Copyright ©2023 by the Magnetics Society of Japan.
This article is licensed under the Creative Commons Attribution International License (CC BY 4.0)
http://creativecommons.org/licenses/by/4.0/

T. Magn. Soc. Jpn. (Special Issues)., 8, 1-5 (2024)<Paper>FeCo-based FeCoBPSiCr Amorphous Alloy and Powder
with High Saturation Magnetic Flux Density and Corrosion ResistanceY. Kajiura, A. Hasegawa, M. Hosono, K. Yoshidome, S. Otsuka *, H. Ohkubo* and H. Matsumoto
Materials Research Center, TDK Corporation, 656-1 Minamihadori, Narita-shi, Chiba 286-0805, Japan*Magnetics Business Group, TDK Corporation,

1-14-38 Takarada, Tsuruoka-shi, Yamagata 997-0011, Japan

Soft magnetic materials are required to obtain high saturation magnetic flux density (Bs) and corrosion resistancefor suppressing the magnetic saturation and supporting usage environments in power supplies of information andcommunication technology (ICT) devices with large current driving. This study investigated the magnetic
characteristics, amorphous stability, and corrosion behavior of (Fe0.7Co0.3)88−x−yB11PxSiyCr1 and conventional
Fe79Si6B13C2 amorphous alloys. A metallic ribbon comprising (Fe0.7Co0.3)82B11P4Si2Cr1 exhibited high Bs of 1.71 T andamorphous forming ability of 87 μm, sufficient to obtain a single amorphous phase even in powder morphology. The(Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy ribbon with a high corrosion potential of −520 mV showed better corrosion
resistance than conventional Fe79Si6B13C2 amorphous alloy with a low corrosion potential of −677 mV in water
immersion tests. The (Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy powder comprised a single amorphous phase andexhibited high Bs of 1.61 T, the same as that of the conventional Fe79Si6B13C2 amorphous alloy powder. Furthermore,
an inductor using the (Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy powder exhibited high DC-bias characteristics andexcellent corrosion resistance compared to that using the conventional Fe79Si6B13C2 amorphous alloy powder. The
results suggested that inductor using the (Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy powder with high DC-bias
characteristic and excellent corrosion resistance contribute to correspond to large current and high reliability of ICT
devices.
Key words: soft magnetic materials, amorphous alloys, powders, corrosion resistance, inductors
1. Introduction
Recently, the demand for the Internet of Things (IoT)
devices has increased significantly owing to the need forminiaturization of electronic devices with highperformance and power density. In particular, inductors
must exhibit excellent DC-bias characteristics
corresponding to high-wattage ICs with large currents
for high-power-density driving. Therefore, instead offerrite materials with saturation magnetic flux density(Bs) < 0.5 T, inductors composed of metallic soft magnetic
materials with Bs > 1.0 T, such as Fe-based amorphousand nanocrystalline alloys, are widely used to suppress
magnetic saturation by a large current 1)-5). Currently,
Fe-based soft magnetic materials are used as magnetic
core materials for inductors. Moreover, achieving a highBs is necessary to obtain high DC-bias characteristics. By
contrast, inductor materials must possess high corrosion
resistance to operate under usage environments.
Moreover, from the perspective of sustainable
development goals (SDGs), materials with high corrosion
resistance are becoming increasingly important because
they can be used for longer periods than those with low
corrosion resistance. However, the corrosion resistance ofa high Bs type Fe-based soft magnetic material is reduced
by including a high Fe content due to achieving a high Bs
Corresponding author: Y. Kajiura(e-mail: yoshiki.kajiura@tdk.com).

6)-7). The corrosion resistance of Fe-based soft magnetic

materials with crystal grains, such as Fe-Si and Fe-based
nanocrystalline alloys, is decreased because corrosion
preferentially progresses from the grain boundaries 8)-9).
Therefore, we focused on Fe-based amorphous alloys
without crystal grains and reported a novel Fe-based(Fe70Co30)BPSiCr alloy with high Bs and corrosion
resistance 10).
2. Experimental procedure
Master alloy ingots with FeCoBPSiCr and FeSiBC
were prepared by induction melting using commercial
raw materials such as Fe (99.9 mass %), Co (99.8 mass %),B (99.5 mass %), Si (99.999 mass %), Cr (99.9 mass %),and pre-alloyed FeP (99.9 mass %) in Ar. A single-roller
melt-spinning method in Ar atmosphere was used to
produce rapidly solidified ribbons with a width of 1 mm
and thickness ranging from 60 µm to 105 µm. The
structures of the as-quenched ribbons were identified by
X-ray diffractometry (XRD) and transmission electron
microscopy (TEM). The amorphous forming ability wasdetermined from the thickness of the as-quenched ribbon
with a single amorphous phase. To this end, as-quenchedribbons of varying thicknesses were fabricated. The
maximum ribbon thickness without crystallization wasdetermined to be the critical thickness. In particular, thedegree of amorphous forming ability was indicated by the
maximum as-quenched ribbon thickness without

