theory used in our experiment can also reach the level of nanometer size. The polyhedral morphology of these particles is due to the macroscopic stacking effect caused by the crystal structure of tetrahedron and octahedron structure. As mentioned above, the samples obtained from my experiments have good macro-structure morphology and standard Zn-Mg spinel crystal structure.

Fig. 4.

The Surface topography characteristic SEM images of Zn_1-xMg_xFe₂O₄; (a), (b), (c) and (d) correspond to x = 0.0, 0.4, 0.8 and 1.0, respectively.

3.2. X-ray photoelectron spectroscopy

The presence of various elemental species and the oxidation state in the sample can be obtained by X-ray photoelectron spectroscopy (XPS), which is an important characterisation technique. The number of unpaired electrons, determined by the oxidation state, governs the magnetic properties of these ferrite materials. The XPS spectra for the Zn_0.6Mg_0.4Fe₂O₄ particles are displayed in Fig. 5 . The full scan plot substantiates the presence of the elements Zn, Mg, Fe, and O as shown as Fig. 5(a). All the individual elemental spectra are fitted using XPS peak fitting software. In Fig. 5(b), the Mg1s peak is observed at 1304.1 eV with Mg²⁺ in Zn_0.6Mg_0.4Fe₂O₄ ferrite. The lower energy peak attributed to A-site Mg²⁺ and higher energy peak to B-site Mg²⁺ [36]. It can be inferred from our experimental results that magnesium ions occupy B-site and force some iron into A-site. Fig. 5(c) displays the Zn2p peak with two distinct peaks at 1021.7 eV and 1045 eV corresponding to Zn2p _3/2 and Zn2p _1/2, respectively. This result confirms the valence state of zinc to be +2 oxidation state. The XPS spectrum for Fe2p, depicting two distinct peaks at 711.3 eV for Fe2p _3/2 and 725.3 eV for Fe2p _1/2, suggest the valence state of iron to be Fe³⁺ with no Fe²⁺. And the presence of a satellite peak of Fe2p _3/2 at around 719.0 eV, above the main peak, has confirmed it [36,37]. This is enough to show that the magnetism of experimental sample particles is caused by the change of iron oxidation state and the occupancy rate of different cations.

Fig. 5.

XPS plots showing (a) full scan (b) Mg1s (c) Zn2p (d) Fe2p of Zn_0.6Mg_0.4Fe₂O₄.

3.3. Thermogravimetric analysis (TGA)

In order to investigate the impact of the heat treatment on Zn_{1- x}Mg_xFe₂O₄ samples, the thermal response recorded by TGA has been analyzed. The thermogravimetric (TG) curves for Zn_{1- x}Mg_xFe₂O₄ have been displayed in Fig. 6 . When x = 0.0, the weight loss is less than one percent, which has occurred in very slow manner with the increase of temperature. For x = 0.4, 0.8, and 1.0 samples, the initial weight loss is occurred in very slow manner and further the sudden weight loss appears within a short range of temperature. The maximum value of weight loss are about 1.5 %, 2% and 2% when x = 0.4, 0.8, and 1.0 respectively. The maximum value and saturation point of weight loss reveal the formation of spinel phase with a second order phase transition, which is usually called as ferritization temperature [38]. For the Zn_{1- x}Mg_xFe₂O₄ samples, the ferritization temperature is considered as 282.8 °C, 304.8 °C, and 341.4 °C for x = 0.4, 0.8, and 1.0, respectively. Above these temperature points, very little weight loss with the increase of temperature indicates the complete formation of spinel phase and a better thermal stability for higher temperature region. Considering that these samples belong to magnetic ferrite materials, it is necessary to explore the magnetization of these spinel ferrite samples after studying the weight loss of them with the change of temperature. The temperature-dependent magnetization curves are helpful for us to understand their low-temperature magnetic behaviors and the magnetic phase transition process.

Fig. 6.

TGA curves of Zn_1-xMg_xFe₂O₄ ferrites.

3.4. Magnetisation

Fig. 7 (a)∼(d) show the magnetization curves of Zn_{1- x}Mg_xFe₂O₄ (x = 0.0, 0.4, 0.8 and 1.0) under ZFC/FC processes (M _ZFC/M _FC) depending on the temperature from 5 K to 400 K. Here, in order to measure the temperature-dependent magnetization of the samples, the applied external field strength is H =100 Oe. In Fig. 7 (a), the M _ZFC curve and M _FC curve of ZnFe₂O₄ show the peak due to strong magnetic interactions at low temperature of 25 K. It indicates that there is an antiferromagnetic super-exchange interaction caused by the reverse magnetic moment of Fe³⁺ ions in the B-sites of ZnFe₂O₄ [39]. Meanwhile, the M _ZFC curve and M _FC curve of ZnFe₂O₄ show slight inversion due to the small particle size by the preparation techniques when the particle size decreases to nanoscale [40]. The spin-glass-like behavior may appear in the field of nanomagnetism when the size of magnetic particles decreases into nanometer-sized scale. From Fig. 7(a)–(d), the M _ZFC curve and M _FC curve of Zn_{1- x}Mg_xFe₂O₄ (x = 0.4, 0.8 and 1.0) are very different from that of ZnFe₂O₄. With the increasing of magnesium ions replacing zinc ions, the ferromagnetic interaction in the system is gradually strengthened, and the process of ferromagnetic and antiferromagnetic competition appears. It can be seen from Fig. 7(c) that when x = 0.8, the magnetic susceptibility of the sample reaches the maximum. The competition between ferromagnetism and antiferromagnetism first strengthens and then weakens, and the system gradually enters a state where ferromagnetism dominates. As the temperature rise, the magnetization of MgFe₂O₄ increases slowly. And the spins of ions are frozen in their own easy-axes at a lower temperature, while the spin-like glass state will be destroyed by the thermal motion with the rise of temperature [40]. Compared with ZnFe₂O₄, Zn_{1- x}Mg_xFe₂O₄ (x = 0.4, 0.8 and 1.0) not only show B-B interactions, but also show stronger A-A interactions and A–B interactions. The M _ZFC curve and M _FC curve of Zn_{1- x}Mg_xFe₂O₄ (x = 0.4, 0.8 and 1.0) show obvious bifurcation, which indicate that the samples show strong temperature-dependent magnetic behaviors. It can be clearly seen that Zn_{1- x}Mg_xFe₂O₄ (x = 0.4, 0.8 and 1.0) have large magnetization values, while the magnetization value of ZnFe₂O₄ basically approaches zero. This fully implies that the substitution of magnesium ions for zinc ions will lead to great changes in the magnetic properties of polycrystalline powder samples.

