Fe-Co Based Target Material and Method for Producing the Same

Abstract

A method for producing a Fe—Co based target material for forming a soft magnetic thin-film comprising the steps of: preparing a first raw-material powder having an Fe:Co weight ratio ranging from 8:2 to 7:3 and a second raw-material powder having an Fe—Co weight ratio ranging from 2:8 to 0:10; mixing the first raw-material powder and the second raw-material powder together to obtain a powder mixture having an Fe:Co weight ratio ranging from 8:2 to 2:8; and applying a pressure of not less than 100 MPa to the powder mixture at a temperature ranging from 1073 to 1473 K for consolidation. At least one additional element selected from the group consisting of Nb, Zr, Ta and Hf is added to either one or both of the first and second raw-material powders in a total amount of 3 to 15 atom % with respect to the total amount of the powder mixture.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 11/789,422 filed on Apr. 24, 2007 which claims priority to Japanese Patent Application No. 128224/2006 filed on May 2, 2006, the entire disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a Fe—Co based target material for forming a soft magnetic thin-film by a sputtering method, and a method for producing the target material.

2. Description of Related Art

The recent progress in the magnetic recording technology is remarkable, and the record densities of magnetic record media are being heightened for increasing capacities of drives. In the magnetic record media for the longitudinal magnetic recording systems currently used worldwide, however, an attempt to realize a high record density leads to refined record bits, which require a high coercivity to such an extent that recording cannot be made with the record bits. In view of this, a perpendicular magnetic recording system is under study as a means of solving these problems and improving the record densities.

The perpendicular magnetic recording system is a system in which a magnetization-easy axis is oriented in the direction vertical to a medium surface in the magnetic film of the perpendicular magnetic record medium, and is suitable for high record densities. In addition, as for the perpendicular magnetic recording system, a two-layered record medium has been developed having a magnetic record film layer where the record sensitivity is improved and a soft magnetic film layer. A CoCrPt—SiO₂ based alloy is generally used in the magnetic record film layer.

On the other hand, it is proposed that a soft magnetic film of a Fe—Co—B based alloy is used as a soft magnetic film of a two-layered record medium. For example, as disclosed in Japanese Patent Laid-Open Publication No. 346423/2004, there is proposed a Fe—Co—B based alloy target material in which the diameter of the maximum inscribed circle which can be drawn in a region with no boride phase in a cross-microstructure is equal to 30 μm or less.

Magnetron sputtering method is generally used for the preparation of the aforementioned soft magnetic film. This magnetron sputtering method is a method in which a magnet is disposed behind a target material to leak the magnetic flux onto a surface of the target material for converging plasma in the leaked magnetic flux region, enabling a high-speed coating. Since the magnetron sputtering method has a feature of leaking the magnetic flux on the sputtering surface of the target material, in the case where magnetic permeability of the target material itself is high, it is difficult to form, on the sputtering surface of the target material, the leaked magnetic flux necessary and sufficient for the magnetron sputtering method. In view of this, Japanese Patent Laid-Open Publication No. 346423/2004 is proposed for a demand for reducing the magnetic permeability of the target material itself as much as possible.

On the other hand, the thickness of the target material can be increased as the magnetic permeability of the target material is lowered. That is, a larger number of thin films can be produced from a single target material, resulting in an improved productivity. However, in the foregoing conventional technique, since the magnetic permeability is not sufficiently low, the maximum thickness of the target material is about 5 mm. If the thickness exceeds 5 mm, leaked magnetic flux is insufficiently created on the surface of the target material, causing a problem that a magnetron sputtering cannot be performed normally.

SUMMARY OF THE INVENTION

The inventors have now found that a target material having a high density and a magnetic permeability lower than the conventional one can be produced by mixing a first raw-material powder having a certain Fe:Co weight ratio and a second raw-material powder having another certain Fe:Co weight ratio so that the resulting Fe:Co weight ratio becomes between 8:2 and 2:8, followed by hot-pressing the powder mixture into which a certain additional element of 3 to 15 atom % is added.

It is therefore an object of the present invention to provide an Fe—Co based target material and a method for producing the Fe—Co based target material capable of ensuring a high density and lowering the magnetic permeability than conventional targets so that the thickness of the target material can be increased to improve productivity of thin-films.

