Improvement of the Magnetization of Barium Hexaferrites Induced by Substitution of Nd³⁺ Ions for Fe³⁺ Ions

Abstract

Barium hexagonal ferrites (BaNd _x Fe_12−x O ₁₉) have been synthesized by initial high-energy milling of the precursors and calcining subsequently. The as-prepared samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM). XRD and SEM examinations reveal that a high-crystallized hexagonal BaNd _x Fe_12−x O ₁₉ with lamellar morphology is obtained when the precursor is calcined at 1200^∘C in air for 3 h. The hexagonal crystalline structure of BaFe₁₂ O ₁₉ is not changed after doping Nd³⁺ ions in BaFe₁₂ O ₁₉. However, lattice parameters a and b values increase with an increase in Nd content at first, then decrease. Nd substitution may improve the magnetic properties of BaNd _x Fe_12−x O ₁₉. BaNd_0.1Fe_11.9 O ₁₉, obtained at 1050^∘C, has the highest specific saturation magnetization value (80.81 emu/g) and magnetic moment (16.21 μ _B); BaNd_0.2Fe_11.8 O ₁₉, obtained at 950^∘C, has the highest coercivity value, 4075.19 Oe.

1 Introduction

Ferrimagnetic materials have three types, which are spinel, garnet, and hexaferrites. Among them, synthesis and structural characterization of hexaferrites have received a considerable amount of attention due to the diversity of structure types as well as to their potential applications, such as multiple-state memory elements, novel memory media, transducers, and new functional sensors [1–3]. Hexaferrites are classified into five types, including M, W, Y, X, and Z type hexaferrites with general formulae BaFe₁₂ O ₁₉, BaMe₂Fe₁₆ O ₂₇, BaMe₂Fe₁₂ O ₂₂, Ba₂Me₂Fe₂₈ O ₄₆, and Ba₂Me₂Fe₂₄ O ₄₁, respectively, where Me represents any divalent element [4]. BaFe₁₂ O ₁₉ is one of the M type hexaferrites, which the unit cell contains 38 O²⁻ ions, 2 Ba²⁺ ions, and 24 Fe³⁺ ions. Sixteen Fe³⁺ ions with upward spin are located in three octahedral sites (2a, 12k, and 2b), whereas the remaining eight Fe³⁺ ions with downward spin are at the tetrahedral and trigonal bipyramidal sites (4f₁ and 4f₂) with a net magnetic moment of 40 μ _B per unit cell [5, 6]. The molecular unit of M type hexaferrites is made of one S and one R block, with an overlap of hexagonally and cubically packed layers; the S block consists of two spinel units and the R block consists of three hexagonal units [4]. BaFe₁₂ O ₁₉ is a very important hard magnetic material, which has many unique properties, such as large specific saturation magnetization (Ms) and coercivity (Hc), high values for the magnetic anisotropy field (HA) and Curie temperature, great chemical stability, and excellent high-frequency microwave absorption materials [7, 8]. Substitutions of Fe³⁺ in BaFe₁₂ O ₁₉ with other metal ions lead to net magnetic moment change per unit and tailor magnetic properties of barium hexaferrites. Therefore, doped barium hexaferrites caused great concern.

