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Effect of sintering conditions on the magnetic disaccommodation in barium M-type hexaferrites
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 304 (2006) e766–e768 www.elsevier.com/locate/jmmm
Effect of sintering conditions on the magnetic disaccommodation in barium M-type hexaferrites Pablo Herna´ndez-Go´meza,, Carlos Torresa, Carlos de Franciscoa, Jose´ Marı´ a Mun˜oza, Oscar Alejosa, Jose´ Ignacio In˜iguezb, Victor Raposob, Oscar Monterob a
Departamento de Electricidad y Electro´nica, Universidad de Valladolid, 47071 Valladolid, Spain b Departamento de Fı´sica Aplicada, Universidad de Salamanca, 37071 Salamanca, Spain Available online 20 March 2006
Abstract The relaxation of the initial magnetic permeability has been measured in polycrystalline hexaferrites with nominal composition BaO 6Fe2O3 (i.e. M-type). The samples have been sintered at different temperatures in CO2 atmosphere and with different manufacturing conditions. In temperature range between 80 and 500 K, the magnetic disaccommodation shows presence of different relaxation processes, depending on both the sintering temperature and sintering time. The analogies and differences between the results obtained are discussed in terms of similar phase formation and different crystallite size. r 2006 Elsevier B.V. All rights reserved. PACS: 75.60Lr; 75.50Gg Keywords: Barium ferrites; Hexaferrites; Magnetic disaccommodation; Magnetic aftereffect
1. Introduction Hexagonal ferrites have been used as permanent magnets from a long time and are still the matter of research due to their good properties in microwave devices and magnetooptic or perpendicular recording media. Among the different stable phases, the M-type compound (AFe12O19) (A: Ba, Sr) is the most studied [1]. In order to tailor the properties for its use in different applications, and regarding magnetic properties, the magnetic after-effect processes have to be taken into account to minimize the losses. In addition, this kind of technique can provide information about the phase changes, defect dynamics, etc. [2]. A valuable measurement technique in this topic is magnetic disaccommodation, i.e. the time evolution of magnetic permeability after sample demagnetization. This kind of magnetic after-effect relaxation phenomenon, due to the time variation of the mobility of domain walls in the return to minimum energy state, origin of whose has been attributed to either the rearrangement or diffusion of Corresponding author. Tel.: +34 983 423895; fax: +34 983 423225.
E-mail address: [email protected] (P. Herna´ndez-Go´mez). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.218
anisotropic point defects [3] (lattice vacancies, interstitials) within the Bloch walls, is strongly temperature dependent. In this paper, we analyse the effect of sintering conditions on the magnetic disaccommodation in BaM hexaferrites. 2. Experimental Series of polycrystalline samples with initial mixtures of BaO 6Fe2O3, i.e. the stoichiometric M-type, were prepared in CO2 sintering atmosphere by means of two different ceramic routes: in A series, the samples were dry milled for 1 h, compacted with deionized water as binder and sintered at top temperature for 4 h, whereas samples of B series were wet milled in ethanol for 2 h, compacted with polyvynil alcohol, and the top temperature at sintering stage was held during 10 h. The relevant parameters are milling and sintering time, and they have been set up in order to obtain similar ceramic and magnetic properties. Measurement of magnetic disaccommodation, i.e., the time evolution of magnetic permeability after sample demagnetization were carried out with a system based on a LCR bridge [4], in temperature range of 80 KoTo500 K. The
ARTICLE IN PRESS P. Herna´ndez-Go´mez et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e766–e768
results have been represented as isochronal curves, i.e., the relative variation of the initial permeability between an initial time t1 ¼ 2 s after demagnetization and different window times t2 ¼ 4, 8, 16, 32, 64 and 128 s in the form: ½mðt1 ; TÞ mðt2 ; TÞ=mðt1 ; TÞ
e767
1275° C 2
(1)
ð%Þ.
3. Results and discussion In Fig. 1, we show the magnetic disaccommodation spectra of BaM samples corresponding to A series sintered at different temperatures, whereas in Fig. 2, the corresponding isochronal spectra of B series are represented. At a lower sintering temperature the disaccommodation amplitude of A samples is low, then different peaks emerge at 140, 200, 235, 300, 380 and 480 K, the last one being the most prominent. At 1330 1C sintering temperature this relaxation process collapses, and the process with peak at 235 K becomes the most important.