Transaction of the Magnetics Society of Japan (Special Issues) Vol.8, No.1, 20241

crystallization, and a thickness of more than 60 µm wasdecided as a minimum critical thickness for powder
production. The densities of the ribbons were measured
using the Archimedean method with kerosene as thesolvent. The Bs was measured using a vibrating sample
magnetometer (VSM) under a maximum applied field of1600 kA/m. Coercivity (Hc) was measured using an
automatic coercive force meter (K-HC1000, Tohoku SteelCo.) under a maximum applied field of ~160 kA/m. The
corrosion resistance was investigated using the corrosion
potential (Ecorr) and corrosion current density (icorr)
measured by linear sweep voltammetry (LSV) using a
potentio-galvanostat. In a 0.5-M NaCl aqueous solution,
the sweep speed for a spontaneous potential was 0.833
mV/s from −300 mV to +300 mV using both Ag/AgCl
reference and Pt counter electrodes. After a 4-h
immersion test in pure water, the appearance of a 20-µmthick metallic ribbon was observed with an optical
microscope. FeCoBPSiCr and FeSiBC powders were
fabricated via gas atomization using high-pressure water.
The average diameter of the powder was measured usinga laser diffraction particle analyzer (HELOS). The
densities of the powders were measured using a He gas
pycnometer with a He purity of 5 N. The particle
morphology of the powders was observed by scanningelectron microscopy (SEM). Particle circularity (C) was
determined as follows:

C = 2√πA/P(1)

where A is the particle area, and P is the perimeter
determined using particle imaging (Morphologi-G3,
Malvern Instruments). The thermal kinetic behavior wasmeasured using differential scanning calorimetry (DSC)
at a heating rate of 0.67 K/s. Inductor with the size of 2.0(L) mm  1.2 (W) mm  0.8 (H) mm were prepared using
the FeCoBPSiCr and FeSiBC powders. The products
were plated with Ni after fabrication using the pressureforming method with a Cu coil and resin-dispersed
FeCoBPSiCr or FeSiBC particles. The permeability (μi)
of the ring core with dimensions of 11 mm in outer
diameter, 6.5 mm in inner diameter, and 2.5 mm inheight at 1 MHz was measured by an impedance
analyzer under a field of 0.13 A/m, and core loss (Pcv) was
measured by a BH analyzer with 10 mT at 2 MHz. The
DC bias characteristics of the inductance (L) were
measured using an LCR meter at a maximum appliedcurrent of 9 A. The products were observed under an
optical microscope.
3. Results
3.1 Dependence of alloy characteristics on thecomposition of (Fe0.7Co0.3)88−x−yB11PxSiyCr1
The dependence of amorphous forming ability and Bs
on the compositions of (Fe0.7Co0.3)88−x−yB11PxSiyCr1 alloys
were investigated. Fig. 1 (a) shows the dependence of