Various methods of synthesizing BaFe₁₂ O ₁₉ and doped BaFe₁₂ O ₁₉ with different magnetic properties have been developed, including high-energy milling method [9–11], solid-state reaction method [12–14], co-precipitation method [15, 16], hydrothermal treatment [7], sol–gel synthesis [17, 18], citrate precursor [19, 20], self assembly method [21], etc. The magnetic properties of hexaferrite samples depend strongly on composition and synthesis conditions. For example, Venkateswaran et al. [9] synthesized barium hexagonal ferrite (BaFe₁₂ O ₁₉) by initial high-energy milling of the precursors, followed by sintering at 950^∘C in air for 5 h. Specific saturation magnetizations (Ms), remanence (Mr), and coercivity (Hc) of the samples are 45 emu/g, 29 emu/g, and 4500 Oe, respectively. Singh et al. [17] prepared BaLa _x Fe_12−x O ₁₉ nanohexaferrites via the sol–gel auto-combustion technique. Specific saturation magnetizations of BaLa _x Fe_12−x O₁₉ increase from 72.00 to 78.72 emu/g when doped La content is increased from x = 0.05 to x = 0.25. Thakur et al. [19] synthesized Sm-doped Ba–Co hexaferrite with composition BaCo_0.8Sm _x Fe_(11.2−x) O ₁₉ (x = 0.2, 0.4, and 0.6) via a citrate precursor method. The BaCo_0.8Sm_0.2Fe₁₁ O ₁₉ sample, sintered at 900^∘C, has the highest specific saturation magnetization (32.55 emu/g) and the highest coercivity (2690.20 Oe) value. However, to the best of our knowledge, the synthesis and magnetic properties of BaNd _x Fe_12−x O ₁₉ by thermal decomposition of oxalates have not been reported in previous studies.

This study aims to prepare BaNd _x Fe_12−x O ₁₉ by calcining oxalates in air and study the effect of composition and calcination temperature on magnetic properties of BaNd _x Fe_12−x O ₁₉. Our results clearly show that the magnetic properties, in particular the specific magnetizations (Ms) and coercivity (Hc) of BaNd _x Fe_12−x O ₁₉, can be improved after doping Nd³⁺ ions.

2 Experimental Procedures

All chemicals used are of reagent-grade purity (purity >99.9 %). The BaNd _x Fe_12−x O ₁₉ was prepared by initial planetary ball milling of the precursors and calcining subsequently. In a typical synthesis (BaFe₁₂ O ₁₉), 1.18 g BaC₂ O ₄⋅ 2H₂O, 9.71 g FeC₂O ⋅ 2H₂O, and 10 ml ethanol were added to a stainless steel ball milling tank of 100 ml. The mass ratio of the sample to the stainless steel ball is about 1/15. Samples were milled at room temperature for 30 min. The grinding velocity was about 350 circles/min. BaFe₁₂ O ₁₉ precursor was obtained after being dried at 80^∘C in air for 5 h. A similar synthesis procedure was used to synthesize other BaNd _x Fe_12−x O ₁₉ precursor. Finally, the BaNd _x Fe_12−x O ₁₉ precursor was calcined over 950^∘C for 3 h at a heating rate of 2^∘C min⁻¹ in air to produce hexagonal BaNd _x Fe_12−x O ₁₉.

X-ray diffraction (XRD) of the prepared sample was carried out using an X ′pert PRO diffractometer equipped with a graphite monochromator and a Cu target. Radiation applied was Cu K α (λ = 0.15406 nm), operating at 40 kV and 50 mA. XRD scans were conducted from 5 to 75^∘ in 2 𝜃, with a step size of 0.01^∘. The morphologies of the synthesis products were observed using a S-3400 scanning electron microscope (SEM). The magnetic properties of the samples were studied by vibrating sample magnetometer (VSM, Lake Shore 7410) at room temperature (RT).

