A. Kojima^ & A. Inoue^ * Faculty of Systems Science and ... permeability at high frequency above 100...

Nanocomposite structure of soft magnetic

Fe-M-B (M = Zr, Hf, Nb), Fe-M-O (M = Zr, Hf,

rare earth) materials

T. Bitoh*, A. Makino\ A. Kojima^ & A. Inoue^* Faculty of Systems Science and Technology,Akita Prefectural University, Japan.Central Research Laboratory, Alps Electric Co., Ltd., Japan.Institute for Materials Research, Tohoku University, Japan.

Abstract

New nanocomposite structure in Fe-M-B (M = Zr, Hf, Nb) and Fe-M-O (M = Zr,Hf, rare earth) systems has been fabricated by use of partial crystallization ofmelt-spun amorphous phase and sputtering-induced partial crystallization, re-spectively. The Fe-M-B ribbons exhibit high magnetic flux density more than 1.5T and good soft magnetic properties. On the other hand, the Fe-M-O films showhigh permeability at high frequency above 100 MHz because of their muchhigher electrical resistively than those of other conventional soft magnetic alloyfilms. These materials have a mixed structure of nanoscale a-Fe grains and anintergranular amorphous thin layer. The grain sizes are 10-15 nm for the Fe-M-Bsystem, and less than 10 nm for the Fe-M-O system, respectively. The inter-granular amorphous phases contain several times higher amounts of M, B and M,O, respectively, than those of the a-Fe nanograins. The good soft magnetic prop-erties should be achieved mainly by the achievement of the decreased apparentmagnetocrystalline anisotropy caused by the nanoscale a-Fe grains and strongferromagnetic exchange coupling between the grains. The intergranular amor-phous phases play an important role in achieving the good soft magnetic proper-ties. The amorphous phases have high thermal stability and high Curie tempera-ture, and suppress the grain growth and mediate the ferromagnetic exchangecoupling between the grains. It can be concluded that the nanocomposite struc-ture is a dominant factor for achieving good soft magnetic properties in both thematerials.

Advances in Composite Materials and Structures VII, C.A. Brebbia, W.R. Blain & W.P. De Wilde (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-825-2

04 Advances in Composite Materials and Structures VII

1 Introduction

Since amorphous phases in Pd-Si, Fe-P-C and (Fe, Co, Ni)-Si-B systems havebeen found to be useful as a precursor to prepare a nanocomposite structure uponcrystallization [1], a great effort has been devoted to the development of newtype of high-functional or high-strength materials by utilizing the formation ofcrystallization-induced nanostructure. In a soft magnetic field, it has been foundin 1988 that the crystallization of Fe-Si-B amorphous alloys containing Nb andCu causes the formation of a nanoscale bcc Fe-Si structure and the bcc Fe-Sialloys exhibit good soft magnetic properties of 1.2-1.4 T for saturation magneticflux density (8$) and high permeability (#,) [2]. The performance of the softmagnetic materials is evaluated mainly by B$ (which indicates the possiblemaximum magnetic energy) and & (which is sharpness of response to magneticfield). Although the good soft magnetic properties are obtained for the nanoscalebcc Feyg^Sio^BgNbgCui alloy, the relatively low Fe concentration leads to thelimitation of 8$ less than 1.4 T. Hence the development of a new soft magneticalloy with high 8$ above 1.5 T has strongly been desired because the simultane-ous achievement of both properties enables the extension of application fields.

We tried to synthesize a nanocomposite soft magnetic material with high B^and have developed nanocomposite Fe-M-B (M = Zr, Hf, Nb) alloys with high B*more than 1.5 T as well as good soft magnetic properties [3]. The formation ofamorphous phases in Fe-metalloid systems such as Fe-Si-B and Fe-P-C alloys bythe melt-spinning method usually occurs in a range of 70-84 at% Fe [4]. On theother hand, amorphous phases have been obtained in the range of 88-91 at% Fefor Fe-transition metal systems such as Fe-Zr and Fe-Hf [5] alloys. In addition, ithas been reported that the addition of a small amount of B to Fe-Zr and Fe-Hfalloys extends the glass formation range [6]. The high volume fraction of thea-Fe phase has been obtained by crystallizing the amorphous Fe-M-B alloyswith the high Fe concentration.

