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Magnetic characterization of SrFe2Fe16O27 ferrite prepared by chemical coprecipitation method
Volume 8. number
MATERIALS
.3,4
LETTERS
May 1989
MAGNETIC CHARACTERIZATION OF SrFezFe,,O,, FERRITE PREPARED BY CHEMICAL COPRECIPITATION METHOD N. SUAREZ I.tlRE,
ALMODOVAR,
Ph~slcs Faculty,
F. LECCABUE .Il.-tSPEC/CNR
La Hahana
J.L. SANCHEZ L’niversliy.
Havana.
LLAMAZARES Cuba
‘, R. PANIZZIERI
Institute,
l’la Chiavari
18/A, 43100 Parma,
Ital)
and Rong Hua XUE Ph,,rrcs Institute.
Received
.Yanllng
8 March
C’niverslt.v. A’anJing, Chrna
1989
Polycrystalline samples of SrFe2Fe1602, have been prepared by thermal decomposition of hydroxide-carbonate coprecipitated salts. The samples were examined by thermomagnetic analysis, Mossbatter spectroscopy, X-ray diffraction, scanning electron microscopy and static magnetic measurements. It was found that this hexafenite is only formed in a narrow temperature range under an inert atmosphere. The intrinsic magnetic properties obtained are: o,= 73.6 emu/g, H,= 18.6 kOe and r,.= 525°C.
1. Introduction The hexagonal ferrites with W-type crystal structure are ferrimagnetic materials, with high magnetic anisotropy, which correspond to the chemical formula (St-, Ba)Me,Fe,,O,, where Me’+ =Fe, Co, Mn, Ni. Zn. In their crystal structure the Fe3+ cations are distributed over the seven crystalline sites of S and R blocks, while the divalent Me*+ cations are located in the S block. The resulting intrinsic magnetic properties depend, to some extent on the divalent ions [ 1 1. These materials show intrinsic magnetic properties which have made them of interest not only for permanent magnets but also for perpendicular recording media and microwave devices. In the recent past we have performed a detailed study of the phase formation, thermal stability, morphology and magnetic properties of W- and X-type hexaferrites prepared by the hydroxide-carbonate coprecipitation method [ l-7 1. After a systematic ’ To whom correspondence
should be addressed.
study of the preparation conditions and magnetostructural properties of these compounds it was found that there are some notable differences in their stability when Me’+ =Cu and Fe are used: this fact is very closely related to the different valencies of such cations. A study of chemical instability of metal-deficient BaFe?-W ferrite was made by Lotgering and Vromans [ 81 and the formation of the W-phase was found to proceed via M and spine1 solid state reactions above 1200°C. In the present work, we report and discuss the results obtained by using the chemical coprecipitation method for the preparation of SrFe?-W polycrystalline powder. The phase formation, morphological grain characteristics and main magnetic properties are also reported.
2. Experimental Powders with nominal composition SrFelFe,,Oz, were prepared from a solution of metal chlorides 127
Volume 8, number
3,4
MATERIALS
1100
1200
HEATING
1300
TEMPERATURE
PC)
Fig. 1. Temperature dependence of the saturation magnetization, crS (A ), and Curie temperature, rc (0 ), for the coprecipitated powders heated at different temperatures for a fixed time of 6 h.
containing ST’+, Fe’+ and Fe’+ in a stoichiometric ratio ( 1: 2: 16) and an alkaline solution of NaOH: Na2C03 following the same procedure reported previously [ 11. The powders were pressed into tablets and then subjected to different thermal treatments always under a high-purity Ar atmosphere in order to avoid the oxidation of divalent iron. After annealing the samples were quenched in cold water. Mijssbauer spectra were measured in an ELSCINT-AME 30A spectrometer in a constant-ac-
(It -12
-10
Fig. 2. RT Mossbauer
128
-8
-6
May 1989
celeration mode and a transmission geometry using a “Co source in Rh matrix. Source and absorber were held at room temperature (RT). Microstructure morphology was observed by using a Cambridge 250 scanning electron microscopy. Magnetic phases and their Curie temperatures ( T,-) were determined from the low-field (60 Oe) thermomagnetic curves. Magnetic properties were derived from hysteresis loops measured by using a PAR 155 vibrating sample magnetometer with a maximum external field of 19.3 kOe. The singular point detection (SPD) technique [ 9 ] was used to determine the anisotropy field (H,). X-ray diffraction was performed on powdered samples with a Philips PW 1050/25 diffractometer employing Co Ku radiation. The relined cell parameters were: a=5.880(1) and c=32.762(9) A.
