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Enhancement of the coercivity of γ-Fe2O3 by hydrothermal treatment
Volume
1 I, number
8.9
MATERIALS
July 1991
LETTERS
Enhancement of the coercivity of y-Fe20, by hydrothermal treatment S.F. Matar and G. Demazeau Laboratoire
Received
de Chitnie
du Solide du C.N.R.S..
35 I. cows de la Lib&ration.
33405 Talence Cedex, France
2 1 April I99 1
Treatment under hydrothermal conditions (P( H,O) = 500 bar, T= 773 K) of doped particulate iron oxide leads to an enhancement of the coercive field. Scanning electron microscopy imaging of the particles tends to show that this phenomenon arises from a control of the subdiviston of mother particles in such conditions.
1. Introduction y-Fez03 magnetic particles for recording are among the first known in magnetic recording history [ 11. Despite the ease of obtaining them as needle-like particles, the shape anisotropy thus induced can only lead to media with H, less than z 400 Oe. Coercivity is actually improved by use of doping with cobalt either near the surface [2] or in the bulk [3]. The additional magnetocrystalline anisotropy role played by cobalt ( CO”:~T,,) - its anisotropy constant K, =440 erg crne3 being the highest among the 3d elements - is at the origin of the enhancement of H,. All magnetic pigments devoted to high-density recording undergo several physico-chemical treatments leading to the final formulation for the application in recording [4]. The use of the properties of water (pure or with oxidizing or reducing agents) beyond its critical pressure and temperature conditions (2 18.3 bar and 647.31 K respectively) is called the hydrothermal method [ 51. In these conditions water is a hypercritical fluid. Hydrothermal treatment can be used for precipitation, decomposition or recrystallization of a compound solubilized (or not) in the hypercritical medium. In general the powders obtained from such reactions are monodispersed, that is. within narrow ranges of size and morphology leading to interesting applications in heterogeneous catalysis for instance [ 61. The role of controlling size and Elsevier Science Publishers
B.V. (North-Holland)
morphology of precursors by use of hydrothermal conditions led us to envisage its use for the treatment of iron oxide particles obtained from oxalate precursors. In this work we report on the possibility of improving the coercive field of iron oxide particles doped in the bulk by use of hydrothermal treatment. In order to achieve a better approach towards the role played by hydrothermal treatment with respect to that of the dopant itself, dopings with different 3d elements were undertaken: besides cobalt which has the highest coupling constant as addressed above, Mn and Ni were selected since their anisotropy constants (32.3 and -4.8 erg cmm3 respectively) are lower than that of Fe itself (45.6 erg cmp3) (values from ref. [7]).
2. Experimental Obtaining high coercivity iron oxide particles goes through three main steps: preparation of oxalate precursors followed by their calcination in air at 573 K and finally a hydrothermal treatment in water at 773 K and 500 bar. It is noteworthy that a direct hydrothermal treatment of oxalate particles - without their calcination - yields non-magnetic, non-acicular particles of a-Fez03. (a) Preparation of oxalate precursors. Pure or substituted iron oxalate, FeCz0,.2Hz0, is used as a 301
Voiume
11.number 8,9
MATERIALS LETTERS
precursor to iron oxide particles. Its preparation consists of a precipitation - or co-precipitation of substituted oxalates - from an aqueous solution of 10% ethylene glycol containing the divalent ion sulfate, with an excess of oxalic acid. This is followed by a separation, washings and drying of the yellow powders. Substitutions using this method can only be achieved for ions with close values of solubility products (i.e. =2x IO-’ [SJ). This applies for the divalent 3d transition ions selected here, leading to quantitative yields 19 1. (b) Calcination of the oxalates. Dry or slightly moist oxalate powders are introduced into 50 ml heatresistant beakers and placed on a hot-plate at 573 K. After stirring the powder during 4 min a dark dot appears at the yellowish surface and expands rapidly ( x2 min) until it covers all the surface then the whole volume of the sample. The beaker is then left to cool down to room temperature. The color of the obtained powders varies according to the amount of dopant: for non-doped oxalates, reddish powders are obtained, whereas doped ( G 10% of transition ion) oxalates yield brown to balk powders. A rough testing of the powders with a magnet shows they are magnetic. (c) Hydrothermal treatment. The calcined powders are introduced into silica tubes within the cylindrical cavity ( 5 30 cm3) of a high-pressure reaction vessel which is subsequently filled with distilled water and sealed. The vessel is then heated up to the reaction temperature. Pressure raising with temperature for such a closed volume, it is regulated by an interplay of a series of high-pressure valves. The reaction conditions are as follows: P= 500bar, T=773 K and t z 1 h at T.
