FE-BASED SOFT MAGNETIC ALLOY

Abstract: Fe-based soft magnetic alloy having excellent soft magnetic characteristics with high saturated magnetic flux density, characterized by fine crystal grains and the general formula (I) (Fe 1-a-b Cu a M b ) 100-c Y c (I) where "M" is at least one rare earth element, "Y" is at least one of: Si, B, P, and C, and "a", "b", and "c", expressed in atomic % are as follows: 0.005 ≦ a ≦ 0.05 0.005 ≦ b ≦ 0.1 15 ≦ c ≦ 28.

Description: This invention relates to Fe-based, soft magnetic alloys. Conventionally, iron cores of crystalline materials such as permalloy or ferrite have been employed in high frequency devices such as switching regulators. However, the resistivity of permalloy is low, so it is subject to large core loss at high frequency. Also, although the core loss of ferrite at high frequencies is small, the magnetic flux density is also small, at best 5,000 G. Consequently, in use at high operating magnetic flux densities, ferrite becomes close to saturation and as a result the core loss is increased. Recently, it has become desirable to reduce the size of transformers that are used at high frequency, such as the power transformers employed in swtiching regulators, smoothing choke coils, and common mode choke coils. However, when the size is reduced, the operating magnetic flux density must be increased, so the increase in core loss of the ferrite becomes a serious practical problem. For this reason, amorphous magnetic alloys, i.e., alloys without a crystal structure, have recently attracted attention and have to some extent been used because they have excellent soft magnetic properties such as high permeability and low coercive force. Such amorphous magnetic alloys are typically base alloys of Fe, Co, Ni, etc., and contain metalloids as elements promoting the amorphous state, (P, C, B, Si, Al, and Ge, etc.). However, not all of these amorphous magnetic alloys have low core loss in the high frequency region. Iron-based amorphous alloys are cheap and have extremely small core loss, about one quarter that of silicon steel, in the frequency region of 50 to 60 Hz. However, they are extremely unsuitable for use in the high frequency region for such applications as in switching regulators, because they have an extremely large core loss in the high frequency region of 10 to 50 kHz. In order to overcome this disadvantage, attempts have been made to lower the magnetostriction, lower the core loss, and increase the permeability by replacing some of the Fe with non-magnetic metals such as Nb, Mo, or Cr. However, the deterioration of magnetic properties due to hardening, shrinkage, etc., of resin, for example, on resin moulding, is large compared to Co-based alloys, so satisfactory performance of such materials is not obtained when used in the high frequency region. Co-based, amorphous alloys have also been used in magnetic components for electronic devices such as saturable reactors, since they have low core loss and high squareness ratio in the high frequency region. However, the cost of Co-based alloys is comparatively high making such materials uneconomical. As explained above, although Fe-based amorphous alloys constitute cheap soft magnetic materials and have comparatively large magnetostriction, they suffer from various problems when used in the high frequency region and are inferior to Co-based amorphous alloys in respect of both core loss and permeability. On the other hand, although Co-based amorphous alloys have excellent magnetic properties, they are not industrially practical due to the high cost of such materials. Consequently, having regard to the above problems, the object of the present invention is to provide an Fe-based, soft magnetic alloy having high saturation magnetic flux density in the high frequency region, with attractive soft magnetic characteristics. The invention is characterized by providing alloys having fine crystal grains and a particular composition. According to the invention there is provided, an Fe-based, soft magnetic alloy having fine crystal grains, as described in formula (I) (Fe 1-a-b Cu a M b ) 100-c Z c ; wherein "M" is at least one element from the following: yttrium and rare earth elements, "Z" is at least one element from the following: Si, B, P, and C, and wherein "a" and "b" are as follows: 0.005 ≦ a ≦ 0.05 0.005 ≦ b ≦ 0.1 and "c", expressed in atomic % is as follows: 15 ≦ c ≦ 28. The area ratio of said fine grains in the range 5 nm to 30 nm (50 Å to 300 Å) is at least 30%. The term "area ratio" of the fine grains, as used herein means the ratio of the surface of the fine grains to the total surface in a plane of the alloy as measured, for example, by photomicrography