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Barium-doped iron oxide pigments for high-density magnetic recording. Thermal stability and magnetic properties
Journal of Magnetism and Magnetic Materials 109 (1992) 127-132 North-Holland
Barium-doped iron oxide pigments for high-density magnetic recording. Thermal stability and magnetic properties Ch. S a r d a
a,
Ch. B o n i n o ~, P. MoUard b a n d A. R o u s s e t ~
"Laboratoire de Chimie des Materiaux Inorganiques URA CNRS 1311, UnirersitFPaul Sabatier, 118 Rte de Narbonne, 31062 Toulouse Ceder, France b Laboratoire Louis Neel, CNRS 166X, 38042 Grenoble Ceder, France Received 18 July 1991; in revised form 17 September 1991
Magnetic particles of barium surface-doped ~t-Fe_,O~ and Co-~-Fe203 were prepared from oxalic precursors to obtain the required morphological and magnetic characteristics for high density magnetic recording. The role of the alkaline-earth ions was studied, especially as they affect morphological and magnetic characteristics, along with the thermal and chemical stabilities of the products. By several methods of analysis, it has been shown that Ba ions are located at the surface of the ~/-Fe20 3 (or Co-?-Fe20 3) particles. Thus the spinel lattice of the particle core, and the resultant magnetic properties, are not modified. Indeed, the evolution of coercive force and magnetization versus oxidation temperatures are comparable with those of free barium pigraents. Barium doping also makes it possible to conserve the spinel lattice above 750°(', and the presence of Fe 2+ ions (with high magnetocrystalline anisotropy) is obsevced up to 250°C.
1. Introduction
Previous work has shown that good recording performance can be obtained using magnetic pigments based on oxalic precursors which enables close control of the morphological characteristics [1]. In this way, 400 Oe pigments with good magnetization can be obtained with iron oxide particles. The coercivity can also be increased by adjusting the amount and the nature of the dopants (e.g. Co, Mn) [2-4]. The thermal stability of these pigments is limited and, at temperatures of about 500 or 600°C, the spinel-like structure is immediately transformed into a corundon-like structure and the magnetic performance in terms of coercivi~, or specific magnetization is lost. ~a the present paper, we show that it is possible to stabilize the spinel-like structure of ~/F e : O 3 up to 750°C, without any major modification of coercivity or magnetization, by doping
with barium ions. This retains large amounts of Fe 2÷ even at temperatures of about 250°C, which are used for textural improvements, tbr example.
2. Sample preparation
The samples are obtained from ferrous oxalic precursors. Precipitation takes place in an alcohol solution, to get the optimum size and shape [5]. The product corresponds to the general formula: ( Fe z*
)C204 • 2H20.
The introduction of barium salts at the same time leads to the formation of a second phase containing only Ba ions: ( Ba 2+ )C204 " I H 2 0 . The presence of this phase can be identified by X-ray diffraction, when barium salts are present in sufficiently large quantities (about 2 wt%).
0304-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
128
Ch. Sarda et aL / Barium-doped iron oxide pigments 8 D
0,50' E ::L ~n
0,40'
7
".:."
Q
6'
..J
5 4 .
°
°
•
°
3
°
°
2
~ °
I
°''° o
i
;
0
3
Fig. I. Average length of oxalate particles versus amount of Ba in weight.
Fig. 2. Average acicularity of oxalate particles versus amount of Ba in weight.
The magnetic oxide particles are generated in a sequence of thermal treatments: - Oxalate decomposition in air (about 400 to 700°C). -Reduction under Na(90%)/H2(10%) atmosphere (300-400°C). - Annealing under N, atmosphere at 400-500°C for one hour to improve particle crystallization. Average crystallites sizes of 45 nm are obtained in this way, resulting in good magnetic characteristics [5]. - Finally, partial or total oxidation treatment at 100 to 400°C results in products with the following composition:
which, after the thermal treatments described above, leads to a mixed oxide:
Ba ,, Fe~3_ 3y)O(,l _ 3y + 8( I -y)). If this is doped with cobalt ions, a mixed oxalate is formed with the ferrous ions: '~'J"
2+
(Fe~'2,Co,.)C204
• 2H20 ,
Bay Fe(3-x×l - y ) C O ( x - x y ) O ( 4 -
3y +tS(i - y ) ) ,
where the Co and Fe ions ferm a solid solution with spinel-like structure (second term of the formula).