3. Physical characterizations (a) X-ray diffraction. For the oxalate precursors, the X-ray diffraction patterns up to 10% doping with Mn, Co and Ni gave evidence of a unique phase of orthorhombic symmetry. No noticeable change in the lattice parameter of the o~horhombic unit cell could be detected 181. The actual presence of the dopant within the oxalate was detected by the X-ray tluorescence method. X-ray diffraction patterns of the powders resulting 302
July 199 1
from calcined oxalates were typical of y-Fe203 except for doping levels lower than 4% where a twophase mixture between y-FezOx and a-Fez,Os was detected. After hydrothermal treatment of the calcined powders, the y-FezOJ phase (brown) was shown to be stable whereas non-doped and slightly doped ( < 4%) ones totally transformed into a-Fez03 (red). (b) Scanning electron microscopy (SEM) characterization. Fig. la shows a micrograph of the particles of a sample of FeCz04.2Hz0 doped with 5 wt% Co. The elongated shape of the particles can be seen and the length to thickness ratio is z 5 : 1. Fig. 1b shows the SEM of the calcined particles. The shape of the mother particle is preserved and starting splitting of the particle along the length can be observed inducing cavities within the particles. The difference of density between the oxalate particle (dz 2.4) and the oxide particle (do 4.5) could be at the origin of the observed porosities. Fig. lc shows the SEM of the powder resulting from a hydrothermal treatment of the oxide shown in fig. 1b. The initial shape (figs. la and lb) is completely lost due to the splitting of the oxide particles into a beam of elongated very fine particles. The same trends were observed for all samples studied. (c) Magnetic characterizations. Magnetic characterizations were undertaken on samples resulting from calcined oxalates and samples obtained after hydrothermal treatment. The values of the coercive field and saturation magnetization can be extracted from the hysteresis loop. This is drawn for a ferro- or ferri-magnetic material initially saturated by a sufficiently high magnetic field. A fully automated hysteresimeter allows us to readily obtain such a loop at room temperature. Fig. 2 shows the hysteresis loops obtained for a 10% doped oxide after calcining the oxalate (fig. 2a) and after its subsequent hydrothermal treatment (fig. 2b). Observed values are within 5% error with respect to a scaled VSM magnetometer. Table 1 summarizes the observed values of coercive field and saturation magnetization of the different samples before and after the hydrothermal treatment. Except for the non-doped sample which yields antiferromagnetic a-Fe20, after hydrothermal treatment, an enhancement of the coercive field can be observed for all samples. However a lowering of
MATERIALS LETTERS
Fig. 1. (a) SEM of iron oxaiate doped with 5% Co. (b) SEM after calcination and ~~dr~t~e~a~ treatment.
ofsamesampk
saturation magnetization is noticed far all treated samples, Two hypotheses could assess this result: either a partial conversion into antiferromagnetic aFe,03 under pressure (the a-corundum form being denser than the y-spine1 one), or a partial reduction of Fe”” into Fe*+ due to the reducing properties of pure water under pressure leading to the formation of Fe&, as a side product. In order to check this latter hypothesis, one further experiment using an aqueous solution of 5% oxidizing HClU, as hypercritical medium was undertaken. The same results were obtained as for the magnetic properties. Hence
July 1991
as in faj after calcinarion. cc) SEM of sample as HI (a)
it could be suggested that water in hypercritical conditions does not play a chemical role but mainly 3 “mechanical” role of controlling the sub-division of the particles. In all cases no extra lines relative to aFe,O, or to Fe,O, in addition to those of y-Fe;O, were observable In the X-ray diffraction patterns of the resuiting powders.
4. Discussion From the SEM micrographs
shown in fig. 1 it ap303
July 1991
MATERIALS LETTERS
Volume 11, number 8,9
t +B
i A. U.)
