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M-type Al-doped strontium ferrite powders (SrAlxFe2n−xO19, n = 5.9) with nominal Al content of x = 0–2.0 are prepared by traditional ceramic technology. The phase identification of the powders, performed using x-ray diffraction, shows the presence of purity hexaferrite structure and absence of any secondary phase. The lattice parameters decrease with increasing x. The average grain size of the powders is about 300 nm–400 nm at Al3+ ion content x = 0–2.0. The room-temperature hysteresis loops of the powders, measured by using vibrating sample magnetometer, show that the specific saturation magnetization (σs) value continuously decreases while the coercivity (Hc) value increases with increasing x, and Hc reaches to 9759 Oe (1 Oe = 79.5775 A/m) at x = 2.0. According to the law of approach saturation, Hc value increases with increasing Al3+ ion content, which is attributed to the saturation magnetization (Ms) decreasing more rapidly than the magnetic anisotropy constant (K1) obtained by numerical fitting of the hysteresis loops. The distribution of Al3+ ions in the hexaferrite structure of SrAlxFe2n−xO19 is investigated by using 57Co Mössbauer spectroscopy. The effect of Al3+ doping on static magnetic properties contributes to the improvement of magnetic anisotropy field.
M-type hexagonal ferrite (MFe12O19, M is divalent cation, such as Ba2+, Sr2+, and so on), is one of the most used hard magnetic materials with applications such as permanent magnets, magnetic recording media due to their low cost, good chemical stabilities, high coercivities, high saturation magnetizations, and large magnetocrystalline anisotropies.[1–5] Recently, the barium ferrite was widely used as magnetic recording media, microwave devices, and electromagnetic wave absorbers.[3,6,7] Furthermore, various methods of fabricating the M-type hexaferrite have been developed including sol-gel method,[8,9] co-precipitation,[10,11] ceramic method,[12,13] and hydrothermal method,[14] citrate auto-combustion,[15] and glycin–nitrate method.[16]
In order to change physical properties, substituting some cations into SrM structure is an effective method to cope with the different applications. A lot of work has been done to modify the magnetic parameters of SrM by substituting Fe3+ with other cations, such as Al3+, Cr3+, Ga3+, and so on.[17–19] The magnetic properties of substituted hexaferrites directly depend on the electronic configuration of the dopant cations as well as their preference to occupy the different Fe sub-lattices of the magnetoplumbite structure.[20] In particular, as is well known, Al-substituted SrM has very large coercivity.[17,21,22] Furthermore, it is important to understand the mechanism for increasing the coercivity by Al3+ doping.
SrM with hexagonal structure and molecular formula SrFe12O19 has a magnetoplumbite (M) structure which was determined by Adelsklod.[23,24] The crystal structure can be separated into two blocks, one block is R Block
In this study, we prepare Al3+-doped SrM by traditional ceramic technology, and the coercivity reaches to 9759 Oe at x = 2.0. We attempt to understand in detail the reasons and the mechanism of enhancing the coercivity and magnetic structure formation of SrAlxFe2n−xO19 as a function of the substituted amounts of dopants in a range (0 ≤ x ≤ 2.0).
SrAlxFe2n−xO19 (n = 5.9 x = 0–2.0) samples were prepared via ball milling appropriate mixtures of SrCO3, Fe2O3 and Al2O3 powders. The amount of each compound was determined for each particular stoichiometry of SrAlxFe2n−xO19 and then mixed in a ball milling cylinder. The mechanical alloying was carried out in a traditional ball mill for 4 h and the volume ratio of alcohol to powders was about 1.5:1. After milling, the powders were placed into cylindrical pellets. Calcination was then carried out for 2 h in air at 1200 °C.
The phase composition and crystal structure were analyzed by using Bruker diffractometer with Cu Kα x-ray diffraction measurement (λ = 1.5406 Å); scanning electron microscopy was employed to analyze the morphologies and microstructures of the samples. Magnetic properties could be obtained by analyzing the data of the vibrating sample magnetometer and the Mössbauer spectroscopy. The Mössbauer spectrum at room temperature was recorded using a Mössbauer spectrometer operating in constant acceleration mode (triangular wave) in transmission geometry. The source employed was 57Co in Rh matrix of strength 50 mCi. The velocity was scaled by using an enriched a-Fe metal foil under an effective nuclear hyperfine field (Heff) of 331 kOe at room temperature.
Figure
The lattice parameters a and c of SrAlxFe2n−xO19 (n = 5.9, x = 0–2.0) are calculated according to the following formula:
The changes of structural parameters, density and volume with composition are given in Table
![]() | Table 1. Values of crystal lattice a and c, the unit-cell volume and the density for SrAlxFe2n−xO19. . |
Particle size and morphology are studied by SEM as shown in Figs.
![]() | Fig. 2. Scanning electron micrographs of SrAlxFe2n−xO19 magnetic powder particles with (a) x = 0, (b) x = 0.8, (c) x = 1.4, (d) x = 2.0. |
In the hexagonal structure of strontium ferrite, Fe3+ ions occupy five sites: 4f1, 2a, 4f2, 12k, and 2b. The contributions of Fe3+ ions located on those sites to magnetic properties are different from each other. After Al3+ ions enter into the lattice and substitute for some Fe3+ ions, the magnetocrystalline anisotropy field is changed. From the Mössbauer spectra as shown in Fig.
![]() | Fig. 3. The room temperature Mössbauer spectra of SrAlxFe2n−xO19 with (a) x = 0, (b) x = 0.8, (c) x = 1.4, and (d) x = 2.0. |
![]() | Table 2. Mössbauer parameters of SrAlxFe2n−xO19 at room temperature. . |
The hysteresis loops of powder samples are measured at room temperature by using a vibrating sample magnetometer in Fig.
The saturation magnetization Ms and magnetic anisotropy constant (K1) can be obtained by fitting the hysteresis loop using Eqs. (
![]() | Table 3. The room temperature magnetic parameters of Al3+ substituted SrAlxFe2n−xO19. . |
The coercivity of M-type ferrite can be expressed as[37]
In Fig.
A series of SrAlxFe2n−xO19 powders (n = 5.9, 0 ≤ x ≤ 2.0) are synthesized by conventional ceramic method. At low substitutions, Al3+ ions mainly occupy the 2a, 4f1, and 12k sites of Fe3+. At x = 2.0, the decrease of magnetizations is attributed to the fact that Fe3+ on 2b sites can hardly be substituted. The magnetic moment per unit cell, saturation magnetization and magnetic anisotropy constant decrease with increasing Al3+ substitution, while the coercivity increases. The enhancement of coercivity is caused by increasing magnetic anisotropy field, which is attributed to the fact that the saturation magnetization (Ms) decreases more rapidly than the magnetic anisotropy constant (K1) with the increase of Al3+ substitution.
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