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Magnetic disaccommodation in Sn substituted magnetite
Journal of Magnetism and Magnetic Materials 226}230 (2001) 1409}1411
Magnetic disaccommodation in Sn substituted magnetite P. HernaH ndez-GoH mez*, K. Bendimya, C. de Francisco, J.M. Mun oz, O. Alejos, C. Torres Departamento de Electricidad y Electro& nica, Universidad de Valladolid, Prado de la Magdalena s/n E-47071 Valladolid, Spain
Abstract The relaxation of the initial magnetic permeability has been measured in polycrystalline Sn-doped magnetite with nominal composition Sn Fe O with x ranging from x"0 to 0.6. In the temperature range between 80 and 500 K, the V \V time decay of the initial permeability after sample demagnetization has been represented by means of isochronal disaccommodation curves, which show the presence of di!erent relaxation processes at 250 K (IV peak), 275 K (IV), 300 K (III), 400 K (II) and 440 K (I). This behavior is explained on the basis of the disaccommodation of vacancy-doped magnetite and another similar tetravalent substitution, as the previously analyzed Ti-doped magnetite. 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnetic after-e!ect; Disaccommodation; Spinel ferrites; Magnetite
Spinel ferrites are widely used for electrotechnical equipment since their discovery in the 1940s [1]. The commercial compositions are obtained by using an adequate cation substitution so that the physical properties required are ful"lled. Regarding magnetic properties, the magnetic relaxations have to be taken into account, in order to minimize the losses. In addition, the study of this kind of processes provides information about the underlying mechanisms governing the dynamic behavior of Bloch walls. Among the di!erent techniques available, the magnetic disaccommodation is a very sensitive one in the detection of small amounts of impurities, defects and lattice vacancies. It consists of the time variation of the mobility of domain walls after a magnetic shock, due to the rearrangement or di!ussion of anisotropic point defects within the Bloch walls. In previous works, we have analyzed the e!ect of the introduction of Ti> cations in both the hexagonal [2] and the cubic lattice [3,4]. In this paper, we analyze the e!ect of another tetravalent cation in the spinel lattice by doping magnetite with Sn>.
* Corresponding author. Fax: #34-983-423225. E-mail address: [email protected] (P. HernaH ndez-GoH mez).
For this work, a series of polycrystalline samples with nominal composition Sn Fe O (0(x(0.6) has V \V been prepared with high-purity starting oxides -Fe O and SnO , mixed in an appropriate ratio for 1 h in an agate mortar, ground and pressed in a cylindrical die. Samples with 5 mm diameter and 15 mm length were sintered in CO atmosphere at 14003 C for 8 h, and rapidly quenched to avoid phase annealing. Thermogravimetric analysis and X-ray di!raction spectra reveal the formation of spinel ferrite as single phase. Magnetic disaccommodation measurements were carried out with an automated system based on a LCR bridge [5] in the 80 K(T(500 K temperature range. The results have been represented as isochronal curves, i.e., the relative variation of the initial permeability after sample demagnetization between an initial time t "2 s and di!erent window times t "4, 8, 16, 32, 64 and 128 s in the form (t , ¹)!(t , ¹) (%). (t , ¹)
(1)
The disaccommodation spectra of the di!erent samples are shown in the Figs. 1 and 2. We can note the e!ect of the progressive introduction of Sn cations in the
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 9 6 3 - X
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P. Herna& ndez-Go& mez et al. / Journal of Magnetism and Magnetic Materials 226 }230 (2001) 1409}1411
Fig. 1. Isochronal disaccommodation spectra of Sn Fe O V \V (0(x(0.1) samples sintered in CO at 14003C. The isochronal curves are formed by using the Eq. (1) with t "2 s and t "4, 8, 16, 32, 64 and 128 s (curves from bottom to top in each graph).
magnetite lattice, and remark the following points: (i) The amplitude of the di!erent processes decreases with the increase of Sn> in the spinel structure, due to the smaller vacancy content with increasing doping rate. (ii) The III (300 K) process which takes place in polycrystalline magnetite splits up into three processes, called III, IV (275 K) and IV (250 K), even for the lowest doping rate analyzed, in a way very similar to the behavior of the Ti-doped magnetite system [3,4] (eventhough they are not well de"ned in our case and are merged into a wide relaxation peak centered at 280 K, they become evident in the "tting stage by using a discrete set of thermally activated exponential terms, yielding activation energies of 0.8, 0.75 and 0.68 eV, resp.). The IV process increases its relative amplitude with the Sn content, whereas the III process diminishes strongly. The origin of these processes is attributed to the presence of Sn> in octahedral sites,
Fig. 2. Isochronal disaccommodation spectra of Sn Fe O V \V (x"0 and 0.1(x(0.6) samples sintered in CO at 14003C, with t and t values as in Fig. 1.