Fig. 2. (a) Hysteresis loop ofcalcined oxalate (y-Fe,O,) doped with 10% Co: H,= 1080 Oe, us=67 emu g-‘; squareness ratio cQ(r,=0.66. (b) Hysteresis loop of same sample as in (a) after hydrothermal treatment at 773 K and 500 bar: H,= 1500 Oe, as=45 emu g-‘: squareness ratio ur,/o;2 0.75. Table 1 Observed values of the coercive field I& (in Oe) and the saturation magnetization o, (in emu g-’ ) for the various samples studied Dopant
10% Mn 5% co
t 0%co lO%Ni
Calcination plus hydrothermal treatment
Calcined oxalates
H,
0%
H,
0,
250 210 810 1080 223
69 65 65 67 60
490 1300 1500 710
43 31 45 43
pears that the transformation of the particle by the hydrothermal treatment is that of the formation of highly acicular particles with a new pa~icularity: the ratio of the length to the thickness is several orders of magnitude higher than z 5 : 1 initially estimated for the mother particles. This explains the high coercivities observed for the different samples studied. The enhancement in H, seems to arise from an improved shape anisotropy, since it is observed not only for Co-doped samples but for Mn- and Ni-doped ones as well. The role of the hydrothermal treatment seems to be that of further splitting of the initially calcined particles into a beam of several smaller particles (fig. 1c), a splitting which occurs along the particles, not across. As a matter of fact, attempting to reduce the obtained powders under hydrogen in order to obtain 304
iron particles led to a drop of coercivity of all samples to around 300 Oe with an increase of saturation magnetization to about 160 emu g-‘. Preliminary investigations of such particles by SEM show a complete loss of the acicular shape seen in fig. lc. 5. Conclusion In this study it was shown that the hydrothermal treatment of initially calcined oxalates can lead to strong enhancement of the coercive field of the particles. This step seems to be crucial in its control of the subsequent splitting of the mother particles into new highly acicular ones likely to be applied in highdensity magnetic recording. References [ I ] S. Luitjens, IEEE Trans. Magn. 26 ( 1990) 6. [2] Y. Imaoka, S. Umeki, Y. Kubota and Y. Tokuoka, IEEE Trans. Magn. 14 ( 1978) 649. [ 31 P. Mallard, P. Taillhades and A. Rousset, IEEE Trans. Magn. 26 (1990) 241. [ 41 G. Podolsky, IEEE Trans. Magn. 23 ( 1987) 39. [5] R.D. Smith, Supercritical Fluid Molecular Spray Film Deposition and Powder Formation, U.S. Patent 4,582,731. [6] Th. Dubois, Ph.D. Thesis, Universitt de Bordeaux 1 (1989). [ 7] A. Herpin, Thtorie du magnCtisme (PUF, Paris, 1968). [S] D. Andriamandroso, Thesis, Universite de Bordeaux 1, (1986). [9] B. Siberchicot, S.F. Matar, L. Fournes, G. Demazeau and P. Hagenmuller, J. Sofid State Chem. 84 ( 1990) 10.
1 I, number
8.9
MATERIALS
July 1991
LETTERS
Enhancement of the coercivity of y-Fe20, by hydrothermal treatment S.F. Matar and G. Demazeau Laboratoire
Received
de Chitnie
du Solide du C.N.R.S..
35 I. cows de la Lib&ration.
33405 Talence Cedex, France
2 1 April I99 1
Treatment under hydrothermal conditions (P( H,O) = 500 bar, T= 773 K) of doped particulate iron oxide leads to an enhancement of the coercive field. Scanning electron microscopy imaging of the particles tends to show that this phenomenon arises from a control of the subdiviston of mother particles in such conditions.
1. Introduction y-Fez03 magnetic particles for recording are among the first known in magnetic recording history [ 11. Despite the ease of obtaining them as needle-like particles, the shape anisotropy thus induced can only lead to media with H, less than z 400 Oe. Coercivity is actually improved by use of doping with cobalt either near the surface [2] or in the bulk [3]. The additional magnetocrystalline anisotropy role played by cobalt ( CO”:~T,,) - its anisotropy constant K, =440 erg crne3 being the highest among the 3d elements - is at the origin of the enhancement of H,. All magnetic pigments devoted to high-density recording undergo several physico-chemical treatments leading to the final formulation for the application in recording [4]. The use of the properties of water (pure or with oxidizing or reducing agents) beyond its critical pressure and temperature conditions (2 18.3 bar and 647.31 K respectively) is called the hydrothermal method [ 51. In these conditions water is a hypercritical fluid. Hydrothermal treatment can be used for precipitation, decomposition or recrystallization of a compound solubilized (or not) in the hypercritical medium. In general the powders obtained from such reactions are monodispersed, that is. within narrow ranges of size and morphology leading to interesting applications in heterogeneous catalysis for instance [ 61. The role of controlling size and Elsevier Science Publishers
B.V. (North-Holland)
morphology of precursors by use of hydrothermal conditions led us to envisage its use for the treatment of iron oxide particles obtained from oxalate precursors. In this work we report on the possibility of improving the coercive field of iron oxide particles doped in the bulk by use of hydrothermal treatment. In order to achieve a better approach towards the role played by hydrothermal treatment with respect to that of the dopant itself, dopings with different 3d elements were undertaken: besides cobalt which has the highest coupling constant as addressed above, Mn and Ni were selected since their anisotropy constants (32.3 and -4.8 erg cmm3 respectively) are lower than that of Fe itself (45.6 erg cmp3) (values from ref. [7]).