3 Results and Discussion

3.1 XRD and SEM Analyses of the Calcined Products

Figure 1 shows XRD patterns of BaNd _x Fe_12−x O ₁₉ calcined at different temperatures for 3 h. Characteristic diffraction peaks of hexagonal BaFe₁₂ O ₁₉ and an unknown phase appeared when BaNd _x Fe_12−x O ₁₉ precursors were calcined at 950^∘C. Characteristic diffraction peaks of the unknown phase become weak and/or disappeared with the increase in calcination temperature. When BaNd _x Fe_12−x O ₁₉ precursors were calcined at 1200^∘C for 3 h, all diffraction peaks in the pattern were in accord with those of hexagonal BaFe₁₂ O ₁₉ with space group P63/mmc(194) from PDF card 43-0002 except for a weak diffraction peak at 28.39^∘ for 2 𝜃. The Nd³⁺-doped ions do not change the hexagonal crystalline structure of BaFe₁₂ O ₁₉ except that the diffraction peaks slightly shift (Fig. 1e). The lattice parameters of the sample were refined by the Rietveld analysis using MDI Jade (ver. 5.0) software. The refined lattice parameters of BaNd _x Fe_12−x O ₁₉, obtained at 1200^∘C, are shown in Table 1. With increase in Nd doping from x = 0, 0.1, 0.2, to 0.3, lattice parameters (a and b values) increase at first, then decrease; the c value decreases at first, then increases. This could be due to the larger Nd³⁺ ion (0.099 nm) [22] substituting the Fe³⁺ ion in a and b axes (with a much smaller ionic radius of 0.067 nm) [23] initially, and then for a higher doping level (x = 0.3), some of the Ba²⁺ ions (0.161 nm) [24] could enter the c axis.

Fig. 1

a–e XRD patterns of BaNd _x Fe_12−x O ₁₉: a BaFe₁₂ O ₁₉, b BaNd_0.1Fe_11.9 O ₁₉, c BaNd_0.2Fe_11.8 O ₁₉, d BaNd_0.3Fe_11.7O₁₉ e Local magnification

Table 1 The lattice parameters of BaNd _x Fe_12−x O ₁₉ (x = 0.0, 0.1, 0.2, and 0.3)

The crystallite size of BaNd _x Fe_12−x O ₁₉ is estimated using the following Scherrer formula [25]:

$$ D= K\lambda /(\beta\text{cos}\theta ) $$

(1)

where D is the crystallite size, K = 0.89 (the Scherrer constant), λ = 0.15406 nm (wavelength of the X-ray used), βis the width at half-maximum intensity, and 𝜃 is the corresponding angle. The d ₍₁₁₄₎ interplanar spacing of BaNd _x Fe_12−x O ₁₉ is determined using the Bragg equation [25]:

$$ d_{\text{(114)}} =\frac{\lambda }{2\sin \theta_{\left( {\text{114}} \right)}}, $$

(2)

The crystallite size (D) of BaNd _x Fe_12−x O ₁₉, obtained at different temperatures, and d ₍₁₁₄₎ interplanar spacing of BaNd _x Fe_12−x O ₁₉, obtained at 1200^∘C, are shown in Figs. 2 and 3, respectively From Fig. 2, the crystallite size of BaNd _x Fe_12−x O ₁₉, obtained at 1200^∘C, exhibits non-linear variation; the crystallite size of BaNd _x Fe_12−x O ₁₉ is between 47.8 and 75.7 nm. By contrast, d ₍₁₁₄₎ interplanar spacing of BaNd _x Fe_12−x O ₁₉ decreases with the increase in Nd content (Fig. 3).

Fig. 2

Dependence of crystallite size of BaNd _x Fe_12−x O ₁₉ on calcination temperature

Fig. 3

Dependence of interplanar spacing (d ₁₁₄) of BaNd _x Fe_12−x O ₁₉ on Nd content

The crystallinity of BaNd _x Fe_12−x O ₁₉ can be estimated by MDI Jade 5.0 software. The crystallinity of BaNd _x Fe_12−x O ₁₉ (x = 0, 0.1, 0.2, and 0.3), obtained at different temperatures, is shown in Fig. 4. The crystallinities of BaNd _x Fe_12−x O ₁₉, obtained at 1200^∘C, decrease with increase in Nd content.

