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2278 IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001
Processing and Properties of Soft Magnetic
Fe Co P B Amorphous Alloy
Martin Hollmark, V. I. Tkatch, Alex Grishin, and S. I. Khartsev
Abstract—The Fe Co P B metallic alloy has been pre-
pared in a glassy state by a melt-spinning process. The 15–25 m
thick as-quenched ribbons have superior soft magnetic properties
compared with those of well-known Fe Ni P B glass: the
saturation magnetization is about 1.45 T, coercive field 4.0 A/m
@ 0.1 Hz, incremental magnetic permeability 90000 @
60 Hz and Curie temperature above 700 K. The crystallization
temperature, determined by differential scanning calorimeter,
is approximately 60 K higher than that for FeNi-based alloy,
indicating the enhanced thermal stability of FeCo-glass.
I. INTRODUCTION
THE DEVELOPMENT of soft magnetic alloys with
enhanced saturation magnetization and reduced coercive
force is of considerable technological interest in view of
magnetic device miniaturization and reducing of energy losses.
In this field of materials science the metallic alloys with
nonequilibrium structure (amorphous and nanocrystalline)
discovered more than 30 years ago are now competitive with
conventional crystalline soft magnetic materials. A further
extension of applications of the amorphous alloys in industrial
scale is mainly dependent on improvement of their magnetic
characteristics, casting properties as well as their thermal
stability. It is known that the highest saturation magnetization
(above 1.8 T) has been achieved in FeCo-based amorphous
alloys [1], thus these alloys are in focus of interest of recent de-
velopments [2]–[4]. In this paper, we report on the replacement
of Ni by Co in well-known Fe Ni P B metallic glass [5]
and how it affects the magnetic properties and thermal stability
of FeCo-metglass.
II. EXPERIMENTAL
The ingot of alloy with nominal composition of
Fe Co P B was prepared in a quartz crucible by in-
ductive (radio-frequency, RF) melting of the mixture of pure
Fe, Co, B and Co P in a protective argon gas atmosphere. A
standard single roller melt spinning technique was used for
rapid solidification. Charges of about 5–10 g were RF melted
under flowing Ar, superheated approximately to 100–120 K
above the melting temperature K, and ejected
through quartz nozzles with round and rectangular orifices onto
the outer surface of the rotating quenching copper wheel. The
casting parameters in these experiments were as follows: the
wheel linear surface velocity was about 30 ms , the ejection
Manuscript received October 13, 2000.
The authors are with the Department of Condensed Matter Physics, Royal
Institute of Technology, S-100 44 Stockholm, Sweden.
Publisher Item Identifier S 0018-9464(01)06761-9.
overpressure was 25–35 kPa, and the gap between the nozzle
and the wheel was 150 m. Before each melt-spinning run,
the outer surface of the copper wheel was abraded with 1200
grit SiC paper. The resultant ribbons of several meters long
had a thickness of 15–25 m and a width ranged from 2 to
8 mm depending on the nozzle slot dimension. The smooth
surfaces and edges of the ribbons as well as the absence of
porosity indicate good casting properties of the Fe Co P B
melt. For comparison, the Fe Ni P B melt-spun ribbons
were prepared under the same processing conditions. The
as-quenched specimens were annealed for 30 min at various
temperatures ranged from 473 to 698 K.
ASiemens D5000 x-ray diffractometer with monochro-
mated Cu radiation was used to examine the structure of
the as-quenched and annealed ribbons. The Curie transition,
thermal stability and crystallization of the rapidly solidified
samples were studied using a Perkin Elmer DSC7 differential
scanning calorimeter under continuous heating with rates from
5 to 40 Kmin . The saturation magnetization was mea-
sured with a vibrating sample magnetometer Oxford VSM3001
in magnetic fields up to 400 kAm . The hysteresis loops were
recorded by an ac – tracer in magnetic fields 8 kAm .
Measurements were performed on straight 30 cm long ribbons
placed into 25 cm long solenoid with 5 cm long pick up and
compensation coils connected in series. Very soft ribbons make
strong distortions of the flux waveform. To reduce this effect
a primary coil is driven by sine generator via ballast resistor.
An output signal was integrated by analog circuit or directly
by digital storage oscilloscope (Infinium, Agilent Technology).