3. Characterization
The morphological analyses were carried out using electron micrographs with counts of more than 100 particles, and were assembled into histograms. The presence of barium ions leads to a decrease in the length and the aeicularity of the oxalate particles (figs. 1 and 2) and, of course, of the resulting oxide grains. The reduction of these morphological characteristics can be explained by the presence of two
Table 1 %Ba
L [o.m]
o't [o.m]
D [:am]
o-o [txm]
L/D
°'l. /D
% sphere
Sw [ma/g]
0 0,3 0.6 0.74 1.1 1.54
0.24 0.276 0.173 0.204 0.239 0.174
0,09 0.116 0.071 0.097 0.122 0.074
0.05 0.052 0.047 0.405 0.0bl 0.067
0.016 0.012 0.014 0.023 0.019
5. I 5.7 4.0 4.8 4.1 2.6
2.5 2.2 2.6 2.0 1.0
2 4 !4 9 11 30
20.0 l&0 16.4 17.7 20.8 15.6
Ch. Sarda et al. / Barium-doped iron oxide pigments
oxalate phases, one containing iron and other transition metal ions, the other containing Ba ions, with different average lengths and acicularities for each one. However, if the percentage of Ba ions remains below 1.5 wt%, the products are compatible with magnetic recording requirements, with only a few particles possessing low acicularity (table 1 and fig. 2), displayed in the table by '% sphere' corresponding to particles with an acicularity ratio less than two. The morphological characterizations of the different samples are summarized in table 1. After thermal treatments, the X-ray diffraction analyses indicated the presence of iron oxide with the spinel-like structure. The chemical composition depends on the temperature of the oxidation treatment, and is somewhere between Fe304 and Fe203. Any Ba-containing phase is displayed by X-ray, but infra-red spectroscopy analysis reveals the presence of barium ferrate BaFe204. The crystallographic parameters determined from X-ray diffraction patterns show no variation of the value of 'a' with amounts of Ba, either for Fe304, or for ~,-Fe~O 3, both with a spinel-like structure (table 2). Indeed, the Ba ion size (0.134 rim) is larger than those of the octahedral or tetrahedral sites of this structure and the substitution of ferrous or ferric ions (0.078 and 0.064 nm respectively) by Ba ions would mean a very. heavy distortion of the spinel phase lattice, and modifications in the crystallographic parameter a. Further analysis by secondary ion mass spectroscopy determined the location of the barium ions. The concentration profiles of the Ba ÷ ions versus the depth of analysis from several samples show a heavy concentration of these ions in the first layers (5 nm) of the particle surface (fig. 3).
12q
_e
O I
Ba +
Depth
(nrn)
Fig. 3. SIMS concentration profile of Ba + ions versus the dcpth of analysis.
At the same time, the Fe ion concentration decreases, going from the bulk material to the surface (fig. 4). It is possible, then, to conclude that the Ba ions are present only near the particles surface. We obtain such particles with a spinel-like structure in the cote (and chemical composition belonging to the Fe304-Fe203 series), and a second phase containing Ba ions in the surperficial layers, probably with the non-magnetic BaFe204 phase ~omposition, as shown by the different analyses carried out [6].
3'
K
_E Table 2 %Ba
0 0.3 0.45 0.58 0.74 1.54
o
Lattice parameter a [nm] Fe304
3'-Fe203
0.8396 0.8306 0.8396 0.8395 0.8394 0.8396
0.8345 0.8346 0.8345 0.8345 0.8346 0.83t4
0 o
5
IO
15
20
2~
Depth
(ran)
30
Fig. 4. SIMS concentration profiles of Ba ÷, Fe~ and F e , O ' ions versus the depth of analysis.
