Magnetic alloy with ultrafine crystal grains and method of producing same

Abstract: There is provided according to the present invention a magnetic alloy with ultrafine crystal grains having a composition represented by the general formula: Fe 100-x-y M x B y (atomic %) wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, 4 ≦ x ≦ 15, 2 ≦ y ≦ 25, and 7 ≦ x + y ≦ 35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 500Å or less, and the crystal grains being based on a bcc structure. It may further contain X (Si, Ge, P, Ga, etc.) and/or T (Au, Co, Ni, etc.). this magnetic alloy has an excellent saturation magnetic flux density, permeability and heat resistance.

Description: The present invention relates to a magnetic alloy with ultrafine crystal grains excellent in magnetic properties and their stability, a major part of the alloy structure being occupied by ultrafine crystal grains, suitable for magnetic heads, etc. Conventionally used as magnetic materials for magnetic parts such as magnetic heads are ferrites, showing relatively good frequency characteristics with small eddy current losses. However, ferrites do not have high saturation magnetic flux densities, so that they are insufficient for high-density magnetic recording of recent magnetic recording media when used for magnetic heads. In order that magnetic recording media having high coercive force for high-density magnetic recording show their performance sufficiently, magnetic materials having higher saturation magnetic flux densities and permeabilities are needed. To meet such demands, thin Fe-A l -Si alloy layers, thin Co-Nb-Zr amorphous alloy layers, etc. are recently investigated. Such attempts are reported by Shibata et al., NHK Technical Report 29 (2), 51-106 (1977), and by Hirota et al., Kino Zairyo (Functional Materials) August, 1986, p. 68, etc. However, with respect to the Fe-A l -Si alloys, both magnetostriction λ s and magnetic anisotropy K should be nearly zero to achieve high permeability. These alloys, however, achieve saturation magnetic flux densities of only 12 kG or so. Because of this problem, investigation is conducted to provide Fe-Si alloys having higher saturation magnetic flux densities and smaller magnetostrictions, but they are still insufficient in corrosion resistance and magnetic properties. In the case of the above Co-base amorphous alloys, they are easily crystallized when they have compositions suitable for higher saturation magnetic flux densities, meaning that they are poor in heat resistance, making their glass bonding difficult. Recently, Fe-M-C (M = Ti, Zr, Hf) layers showing high saturation magnetic flux densities and permeabilities were reported in Tsushin Gakkai Giho (Telecommunications Association Technical Report) MR89-12, p. 9. However, carbon atoms contained in the alloy are easily movable, causing magnetic aftereffect, which in turn deteriorates the reliability of products made of such alloys. A magnetic alloy with the features including in the first part of claim 1 is known from Journal of Applied Physics, vol. 62, No. 5, pages 1948-1951. The alloy known from this document contains considerable amounts of Cr and Ni and an accordingly small amount of Fe. It is a superparamagnetic alloy in which the fine bcc phase is dispersed in a non-magnetic matrix, so that it is not easily magnetised in a magnetic field. JP-A-59-150404 discloses an electromagnetic device comprising a core made of a low-loss magnetic, boron containing Fe alloy, which is amorphous. OBJECT AND SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a magnetic alloy having excellent magnetic properties, heat resistance and reliability. As a result of intense research in view of the above object, the inventors have found that a magnetic alloy based on Fe, M and B (M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn), at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 50nm or less, and the crystal grains being based on a bcc structure, has high saturation magnetic flux density and permeability and also good heat resistance, suitable for magnetic cores. The present invention defined in claim 1 has been made based upon this finding. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 (a) is a graph showing an X-ray diffraction pattern of the alloy of the present invention before heat treatment: Fig. 1 (b) is a graph showing an X-ray diffraction pattern of the alloy of the present invention heat-treated at 600°C; Fig. 2 (a) is a graph showing the relation between a saturation magnetic flux density (B₁₀) and a heat treatment temperature; and Fig. 2 (b) is a graph showing the relation between an effective permeability (µ e1k ) and a heat treatment temperature; Fig. 3 is a graph showing the relation between a magnetic flux density B and a magnetic field intensity with respect to the alloy of the present invention; and Fig. 4 is a graph showing the relation between a magnetic flux density B and a magnetic field intensity with respect to the alloy of the present invention. DETAILED DESCRIPTION OF THE INVENTION In the above magnetic alloy of the present invention, B is an indispensable element, which is dissolved in a bcc Fe, effective for making the crystal grains ultrafine and controlling the alloy's magnetostriction and magnetic anisotropy. M is at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, which is also an indispensable element. By the addition of both M and B, the crystal grains can be made ultrafine, and the alloy's heat resistance can be improved. The M content (x), the B content (y) and the total content of M and B (x + y) should meet the following requirements: 4 ≦ x ≦ 15, 2 ≦ y ≦ 25, and 7 ≦ x + y ≦ 35. When x and y are lower than the above lower limits, the alloy has poor heat resistance. On the other hand, when x and y are larger than the above upper limits, the alloy has poor saturation magnetic flux density and soft magnetic properties. Particularly, the preferred ranges of x and y are: 5 ≦ x ≦ 15, 10 < y ≦ 20, and 15 < x + y ≦ 30. With these ranges, the alloys show excellent heat resistance. According to another aspect of the present invention, the above composition may further contain at least one element (X) selected from Si, Ge, P, Ga, Al and N, and at least one element (T) selected from Au, platinum group elements, Co, Ni, Sn, Be, Mg, Ca, Sr and Ba. Accordingly, the following alloys are also included in the present application. The magnetic alloy with ultrafine crystal grains according to another embodiment of the present invention has a composition represented by the general formula: Fe 100-x-y-z M x B y X z (atomic %) wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, X represents at least one element selected from Si, Ge, P, Ga, Al and N, 4 ≦ x ≦ 15, 2 ≦ y ≦ 25, 0 < z ≦ 10, and 7 ≦ x + y + z ≦ 35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 50nm or less, and the crystal grains being based on a bcc structure. The magnetic alloy with ultrafine crystal grains according to a further embodiment of the present invention has a composition represented by the general formula: Fe 100-x-y-b M x B y T b (atomic %) wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, T represents at least one element selected from Au, platinum group elements, Co, Ni, Sn, Be, Mg, Ca, Sr and Ba, 4 ≦ x ≦ 15, 2 ≦ y ≦ 25, 0 < b ≦ 10, and 7 ≦ x + y + b ≦ 35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 50nm or less, and the crystal grains being based on a bcc structure. The magnetic alloy with ultrafine crystal grains according to a still further embodiment of the present invention has a composition represented by the general formula: Fe 100-x-y-z-b M x B y X z T b (atomic %) wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, X represents at least one element selected from Si, Ge, P, Ga, Al and N, T represents at least one element selected from Au, platinum group elements, Co, Ni, Sn, Be, Mg, Ca, Sr and Ba, 4 ≦ x ≦ 15, 2 ≦ y ≦ 25, 0 < z ≦ 10, 0 < b ≦ 10, and 7 ≦ x + y + z + b ≦ 35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 50nm or less, and the crystal grains being based on a bcc structure. With respect to the element X, it is effective to control magnetostriction and magnetic anisotropy, and it may be added in an amount of 10 atomic % or less. When the amount of the element X exceeds 10 atomic %, the deterioration of soft magnetic properties takes place. The preferred amount of X is 0.5-8 atomic %. With respect to the element T, it is effective to improve corrosion resistance and to control magnetic properties. The amount of T (b) is preferably 10 atomic % or less. When it exceeds 10 atomic %, extreme decrease in a saturation magnetic flux density takes place. The preferred amount of T is 0.5-8 atomic %. The above-mentioned alloy of the present invention has a structure based on crystal grains having an average grain size of 