Then we have found that dissolution of a large amount of oxygen into theremaining amorphous phase is effective for a drastic increase in electrical resis-tivity (p) of sputtered Fe-Hf-O, Fe-Zr-O and Fe-(rare earth elements)-O films,leading to the achievement of good high-frequency permeability in the high fre-quency range of 1-1000 MHz [7]. In this paper, we report our recent experimen-tal results on two kinds of the nanocomposite soft magnetic materials.

2 Experimental procedure

The Fe-M-B alloys "NANOPERM" with 15 mm in width and 15-25 jim inthickness were obtained by crystallizing an amorphous phase produced by singleroller melt-spinning method. The crystallization was carried out by keeping thesamples at 748-1073 K for 60-3600 s in an evacuated state. The Fe-M-O filmswith 2-3 p,m in thickness were deposited onto indirectly water-cooled glass sub-strates by an rf reactive sputtering technique in a mixed atmosphere of pure ar-


Advances in Composite Materials and Structures VII 35

gon and oxygen. The deposition was performed in no magnetic field or a staticmagnetic field to induce in-plane uniaxial magnetic anisotropy. The films weresubjected to annealing for 10.8x10 s under a uniaxial magnetic field of 160kA/m at lower temperatures than crystallization temperature of an amorphousphase in an evacuated atmosphere.

3 Results and discussion

3.1 Microstructure

Figures l(a) and 2(a) show high-resolution transmission electron microscopy(HRTEM) images for the crystallized Feg^Nb^Bg alloy and the as-depositedFe55HfnOs4 film. The selected area electron diffraction (SAED) patterns takenfrom the regions 1 and 2 for the Fe Hf Og film are shown in figs. 2(b) and 2(c),respectively. The SAED pattern of region 2 for the Fe-Hf-O shows a halo patternand clarifies the region is an amorphous phase. For the Fe-Nb-B, the SAED pat-tern is not given here, it is confirmed that the region 2 has an amorphous struc-ture. The HRTEM images reveal a detailed structure consisting of a mixture ofthe nearly spherical a-Fe grains with sizes of about 10 nm for the Fe-Nb-B andof less than 10 nm for the Fe-Hf-O, respectively, and the amorphous region lessthan 5 nm wide surrounding the grains.

2001o 100

n

. (b) Region 1 EDSBeem: 0.6 nm Fe

COFe I

¥... I. I

. (c) Region 2 EDSBeem:d>0.6nm

COFe

icV, I . . '

100 150 200 250 300 50 100 150 200 250 300Energy loss, E I eV

Figure 1: HRTEM image, EDS and EELS spectra taken from a-Fe phase (re-gion 1) and amorphous phase (region 2) for Fe^Nb^Bg alloy annealedat 923 K for 3.6 ks.


36 Advances in Composite Materials and Structures VII

Figure 2: HRTEM image, SAED patterns and EDS spectra taken from a-Fephase (region 1) and amorphous phase (region 2) for as-deposited

film.

The composition for the a-Fe and the intergranular amorphous matrix is ex-amined by energy-dispersive spectroscopy (EDS) and electron energy loss spec-troscopy (EELS) analysis. Figures l(b) and 2(d) show the EDS spectra of thea-Fe grain and figs. l(c) and 2(e) show those of the intergranular amorphousphases. Figures l(d) and l(e) show the EELS spectra of the a-Fe grain and of theintergranular amorphous phase, respectively. The EDS and EELS spectra for theFe-M-B and the Fe-M-O reveal that the concentrations of M, B or M, O in theintergranular amorphous phases are considerably higher than those in the a-Fegrains.

Figure 3 schematically shows the structure of (a) crystallized the F-M-B rib-bon (NANOPERM) and (b) the as-deposited Fe-M-O film. Both the materials

(a) Fe-M-B ribbon (NANOPERM) (b) Fe-M-O filma-Fe grains

10nm 10nmAmorphous matrix

Figure 3: Schematic illustration of microstructure for nanocomposite (a)Fe-M-B ribbon (NANOPERM) and (b) Fe-M-O film.



Table 1. Grain size (D), saturation magnetic flux density (BJ, permeabilitycoercivity (#J and core loss (W) of nanocomposite Fe-M-B alloy rib-bons (NANOPERM) and conventional Fe-Si-B amorphous alloy ribbon.