µ (t1,T) - µ (t2,T) /µ (t1,T) (%)
When the time window is similar to the relaxation time this curve exhibit a maximum, so that different relaxation processes are disclosed. 0 6
1300° C
4
2
0 6 1330° C 4
0.3 2
1275° C 0.2
0 100
0.1
200
300 Temperature (K)
400
500
µ (t1,T) - µ (t2,T) / µ (t1,T) (%)
Fig. 2. Isochronal spectra of B series of BaM samples.
0.0 1300° C
4
2
0 4
1330° C
2
0 100
200
300 Temperature (K)
400
Fig. 1. Isochronal spectra of A series of BaM samples.
500
On the other hand, for B samples peaks at 180, 235, 300 and 430 K are clearly defined even at the lower sintering temperature analysed. At 1330 1C the position of peaks is the same than A samples (i.e. 165, 235, 300 and 380 K), but in this case the most important relaxation process corresponds to the one with peak at temperature of 380 K. The assignment of the different relaxation processes to the reorientation or diffusion in different interstitial sites of the magnetoplumbite structure have already been discussed in Ref. [5]; roughly we have stated that the low-temperature processes are connected to the metallic sites in R blocks, and processes over the room temperature are caused by the relaxations in spinel S blocks. Comparing the dependence of the sintering temperature, the results of two ceramic routes are similar, with M-type formation and a phase transition at 1330 1C due to decomposition into X- and Z-phases [6]. In B samples it seems that the reduction of initial grain size in raw ferrite obtained by increased milling time leads to an increase in the reaction velocity at low sintering temperatures, so that the formation reaction of M hexaferrite is favoured.
ARTICLE IN PRESS e768
P. Herna´ndez-Go´mez et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e766–e768
The main differences between the two series of samples are the following: (i) the amplitude of disaccommodation is higher in B series, and (ii) the most prominent relaxation process is always that at a higher temperature, whereas in A series the low-temperature peaks develop with the sintering temperature. Concerning (i), and taking into account that the strength of this kind of magnetic relaxation processes are dependent on the presence of both lattice vacancies and ferrous cations, it can be said that the increased sintering time in B samples promotes the increase in crystallite size, allowing a higher defect concentration in the hexagonal structure. This fact, together with increased ferrous content introduced during the increased milling time, is responsible for higher disaccommodation amplitude observed in all B samples regarding the corresponding A series. With regard to (ii), the reason of this behaviour can be found in the sintering time: in B samples, the increased annealing time at top sintering temperature will allow ferrous cations to be placed in the most favourable metallic sites, which correspond to the octahedral sites in S blocks, thus enhancing the high temperature processes. The possibility of a lower degree of decomposition into different hexagonal phases could also be envisaged, assuming that 240 K peak is connected also to reorientations of anisotropic ferrous cations in the octahedral sites
located in T blocks of Z phase [5]. Anyway, a wide temperature range (almost from 350 to 500 K) of the preponderant relaxation processes in B samples points to a diffussive nature of them, making necessary further research. Acknowledgements Assistance of Miss A. Arguello in sample preparation is acknowledged. This work has been supported in part by Ministerio de Educacio´n y Ciencia (MAT 2004-04688-C0202) and Junta de Castilla y Leo´n. References [1] H. Kojima, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol.3, North-Holland, Amsterdam, 1982, pp. 305–391. [2] P. Herna´ndez-Go´mez, C. De Francisco, C. Torres, J. In˜iguez, V. Raposo, J.M. Perdigao, A.R. Ferreira, Phys. Status Solidi C 1 (7) (2004) 1792. [3] F. Walz, V.A.M. Brabers, S. Chikazumi, H. Kronmu¨ller, M.O. Rigo, Phys. Status Solidi B 110 (1982) 471. [4] C. de Francisco, J. In˜iguez, J.M. Mun˜oz, J. Ayala, IEEE Trans. Magn. 23 (1987) 1866. [5] P. Herna´ndez-Go´mez, J. Mun˜oz, C. Torres, C. de Francisco, O. Alejos, J. Phys. D 36 (9) (2003) 1062. [6] S.M. Lim, Y. Nakamura, J. Jpn. Inst. Met. 56 (1992) 1422.