or by microscopic examination of ground and polished specimens. Advantageously, at least 80% of the fine grains are in the range of 50 Å to 300 Å. Thus, a characteristic of the invention is that fine crystal grains are present in an alloy having the aforesaid composition. Preferred embodiments are now described in detail below. An alloy in accordance with the invention contains Fe, Cu, at least one of yttrium and rare earth element(s) and at least one of Si, B, P, and C, in accordance with the above definition. It is important that alloys according to the invention contain the aforementioned components in the amounts and proportions described in order to obtain the advantageous properties characteristic of the new alloy. For example, copper is an element that is effective in increasing corrosion resistance and preventing coarsening of the crystal grains, as well as in improving soft magnetic characteristics such as core loss and permeability. However, if the amount of Cu used is too small, the benefit of the addition is not obtained. On the other hand, if the amount of Cu used is too large, the magnetic properties are adversely affected. A range of 0.005 to 0.05 of Cu, preferably 0.01 to 0.04 has been found to be effective for the value of a in the above formula. At least one of yttrium and rare earth elements, "M", is required to improve soft magnetic characteristics such as reduced core loss, improved magnetic characteristics with respect to change of temperature, and to make the crystal grain size more uniform. However, if the amount of "M" used is too small, the benefit of the addition is not obtained. On the other hand, if the amount used is too large, the Curie temperature becomes low, adversely affecting the magnetic characteristics. A range of 0.005 to 0.1 is therefore selected for b in the above formula. Preferably the range is 0.01 to 0.08, and even more preferably 0.02 to 0.05. Combined addition of Cu and rare earth element(s) and/or yttrium results in the benefit that the magnetic characteristics with respect to temperature variation are improved. At least one of Si, B, P and C (designated "Z" in formula (I)) is effective in obtaining the amorphous condition of the alloy during manufacture, or in directly segregating fine crystals. If too little "Z" is used, the benefit of superquenching is lost, and the aforementioned condition is not obtained. On the other hand, if too much is used, the saturation magnetic flux density is lowered with the result that the aforesaid condition becomes difficult to obtain and superior magnetic properties are therefore not obtained. An amount of "Z" in the range 15 to 28 atomic % is therefore selected. Preferably the amount is 18 to 26 atomic %. It is also desirable that the ratio (Si, C) to (B, P) is preferably greater than 1. It is thus preferred that the atomic ratio(s) Si:B and/or C:P is > 1, whichever may be present. The Fe-based soft magnetic alloy of this invention may be obtained by the following method: An amorphous alloy thin strip is obtained by liquid quenching or from a quenched powder obtained by the atomizing method. The alloy is heat treated for from one minute to 10 hours, preferably 10 minutes to 5 hours at a temperature from 50C o below the crystallization temperature to 120C o above the crystallization temperature, preferably from 30C o below to 100C o above the crystallization temperature of the amorphous alloy, to segregate the required fine crystals. An alternative method of directly segregating the fine crystals is by controlling quenching rate in the liquid quenching method. As indicated previously, it is important that the alloy contain fine crystal grains. However, if the amount of fine crystal grains in the alloy of this invention is too small, i.e. if the amorphous phase is great there is a tendency toward increased deterioration of the magnetic properties on resin moulding, with resulting increased core loss, lower permeability, and increased magnetostriction. The amount of fine crystal grains in the alloy is at least 30% in terms of area ratio, preferably at least 40% and may be greater than 50%. It has also been determined that if the crystal grain size in these fine crystal grains is too small, the maximum improvement in magnetic properties is not obtained. On the other hand, if it is too large, the magnetic properties are adversely affected. It is therefore preferable that the proportion of crystal grains of grain size 50 Å to 300 Å should be at least 80%. Fe-based soft magnetic alloys of this invention can have excellent soft magnetic characteristics at high frequency. They can further have excellent characteristics for use in magnetic components such