Influence of sintering atmosphere on the magnetic after-effect in strontium ferrites

Journal of Magnetism and Magnetic Materials 157/158 (1996) 123-124

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Influence of sintering atmosphere on the magnetic after-effect in strontium ferrites P. Hernfindez

a,

C. de Francisco a,* , J.M. Mufioz a, J. Ifiiguez b, L. Torres b, M. Zazo h

a Dpto Electricidad y Electrdnica, Facultad de Ciencias, Universidad de Valladolid, Prado de la Magdalena s / n, 47071 Valladolid, Spain b Dpto. Fisica Aplicada. Facultad de Fisica Universidad de Salamanca, 37071 Salamanca, Spain

Abstract The relaxation of the initial permeability after sample demagnetization has been measured in polycrystalline strontium ferrite samples with nominal composition SrO • 6Fe203 (M-phase), sintered under different oxygen pressures, and has been represented by isochronal curves. Very different behaviours are observed. With lower oxygen pressures the isochronal spectra are similar to the curves obtained for barium ferrites, with relaxation peaks at room temperature. In higher oxygen partial pressures, the relaxation peaks are found at lower temperatures. Keywords: Magnetic after-effect; Permeability disaccommodation; Hexagonal ferrites; Strontium ferrites

In many ferrimagnetic oxides a time variation of the mobility of domain walls has been observed after a magnetic shock, as a consequence of microstructural changes of the defect arrangement or the electronic states within the domain walls. This phenomenon is called the magnetic after-effect, and its origin has been ascribed to the rearrangement of point defects within the Bloch walls by a reorientation of their symmetry axes (orientational processes) or to a long range cation diffusion via lattice vacancies (diffusion processes) [1]. Similar relaxation processes have been observed in magnetic oxides containing ferrous and ferric cations. In addition, it has been stated that the presence of lattice defects, controlled by carefully choosing the firing conditions, plays a decisive role in magnetic after-effect processes. The change in the wall mobility can be observed as a time decay of the initial magnetic permeability after a demagnetization of the sample. This phenomenon is called disaccommodation, and constitutes a powerful tool for the investigation of structural defects both in crystalline and amorphous structures. Hexagonal ferrites are a large family of ferrimagnetic oxides with notable applications in microwave devices and as permanent magnets, and show promise in magnetic and magneto-optic recording media. Thus a complete study of their magnetic properties is very important in order to obtain a good performance in these different devices.

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Studies of the magnetic after-effect by means of disaccommodation have been carried out on many cubic ferrites [2,3], but there is little information about this kind of technique in ferrites with hexagonal structures, especially in the S r O - F e O . Fe203 system. In this paper we begin the study of this system by analyzing its most significant compound, the M-phase, with nominal composition SrFe12O19, and sintered under different conditions. The most typical way to prepare ceramic compounds is the solid state reaction, at high temperatures, of the constituent oxides. In the present work, raw powders of high purity (over 99%) Fe203 and SrCO 3 were mixed in order to obtain a S r - F e ratio of 1:12. The mixtures were milled, prefired in air at 900°C, and pressed in a cylindrical die. Samples 5 mm in diameter and 15 mm long were sintered at 1300°C for 8 h under different partial oxygen pressures ranging between 1 and 4 × 10 - 4 atm. Finally, the polycrystalline samples were rapidly quenched in an air stream to avoid vacancy annealing. Magnetic after-effect measurements were carried out with a computer-aided system based on an automatic LCR bridge [4]. In the measuring process, the time decay of the initial permeability after a demagnetization of the sample was recorded in the temperature range 80-420 K. The experimental results were represented as isochronal curves, i.e. the relative variation of the initial permeability,

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124

P. Herndndez et al. / Journal of Magnetism and Magnetic Materials 157/158 (1996) 123-124

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TEMPERATURE (K) Fig. 1. Isochronal curves for Sr-M samples sintered at 1300°C in different partial pressures of oxygen, t 1 = 2 s and t 2 = 4, 8, 16, 32, 64 and 128 s (curves from bottom to top in each graph).

Fig. 1 shows the magnetic after-effect measurements, represented by isochronal curves. It is clear that the behaviour is completely different above and below Po2 = 0.21 atm. Samples sintered in air and in oxygen atmospheres exhibit isochronal spectra with peaks below room temperature. It is noteworthy that in B a - M samples with these sintering atmospheres, ferrous cations are not expected, and there are no relaxation peaks. However, in S r - M samples two peaks at 95 and 135 K exist, together with a large peak between 160 and 300 K. W h e n the oxygen partial pressure is decreased, the behaviour becomes similar to that of barium hexagonal samples with the W structure [5,6]. In this case, two peaks emerge at higher temperatures, 300 and 380 K, and a peak at 180 K in the sample sintered at P % = l 0 -2 atm. The experimental results have been processed using the analytical techniques described in Ref. [7]. With these techniques

we found that the 95 K peak could be fitted by the superposition of two Debye processes with activation energies close to 0.24 and 0.26 eV and pre-exponentiai factors 10 -12 s. The 135 K peak can also be fitted by the superposition of two Debye processes with activation energies of 0.34 and 0.36 eV. The wide peak can be approximated using several Debye-type processes, with activation energies ranging between 0.46 and 0.66 eV. Samples sintered at higher oxygen pressures can be fitted using parameters identical to those of B a - W compounds [5]. In our view, the quenching process is the main reason for this difference. In samples sintered at lower oxygen partial pressures, quenching has no effect because the sintering temperature is sufficient to obtain the W hexagonal compound [8], and the isochronai spectra are similar to those of the corresponding B a t . F e O . F e 2 0 3 system [5]. In the sample sintered with the lowest oxygen partial pressure, the vacancy content is increased, thus leading to an increment in the amplitude of the disaccommodation peaks. However, the quenching process strongly affects samples sintered with elevated oxygen partial pressures, owing to the formation of a mixed phase sample containing SrFelzO19, SrFe204 and SrTFe10022 [9]. The hexagonal compound SrFel2019 does not contribute to the disaccommodation, because it does not have ferrous cations in its structure. Therefore, Sr7Fea0022 and SrFe204 compounds are responsible for the low-temperature peaks. Nevertheless, further research on this system is needed in order to clarify this effect in a material in which ferrous cations are not expected. References

[1] F. Walz, V.A.M. Brabers, S. Chikazumi, H. KrEinmuller and M.O. Rigo, Phys. Status Solidi (b) 110 (1982) 471. [2] C. de Francisco, J.M. Mufioz, J. Ayala and J. Ifiiguez, Phys. Status Solidi (a) 108 (1988) 721. [3] H. KronmiJller and F. Walz, Philos. Mag. B 42 (1980) 433. [4] C. de Francisco, J. Ifiiguez, J.M. Mufioz and J. Ayala, IEEE Trans. Magn. 23 (1987) 1866. [5] C. de Francisco, J.M. Mufioz, R. Tortes, L. Torres, J. Ifiiguez and M. Zazo, IEEE Trans. Magn. 29 (1993) 3523. [6] F. Walz, J. Rivas, D. Martinez and H. Kronmiiller, Phys. Status Solidi (a) 143 (1994) 137. [7] C. de Francisco, J. Ifiiguez and J.M. Mufioz, IEEE Trans. Magn. 25 (1989) 4413. [8] Y. Goto and T. Takahashi, J. Jpn. Soc. Powder Met. 17 (1971) 193. [9] F. Haberey and A. Kockel, IEEE Trans. Mag. 12 (1976) 983.