D(nm)

FP» 7r MK R 1185 5 2 4 8 5 1 1

FeygSigB^ (amor.)* 1 kHz, 0.4 A/m**50Hz, 1.4 T

(T)1.70

1.52

1.64

1.56

*

30

51

60

10

*e

000

000

000

000

H(A/5.

4.

3.

3.

cm)8

8

0

5

(W/kg)0.21

0.14

0.09

0.28

have a similar nanocomposite structure composed of the nanoscale a-Fe grainsand the intergranular amorphous matrix with the several times higher amounts ofM, B and M, O, respectively, than those of the a-Fe nanograins.

3.2 Soft magnetic properties

The magnetic properties of the Fe-M-B ribbons "NANOPERM" are summarizedin Table 1. The NANOPERM exhibits high B, of 1.5-1.7 T, high permeability(#.) at 1kHz of 30,000-60,000 and low core losses (W) at 1.4 T and 50 Hz of0.09-0.21 W/kg. It is to be noted that these values are superior to those of a con-ventional FejgSigBig amorphous alloy ribbon. The relationship between B$ and ^at 1kHz for the NANOPERM and the conventional soft magnetic materials isshown in fig. 4. The NANOPERM is found to be situated in the most right and

_ 10*

g 10"

10*

NanocompositeFe-M-B alloys

Fe-AI-Si "NANOPERM"alloys

. Ni-Fe alloys

2.50.5 1.0 1.5 2.0Saturation magnetic flux density, B$ I T

Figure 4: Relationship between 8$ and at 1 kHz for nanocomposite Fe-M-Balloy ribbons (NANOPERM) and conventional soft magnetic materi-als.


38 Advances in Composite Materials and Structures III

Table 2. Saturation magnetic flux density (BJ, coercivity (H , electrical resistiv-ity (p) and structure of nanocomposite Fe-M-O films and conventionalsoft magnetic films.

mCa uf O i orcggfiljjL/24 l.Z.

C--O44 jFeig iHfi4 5022.1 1.1

FeagYzzOio 0.9

*• 72' 10 18 1.1vrj Cc& f\ Qliloii^CiQ U.oCogsTanHffi 0.8

side among all the materials.)PERM is best material for the acti

HelA/m)15473185

211207322

402433

Thereforlievement

P(uQm)

9.16.3

15.1

6.622.69.3

1.30.21.7

e, it canof high Bg

Struct

amor.amor.amor.

amor.amor.amor.

bccfeeamorf

beand

ure

+ bcc+ bcc+ bcc

+ bcc+ bcc+ bcc

)hous

saidhi%hf

that the*e, simul-

upperNANtaneously

The magnetic properties and structure of the Fe-M-O films are summarizedin Table 2. Figure 5 shows the frequency dependence of the real part of perme-ability (X) and the quality factor (Q = ju'/ //", then //" is the complex part of

10000

10 100Frequency, f I MHz

1000

Figure 5: Frequency dependence of ju' and Q for nanocomposite Fe-Hf-O andCo-Fe-Hf-O films compared with conventional soft magnetic films.


Advances in Composite Materials and Structures 171 gg

permeability that related to the energy loss) for the Fe-M-0 films. As the ordi-nary metallic films such as Fe-Al-Si, Ni-Fe and Co based amorphous alloys havelow electrical resistively smaller than 2 jiQm, drastically decreases at the fre-quency below 100 MHz where an eddy current loss become relatively large. Onthe contrary, nanocomposite Fe-M-O films with high p larger than 6 p,Om, asshown in Table 2, exhibit superior n frequency characteristics, and moreover, Qof the films is much higher than those of the ordinary ones.

3.3 Theoretical model

The theoretical model (so-called random anisotropy model) for the soft magneticmaterials with the nanocomposite structure was proposed by Herzer [8, 9]. Themagnetic properties of an assembly of small crystalline grains strongly dependon the counterplay of local magnetic anisotropy energy and ferromagnetic ex-change energy. For vary small grains, the ferromagnetic exchange interactionincreasingly forces the magnetic moments to align parallel, thus, impeding themagnetization to follow the easy directions of each individual grain. Conse-quently, the effective anisotropy for the magnetic behavior is an average overseveral grains, thus, reduced in magnitude