Fig. 4

Dependence of crystallinity of BaNd _x Fe_12−x O ₁₉ on calcination temperature and Nd content

Lattice strains of the BaNd _x Fe_12−x O ₁₉ are determined using the following Williamson–Hall formula [25]:

$$ \varepsilon =\frac{\beta }{4\tan \theta }, $$

(3)

where β is the full-width at half maximum (in radian) of the peaks, 𝜃 is the peak position, and ε is the lattice strain of the structure. Lattice strains of BaNd _x Fe_12−x O ₁₉, obtained at 1200^∘C, are shown in Fig. 5. The lattice strain of BaNd _x Fe_12−x O ₁₉ increases with an increase in Nd content (0 ≤ x ≤ 0.2) at first and then decreases (x = 0.3). The lattice strain exists in the BaFe₁₂ O ₁₉, attributed that Ba²⁺ (0.161 nm) [24] and Fe²⁺ (0.067 nm) [23] ions have different ionic radii and/or charge, resulting in the distortion of octahedral, tetrahedral, and trigonal bipyramidal. The substitution of Fe³⁺ ions in a and b axes of the hexagonal by Nd³⁺ ions with larger ionic radii can increase the distortion of hexagonal in BaNd _x Fe_12−x O ₁₉, resulting in the increase of lattice strain in BaNd _x Fe_12−x O ₁₉ with increasing Nd content. However, for a higher doping level (x = 0.3), the replacement of some Fe³⁺ ions in the c axis by Nd³⁺ ions decreases the distortion in BaNd _x Fe_12−x O ₁₉, resulting in the decrease of lattice strain.

Fig. 5

Dependence of lattice strain of BaNd _x Fe_12−x O ₁₉ on Nd content

The morphologies of the calcined products are shown in Fig. 6. BaNd _x Fe_12−x O ₁₉ (x = 0, 0.1, 0.2, and 0.3) samples, obtained at 1050 and 1200^∘C, are composed of approximately lamellar grains. The particle size increases with an increase of calcination temperature. The particle size of BaNd _x Fe_12−x O ₁₉, obtained at 1200^∘C, is mainly between 500 nm and 3 μ m.

Fig. 6

SEM images of BaNd _x Fe_12−x O ₁₉: BaFe₁₂ O ₁₉ (a 1050^∘C, b 1200^∘C), BaNd_0.1Fe_11.9 O ₁₉ (c1050^∘C, d 1200^∘C), BaNd_0.2Fe_11.8 O ₁₉ (e 1050^∘C, f 1200^∘C), and BaNd_0.3Fe_11.7 O ₁₉ (g 1050^∘C, h 1200^∘C)

3.2 Magnetic Properties of BaNd _x Fe_12−x O ₁₉

Figure 7 shows RT magnetic hysteresis loops of the as-prepared BaNd _x Fe_12−x O ₁₉ samples. Effects of Nd content and calcination temperature on specific magnetization are presented in Fig. 8. Dependence of specific saturation magnetizations of BaNd _x Fe_12−x O ₁₉ on Nd content exhibits non-linear variation. BaNd_0.1Fe_11.9 O ₁₉, obtained at 1050^∘C, has the highest specific saturation magnetization value (80.81 emu/g); BaNd_0.3Fe_11.7 O ₁₉, obtained at 1200^∘C, has the lowest specific saturation magnetization value (64.22 emu/g). The evolution of specific saturation magnetization of BaNd _x Fe_12−x O ₁₉, obtained at 1200^∘C, can be explained as follows: The magnetic moments per ion for Ba²⁺, Nd³⁺, and Fe³⁺ ions are 0, 3, and 5 μ _B, respectively. When Fe³⁺ ions in hexagonal BaFe₁₂ O ₁₉ are partially substituted by Nd³⁺ ions, Nd³⁺ ions preferentially fill the tetrahedral and trigonal bipyramidal sites, resulting in the increase in the net magnetic moment and/or specific saturation magnetization of BaNd _x Fe_12−x O ₁₉ with the increase of Nd content initially, and then for a higher doping level (x = 0.3), some of the Nd³⁺ could enter the octahedral sites, resulting in the decrease of the net magnetic moment and/or specific saturation magnetization of BaNd _x Fe_12−x O ₁₉.