Ch. Sarda et at. / Barium-doped iron oxide pigments
130
4. Thermal stability We studied the stability of the various phases by thermal analysis with a differential scanning calorimeter. The curves indicate a major increase in the temperature at which the spinel-like structure ('y-Fe203) changes to the corundon-like one (ot-Fe203) (fig. 5). Indeed, an increase of 250°C in the exothermic peak is observed with only 2.5 wt% of barium. This phenomenon can be explained by a gradual covering of the particle with a single layer of Ba ions, leading to a freezing of the ~ / ~ ct transformal;on, generated by the nucleation of aFe203 m the superficial layers. The presence of 2.5 wt% of the doping ion would fully cover particles with layers of composition BaFe204. When the amount of barium is greater than 2.5 wt%, no variation in the peak temperature is noted. In this case, the doping ions would be in a multilayer arrangement, with no influence on the innermost layers of the ferrite core. The surface layer of Ba ions also increases the oxidation temperature of Fe304 in ~,-Fe20 a. In this case the phenomenon is less sensitive, since a variation of about 80°C is only observed when the amount of Ba rises fi'om 0 to 2.5 wt% (fig. 6). For this reaction, the additive ions constitute a diffusion barrier for 0 2- (from the air) needed to oxidize the ferrous ions into ferric ions according to the reaction: 4
F e 3 0 4 + 0 2 --~ 6 F e 2 0 3.
(1)
250'
O
g I'200'
150
Fig, 6. FesO 4 oxidation temperature versus the percent weight of barium.
For magnetic recording applications, barium doping is of great interest, as it provides a way of high treatment temperatures are used for textural improvements. Since it is difficult to accurately control the amount of this ion by heat treatment, no close relation can be put forward with the percentage of Ba ions. However, the effect of Ba doping versus oxidation temperature can be observed for samples containing about 1 wt% of the additive ion (fig. 7).
100 ¸ 4. OI Q
u. 8 0 O %
8OO
m Without Ba
60
O
o With Ba
xO\X\
700 40
o
Xxo
I60O
20
0
5OO"
75
125
175
225
0-1 27'5 T OX ( ° C )
400
i
O
z
:~
% Ba
a
Fig. 5. ~, ~ (-~ Fc203 transformation temperature versus the percent weight of barium.
Fig. 7. Fe 2 ' content versus the temperature of oxidation treatment, tbr samples with and without barium, The Fe 2÷ content on the y-axis represents the amount of ferrous ions with respect to FesO 4, i.e. 100% corresponds to Fe304, 0% to ~/-Fe203.
Ch. Sarda et al. / Barium-doped iron oxide pigments
131
5. Magnetic properties The principal interest for Ba doping at the particle surface is to prevent the spinel structure of the iron oxide from altering, and thereby, the magnetic properties.
5.1. Coercive force The variation in the coercivity H c versus the oxidation temperature remains the same as in the case of pure iron oxide pigments (fig. 8). For low temperature oxidation treatments (less than 250"C) the coercive force decreases in close relation to the amount of ferrous ions, possessing higher magneto-crystalline anisotropy than ferric ions. Above this temperature, the H c curve becomes constant, denoting the presence of ferric ions alone in the spinel-like structure of the particle core. The coercive forces observed (about 300-350 Oe) are in the same range as that of the commercial products, but in this case the H¢ values are available up to 450°C.
5.2. Remanent and saturation magnetization For specific magnetizations (remanent and saturation magnetizations), we observe the same type of variation versus temperature treatmcnt with two separate domains (fig. 9). Up to oxidation
= 0,3 %Ba • 0,4 & 0,6
=o
.\
~ = ~
80 ,.5
E
ol A
0,4s % Ba1 i: 0,3 0.6 .. 0.8 1,1
75
35 A
I
~P
I00
2~10
sop
u
400 Tox (°C}
Fig. 9. Specific magnetizations or and o~ versus oxidation temperature.
temperature of 250°C, magnetization corresponds to the presence of ferrous ions whose concertration decreases as oxidation temperature increases. Above a oxidation temperature of 250°C, the particle core is only composed of ferric ions (and oxygen) and no variation can be performed in this domain. The presence of the barium ions causes at least a decrease of the specific magnetization values attributed to the increase in the amount of Ba ions which, along with part of the Fe ions, form a non-magnetic phase.
700
wt%0A Co:2,6 Co:2,7 Ba:l,9 Ba:0.6
650" ° ' " 0
= Co:2,2 Ba:0,6
,0,8
o
i=
30
7/I /
1.1
o 600 -r
o "I"
-,..-..~.,.~ . ~ g
100
200
300
450
400
503
Tox
(°C)
Fig. 8. Coercive force versus oxidation temperature.
xoo
=oo
soo
400 Tox (oC)
Fig. 10. Coercive force versus oxidation temperature for Codoped "y-Fe203.
