Patent Application
20080057352
a) is a TEM image of powder particles obtained in Example 1.
b) is a TEM image of powder particles obtained in Example 2.
c) is a TEM image of powder particles obtained in Example 3.
d) is a TEM image of powder particles obtained in Example 4.
e) is a TEM image of powder particles obtained in Example 5.
As described in the References 1 to 3, ε-Fe2O3 nanoparticles can be obtained by a process step that combines the reverse micelle method and the sol-gel method, and a heat treatment (baking) process step. In the reverse micelle method, two surfactant-containing micelle solutions, comprising micelle solution I (starting material micelle) and micelle solution II (neutralizer micelle), are mixed together, thereby promoting a reaction that precipitates iron hydroxide in the micelle. The sol-gel method provides the iron hydroxide particles generated in the micelle with a silica coating. The coated particles are then separated from the solution and subjected to heat treatment in air at a predetermined temperature in the range 700˜1300° C. to thereby obtain ε-Fe2O3 crystal particles.
More specifically, this is carried out as follows, for example.
Iron (III) nitrate as iron source, aluminum (III) nitrate 9-hydrate as a source of Al for replacing a portion of the iron, and a surfactant (cetyl trimethyl ammonium bromide, for example) are dissolved in the aqueous phase of a micelle solution I whose oil-phase is n-octane, and an aqueous solution of ammonia is used as the aqueous phase of a micelle solution II whose oil-phase is also n-octane. An appropriate amount of a nitrate of an alkaline earth metal (such as Ba, Sr, Ca, etc.) can be dissolved in the aqueous phase of the micelle solution I as a shape control agent. The presence of this shape control agent makes it possible to ultimately obtain rod-shaped crystals of Al-containing ε-Fe2O3. When the shape control agent is not present, granular Al-containing ε-Fe2O3 crystal is obtained.
After the micelle solutions I and II have been combined, the sol-gel method is used in parallel. Specifically, a silane (tetraethylorthosilane, for example) is dripped into the combined solution while the solution is stirred to promote the reaction that generates the iron hydroxide in the micelle. As a result, the surface of the iron hydroxide microparticles precipitated in the micelle is coated with silica produced by the hydrolysis of the silane. It is thought that the iron hydroxide has a portion of its Fe replaced with Al, or otherwise that the iron hydroxide and aluminum hydroxide are in a mixed state or that they are in a state of the iron hydroxide being coated with the aluminum hydroxide. Next, silica-coated particles of the iron hydroxide powder entraining Al are separated from the solution and, after being washed with water and dried, are placed in a furnace and heat-treated (baked) in air at 700 to 1300° C. (973 to 1573K), more preferably 900 to 1200° C. (1173 to 1473K), and even more preferably at 950 to 1150° C. (1223 to 1423K). This heat treatment promotes the oxidation reaction in the iron hydroxide particles inside the silica coating, thereby converting the microparticles of iron hydroxide particles entraining Al into microparticles of Al-containing ε-Fe2O3. At the time of the oxidation reaction, the presence of the silica coating not only contributes to the formation of crystals having the same space group as ε-Fe2O3 rather than α-Fe2O3 or γ-Fe2O3 but also works to prevent the particles from sintering together. Moreover, the copresence of an appropriate amount of an alkaline earth metal facilitates the growth of rod-shaped Al-containing ε-Fe2O3 particles.
