ESTUDIOS MAGNÉTICOS Y ESTRUCTURALES DE HEXAFERRITAS DE BARRIO SUSTITUIDAS (Sn, Ni –Sn) SINTETIZADAS POR MOLIENDA DE BOLAS

ALVARO GONZÁLEZ-ANGELES
Facultad de Ingeniería, Universidad Autónoma de Baja California, México, gangelesa@yahoo.com.mx

ANNA GRUSKOVÁ
Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, Bratislava, Slovakia

JOZEF SLÁMA
Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, Bratislava, Slovakia

JORGE LÓPEZ-CUEVAS ]]> Cinvestav-Saltillo, Coahuila, México

ISRAEL SAUCEDA-MEZA
Facultad de Ingeniería, Universidad Autónoma de Baja California, México

JESÚS MÁRQUEZ-GONZÁLEZ
Facultad de Ingeniería, Universidad Autónoma de Baja California, México

NUN PITALÚA-DÍAZ
Departamento de Ingeniería Industrial-Programa ingeniería Mecatrónica, Universidad de Sonora, México

Recibido para revisar mayo 27 de 2008, aceptado marzo 9 de 2009, versión final marzo 29 de 2009

PALABRAS CLAVE: Materiales magnéticos, Propiedades magnéticas, Materiales nanocristalinos, Hexaferritas sustituidas.

ABSTRACT: Samples of BaFe_12-2x(SnSn)_xO₁₉ and BaFe_12-2x(NiSn)_xO₁₉ were processed by high energy milling with 0 ≤ x ≤ 0.4 and 0 ≤ x ≤ 0.3 respectively. Sn²⁺-Sn⁴⁺ mixture considerably improved the saturation magnetization, M_s, (from 60.9 to 66.15 A.m²/kg) and diminished quickly the intrinsic coercivity, H_ci, (381.1-72.33 kA/m) to suitable values for high-density magnetic recording applications, with low substitution values. On the other hand, the Ni²⁺-Sn⁴⁺ mixture also decreased the coercivity rapidly (diminution ~ 75%) however without enhancing the saturation magnetization. The shown differences on magnetic properties were mainly due to both magnetic nature of divalent ion and secondary phase presence. Mixtures with Sn²⁺ as partner ion diminished markedly to T_c (~21%).

KEYWORDS: Magnetic materials, Magnetic properties, Nanocrystalline materials, substituted hexaferrites.

1. INTRODUCTION

At present, the development of microwave communications technology and the need for anti-electromagnetic interference coatings has induced to an intense study of electromagnetic wave absorbing materials in last years [1, 2]. The most studied materials have been ferrites with spinel, garnet and hexagonal structure. Barium hexagonal ferrite,

BaM, is one of the most important hard magnetic materials widely used in many applications. For its high stability, excellent high-frequency response and narrow switching field distribution, BaM has been studied extensively during the last few years. Despite a considerable amount of research published over the past decade related to the magnetic properties and microstructure of substituted barium ferrites, there have been only a few successful breakthroughs regarding to discovery of new compositions, with excellent properties for special applications as high frequency microwave absorbers [3-5].

In order to obtain M-type barium ferrites with improved characteristics and planar anisotropy for applications in the field of high frequency microwave absorbers is necessary to substitute Fe³⁺ by other trivalent ions or cation mixtures [6, 7].

In this study, we compare the effect of two different partner ions on barium hexaferrite magnetic properties, in an attempt to understand both their magnetic behavior and their possible application on high-density magnetic recording and microwave absorption.

2. EXPERIMENT

Substituted hexaferrite powders BaFe_12-2x(M)_2xO₁₉ with composition of 0 £ x £ 0.3 and 0 £ x £ 0.4 were synthesized by high energy milling (where M = Ni²⁺- Sn⁴⁺ and Sn²⁺ - Sn⁴⁺). As raw materials were used BaCO₃, Fe₂O₃, NiO, SnO and SnO₂, all with purity of 98%. The milling process was performed in a Segvari attritor mill using a ball/powder mass ratio of 15 and a Fe/Ba molecular ratio of 10. The starting materials were milled for 28 h in air using 250 ml of benzene to avoid agglomeration of the powders at the mill bottom and to assure active participation of them in milling process. The as-milled powders were annealed in air at 1050ºC for 1.5 hours.

Thermo-magnetic measurements were performed to obtain both Curie temperature, T_c, and magnetic susceptibility curves, χ, for all samples using the bridge method in an alternating magnetic field of 421 A/m at 920 Hz. Crystalline phase identification in the samples was performed using an X-pert Phillips diffractometer with Cu-K_α radiation source. The magnetic properties of polycrystalline samples were measured in a Lake Shore 7300 vibrating sample magnetometer at room temperature applying an external magnetic field of 1.2 T. Mössbauer spectroscopy analyses were carried out to study the cationic distribution on hexagonal structure using a spectrometer with conventional constant acceleration mode and ⁵⁷Co/Rh γ-ray source. Mössbauer spectra were fitted by NORMOS software.