132
Ch. Sarda et al. / Barium-doped iron oxide pigments
5. 3. Optimization of the magnetic properties The magnetic characteristics (notably coercive force) are strongly affected by: - % of Co 2+,
- % o f F e 2+ and - thermal treatments. Fig. 10 shows the influence of these parameters on the coercivity. It may still be improved notably by thermal treatments, permitting the establishment of directional order [8]. In this way, we have obtained a value of coercive force of 900 Oe by an appropriate thermal treatment for pigments with 3 wt% of Co and 1 5 wt% of Ba [6].
6. Conclusion
The analyses show that the Ba 2+ ions are located in the superficial layers of the particles, forming a BaFe204 phase. The presence of this phase does not affect the magnetic prc,perties but stabilizes the spinel-like structure: the temperature of o:ddation of the ferrous ions increases up
to 250°C, and the temperature of the ~,-0t transformation is raised up to 750°C. The presence of Ba ions permits us to control more easily the size and the shape of the aeieular particles, and to improve the regularity of the morphology. The magnetic characteristics of these compounds are compatible with the constraints imposed by high-density magnetic recording. References [1] Ph. Tailhades, M. Brieu, P. Mollard, A Rousset and Y. Chassaigne, IEEE Trans. Magn. MAG-26 (1990) 63. [2] A. Rousset, Ch. Bonino, M. Gougeon ~nd P, Mollard, J. Chem. Res. (S) (1987) 140. [3] Ph. Tailhades, Ch. Sarda, Ch. Bonino, P, Mollard and A. Rousset, L Magn. Magn, Mater. 89 (19!)0) 33. [4] A. Rousset, P. Mollard, Ch. Bonino, M. Gougeon and Ph. Tailhades, Europatent 86905 854-5. [5] A.E. Berkowitz and W.J. Schuele, J. Appl. Phys. 39 (1968) 126. [6] Ch. Sarda, Thesis, Toulouse, Frm~ce (1990). [7l A. Rousset, Ch. Bonino, M. Goug¢on and P. Mollard, IEEE Trans. Magn. MAG-23 (1987) 77. [8] Ph. Tailhades, P. Mollard, A. Rousset and M. Gougeon, IEEE Trans. Magn. MAG-26 (1990) !822.
Barium-doped iron oxide pigments for high-density magnetic recording. Thermal stability and magnetic properties Ch. S a r d a
a,
Ch. B o n i n o ~, P. MoUard b a n d A. R o u s s e t ~
"Laboratoire de Chimie des Materiaux Inorganiques URA CNRS 1311, UnirersitFPaul Sabatier, 118 Rte de Narbonne, 31062 Toulouse Ceder, France b Laboratoire Louis Neel, CNRS 166X, 38042 Grenoble Ceder, France Received 18 July 1991; in revised form 17 September 1991
Magnetic particles of barium surface-doped ~t-Fe_,O~ and Co-~-Fe203 were prepared from oxalic precursors to obtain the required morphological and magnetic characteristics for high density magnetic recording. The role of the alkaline-earth ions was studied, especially as they affect morphological and magnetic characteristics, along with the thermal and chemical stabilities of the products. By several methods of analysis, it has been shown that Ba ions are located at the surface of the ~/-Fe20 3 (or Co-?-Fe20 3) particles. Thus the spinel lattice of the particle core, and the resultant magnetic properties, are not modified. Indeed, the evolution of coercive force and magnetization versus oxidation temperatures are comparable with those of free barium pigraents. Barium doping also makes it possible to conserve the spinel lattice above 750°(', and the presence of Fe 2+ ions (with high magnetocrystalline anisotropy) is obsevced up to 250°C.
1. Introduction
Previous work has shown that good recording performance can be obtained using magnetic pigments based on oxalic precursors which enables close control of the morphological characteristics [1]. In this way, 400 Oe pigments with good magnetization can be obtained with iron oxide particles. The coercivity can also be increased by adjusting the amount and the nature of the dopants (e.g. Co, Mn) [2-4]. The thermal stability of these pigments is limited and, at temperatures of about 500 or 600°C, the spinel-like structure is immediately transformed into a corundon-like structure and the magnetic performance in terms of coercivi~, or specific magnetization is lost. ~a the present paper, we show that it is possible to stabilize the spinel-like structure of ~/F e : O 3 up to 750°C, without any major modification of coercivity or magnetization, by doping
with barium ions. This retains large amounts of Fe 2÷ even at temperatures of about 250°C, which are used for textural improvements, tbr example.
