Recent Development of Soft Magnetic Materials. K. I. ARAI õ and K. ISHIYAMA õ ABSTRACT

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1 Int. J. of The Soc. of Mat. Eng. for Resources Vol. 4 No `95 (1996) Review Recent Development of Soft Magnetic Materials by K. I. ARAI õ and K. ISHIYAMA õ ABSTRACT Three large topics for recent soft magnetic materials, such as ultra thin grain oriented silicon steels, nanocrystalline magnetic materials and 6.5wt% Si- Fe alloy sheets are reported. The ultra thin grain oriented silicon steels are known to have extremely low iron loss. Lately, it is reported the domain width of the ultra thin grain oriented silicon steels can be controlled by changing grain size. By this method very low iron loss can be realized without additional domain refining technique. The nanocrystaline magnetic materials are made by recrystallizing the amorphous materials. Since the effective magnetic anisotropy of the material is small, the nanocrystaline materials have fine soft magnetic properties. The 6.5wt% Si-Fe alloy has properties of small magnetostriction and poor ductility. Recently the sheets are commercially developed and used for high frequency transformers. Key Words: Ultra thin grain oriented silicon steel sheet, Nanocrystalline magnetic material, 6.5 wt% Si-Fe alloy sheet INTRODUCTION To realize a good soft magnetic materials, it is need to have higher magnetic induction, larger permeability, lower iron loss, lower magnetostriction, controlled magnetic anisotropy, and so on. However these factors does not stand together. For high magnetic induction, iron based materials are used. However, the materials have large magnetic anisotropy because of the large crystalline anisotropy constant of iron. Two methods to reduce the magnetic anisotropy of iron based materials are investigated. One method is based upon controlling grain-orientation, while an other technique consists in reducing the grain size to nm order scale [1] [2]. A typical material obtained through the former method is grain oriented silicon steels, while nanocrystalline magnetic materials are an example of the latter method's results. On the other hand, the magnetostriction is also important properties for soft magnetic materials. A 6.5wt% Si-Fe alloy is known to have zero magnetostriction [3]. Therefore using the 6.5wt% Si-Fe Received July 12, 1994 Research Institute õ of Electrical Communication, Tohoku University. Sendai , JAPAN 89

2 90 K. I. ARAI and K. ISHIYAMA Table 1. Magnetic properties of conventional grain oriented silicon steel, amorphous sheet and ultra thin grain oriented silicon steels [6]. alloy we can obtain a soft magnetic material with very small magnetosrtiction. In this paper, we report recent developments of soft magnetic materials, particularly the ultra thin grain oriented silicon steels, the nanocrystalline magnetic materials and the 6.5wt% Si-Fe alloy sheets. ULTRA THIN GRAIN ORIENTED SILICON STEELS One method to reduce the effective magnetic anisotropy is controlling the grain orientation. The materials having grains which easy axis oriented to one direction have large permeability to the direction of the grain orientation. The typical material of this theory is grain oriented 3wt% silicon steel Fig. 1 Thickness dependence of DC magnetic properties of ultra thin grain oriented silicon steels [8]. Fig. 2 Frequency dependence of iron loss of ultra thin grain oriented silicon steels and Fe based amorphous ribbon [9].

3 Vol. 4 No. 1 (1996) Recent Development of Soft Magnetic Materials 91 that has (110) [001] texture [4]. This material is widely used as the magnetic core material for transformers. Recently, a method to obtain ultra thin and highly oriented grain oriented silicon steels were investigated [5] [6]. Ultra thin grain oriented silicon steels can be made by cold rolling and annealing the conventional grain oriented silicon steels. The motive force of the selective grain growth of (110) [001] is a difference of surface energy between (110) and other planes [7] [8]. By this method, the ultra thin grain oriented silicon steels can be obtained with the thickness from 5 ft m to 120 ft m. Fig. 1 shows DC magnetic properties of the ultra thin grain oriented silicon steels. The magnetic induction at 800A/m of the sheets are over 1.9T, and the coercive forces are several A/m in every thickness.the iron losses of ultra thin grain oriented silicon steel sheets with domain refining treatment are extremely low [9] [10]. Table 1 shows the magnetic properties of the conventional grain oriented silicon steels, iron based amorphous ribbon and ultra thin grain oriented silicon steels. The iron losses of 3 2ƒÊm thick grain oriented silicon steel were as follows; W13/50 = 0.13 W/kg, W17/5o = 0.21 W/kg. These values are almost 1/5 of the conventional grain oriented silicon steels. Fig. 2 shows the frequency dependence of the iron losses of the ultra thin grain oriented silicon steels and the iron based amorphous ribbon (2605S2, 28ƒÊm). The iron losses of domain refined ultra thin grain oriented silicon steel is almost same or lower than that of the iron based amorphous ribbon. It was reported that the grain size of the ultra thin grain oriented silicon steels can be controlled by changing the conditions of the cold rolling and the annealing [111. It was discovered that decreasing the grain size increased the number of the magnetic domain walls. Therefore the eddy current losses decreasd with decreasing the grain size. On the other hand, the hysteresis losses increased with decreasing the grain size. Therefore the iron losses of the ultra thin grain oriented silicon steel sheets can be minimized selecting an optimun grain size, because the iron loss is the sum of the hysteresis loss and the eddy current loss. Fig.3 shows the grain size dependence of the iron losses, the hysteresis losses and the eddy current losses at a frequency of 50Hz. The iron loss of the sheet is minimized at the grain size of 0.8mm. Fig. 3 Grain size dependence of iron loss of ultra thin grain oriented silicon steels [11].