The present invention provides a method for producing a Fe—Co based target material, comprising the steps of:

preparing a first raw-material powder having an Fe:Co weight ratio ranging from 8:2 to 7:3 and a second raw-material powder having an Fe—Co weight ratio ranging from 2:8 to 0:10;

mixing the first raw-material powder and the second raw-material powder together to obtain a powder mixture having an Fe:Co weight ratio ranging from 8:2 to 2:8; and

applying a pressure of not less than 100 MPa to the powder mixture at a temperature ranging from 1073 to 1473 K for consolidation,

wherein at least one additional element selected from the group consisting of Nb, Zr, Ta and Hf is added to either one or both of the first and second raw-material powders in a total amount of 3 to 15 atom % with respect to the total amount of the powder mixture.

The present invention also provides an Fe—Co based target material, which is obtainable by the above method, wherein the Fe—Co based target material is made of an Fe—Co based alloy comprising:

85 to 97 atom % of Fe and Co, which have a Fe:Co weight ratio ranging from 8:2 to 2:8; and

3 to 15 atom % of at least one additional element selected from the group consisting of Zr, Nb, Ta and Hf.

DETAILED DESCRIPTION OF THE INVENTION

Method for Producing Fe—Co Based Target Material

First of all, in the method for producing a Fe—Co based target material according to the present invention, a first raw-material powder having an Fe:Co weight ratio ranging from 8:2 to 7:3 and a second raw-material powder having a Fe—Co weight ratio ranging from 2:8 to 0:10 are prepared. As far as the composition of each of the first and second raw-material powders falls within the above range, the magnetic characteristics (magnetic permeability) are meaningfully reduced.

Then, the first raw-material powder and the second raw-material powder are mixed together to obtain a powder mixture having a Fe:Co weight ratio ranging from 8:2 to 2:8. As far as the weight ratio falls within the above range, an increase in magnetic characteristics (magnetic permeability) can be prevented.

In the production method of the present invention, at least one additional element selected from the group consisting of Nb, Zr, Ta and Hf is added to either one or both of the first and second raw-material powders in a total amount of 3 to 15 atom %, desirably, 5 to 10 atom %, with respect to the total amount of the powder mixture. The additional element should be added to one or both of the first and second raw-material powders in advance prior to the mixing. The additional element of less than 3 atom % leads to difficulty in amorphization even if the additional element is added to the Fe—Co powder mixture. The additional element of more than 15 atom % leads to a reduced saturation flux density. According to a preferred aspect of the present invention, two additional elements are preferably selected from the group consisting of Nb, Zr, Ta and Hf.

According to a preferred aspect of the present invention, it is preferred that, prior to the mixing step, the additional element is added to the first raw-material powder in a total amount of not less than 1 atom % with respect to the total amount of the first raw-material powder, and/or, the additional element is added to the second raw-material powder in a total amount of not less than 1 atom % with respect to the total amount of the second raw-material powder. This makes it possible to fully achieve the effect of a reduced magnetic permeability provided by the additional element.

The powder mixture thus obtained is consolidated by applying a pressure of not less than 100 MPa, preferably of 100 MPa to 500 MPa, at a temperature of 1073 to 1473 K to obtain a Fe—Co based target material. The consolidating process usable in the method of the present invention is not particularly limited and may be any process, for example, hot consolidation such as HIP and hot pressing, as far as the target material can be consolidated with a high density. The method for producing powder is not limited to any technique, and includes gas atomizing, water atomizing, and casting-pulverizing. The reason why the consolidating temperature is selected to the above range is that the density of the target material does not reach 100% when the consolidating temperature is less than 1073 K, and that when it exceeds 1473 K the diffusion between particles is extremely promoted so that plenty of phases having strong magnetic characteristics are formed. The reason why the consolidating pressure is selected to the above range is that when the consolidating pressure is less than 100 MPa, the density does not reach 100%. While there is no problem as far as the consolidating pressure is high, the upper limit of the consolidating pressure is preferably at 500 MPa from the viewpoint of cost and productivity.

Fe—Co Based Target Material

The Fe—Co based target material obtainable by the production method of the present invention is made of an Fe—Co based alloy comprising 85 to 97 atom % of Fe and Co, which have a Fe:Co weight ratio ranging from 8:2 to 2:8; and 3 to 15 atom % of at least one additional element selected from the group consisting of Zr, Nb, Ta and Hf. The Fe—Co based target material thus produced has a high density and a lower magnetic permeability than those of conventional target materials. According to a preferred aspect of the present invention, the Fe—Co based target material has a magnetic permeability between 10 and 200. As a result, the thickness of the target material can be increased to improve productivity of thin-films. The magnetic permeability of the target material of not more than 200 makes it possible to increase the thickness of the target material, while the magnetic permeability of not less than 10 results in satisfactory characteristics as a magnetic material.