I
I
LETTERS
3. Results and discussion Fig. 1 shows the saturation magnetization ( crs) and Curie temperature as a function of the heating temperature in the 1100-l 300°C range. With the exception of the sample heated at 1250°C the H, values measured are negligible. Above and below this temperature T, values are only little lower than those
,a -4YELOCi~
spectra for coprecipitated
(m/05,
powders
2
4
6
8
10
12
heated at (a) 1200, (b) 1250, and (c) 1300°C for 6 h
Volume 8. number
MATERIALS
3.4
May 1989
LETTERS
,b
I
-12
1.00t L 0.981 L
-10
-a
-6
-4 "ELLlcl~:
(mm/j
.'........~.'~.';..:"._..,,,~, ,,,...'.'i; ,;;:“‘.‘.‘.., ; '. .. : . . '.. .
0.96>
2
4
6
a
10
12
1
<......'.'.. ... ,.,.,."..~',. .,,..'.. ..J,~.,.~ ../.'.",",,~.., .., ,' '. . . .,". . .
..
.
'. ,.
'.
z go.94
..
z so.92 1
I
-12
-10
I
h
-a
-6
1
"L&Y
1
-;m/3
1
I
I
1
I
1
0
2
4
6
a
10
,C 12
Fig. 2. Continued.
reported for magnetite (Fe,O,) and a, values are relatively high. All these data suggest that these samples contain a high percentage of magnetite. Mijssbauer spectra gave additional information on this point. In fig. 2 the MSssbauer spectra for samples heated at 1200, 1250 and 1300°C for 6 h are
reported, respectively. The fig. 2a spectrum has been interpreted as a superposition of non-stoichiometric magnetite (Fe3’),(Fe::-~,,Fe::+2x)[ with a Fe*+ deficient
lxh04 1 and a-Fe20,.
The fig. 2b spec129
Volume 8, number 3,4
MATERIALS LETTERS
trum matches well with that previously reported for the pure SrFe,-W hexaferrite [ IO,1 11. By increasing the sintering temperature (13OO”C), a similar situation to the one observed at 1200°C was found (i.e. the “fitting” of Mossbauer spectra gives three sextets that correspond: two of these to the presence of Fe’+deficient non-stoichiornetric magnetite and one to aFez03 phase in z 5wt%) suggesting that the SrFezW compound is only thermodynamically formed in a very narrow temperature range under critical conditions. Kojima et al. [ 111 showed that there is a strong difference between Ba and SrFe,-W ferrites (prepared by standard ceramic methods by firing at 1400°C in air) in the phase formation under different oxygen partial pressure and temperatures: i.e. for SrFe,-W the limit of tiring temperature was reduced to 1250°C only by using a Po,/(Po, +PN2)= 10w6. This system shows some differences in phase formation in comparison with other W-type hexaferrites in which the spine1 oxide always coexists with the M-type hexaferrites and the formation of W phase passes through the solid-state reaction of M and spi-
May 1989
nel phases [2,3,7], the only exception being the SrMn2-W ferrite [ 41. In fig. 3 the results of our thermomagnetic analysis are reported. Fig. 3b clearly shows the presence of one well-defined magnetic phase with T,= 525 “C; T, values appearing in figs. 3a and 3c are attributed to the presence of magnetite (7’,~585~C). Data for the complete magnetic characterization of this compound are presented in table 1 together with data reported up to now in the literature, both for SrFez-W [ 12 ] and for BaFe*-W ferrites [ 11,13 1. Unfortunately, even if the starting coprecipitated powders with grain size of about 0.075 urn are employed, the temperature of formation of the W-phase is too high ( GZ1250°C), resulting in grain growth (> 20 pm) and a consequent decrease of the coerTable 1 The main magnetic properties of single-phase SrFe2Fe160Z, hexaferrite prepared by the chemical coprecipitation method
SrFe,-W [ 111 our work BaFe,-W [ 121 BaFe,-W [lo]
0,
H,
TC
,f&
(emu/g)
WeI
(“Cl
(Oe)
77 73.6 91.5 a’ 66.8 75.1
19.5 18.6 20.8 a’ 15 17.6
507 525
30 55
534 503
380
‘) Measured at LNT. The polycrystalline samples of refs. [lo121 were prepared by the classical ceramic method.