2

Fig. 1 Dependence of (a) amorphous forming ability and (b)Bs on the compositions of the (Fe0.7Co0.3)88-x-yB11PxSiyCr1
alloys.
amorphous forming ability on the compositions of(Fe0.7Co0.3)88−x−yB11PxSiyCr1 alloys. These alloys exhibitedhigh amorphous forming ability ranging from 61 μm to105 μm in the wide composition range. In particular, ahigh amorphous forming ability above 80 µm wasobserved for P contents of (3.5 to 4.5) at. % and Si
contents of (1.5 to 2.0) at. %. Fig. 1 (b) shows thedependence of Bs on the compositions of the(Fe0.7Co0.3)88−x−yB11PxSiyCr1 alloys. A high Bs of more than
1.7 T was observed at high FeCo contents of more than
82 at. %, Si contents of (1 to 3) at. % and P contents of(3 to 4) at. %. From the above results, it is considered thatcomposition optimizations for P, Si, and FeCo in the alloy
are essential to achieve a high Bs and amorphous forming
ability. Therefore, these results suggest that the(Fe0.7Co0.3)82B11P4Si2Cr1 alloy has the optimum
composition because it exhibits a high amorphous
forming ability of 87 µm with a Bs >1.7 T.
The magnetic characteristics, corrosion potential, andcorrosion current density of the (Fe0.7Co0.3)82B11P4Si2Cr1
alloy and the high-Bs-type conventional Fe79Si6B13C2
amorphous alloy with images of the metallic ribbon thatappeared after the immersion tests are listed in Table 1.
The (Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy showed
the favorable magnetic characteristics compared to theconventional Fe79Si6B13C2 amorphous alloy and a higher
amorphous forming ability than the conventional alloy.
In addition, the (Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy
exhibited higher corrosion resistance than theconventional Fe79Si6B13C2 amorphous alloy in theappearance of the metallic ribbon after the immersion
tests. In contrast, although the ribbon appearance of the(Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy clearly showed
Table 1 Magnetic characteristics of Bs and Hc, corrosion
characteristics of Ecorr and icorr, density of ribbons and optical
microscope images of metallic ribbons in the(Fe0.7Co0.3)82B11P4Si2Cr1 and conventional Fe79Si6B13C2
amorphous alloys after 4-h immersion tests in pure water.

Transaction of the Magnetics Society of Japan (Special Issues) Vol.8, No.1, 2024

Fig. 2 Anodic polarization curves in 0.5 M NaCl aqueous
solution at 0.833 mV/s sweep rate of the(Fe0.7Co0.3)82B11P4Si2Cr1 and the conventional Fe79Si6B13C2
alloy metallic ribbons.
a metallic color on the surface, the conventional
Fe79Si6B13C2 amorphous alloy showed significant red ruston the surface. Fig. 2 shows the anodic polarization
curves in 0.5-M NaCl aqueous solution at 0.833 mV/s
sweep rate of the (Fe0.7Co0.3)82B11P4Si2Cr1 andconventional Fe79Si6B13C2 alloy metallic ribbons. High
Ecorr of −520 mV and low icorr of 23.7 µA/cm2 of the(Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy led to theexcellent corrosion resistance compared to theconventional Fe79Si6B13C2 amorphous alloy. Therefore,
from the above results, the (Fe0.7Co0.3)82B11P4Si2Cr1
amorphous alloy showed higher Bs and corrosion
resistancethantheconventional
Fe79Si6B13C2
amorphous alloy.
3.2 Powdarization of (Fe0.7Co0.3)82B11P4Si2Cr1
amorphous alloy
The morphology of the (Fe0.7Co0.3)82B11P4Si2Cr1
amorphous alloy powder was investigated. Fig. 3 shows
the SEM images of the (Fe0.7Co0.3)82B11P4Si2Cr1
amorphous alloy powder, whereas Fig. 4 shows theparticle circularities of (Fe0.7Co0.3)82B11P4Si2Cr1 and thepowders.
The
conventional
Fe79Si6B13C2 alloy(Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy powder
consisted of spherical particles with smooth surfaces (Fig.
3). In addition, the (Fe0.7Co0.3)82B11P4Si2Cr1 amorphousalloy powder showed a high circularity of 0.981 comparedto the conventional Fe79Si6B13C2 alloy powder with a low

Fig. 3 SEM images of the (a) powder and (b) particle
morphology
inthe(Fe0.7Co0.3)82B11P4Si2Cr1
amorphous alloy powder.

Fig. 4 Particle circularity of the (Fe0.7Co0.3)82B11P4Si2Cr1
and the conventional Fe79Si6B13C2 alloy powders.
circularity of 0.970, as shown in Fig. 4.
Fig. 5 shows the melting point (Tm) and liquid phase
point (Tl) of (Fe0.7Co0.3)82B11P4Si2Cr1 and conventional
Fe79Si6B13C2 alloy powders. Low Tm and Tl were observed
for the (Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy powder
compared to those of the conventional Fe79Si6B13C2
amorphous alloy powder. The viscosity of the melting
alloy strongly depends on the ΔT (= T − Tm, where T
indicates the molten metal temperature duringatomization), and the viscosity decreases by increasing
ΔT. This suggests that the (Fe0.7C0.3)82B11P4Si2Cr1
amorphous alloy powder with the low Tm of 1270 K is low
viscosity of molten metal during atomization compared tothe conventional Fe79Si6B13C2 amorphous alloy powder
with high Tm of 1360 K 11),12). Moreover, sphericalparticles are easily obtained in case the alloy with low
viscosity in the gas-atomized process 13). Therefore, it isconsidered that particles with high circularity wereobtained by the viscosity of the (Fe0.7Co0.3)82B11P4Si2Cr1
amorphous alloy molten metal during atomization, which
was low due to the low Tm of the (Fe0.7Co0.3)82B11P4Si2Cr1
amorphous alloy.
Fig.
6
shows
a
TEM
image
of
the
(Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy particles withthe selected area electron diffraction (SAED) pattern.
The (Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy powder
was composed of a single amorphous phase. The XRD

Fig. 5 DSC curves of the (Fe0.7Co0.3)82B11P4Si2Cr1 and theconventional Fe79Si6B13C2 alloy powders.

