Improving soft magnetic properties of Mn-Zn ferrite by rare earth ions doping

As a typical soft magnetic material, Mn-Zn ferrites exhibit high performance-cost ratio and excellent soft magnetic properties, which make it widely used in home appliances, communication, computer and other electronic industries.^1,2 With the rapid development of industry, advanced properties of Mn–Zn ferrites such as high permeability and low loss are required in response to the progress of electronic equipment towards miniaturization, light-weight and multifunction.³ In addition to the formulation and processing, additives is one of the key factors that influence the performances of Mn-Zn ferrites, and doping is an important way to improve their electromagnetic properties. As we know, Mn-Zn ferrite is a typical spinel structure, which contains 64 tetrahedral site (A-site) and 32 octahedral site (B-site) in each unit cell. Only 8 A-site and 16 B-site were occupied by metal cations, leading to 72 vacancies in the unit cell. This kind of crystal structure allows the introduction of different metallic ions including the rare earth ions into the lattice, thus modifying the structure and magnetic properties.^4,5

In recent years, chemical doping has been well employed in the development of ferrites. There are generally three types of additives, which affecting the microstructure and properties of Mn-Zn ferrites in different ways.⁶ The additives like SiO₂, CaO, and Nb₂O₅ can form a high-resistivity layers in the grain boundary, which can increase the electrical resistivity of the ferrite, thus reduce the eddy-current loss, and finally reduce the total power loss.^7–10 Some additives like TiO₂, MgO, and CoO can enter into the spinel lattice to modify the microstructure and intrinsic properties of the ferrites.^11,12 Moreover, low-melting point oxides such as V₂O₅, MoO₃, and Bi₂O₃ can be added to promote grain growth through creating a thin layer of liquid phase during sintering.^13–15 The effect of these traditional additives on the magnetic properties of Mn-Zn ferrite has been well studied.

Recently, rare earth element has received much attention from ferrite industry due to their unique physical properties. The influences of rare-earth oxides doping on various ferrites such as manganese ferrite, Mg-Zn ferrite, Ni-Zn ferrite and Mn-Zn ferrite had been investigated.^16–18 However, the dependences of the rare earth doping on the soft magnetic properties of Mn-Zn ferrites have not been fully understood, yet. In this work, we use industrial pre-sintered powders as the raw materials, and prepare Mn-Zn ferrites with different Sm₂O₃, Gd₂O₃, Ce₂O₃ or Y₂O₃ doping concentrations by traditional ceramic technology. The effects of these individual rare earth ion doping on the structure and magnetic properties of Mn-Zn ferrite have been studied.

Mn-Zn ferrites doped with different Sm₂O₃, Gd₂O₃, Ce₂O₃ or Y₂O₃ (0.00, 0.01, 0.03, 0.05 0.07 wt.%) were prepared by the conventional ceramic process. Industrial pre-sintered Mn-Zn ferrite powders and RE₂O₃ (RE=Sm, Gd, Ce and Y) powders of purity 99.99% were used as raw materials. The steps to prepare RE doped Mn-Zn ferrites using the powders were as follows. (I) adding a certain amount of Sm₂O₃, Gd₂O₃, Ce₂O₃ or Y₂O₃ into pre-sintered powders and milling for 4 h in a ball miller with the weight ratio of ball, powder and water of 4:1:1. (II) After dried, the mixed powders were added with 10-12 wt.% of the 10 wt.% PVA solution for manual granulating. (III) The granulated powders were then pressed into toroid-shape samples with the external diameter of 20 mm, internal diameter of 12 mm, and height of 4 mm under a pressure of 75 MPa. (IV) Finally, the samples were sintered at 1350°C for 2 h, then slowly cooled in the tube furnace. The sintering and cooling was performed in a controlled oxygen and nitrogen atmosphere.

The phase constitution of the samples were identified by Philips X’Pert MPD X-ray diffractometer with Cu-K_α1 radiation (⁠$λ$=0.154056 nm). The microstructure of the samples was observed using a scanning electron microscope (SEM, Quanta 200, FEI). The magnetic properties including amplitude permeability, loss and coercivity were tested by soft magnetic measuring device (MATS-2010SA, Hunan Linkjoin Technology Co.) at 200 mT and 100 kHz for the Mn-Zn ferrites doped by Sm₂O₃ or Gd₂O₃, while at 100 mT and 100 kHz for the samples doped by Ce₂O₃ or Y₂O₃.