4 92 K. I. ARAI and K. ISHIYAMA Table 2. Magnetic properties of nanocrystalline soft magnetic materials. (Ta: annealing temperature for recrystallization) The ultra thin grain oriented silicon steels still have some problems for mass production. largest problems are about coating and impurity. To increase the packing density of the core of the transformers, the coatings of the ultra thin silicon steels need to be thin, flat and well-insulated. Therefore the development of new coating material or system must be needed. In addition, it is re ported that the contents of impurities have large effect for annealing time for obtaining the ultra thin grain oriented silicon steels [8]. With reducing the impurities of Cu from 600 ppm to 5ppm, the required annealing time becomes shorter from 5 hours to 10 minutes. Therefore high purity grain oriented silicon steels for starting material are required. However the conventional grain oriented silicon steels have impurities for obtaining the (110) [001] texture as inhibiter. Therefore it is need to purification process, but the process will not fit to mass produce. The NANOCRYSTALLINE MAGNETIC MATERIALS Amorphous Fe-Si-B-Cu-Nb ribbons made by rapid quenching method were reported to show excel lent soft magnetic properties after an annealing in optimum condition [12]. By the annealing, the sheets were recrystallized and the size of the grains was about 10nm. The fine grains were obtained by the inhomogeneity of the composition in the material. In this material, Fe-Si fine grains were surrounded by an amorphous phase enriched in B and Nb that has high recrystallization temperature [13] [14]. This material called Finemet has large permeability, relatively high magnetic induction, and low iron losses. Fig.4 shows the frequency dependence of the permeability. From this figure, it is real ized the Finemet has excellent soft magnetics. However mechanical properties of this material are worse because of its brittleness. In addition, this material has poor thermal stability because Fe3 B and other grains are recrystallized in higher temperature. After the development of Finemet, another iron based nanocrystalline soft magnetic materials are investigated not only ribbon form but thin films. Thin film nanocrystalline materials in Fe-M-C (M=Zr, Hf, Ta, etc.) systems are studied [15] [16]. These materials have large saturation magneti zation and large permeability at high frequency. In this system, crystalline M-C particles inhibit the grain growth of Fe. Table 2 shows magnetic properties of nanocrystalline soft magnetic materials.

5 Vol. 4 No. 1 (1996) Recent Development of Soft Magnetic Materials 93 Fig. 4 Frequency dependence of permeability for Finemet and other materials [12]. Fig. 5 Induction dependence of transformer noise [261. The studies of the nanocrystalline materials were mainly for ribbon form or thin film materials. To obtain a bulk form nanocrystalline material, an extruded alloy using a Fe-Nb-B powder was investigated [20]. This material is reported to have 64A/m of He and 1.5T of Bs. The theories for the origin of the soft magnetic properties of the nanocrystalline materials are studied. It is generally predicted that decreasing the grain size makes the coercive force large and makes the soft magnetic properties worse [21]. However, if the grains are very fine, the material shows excellent soft magnetic properties. There are two theories for the soft magnetic properties of nanocrystalline materials. One is ripple theory [1] [2]. The other is random anisotropy model [22] [23]. These theories say that the magnetic coupling energy between the grains makes the effective magnetic anisotropy small if the grain size is very fine. Using these models, the soft magnetic properties of the nanocrystalline materials were explained. The nanocrystalline soft materials show excellent properties at high frequency. However these materials have problems to be solved such as poor thermal stability, mechanically brittleness and low saturation magnetization. 6.5wt% Si-Fe ALLOYS In the Si-Fe alloy system, the magnetostriction goes to zero at the composition of 6.5wt% Si-Fe [3]. In addition, the magnetocrystalline anisotropy constant is about 1/2 comparing the 3wt% Si-Fe alloy. Therefore the 6.5wt% Si-Fe alloy has good soft magnetic properties. However the alloy was not commercially produced because it is very hard and brittle. In 1988, a commercial scale production of 6.5wt% Si-Fe alloy sheets were developed [24]. The sheets with thickness range from 0.1 to 0.5 mm were menufactured by a rolling process or a chemical vapor deposition (CVD) according to the thickness. Using the sheets, stamping is essentially