Journal of Magnetism and Magnetic Materials 304 (2006) e766–e768 www.elsevier.com/locate/jmmm
Effect of sintering conditions on the magnetic disaccommodation in barium M-type hexaferrites Pablo Herna´ndez-Go´meza,, Carlos Torresa, Carlos de Franciscoa, Jose´ Marı´ a Mun˜oza, Oscar Alejosa, Jose´ Ignacio In˜iguezb, Victor Raposob, Oscar Monterob a
Departamento de Electricidad y Electro´nica, Universidad de Valladolid, 47071 Valladolid, Spain b Departamento de Fı´sica Aplicada, Universidad de Salamanca, 37071 Salamanca, Spain Available online 20 March 2006
Abstract The relaxation of the initial magnetic permeability has been measured in polycrystalline hexaferrites with nominal composition BaO 6Fe2O3 (i.e. M-type). The samples have been sintered at different temperatures in CO2 atmosphere and with different manufacturing conditions. In temperature range between 80 and 500 K, the magnetic disaccommodation shows presence of different relaxation processes, depending on both the sintering temperature and sintering time. The analogies and differences between the results obtained are discussed in terms of similar phase formation and different crystallite size. r 2006 Elsevier B.V. All rights reserved. PACS: 75.60Lr; 75.50Gg Keywords: Barium ferrites; Hexaferrites; Magnetic disaccommodation; Magnetic aftereffect
1. Introduction Hexagonal ferrites have been used as permanent magnets from a long time and are still the matter of research due to their good properties in microwave devices and magnetooptic or perpendicular recording media. Among the different stable phases, the M-type compound (AFe12O19) (A: Ba, Sr) is the most studied [1]. In order to tailor the properties for its use in different applications, and regarding magnetic properties, the magnetic after-effect processes have to be taken into account to minimize the losses. In addition, this kind of technique can provide information about the phase changes, defect dynamics, etc. [2]. A valuable measurement technique in this topic is magnetic disaccommodation, i.e. the time evolution of magnetic permeability after sample demagnetization. This kind of magnetic after-effect relaxation phenomenon, due to the time variation of the mobility of domain walls in the return to minimum energy state, origin of whose has been attributed to either the rearrangement or diffusion of Corresponding author. Tel.: +34 983 423895; fax: +34 983 423225.
E-mail address: [email protected] (P. Herna´ndez-Go´mez). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.218
anisotropic point defects [3] (lattice vacancies, interstitials) within the Bloch walls, is strongly temperature dependent. In this paper, we analyse the effect of sintering conditions on the magnetic disaccommodation in BaM hexaferrites. 2. Experimental Series of polycrystalline samples with initial mixtures of BaO 6Fe2O3, i.e. the stoichiometric M-type, were prepared in CO2 sintering atmosphere by means of two different ceramic routes: in A series, the samples were dry milled for 1 h, compacted with deionized water as binder and sintered at top temperature for 4 h, whereas samples of B series were wet milled in ethanol for 2 h, compacted with polyvynil alcohol, and the top temperature at sintering stage was held during 10 h. The relevant parameters are milling and sintering time, and they have been set up in order to obtain similar ceramic and magnetic properties. Measurement of magnetic disaccommodation, i.e., the time evolution of magnetic permeability after sample demagnetization were carried out with a system based on a LCR bridge [4], in temperature range of 80 KoTo500 K. The
ARTICLE IN PRESS P. Herna´ndez-Go´mez et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e766–e768
results have been represented as isochronal curves, i.e., the relative variation of the initial permeability between an initial time t1 ¼ 2 s after demagnetization and different window times t2 ¼ 4, 8, 16, 32, 64 and 128 s in the form: ½mðt1 ; TÞ mðt2 ; TÞ=mðt1 ; TÞ
e767
1275° C 2
(1)
ð%Þ.
3. Results and discussion In Fig. 1, we show the magnetic disaccommodation spectra of BaM samples corresponding to A series sintered at different temperatures, whereas in Fig. 2, the corresponding isochronal spectra of B series are represented. At a lower sintering temperature the disaccommodation amplitude of A samples is low, then different peaks emerge at 140, 200, 235, 300, 380 and 480 K, the last one being the most prominent. At 1330 1C sintering temperature this relaxation process collapses, and the process with peak at 235 K becomes the most important.
