Crystallization of amorphous alloys
induced by the rf magnetic field
Michael Kopcewicz
Institute of Electronic Materials Technology,
01-919 Warszawa, Wólczyńska Street 133, Poland
Introduction
Crystallization is of crucial importance for amorphous alloys.
Excellent soft magnetic properties of amorphous alloys dramatically
deteriorate upon crystallization.
The common origin of crystallization is due to thermal effects [1,2].
However, the crystallization may result also from nonthermal effects,
e.g., those associated with mechanical deformations.
An unusual effect of crystallization of amorphous phase
(containing Co) was noticed in the rf-Mössbauer study of
Fe81-x-yNixCoyZr7B12 alloys [1].
The rf field induced crystallization is discussed for the x=30, y=10
(Fe41Ni30Co10Zr7B12) and x=20, y=20 (Fe41Ni20Co20Zr7B12) alloys and is
compared with that of the x=40, y=0 and x=50, y=0 Co-free alloys.
The rf-Mössbauer technique (see e.g., [3-5]):
- the ferromagnetic sample is exposed to the radio-frequency (rf)
magnetic field that may induce the rf-collapse and rf-sideband effects.
This technique allows us to follow the crystallization of certain
amorphous alloys induced by the rf field.
In particular the rf-sidebands effect is relevant to the present study.
The rf-sidebands effect:
- frequency modulation of Mössbauer -radiation due to rf-induced
vibrations of Mössbauer atoms via magneto-acoustic coupling magnetostriction;
Sideband positions are given by nr.
Intensities: frequency modulation (FM) model;
Rf-sidebands can be observed in ferromagnetic magnetostrictive
materials (below the Curie point).
Experimental procedure
Samples: Amorphous alloys: Fe41Ni30Co10Zr7B12 (Co10 alloy),
Fe41Ni20Co20Zr7B12 (Co20 alloy)
and Fe41Ni40Zr7B12 and Fe31Ni50Zr7B12 (Co-free alloys)
are prepared by the melt quenching technique.
The ribbons were 3-5 mm wide and about 25 m thick.
The Mössbauer spectra: recorded at room temperature in the
absence of the external radio-frequency (rf) field before and after
the exposure to the rf field.
The 57Co-in- Rh source of 25 mCi activity was used.
The rf-Mössbauer measurements were performed during
exposure of the samples to a rf magnetic field with a frequency of
61 MHz and intensity of 20 Oe.
The Mössbauer investigations were accompanied by the
measurements of magnetostriction constants of the alloys
studied.
The saturation magnetostriction constant s was measured at
room temperature using a strain modulated ferromagnetic
resonance method (SMFMR) [6, 7].
Results
All Co-containing amorphous alloys studied revealed the onset of
crystallization of an amorphous phase when exposed to the rf
field of intensity up to 20 Oe at 61 MHz.
The rf field induced crystallization effect depends strongly on the
sample composition, in particular on the Co-content.
One can easily observe this crystallization effect by comparing the
Mössbauer spectra recorded before and after the rf field exposure.
As examples, the spectra recorded for the Fe41Ni30Co10Zr7B12,
Fe41Ni20Co20Zr7B12 alloys (Co-containing alloys designated as
Co10 and Co20, respectively), and the Co-free Fe41Ni40Zr7B12 alloy
are shown in the next frame.