Fig. 3. Initial permeability versus temperature for coprecipitate powders heated at (A) 660, (B) 1200, (C) 1250, and (D) 1300”Cfor6h.
130
Fig. 4. SEM micrograph of SrFe,-W ferrite. The coprecipitated powder was pressed and heated at 1250°C for 6 h under Ar atmosphere.
Volume 8, number
3,4
cive field. Fig. 4 reports a SEM micrograph SrFe2-W ferrite.
MATERIALS
for the
May 1989
LETTERS
References [ 1 ] F. Leccabue, G. Salviati. N. Suarez, G. Albanese and G. Leo, IEEE Trans. Magn. MAG-24
4. Conclusion
[ 21 F. Leccabue,
R. Panizzieri,
Sanchez, J. Appl. Phys. 59
Our results indicate that the SrFe2-W monophasic compound exists only in a narrow temperature range and is very sensitive to preparation conditions. We think that this situation is due to different diffusion mechanisms for Sr2+ and Fe3+ ions together with the critical conditions required for adjusting the Fe’+/ Fe’+ ratio for the phase formation of this compound. In contrast with several other W-types of hexagonal ferrites and with that proposed by Lotgering and Vromens [ 8 1, the M-type hexaferrite is not formed together with the spine1 oxides during the formation of SrFe2-W ferrite.
[3] F. Leccabue. Sanchez,
( 1988 ) 1850.
G. Salviati, G. Albanese and J.L.
( 1986 ) 2 114.
R. Panizzieri,
G. Albanese,
Mater. Sci. Monographs
[4] F. Leccabue.
0. Ares Muzio,
61 (1987)
2600.
[6] F. Leccabue.
R. Panizzieri,
G. Bocelli,
G. Calestam.
Rizzoli and N. Suarez, J. Magn. Magn. Mater.
C.
68 (1987)
365. [7] F. Leccabue.
R. Panizzieri,
G. Albanese.
Suarez, Mater. Res. Bull. 23 (1988)
G. Leo and N.
263.
[ 8 ] F.K. Lotgering and P.H.G.M. Vromans. J. 4m. Ceram. Sot. 60 (1977)
416. J. Appl. Phys. 45
[ IO] A.M. van Diepen and F.K. Lotgering,
The authors would like to thank the International Centre for Theoretical Physics (Trieste) for the interchange possibility among the researchers.
2019.
M. Safei Eldin. G. Calestani
and G. Albanese, J. Magn. Magn. Mater. 68 ( 1987) 201. [ 51 F. Leccabue, G. Albanese and 0. Ares Muzio. J. Appl. Phys.
[9] G. Asti and S. Rinaldi,
Acknowledgement
0. Ares and J.L.
38B (1987)
27 (1978)
( 1974) 3600.
Solid State Commun.
255.
[ 111H. KoJima, C. Miyakawa, T. Sato and K. Goto. Japan. J. Appl. Phys. 24 ( 1985) 5 I. [ 121 S. Dey and R. Valenzuela, J. Appl. Phys. 55 ( 1984) 2340. [ 13 1H. Lu. Y. Du, T. Wang, H. Hu and R. Xue. Advan. Ceram. 16 (1984)
561.
