Influence of La–Mn substitutions on magnetic properties of M-type strontium hexaferrites

M-type barium hexaferrites (SrM: SrFe₁₂O₁₉) have been extensively used in the fields of permanent magnets, perpendicular recording media, and high-frequency microwave absorption materials due to the magnetic properties, high coercivity, large magnetic energy product, excellent chemical stability and low cost included.^1–8 

The crystal structure of SrM is stacked alternatively by spinel (S=Fe₆O₈²⁺) and hexagonal (R=MFe₆O₁₁^2-) blocks in the form RSR*S* (*denoting 180° rotation around the hexagonal c-axis).⁹ The O^2- ions exist as close-packed blocks, with the M²⁺ ion substituting for the O^2- ion in R block. The Fe³⁺ ions are distributed in the five interstitial sites of the close-packed blocks, i.e. three octahedral sites (2a, 12k and 4f₂), one tetrahedral site (4f₁), and one trigonal bipyramid site (2b). Three parallel (2a, 12k and 2b) and two antiparallel (4f₁ and 4f₂) sublattices, which are coupled by superexchange interactions through the O^2- ions, form the ferrimagnetic structure.^10,11 Therefore, each magnetic sublattice will give a specific contribution to the total magnetic moment and to the magnetocrystalline anisotropy of SrM.

SrM has been extensively studied due to its high technological interest in permanent magnets. On the one hand, the improved intrinsic magnetic properties of SrM can be obtained by the partial substitution of Sr and Fe. But most of the substitutions, such as Co–Sn,¹² La–Co,¹³ Co–Ti,¹⁴ Mn–Co–Zr¹⁵ and Ir–Co¹⁶ substitutions, will reduce the magnetocrystalline anisotropy of SrM. As a result, the coercivities of the samples are below 1 kOe, which is a disadvantage for the application of permanent magnets. On the other hand, since the magnetic properties of the samples depend strongly on the size and shape of the grains, lots of efforts have been devoted to developing a synthesis method to fabricate homogeneous M-type ferrite grains, such as sol-gel method,^17,18 water-in-oil microemulsion method,¹⁹ solvothermal route,^20,21 and molten-salt method.²² It is well-known that the chemical coprecipitation method is usually used to synthesize magnetic oxides owing to its well-control of grain size.

In this paper, we selected the combination of La³⁺ and Mn²⁺ ions to substitute SrM in order to improve the magnetic properties. As a result, single-phase M-type Sr_1−xLa_xFe_12−xMn_xO₁₉ (SLFMO) hexagonal ferrite platelets were successfully prepared by a modified chemical coprecipitation method. In the experimental processing, the ammonium oxalate monohydrate was used as a precipitator instead of alkali to control effectively the nucleation and growth of grains. In conclusion, the grain size, morphology, magnetic properties of SLFMO hexaferrites were successfully controlled.

The series of M-type barium hexaferrites with the nominal formula of Sr_1−xLa_xFe_12−xMn_xO₁₉ were prepared by the chemical coprecipitation method. Firstly, Sr(NO₃)₂, La(NO₃)₃$⋅$6H₂O, Mn(AC)₂$⋅$4H₂O, and Fe(NO₃)₃$⋅$9H₂O were dissolved stoichiometrically in deionized water by gentle heating. Then, the aqueous mixture was slowly poured into (NH₄)₂C₂O₄·H₂O solution and stirred for forty minutes using magnetic stirrer. The gelatinous precipitates were filtered and washed for several times using deionized water until the pH value of the solution became neutral. The dried powders were sintered at 500 °C for 8 h in air, and then sintered at 1100 °C for 12 h in air.

The crystal structure was characterized by X-ray diffractometer (XRD, Philips designed, X’pert PRO type) with CuK_α radiation (wavelength λ=1.54056 Å) at room temperature. The morphology of grains was investigated by field-emission scanning electronic microscopy (FE-SEM). Magnetization measurements from 380 to 800 K were performed (1.8 K ≤ T ≤ 1000 K, 0 kOe ≤ H ≤ 90 kOe). All the magnetic loops were measured by using a vibrating-sample magnetometer (VSM) accompanied in Quantum Design physical properties measurement system (PPMS) with an applied field (-40 kOe ≤ H ≤ 40 kOe) at room temperature.

The phase and purity of the samples are tested by XRD patterns. All of diffraction peaks of Sr_1−xLa_xFe_12−xMn_xO₁₉ (x = 0.0, 0.1, 0.2, 0.3 and 0.4) hexaferrites can be well indexed on the basis of a hexagonal magnetoplumbite crystal unit cell with a space group of P6₃/mmc in figure 1. The lattice parameters a, c, and c/a ratio are calculated as listed in Table I. It is obvious that the lattice constants a, c, and c/a ratio increase with increasing La-Mn substitution ratio x. These variations can mainly be explained on the basis of the ionic radii of substituted ions.

