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Magnetic disaccommodation in Sr hexagonal ferrites with X-phase (2SrO·15Fe2O3) initial composition
Physica B 320 (2002) 267–269
Magnetic disaccommodation in Sr hexagonal ferrites with X-phase (2SrO 15Fe2O3) initial composition a, ! * a, P. Herna! ndez-Gomez *, P.G. Bercoffb, O. Alejosa, C. Torresa, J.M. Munoz c * C. de Franciscoa, J.I. Iniguez , H.R. Bertorellob a
! Departamento de Electricidad y Electronica, Prado de la Magdalena s/n, Universidad de Valladolid, E-47071 Valladolid, Spain b ! ! Departamento de Ciencia de Materiales, Universidad Nacional de Cordoba, 5000 Cordoba, Argentina c Departamento de F!ısica Aplicada, Universidad de Salamanca, 37071 Salamanca, Spain
Abstract The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition of X-phase (2SrO 15Fe2O3) in the temperature range between 80 and 500 K. The time decay of the initial permeability after sample demagnetization is represented by means of isochronal disaccommodation curves which show the presence of different disaccommodation processes whose maxima lie at 380, 300 and 160 K (resp. A, B and D peaks). This behaviour is explained regarding the spectra corresponding to barium ferrites in order to ascribe the different relaxation processes found for ionic transitions in the cationic sites within the hexagonal structure. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic after effect; Disaccommodation; Strontium ferrites; Hexaferrites
Hexagonal ferrites have been widely used as permanent magnets since their discovery due to their rather good energy product and low cost. In addition, they show promising properties for their use as magnetic and magneto-optic recording media as well as in microwave devices [1]. The physical properties needed are fulfilled choosing the most appropriate phase and carrying out adequate cation substitutions [2]. Regarding magnetic properties, the magnetic relaxations have to be taken into account, in order to minimize the losses. In addition, a study of this kind of processes provides information about the under*Corresponding author. Fax: +34-983-423225. E-mail address: [email protected] ! (P. Hern!andez-Gomez).
lying mechanisms governing the dynamic behaviour of Bloch walls. Among the different techniques available, the magnetic disaccommodation is a very sensitive one in the detection of small amounts of impurities, defects and lattice vacancies. It consists in the time decrease of the magnetic permeability after a demagnetization. This effect has its origin in the time variation of the mobility of domain walls after a magnetic shock, due to the rearrangement or diffusion of anisotropic point defects within the Bloch walls. This technique has been usually carried out in spinel ferrites [3], but it is also detectable in garnets [4] and hexagonal ferrites [5,6]. In this paper we analyze the magnetic disaccommodation observed in Sr hexaferrites with X-type initial stoichiometry, for which there is very scarce information [7,8].
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 0 7 0 8 - 1
268
! ! P. Hernandez-G omez et al. / Physica B 320 (2002) 267–269
For this work several polycrystalline samples with initial composition 2SrO 15Fe2O3 have been prepared with high purity a-Fe2O3 and SrCO3 powders, mixed in the appropriate ratio and milled for 1 h in an agate mortar. After calcination at 9501C in air during 12 h, samples with 5 mm in diameter and 15 mm in length, which were pressed in a cylindrical die, were sintered at different sintering temperatures both in air as well as CO2 atmosphere during 4 h, and rapidly quenched to avoid phase annealing. Magnetic disaccommodation measurements were carried out with an automated system based on a LCR bridge [9] in the 80 KoTo500 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 ¼ 2 s and different window times t2 =4, 8, 16, 32, 64 and 128 s in the form mðt1 ; TÞ mðt2 ; TÞ ð%Þ: mðt1 ; TÞ
ð1Þ
Fig. 1. Isochronal disaccommodation spectra of Sr2Fe30O46 samples sintered in air at different temperatures. The isochronal curves are formed by using Eq. (1) with t1 ¼ 2 s and t2 ¼ 4; 8, 16, 32, 64 and 128 s (curves from bottom to top in each graph).