The XRD patterns of the Mn-Zn ferrites doped with different Gd₂O₃, Sm₂O₃, Ce₂O₃ or Y₂O₃ are shown in Fig. 1(a)–(d), respectively. All samples with doping contents up to 0.07 wt.% have the single spinel phase structure without other impurity phase precipitation. The results confirm that the RE⁺³ (RE=Sm, Gd, Ce and Y) ions can enter into the spinel lattice when the doping content is very small, and the rare earth ions do not easily aggregate on the grain boundaries to form secondary phase.

Fig. 2 shows the scanning electron micrographs (SEM) taken from the fracture surfaces of several investigated Gd₂O₃ or Sm₂O₃ doped samples. It can be clearly observed that the grain size of doped samples are all smaller that of un-doped one. The large variation of the grain size with the substituted ions can be explained as follows. The radius of Gd (0.938Å) and Sm ions (0.96 Å) are larger than that of Fe ions (0.64 Å). When some Fe ions in ferrite lattices are substituted by Gd or Sm ions, the lattice parameters will be changed, which then generate lattice strains and internal stress. Such a stress can hinder the growth of grains.¹⁹ Furthermore, un-doped samples show abnormal grain growth, and some irregular large pores can be observed in some regions, as displayed in Fig. 2(a). The samples doped with Gd or Sm ions have a more uniform microstructure with lower porosity. Since the grain growth rate is reduced, the grains could evolve steadily and uniformly. The mean grain sizes for the samples with 0.01 wt.% and 0.05 wt.% Gd₂O₃ are around 10 μm. The mean grain sizes for the samples with 0.01 wt.%, 0.03 wt.% and 0.05 wt.% Sm₂O₃ are around 8∼10 μm. With the further increase of substituted ion concentration, the aggravated lattice distortion have a bad effect on the microstructure of Mn-Zn ferrites. Abnormally grown grains were obtained and many pore are located inside the grains, as shown in Fig. 2(f) for 0.07 wt.% Sm²O³ doping. Based on our observations, the ferrites doped with 0.01∼0.05 wt.% for Sm₂O₃ or Gd₂O₃ have relatively good microstructures, including relatively uniform grain, very few pores. Similarly, the effects of Ce₂O₃ or Y₂O₃ doping on the microstructure were also studied, the results (not shown here) indicated that the optimized microstructure are associated with around 0.03 wt.% doping for both oxides.

The amplitude permeability (μ_a) of Mn-Zn ferrites doped with different Sm₂O₃, Gd₂O₃, Ce₂O₃ or Y₂O₃ are presented in Fig. 3. For Sm₂O₃ and Gd₂O₃ doping, the permeability reaches a maximum when the doping content is 0.01 wt%, then decreases with further increase of doping. Whereas for Ce₂O₃ or Y₂O₃, the permeability reaches a maximum at 0.03 wt.%, followed by a little decrease with higher amounts of doping. Basically, the increase of permeability of Mn-Zn ferrites with the RE doping is due to the optimization of microstructure. The permeability is correlated to two different magnetizing mechanisms, i.e. the spin rotational magnetizing inside the domains and the domain wall motion.²⁰ Mn-Zn ferrites doped with 0.01 wt.% Gd and Sm have a homogeneous microstructure and lower porosity, which make the domain wall movement and domain rotation easy. At the same time, with more doping of Sm₂O₃ and Gd₂O₃, the grain size becomes non-uniform, which hinders the domain wall movement and domain rotation and results in the decrease of permeability.

Fig. 4 shows the variation of coercivity with different doping of RE₂O₃. It is clear that the coercivity of Mn-Zn ferrites doped with Gd₂O₃ and Sm₂O₃ reach a lowest value at the concentration of 0.01 wt.%. For Ce₂O₃ or Y₂O₃ doping, the concentration of 0.03 wt.% leads to the lowest coercivity, although 0.07 wt.% doping gave somehow similar values. It is known that the coercivity is caused by the resistance of domain wall displacement, which is strongly influenced by the microstructure. The pores (especially irregular large pores) on grain boundaries may hinder the domain wall motion. As a result, the coercivity of the samples without additive is higher than that of substituted ones. Previous work reported that there was a limited solubility of rare earth ions in the spinel structure.²¹ When the doping content of rare earth ions exceeds the solubility, some ions cannot enter the lattice but precipitate as secondary phase at the grain boundaries. The presence of impurities will inhibit the grain growth and hinder the motion of domain walls, although they are not detected by our XRD characterization. All these factors may result in the increase of coercivity with large doping amounts.