lowering the activation energies of Fe> jumps into vacancies (III peak), in two di!erent ways depending on whether the Sn cation is in the same octant where both the vacancy and the ferrous cation are (IV peak) or in a neighbor octant (IV peak) [4]. (iii) Two additional processes emerge at higher temperatures for substituted samples, the so-called II (385}400 K) and I (420}440 K) peaks. Regarding the titanomagnetite system, these peaks also shift towards higher temperatures with the increasing amount of Sn doping, but they do not move out of the temperature range tested. These processes are associated with vacancy-mediated long-range di!usion of Sn ions within the Bloch walls, and they become preponderant with higher substitution rates, thus indicating that the tendency to form Sn>}Fe> pairs is lower than in the case of Ti-doped magnetite. In this study we have shown that the magnetic aftere!ect behavior of Sn substituted magnetite reveals some
P. Herna& ndez-Go& mez et al. / Journal of Magnetism and Magnetic Materials 226 }230 (2001) 1409}1411
di!erences regarding the Ti-doped magnetite system with similar valence and site occupation features, especially at the higher temperatures tested. This work has been partially supported by &Junta de Castilla y Leon', project VA-06/00B.
References [1] V.A.M. Brabers, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 8, North-Holland, Amsterdam, 1995, (Chapter 3).
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[2] P. HernaH ndez-GoH mez, C. De Francisco, J.M. Mun oz, O. Alejos, C. Torres, P.G. Berco!, H.R. Bertorello, J. Appl. Phys. 87 (9) (2000) 6250. [3] K. Bendimya, C. de Francisco, P. HernaH ndez, O. Alejos, J.M. Mun oz, J. Phys. IV France C1 (1997) 605. [4] L. Torres, F. Walz, K. Bendimya, C. de Francisco, H. Kronmuller, Phys. Stat. Sol. A 161 (1997) 289. [5] C. de Francisco, J. In iguez, J.M. Mun oz, J. Ayala, IEEE Trans. Magn. 23 (1987) 1866.
Magnetic disaccommodation in Sn substituted magnetite P. HernaH ndez-GoH mez*, K. Bendimya, C. de Francisco, J.M. Mun oz, O. Alejos, C. Torres Departamento de Electricidad y Electro& nica, Universidad de Valladolid, Prado de la Magdalena s/n E-47071 Valladolid, Spain
Abstract The relaxation of the initial magnetic permeability has been measured in polycrystalline Sn-doped magnetite with nominal composition Sn Fe O with x ranging from x"0 to 0.6. In the temperature range between 80 and 500 K, the V \V time decay of the initial permeability after sample demagnetization has been represented by means of isochronal disaccommodation curves, which show the presence of di!erent relaxation processes at 250 K (IV peak), 275 K (IV), 300 K (III), 400 K (II) and 440 K (I). This behavior is explained on the basis of the disaccommodation of vacancy-doped magnetite and another similar tetravalent substitution, as the previously analyzed Ti-doped magnetite. 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnetic after-e!ect; Disaccommodation; Spinel ferrites; Magnetite
Spinel ferrites are widely used for electrotechnical equipment since their discovery in the 1940s [1]. The commercial compositions are obtained by using an adequate cation substitution so that the physical properties required are ful"lled. Regarding magnetic properties, the magnetic relaxations have to be taken into account, in order to minimize the losses. In addition, the study of this kind of processes provides information about the underlying mechanisms governing the dynamic behavior of Bloch walls. Among the di!erent techniques available, the magnetic disaccommodation is a very sensitive one in the detection of small amounts of impurities, defects and lattice vacancies. It consists of the time variation of the mobility of domain walls after a magnetic shock, due to the rearrangement or di!ussion of anisotropic point defects within the Bloch walls. In previous works, we have analyzed the e!ect of the introduction of Ti> cations in both the hexagonal [2] and the cubic lattice [3,4]. In this paper, we analyze the e!ect of another tetravalent cation in the spinel lattice by doping magnetite with Sn>.
* Corresponding author. Fax: #34-983-423225. E-mail address: [email protected] (P. HernaH ndez-GoH mez).