The measurements were performed at frequencies ranged from
0.1 to 60 Hz. The incremental magnetic permeability @ 60 Hz
was estimated as a derivative of the magnetization curve:
.
III. RESULTS AND DISCUSSION
Structure and Thermal Stability: The curve 1 in Fig. 1 dis-
plays the diffraction pattern of the as-quenched Fe Co P B
ribbon. It contains only two broad halos at Bragg angles 22.4
and 40.5 indicating the absence of the long-range crystalline
order. The size of coherently scattering regionscalculated from
the half-width of the first halo using Scherrer formula is about
1.6 nm, which is typical for amorphous metallic alloys.
The transformations of the Fe Co P B melt-spun ribbon
occurring during heat treatment are evident from DSC curves
shown in Fig. 2. A broad exothermic maximum between 500
and 735 K is attributed to the structural relaxation processes in
as-quenched glass while endothermic peak at 738 K is associ-
ated with the glass transition. A small peak at 618 K indicating
0018–9464/01$10.00 © 2001 IEEE
HOLLMARK et al.: PROCESSING AND PROPERTIES OF SOFT MAGNETIC Fe Co P B AMORPHOUS ALLOY 2279
Fig. 1. – x-ray diffraction patterns of Fe Co P B melt-spun ribbons
in Cu radiation. 1—as-spun amorphous ribbon; 2—the ribbon has been
rapidly cooled from the temperature just above the crystallization peak at the
DSC scan in Fig. 2.
Fig. 2. Differential scanning calorimeter (DSC) traces for as-quenched
Fe Ni P B and Fe Co P B amorphous alloys recorded at the heating
rate of 40 K/min. The Curie temperature and glass transition temperature
in the FeCo-based glass are marked by arrows in the insert.
changes in specific heat is a Curie point [6]. The single very
sharp exothermic peak at 750 K clearly indicates the one-stage
crystallization process of Fe Co P B alloy. Note that as
is seen in Fig. 2 the DSC scan of the Fe Ni P B glass is
similar to that of the Fe Co P B but is shifted to lower
temperatures.
Thevaluesof the Curietemperature of the Fe Co P B
glass and the temperatures of crystallization peak for both
glasses were estimated from the DSC scans obtained at different
heating rates and listed in Table I.
Both temperatures depend on the heating rate: the values of
the crystallization temperature increase while the Curie point
TABLE I
CURIE TEMPERATURE AND CRYSTALLIZATION TEMPERATURE
MEASURED AT DIFFERENT HEATING RATES
in the Fe Co P B glassy alloy lowers about 100 K with
a heating rate rising from 5 to 40 Kmin . The observed be-
havior of the Curie temperature is typical for the as-quenched
specimens of metallic glasses and is caused by the structural re-
laxation processes mainly by stress relief [6]. It should be noted
that at a heating rate of 5 Kmin the Curie transition in the
Fe Co P B glass occurs in the close vicinity of crystalliza-
tion. It means that this glass is impossible to anneal in paramag-
netic state in contrast with the Fe Ni P B glass with the
Curie temperature (508 K [6]) appreciably lower than the crys-
tallization temperature.
Analysis of the DSC data showed that the temperature of the
crystallization peak of amorphous Fe Co P B is approx-
imately 60 K higher than the crystallization temperatures of
Fe Ni P B glass in the used range of the heating rates. It
should be also pointed out, that although the reduced crystal-
lization temperatures of the FeCo-based glass, which
characterize the glass-forming ability of melt, are rather high
(0.55–0.57), they are still somewhat lower than those of the
Fe Ni P B amorphous phase (0.57–0.59).
To clarify the mechanism of Fe Co P B glass crystal-
lization, the x-ray diffraction studies were carried out with sam-
plescooled from thetemperature just above the exothermicpeak
in DSC scan. Analysis of the diffraction patterns of the crys-
tallized specimens shown in Fig. 1 revealed that they contain
peaks of two phases: f.c.c. (marked by arrows) and b.c. tetrag-
onal of Fe P-type. Such structure is quite similar to that iden-
tified in crystallized Fe Ni P B amorphous alloys [7], [8].