50nm or less. Particularly when the average grain size is 20nm or less, excellent soft magnetic properties can be obtained. In the present invention, ultrafine crystal grains should be at least 50% of the alloy structure, because if otherwise, excellent soft magnetic properties would not be obtained. Depending upon the heat treatment conditions, an amorphous phase may remain partially, or the alloy structure may become 100% crystalline. In either case, excellent soft magnetic properties can be obtained. The reason why excellent soft magnetic properties can be obtained in the magnetic alloy with ultrafine crystal grains of the present invention are considered as follows: In the present invention, M and B form ultrafine compounds based on bcc Fe and uniformly dispersed in the alloy structure by a heat treatment, suppressing the growth of such crystal grains. Accordingly, the magnetic anisotropy is apparently offset by this action of making the crystal grains ultrafine, resulting in excellent soft magnetic properties. According to a further aspect of the present invention, there is provided a method of producing a magnetic alloy with ultrafine crystal grains comprising the steps of producing an amorphous alloy having either one of the above-mentioned compositions, and subjecting the resulting amorphous alloy to a heat treatment to cause crystallization, thereby providing the resulting alloy having a structure, at least 50% of which is occupied by crystal grains based on a bcc Fe solid solution and having an average grain size of 50nm or less. The amorphous alloy is usually produced by a liquid quenching method such as a single roll method, a double roll method, a rotating liquid spinning method, etc., by a gas phase quenching method such as a sputtering method, a vapor deposition method, etc. The amorphous alloy is subjected to a heat treatment in an inert gas atmosphere, in hydrogen or in vacuum to cause crystallization, so that at least 50% of the alloy structure is occupied by crystal grains based on a bcc structure solid solution and having an average grain size of 50nm or less. The heat treatment according to the present invention is preferably conducted at 450°C-800°C. When the heat treatment is lower than 450°C, crystallization is difficult even though the heat treatment is conducted for a long period of time. On the other hand, when it exceeds 800°C, the crystal grains grow excessively, failing to obtain the desired ultrafine crystal grains. The preferred heat treatment temperature is 500-700°C. Incidentally, the heat treatment time is generally 1 minute to 200 hours, preferably 5 minutes to 24 hours. The heat treatment temperatures and time may be determined within the above ranges depending upon the compositions of the alloys. Since the alloy of the present invention undergoes a heat treatment at as high a temperature as 450-800°C, glass bonding is easily conducted in the production of magnetic heads, providing the resulting magnetic heads with high reliability. The heat treatment of the alloy of the present invention can be conducted in a magnetic field. When a magnetic field is applied in one direction, a magnetic anisotropy in one direction can be given to the resulting heat-treated alloy. Also, by conducting the heat treatment in a rotating magnetic field, further improvement in soft magnetic properties can be achieved. In addition, the heat treatment for crystallization can be followed by a heat treatment in a magnetic field. The present invention will be explained in further detail by way of the following Examples, without intending to restrict the scope of the present invention. Example 1 An alloy melt having a composition (atomic %) of 7% Nb, 18 % B and balance substantially Fe was rapidly quenched by a single roll method to produce a thin amorphous alloy ribbon of 18 µm in thickness. The X-ray diffraction pattern of this amorphous alloy before a heat treatment is shown in Fig. 1 (a). It is clear from Fig. 1 (a) that this pattern is a halo pattern peculiar to an amorphous alloy. Next, this thin alloy ribbon was subjected to a heat treatment at 600°C for 1 hour in a nitrogen gas atmosphere to cause crystallization, and then cooled to room temperature. The X-ray diffraction pattern of the alloy obtained by the heat treatment at 600°C is shown in Fig. 1 (b). As a result of X-ray diffraction analysis, it was confirmed that the alloy after a 600°C heat treatment