The disaccommodation spectra of the samples sintered in air is shown in Fig. 1. The sample sintered at 13001C does not show disaccommodation processes, due to the formation of M-type (SrFe12O19) hexaferrite as main phase and hematite as secondary phase. Magnetic disaccommodation phenomena are related to the presence of both ferrous cations and lattice vacancies within the sample. In the M-phase all the iron cations are in trivalent state so that magnetic after-effect cannot occur. However, with the reduction process in the X-phase formation (Sr2Fe30O46), which occur at 13501C in air [8], three relaxation processes emerge at higher sintering temperatures: the A process centred at 380 K, the B peak at 300 K and the D peak at 160 K, following the notation employed in [6]. The B process is closely related to the III process which appears in polycrystalline magnetite [3]. The origin of this ionic relaxation process lie in the reorientation of the local symmetry axis of the vacancy located in an octahedral site due to the jump of a ferrous cation from a neighbour octahedral site towards the vacancy. The hexagonal ferrites are built by piling up two types of blocks: one of them with spinel-like cubic structure S(Fe6O8) and the other with hexagonal packing R(BaFe6O11). In the X-phase the stacking sequence is RSSR*S* (*denotes a 1801 rotation around the c-axis), and a close inspection reveals that the octahedral site arrangements in the two consecutive S blocks in hexaferrites and in spinel ferrites are similar; in addition, there are ferrous cations in X-type hexaferrites, so that the mechanism which governs the III process takes place within the hexaferrite structure, with activation energy of about 0.8 eV. In a similar way the A process is ascribed to reorientation of anisotropic Fe2+ cations with an increased activation energy (about 1 eV) regarding the B peak due to the presence of the Sr ion. Moreover, the D process is attributed to anisotropic rearrangement of ferrous cations in the octahedral face-sharing sites located in R blocks, with an activation energy of 0.4 eV. The behaviour with CO2 sintering atmosphere is shown in Fig. 2, is very similar. X-type hexaferrite is obtained at 12751C sintering temperature [7], but in this case the amplitude of the different
! ! P. Hernandez-G omez et al. / Physica B 320 (2002) 267–269
269
able in Sr hexaferrites regarding Ba ferrites. It seems that the smaller size of the Sr ion reduces the cationic site spacing in the R blocks, making unfavourable the existence of Fe2+ in these sites, due to their higher cationic radii regarding Fe3+. On the other hand, the onset of another relaxation process at 500 K is discerned, probably due to diffusion of lattice vacancies. Extension of the temperature range in the measurements would get deeper insight in this type of behaviour in the ferrites of the SrO–Fe2O3 system.
Acknowledgements This work has been partially supported by ‘‘Junta de Castilla y Leon’’, project VA-06/00B. ! The authors are indebted to Mr. J.A. Gomez Garc!ıa for their assistance in the preparation of samples. Fig. 2. Isochronal disaccommodation spectra of Sr2Fe30O46 samples sintered in CO2 at different temperatures. The isochronal curves are formed by using Eq. (1) with t1 ¼ 2 s and t2 ¼ 4; 8, 16, 32, 64 and 128 s (curves from bottom to top in each graph).
disaccommodation peaks is lower due to the smaller vacancy content provided by the reducing sintering atmosphere. Thermogravimetric results show a phase transition at 13201C so that the sample sintered at 13301C is of W-type (SrFe18O27) [2]. However, the C disaccommodation process, which emerged at 240 K in Ba–W hexaferrites [6] and was ascribed to an ionic process in bypyramidal trigonal sites located in the R blocks, does not take place in Sr ferrites. This fact together with the low amplitude of the D peak suggests that relaxation processes in R blocks are not favour-
References [1] M.P. Sharrock, IEEE Trans. Magn. 25 (6) (1989) 4374. [2] H. Kojima, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, Vol. 3, Amsterdam, North-Holland, 1982, pp. 305–391. [3] F. Walz, V.A.M. Brabers, S. Chikazumi, H. Kronmuller, M.O. Rigo, Phys. Stat. Sol. B 110 (1982) 471. * [4] L. Torres, M. Zazo, J. Iniguez, C. de Francisco, J.M. * Munoz, P. Hern!andez, Appl. Phys. Lett. 68 (4) (1996) 564. * * [5] P. Hern!andez, C. de Francisco, J.M. Munoz, J. Iniguez, L. Torres, M. Zazo, J. Magn. Magn. Mater. 157/158 (1996) 123. ! * [6] P. Hern!andez-Gomez, C. De Francisco, J.M. Munoz, O. Alejos, C. Torres, P.G. Bercoff, H.R. Bertorello, J. Appl. Phys. 87 (9) (2000) 6250. [7] S. Dey, R. Valenzuela, J. Appl. Phys. 55 (6) (1984) 2340. [8] F. Leccabue, G. Albanese, O. Ares Muzio, J. Appl. Phys. 61 (7) (1987) 2600. * * [9] C. de Francisco, J. Iniguez, J.M. Munoz, J. Ayala, IEEE Trans. Magn. 23 (1987) 1866.