Among polymorphs composed of Fe2O3 but differing in crystal structure, α-Fe2O3 and γ-Fe2O3 are the most common. Other iron oxides include FeO and Fe3O4. When the aforesaid Al-containing ε-Fe2O3 is synthesized, iron oxide crystals (impurity crystals) whose space groups differ from that of the ε-Fe2O3 may intermingle therewith. Although the inclusion of such impurity crystals is undesirable from the viewpoint of fully bringing out the properties of the Al-containing ε-Fe2O3 crystal, their presence is acceptable within levels that do not impair the effect of the invention. When the Al-containing ε-Fe2O3 crystal accounts for 75 mole % or more of the iron oxide phase, for example, the magnetic material exhibits excellent magnetic properties difficult to achieve with conventional magnetic materials and can be utilized in a broad range of magnetic applications. Even when the Al-containing ε-Fe2O3 crystal accounts for between 50 mole % and less than 75 mole % of the iron oxide phase, the magnetic material can still serve a purpose so long as it has a saturation magnetization θs of 2 emu/g (2 A·m2/kg) or greater, because it enables reading of written signals at high intensity by use of the high-sensitivity magnetic read head known as the GMR (giant magnetoresistive) head or the still more sensitive TMR head that utilizes the tunnel effect.
It was found that the coercive force Hc of the invention magnetic material comprising an iron oxide phase having Al-containing ε-Fe2O3 as its principal phase can be controlled by varying the Al content of the iron oxide phase (see Table 1 below). When x in Al:Fe=x:(2−x) used to express the molar ratio of Al to Fe in the iron oxide phase is 0, Hc may become as high as 20 kOe. No currently available magnetic head would be capable of writing recording magnetization to a magnetic recording medium using such a magnetic material. On the other hand, when x exceeds 1, Hc may become about 0 Oe, which is unsuitable for magnetic recording because recorded data would not be stably retained at room temperature.
The saturation magnetization as was observed to exhibit a behavior of assuming maximum value in the composition range in which the value of x in the molar ratio of Al to Fe in the iron oxide phase having Al-containing ε-Fe2O3 as principal phase was near 0.37 (see Table 1 below). In applications that require high saturation magnetization, therefore, it is advantageous to adopt a composition whose x is in the range of 0.25 to 0.45, although the tradeoff with coercive force must also be taken into account.
The lattice constants of the Al-containing ε-Fe2O3 crystal were observed to exhibit a behavior of growing smaller with increasing amount of Al substitution (see
These behaviors were also observed for the Ga-containing ε-Fe2O3 crystal taught in Japanese Patent Application 2006-96907. However, when Ga is used as the substitutional element, more than about 40 at. % of the Fe atoms have to be replaced with Ga in order to reduce coercive force Hc to a readily usable level of, say, 10 kOe (7.96×105 A/m) or less. This leads to a marked increase in cost. On this and other points, Al is, from the industrial viewpoint, a very suitable element for use as a substitutional element because of its low price and plentiful reserves, and the fact that it is a chemically stable metal low in toxicity.
The notation ε-AlyFe2-yO3 can be used to reflect the Al substitution ratio of the Al-containing ε-Fe2O3 crystal. Here, it should be noted that the correlation between x of Al:Fe=x:(2−x) used to express the molar ratio of Al to Fe in the iron oxide phase and y in the ε-AlyFe2-yO3 crystal in the iron oxide phase may be somewhat shifted when impurity crystal is present in the iron oxide phase. However, in the case of x=0.45 and x=0.53 in
The magnetic material of the present invention is typically in the form of the magnetic powder obtained by the process discussed above. The diameter of the particles constituting the magnetic powder can be controlled by, for example, regulating the temperature of the heat treatment (baking) in the foregoing process. The average diameter of the magnetic powder particles measured from a TEM image of the particles is preferably 5 to 200 nm, more preferably 5 to 100 nm and still more preferably 10-100 nm. The average particle diameter of most magnetic recording tapes for data backup on the market today is 200 nm or less, which is not considered sufficiently fine. The magnetic powder of this invention can meet the need felt for magnetic recording tapes having still finer magnetic particles. A magnetic layer for magnetic recording constituted using the invention magnetic powder can achieve a high magnetic recording density because the magnetic powder is so fine that the individual particles can assume single domain structures.
However, it should be noted that a magnetic powder containing many particles under 5 nm in diameter experiences magnetic property degradation owing to the superparamagnetic effect of the particles. Particles under 5 nm in diameter, preferably under 10 nm in diameter, are therefore best removed as much as possible.
