The Cation Distributions of Zn-doped Normal Spinel MgFe₂O₄ Ferrite and Its Magnetic Properties

Abstract

Determining the exact occupation sites of the doping ions in spinel ferrites is vital for tailoring and improving their magnetic properties. In this study, the distribution and occupation sites of cations in MgFe₂O₄ and Zn-doped MgFe₂O₄ ferrite are imaged by Cs-STEM. The experimental STEM images along [001], [011] and [111] orientations suggest that the divalent Mg²⁺ cations occupy all A sites, and the trivalent Fe³⁺ cations occupy all B sites in MgFe₂O₄ ferrite prepared by electrospinning, which is consistent with the normal spinel structure. We further clarify that the preferred sites of dopant Zn²⁺ ions are Fe³⁺ crystallographic sites in the Zn-doped MgFe₂O₄ ferrite nanofibers. Magnetic measurements show that Zn doping affects the spin states of the Fe³⁺, and the Fe³⁺-O²⁻-Fe³⁺ super-exchange interaction leads to enhancements in the magnetization and reduction in the Curie temperature. Our work should contribute a significant step toward eventually realizing the practical application of doped spinel ferrites.

1. Introduction

The direct imaging of atomic structures, especially in the site preference of substituted ions, is vital for magnetic materials to correctly explain their magnetic performance and provide guidance for potential commercial applications, including microwave devices, magnetic storage media, ferrofluids and biomedical devices [1,2,3,4,5,6,7]. It is known that the structural, electrical and magnetic properties of materials are highly sensitive to the conditions of their preparation, compositions and magnetic interactions, which strongly depend on the distribution of cations [8,9,10,11,12]. Magnetic spinel ferrites are of great interest in addressing the fundamental relationship between magnetic properties and their crystal structures, and they have received significant attention for their unique magnetic properties, such as a high Curie temperature, large magnetocrystalline anisotropy, a large Kerr effect and low magnetic losses [13,14,15,16]. Spinel ferrites are represented as [M_1−xFe_x]_tet[M_xFe_(2−x)]_octO₄ with two different co-ordination polyhedra for cations, namely tetrahedral and octahedral sites, where x represents the degree of inversion if (x = 0) is normal, (x = 1) is inverse and (0 < x < 1) is partially inversed. Twenty-four metallic ions occupied two interstitial crystallographic sites, including tetrahedral and octahedral sites. They showed a distribution of parallel and antiparallel magnetic moments in two sub-lattices, which resulted in a net magnetic moment per formula unit and eventually determined the unique magnetic properties of the spinel ferrites. Their magnetic properties could be systematically varied by changing the identity of the divalent M²⁺ cation or by a partial substitution while maintaining the basic crystal structure.

Biocompatible magnesium ferrite (MgFe₂O₄), an important functional soft magnetic (M_s ~ 27.8 emu/g; H_c ~ 9 Oe; T_c ~ 713 K) [17,18] and an n-type semiconductor material with a direct band gap of 1.9 eV [19], is an important member of the spinel family. It is widely used in transformer cores, humidity sensors, catalysts, coil cores, heterogeneous catalysis and sensors [20,21]. Recently, it has been reported in the literature that this ferrite could be applied to thermal coagulation treatment using an alternating magnet [22]. Many attempts have been made to reduce their cost and further improve their magnetic performance. Among these, ion doping has proven to be an effective method to tailor the magnetic performance of spinel ferrites [23,24]. The magnetic and dielectric properties of substituted ferrites directly depend on the electronic configuration of the dopant cations and on their preference for occupying the different Fe³⁺ sub-lattices of the spinel structure, such as Zn_xMg_1−xFe₂O₄ [25,26,27,28]. Non-magnetic Zn²⁺ ions are selected as doping ions to further improve the magnetic performance of the MgFe₂O₄ ferrite. However, the preferred site of the dopant Zn atoms is indistinct, so a satisfactory explanation of the magnetic properties of Zn-doped MgFe₂O₄ is still a matter of debate. Therefore, it is necessary to image their specific distributions in a sub-ångstrom resolution. Among the techniques for detecting the distributions and the preferred site of the doping ions [29,30,31,32,33,34,35], aberration-corrected transmission electron microscopy (Cs-corrected TEM) and scanning transmission electron microscopy (Cs-corrected STEM) equipped with energy-dispersive X-ray analysis (EDX) have recently made significant progress in space and time resolution. These techniques achieve a surprisingly high spatial resolution below 0.5 Å and readily realize a direct imaging and electronic-state detection of individual atoms and even chemical bonds. In this study, we employed Cs-STEM to systematically study the microstructure of MgFe₂O₄ nanofibers and the influence of the substitution and occupancy of doped Zn²⁺ on their magnetic properties.