Transaction of the Magnetics Society of Japan (Special Issues) Vol.8, No.1, 20243

Table 2 Average diameter, density of powders,packing density, magnetic characteristics of Bs andHc, electro-magnetic characteristics of μi, and Pcv inthe (Fe0.7Co0.3)82B11P4Si2Cr1 and the conventional
Fe79Si6B13C2 amorphous alloy powders.

Fig. 6 TEM image of the (Fe0.7Co0.3)82B11P4Si2Cr1
amorphous alloy particle with SAED pattern.

3.3 Electro-magnetic characteristics of(Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy powder
Table 2 lists the representative characteristics of the(Fe0.7Co0.3)82B11P4Si2Cr1 and conventional Fe79Si6B13C2
alloy powders. The (Fe0.7Co0.3)82B11P4Si2Cr1 amorphousalloy powder showed the same Bs level (1.61 T) as that of
the conventional Fe79Si6B13C2 alloy powder. However, aslightly higher permeability was observed in the(Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy powder
because Hc of the (Fe0.7Co0.3)82B11P4Si2Cr1 amorphousalloy powder was lower than that of the conventional
Fe79Si6B13C2 alloy powder. In addition, although Bs of thepowders decreased compared with that of the metallic
ribbons, it is considered that Bs decrease led to a decreasein the density of the powders. In gas atomization, hollow

particles including air are observed especially in the
powder, which is presumed the cause of the decrease in
the density of the powders.
Fig. 8 shows the DC-bias characteristics of inductors
with the dimensions of 2.0 (L) mm  1.2 (W) mm  0.8 (H)mm fabricated by using the (Fe0.7Co0.3)82B11P4Si2Cr1 andconventional Fe79Si6B13C2 amorphous alloy powders.
Moreover, their appearances are shown in Fig. 9. The
inductor fabricated using the (Fe0.7Co0.3)82B11P4Si2Cr1
amorphous alloy powder showed slightly better DC-bias
characteristics than those fabricated using theconventional Fe79Si6B13C2 amorphous alloy powder. It isconsidered that the (Fe0.7Co0.3)82B11P4Si2Cr1 amorphousalloy powder exhibited a slightly high DC bias
characteristic in the inductor consisting of the integrally
molded structure with Cu coil and magnetic core becausea packing condition of that was improved by the highcircularity of the (Fe0.7Co0.3)82B11P4Si2Cr1 amorphousalloy powder. However, further investigation is required
regarding the relationship between the particle
circularity and DC-bias characteristics.
In addition, the inductor fabricated using the(Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy powder
exhibited excellent corrosion resistance compared withthat fabricated using the conventional Fe79Si6B13C2
amorphous alloy powder.

Fig. 7 XRD patterns of the (Fe0.7Co0.3)82B11P4Si2Cr1
and the conventional Fe79Si6B13C2 alloy powders.

Fig.
8DC-biascharacteristics
ofthe(Fe0.7Co0.3)82B11P4Si2Cr1 and the conventional
Fe79Si6B13C2 amorphous alloy powders.

patterns of the (Fe0.7Co0.3)82B11P4Si2Cr1 and conventional
Fe79Si6B13C2 alloy powders are shown in Fig. 7. The(Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy powder showsa halo pattern, indicating an amorphous structure withthe same level of amorphous stability as that of theconventional Fe79Si6B13C2 alloy powder. Therefore, the(Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy with anamorphous forming ability of more than 80 μm can be
fabricated as the amorphous powder consists of a single
amorphous phase.