2. Sample preparation
The samples are obtained from ferrous oxalic precursors. Precipitation takes place in an alcohol solution, to get the optimum size and shape [5]. The product corresponds to the general formula: ( Fe z*
)C204 • 2H20.
The introduction of barium salts at the same time leads to the formation of a second phase containing only Ba ions: ( Ba 2+ )C204 " I H 2 0 . The presence of this phase can be identified by X-ray diffraction, when barium salts are present in sufficiently large quantities (about 2 wt%).
0304-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
128
Ch. Sarda et aL / Barium-doped iron oxide pigments 8 D
0,50' E ::L ~n
0,40'
7
".:."
Q
6'
..J
5 4 .
°
°
•
°
3
°
°
2
~ °
I
°''° o
i
;
0
3
Fig. I. Average length of oxalate particles versus amount of Ba in weight.
Fig. 2. Average acicularity of oxalate particles versus amount of Ba in weight.
The magnetic oxide particles are generated in a sequence of thermal treatments: - Oxalate decomposition in air (about 400 to 700°C). -Reduction under Na(90%)/H2(10%) atmosphere (300-400°C). - Annealing under N, atmosphere at 400-500°C for one hour to improve particle crystallization. Average crystallites sizes of 45 nm are obtained in this way, resulting in good magnetic characteristics [5]. - Finally, partial or total oxidation treatment at 100 to 400°C results in products with the following composition:
which, after the thermal treatments described above, leads to a mixed oxide:
Ba ,, Fe~3_ 3y)O(,l _ 3y + 8( I -y)). If this is doped with cobalt ions, a mixed oxalate is formed with the ferrous ions: '~'J"
2+
(Fe~'2,Co,.)C204
• 2H20 ,
Bay Fe(3-x×l - y ) C O ( x - x y ) O ( 4 -
3y +tS(i - y ) ) ,
where the Co and Fe ions ferm a solid solution with spinel-like structure (second term of the formula).
3. Characterization
The morphological analyses were carried out using electron micrographs with counts of more than 100 particles, and were assembled into histograms. The presence of barium ions leads to a decrease in the length and the aeicularity of the oxalate particles (figs. 1 and 2) and, of course, of the resulting oxide grains. The reduction of these morphological characteristics can be explained by the presence of two
Table 1 %Ba
L [o.m]
o't [o.m]
D [:am]
o-o [txm]
L/D
°'l. /D
% sphere
Sw [ma/g]
0 0,3 0.6 0.74 1.1 1.54
0.24 0.276 0.173 0.204 0.239 0.174
0,09 0.116 0.071 0.097 0.122 0.074
0.05 0.052 0.047 0.405 0.0bl 0.067
0.016 0.012 0.014 0.023 0.019
5. I 5.7 4.0 4.8 4.1 2.6
2.5 2.2 2.6 2.0 1.0
2 4 !4 9 11 30
20.0 l&0 16.4 17.7 20.8 15.6
Ch. Sarda et al. / Barium-doped iron oxide pigments
oxalate phases, one containing iron and other transition metal ions, the other containing Ba ions, with different average lengths and acicularities for each one. However, if the percentage of Ba ions remains below 1.5 wt%, the products are compatible with magnetic recording requirements, with only a few particles possessing low acicularity (table 1 and fig. 2), displayed in the table by '% sphere' corresponding to particles with an acicularity ratio less than two. The morphological characterizations of the different samples are summarized in table 1. After thermal treatments, the X-ray diffraction analyses indicated the presence of iron oxide with the spinel-like structure. The chemical composition depends on the temperature of the oxidation treatment, and is somewhere between Fe304 and Fe203. Any Ba-containing phase is displayed by X-ray, but infra-red spectroscopy analysis reveals the presence of barium ferrate BaFe204. The crystallographic parameters determined from X-ray diffraction patterns show no variation of the value of 'a' with amounts of Ba, either for Fe304, or for ~,-Fe~O 3, both with a spinel-like structure (table 2). Indeed, the Ba ion size (0.134 rim) is larger than those of the octahedral or tetrahedral sites of this structure and the substitution of ferrous or ferric ions (0.078 and 0.064 nm respectively) by Ba ions would mean a very. heavy distortion of the spinel phase lattice, and modifications in the crystallographic parameter a. Further analysis by secondary ion mass spectroscopy determined the location of the barium ions. The concentration profiles of the Ba ÷ ions versus the depth of analysis from several samples show a heavy concentration of these ions in the first layers (5 nm) of the particle surface (fig. 3).