Sputtering Using Fe—Co Based Target Material

As described above, magnetron sputtering method is generally used for forming soft magnetic films. This magnetron sputtering method is a method in which a magnet is disposed behind a target material to leak the magnetic flux onto a surface of the target material for converging plasma in the leaked magnetic flux region, enabling a high-speed coating. This magnetron sputtering device has a feature that a magnet is disposed behind the target material to trap γ electrons in the vicinity of the target material by the application of a magnetic field, aimed at solving the drawback of bipolar DC glow discharge sputtering devices. Since the γ electron has such an orbit as to be entangled with the lines of magnetic force, the plasma concentrates in the vicinity of the target material to reduce damages to the substrate. In addition, since the moving distance of the γ electron becomes long, it is possible to perform a high-speed sputtering at a low gas pressure.

EXAMPLES

Examples of the present invention will be in detail explained hereinafter.

As shown in Table 1, Fe—Co based alloys were produced by gas-atomizing methods or casting method. The gas-atomizing methods were carried out on condition that the type of gas was an argon gas, the nozzle diameter was 6 mm and the gas pressure was 5 MPa. On the other hand, the casting methods were carried out by melting the alloys in a ceramic vessel (diameter: 200 mm; length: 30 mm) and then pulverizing the alloys to powders. Powders thus produced were classified into 500 μm or less and each powder was stirred for one hour by a V-type mixer.

Each powder thus produced was filled in an enclosing vessel made of a SC steel having a diameter of 200 mm and a height of 100 mm and was encapsulated with vacuum evacuation at an ultimate vacuum of 10⁻¹ Pa or less, followed by an HIP (hot isostatic pressing) at a temperature of 1173K under a pressure of 150 MPa for a holding time of 5 hours. Next, the resultant consolidated bodies were subjected to lathing and wire-cutting to provide final shapes to obtain target materials having outer diameters of 180 mm and thicknesses of 3 to 10 mm. Properties of the above target materials are shown in Table 2.

	TABLE 1
	Total Formulation After Mixing
	Additional	Raw-Material Powder A	Raw-Material Powder B
	Composition	Element (at %)	Composition		Composition
	Ratio	Total of	Ratio	Additional Element (at %)	Ratio	Additional Element (at %)
No.	Fe:Co Ratio	Zr, Nb, Ta, Hf	Fe	Co	Zr	Nb	Ta	Hf	Fe	Co	Zr	Nb	Ta	Hf
1	8:2	7	74.2	25.8	4	3	—	—	—	—					Examples of
2	7:3	7	71	29	4	3	—	—	0	100	1	1			Present
3	6:4	7	76	24	4	3	—	—	0	100	0	1			Invention
4	4:6	7	77	23	—	—	3	3	0	100			1	0
5	3:7	10	73	27	4	3	—	—	0	100	2	1
6	8:2	15	70	30	—	4	5	5	11	89		0	0	1
7	7:3	14	71	29	6	8	—	—	0	100	0	0
8	6:4	12	76	24	6	6	—	—	11	89	0	0
9	4:6	15	77	23	—	—	7	7	9	91			0	1
10	3:7	5	73	27	1	2	—	—	13	87	1	1
11	7:3	14	71	29	6	8	—	—	0	100	0	0
12	6:4	14	76	24	6	6	—	—	11	89	1	1
13	4:6	9	77	23	—	—	4	4	9	91			1	0
14	1:8	11	100	0	—	4	5	5	16	84		1	1	1	Comparative
15	9:1	7	73	27	4	3	—	—	0	100	0	0			Examples
16	3:7	16	70	30	8	8	—	—	0	100	0	0
17	3:7	16	70	30	—	—	8	8	0	100			0	0
18	7:3	15	89	11	—	4	5	5	11	89			1	0
19	7:3	10	76	24	—	4	5	5	22	78			1	1
20	3:7	9	73	27	4	3	—	—	0	100	1	1
21	7:3	14	69	31	6	8	—	—	0	100	1	1
(Underlined part is out of the range of the conditions of the present invention)