4

Transaction of the Magnetics Society of Japan (Special Issues) Vol.8, No.1, 2024

Fig. 9 Optical microscope images of products with thedimensions of 2.0 mm (L)  1.2 (W) mm  0.8 (H) mm using
the (Fe0.7Co0.3)82B11P4Si2Cr1 and the conventional
Fe79Si6B13C2 amorphous alloy powders.
Even after cutting and electrode plating, the inductor
fabricated using the (Fe0.7Co0.3)82B11P4Si2Cr1 amorphousalloy powder exhibited a gray color with the same color
as the powder itself. In contrast, red rust was clearlyobserved on the surface of the product fabricated using
the conventional Fe79Si6B13C2 amorphous alloy powder.
From the above results, the (Fe0.7Co0.3)82B11P4Si2Cr1
amorphous alloy with Bs of more than 1.7 T is recognizedthat the alloy shows a high Bs of 1.61 T and an excellent
corrosion resistance in also the powder. Therefore, theinductor fabricated using (Fe0.7Co0.3)82B11P4Si2Cr1
amorphousalloy
powdercontributetotheminiaturization of electronic devices by exhibiting highDC-bias characteristics. Moreover, their excellent
corrosion resistance contributes to maintaining a highreliability in various usage environments.
4. Conclusions
The magnetic characteristics, amorphous stability, andcorrosion
behaviorof(Fe0.7Co0.3)88-x-yB11PxSiyCr1
amorphous alloys and powder were investigated in thisstudy. The conclusions drawn from the results are as
follows.
(1) (Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy exhibiteda high amorphous forming ability of 87 µm, which is
required for powderization, and a high Bs of 1.71 T.

(2) With regard to the corrosion behavior, the(Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy with a highEcorr value of −520 mV and a low icorr value of 23.7 µA/cm2
showed a higher corrosion resistance than commercial
Fe79Si6B13C2 amorphous alloy without red rust on themetallic ribbon surface when immersed in pure water.
(3) The (Fe0.7Co0.3)82B11P4Si2Cr1 amorphous alloy
powder consisted of a single amorphous phase, as
observed from the TEM images and XRD and SAED
patterns.
(4) An inductor using the (Fe0.7Co0.3)82B11P4Si2Cr1
amorphous alloy powder exhibited high DC-bias
characteristics and excellent corrosion resistancecompared to that using the commercial Fe79Si6B13C2
amorphous alloy powder.
References
1)
2)
3)
4)
5)
6)

7)
8)
9)
10)
11)
12)
13)

F. Luborsky, J. Becker, J. Walter, H. Liebermann: IEEE Trans. Magn.,
15, 1146 (1979).
A. Kojima, S. Ito, A. Makino and A. Inoue: Mater. Trans. JIM, 42,1535 (2001).
Y. Yoshizawa, S. Oguma and K. Yamauchi: Appl. Phys., 64, 6044
(1988).
A. Urata, H. Matsumoto, S. Yoshida and A. Makino: IEEE Trans.
Magn., 47, 3177 (2011).
H. Matsumoto, Y. Kajiura, M. Hosono, A. Hasegawa, H. Kumaoka,
K. Yoshidome, S. Mori: AIP Advances, 12, 035312 (2022).
D.D. Coimbrao, G. Zepon, G.Y. Koga, D.A. Godoy Perez, F.H. Paesde Almeida, V. Roche, J.-C. Lepretre, A.M. Jorge Jr., C.S. Kiminami,
C. Bolfarini, A. Inoue, W.J. Botta: J. Alloys Compd., 826, 154123(2020).
G. Palumbo, D. Dunikowski, R. Wirecka, T. Mazur, U. LelekBorkowska, K. Wawer, J. Banas: Materials, 14, 5084 (2021).
Y. Han, F.L. Kong, F.F. Han, A. Inoue, S.L. Zhu, E. Shalaan, F. AlMarzouki: Intermetallics, 76, 18 (2016).
D.D. Xu, B.L. Zhou, Q.Q. Wang, J. Zhou, W.M. Yang, C.C. Yuan, L.
Xue, X.D. Fan, L.Q. Ma, B.L. Shen: Corros. Sci., 138, 20 (2018).
A. Hasegawa, Y. Kajiura, M. Hosono, K. Yoshidome, H. Matsumoto:
AIP Advances, 13, 025120 (2023).
M. Hirai: ISIJ International, 33, 1182, (1993).
V. S. Tsepelev and Y. N. Starodubtsev: Nanomaterials, 11, 108(2021).
E. S. Barreto, M. Frey, J. Wegner, A. Jose, N. Neuber, R. Busch, S.
Kleszczynski, L. Madler and V. Uhlenwinkel: Materials & Design,
215, 110519 (2022).

Received Oct. 12, 2023; Accepted Dec. 9, 2023

Transaction of the Magnetics Society of Japan (Special Issues) Vol.8, No.1, 20245