Determination of average particle diameter can be done by measuring the largest diameter (the long-axis diameter, in the case of rod-shaped particles) of individual particles in a 600,000×TEM image. The average of the particle diameters found for 300 isolated particles is defined as the average particle diameter of the powder concerned. This value will sometimes be called the “TEM average particle diameter” in the following. The magnetic powder of this invention is preferably one whose TEM average particle diameter is 100 nm or less and whose particles each has a single domain structure.
Although the iron oxide phase of the magnetic material ideally consists of a single-phase composition represented by the general formula ε-AlxFe2-xO3, 0≦x≦1, impurity crystals of crystal structures differing from this (α-Fe2O3 etc.) may, as mentioned earlier, get mixed into the iron oxide phase. The inclusion of such impurity crystals is acceptable within levels that do not impair the effect of the invention. The iron oxide phase may also contain other impurities whose inclusion cannot be avoided during production as well as elements added out of necessity. In addition, nonmagnetic compounds and the like other than the iron oxide phase may also adhere. The inclusion of these elements and compounds is acceptable within levels that do not impair the effect of the invention.
For example, as explained earlier, when Al-containing ε-Fe2O3 is synthesized by a combination of the reverse micelle method and the sol-gel method, the copresence of an appropriate amount of an alkaline earth metal in the micelle makes it easier to ultimately obtain rod-shaped crystals. The alkaline earth metal (such as Ba, Sr, Ca, etc.) added as a shape control agent may remain in the surface region of produced crystal, so that the magnetic material according to the invention may contain such a shape control agent. Thus the invention magnetic material may include at least one alkaline earth metal element. Alkaline earth metal element will sometime be denoted by M in the following. The content thereof expressed as M/(Fe+Al)×100 is at most within the range of not greater than 20 mass %. An alkaline earth metal generally need not be present at a content of greater than 20 mass % to fulfill the function of a shape control agent. A content of 10 mass % or less is preferable.
The silica coating formed on the surface of the iron hydroxide microparticles by the sol-gel method may also remain on the surface of the powder particles after heat treatment (baking). For reasons such as set out below, the presence of a nonmagnetic compound like silica on the surface of the powder particles may be advantageous in the point of improving durability, weatherability, reliability and the like during handling of the magnetic powder or when it is used as a magnetic material in different applications.
Although the fact that Al-containing ε-Fe2O3 is an oxide means that the powder particles of this invention have higher oxidation resistance than metal magnetic particles, Fe itself is an element that reacts readily and, therefore, it is not easy to completely prevent it from complexing or reacting with acid. For example, when a magnetic tape is used for a long time or under high temperature and humidity conditions, the magnetic particles may react with resin and/or dispersant in the tape to form metal complexes. If the metal complexes produced adhere to the surface of the magnetic head, the spacing between the tape and head widens to decrease the recording signal strength. In the worst case, reading of recorded data may become impossible. There may also be cases in which H2S, Cl2, NO2 and other such gas components in the air together with moisture produce corrosive acidic gases, that corrode the magnetic particles. The presence of a chemically stable nonmagnetic compound like silica on the iron oxide phase having Al-containing ε-Fe2O3 as its principal phase produces strong resistance to complexing and acid, thereby enabling the magnetic material to exhibit excellent durability, weatherability and reliability. Nonmagnetic compounds offering these capabilities are not limited to silica but also include, for example, heat-resistant compounds such as alumina and zirconia.
However, an excessive amount of adhered nonmagnetic compound is undesirable because it tends to exacerbate adverse effects, e.g., to cause severe aggregation among the particles. Studies done taking Si02 as an example showed that the amount of nonmagnetic compound present, expressed as Si/(Fe+Al)×100, is preferably 100 mass % or less.
In some applications, the magnetic powder of this invention can be used in a mixture with a powder whose iron oxide phase is ε-Fe2O3 (powder not added with the substitutional element).