The basic idea of the random anisotropy model starts form the assembly offerromagnetically coupled grains of size D with the magnetocrystalline anisot-ropy constant KI oriented at random. The effective anisotropy affecting themagnetization process results from averaging over the N = (L /D) grains withinthe volume V = L^ of the ferromagnetic exchange length. The exchangelength is given by

(1)

where A is the exchange stiffness constant. For a finite number (N) of grainsthere will always be some easiest direction determined by statistical fluctuations.Consequently, the resulting anisotropy density ( (K} ) is determined by the meanfluctuation amplitude of the anisotropy energy, i.e.,

_

#

The most significant feature of the above analysis is the strong variation ofwith D*. It is kmportional to (K)mentally [9, 11].

with D*. It is known that coercivity (H^ is proportional to (K}, and is pro-portional to (K)~ [10]. The D^ dependence of H<. had been confirmed experi-


4Q Advances in Composite Materials and Structures III

3.4 Role of intergranular amorphous matrix

As described above, it is predicted that the soft magnetic properties of the nano-composite materials are improved with decreasing D. However, the experimentalresults are more complicate. As shown in Table 1, the nanocompositeFegs.sZ l Bg.s alloy exhibits the highest fa and lowest H^. However, D of theFegg^Z^NbdBgj alloy is intermediate between those of the FegoZ Bg and theFeg^Nb^Bg alloys. Furthermore, an interesting effect of the low temperature agingon the soft magnetic properties for the nanocomposite Fe-Zr-Nb-B alloys hasbeen found. Figure 6 shows the change in H^ of the crystallized Feg Z l Bg.salloy as a function of the aging time (). The aging temperature (T ) is 593 K,which is far below the crystallization temperature (« 763 K) of the alloy. The Hdecreases gradually with increasing t^ and exhibits a minimum around ^ «10 -10 s. This behavior cannot be explained the change of D because D shouldbe slightly increased monstrously with increasing f%. These results suggest thatthe properties of the intergranular amorphous phase should be emphasized.

The nanocomposite soft magnetic Fe-M-B and Fe-M-O materials consist ofthe nanoscale a-Fe grains embedded in the intergranular amorphous matrix withthe large amounts of M, B or M, O. The intergranular coupling in the materials ismediated by the intergranular amorphous matrix. If the Curie temperature (T ) ofthe amorphous matrix is low, it cannot fully mediate the intergranular couplingowing to the thermal fluctuation of the magnetic moments in the amorphous ma-trix. Therefore, both the small D and the high 7^ of the amorphous matrix arenecessary to obtain the good soft magnetic properties. The intergranular amor-phous matrix contains the large amounts of solute elements and exhibits high 7%.Figure 7 shows the annealing temperature dependence of TC on the amorphousmatrix and fa of the typical nanocomposite Fe-M-B alloys. The Fegs.sZ r Bg.salloy exhibits highest (amor.) [12]. This is a reason why the Fe-Zr-Nb-B alloyshows the best soft magnetic properties. Furthermore, it is confirmed that theintergranular amorphous matrix has high thermal stability and suppresses thegrain growth [11].

c_ 4E

J a'

|0)oU o

Aging time, f^ / h) 1 10 10*

\\ | 1 1 1 1 MM) 1 1 1 1 Illljaged at 593 Kin air

\ \ i i 1 1 1 ul i i i 1 1 1 ni i iii

,0»

:

-

10* 10^ 10^Aging time, / s

Figure 6: Change in HC as a function of aging time (^) at 593 K in air for nano-composite Feg5.sZr2Nb4Bg 5 alloy.



| amorphous /a-Fe + amrphous^^ o-k

as-Q 700 800 900 1000Annealing temperature, 7" / K

1100

Figure 7: Change in TC of amorphous matrix and as a function of annealingtemperature (7%) for nanocomposite Fe oZr Bg, Feg^Nb^Bg andFe85.5Zr2Nb4Bs.5 alloys. Data of annealed structure are also shown.

For the Fe-M-O film, oxygen preferentially combines with M element [13].The chemical combination O with the M element, which is mainly contained inthe amorphous matrix, increases p of the amorphous matrix. The high p of theFe-M-O films results from the existence of the amorphous matrix with largeamounts of M and O elements [7,13].