In this study, the occupation sites and the occupancies of trace dopants in Zn-doped MgFe₂O₄ nanofibers were directly and precisely observed by Cs-STEM, which demonstrated that dopant Zn²⁺ cations preferred to randomly occupy Fe³⁺ crystallographic sites rather than Mg²⁺ sub-lattices to modify magnetic properties. The findings indicated that the Zn²⁺ substitution results in an increase in magnetization and a decrease in the Curie temperature. These results offer insights into interactions between dopant atoms and the magnetic properties in spinel structures and provide guidance for designing magnetoelectric multiferroics applications.

2. Materials and Methods

Synthesis of MFO and MZFO nanofibers. Single-particle-chain normal spinel MgFe₂O₄ (MFO) and MgZn_0.2Fe_1.8O₄ (MZFO) nanofibers were synthesized by electrospinning. A typical precursor solution was used (A.R., Alfa-Aesar Inc., Ward Hill, NJ, USA), which contained 0.1 mmol of magnesium nitrate hexahydrate, 0.2 mmol of iron nitrite nonahydrate (0.1 mmol magnesium nitrate hexahydrate, 0.18 mmol iron nitrite nonahydrate and 0.02 mmol zinc nitrate hexahydrate); 5 mL of N, N-dimethyl formamide (DMF, A.R., Tianjin Chemical Corp., Tianjin, China) was dissolved into a glass container marked as A solution. Next, 1 mL of A solution, 1 mL of ethanol alcohol and 0.1786 g of poly vinylpyrrolidone (PVP, Mw ≈ 1,300,000, Alfa-Aesar Inc., Ward Hill, NJ, USA) were dissolved into a 5 mL vessel combined with sufficient stirring for 4~5 h. A typical precursor solution was successfully obtained. The electrospinning process was performed with 18 kV DC voltage with a 15 cm gap between the needle tip (Jinan Qinlu Pharmaceutical Technology Co., Ltd., Jinan, China) and the collector; the feed rate was 0.4 mL⋅h⁻¹. The electrospun polymer composite nanofibers were first heated at 200 °C for 2 h and then calcined at 900 °C for 3 h with a heating rate of 1 °C⋅min⁻¹ in air. The samples were finally cooled to room temperature with the same rate of 1 °C⋅min⁻¹.

Characterization of MFO and MZFO nanofibers. The morphology and atomic-level crystal structures of the individual single-particle-chain MFO and MZFO nanofibers were systemically characterized with an aberration-corrected scanning transmission electron microscope (FEI Titan Cubed Themis G2 300, FEI, Hillsboro, OR, USA), which operated at 300 kV and was equipped with a monochromator, Gatan image filter (GIF Quantum ER/965, Gatan, CA, USA), EDX (Bruke EDX, Bruke, VA, USA). And an X-ray diffraction instrument (XRD, Philips X’pert ProMPD, Almelo, The Netherlands). Magnetic properties were investigated using a superconducting quantum interference magnetometer (SQUID, MPMS XL-7, Quantum Design, San Diego, CA, USA) and a magnetic property measurement system (MPMS, SQUID-VSM, Quantum Design, San Diego, CA, USA).