When the invention magnetic powder comprising particles having Al-containing ε-Fe2O3 crystal as their principal phase is to be used to produce the magnetic layer of a coated-type magnetic recording medium, it suffices to affix powder particles having an average diameter of from 5 to 200 nm, as determined by measuring the diameters of individual particles from a TEM image, to a substrate, with the easy magnetization axes of the particle crystals at each site oriented in a prescribed direction.
The magnetic powder of this invention can also be used to produce a magnetic layer suitable for thermally-assisted magnetic recording or next-generation magneto-optical recording. In thermally-assisted magnetic recording, data is recorded on a medium having a high Hc by using a laser to heat the recording medium to thereby lower the Hc enough to write the data. The written data bits are then stably retained at a temperature around room temperature at which the Hc is high. Thermally-assisted magnetic recording is regarded as the ultrahigh-density magnetic recording technology of the future. In magneto-optical recording, a laser beam is used to heat up localized areas of the medium to lower the Hc and enable data to be magnetically written. Stored data is read by projecting a light beam onto the medium and utilizing differences in the polarization angle of the reflected beam due to the direction of magnetization. Thus, it is a magnetic recording system that utilizes the magneto-optical effect. In both thermally-assisted magnetic recording and magneto-optical recording, the recording medium is repeatedly heated and cooled, so the magnetic material used has to be excellent in chemical stability against oxidation corrosion and in thermal stability against crystal transformation and crystallization. The invention magnetic material having Al-containing ε-Fe2O3 as its principal phase satisfies the requirement for chemical stability because it is an iron oxide obtained by oxidizing the iron to the trivalent level, and is also far superior in thermal stability to amorphous magnetic materials capable of achieving high coercive force. Moreover, the fact that the invention magnetic material can be controlled in coercive force Hc and saturation magnetization σs by controlling the amount of Al substitution enables it to be provided as a material suitable for thermally-assisted magnetic recording and next-generation magneto-optical recording.
Particular problems, in the case of thermally-assisted magnetic recording applied to hard disks, are those of the head-disk interface and the heating up of the head. Other problems arise because the magnetic head is separated from the disc surface by only several tens of nanometers, and because the disk surface is coated with lubricant to prevent wear to head. A particular issue is that the lubricant, being organic, is likely to deteriorate if exposed to high temperatures for even a short time. In most cases, a fluorine-base liquid lubricant is used, and although it has a relatively high heat resistance for an organic material, it can only resist temperatures of up to 300° C. (573 K). Taking into consideration the repeated heatings it will be subjected to, even if each heating period is short, it is desirable that its temperature does not exceed 200° C. (473 K). Because such a maximum heating temperature must be set with consideration to the magnetic phase transition temperature of the recording medium, it is desirable from the standpoint of the head-disk interface for the recording medium to have a low magnetic phase transition temperature, a requirement that can be met by the magnetic material of the present invention.
Thus, the usefulness of the magnetic material of the present invention goes beyond its application in high-density magnetic recording media. Namely, owing to its stability as an oxide substance and its outstanding magnetic properties, it also has potential for use in many other applications, including electromagnetic wave absorption materials, nanoscale electronics materials, permanent magnet materials, biomolecular marker agents, and drug carriers.
The synthesis of the Al-containing ε-Fe2O3 crystal of the invention has been described in the foregoing with reference to the use of a reverse micelle method to fabricate the precursor iron hydroxide and aluminum hydroxide microparticles. However, the fabrication method is not limited to the reverse micelle method and any available method capable of fabricating the precursors to a size of under a few hundred nanometers can be adopted. Similarly, while the use of the sol-gel method has been described with reference to providing the precursor particles with a heat-resistant silica coating, the fabrication of the coating is not limited to the described method and another method capable of providing the particles with a heat-resistant coating can be adopted. For example, a powder comprising particles having an iron oxide phase whose principal phase is Al-containing ε-Fe2O3 crystal can also be obtained by forming a heat-resistant coating of alumina, zirconia or the like on the precursor microparticles and subjecting the coating to heat treatment at a predetermined temperature.