The similarity of both the DSC data and the structures of crys-
tallized specimens of FeNi- and FeCo-based glasses indicates
that Fe Co P B glass transforms into crystalline state via
eutectic reaction. In this context formation of an austenite at
temperatures 725–750 K is somewhat surprising, because in
equilibrium Fe Co alloy f.c.c. structure is stable only at tem-
peratures above 1250 K [9].
Magnetic Properties: Fig. 3 shows hysteresis – loops
of the as-quenched Fe Co P B and Fe Ni P B glassy
ribbons. It is seen that the replacement of Ni by Co causes an
increase of the saturation magnetization from 0.84 to 1.45 T
while the coercive fields (9.4 and 9.9 Am @ 10 Hz) are
rather close. Quasistatic coercive field at 0.1 Hz has been
measured directly recording the maxima of . In as-cast
Fe Co P B and Fe Ni P B ribbons 4.0 and
1.6 Am at 0.1 Hz, respectively. The maximum of the incre-
mental magnetic permeability in the as-spun
Fe Co P B ribbons was found to be about 90 000 at 60 Hz.
2280 IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001
Fig. 3. Hysteresis – loops of the as-quenched Fe Co P B and
Fe Co P B melt-spun glassy ribbons recorded by a vibrating sample
magnetometer (a) and an ac – loop tracer @ 10 Hz (b) in magnetic field
parallel to ribbon surface.
Hysteretic loss in saturated state has been calculated as an
area within the saturated – loop:
where is a frequency and g/cm is taken for the
FeCo-alloy density. The hysteretic losses in saturated regime
for the as-quenched FeCo-based ribbon were estimated to be
0.56 W/kg @ 60 Hz. Although this value is higher than the core
losses for heat treated FeSiB amorphous ribbon @ 60 Hz and
1.4 T, but it is a half of those in a silicon steel [10].
Mechanism and Kinetics of Crystallization: The detailed
consideration of crystallization kinetics of the Fe Ni P B
glass carried out in the frame of the classical theory showed
that this process took place by nonsteady state homogeneous
nucleation and linear growth of eutectic colonies [8]. The
technique of the experimental data analysis developed in this
work enables calculations of all thermodynamical and kinetic
parameters, which determine the thermal stability of the glass.
In view of close similarity of crystallization behavior of FeCo-
and FeNi-based glasses it is interesting to establish the main
factors responsible for the improved thermal stability of glassy
structure in Fe Co P B alloy. The starting data of such
analysis were the experimental kinetic crystallization curves
calculated from the DSC scans and the value of the melting
temperature.
The results of the detailed analysis of the experiments
presented in [11] showed that the value of the interfacial
diffusion coefficient as well as the rates of crystal growth in
the Fe Co P B and Fe Ni P B glasses are very close,
whereas the magnitude of the nucleus-glass interface tension
in FeCo-based glass is higher (0.2 Jm ) compared with
0.147 Jm earlier reported for the FeNi-based counterpart
[8]. Correspondingly, the increased barrier for nuclei formation
results in lowering of the nucleation frequency by about five
orders of magnitude and leads to highly nonsteady character
of the nucleation process, which is clearly displayed by the
very sharp exothermic peak in DSC scan (Fig. 2). Both of
these factors, the decreased nucleation rate and nonsteady
character of nucleation process, are responsible for retarding
crystallization of the Fe Co P B amorphous alloy under
continuous heating in comparison with the transformation of
the FeNi-based glass.
IV. CONCLUSION
In summary, Fe Co P B glassy ribbons have been
prepared using a single roller melt-spinning process. In liquid
state FeCo-based alloy has good casting characteristics while
the as cast 15–25 m thick ribbons exhibit high magnetic
performance: dc saturation magnetization as high as 1.45 T, the
coercive field 4.0 Am @ 0.1 Hz, the maximum of differential
magnetic permeability about 10 , and hysteretic losses in
saturated regime about 0.56 W/kg @ 60 Hz. Fe Co P B
glass is crystallized via eutectic reaction at temperatures
about 60 K higher than its FeNi-based counterpart. Enhanced
thermal stability of the FeCo-glass is caused by enlarged
nucleus-glass interfacial tension as high as 0.2 J m compared
with 0.147 Jm in FeNi-based alloy.
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