had a structure mostly constituted by ultrafine crystal grains made of a bcc Fe solid solution having a small half-width. As a result of transmission electron photomicrography, it was confirmed that the alloy after the heat treatment had a structure mostly constituted by ultrafine crystal grains having an average grain size of 10nm or less. Incidentally, in the present invention, the percentage of ultrafine crystal grains is determined by a generally employed intersection method. In this method, an arbitrary line (length = L) is drawn on a photomicrograph such that it crosses crystal grains in the photomicrograph. The length of each crystal grains crossed by the line (L₁, L₂, L₃ ... L n ) is summed to provide a total length (L₁ + L₂ + L₃ + ... + L n ), and the total length is divided by L to determine the percentage of crystal grains. Where there are a large percentage of crystal grains in the alloy structure, it appears from the photomicrograph that the structure is almost occupied by crystal grains. However, even in this case, some percentage of an amorphous phase exists in the structure. This is because the periphery of each crystal grain looks obscure in the photomicrograph, suggesting the existence of an amorphous phase. Where there are a large percentage of such crystal grains, it is generally difficult to express the percentage of crystal grains by an accurate numerical value. Accordingly, in Examples, "substantially" or "mostly" is used. Next, a toroidal core produced by the amorphous alloy of this composition was subjected to a heat treatment at various heat treatment temperatures without applying a magnetic field to measure a dc B-H hysteresis curve by a dc B-H tracer and an effective permeability µ e1k at 1 kHz by an LCR meter. The heat treatment time was 1 hour, and the heat treatment atmosphere was a nitrogen gas atmosphere. The results are shown in Figs. 2 (a) and (b). Fig. 3 shows the dc B-H hysteresis curve of Fe₇₅Nb₇B₁₈ heated at 630°C for 1 hour, in which B₁₀ = 12.1 kG, Br/B₁₀ = 24%, and Hc = 0.103 Oe. It can be confirmed that at a heat treatment temperature higher than the crystallization temperature at which bcc Fe phases are generated, high saturation magnetic flux density and high permeability are obtained. Thus, the alloy of the present invention can be obtained by crystallizing the corresponding amorphous alloy. The alloy of the present invention has extremely reduced magnetostriction than the amorphous counterpart, meaning that it is suitable as soft magnetic materials. The alloy of the present invention shows higher saturation magnetic flux density than the Fe-Si-A l alloy, and its µ e1k exceeds 10000 in some cases. Therefore, the alloy of the present invention is suitable for magnetic heads for high-density magnetic recording, choke cores, high-frequency transformers, sensors, etc. Example 2 Thin heat-treated alloy ribbons of 5 mm in width and 15 µm in thickness having the compositions shown in Table 1 were produced in the same manner as in Example 1. It was measured with respect to B₁₀ and Hc by a dc B-H tracer, an effective permeability µ e1k at 1 kHz by an LCR meter, and a core loss Pc at 100 kHz and at 0.2 T by a U-function meter. The average crystal grain size and the percentage of crystal grains were determined by using the photomicrographs of the alloy structures. The results are shown in Table 1. Any of the heat-treated alloys had crystal grains based on a bcc structure and having an average grain size of 50nm or less. The dc hysteresis curve of No. 1 alloy (Fe₇₉Nb₇B₁₄) shown in Table 1 is shown in Fig. 4, in which B₁₀ = 1.25 T, Br/B₁₀ = 72%, and Hc = 16 A/m. The alloys of the present invention show saturation magnetic flux densities equal to or higher than those of the Fe-Si-Al alloy and the Co-base amorphous alloy, and also have higher µ e1k than those of the Fe-Si, etc. Accordingly, the alloys of the present invention are suitable as alloys for magnetic heads. Example 3 Thin amorphous alloy ribbons of 5 mm in width and 15 µm in thickness having the compositions shown in Table 2 were produced by a single roll method. Next, each of these thin alloy ribbons was formed into a toroidal core of 19 mm in outer diameter and 15 mm in inner diameter, and subjected to a heat treatment at 550°C-700°C in an Ar gas atmosphere to cause crystallization. As a result of X-ray diffraction analysis and transmission electron photomicrography, it was