2. Experimental Obtaining high coercivity iron oxide particles goes through three main steps: preparation of oxalate precursors followed by their calcination in air at 573 K and finally a hydrothermal treatment in water at 773 K and 500 bar. It is noteworthy that a direct hydrothermal treatment of oxalate particles - without their calcination - yields non-magnetic, non-acicular particles of a-Fez03. (a) Preparation of oxalate precursors. Pure or substituted iron oxalate, FeCz0,.2Hz0, is used as a 301
Voiume
11.number 8,9
MATERIALS LETTERS
precursor to iron oxide particles. Its preparation consists of a precipitation - or co-precipitation of substituted oxalates - from an aqueous solution of 10% ethylene glycol containing the divalent ion sulfate, with an excess of oxalic acid. This is followed by a separation, washings and drying of the yellow powders. Substitutions using this method can only be achieved for ions with close values of solubility products (i.e. =2x IO-’ [SJ). This applies for the divalent 3d transition ions selected here, leading to quantitative yields 19 1. (b) Calcination of the oxalates. Dry or slightly moist oxalate powders are introduced into 50 ml heatresistant beakers and placed on a hot-plate at 573 K. After stirring the powder during 4 min a dark dot appears at the yellowish surface and expands rapidly ( x2 min) until it covers all the surface then the whole volume of the sample. The beaker is then left to cool down to room temperature. The color of the obtained powders varies according to the amount of dopant: for non-doped oxalates, reddish powders are obtained, whereas doped ( G 10% of transition ion) oxalates yield brown to balk powders. A rough testing of the powders with a magnet shows they are magnetic. (c) Hydrothermal treatment. The calcined powders are introduced into silica tubes within the cylindrical cavity ( 5 30 cm3) of a high-pressure reaction vessel which is subsequently filled with distilled water and sealed. The vessel is then heated up to the reaction temperature. Pressure raising with temperature for such a closed volume, it is regulated by an interplay of a series of high-pressure valves. The reaction conditions are as follows: P= 500bar, T=773 K and t z 1 h at T.
3. Physical characterizations (a) X-ray diffraction. For the oxalate precursors, the X-ray diffraction patterns up to 10% doping with Mn, Co and Ni gave evidence of a unique phase of orthorhombic symmetry. No noticeable change in the lattice parameter of the o~horhombic unit cell could be detected 181. The actual presence of the dopant within the oxalate was detected by the X-ray tluorescence method. X-ray diffraction patterns of the powders resulting 302
July 199 1
from calcined oxalates were typical of y-Fe203 except for doping levels lower than 4% where a twophase mixture between y-FezOx and a-Fez,Os was detected. After hydrothermal treatment of the calcined powders, the y-FezOJ phase (brown) was shown to be stable whereas non-doped and slightly doped ( < 4%) ones totally transformed into a-Fez03 (red). (b) Scanning electron microscopy (SEM) characterization. Fig. la shows a micrograph of the particles of a sample of FeCz04.2Hz0 doped with 5 wt% Co. The elongated shape of the particles can be seen and the length to thickness ratio is z 5 : 1. Fig. 1b shows the SEM of the calcined particles. The shape of the mother particle is preserved and starting splitting of the particle along the length can be observed inducing cavities within the particles. The difference of density between the oxalate particle (dz 2.4) and the oxide particle (do 4.5) could be at the origin of the observed porosities. Fig. lc shows the SEM of the powder resulting from a hydrothermal treatment of the oxide shown in fig. 1b. The initial shape (figs. la and lb) is completely lost due to the splitting of the oxide particles into a beam of elongated very fine particles. The same trends were observed for all samples studied. (c) Magnetic characterizations. Magnetic characterizations were undertaken on samples resulting from calcined oxalates and samples obtained after hydrothermal treatment. The values of the coercive field and saturation magnetization can be extracted from the hysteresis loop. This is drawn for a ferro- or ferri-magnetic material initially saturated by a sufficiently high magnetic field. A fully automated hysteresimeter allows us to readily obtain such a loop at room temperature. Fig. 2 shows the hysteresis loops obtained for a 10% doped oxide after calcining the oxalate (fig. 2a) and after its subsequent hydrothermal treatment (fig. 2b). Observed values are within 5% error with respect to a scaled VSM magnetometer. Table 1 summarizes the observed values of coercive field and saturation magnetization of the different samples before and after the hydrothermal treatment. Except for the non-doped sample which yields antiferromagnetic a-Fe20, after hydrothermal treatment, an enhancement of the coercive field can be observed for all samples. However a lowering of
MATERIALS LETTERS
Fig. 1. (a) SEM of iron oxaiate doped with 5% Co. (b) SEM after calcination and ~~dr~t~e~a~ treatment.