Magnetic disaccommodation in Sr hexagonal ferrites with X-phase (2SrO 15Fe2O3) initial composition a, ! * a, P. Herna! ndez-Gomez *, P.G. Bercoffb, O. Alejosa, C. Torresa, J.M. Munoz c * C. de Franciscoa, J.I. Iniguez , H.R. Bertorellob a
! Departamento de Electricidad y Electronica, Prado de la Magdalena s/n, Universidad de Valladolid, E-47071 Valladolid, Spain b ! ! Departamento de Ciencia de Materiales, Universidad Nacional de Cordoba, 5000 Cordoba, Argentina c Departamento de F!ısica Aplicada, Universidad de Salamanca, 37071 Salamanca, Spain
Abstract The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition of X-phase (2SrO 15Fe2O3) in the temperature range between 80 and 500 K. The time decay of the initial permeability after sample demagnetization is represented by means of isochronal disaccommodation curves which show the presence of different disaccommodation processes whose maxima lie at 380, 300 and 160 K (resp. A, B and D peaks). This behaviour is explained regarding the spectra corresponding to barium ferrites in order to ascribe the different relaxation processes found for ionic transitions in the cationic sites within the hexagonal structure. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic after effect; Disaccommodation; Strontium ferrites; Hexaferrites
Hexagonal ferrites have been widely used as permanent magnets since their discovery due to their rather good energy product and low cost. In addition, they show promising properties for their use as magnetic and magneto-optic recording media as well as in microwave devices [1]. The physical properties needed are fulfilled choosing the most appropriate phase and carrying out adequate cation substitutions [2]. Regarding magnetic properties, the magnetic relaxations have to be taken into account, in order to minimize the losses. In addition, a study of this kind of processes provides information about the under*Corresponding author. Fax: +34-983-423225. E-mail address: [email protected] ! (P. Hern!andez-Gomez).
lying mechanisms governing the dynamic behaviour of Bloch walls. Among the different techniques available, the magnetic disaccommodation is a very sensitive one in the detection of small amounts of impurities, defects and lattice vacancies. It consists in the time decrease of the magnetic permeability after a demagnetization. This effect has its origin in the time variation of the mobility of domain walls after a magnetic shock, due to the rearrangement or diffusion of anisotropic point defects within the Bloch walls. This technique has been usually carried out in spinel ferrites [3], but it is also detectable in garnets [4] and hexagonal ferrites [5,6]. In this paper we analyze the magnetic disaccommodation observed in Sr hexaferrites with X-type initial stoichiometry, for which there is very scarce information [7,8].
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 0 7 0 8 - 1
268
! ! P. Hernandez-G omez et al. / Physica B 320 (2002) 267–269
For this work several polycrystalline samples with initial composition 2SrO 15Fe2O3 have been prepared with high purity a-Fe2O3 and SrCO3 powders, mixed in the appropriate ratio and milled for 1 h in an agate mortar. After calcination at 9501C in air during 12 h, samples with 5 mm in diameter and 15 mm in length, which were pressed in a cylindrical die, were sintered at different sintering temperatures both in air as well as CO2 atmosphere during 4 h, and rapidly quenched to avoid phase annealing. Magnetic disaccommodation measurements were carried out with an automated system based on a LCR bridge [9] in the 80 KoTo500 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 ¼ 2 s and different window times t2 =4, 8, 16, 32, 64 and 128 s in the form mðt1 ; TÞ mðt2 ; TÞ ð%Þ: mðt1 ; TÞ
ð1Þ
Fig. 1. Isochronal disaccommodation spectra of Sr2Fe30O46 samples sintered in air at different temperatures. The isochronal curves are formed by using Eq. (1) with t1 ¼ 2 s and t2 ¼ 4; 8, 16, 32, 64 and 128 s (curves from bottom to top in each graph).