12q
_e
O I
Ba +
Depth
(nrn)
Fig. 3. SIMS concentration profile of Ba + ions versus the dcpth of analysis.
At the same time, the Fe ion concentration decreases, going from the bulk material to the surface (fig. 4). It is possible, then, to conclude that the Ba ions are present only near the particles surface. We obtain such particles with a spinel-like structure in the cote (and chemical composition belonging to the Fe304-Fe203 series), and a second phase containing Ba ions in the surperficial layers, probably with the non-magnetic BaFe204 phase ~omposition, as shown by the different analyses carried out [6].
3'
K
_E Table 2 %Ba
0 0.3 0.45 0.58 0.74 1.54
o
Lattice parameter a [nm] Fe304
3'-Fe203
0.8396 0.8306 0.8396 0.8395 0.8394 0.8396
0.8345 0.8346 0.8345 0.8345 0.8346 0.83t4
0 o
5
IO
15
20
2~
Depth
(ran)
30
Fig. 4. SIMS concentration profiles of Ba ÷, Fe~ and F e , O ' ions versus the depth of analysis.
Ch. Sarda et at. / Barium-doped iron oxide pigments
130
4. Thermal stability We studied the stability of the various phases by thermal analysis with a differential scanning calorimeter. The curves indicate a major increase in the temperature at which the spinel-like structure ('y-Fe203) changes to the corundon-like one (ot-Fe203) (fig. 5). Indeed, an increase of 250°C in the exothermic peak is observed with only 2.5 wt% of barium. This phenomenon can be explained by a gradual covering of the particle with a single layer of Ba ions, leading to a freezing of the ~ / ~ ct transformal;on, generated by the nucleation of aFe203 m the superficial layers. The presence of 2.5 wt% of the doping ion would fully cover particles with layers of composition BaFe204. When the amount of barium is greater than 2.5 wt%, no variation in the peak temperature is noted. In this case, the doping ions would be in a multilayer arrangement, with no influence on the innermost layers of the ferrite core. The surface layer of Ba ions also increases the oxidation temperature of Fe304 in ~,-Fe20 a. In this case the phenomenon is less sensitive, since a variation of about 80°C is only observed when the amount of Ba rises fi'om 0 to 2.5 wt% (fig. 6). For this reaction, the additive ions constitute a diffusion barrier for 0 2- (from the air) needed to oxidize the ferrous ions into ferric ions according to the reaction: 4
F e 3 0 4 + 0 2 --~ 6 F e 2 0 3.
(1)
250'
O
g I'200'
150
Fig, 6. FesO 4 oxidation temperature versus the percent weight of barium.
For magnetic recording applications, barium doping is of great interest, as it provides a way of high treatment temperatures are used for textural improvements. Since it is difficult to accurately control the amount of this ion by heat treatment, no close relation can be put forward with the percentage of Ba ions. However, the effect of Ba doping versus oxidation temperature can be observed for samples containing about 1 wt% of the additive ion (fig. 7).
100 ¸ 4. OI Q
u. 8 0 O %
8OO
m Without Ba
60
O
o With Ba
xO\X\
700 40
o
Xxo
I60O
20
0
5OO"
75
125
175
225
0-1 27'5 T OX ( ° C )
400
i
O
z
:~
% Ba
a
Fig. 5. ~, ~ (-~ Fc203 transformation temperature versus the percent weight of barium.
Fig. 7. Fe 2 ' content versus the temperature of oxidation treatment, tbr samples with and without barium, The Fe 2÷ content on the y-axis represents the amount of ferrous ions with respect to FesO 4, i.e. 100% corresponds to Fe304, 0% to ~/-Fe203.
Ch. Sarda et al. / Barium-doped iron oxide pigments
131
5. Magnetic properties The principal interest for Ba doping at the particle surface is to prevent the spinel structure of the iron oxide from altering, and thereby, the magnetic properties.