6 94 K. I. ARAI and K. ISHIYAMA Table 3. Comparison of magnetic properties between 6.5%Si-Fe and conventional silicon steels [25]. possible. Table 3 shows the comparison of magnetic properties between the 6.5wt% Si-Fe and the conventional silicon steels [25]. The iron loss of 6.5wt% Si-Fe is smaller than that of conventional nonoriented silicon steels. In frequency range of over 400Hz, it is lower than that of grain oriented silicon steels. The reason of the low iron loss is the high resistivity of 82 Lt Q cm comparing that of the conventional 3wt% silicon steels. In addition, low noise transformers can be made by using the 6.5wt% Si-Fe alloy sheets. Fig. 5 shows the induction dependence of the noise of a trial transformer made by 6.5wt% Si-Fe alloy sheets and the conventional silicon steels [26] Because the 6.5wt% Si-Fe alloy sheets have low magnetostriction (0.6x10-0, the noise of the transformer made of the 6.5wt% Si-Fe is lower than that made of conventional silicon stells. However, the transformers made of the 6.5wt% Si-Fe alloy will become larger than that made of conventional silicon steels because the saturation magnetizaition of the 6.5wt% Si-Fe is low (1.8T). Therefore the 6.5wt% Si-Fe will be used for cores of special transformers such as for low noise or for high frequency. CONCLUSION We report about three soft magnetic materials newly developed. These materials have excellent magnetic properties, but have some problems to be solved. The ultra thin grain oriented silicon steels have highly oriented (110) [001] texture and show the extremely low iron losses at frequencies of 50Hz-lkHz. Therefore they are promising materials for the core material of the transformers in the commercial ot higher frequency. However there are the problems about the coating and the impurity of the sheets to be clarified for mass production. The nanocrystalline soft materials have the properties of large permeability, low iron loss and small magnetostriction. However the low saturation magnetizaition and the poor mechanically properties of the materials need to be improved. The 6.5wt% Si-Fe alloy has the lower magnetostriction, the lower anisotropy constant and the higher resistance than 3wt% Si-Fe. Therefore the transformers made of the 6.5wt% Si-Fe alloy will have the low noise and the low loss in higher than commercial frequencies. However since the

7 Vol. 4 No. 1 (1996) Recent Development of Soft Magnetic Materials 95 saturation magnetizaition of the alloy is low, cautions for application is needed. References [1] H. Hoffmann: IEEE Trans. Magn., MAG-4 (1968), 32. [2] H. Hoffmann: IEEE Trans. Magn., MAG-9 (1973), 17. [3] R. M. Bozorth: ferromagnetism (Van Nostrand, Amsterdam, 1951), 30. [4] N. P. Goss: U. S. Patent (1934). [5] K. I. Arai and K. Ishiyama: J. Appl. Phys., 64 (1988), [6] K. I. Arai, K. Ishiyama and H. Mogi: IEEE Trans. Magn., 25 (1989), [7] G. A. Wiener: J. Appl. Phys., 35 (1964), 856. [8] K. Ishiyama, K. I. Arai and T. Honda: J. Appl. Phys., 70 (1991), [9] K. I. Arai, H. Mogi and K. Ishiyama: IEEE Trans. Magn., 26 (1990), [10] K. I. Arai, H. Satoh, S. Agatsuma and K. Ishiyama: IEEE Trans. Magn., 26 (1991), [11] Y. H. Kim, M. Ohkawa, K. Ishiyama and K. I. Arai: IEEE Trans. Magn., 28 (1993), [12] Y. Yoshizawa, S. Oguma and K. Yamauchi: J. Appl. Phys., 64 (1988), [13] K. Hiraga and 0. Kohmoto: Mater. Trans. JIM, 11 (1991), 868. [14] K. Hono, K. Hiraga, Q. Wang, A. Inoue and T. Sakurai: Acta. Metall. Matter, 40 (1992), [15] N. Hasegawa and M. Saito: J. Magn. Soc. Jpn., 14 (1990), 313. (in japanese) [16] N. Hasegawa and M. Saito: IEEE TJMJ 6 (1991), 91. [17] K. Suzuki, N. Kataoka, A. Inoue, A. Makino and T. Masumoto: Marter. Trans. JIM., 31 (1990), 743. [18] K. Suzuki, A. Makino, N. Kataoka, A. Inoue and T. Masumoto: Marer. Trans. JIM., 32 (1991), 93. [19] A. Makino, K. Suzuki, A. Inoue and T. Masumoto: Mater. Trans. JIM., 33 (1992), 80. [20] A. Kojima, H. Horikiri, Y. Kawamura, A. Makino, A. Inoue and T. Masumoto: Digests of the 17th Annual Conference on Magnetics in Japan, (1993), 13pB-2. (in Japanese) [21] M. Kersten: Z. Phys., 44 (1943), 63. [22] G. Herzer: IEEE Trans. Magn., 25 (1989), [23] G. Herzer: IEEE Trans. Magn., 26 (1990), [24] Y. Takada, M. Abe, S. Masuda and J. Inagaki: J. Appl. Phys., 64 (1988) [25] Y. Tanaka, M. Abe and F. Fujita: MRS Int'l. Mtg. on Adv. Mats. 11 (1989),199. [26] H. Ninomiya, K. Turu, A. Hiura, Y. Tanaka, Y. Takada and Y. Masuda: J. Magn. Soc. Jpn., 14 (1990), 201. (in Japanese) [27] Y. Tanaka, Y. Takada, M. Abe and S. Masuda: J. Mag. Mag. Mat., 83 (1990), 375.