0.99
after
rf field
-6
1.00
a
a
0.98
1.00
1.00
d
0.98
before
rf field
0.98 before
before
before
rf field rf fieldrf field
d
a
-3
0
3
6
-6
-3
0
Fe41Ni40Zr7B12
FeNi
Fe41Ni20
FeCo
Ni41
Co
Zr
BZr
NiCo
Zr12
B12
NiZr
30
10
7Co
12
41
3020
10
7B12
4120
207Co
20Zr7B12
Co10Zr7B12 Fe41Fe
7B
20
1.00
after
rf field
1.00
1.00
d
0.98
g
0.98
before
rf field
0.96
before
rf field
b
b
0.995
1.00
in rf0.995
fieldin rf in
rf field
field
c
3
6
-3 -6 -3
0 -3 03 0 36 3 6
-6
6-6
1.00
1.00
0
before
1.00
before
before
i
0.96
-3 -6 0 -3 3 0 6 3
g
1.00
0.98
after
rf field
Velocity [mm/s]
g
in rf field
f
0.98
Fe41Ni4Fe
B12
0Zr
7Ni
41
40Zr7B12
Fe41Ni40Zr7B12
1.00
f
Velocity [mm/s]
h
0.90
c
after
rf field
0.95
0.99
in rf field
0.98
0.99
after after after
rf field rf field
rf field
-6
f
e
0.99
1.00
1.00
1.00
e
b
0.99
in rf field
c
0.99
e
1.00
1.00
Transmission
1.000
Transmission
Transmission
Transmission
1.00
1.000
g
6
after
rf field
-6
-3
0
3
Velocity [mm/s]
6
The spectra of all amorphous alloys recorded before the rf field exposure reveal the
shapes typical of ferromagnetic amorphous alloys (broadened sextets), (Figs 1a, 1d,
1g).
The spectra recorded for the Co-containing samples after the rf field exposure clearly
contain two spectral components (Figs. 1c, 1f):
(i) a well-resolved sextet with narrow lines (the hyperfine field of about 36 T)
characteristic of the crystalline bcc-FeCo phase, formed due to the rf induced
crystallization of amorphous alloys,
(ii) the broadened sextet characteristic of the retained amorphous phase.
The spectra recorded for the Co-free amorphous alloys before and after rf field
exposure are almost identical and consist of a broadened sextet characteristic of the
amorphous alloy. They do not show any evidence of the crystallization effect
(Figs. 1g and 1i).
The rf-Mössbauer spectra recorded during the rf field exposure (Figs. 1b, 1e and
1h) consist of the rf-collapsed central component accompanied by intense rfsidebands.
The relative intensity of the rf-sideband lines provides direct information on the
magnetostriction of the alloy studied.
Intensities of the rf-sidebands are particularly large for the
Co-containing alloys (Fig.1b, 1e).
The rf-sidebands vanish for zero magnetostriction alloys.
- Large intensities of the rf-sidebands observed for Co-containing
alloys (Figs. 1b, 1e) strongly suggest that the magnetostriction of
these alloys is large.
- The rf-sidebands observed for Co-free sample (Fig. 1h) are
significantly smaller, suggesting a significantly smaller
magnetostriction.
In order to suggest the origin of the rf field induced crystallization of
the Co-containing alloys it is necessary to estimate the temperature of
the sample during the exposure to the rf field applied.
- It is assumed that the entire center shift of the spectrum recorded during the rf
field exposure is caused by the rf-heating effect and can be described by the
second order Doppler (SOD) shift.
- The Debye temperature of the sample material must be known.
- It is assumed that the typical Debye temperatures of Fe-based amorphous alloys
are close to the room temperature [8].
The SOD factor can be determined for the relevant alloys from the linear
dependence of the center shifts of the spectra vs. sample temperatures .
The temperature of the samples during the rf field exposure was estimated by
comparing the changes of center shifts of the rf-Mössbauer spectra (Figs. 1b,
1e, 1h) with those recorded for the same samples at room temperature in the
absence of the rf field (Figs. 1a, 1d, 1g) and by dividing these differences by the
SOD factor.
It was found that the temperature of the Co-free amorphous sample during the
exposure to the rf field of the intensity of 20 Oe was about 200oC and 230oC for
Fe31Ni50Zr7B12 and Fe41Ni40Zr7B12 samples, respectively.
The temperatures of the amorphous Co-containing samples were higher, about
260oC and 290oC for Fe41Ni30Co10Zr7B12 and Fe41Ni20Co20Zr7B12 alloys,
respectively.
Thus, these temperatures of the samples were much lower than the
temperatures of the first step of crystallization of the corresponding
amorphous alloys (about 470oC and about 490-500oC for the Co-free alloy
and Co-containing alloys, respectively).
The origin of the crystallization effect induced by the rf field as resulting from
heating the sample can be excluded as a major mechanism that causes the
crystallization.