The disaccommodation spectra of the samples sintered in air is shown in Fig. 1. The sample sintered at 13001C does not show disaccommodation processes, due to the formation of M-type (SrFe12O19) hexaferrite as main phase and hematite as secondary phase. Magnetic disaccommodation phenomena are related to the presence of both ferrous cations and lattice vacancies within the sample. In the M-phase all the iron cations are in trivalent state so that magnetic after-effect cannot occur. However, with the reduction process in the X-phase formation (Sr2Fe30O46), which occur at 13501C in air [8], three relaxation processes emerge at higher sintering temperatures: the A process centred at 380 K, the B peak at 300 K and the D peak at 160 K, following the notation employed in [6]. The B process is closely related to the III process which appears in polycrystalline magnetite [3]. The origin of this ionic relaxation process lie in the reorientation of the local symmetry axis of the vacancy located in an octahedral site due to the jump of a ferrous cation from a neighbour octahedral site towards the vacancy. The hexagonal ferrites are built by piling up two types of blocks: one of them with spinel-like cubic structure S(Fe6O8) and the other with hexagonal packing R(BaFe6O11). In the X-phase the stacking sequence is RSSR*S* (*denotes a 1801 rotation around the c-axis), and a close inspection reveals that the octahedral site arrangements in the two consecutive S blocks in hexaferrites and in spinel ferrites are similar; in addition, there are ferrous cations in X-type hexaferrites, so that the mechanism which governs the III process takes place within the hexaferrite structure, with activation energy of about 0.8 eV. In a similar way the A process is ascribed to reorientation of anisotropic Fe2+ cations with an increased activation energy (about 1 eV) regarding the B peak due to the presence of the Sr ion. Moreover, the D process is attributed to anisotropic rearrangement of ferrous cations in the octahedral face-sharing sites located in R blocks, with an activation energy of 0.4 eV. The behaviour with CO2 sintering atmosphere is shown in Fig. 2, is very similar. X-type hexaferrite is obtained at 12751C sintering temperature [7], but in this case the amplitude of the different
! ! P. Hernandez-G omez et al. / Physica B 320 (2002) 267–269
269
able in Sr hexaferrites regarding Ba ferrites. It seems that the smaller size of the Sr ion reduces the cationic site spacing in the R blocks, making unfavourable the existence of Fe2+ in these sites, due to their higher cationic radii regarding Fe3+. On the other hand, the onset of another relaxation process at 500 K is discerned, probably due to diffusion of lattice vacancies. Extension of the temperature range in the measurements would get deeper insight in this type of behaviour in the ferrites of the SrO–Fe2O3 system.
Acknowledgements This work has been partially supported by ‘‘Junta de Castilla y Leon’’, project VA-06/00B. ! The authors are indebted to Mr. J.A. Gomez Garc!ıa for their assistance in the preparation of samples. Fig. 2. Isochronal disaccommodation spectra of Sr2Fe30O46 samples sintered in CO2 at different temperatures. The isochronal curves are formed by using Eq. (1) with t1 ¼ 2 s and t2 ¼ 4; 8, 16, 32, 64 and 128 s (curves from bottom to top in each graph).
disaccommodation peaks is lower due to the smaller vacancy content provided by the reducing sintering atmosphere. Thermogravimetric results show a phase transition at 13201C so that the sample sintered at 13301C is of W-type (SrFe18O27) [2]. However, the C disaccommodation process, which emerged at 240 K in Ba–W hexaferrites [6] and was ascribed to an ionic process in bypyramidal trigonal sites located in the R blocks, does not take place in Sr ferrites. This fact together with the low amplitude of the D peak suggests that relaxation processes in R blocks are not favour-
References [1] M.P. Sharrock, IEEE Trans. Magn. 25 (6) (1989) 4374. [2] H. Kojima, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, Vol. 3, Amsterdam, North-Holland, 1982, pp. 305–391. [3] F. Walz, V.A.M. Brabers, S. Chikazumi, H. Kronmuller, M.O. Rigo, Phys. Stat. Sol. B 110 (1982) 471. * [4] L. Torres, M. Zazo, J. Iniguez, C. de Francisco, J.M. * Munoz, P. Hern!andez, Appl. Phys. Lett. 68 (4) (1996) 564. * * [5] P. Hern!andez, C. de Francisco, J.M. Munoz, J. Iniguez, L. Torres, M. Zazo, J. Magn. Magn. Mater. 157/158 (1996) 123. ! * [6] P. Hern!andez-Gomez, C. De Francisco, J.M. Munoz, O. Alejos, C. Torres, P.G. Bercoff, H.R. Bertorello, J. Appl. Phys. 87 (9) (2000) 6250. [7] S. Dey, R. Valenzuela, J. Appl. Phys. 55 (6) (1984) 2340. [8] F. Leccabue, G. Albanese, O. Ares Muzio, J. Appl. Phys. 61 (7) (1987) 2600. * * [9] C. de Francisco, J. Iniguez, J.M. Munoz, J. Ayala, IEEE Trans. Magn. 23 (1987) 1866.