- Email: [email protected]
Influence of stoichiometry on the magnetic disaccommodation in M-type Sr hexaferrites
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e1843–e1844
Influence of stoichiometry on the magnetic disaccommodation in M-type Sr hexaferrites a, ! Pablo Herna! ndez-Gomez *, Carlos Torresa, Carlos de Franciscoa, Jose! Mar!ıa Mun˜oza, Oscar Alejosa, Jose! Ignacio In˜iguezb, Victor Raposob a
! Departamento de Electricidad y Electronica, Universidad de Valladolid, Prado de la Magdalena s/n Valladolid 47071, Spain b Departamento de F!ısica Aplicada, Universidad de Salamanca, Salamanca 37071, Spain
Abstract The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition SrO nFe2O3 (n=5.7, 6), prepared by means of standard ceramic techniques in air as well as CO2 sintering atmospheres. The isochronal disaccommodation spectra show the presence of different relaxation processes, depending on both the sintering atmosphere and especially the initial composition, and associated to ionic reorientations of ferrous cations and lattice vacancies in the different metallic sites within the spinel (S) and hexagonal (R) blocks of the close packed lattice. r 2003 Elsevier B.V. All rights reserved. PACS: 75.60Lr; 75.50 Gg Keywords: Strontium ferrite; Hexaferrite; Magnetic disaccommodation; Magnetic aftereffect
1. Introduction Hexagonal ferrites are widely used as permanent magnets and show promising properties in microwave devices and magneto-optic or perpendicular recording media. Among the different stable phases, the M-type (AFe12O19) (A: Ba, Sr) was the first discovered and most studied [1]. In order to tailor the properties for its use in the different applications, and regarding magnetic properties, the magnetic after-effect processes have to be taken into account to minimize the losses. A valuable measurement technique in this topic is the magnetic disaccommodation, i.e. the time variation of the mobility of domain walls after a magnetic shock, which is shown by a temporal evolution of the magnetic permeability after sample demagnetization. This relaxation phenomenon whose origin has been attributed to *Corresponding author. Tel.: +34-983-423895; fax: +34983-423225. E-mail address: [email protected] ! (P. Hern!andez-Gomez).
either the rearrangement or diffusion of anisotropic point defects [2] (lattice vacancies, interstitials) within the Bloch walls, is strongly temperature dependent. In this paper the effect of stoichiometry in the neighbourhood of Sr M-type hexaferrites is studied.
2. Experimental Series of polycrystalline samples with compositions SrO nFe2O3 (n=6, 5.7) were prepared by means of standard ceramic techniques, as described earlier [3], in air or CO2 sintering atmospheres. Magnetic disaccommodation measurements were carried out with a system based on a LCR bridge [4], in the 80 K o T o 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 t1=2 s and different window times t2=4, 8, 16, 32, 64 and 128 s in the form: ½mðt1 ; TÞ mðt2 ; TÞ=mðt1 ; TÞ ð%Þ:
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.887
ð1Þ
ARTICLE IN PRESS ! ! P. Hernandez-G omez et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e1843–e1844
e1844
(a) SrO .5.7 Fe 2O3
1.0
0.5
0.0 3
0.0 100
1300 ˚C
[µ(t1, T) - µ (t2, T) ] / µ (t1, T) (%)
[µ(t 1, T) - µ (t 2, T) ] / µ (t1, T) (%)
1300 ˚C
0.5
2 1 0 0.4
(b) SrO .6 Fe 2O3 1275 ˚C
0.0 1
(a) SrO.5.7 Fe2O3
1.0
1275 ˚C
A0 1300 ˚C
A B
0.4
200
400
500
(b) SrO.6 Fe2O3 1300 ˚C
0.0 0.4
1340 ˚C
0.0 0.4
B
1380 ˚C
D
A
D
0
F
100
300
200
300
400
500
E
0.0
TEMPERATURE (K)
100
200
300
400
TEMPERATURE (K)
Fig. 1. Isochronal spectra of Sr hexaferrites sintered in CO2.
Fig. 2. Isochronal spectra of Sr hexaferrites sintered in air.