TABLE 2
	Consolidating	Consolidating		Relative
	Temperature	Pressure	Magnetic	Density
No.	(K)	(MPa)	Permeability	(%)
1	1173	150	100	100	Examples of
2	1173	200	90	100	Present
3	1223	180	85	100	Invention
4	1123	220	110	100
5	1223	220	120	100
6	1173	180	100	100
7	1173	200	90	100
8	1223	220	85	100
9	1123	200	110	100
10	1223	250	120	100
11	1073	200	90	100
12	1373	180	85	100
13	1123	200	110	100
14	1223	200	200	100	Comparative
15	1223	200	250	100	Examples
16	1223	180	5	100
17	1223	280	5	100
18	1223	200	250	100
19	1223	200	220	100
20	1523	200	250	100
21	1023	200	240	96
(Underlined part is out of the range of the conditions of the present invention)

In order to evaluate the characteristics of the target materials thus produced, the following measurements were carried out.

(1) Magnetic Permeability Measurements

Preparation of ring specimens: outer diameter of 15 mm, inner diameter of 10 mm, height of 5 mm

Apparatus: BH tracer

Applied magnetic field: 8 kA/m

Measurement item: mean magnetic permeability between zero and 100 A/m(Oe)

(2) Density

The density measurement was made by the use of Archimedes' principle to calculate a relative density (ratio of a measured density to a calculated density).

As shown in Table 1, No. 1 to No. 13 are examples of the present invention, and No. 14 to No. 21 are comparative examples. In comparative example No. 14, the Fe:Co ratio as a whole after the mixing step is 1:8, which indicates a low Fe content and a high Co content, resulting in a high magnetic permeability as a magnetic characteristic. In comparative example No. 15, the Fe:Co ratio as a whole after the mixing step is 9:1, which indicates a high Fe content and a low Co content, resulting in a high magnetic permeability as the magnetic characteristic. In comparative example No. 16, the content of Zr and Nb as the additional elements is high, resulting in a low magnetic permeability. In comparative example No. 17, the content of Ta and Hf as the additional elements is high, resulting in a low magnetic permeability.

In comparative example No. 18, the first raw-material powder has the high Fe ratio and the low Co ratio, resulting in a high magnetic permeability. In comparative example No. 19, the second raw-material powder has the low Fe ratio and the high Co ratio, resulting in a high magnetic permeability. In comparative example No. 20, phases with strong magnetic characteristics are formed by a reaction due to the high consolidating temperature, resulting in a high magnetic permeability. In comparative Example No. 21, the first raw-material powder has the low Fe ratio and the high Co ratio, resulting in a high magnetic permeability, and the relative density is inferior because of the low consolidating temperature.

In contrast, it can be seen that all examples No. 1 to No. 13 of the present invention are superior in magnetic permeability as a magnetic characteristic because examples No. 1 to No. 13 satisfy the conditions of the present invention.

As described above, a Fe:Co ratio is determined for each of the first and second raw-material powders, and the total Fe:Co ratio is controlled to fall within an optimum range for the powder mixture to which a certain amount of an additional element is added. This leads to a magnetic permeability as a magnetic characteristic which is lower than the conventional one, making it possible to increase the thickness of the target material and thus to improve productivity. In consequence, significantly advantageous effects are attained.

Claims

1. A method for producing a Fe—Co based target material, comprising the steps of:

preparing a first raw-material powder having an Fe:Co weight ratio ranging from 8:2 to 7:3 and a second raw-material powder having an Fe—Co weight ratio ranging from 2:8 to 0:10;

mixing the first raw-material powder and the second raw-material powder together to obtain a powder mixture having an Fe:Co weight ratio ranging from 8:2 to 2:8; and

applying a pressure of not less than 100 MPa to the powder mixture at a temperature ranging from 1073 to 1473 K for consolidation,

wherein at least one additional element selected from the group consisting of Nb, Zr, Ta and Hf is added to either one or both of the first and second raw-material powders in a total amount of 3 to 15 atom % with respect to the total amount of the powder mixture.

2. The method according to claim 1, wherein, prior to the mixing step, the additional element is added to the first raw-material powder in a total amount of not less than 1 atom % with respect to the total amount of the first raw-material powder.

3. The method according to claim 1, wherein, prior to the mixing step, the additional element is added to the second raw-material powder in a total amount of not less than 1 atom % with respect to the total amount of the second raw-material powder.

4. The method according to claim 2, wherein, prior to the mixing step, the additional element is added to the second raw-material powder in a total amount of not less than 1 atom % with respect to the total amount of the second raw-material powder.