3. Results and Discussions

Single-particle-chain MgFe₂O₄ nanofibers (MFO) and Zn-doped MFO nanofibers (MZFO) were prepared using the electrospinning method. All nanofibers had a continuous structure and uniform chemical composition (ESI Figure S1). The average diameter of individual particles on the MFO and MZFO nanofibers was approximately 100 nm, with a single-crystal structure ranging from 90 to 105 nm (ESI Figures S2 and S3). MFO nanofibers were found to have a spinel structure, and Zn elements were successfully doped into the MFO nanofibers to form a single-phase MZFO with no impurity phase (ESI Figure S4). Figure 1a,b show the typical unit cell structure model of the ideal normal spinel MFO ferrite. It has a cubic unit cell structure containing 56 ions in total, in which the 32 O anions are closely packed, and 24 metal cations can be distributed in two different crystallographic sites, including a tetrahedral (A) site and an octahedral (B) site. As shown in Figure 1a, in the MFO unit cell ‘ball-and-stick’ model of the normal spinel structure, all the divalent Mg²⁺ cations occupy tetrahedral A sites, and all the trivalent Fe³⁺ cations occupy the octahedral B sites, in which the orange balls represent Mg cations (the atomic radius is 1.6 Å) and the bright blue spheres represent Fe cations (the atomic radius is 1.27 Å). Figure 1b shows the polyhedral model, which provides the corresponding polyhedral sites where different atoms are located.

The atomic-scale cation occupations and distributions of individual MFO single-particle-chain nanofibers were then captured using Cs-STEM. As shown in Figure 2a–c, atomic-scale images of an individual MFO nanoparticle, along with [001], [011] and [111] orientations, were taken by HAADF-STEM (Cubed Titan G2 60-300, FEI, Hillsboro, OR, USA).

It is noted that the contrast of the HAADF-STEM image is sensitive to the effective atomic number Z (Z_Mg < Z_Mg–Fe < Z_Fe) and the number of atoms (the length of the column). The atomic number of iron (Z = 26) was significantly higher than magnesium (Z = 12) when the number of atoms on the atomic column was the same or similar, so the atomic columns of pure Fe and mixed Mg-Fe were much brighter than the pure Mg atomic columns. However, when the number of atoms on the atomic column greatly differed, the contrast of images of different atomic columns was sensitive to the number of atoms rather than the atomic number. Figure 1a shows that the Mg atomic column located in the tetrahedral sites is clearly darker than the surrounding Fe atomic column, and the Mg atoms (1.6 Å) are easily distinguished because they are larger in size than the Fe atoms (1.27 Å). The atomic images in the three crystal axes are all consistent with the cation occupancy and distribution of the ideal normal spinel structure. The corresponding unit cell models are shown in Figure 2g–i, which clearly show that all tetrahedral sites are occupied by Mg cations, and all octahedral sites are occupied by Fe cations. Figure 2d–f further show the line intensity profiles of ‘1’, ‘2’ and ‘3’ marked in the light blue color, in which ‘Mg’ represents the pure Mg atomic column at the tetrahedral A sites, ‘Fe’ represents the pure Fe atomic column in the octahedral B sites, and ‘Mg & Fe’ represents mixed Mg-Fe columns. The intensity distribution of the atomic columns further indicates that the Fe atomic columns at the octahedral B sites are brighter than the pure Mg atomic columns at the tetrahedral A sites, which is consistent with the theoretical atomic occupation of the ideal normal spinel MFO structure.