For this work, a series of polycrystalline samples with nominal composition Sn Fe O (0(x(0.6) has V \V been prepared with high-purity starting oxides -Fe O and SnO , mixed in an appropriate ratio for 1 h in an agate mortar, ground and pressed in a cylindrical die. Samples with 5 mm diameter and 15 mm length were sintered in CO atmosphere at 14003 C for 8 h, and rapidly quenched to avoid phase annealing. Thermogravimetric analysis and X-ray di!raction spectra reveal the formation of spinel ferrite as single phase. Magnetic disaccommodation measurements were carried out with an automated system based on a LCR bridge [5] in the 80 K(T(500 K temperature range. The results have been represented as isochronal curves, i.e., the relative variation of the initial permeability after sample demagnetization between an initial time t "2 s and di!erent window times t "4, 8, 16, 32, 64 and 128 s in the form (t , ¹)!(t , ¹) (%). (t , ¹)
(1)
The disaccommodation spectra of the di!erent samples are shown in the Figs. 1 and 2. We can note the e!ect of the progressive introduction of Sn cations in the
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 9 6 3 - X
1410
P. Herna& ndez-Go& mez et al. / Journal of Magnetism and Magnetic Materials 226 }230 (2001) 1409}1411
Fig. 1. Isochronal disaccommodation spectra of Sn Fe O V \V (0(x(0.1) samples sintered in CO at 14003C. The isochronal curves are formed by using the Eq. (1) with t "2 s and t "4, 8, 16, 32, 64 and 128 s (curves from bottom to top in each graph).
magnetite lattice, and remark the following points: (i) The amplitude of the di!erent processes decreases with the increase of Sn> in the spinel structure, due to the smaller vacancy content with increasing doping rate. (ii) The III (300 K) process which takes place in polycrystalline magnetite splits up into three processes, called III, IV (275 K) and IV (250 K), even for the lowest doping rate analyzed, in a way very similar to the behavior of the Ti-doped magnetite system [3,4] (eventhough they are not well de"ned in our case and are merged into a wide relaxation peak centered at 280 K, they become evident in the "tting stage by using a discrete set of thermally activated exponential terms, yielding activation energies of 0.8, 0.75 and 0.68 eV, resp.). The IV process increases its relative amplitude with the Sn content, whereas the III process diminishes strongly. The origin of these processes is attributed to the presence of Sn> in octahedral sites,
Fig. 2. Isochronal disaccommodation spectra of Sn Fe O V \V (x"0 and 0.1(x(0.6) samples sintered in CO at 14003C, with t and t values as in Fig. 1.
lowering the activation energies of Fe> jumps into vacancies (III peak), in two di!erent ways depending on whether the Sn cation is in the same octant where both the vacancy and the ferrous cation are (IV peak) or in a neighbor octant (IV peak) [4]. (iii) Two additional processes emerge at higher temperatures for substituted samples, the so-called II (385}400 K) and I (420}440 K) peaks. Regarding the titanomagnetite system, these peaks also shift towards higher temperatures with the increasing amount of Sn doping, but they do not move out of the temperature range tested. These processes are associated with vacancy-mediated long-range di!usion of Sn ions within the Bloch walls, and they become preponderant with higher substitution rates, thus indicating that the tendency to form Sn>}Fe> pairs is lower than in the case of Ti-doped magnetite. In this study we have shown that the magnetic aftere!ect behavior of Sn substituted magnetite reveals some
P. Herna& ndez-Go& mez et al. / Journal of Magnetism and Magnetic Materials 226 }230 (2001) 1409}1411
di!erences regarding the Ti-doped magnetite system with similar valence and site occupation features, especially at the higher temperatures tested. This work has been partially supported by &Junta de Castilla y Leon', project VA-06/00B.
References [1] V.A.M. Brabers, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 8, North-Holland, Amsterdam, 1995, (Chapter 3).
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[2] P. HernaH ndez-GoH mez, C. De Francisco, J.M. Mun oz, O. Alejos, C. Torres, P.G. Berco!, H.R. Bertorello, J. Appl. Phys. 87 (9) (2000) 6250. [3] K. Bendimya, C. de Francisco, P. HernaH ndez, O. Alejos, J.M. Mun oz, J. Phys. IV France C1 (1997) 605. [4] L. Torres, F. Walz, K. Bendimya, C. de Francisco, H. Kronmuller, Phys. Stat. Sol. A 161 (1997) 289. [5] C. de Francisco, J. In iguez, J.M. Mun oz, J. Ayala, IEEE Trans. Magn. 23 (1987) 1866.