Hyperfine Interact (2008) 184:135–141 DOI 10.1007/s10751-008-9778-6
(Ni, Zn, Sn) Ru and (Ni, Sn) Sn substituted barium ferrite prepared by mechanical alloying A. González-Angeles · J. Lipka · A. Grusková · V. Janˇcárik · I. Tóth · J. Sláma
Published online: 5 October 2008 © Springer Science + Business Media B.V. 2008
Abstract NiRu, ZnRu, SnRu and SnSn mixtures considerably improved the saturation magnetization, Ms with low substitution values; diminishing quickly at the same times the coercivity, Hci to suitable values for high-density magnetic recording applications. On the other hand, the NiSn mixture also decreased the coercivity rapidly however without enhancing the saturation magnetization. The shown differences on magnetic properties were mainly due both to magnetic nature of divalent ion and to secondary phase apparitions. The mixtures with Sn2+ as partner ion diminished markedly to Tc . The tetravalent Ru4+ ion has a special effect on magnetic properties of hexagonal ferrites (increases Ms and diminishes fast Hci with low substitutions). Keywords Magnetic materials · Magnetic properties · Nanocrystalline materials PACS 75.50.-y · 75.75.+a · 81.07.Bc
1 Introduction Nowadays, 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]. The most studied materials have been ferrites with spinel, garnet and hexagonal structure. Barium hexagonal ferrite is anisotropic and has a larger intrinsic magneto-crystalline anisotropy field. Because of their anisotropic plane, the natural resonance occurs in GHz range
A. González-Angeles (B) Facultad de Ingenieria, Universidad Autónoma de Baja California, Blvd. Benito Juárez s/n, Cp 21280, Mexicali, B. C., México e-mail:
[email protected] J. Lipka · A. Grusková · V. Janˇcárik · I. Tóth · J. Sláma Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, Ilkoviˇcova 3, 81219 Bratislava, Slovak Republic
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and therefore, it is possible to use these materials as high frequency microwave absorbers [2–4]. To obtain M-type barium ferrites with planar anisotropy and to vary their magnetic properties is necessary to substitute Fe3+ by other trivalent ions or cationic mixtures [5, 6]. Also, it has been explored the ferrite composites field to synthesize new magnetic materials with improved characteristics for emergent technological applications [1, 7]. In this study, we compare the effect of different mixtures 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 hexaferrites powders Ba(M)2x Fe12−2x O19 with composition of 0≤ x ≤ 0.3 were synthesized by high energy milling where M = (Ni2+ , Zn2+ , Sn2+ )–Ru4+ and (Ni2+ , Sn2+ )–Sn4+ . As raw materials were used BaCO3 , Fe2 O3 , NiO, ZnO, SnO, SnO2 and RuO2 , all with purity of 98%. (ASC reagent, Aldrich Co.) The milling process was performed in a Segvary 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 1050o C with a soaking time of 1.5 h. Thermomagnetic measurements were performed to obtain both Curie temperature, Tc , and magnetic susceptibility curves, χ (θ), for all samples using the bridge method in an alternating magnetic field of 421 A/m at 920 Hz. The identification of crystalline phases in the samples was performed using an X-pert Phillips diffractometer with Cu-Kα 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 both conventional constant acceleration mode and 57 Co/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. 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. Mössbauer spectra at room temperature for all analyzed samples are shown in Fig. 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.
Intensity (a. u.)