3. Results and discussion The isochronal curves for samples sintered in CO2 and air are shown in Figs. 1 and 2, respectively. The results for both initial compositions in CO2 sintered samples looks similar. We observe peaks A and B (Tmax ¼ 380 and 300 K resp), related to ionic reorientations in octahedral sites in S blocks (activation energies E ¼ 1 and 0.8 eV resp) [3]. Another process (A0 ), related to vacancy-mediated diffusion of anisotropic cations, appears at 450–500 K and has increasing amplitude with sintering temperature, due to the higher vacancy content. Their high amplitudes together with the thermal dependence of magnetic permeability points to the formation of some amount of Sr2Fe30O46 (X phase) in these samples. In SrO 6Fe2O3 samples it also appears the D peak (Tmax ¼ 165 K, E ¼ 0:5 eV) which is associated with a similar reorientation process involving octahedral sites in R blocks [3]. It seems that deviation from stoichiometry reduces the amount of Fe2+ cation and increases the vacancy content, thus enhancing S block-related relaxation processes. On the other hand, the results for air sintering atmosphere (Fig. 2) are rather different. Up to five processes emerge for stoichiometric samples when sintering above 1340 C, the aforementioned A, B and D peaks, together with peaks E and F (Tmax ¼ 130; 90 K; E ¼ 0:35; 0.25 eV). The origin of these peaks is not yet clear. E process is also found at 145 K in M-type barium ferrites and it
could be related to interstitials in R blocks, according to the different size of Ba and Sr ions [3]. F process could be of electronic nature due to the low activation energy, but further research at lower temperatures is needed. Finally, as no ferrous cations are expected at 1300 C in air, the curves shown in Fig. 2a suggest that the temperature transition to liquid phase has just been reached for this composition [3], thus allowing the formation of some amount of X phase with a small proportion of Fe2+ and high vacancy content. Acknowledgements This work has been partially supported by ‘‘Junta de ! Castilla y Leon’’, project Ref. SA/010/03.
References [1] H. Kojima, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials Vol. 3, North-Holland, Amsterdam, 1982, pp. 305–391. [2] F. Walz, V.A.M. Brabers, S. Chikazumi, H. Kronmuller, . M.O. Rigo, Phys. Stat. Sol. B 110 (1982) 471. ! [3] P. Hern!andez-Gomez, J. Mun˜oz, C. Torres, C. de Francisco, O. Alejos, J. Phys. D 36 (9) (2003) 1062. [4] C. de Francisco, J. In˜iguez, J.M. Mun˜oz, J. Ayala, IEEE Trans. Mag. 23 (1987) 1866.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e1843–e1844
Influence of stoichiometry on the magnetic disaccommodation in M-type Sr hexaferrites a, ! Pablo Herna! ndez-Gomez *, Carlos Torresa, Carlos de Franciscoa, Jose! Mar!ıa Mun˜oza, Oscar Alejosa, Jose! Ignacio In˜iguezb, Victor Raposob a
! Departamento de Electricidad y Electronica, Universidad de Valladolid, Prado de la Magdalena s/n Valladolid 47071, Spain b Departamento de F!ısica Aplicada, Universidad de Salamanca, Salamanca 37071, Spain
Abstract The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition SrO nFe2O3 (n=5.7, 6), prepared by means of standard ceramic techniques in air as well as CO2 sintering atmospheres. The isochronal disaccommodation spectra show the presence of different relaxation processes, depending on both the sintering atmosphere and especially the initial composition, and associated to ionic reorientations of ferrous cations and lattice vacancies in the different metallic sites within the spinel (S) and hexagonal (R) blocks of the close packed lattice. r 2003 Elsevier B.V. All rights reserved. PACS: 75.60Lr; 75.50 Gg Keywords: Strontium ferrite; Hexaferrite; Magnetic disaccommodation; Magnetic aftereffect
1. Introduction Hexagonal ferrites are widely used as permanent magnets and show promising properties in microwave devices and magneto-optic or perpendicular recording media. Among the different stable phases, the M-type (AFe12O19) (A: Ba, Sr) was the first discovered and most studied [1]. In order to tailor the properties for its use in the different applications, and regarding magnetic properties, the magnetic after-effect processes have to be taken into account to minimize the losses. A valuable measurement technique in this topic is the magnetic disaccommodation, i.e. the time variation of the mobility of domain walls after a magnetic shock, which is shown by a temporal evolution of the magnetic permeability after sample demagnetization. This relaxation phenomenon whose origin has been attributed to *Corresponding author. Tel.: +34-983-423895; fax: +34983-423225. E-mail address: [email protected] ! (P. Hern!andez-Gomez).
either the rearrangement or diffusion of anisotropic point defects [2] (lattice vacancies, interstitials) within the Bloch walls, is strongly temperature dependent. In this paper the effect of stoichiometry in the neighbourhood of Sr M-type hexaferrites is studied.