In this Example, ε-Al0.29Fe1.71O3 was synthesized by the following procedure:
A micelle solution I and a micelle solution II were prepared.
Preparation of Micelle Solution I
6 ml of pure water, 18.3 ml of n-octane and 3.7 ml of 1-butanol were put into a Teflon (Trade Mark) flask, to which were added 0.00240 mole of iron (III) nitrate 9-hydrate and 0.00060 mole of aluminum (III) nitrate 9-hydrate, and the solution was dissolved at room temperature under fully stirring. An amount of a surfactant, cetyl trimethyl ammonium bromide, sufficient to bring the water/surfactant molar ratio to 30 was dissolved in the solution under stirring, thereby obtaining the micelle solution I.
The preparation had an Al to Fe molar ratio expressed as Al:Fe=x:(2−x) where x was 0.40.
Preparation of Micelle Solution II
2 ml of a 25% solution of aqueous ammonia was stirred into 4 ml of pure water. That evening, 18.3 ml of n-octane and 3.7 ml of 1-butanol were added, followed under fully stirring. Cetyl trimethyl ammonium bromide was added to and dissolved in the solution as surfactant in an amount that brought the (pure water+water present in the aqueous ammonia)/surfactant molar ratio to 30, to thereby obtain the micelle solution II.
The micelle solution II was then dripped into the micelle solution I under stirring. After this was completed, stirring of the mixed solution was continued for 30 minutes.
1.5 ml of tetraethoxysilane was added to the mixed solution obtained in Step 2 under stirring. The stirring was continued for about one day.
The solution obtained by Step 3 was centrifuged and the solid component thus separated was recovered and washed a plurality of times using a mixed solution of chloroform and methanol.
After the solid component obtained in Step 4 was dried, it was placed in a furnace and subjected to 4 hours of heat treatment at 1150° C. in air.
The heat-treated powder obtained in Step 5 was added to, and stirred for 24 hours in, one liter of 2 mole/L aqueous solution of NaOH to remove any silica that might be present on the surface of the particles. The particles were then filtered, washed and dried.
The above Steps 1 to 6 afforded a magnetic powder specimen. A TEM image of the powder is shown in
The specimen was subjected to powder X-ray diffraction (XRD: Rigaku Corporation, RINT 2000, using CuKa radiation at a voltage of 40 kV and a current of 30 mA) to obtain the diffraction pattern shown at the bottom of
While the preparation from which the specimen was obtained had an Al to Fe molar ratio expressed as Al:Fe=x:(2−x) where x was 0.40, fluorescent X-ray analysis (JEOL Ltd. JSX-3220) of the specimen gave x=0.29, indicating that Al-containing ε-Fe2O3 having the composition ε-Al0.29Fe1.71O3 was synthesized. The reason for the Al concentration being lower than in the preparation is thought to be that some of the added Al, which readily reacts with Si, was consumed by reaction with Si present on the particle surface in the sol-gel process to form part of a nonmagnetic compound that was dissolved and removed in Step 6. The same reasoning is thought to apply to the Examples that follow.
The magnetization hysteresis loop of the specimen at room temperature (300 K) was measured. The results are shown in
In this Example, ε-Al0.37Fe1.63O3 was synthesized.
This was done by the same procedure as Example 1, except that in the preparation of the micelle solution I, the amount of iron (III) nitrate 9-hydrate added was changed from 0.00240 mole to 0.00225 mole, and the amount of aluminum (III) nitrate 9-hydrate added was changed from 0.00060 mole to 0.000750 mole. The preparation had an Al to Fe molar ratio expressed as Al:Fe=x:(2−x) where x was 0.50.