In parallel with the observed approach, the specific preferred sites of the dopant Zn atoms in the MFO nanofibers were further imaged. Figure 3a–c show typical HADDF-STEM atomic images of the MZFO nanofibers along the [001], [011] and [111] orientations. It is clear that MZFO nanofibers retain a spinel crystal structure in contrast to the pure MFO nanofibers. Figure 3a shows the atomic STEM image of the MZFO nanofibers projected along the [001] orientation, revealing that the overall atomic image contrast of MZFO nanofibers is essentially the same as that of the MFO nanofibers. It should be noted that the number of atoms on each column is the same in this direction, which presents an ideal situation to identify the preferred sites of dopant atoms. It can be seen that the brightness of the tetrahedral Mg atomic columns is nonetheless darker, but the octahedral Fe atomic column is brighter than that of the MFO. The line intensity distribution was obtained from the blue rectangular box ‘1’. As Figure 3d shows, the difference between the intensities of the two peaks is obviously larger than that of the MFO nanofibers (Figure 2d). Due to the atomic number Z (30) of the Zn atom being greater than the atomic number Z (26) of the Fe atom, the brightness of the octahedral atomic column increases. If the Zn atoms replace the Mg atoms at the tetrahedral sites, the brightness of the atomic column at the tetrahedral sites inevitably increases, thereby reducing the intensity difference between the two peaks. Furthermore, the atomic radius of a Zn ion (1.39 Å) and that of an Fe ion (1.27 Å) are relatively similar, making it easier for atomic substitution to occur. Therefore, it is believed that the dopant Zn atoms randomly replace the Fe atoms at the octahedral sites. The corresponding atomic unit cell model is shown in Figure 3g, in which the blue atoms represent the doping Zn atoms. Figure 3b shows the atomic image along the [011] crystal orientation, showing that the octahedral sites of the atomic column, marked by the yellow dashed circle in the image, are brighter than other octahedral atoms, and its corresponding sites are marked with the numbers ‘2’ and ‘4’ in the atomic unit cell model (Figure 3h). Moreover, the atomic tailing found around the Fe atoms at the octahedral sites, indicated by the yellow dashed rectangular box, may be caused by the irregular occupancy of the doping Zn atoms, which prevented the Zn-Fe atomic columns from being arranged in a straight line. The corresponding positions are indicated by the number ‘5’ in the atomic unit cell model (Figure 3h). Figure 3e is the line intensity distribution diagram shown in the blue rectangle ‘2’ in Figure 3b, and it indicates the specific distribution of the atomic column intensity at the tetrahedral and octahedral sites. Tilting the nanofiber to the [111] crystal axis, shown in Figure 3c, clearly shows that there are six octahedral Fe atomic columns around the Mg-Fe mixed atomic column, forming a regular hexagon, represented by the numbers ‘1–6’ in the atomic unit cell model (Figure 3i). These atomic columns should show the same brightness because each atomic column contains the same number of atoms. However, the octahedral atomic column marked by the yellow dashed circle in Figure 3c is obviously brighter than the surrounding columns. Its atomic unit cell model is indicated by the numbers ‘3’ and ‘6’ in Figure 3i. In addition, the line intensity analysis of the Fe atoms at the octahedral site marked by rectangle ‘3’ in Figure 3c further indicates the specific substitution position of dopant Zn atoms, which match well with the peak intensity distribution shown in Figure 3f and the unit cell model numbered as ‘1’ and ‘6’ atomic columns in Figure 3i. The Fe atom columns with the same number of atoms have different contrasts due to the substitution of Zn atoms, which makes pure Fe atom columns become Zn-Fe mixed-atom columns, further contributing to the increase in the brightness of the atomic column. The above results further prove that the MZFO nanofibers retain a normal spinel crystal structure, and the Zn substitutions occur in the Fe atomic columns at the octahedral sites.

In order to further verify the correlation between crystal structure and magnetic properties, SQUID and MPMS technologies were used to characterize the magnetic properties of the normal spinel MFO nanofibers and Zn-doped MFO nanofibers. The external magnetic field (3 T) dependence on the magnetization of the MZFO and MFO nanofibers at 300 K and 5 K is shown in Figure 4a,b, respectively. A quantitative analysis indicated that the saturation magnetization (M_s) and coercivity (H_c) of MZFO nanofibers at 300 K were 54.7 emu/g (2.30 μ_B/fu) and 82 Oe, respectively, and the saturation magnetization (M_s) and coercivity (H_c) at 5 K were 92.2 emu/g (3.87 μ_B/fu) and 145 Oe, respectively, compared to the pure MFO nanofibers with 42.3 emu/g (1.78 μ_B/fu) 102 Oe at 300 K and 60.8 emu/g (2.55 μ_B/fu) 167 Oe at 5 K. It was found that the saturation magnetization (μ_B/f.u.) of MZFO nanofibers increased by 0.52 μ_B/f.u. and 1.32 μ_B/f.u. at 300 K and 5 K, respectively, and the coercivity had a slight tendency to increase.