as magnetic cores for use at high frequency, for example in magnetic heads, thin film heads, high power radio frequency transformers, saturable reactors, common mode choke coils, normal mode choke coils, high voltage pulse noise filters, magnetic switches used in laser power sources, etc., and magnetic materials for various types of sensors, such as power source sensors, direction sensors, and security sensors. The invention will now be described and illustrated by way of non-limiting examples below. Examples 1 to 12 - General Procedure Amorphous alloy thin strips of strip thickness about 18 micron were obtained by the single roll method from alloys having atomic compositions shown in Table I. The amorphous alloy thin strips thus obtained were wound to form a toroidal magnetic core of external diameter 18 mm, internal diameter 12 mm, height 4.5 mm. Heat treatment was then performed for about 1 hour at a temperature of about 30C o higher than the crystallization temperature of each alloy (measured at rate of temperature rise of 10C o /minute). The toroidal magnetic cores produced were then used for measurement. Also, for comparison, magnetic cores were manufactured by carrying out heat treatment for about 1 hour at a temperature about 70C o lower than the crystallization temperature of the samples, on magnetic cores after the aforementioned winding. The ratio of fine crystal grains in the thin strips constituting the magnetic cores obtained, and the ratio of fine crystal grains of 50 Å to 300 Å in the aforesaid crystal grains are respectively shown as A and B (%) in Table I. Table I also shows the mean values obtained when the core loss, magnetostriction, permeability at 1 kHz, 2 mOe and saturation magnetic flux density were measured after heat treatment at B = 2 G, F= 100 kHz, using, respectively, 5 samples of magnetic cores of the invention in which fine crystal grains were present, and magnetic cores of the comparative examples in which fine crystal grains were not present. It is clear from Table I that, with the presence of fine crystals, the alloy of the invention shows excellent soft magnetic characteristics at high frequency, with low core loss, low magnetostriction and high permeability, compared to iron cores of thin strips of composition not having fine crystals. Furthermore, when these magnetic cores were subject to impregnation hardening by epoxy resin, the increased core loss of those cores having fine crystal grains and a composition according to the invention was in each case less than 5%. i.e., excellent magnetic properties were retained. In contrast, the core loss increase of magnetic cores produced using comparative alloys and amorphous alloy thin strips was about three times. Thus, the superior performance with this invention is particularly surprising. It is apparent from the foregoing examples that an Fe-based soft magnetic alloy having the desired alloy composition and fine crystal grains in accordance with the invention possesses excellent soft magnetic characteristics with high saturation magnetic flux density in the high frequency region.

Claims: An Fe-based, soft magnetic alloy having fine crystal grains, defined by formula (I): (Fe 1-a-b Cu a M b ) 100-c Z c ; (I) wherein "M" is at least one element from the following: yttrium and rare earth elements; "Z" is at least one element from the following: Si, B, P, and C; and wherein "a" and "b", are as follows: 0.005 ≦ a ≦ 0.05 0.005 ≦ b ≦ 0.1 and "c", expressed in atomic % is as follows: 15 ≦ c ≦ 28 and the area ratio of said fine grains in the range of 5 nm (50 Å) to 30 nm (300 Å) is at least 30%. An alloy according to claim 1 wherein at least 80% of said fine grains are in the range of 5-30 nm (50 Å to 300 Å). An alloy according to claim 1 or 2 wherein the ratio Si:B, and/or C:P is > 1. An alloy according to any preceding claim wherein "a" is 0.01 to 0.04. An alloy according to any preceding claim wherein "b" is 0.01 to 0.08. An alloy according to claim 5, wherein "b" is 0.02 to 0.05. An alloy according to any preceding claim wherein "c" is 18 to 26 Atomic %. A method of treating an Fe-based soft magnetic alloy according to any preceding claim which comprises heat treating said alloy for a period of from one minute to ten hours at a temperature from 50C° below the crystallization temperature to 120C° above the crystallization temperature to segregate fine crystal grains. A method according to claim 8 wherein said alloy is heat treated for a period of ten minutes to five hours.

Inventors: OKAMURA, MASAMI C/O INTELLECTUAL PROPERTY DIVISION, SAWA, TAKAO C/O INTELLECTUAL PROPERTY DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H01F1
IPC codes: H01F1, C22C45