µ (t1,T) - µ (t2,T) /µ (t1,T) (%)
When the time window is similar to the relaxation time this curve exhibit a maximum, so that different relaxation processes are disclosed. 0 6
1300° C
4
2
0 6 1330° C 4
0.3 2
1275° C 0.2
0 100
0.1
200
300 Temperature (K)
400
500
µ (t1,T) - µ (t2,T) / µ (t1,T) (%)
Fig. 2. Isochronal spectra of B series of BaM samples.
0.0 1300° C
4
2
0 4
1330° C
2
0 100
200
300 Temperature (K)
400
Fig. 1. Isochronal spectra of A series of BaM samples.
500
On the other hand, for B samples peaks at 180, 235, 300 and 430 K are clearly defined even at the lower sintering temperature analysed. At 1330 1C the position of peaks is the same than A samples (i.e. 165, 235, 300 and 380 K), but in this case the most important relaxation process corresponds to the one with peak at temperature of 380 K. The assignment of the different relaxation processes to the reorientation or diffusion in different interstitial sites of the magnetoplumbite structure have already been discussed in Ref. [5]; roughly we have stated that the low-temperature processes are connected to the metallic sites in R blocks, and processes over the room temperature are caused by the relaxations in spinel S blocks. Comparing the dependence of the sintering temperature, the results of two ceramic routes are similar, with M-type formation and a phase transition at 1330 1C due to decomposition into X- and Z-phases [6]. In B samples it seems that the reduction of initial grain size in raw ferrite obtained by increased milling time leads to an increase in the reaction velocity at low sintering temperatures, so that the formation reaction of M hexaferrite is favoured.
ARTICLE IN PRESS e768
P. Herna´ndez-Go´mez et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e766–e768
The main differences between the two series of samples are the following: (i) the amplitude of disaccommodation is higher in B series, and (ii) the most prominent relaxation process is always that at a higher temperature, whereas in A series the low-temperature peaks develop with the sintering temperature. Concerning (i), and taking into account that the strength of this kind of magnetic relaxation processes are dependent on the presence of both lattice vacancies and ferrous cations, it can be said that the increased sintering time in B samples promotes the increase in crystallite size, allowing a higher defect concentration in the hexagonal structure. This fact, together with increased ferrous content introduced during the increased milling time, is responsible for higher disaccommodation amplitude observed in all B samples regarding the corresponding A series. With regard to (ii), the reason of this behaviour can be found in the sintering time: in B samples, the increased annealing time at top sintering temperature will allow ferrous cations to be placed in the most favourable metallic sites, which correspond to the octahedral sites in S blocks, thus enhancing the high temperature processes. The possibility of a lower degree of decomposition into different hexagonal phases could also be envisaged, assuming that 240 K peak is connected also to reorientations of anisotropic ferrous cations in the octahedral sites
located in T blocks of Z phase [5]. Anyway, a wide temperature range (almost from 350 to 500 K) of the preponderant relaxation processes in B samples points to a diffussive nature of them, making necessary further research. Acknowledgements Assistance of Miss A. Arguello in sample preparation is acknowledged. This work has been supported in part by Ministerio de Educacio´n y Ciencia (MAT 2004-04688-C0202) and Junta de Castilla y Leo´n. References [1] H. Kojima, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol.3, North-Holland, Amsterdam, 1982, pp. 305–391. [2] P. Herna´ndez-Go´mez, C. De Francisco, C. Torres, J. In˜iguez, V. Raposo, J.M. Perdigao, A.R. Ferreira, Phys. Status Solidi C 1 (7) (2004) 1792. [3] F. Walz, V.A.M. Brabers, S. Chikazumi, H. Kronmu¨ller, M.O. Rigo, Phys. Status Solidi B 110 (1982) 471. [4] C. de Francisco, J. In˜iguez, J.M. Mun˜oz, J. Ayala, IEEE Trans. Magn. 23 (1987) 1866. [5] P. Herna´ndez-Go´mez, J. Mun˜oz, C. Torres, C. de Francisco, O. Alejos, J. Phys. D 36 (9) (2003) 1062. [6] S.M. Lim, Y. Nakamura, J. Jpn. Inst. Met. 56 (1992) 1422.