2. Experimental Series of polycrystalline samples with compositions SrO nFe2O3 (n=6, 5.7) were prepared by means of standard ceramic techniques, as described earlier [3], in air or CO2 sintering atmospheres. Magnetic disaccommodation measurements were carried out with a system based on a LCR bridge [4], in the 80 K o T o 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 t1=2 s and different window times t2=4, 8, 16, 32, 64 and 128 s in the form: ½mðt1 ; TÞ mðt2 ; TÞ=mðt1 ; TÞ ð%Þ:
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.887
ð1Þ
ARTICLE IN PRESS ! ! P. Hernandez-G omez et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e1843–e1844
e1844
(a) SrO .5.7 Fe 2O3
1.0
0.5
0.0 3
0.0 100
1300 ˚C
[µ(t1, T) - µ (t2, T) ] / µ (t1, T) (%)
[µ(t 1, T) - µ (t 2, T) ] / µ (t1, T) (%)
1300 ˚C
0.5
2 1 0 0.4
(b) SrO .6 Fe 2O3 1275 ˚C
0.0 1
(a) SrO.5.7 Fe2O3
1.0
1275 ˚C
A0 1300 ˚C
A B
0.4
200
400
500
(b) SrO.6 Fe2O3 1300 ˚C
0.0 0.4
1340 ˚C
0.0 0.4
B
1380 ˚C
D
A
D
0
F
100
300
200
300
400
500
E
0.0
TEMPERATURE (K)
100
200
300
400
TEMPERATURE (K)
Fig. 1. Isochronal spectra of Sr hexaferrites sintered in CO2.
Fig. 2. Isochronal spectra of Sr hexaferrites sintered in air.
3. Results and discussion The isochronal curves for samples sintered in CO2 and air are shown in Figs. 1 and 2, respectively. The results for both initial compositions in CO2 sintered samples looks similar. We observe peaks A and B (Tmax ¼ 380 and 300 K resp), related to ionic reorientations in octahedral sites in S blocks (activation energies E ¼ 1 and 0.8 eV resp) [3]. Another process (A0 ), related to vacancy-mediated diffusion of anisotropic cations, appears at 450–500 K and has increasing amplitude with sintering temperature, due to the higher vacancy content. Their high amplitudes together with the thermal dependence of magnetic permeability points to the formation of some amount of Sr2Fe30O46 (X phase) in these samples. In SrO 6Fe2O3 samples it also appears the D peak (Tmax ¼ 165 K, E ¼ 0:5 eV) which is associated with a similar reorientation process involving octahedral sites in R blocks [3]. It seems that deviation from stoichiometry reduces the amount of Fe2+ cation and increases the vacancy content, thus enhancing S block-related relaxation processes. On the other hand, the results for air sintering atmosphere (Fig. 2) are rather different. Up to five processes emerge for stoichiometric samples when sintering above 1340 C, the aforementioned A, B and D peaks, together with peaks E and F (Tmax ¼ 130; 90 K; E ¼ 0:35; 0.25 eV). The origin of these peaks is not yet clear. E process is also found at 145 K in M-type barium ferrites and it
could be related to interstitials in R blocks, according to the different size of Ba and Sr ions [3]. F process could be of electronic nature due to the low activation energy, but further research at lower temperatures is needed. Finally, as no ferrous cations are expected at 1300 C in air, the curves shown in Fig. 2a suggest that the temperature transition to liquid phase has just been reached for this composition [3], thus allowing the formation of some amount of X phase with a small proportion of Fe2+ and high vacancy content. Acknowledgements This work has been partially supported by ‘‘Junta de ! Castilla y Leon’’, project Ref. SA/010/03.
References [1] H. Kojima, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials Vol. 3, North-Holland, Amsterdam, 1982, pp. 305–391. [2] F. Walz, V.A.M. Brabers, S. Chikazumi, H. Kronmuller, . M.O. Rigo, Phys. Stat. Sol. B 110 (1982) 471. ! [3] P. Hern!andez-Gomez, J. Mun˜oz, C. Torres, C. de Francisco, O. Alejos, J. Phys. D 36 (9) (2003) 1062. [4] C. de Francisco, J. In˜iguez, J.M. Mun˜oz, J. Ayala, IEEE Trans. Mag. 23 (1987) 1866.