3. RESULTS AND DISCUSSION

The X-ray diffraction patterns analyses (Fig. 1) showed the apparition of SnO₂ as secondary phase for x ≥ 0.3 Sn²⁺-Sn⁴⁺ samples. The secondary phase formation occurred presumably for SnO oxidation due to milling conditions, which can be controlled by reducing the staring oxides quantity (SnO). Magnetoplumbite phase was confirmed for the rest of mixtures; surprisingly no other secondary phases were detected, at least within the errors inherent to X-ray diffractometer.

Figure 1. X-ray diffraction patterns of (Sn, Ni)²⁺-Sn⁴⁺ substituted samples for x = 0.4 and 0.3 respectively. It can see the nucleation of SnO₂ as secondary phase for SnSn mixture

Mössbauer spectra at room temperature for all analyzed samples are shown in Figure 2. It can be observed that the intensity and shape of the spectra changed as the substitution level increased, indicating that the substitution of iron ions by substituting cations took place.

Figure 2. Room temperature ⁵⁷Fe Mössbauer spectra for BaM(Sn, Ni)-Sn mixtures with composition of x = 0.3. Note the change in the intensity and broad as the substitution increased as compared to pure magnetoplumbite (x = 0)

The realized studies threw that both samples presented similar tendencies to substitute of ion Fe³⁺ namely, both divalent partner ions (Ni²⁺ and Sn²⁺) possess special preference for octahedral sites (4f₂ and 2a), whilst tetravalent partner ions (Sn⁴⁺) prefer to replace iron ions on both bipyramidal (2b) and tetrahedral (4f₁) sites.

Nevertheless, it is clear that further Mössbauer studies with ¹¹⁹Sn γ-ray sources would be necessary to determine more exactly the site preferences of the ions. Moreover, to observe the canting evolution on spin structure (change from axial to planar anisotropy), Mössbauer analyses with an external magnetic field applied parallel to γ-ray direction would be of special interest as well.

Thermomagnetic measurements (Fig. 3) were performed to determinate both the dependence of magnetic susceptibility with temperature (χ (T)) and the Curie temperature (T_c). It could be observed how χ was affected for all studied samples, besides; it was possible to detect that mixtures with Sn²⁺ as partner ion diminished markedly to T_c, (SnSn ~ 21% as distinct from NiSn ~ 1.8%) this can be related to the gradually magnetic collinearity break down after replacing iron ions by diamagnetic ions (Sn²⁺) [8]. NiSn curves also showed the appearance of another peak at ~ 580 ºC, which correspond to spinel compound nucleation as secondary phase. This phase is most probably NiFe₂O₄, which possess a T_c of 585 ºC.

Figure 3. Magnetic Susceptibility temperature dependence studies for BaM(Sn, Ni)-Sn substituted materials. Note the nucleation of Ni spinel (NiFe₂O₄) as secondary phase at ~ 580ºC in BaM(NiSn) sample

The figure 4 shows magnetic properties behavior as a function of substitution for all studied samples. For substituted SnSn sample, it can see how saturation magnetization was enhanced (increase ~ 9%) to reach a maximum at x = 0.2, diminishing after. Also it could be observed how M_s for NiSn sample remained almost constant for all substitution range. Taking into account that both mixtures possess similar preferences to replace iron ions into hexagonal structure, therefore, we believe that both the magnetic nature divalent partner ions (Ni²⁺ is magnetic ~2µ_B and Sn²⁺ diamagnetic) and secondary phase nucleation are responsible of M_s behavior.

Figure 4. Magnetic properties behavior of all synthesized samples as a function of the substitution

4. CONCLUSIONS

Syntheses of two different cationic mixtures were studied, aiming to obtain materials useful for high-density magnetic recording. In this aspect, we can highlight that Sn-Sn mixtures diminished the coercivity (H_ci) rapidly to suitable values for high-density magnetic recording applications; increasing at the same time the saturation magnetization (M_s) with low substitution values. Mixtures with Sn²⁺ as partner ion diminished markedly to T_c, (SnSn ~ 21% as distinct from NiSn ~ 1.8%) this can be related to the gradually magnetic collinearity break down after replacing iron ions by diamagnetic ions (Sn²⁺), taking as consequence the superexchange interaction decrease. Alternatively, NiSn mixtures showed to be promissory materials for microwave absorption applications.

To determine more exactly the site preferences of substituting ions further Mössbauer studies with ¹¹⁹Sn γ-ray sources would be necessary. Moreover, to observe the canting evolution on spin structure (change from axial to planar anisotropy), Mössbauer analyses with an external magnetic field applied parallel to γ-ray direction would be of special interest as well.

5. ACKNOWLEDGMENTS

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