It is inferred that the rf-crystallization effect is of nonthermal origin.
While the common origin of conventional crystallization of
amorphous alloys is usually thermal, crystallization may also
originate from the nonthermal effects related to mechanical
deformations.
The crystallization of Co-containing amorphous phase, observed
earlier [9], was related to mechanical deformations induced
during the high-energy ball milling.
Also in that case the concept of a high effective local heating
during the milling process was ruled-out.
It is concluded that the rf field induced crystallization of amorphous alloys
observed here is of magnetostriction origin.
Since, the crystallization effect caused by the rf field is particularly
pronounced in the amorphous alloys that reveal significantly large
magnetostriction, the crystallization effect was attributed to mechanical
deformations induced in the sample via magnetostriction (rf-sidebands effect).
The rf field forced, via magnetostriction, enhanced vibrations of atoms as a
result of which the amorphous structure was destabilized and partly crystallized.
These vibrations cause the frequency modulation of the Mössbauer gamma
radiation and the rf-sideband lines are formed in the rf-Mössbauer spectrum.
When magnetostriction of the alloy is large (large rf-sidebands effect) then the
rf field induced crystallization effect is strong.
When the rf-sidebands effect is decreased because of small
magnetostriction then the rf-induced crystallization does not occur.
The Mössbauer results are well supported by direct
measurements of the saturation magnetostriction constants (s)
[10] performed by using a strain modulated ferromagnetic
resonance (SMFMR).
The smallest value of s was obtained for Co-free Fe31Ni50Zr7B12
alloy (s10x10-6).
The magnetostriction constant of Fe41Ni40Zr7B12 alloy: s11x10-6.
The magnetostriction constants, determined for Co-containing
alloys, were significantly larger:
s15x10-6 for Fe41Ni30Co10Zr7B12
s23x10-6 for Fe41Ni20Co20Zr7B12.
The magnetostriction constant data obtained fully agree with
the Mössbauer results.
Conclusion
The rf field induced crystallization effect, observed in
high magnetostriction Co-containing amorphous alloys, was
attributed to mechanical deformations induced in the
sample via magnetostriction (rf-sidebands effect).
It did not occur in the Co-free amorphous alloys with smaller
magnetostriction.
The presence of Co is important for this effect, because Co
creates significantly larger magnetostriction constants of the
Co-containing amorphous alloys.
References
1. M. Kopcewicz and T. Kulik, J. Appl. Phys. 99, 08F112 (2006).
2. M. Kopcewicz, J. Latuch and T. Kulik, Phys. Stat. Sol (a) 204, 3179 (2007)/ DOI
10.1002/ pssa.200723053.
3. M. Kopcewicz, Strutural Chem. 2, 313 (1991).
4. M. Kopcewicz, in G.J. Long, F. Grandjean (Eds) "Mössbauer Spectroscopy
Applied to Inorganic Chemistry" vol. 3, Plenum, N. York, London, 1989, p. 243.
5. M. Kopcewicz, in Y. Liu, D.J. Sellmyer and D. Shindo (Eds.),"Handbook of
Advanced Magnetic Materials" vol. 2, Tsingua Univ. Press and Springer, 2006, p.
151.
6. J. Wosik, K. Nesteruk, W. Zbieranowski and A. Sienkiewicz, J. Phys. E: Sci.
Instrum. 11, 1200 (1978).
7. R. Żuberek, K. Fronc, A. Szewczyk and H. Szymczak, J. Magn. Magn. Mater. 260,
386 (2003).
8. M. Kopcewicz, B. Kopcewicz and U. Gonser, J. Magn. Magn. Mater. 66, 79
(1987).
9. M.L. Trudeau, R. Schulz, D. Dassault, A. Van Neste, Phys. Rev. Lett. 64, 99
(1990).
10. R. Żuberek, unpublished data.
These results were presented at the Magnetism and Magnetic
Materials Conference (MMM-2007), Tampa, FL, (USA) in
November 2007
and and were published in the Journal of Applied Physics 103,
07E717 (2008).