Fig. 7.

(a)∼(d): The temperature-dependent magnetization of Zn_1-xMg_xFe₂O₄ (x = 0.0, 0.4, 0.8 and 1.0) under ZFC/FC processes (M_ZFC/M_FC) from 5 to 400 K.

Fig. 8(a) and (b) shows the magnetic hysteresis ((M−H)) loops of Zn_1-xMg_xFe₂O₄ (x = 0.0, 0.4, 0.8, 1.0) at 5 K and 300 K, respectively. Interestingly, the magnetization value of the sample increases first and then decreases with the increase of x. when x = 0.8 (Zn_0.2Mg_0.8Fe₂O₄), the value of M _s reaches a maximum as 85.566 emu/g. As shown as Fig. 8(c), the intrinsic coercivity (H _c) of the sample also shows a change rule of increasing first and then decreasing at 5 K. All relevant values are listed in Table 1. Fig. 8(b) reveals that ZnFe₂O₄ shows a fully paramagnetic state at 300 K, while the ‘S’-like shape of hysteresis loops at 5 K as shown as Fig. 8(a). This phenomenon shows that due to the small size of the powder, zinc ions do not occupy all A-sites, and very few occupy B-sites. In bulk ZnFe₂O₄ ferrite, Zn²⁺ ions fully fill A-sites and Fe³⁺ ions fully fill B-sites, respectively. Herein there is no A–B interaction and the negative B-B interaction caused by Fe³⁺ ions just has antiparallel moment resulting no net moment [24]. After Mg²⁺ ions substitute Zn²⁺ ions, Mg²⁺ ions can enters B-sites, forcing part of Fe³⁺ ions originally occupying B-sites to A-sites. Fe³⁺ ions concentration at A-sites, A-A interaction, A–B super-exchange interaction are enhanced and B-B interaction is weakened [41]. For this reason, the magnetization of Zn_{1- x}Mg_xFe₂O₄ (x = 0.0, 0.4, 0.8, 1.0) will increases with the increase of x and increased up to x = 0.8. As mentioned above, Mg²⁺ ions have no moment as same as Zn²⁺ ions. A–B super-exchange interaction and B-B interaction mainly contribute to Fe³⁺ ions. As the substitution amount of Mg²⁺ ions continues to increase, the Fe³⁺ ions in the B-sites enter the A-sites more and more, and then the Fe³⁺ ions in the B-sites decrease, which will eventually lead to the A–B interaction instead beginning to decrease. When Mg²⁺ ions continuously substitute Zn²⁺ ions, antiferromagnetism decays all the time, and ferromagnetism goes through the process of increasing at first and then decreasing, all of which are due to the change of magnetic Fe³⁺ ions occupation. The results of H _c shown in Fig. 8(c) also confirm this conclusion. Finally, the M _s of Zn_{1- x}Mg_xFe₂O₄ will decrease for x > 0.8, which is attributed to the decrease of Fe³⁺ ions of B-sites [24].

Fig. 8.

(a) and (b): The comparison chart of hysteresis loops of Zn_1-xMg_xFe₂O₄ at 5 K and 300 K; (c): A partially enlarged chart near the zero point of (a).

4. Conclusions

In conclusion, the magnetic properties of Zn_{1- x}Mg_xFe₂O₄ (x = 0.0, 0.4, 0.8, 1.0) have been systematically investigated. The results of XPS, the curves of M–T and (M−H) suggest that the substitution of Mg²⁺ ions for Zn²⁺ ions will cause Fe³⁺ ions to change from the original B-sites to the A-sites. Thus strengthened A–B super-exchange interaction and weaken B-B interaction. With the increase of x, the ferromagnetism of the sample system increases and the antiferromagnetism decreases gradually. The magnetization value of Zn_{1- x}Mg_xFe₂O₄ (x = 0.0, 0.4, 0.8, 1.0) increases first and then decreases with the increase of x. when x = 0.8 (Zn_0.2Mg_0.8Fe₂O₄), the value of M _s reaches a maximum as 85.566 emu/g. The magnetic properties of Zn_{1- x}Mg_xFe₂O₄ (x = 0.0, 0.4, 0.8, 1.0) change obviously and show strong sensitivity to Mg²⁺ ions due to the different occupancy preference of Mg²⁺ and Zn²⁺ ions.

Declaration of Competing Interest

The authors reported no declarations of interest