A TEM image of the specimen (magnetic powder) obtained is shown in
The specimen was subjected to powder X-ray diffraction under the same conditions as set out in Example 1. The diffraction pattern obtained is the second from the bottom in
While the preparation from which the specimen was obtained had an Al to Fe molar ratio expressed as Al:Fe=x:(2−x) where x was 0.50, fluorescent X-ray analysis of the specimen in the manner set out above gave x=0.37, indicating that Al-containing ε-Fe2O3 having the composition ε-Al0.37Fe1.63O3 was synthesized.
The magnetization hysteresis loop of the specimen at room temperature (300 K) was measured by the method set out in Example 1. The results are shown in
In this Example, ε-Al0.45Fe1.55O3 was synthesized.
This was done by the same procedure as Example 1, except that in the preparation of the micelle solution I, the amount of iron (III) nitrate 9-hydrate added was changed from 0.00240 mole to 0.00210 mole, and the amount of aluminum (III) nitrate 9-hydrate added was changed from 0.00060 mole to 0.00090 mole. The preparation had an Al to Fe molar ratio expressed as Al:Fe=x:(2−x) where x was 0.60.
A TEM image of the specimen (magnetic powder) obtained is shown in
The specimen was subjected to powder X-ray diffraction under the same conditions as set out in Example 1. The diffraction pattern obtained is the third from the bottom in
While the preparation from which the specimen was obtained had an Al to Fe molar ratio expressed as Al:Fe=x:(2−x) where x was 0.60, fluorescent X-ray analysis of the specimen in the manner set out above gave x=0.45, indicating that Al-containing ε-Fe2O3 having the composition ε-Al0.45Fe1.55O3 was synthesized.
The magnetization hysteresis loop of the specimen at room temperature (300 K) was measured by the method set out in Example 1. The results are shown in
In this Example, ε-Al0.53Fe1.47O3 was synthesized.
This was done by the same procedure as Example 1, except that in the preparation of the micelle solution I, the amount of iron (III) nitrate 9-hydrate added was changed from 0.00240 mole to 0.001950 mole, and the amount of aluminum (III) nitrate 9-hydrate added was changed from 0.00060 mole to 0.001050 mole. The preparation had an Al to Fe molar ratio expressed as Al:Fe=x:(2−x) where x was 0.70.
A TEM image of the specimen (magnetic powder) obtained is shown in
The specimen was subjected to powder X-ray diffraction under the same conditions as set out in Example 1. The diffraction pattern obtained is the top one in
While the preparation from which the specimen was obtained had an Al to Fe molar ratio expressed as Al:Fe=x:(2−x) where x was 0.70, fluorescent X-ray analysis of the specimen in the manner set out above gave x=0.53, indicating that Al-containing ε-Fe2O3 having the composition ε-Al0.53Fe1.47O3 was synthesized.
The magnetization hysteresis loop of the specimen at room temperature (300 K) was measured by the method set out in Example 1. The results are shown in
In this example, ε-Fe2O3 without added substitutional element was synthesized.
This was done by the same procedure as Example 1, except that in the preparation of the micelle solution I, the amount of iron (III) nitrate 9-hydrate added was changed from 0.00240 mole to 0.0030 mole, and no aluminum (III) nitrate 9-hydrate was added. The preparation had an Al to Fe mole ratio expressed as Al:Fe=x:(2−x) where x was 0.
A TEM image of the specimen (magnetic powder) obtained is shown in
The specimen was subjected to powder X-ray diffraction under the same conditions as set out in Example 1. The diffraction pattern obtained is show in
Table 1 shows that the coercive force Hc of a magnetic material whose principal phase is Al-containing ε-Fe2O3 can be lowered by increasing the amount of Al substituted for a portion of the Fe (i.e., by increasing the value of x). Moreover, in the analysis results, the saturation magnetization as is seen to maximize around x=0.37. In other words, the magnetic properties can be controlled by controlling the Al content. Moreover, it can be seen from
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-234958 | Aug 2006 | JP | national |