ofsamesampk
saturation magnetization is noticed far all treated samples, Two hypotheses could assess this result: either a partial conversion into antiferromagnetic aFe,03 under pressure (the a-corundum form being denser than the y-spine1 one), or a partial reduction of Fe”” into Fe*+ due to the reducing properties of pure water under pressure leading to the formation of Fe&, as a side product. In order to check this latter hypothesis, one further experiment using an aqueous solution of 5% oxidizing HClU, as hypercritical medium was undertaken. The same results were obtained as for the magnetic properties. Hence
July 1991
as in faj after calcinarion. cc) SEM of sample as HI (a)
it could be suggested that water in hypercritical conditions does not play a chemical role but mainly 3 “mechanical” role of controlling the sub-division of the particles. In all cases no extra lines relative to aFe,O, or to Fe,O, in addition to those of y-Fe;O, were observable In the X-ray diffraction patterns of the resuiting powders.
4. Discussion From the SEM micrographs
shown in fig. 1 it ap303
July 1991
MATERIALS LETTERS
Volume 11, number 8,9
t +B
i A. U.)
Fig. 2. (a) Hysteresis loop ofcalcined oxalate (y-Fe,O,) doped with 10% Co: H,= 1080 Oe, us=67 emu g-‘; squareness ratio cQ(r,=0.66. (b) Hysteresis loop of same sample as in (a) after hydrothermal treatment at 773 K and 500 bar: H,= 1500 Oe, as=45 emu g-‘: squareness ratio ur,/o;2 0.75. Table 1 Observed values of the coercive field I& (in Oe) and the saturation magnetization o, (in emu g-’ ) for the various samples studied Dopant
10% Mn 5% co
t 0%co lO%Ni
Calcination plus hydrothermal treatment
Calcined oxalates
H,
0%
H,
0,
250 210 810 1080 223
69 65 65 67 60
490 1300 1500 710
43 31 45 43
pears that the transformation of the particle by the hydrothermal treatment is that of the formation of highly acicular particles with a new pa~icularity: the ratio of the length to the thickness is several orders of magnitude higher than z 5 : 1 initially estimated for the mother particles. This explains the high coercivities observed for the different samples studied. The enhancement in H, seems to arise from an improved shape anisotropy, since it is observed not only for Co-doped samples but for Mn- and Ni-doped ones as well. The role of the hydrothermal treatment seems to be that of further splitting of the initially calcined particles into a beam of several smaller particles (fig. 1c), a splitting which occurs along the particles, not across. As a matter of fact, attempting to reduce the obtained powders under hydrogen in order to obtain 304
iron particles led to a drop of coercivity of all samples to around 300 Oe with an increase of saturation magnetization to about 160 emu g-‘. Preliminary investigations of such particles by SEM show a complete loss of the acicular shape seen in fig. lc. 5. Conclusion In this study it was shown that the hydrothermal treatment of initially calcined oxalates can lead to strong enhancement of the coercive field of the particles. This step seems to be crucial in its control of the subsequent splitting of the mother particles into new highly acicular ones likely to be applied in highdensity magnetic recording. References [ I ] S. Luitjens, IEEE Trans. Magn. 26 ( 1990) 6. [2] Y. Imaoka, S. Umeki, Y. Kubota and Y. Tokuoka, IEEE Trans. Magn. 14 ( 1978) 649. [ 31 P. Mallard, P. Taillhades and A. Rousset, IEEE Trans. Magn. 26 (1990) 241. [ 41 G. Podolsky, IEEE Trans. Magn. 23 ( 1987) 39. [5] R.D. Smith, Supercritical Fluid Molecular Spray Film Deposition and Powder Formation, U.S. Patent 4,582,731. [6] Th. Dubois, Ph.D. Thesis, Universitt de Bordeaux 1 (1989). [ 7] A. Herpin, Thtorie du magnCtisme (PUF, Paris, 1968). [S] D. Andriamandroso, Thesis, Universite de Bordeaux 1, (1986). [9] B. Siberchicot, S.F. Matar, L. Fournes, G. Demazeau and P. Hagenmuller, J. Sofid State Chem. 84 ( 1990) 10.