4 Conclusion and further works

We have developed the new type of nanocomposite soft magnetic materials inthe Fe-M-B (M = Zr, Hf, Nb) ribbons "NANOPERM" and the Fe-M-O (M = Zr,Hf, rare earth) films. The NANOPERM shows high B* above 1.5 T as well asgood soft magnetic properties. The Fe-M-O films with high p exhibit goodhigh-frequency permeability in the high frequency range of 1-100 MHz. TheNANOPERM and the Fe-M-O films are expected to be used widely in thesemagnetic application fields.

As mentioned above, the soft magnetic properties of the nanocomposite ma-terials are strongly related to the nanocomposite structure of the materials. How-ever, several open questions are remained. Detailed theoretical studies are nec-essary to obtain a useful guideline for the development of the nanocompositematerials with the superior soft magnetic properties. We believe the computermethod will reveal the detailed relationship between the nanocomposite structure(the grain size and their distribution, the magnetic and electrical properties of theintergranular amorphous matrix, etc.) and the soft magnetic properties, and willmake a great contribution to development of new nanocomposite materials withthe superior soft magnetic properties.


A 2 Advances in Composite Materials and Structures VI'I

References

[1] Masumoto T., Waseda Y., Kimura H. & Inoue A., Thermal instability andcrystallization characteristics of amorphous meta-metalloid system, TheScience Reports on the Research Institute ofTohoku University, A26(2), pp.21-35, 1976

[2] Yoshizawa Y, Oguma S. & Yamauchi K., New Fe-based soft magnetic al-loys composed of ultrafine gain structure, Journal of Applied Physics,64(10), pp. 6044-6046,1988.

[3] Makino A., Suzuki K., Inoue A., Hirotsu Y. & Masumoto T., Magneticproperties and microstructure of nanocrystalline bcc Fe-M-B (M = Zr, Hf,Nb) alloys, Journal of Magnetism & Magnetic Materials, 133, pp. 329-333,1994.

[4] Masumoto T., Suzuki K., Fujimori Y. & Hashimoto K., Materials Science ofAmorphous Metals, Ohmu: Tokyo, pp. 281-283, 1982.

[5] Inoue A. Kobayashi K. & Masumoto T., Mechanical properties and thermalstability of Hf-poor (Fe, Co, Ni)-Hf binary amorphous alloys, Proc. of Conf.on Metallic Glasses, Science & Technology, eds. C. Hargitai et al., CentralResearch Institute of Physics: Budapest, vol. II, pp. 217-222, 1980.

[6] Ohnuma S., Nose M., Shirakawa K., & Masumoto T., Magnetic propertiesof amorphous Fe-Zr-B alloys, The Science Reports on the Research Insti-tute ofTohoku University, A29(2), pp. 254-264, 1981.

[7] Makino A. & Hayakawa Y, Sputtered Fe-Hf-0 films with high electricalresistivity and good soft magnetic properties, Materials Science & Engi-neering, A181/A182, pp. 1020-1024, 1994.

[8] Herzer G., Grain structure and magnetism of nanocrystalline ferromagnets,IEEE Transactions on Magnetics, 25(5), pp. 3327-3329, 1989.

[9] Herzer G., Grain size dependence of coercivity and permeability innanocrystalline ferromagnets, IEEE Transactions on Magnetics, 26(5), pp.1397-1402, 1990.

[10] Bozorth R.M., Ferromagnetism, D. Van Nostrand: Princeton, ch. 18, pp.811-837, 1951.

[11] Makino A., Inoue A. & Masumoto T., Nanocrystalline soft magneticFe-M-B (M = Zr, Hf, Nb) alloys produced by crystallization of amorphousphase, Materials Transaction, JIM, 36(7), pp. 924-938, 1995.

[12] Makino A., Bitoh T., Kojima A. Inoue A. & Masumoto T, Magnetic prop-erties of zero-magnetostrictive nanocrystalline Fe-Zr-Nb-B alloys with highmagnetic induction, Journal of Magnetism & Magnetic Materials, 2000, inpress.

[13] Hayakawa Y, Makino A., Fujimori F. & Inoue A., High resistive nanocrys-talline Fe-M-O (M = Hf, Zr, rare-earth metals) soft magnetic films forhigh-frequency applications, Journal of Applied Physics, 81(8), pp.3747-3752,1997.


A. Kojima^ & A. Inoue^ * Faculty of Systems Science and ... permeability at high frequency above 100...

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