131
MATERIALS
.3,4
LETTERS
May 1989
MAGNETIC CHARACTERIZATION OF SrFezFe,,O,, FERRITE PREPARED BY CHEMICAL COPRECIPITATION METHOD N. SUAREZ I.tlRE,
ALMODOVAR,
Ph~slcs Faculty,
F. LECCABUE .Il.-tSPEC/CNR
La Hahana
J.L. SANCHEZ L’niversliy.
Havana.
LLAMAZARES Cuba
‘, R. PANIZZIERI
Institute,
l’la Chiavari
18/A, 43100 Parma,
Ital)
and Rong Hua XUE Ph,,rrcs Institute.
Received
.Yanllng
8 March
C’niverslt.v. A’anJing, Chrna
1989
Polycrystalline samples of SrFe2Fe1602, have been prepared by thermal decomposition of hydroxide-carbonate coprecipitated salts. The samples were examined by thermomagnetic analysis, Mossbatter spectroscopy, X-ray diffraction, scanning electron microscopy and static magnetic measurements. It was found that this hexafenite is only formed in a narrow temperature range under an inert atmosphere. The intrinsic magnetic properties obtained are: o,= 73.6 emu/g, H,= 18.6 kOe and r,.= 525°C.
1. Introduction The hexagonal ferrites with W-type crystal structure are ferrimagnetic materials, with high magnetic anisotropy, which correspond to the chemical formula (St-, Ba)Me,Fe,,O,, where Me’+ =Fe, Co, Mn, Ni. Zn. In their crystal structure the Fe3+ cations are distributed over the seven crystalline sites of S and R blocks, while the divalent Me*+ cations are located in the S block. The resulting intrinsic magnetic properties depend, to some extent on the divalent ions [ 1 1. These materials show intrinsic magnetic properties which have made them of interest not only for permanent magnets but also for perpendicular recording media and microwave devices. In the recent past we have performed a detailed study of the phase formation, thermal stability, morphology and magnetic properties of W- and X-type hexaferrites prepared by the hydroxide-carbonate coprecipitation method [ l-7 1. After a systematic ’ To whom correspondence
should be addressed.
study of the preparation conditions and magnetostructural properties of these compounds it was found that there are some notable differences in their stability when Me’+ =Cu and Fe are used: this fact is very closely related to the different valencies of such cations. A study of chemical instability of metal-deficient BaFe?-W ferrite was made by Lotgering and Vromans [ 81 and the formation of the W-phase was found to proceed via M and spine1 solid state reactions above 1200°C. In the present work, we report and discuss the results obtained by using the chemical coprecipitation method for the preparation of SrFe?-W polycrystalline powder. The phase formation, morphological grain characteristics and main magnetic properties are also reported.
2. Experimental Powders with nominal composition SrFelFe,,Oz, were prepared from a solution of metal chlorides 127
Volume 8, number
3,4
MATERIALS
1100
1200
HEATING
1300
TEMPERATURE
PC)
Fig. 1. Temperature dependence of the saturation magnetization, crS (A ), and Curie temperature, rc (0 ), for the coprecipitated powders heated at different temperatures for a fixed time of 6 h.
containing ST’+, Fe’+ and Fe’+ in a stoichiometric ratio ( 1: 2: 16) and an alkaline solution of NaOH: Na2C03 following the same procedure reported previously [ 11. The powders were pressed into tablets and then subjected to different thermal treatments always under a high-purity Ar atmosphere in order to avoid the oxidation of divalent iron. After annealing the samples were quenched in cold water. Mijssbauer spectra were measured in an ELSCINT-AME 30A spectrometer in a constant-ac-
(It -12
-10
Fig. 2. RT Mossbauer
128
-8
-6
May 1989
celeration mode and a transmission geometry using a “Co source in Rh matrix. Source and absorber were held at room temperature (RT). Microstructure morphology was observed by using a Cambridge 250 scanning electron microscopy. Magnetic phases and their Curie temperatures ( T,-) were determined from the low-field (60 Oe) thermomagnetic curves. Magnetic properties were derived from hysteresis loops measured by using a PAR 155 vibrating sample magnetometer with a maximum external field of 19.3 kOe. The singular point detection (SPD) technique [ 9 ] was used to determine the anisotropy field (H,). X-ray diffraction was performed on powdered samples with a Philips PW 1050/25 diffractometer employing Co Ku radiation. The relined cell parameters were: a=5.880(1) and c=32.762(9) A.