The morphology of the samples can be examined by FE-SEM. Typical FE-SEM images of SLFMO hexaferrites are shown in figure 2. It is clearly seen that the grains are regular hexagonal platelets with rather homogenous grain sizes between 0.7 μm and 1.4 μm. Furthermore, the shape and diameters of most the grains are almost independent of the La-Mn substitution ratio x.

In order to investigate the magnetic properties of the samples, the magnetic hysteresis loops M (H) are measured in the figure 3. The M (H) loops of the SLFMO hexaplatelets for x= 0.0, 0.1, 0.2, 0.3 and 0.4 at 300 K under an external field varying up to 40 kOe are shown in figure 3. From the figure 3 it can be concluded that SLFMO hexaplatelets exhibit decreased saturation magnetization (M_s) values with increasing x. This result can be explained that the weakness of tetrahedral–octahedral superexchange interactions due to the high concentration of La-Mn substitution. It is well-known for M-type hexaferrite that there are five different interstitial sublattices, that is, three spins-up (2a, 12k and 2b) and two spins-down (4f₁ and 4f₂) sublattices according to Gorter’s model.²³ It can be understood that trivalent La³⁺ ions substitution for bivalent Sr²⁺ ions will lead to the Fe³⁺ irons unbalanced distribution, meanwhile, Mn²⁺ ions substitute for Fe³⁺ ions, which will attenuate the superexchange interactions of Fe³⁺ ions between tetrahedral (4f₁) and octahedral (4f₂) sites of the S block. Therefore, the substitution of La-Mn results in the decrease of the total magnetic moments.

The coercivity for a ferrimagnet can be reflected by coercivity field H_c. The value refers to the intensity of the magnetic field required to reduce the magnetization of the magnetic sample to zero, after the magnetization of the sample has reached saturation. The variations of H_c values for our samples are exhibited in figure 4. It is found that value of H_c increases at low substitution (x ≤ 0.1), reaches a maximum at x = 0.1 and then decreases at x ≥ 0.1. There are the following possible reasons for the above variation of H_c. On the one hand, the coercivity for the disordered particles in M-type hexaferrite can be estimated through the following formula:²⁴ 

Where, K₁ is the magnetocrystalline anisotropy constant, M_s is the saturation magnetization. In our experiment, the substitution of Mn²⁺ ions for Fe³⁺ ions will lead to the increase of magnetocrystalline anisotropy constant K₁ of SLFMO hexaferrites. Larger magnetocrystalline anisotropy can contribute to the higher intrinsic coercivity. Based on above discussion about M_s, the value of M_s gradually decreases with an increasing La-Mn substitution ratio x, while the values of H_c and M_s basically accord with the inverse relation. Therefore, H_c value should increase theoretically for x ≤ 0.1. On the other hand, for x⩾0.1, an excessive amount of Mn²⁺ ions may probably destroy the regular arrangement of the Fe³⁺ ions, weaken the superexchange interactions of Fe³⁺ ions, which leads to a decrease of H_c.

In this paper, the grain sizes of SLFMO hexagonal platelets are between 0.7 μm and 1.4 μm according to our FE-SEM observations. The critical size of a monodomain particle estimated is about 1.0 μm by using the Kittel’s theory,²⁵ which is near to the mean grain size of our samples. So the grains could be described as a transformation from monodomain to multidomain state. In such a state there exists a complex relationship between the coercivity and grain size. For our SLFMO hexaplatelets, the formation multi-domain and the movement of the domain walls can also affect the variation of the coercivity H_c.

The chemical coprecipitation method was used to prepare M-type SLFMO hexaferrites. The XRD results showed that all the derived samples were pure phase (space group: P6₃/mmc). FE-SEM observations exhibited that the grains were regular hexagonal platelets with grain sizes from 0.7 μm to 1.4 μm. The magnetic properties including room temperature M_s and H_c, were systematically investigated. The variations of M_s and H_c might be ascribed to the effects of La-Mn substitutions.

This work was financially supported by the National Nature Science Foundation of China (Grant No. U16321611, and 1274314), Anhui Provincial Natural Science Foundation, China (Grant No. 1508085MA18), The Project of the introduction of leading talents in Anhui Provincial Universities (Grant No. gxfxZD2016188), and 136 talent project of Hefei Normal University (Grant No. 2014136JKC08).