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o
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(1116)
(220)
(217) (2011)
(2014)
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° - SnO 2
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Fig. 1 X-ray diffraction patterns for all substituted samples at x = 0.3
(110) (107) (114) (203)
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Fig. 2 57 Fe Mössbauer spectra at room temperature for all x = 0.3 synthesized samples. The x = 0 sample is included for comparison
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Fig. 3 Temperature dependence of χ (θ ) for all obtained samples with x = 0.3
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The realized studies threw that Ni2+ ions possess special preference for octahedral sites (4f2 and 2a), Sn4+ ions prefer to replace iron ions on both bipyramidal (2b) and tetrahedral (4f1 ) sites, Zn2+ preferred tetrahedral (4f1 ) sites although it could occupy octahedral sites at higher substitutions whilst Sn2+ ions presented similar tendencies to substitute octahedral sites (4f2 and 2a) than those by Ni2+ ions. Ru4+ showed some differences on site preferences, for ZnRu occupied first octahedral sites (4f2 and 2a) and after bipyramidal (2b) sites, for NiRu and SnRu samples preferred to substitute both bipyramidal (2b) and tetrahedral (4f1 ) sites. Nevertheless, it is clear that further Mössbauer studies with 119 Sn and 99 Ru γ -ray sources would be necessary to determine more exactly the site preferences of both ions. Moreover, to observe the canting evolution on spin structure (change from axial to planar anisotropy), Mössbauer analyses with a 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 (θ) and the Curie temperature, Tc . 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 Tc , (SnSn ∼21% and SnRu ∼12% as distinct from NiRu ∼1.6%, NiSn ∼1.8% and ZnRu ∼3.6%). In addition, NiSn and NiRu curves showed the appearance of another peak at ∼580◦ C, which correspond to spinel compound nucleation as secondary phase. This phase is most probably NiFe2 O4 , which posses a Tc of 585◦ C. The Fig. 4 shows Ms behavior as a function of substitution for all studied samples. It can see the substitution effect on saturation magnetization. Also it can be observed that all samples similarly enhanced Ms at low substitution (except NiSn sample, which remained almost constant for all substitution range), but as x increased, Ms curves showed different trend. This Ms behavior was due to both magnetic nature of divalent ions and secondary phases nucleation.
(Ni, Zn, Sn) Ru and (Ni, Sn) Sn substituted barium ferrite Fig. 4 Saturation magnetization, Ms , as a function of the substitution
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Fig. 5 Remanent magnetization, Mr , tendency as x increased for all samples
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It is important to point out the cationic substitution effect on remanent magnetization (Fig. 5) (important parameter for high density magnetic recording). It could be observed that Mr decreased slightly, remaining almost constant up to x = 0.2 for
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Fig. 6 Substitution effect on intrinsic coercivity (Hci )
400
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Ni-Sn Sn-Sn 100
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Ni-Ru 0 0
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SnSn, SnRu and NiSn samples (diminution ∼4%, 8% and 10% respectively). NiRu and ZnRu samples diminished Mr enormously as x increased (reduction ∼36% and 39% respectively). From Fig. 6, it can be observed the caused effect on intrinsic coercivity, Hci , by all experimented substitution. It can be seen that Hci suffered a drastic reduction for NiRu and ZnRu samples with x substitution (drop ∼90% and 87% respectively). The rest of mixtures also diminished Hci as x increased, but more smoothly. In addition, observing the obtained results, it can say that the tetravalent Ru4+ ion has a special effect on magnetic properties of hexagonal ferrites (increases Ms and diminishes so fast Hci with low substitutions).
4 Conclusions In substituted hexaferrites synthesis, different cationic mixtures were studied, aiming to obtain materials useful for high-density magnetic recording. In this aspect, we can highlight that ZnRu, NiRu and Sn–Sn mixtures diminished the coercivity (Hci ) rapidly to suitable values for high-density magnetic recording applications; increasing at the same time the saturation magnetization (Ms ) with low substitution values. On the other hand, ZnRu and NiRu mixtures also reduced the remanent magnetization, which is an important parameter to high-density magnetic recording. From this point of view, the SnSn substituted samples showed the most suitable properties for magnetic recording, in the 0 ≤ x ≤ 0.2 range. Alternatively, NiSn, SnRu, ZnRu and NiRu mixtures showed to be promissory materials for microwave absorption applications. To determine more exactly the site preferences of substituting ions further Mössbauer studies with 119 Sn and 99 Ru γ -ray sources would be necessary. Moreover,
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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. Finally, the results for the magnetic properties of substituted hexaferrites obtained in this investigation were similar to those reported in the literature, which highlights the possible application of these new compounds on high-density magnetic recording and microwave absorption, as well as the use of the mechanical alloying technique for the production of ceramic powders at great scale. Acknowledgements We would like to thanks both CONACyT-México and VEGA,—Slovak Republic (projects No. G-1/3096/06 and G-1/3189/06) for given support to carry out this work, respectively.
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