I
I
LETTERS
3. Results and discussion Fig. 1 shows the saturation magnetization ( crs) and Curie temperature as a function of the heating temperature in the 1100-l 300°C range. With the exception of the sample heated at 1250°C the H, values measured are negligible. Above and below this temperature T, values are only little lower than those
,a -4YELOCi~
spectra for coprecipitated
(m/05,
powders
2
4
6
8
10
12
heated at (a) 1200, (b) 1250, and (c) 1300°C for 6 h
Volume 8. number
MATERIALS
3.4
May 1989
LETTERS
,b
I
-12
1.00t L 0.981 L
-10
-a
-6
-4 "ELLlcl~:
(mm/j
.'........~.'~.';..:"._..,,,~, ,,,...'.'i; ,;;:“‘.‘.‘.., ; '. .. : . . '.. .
0.96>
2
4
6
a
10
12
1
<......'.'.. ... ,.,.,."..~',. .,,..'.. ..J,~.,.~ ../.'.",",,~.., .., ,' '. . . .,". . .
..
.
'. ,.
'.
z go.94
..
z so.92 1
I
-12
-10
I
h
-a
-6
1
"L&Y
1
-;m/3
1
I
I
1
I
1
0
2
4
6
a
10
,C 12
Fig. 2. Continued.
reported for magnetite (Fe,O,) and a, values are relatively high. All these data suggest that these samples contain a high percentage of magnetite. Mijssbauer spectra gave additional information on this point. In fig. 2 the MSssbauer spectra for samples heated at 1200, 1250 and 1300°C for 6 h are
reported, respectively. The fig. 2a spectrum has been interpreted as a superposition of non-stoichiometric magnetite (Fe3’),(Fe::-~,,Fe::+2x)[ with a Fe*+ deficient
lxh04 1 and a-Fe20,.
The fig. 2b spec129
Volume 8, number 3,4
MATERIALS LETTERS
trum matches well with that previously reported for the pure SrFe,-W hexaferrite [ IO,1 11. By increasing the sintering temperature (13OO”C), a similar situation to the one observed at 1200°C was found (i.e. the “fitting” of Mossbauer spectra gives three sextets that correspond: two of these to the presence of Fe’+deficient non-stoichiornetric magnetite and one to aFez03 phase in z 5wt%) suggesting that the SrFezW compound is only thermodynamically formed in a very narrow temperature range under critical conditions. Kojima et al. [ 111 showed that there is a strong difference between Ba and SrFe,-W ferrites (prepared by standard ceramic methods by firing at 1400°C in air) in the phase formation under different oxygen partial pressure and temperatures: i.e. for SrFe,-W the limit of tiring temperature was reduced to 1250°C only by using a Po,/(Po, +PN2)= 10w6. This system shows some differences in phase formation in comparison with other W-type hexaferrites in which the spine1 oxide always coexists with the M-type hexaferrites and the formation of W phase passes through the solid-state reaction of M and spi-
May 1989
nel phases [2,3,7], the only exception being the SrMn2-W ferrite [ 41. In fig. 3 the results of our thermomagnetic analysis are reported. Fig. 3b clearly shows the presence of one well-defined magnetic phase with T,= 525 “C; T, values appearing in figs. 3a and 3c are attributed to the presence of magnetite (7’,~585~C). Data for the complete magnetic characterization of this compound are presented in table 1 together with data reported up to now in the literature, both for SrFez-W [ 12 ] and for BaFe*-W ferrites [ 11,13 1. Unfortunately, even if the starting coprecipitated powders with grain size of about 0.075 urn are employed, the temperature of formation of the W-phase is too high ( GZ1250°C), resulting in grain growth (> 20 pm) and a consequent decrease of the coerTable 1 The main magnetic properties of single-phase SrFe2Fe160Z, hexaferrite prepared by the chemical coprecipitation method
SrFe,-W [ 111 our work BaFe,-W [ 121 BaFe,-W [lo]
0,
H,
TC
,f&
(emu/g)
WeI
(“Cl
(Oe)
77 73.6 91.5 a’ 66.8 75.1
19.5 18.6 20.8 a’ 15 17.6
507 525
30 55
534 503
380
‘) Measured at LNT. The polycrystalline samples of refs. [lo121 were prepared by the classical ceramic method.