Fig. 7

a–d M–H (magnetization–hysteresis) loops of BaNd _x Fe_12−x O ₁₉ samples obtained at different temperatures in air for 3 h

Fig. 8

Dependence of specific saturation magnetization of BaNd _x Fe_12−x O ₁₉ on Nd content and calcination temperature

Dependence of remanence (Mr) and coercivity (Hc) on Nd content and calcination temperature is shown in Fig. 9. From Fig. 9a, remanences of BaNd _x Fe_12−x O ₁₉ decrease with an increase in calcination temperature except for 950^∘C; that of BaNd _x Fe_12−x O ₁₉, obtained at 1200^∘C, decreases with an increase in Nd content. By contrast, coercivity of BaNd _x Fe_12−x O ₁₉ decreases with an increase in calcination temperature (Fig. 9b). Dependence of coercivity of BaNd _x Fe_12−x O ₁₉ on Nd content exhibits non-linear variation. BaNd_0.2Fe_11.8 O ₁₉, obtained at 950^∘C, has the highest coercivity value (4075.19 Oe); BaNd_0.2Fe_11.8 O ₁₉, obtained at 1200^∘C, has the lowest coercivity value (481.2 Oe). Dependence of squareness (Mr/Ms) on Nd content is shown in Fig. 10a. Squareness (Mr/Ms) of BaNd _x Fe_12−x O ₁₉ decreases with an increase of calcination temperature; that of BaNd _x Fe_12−x O ₁₉, obtained at 1200^∘C, decreases with an increase in Nd content. BaNd_0.3Fe_11.7 O ₁₉ has the lowest Mr/Ms, 0.3625.

Fig. 9

a, b Dependence of remanence (Mr) and coercivity (Hc) of BaNd _x Fe_12−x O ₁₉ on Nd content and calcination temperature

Fig. 10

a, b Dependence of squareness (Mr/Ms) and magnetic moment (η _B) of BaNd _x Fe_12−x O ₁₉ on Nd content and calcination temperature

The magnetic moment of BaNd _x Fe_12−x O ₁₉ samples obtained at 1200^∘C is estimated using the following formula [26]:

$$ \eta_{\mathrm{B}} = M \times Ms/5585, $$

(4)

where M is the molecular weight of the composition, Ms is the specific saturation magnetization (emu/g), and η _B is the magnetic moment (μ _B). The results show that dependence of the magnetic moment of BaNd _x Fe_12−x O ₁₉ on Nd content exhibits non-linear variation (Fig. 10b). BaNd_0.1Fe_11.9 O ₁₉, obtained at 1050^∘C, has the largest magnetic moment value (16.21 μ _B); BaNd_0.3Fe_11.7 O ₁₉, obtained at 1200^∘C, has the lowest magnetic moment value (13.09 μ _B).

4 Conclusions

Barium hexagonal ferrites (BaNd _x Fe_12−x O ₁₉) have been successfully synthesized by ball milling a mixture of oxalates at first, followed by calcination in air. XRD and SEM examinations indicate that a high-crystallized hexagonal BaNd _x Fe_12−x O ₁₉ with lamellar morphology is obtained when the precursor is calcined above 950^∘C in air for 3 h. The hexagonal crystalline structure of BaFe₁₂ O ₁₉ is not changed after doping Nd³⁺ ions in BaFe₁₂ O ₁₉. However, lattice parameters a and b values increase at first, then decrease; the c value decreases at first, then increases. The lattice strain of BaNd _x Fe_12−x O ₁₉ increases with an increase in Nd content (0 ≤ x ≤ 0.2) at first and then decreases (x = 0.3). Specific saturation magnetization and the magnetic moment of BaNd _x Fe_12−x O ₁₉ exhibit non-linear variation. Nd substitution may improve magnetic properties of BaNd _x Fe_12−x O ₁₉. BaNd_0.1Fe_11.9 O ₁₉, obtained at 1050^∘C, has the highest specific saturation magnetization value (80.81 emu/g) and magnetic moment (16.21 μ _B); BaNd_0.2Fe_11.8 O ₁₉, obtained at 950^∘C, has the highest coercivity value, 4075.19 Oe.