confirmed that the alloys after the heat treatment had structures mostly constituted by ultrafine crystal grains based on a bcc structure and having an average grain size of 50nm or less. With respect to newly prepared thin amorphous alloy ribbons having the above-mentioned compositions, they were formed into toroidal cores in the same manner as above and measured on effective permeability µ e1k at 1 kHz. Next, they were subjected to a heat treatment at 600°C for 30 minutes and cooled to room temperature. Their effective permeabilities (µ e1k ³⁰) at 1 kHz were also measured. The values of µ e1k ³⁰/µ e1k are shown in Table 2. It is clear from Table 2 that the alloys of the present invention show extremely larger µ e1k ³⁰/µ e1k than those of the conventional materials, and so excellent heat resistance, suffering from less deterioration of magnetic properties even at as high a temperature as 600°C. Accordingly, they are suitable as magnetic materials for magnetic heads needing glass bonding, sensors operated at high temperature, etc. Incidentally, in the alloy of the present invention, the larger the B content, the larger the value of µ e1k ³⁰/µ e1k . In addition, when the M content is smaller than the lower limit of the range of the present invention, µ e1k ³⁰/µ e1k is low, meaning that the heat resistance is poor. Example 4 Alloy layers having compositions shown in Table 3 were produced on fotoceram substrates by a sputtering method, and subjected to a heat treatment at 550-700°C for 1 hour to cause crystallization. At this stage, their µ e1M ⁰ was measured. As a result of X-ray diffraction analysis and transmission electron photomicrography, it was confirmed that the alloys after the heat treatment had structures mostly constituted by ultrafine crystal grains based on a bcc structure and having an average grain size of 50nm or less. Next, these alloys were introduced into an oven at 550°C, and kept for 1 hour and cooled to room temperature to measure their µ e1M ¹. Their µ e1M ¹/µ e1M ⁰ ratios are shown in Table 3. The alloy layers of the present invention show µ e1m ¹/µ e1M ⁰ closer to 1 than the alloys of Comparative Examples, and suffer from less deterioration of magnetic properties even at a high temperature, showing better heat resistance. Thus, the alloys of the present invention are suitable for producing high-reliability magnetic heads. According to the present invention, magnetic alloy with ultrafine crystal grains having excellent saturation magnetic flux density, permeability and heat resistance can be produced.

Claims: A magnetic boron containing iron alloy with ultrafine crystal grains based on a bcc structure, characterised in that it has a composition represented by the general formular Fe 100-x-y-z-b M x B y X z T b (atomic %) wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn; X represents at least one element selected from Si, Ge, P, Ga, Al and N; T represents at least one element selected from Au, the platinum group, Co, Ni, Sn, Be, Mg, Ca, Sr and Ba; 4 ≦ x ≦ 15; 2 ≦ y ≦ 25; 0 ≦ z ≦ 10, 0 ≦ b ≦ 10; and 7 ≦ x + y + z + b ≦ 35; and that at least 50% of the alloy structure are occupied by crystal grains having an average grain see of 50 nm or less. The alloy of claim 1, wherein the balance of the alloy structure is composed of an amorphous phase. The alloy of claim 1, which is substantially composed of a crystalline phase. The alloy of any one of claims 1 to 3, wherein 10 < y ≦ 20. The alloy of any one of claims 1 to 4, wherein said crystal grains have an average grain size of 20 nm or less. A method of producing a magnetic alloy with ultrafine crystal grains, comprising the steps of producing an amorphous alloy having the composition recited in any one of claims 1 to 5, and subjecting the resulting amorphous alloy to a heat treatment to cause crystallization, thereby providing the resulting alloy with a structure at least 50% of which are occupied by crystal grains having an average grain size of 50 nm or less. The method of claim 6, wherein said amorphous alloy is subjected to a magnetic field during said heat treatment.

Inventors: BIZEN YOSHIO, NISHIYAMA TOSHIKAZU, SUWABE SHIGEKAZU, YAMAUCHI KIYOTAKA, YOSHIZAWA YOSHIHITO, BIZEN, YOSHIO, NISHIYAMA, TOSHIKAZU, SUWABE, SHIGEKAZU, YAMAUCHI, KIYOTAKA, YOSHIZAWA, YOSHIHITO
Applicants: HITACHI METALS LTD, HITACHI METALS, LTD.
Application year: 1990
Main classification: H01F1
IPC codes: H01F1, C22C45