Fig. 3. Initial permeability versus temperature for coprecipitate powders heated at (A) 660, (B) 1200, (C) 1250, and (D) 1300”Cfor6h.
130
Fig. 4. SEM micrograph of SrFe,-W ferrite. The coprecipitated powder was pressed and heated at 1250°C for 6 h under Ar atmosphere.
Volume 8, number
3,4
cive field. Fig. 4 reports a SEM micrograph SrFe2-W ferrite.
MATERIALS
for the
May 1989
LETTERS
References [ 1 ] F. Leccabue, G. Salviati. N. Suarez, G. Albanese and G. Leo, IEEE Trans. Magn. MAG-24
4. Conclusion
[ 21 F. Leccabue,
R. Panizzieri,
Sanchez, J. Appl. Phys. 59
Our results indicate that the SrFe2-W monophasic compound exists only in a narrow temperature range and is very sensitive to preparation conditions. We think that this situation is due to different diffusion mechanisms for Sr2+ and Fe3+ ions together with the critical conditions required for adjusting the Fe’+/ Fe’+ ratio for the phase formation of this compound. In contrast with several other W-types of hexagonal ferrites and with that proposed by Lotgering and Vromens [ 8 1, the M-type hexaferrite is not formed together with the spine1 oxides during the formation of SrFe2-W ferrite.
[3] F. Leccabue. Sanchez,
( 1988 ) 1850.
G. Salviati, G. Albanese and J.L.
( 1986 ) 2 114.
R. Panizzieri,
G. Albanese,
Mater. Sci. Monographs
[4] F. Leccabue.
0. Ares Muzio,
61 (1987)
2600.
[6] F. Leccabue.
R. Panizzieri,
G. Bocelli,
G. Calestam.
Rizzoli and N. Suarez, J. Magn. Magn. Mater.
C.
68 (1987)
365. [7] F. Leccabue.
R. Panizzieri,
G. Albanese.
Suarez, Mater. Res. Bull. 23 (1988)
G. Leo and N.
263.
[ 8 ] F.K. Lotgering and P.H.G.M. Vromans. J. 4m. Ceram. Sot. 60 (1977)
416. J. Appl. Phys. 45
[ IO] A.M. van Diepen and F.K. Lotgering,
The authors would like to thank the International Centre for Theoretical Physics (Trieste) for the interchange possibility among the researchers.
2019.
M. Safei Eldin. G. Calestani
and G. Albanese, J. Magn. Magn. Mater. 68 ( 1987) 201. [ 51 F. Leccabue, G. Albanese and 0. Ares Muzio. J. Appl. Phys.
[9] G. Asti and S. Rinaldi,
Acknowledgement
0. Ares and J.L.
38B (1987)
27 (1978)
( 1974) 3600.
Solid State Commun.
255.
[ 111H. KoJima, C. Miyakawa, T. Sato and K. Goto. Japan. J. Appl. Phys. 24 ( 1985) 5 I. [ 121 S. Dey and R. Valenzuela, J. Appl. Phys. 55 ( 1984) 2340. [ 13 1H. Lu. Y. Du, T. Wang, H. Hu and R. Xue. Advan. Ceram. 16 (1984)
561.
131