The increase in saturation magnetization M_s and coercivity H_c should relate to the substitution of Zn²⁺ cations for Fe³⁺ cations at the octahedral sites, including the occupied sites; canted spin structure; and the super-exchange interaction between the cations and the O^2- anions [27], of which the exchange interactions should be the main cause. According to Neel’s theory of the sub-lattice [36], the cations located at the tetrahedral and octahedral sites indicate two sub-lattice positions, A and B, respectively, and the magnetic moments of cations at the tetrahedral A sites and the octahedral B sites are aligned in antiparallel in the spinel structure. The net magnetization (M_net) is calculated by the following:

M_net = ∑M_B − ∑M_A

(1)

where ∑M_B and ∑M_A represent the moment sum of cations located at octahedral sites and tetrahedral sites, respectively. The magnetization of the nanofibers depends on the distribution of the magnetic Fe³⁺ ions among the two sites because the Mg²⁺ and Zn²⁺ ions are non-magnetic.

The exchange interactions between two sub-lattices have different intensities; the antiferromagnetic A-B interaction is stronger than the ferromagnetic B-B interaction and A-A interaction, and the ferromagnetic A-A interaction is the weakest. In the normal spinel MgFe₂O₄ ferrites, all non-magnetic Mg²⁺ ions occupy the tetrahedral A sites, and all magnetic Fe³⁺ ions occupy the octahedral B sites. In the present study, Zn-doped MgFe₂O₄ ferrites, non-magnetic Zn²⁺ ions preferred occupying the octahedral Fe³⁺ site. A small amount of Zn²⁺ ions substituting Fe³⁺ ions increased the valence of the surrounding trivalent Fe³⁺ ions to Fe^3+x ions, which may have contributed to the increase in the saturation magnetization. The coercivity was related to magnetocrystalline anisotropy (K₁ ~ −2.46 × 10⁴ erg/cm³ for MgFe₂O₄) and crystallite size in spinel magnetic ferrites. Therefore, the slight increase in coercivity could be attributed to the enhancement in magnetocrystalline anisotropy of ferrites due to the substitutions of Zn²⁺ ions in MgFe₂O₄ ferrite. The minor change in coercivity means that Zn doping does not change the soft magnetic properties of MgFe₂O₄ ferrite.

The Curie temperature of MFO and MZFO nanofibers was further measured in the temperature range of 300–950 K at the applied external magnetic field of 3 T, as shown in Figure 4d. The first order derivative was taken to find out the T_c. It was evident that the T_c of MFO nanofibers was 725 K, which was higher than the value reported for MFO (713 K) in the literature [18]. When the Zn²⁺ ions were doped, the T_c of MZFO ferrite dropped to 690 K. The substitution of Zn ions for magnetic Fe ions reduced the Fe³⁺-O^2--Fe³⁺ super-exchange interaction, eventually lowering the Curie temperature. The paramagnetic Curie point T_p of the MFO and MZFO nanofibers were 904 K and 852 K, respectively, as shown in Figure 4d. When T > T_p, the nanofibers were in a completely pure paramagnetic state.

4. Conclusions

In summary, we proved that the MFO nanofibers synthesized by electrospinning have a normal spinel structure by directly observing the occupancy and distribution of cations using Cs-STEM. The atomic HAADF-STEM images, combined with the theoretical atomic model and line intensity distribution, revealed that the dopant Zn²⁺ cations only randomly substitute for the Fe³⁺ sites rather than non-magnetic Mg²⁺ sub-lattices sites. Furthermore, the magnetic characterization showed that the Zn doping affects the spin states of the Fe³⁺, and the Fe³⁺-O²⁻-Fe³⁺ super-exchange interaction leads to an enhancement in magnetization and a reduction in the Curie temperature. We demonstrated that the doping of cations in spinel ferrites is an effective method to improve magnetic properties through tailoring the spin states of the cations and the occupation sites of the trace dopants, which provides guidance to modify the magnetic properties for the development of the next generation of multi-functional magneto-electric devices.