5.1. Coercive force The variation in the coercivity H c versus the oxidation temperature remains the same as in the case of pure iron oxide pigments (fig. 8). For low temperature oxidation treatments (less than 250"C) the coercive force decreases in close relation to the amount of ferrous ions, possessing higher magneto-crystalline anisotropy than ferric ions. Above this temperature, the H c curve becomes constant, denoting the presence of ferric ions alone in the spinel-like structure of the particle core. The coercive forces observed (about 300-350 Oe) are in the same range as that of the commercial products, but in this case the H¢ values are available up to 450°C.
5.2. Remanent and saturation magnetization For specific magnetizations (remanent and saturation magnetizations), we observe the same type of variation versus temperature treatmcnt with two separate domains (fig. 9). Up to oxidation
= 0,3 %Ba • 0,4 & 0,6
=o
.\
~ = ~
80 ,.5
E
ol A
0,4s % Ba1 i: 0,3 0.6 .. 0.8 1,1
75
35 A
I
~P
I00
2~10
sop
u
400 Tox (°C}
Fig. 9. Specific magnetizations or and o~ versus oxidation temperature.
temperature of 250°C, magnetization corresponds to the presence of ferrous ions whose concertration decreases as oxidation temperature increases. Above a oxidation temperature of 250°C, the particle core is only composed of ferric ions (and oxygen) and no variation can be performed in this domain. The presence of the barium ions causes at least a decrease of the specific magnetization values attributed to the increase in the amount of Ba ions which, along with part of the Fe ions, form a non-magnetic phase.
700
wt%0A Co:2,6 Co:2,7 Ba:l,9 Ba:0.6
650" ° ' " 0
= Co:2,2 Ba:0,6
,0,8
o
i=
30
7/I /
1.1
o 600 -r
o "I"
-,..-..~.,.~ . ~ g
100
200
300
450
400
503
Tox
(°C)
Fig. 8. Coercive force versus oxidation temperature.
xoo
=oo
soo
400 Tox (oC)
Fig. 10. Coercive force versus oxidation temperature for Codoped "y-Fe203.
132
Ch. Sarda et al. / Barium-doped iron oxide pigments
5. 3. Optimization of the magnetic properties The magnetic characteristics (notably coercive force) are strongly affected by: - % of Co 2+,
- % o f F e 2+ and - thermal treatments. Fig. 10 shows the influence of these parameters on the coercivity. It may still be improved notably by thermal treatments, permitting the establishment of directional order [8]. In this way, we have obtained a value of coercive force of 900 Oe by an appropriate thermal treatment for pigments with 3 wt% of Co and 1 5 wt% of Ba [6].
6. Conclusion
The analyses show that the Ba 2+ ions are located in the superficial layers of the particles, forming a BaFe204 phase. The presence of this phase does not affect the magnetic prc,perties but stabilizes the spinel-like structure: the temperature of o:ddation of the ferrous ions increases up
to 250°C, and the temperature of the ~,-0t transformation is raised up to 750°C. The presence of Ba ions permits us to control more easily the size and the shape of the aeieular particles, and to improve the regularity of the morphology. The magnetic characteristics of these compounds are compatible with the constraints imposed by high-density magnetic recording. References [1] Ph. Tailhades, M. Brieu, P. Mollard, A Rousset and Y. Chassaigne, IEEE Trans. Magn. MAG-26 (1990) 63. [2] A. Rousset, Ch. Bonino, M. Gougeon ~nd P, Mollard, J. Chem. Res. (S) (1987) 140. [3] Ph. Tailhades, Ch. Sarda, Ch. Bonino, P, Mollard and A. Rousset, L Magn. Magn, Mater. 89 (19!)0) 33. [4] A. Rousset, P. Mollard, Ch. Bonino, M. Gougeon and Ph. Tailhades, Europatent 86905 854-5. [5] A.E. Berkowitz and W.J. Schuele, J. Appl. Phys. 39 (1968) 126. [6] Ch. Sarda, Thesis, Toulouse, Frm~ce (1990). [7l A. Rousset, Ch. Bonino, M. Goug¢on and P. Mollard, IEEE Trans. Magn. MAG-23 (1987) 77. [8] Ph. Tailhades, P. Mollard, A. Rousset and M. Gougeon, IEEE Trans. Magn. MAG-26 (1990) !822.