The magnetic power loss of the Mn-Zn ferrites doped with different RE₂O₃ are shown in Fig. 5. It is found that a small amount of doping of Gd, Sm, Ce and Y ions can significantly reduce the loss of Mn-Zn ferrites. For examples, compared with the pure Mn-Zn ferrite, the loss of the sample doped with 0.01 wt.% Sm₂O₃ reduced from 708 W/kg to 316 W/kg (at 200 mT, 100 kHz), showing a decrease of 55.4%; while the loss of the sample doped with 0.03 wt.% Ce₂O₃ reduced from 125 W/kg to 73 W/kg (at 100 mT, 100 kHz), almost 42.4% decrease. The total power loss of ferrite generally included hysteresis loss (P_h), eddy current loss (P_e) and residual loss (P_r), which can be expressed as the following equation:^22,23

where K_H and K_E are the constants related to hysteresis loss and eddy current loss, respectively, B is the magnetic flux density, f is the frequency and ρ is the electrical resistivity. Usually, the hysteresis loss and eddy current loss dominate, and the residual loss can be neglected at frequencies below 100 kHz. Hysteresis loss is related to the area of the hysteresis loop, which is mainly determined by the permeability and coercivity. The samples with the optimized doping (0.01 wt.% for Sm₂O₃ or Gd₂O₃ and 0.03 wt.% for Ce₂O₃ or Y₂O₃) showed the highest permeability and the lowest coercivity, making it exhibits the lowest loss among all compositions. Moreover, as described above, a small amount of rare earth ion doping has a strong effect to refine grain size, which thus leads to the increase of the electrical resistivity. And the high resistivity contributes to reduced eddy current loss, which is also beneficial for the decrease of the total power loss. In addition, from Figs. 3b, 4b and 5b, 0.07 wt.% Ce₂O₃ or Y₂O₃ doping give almost similar values of coercivity, permeability and magnetic loss as 0.03 wt.% doping, but if we think about obtaining good properties with the least RE doping, 0.03 wt.% is considered as the optimized value for these additions.

Over all, based on our results, the rare earth element is very useful for the improvement of soft magnetic properties of spinel ferrites. Doping a small amount of Sm₂O₃, Gd₂O₃, Ce₂O₃ or Y₂O₃ after pre-sintering during the preparation of Mn-Zn ferrites is beneficial for their improved permeability, reduced coercivity and magnetic loss. For an example, the Mn-Zn ferrite doped with 0.01 wt.% Sm₂O₃ shows the optimized microstructure and the magnetic properties with permeability, loss, and coercivity of 2586, 316 W/kg, and 24A/m, respectively, at 200 mT and 100 kHz.

The effects of RE₂O₃ (RE=Sm, Gd, Ce and Y) doping on the microstructure and magnetic properties of Mn-Zn ferrite have been investigated. The single spinel phase structure can be maintained with the doping amounts up to 0.07 wt.%. A refined grain structure and uniform grain size distribution can be obtained in the optimally doped materials. With a small amount of doping, the permeability increases significantly and the magnetic loss and coercivity decrease, but further increase of the doping content leads to a reduced permeability and increased loss and coercivity. To achieve excellent magnetic properties, the optimized doping amount for Sm₂O₃ or Gd₂O₃ is 0.01 wt.%, while for Ce₂O₃ or Y₂O₃ is 0.03 wt.%. The sample with 0.01 wt.% Sm₂O₃ shows the magnetic properties with permeability, loss, and coercivity of 2586, 316W/kg, and 24A/m, respectively, at 200 mT and 100 kHz.

This work is financial supported by the joint project of the Foshan Shunde Midea Electrical Heating Appliances Manufacturing Co. Ltd. and the South China University of Technology (Grant no. SHDQ2015120095).