Nanocrystalline alloys:
I. Crystallization
M. Miglierini et al.
Department of Nuclear Physics and Technology
Slovak University of Technology
Ilkovicova 3,812 19 Bratislava, Slovakia
E-mail: marcel.miglierini@stuba.sk
http://www.nuc.elf.stuba.sk/bruno
Nanocrystalline alloys prepared by controlled annealing from rapidly quenched
amorphous ribbons exhibit an interesting class of materials from the point of view of
their magnetic properties [1]. Resulting magnetic parameters, which are superior to
those of conventional transformer steels and/or amorphous materials, are ensured by a
presence of crystalline grains several nanometres in size embedded in the amorphous
residual phase [2]. Magnetic parameters of amorphous alloys are frequently
deteriorated in the process of their practical employment by elevated temperature
especially during prolonged operational times. On the other hand, nanocrystalline
alloys are in fact already partially crystallized and from this point of view their
structure is more resistant to such external effects and that is why it is more stable.
Nevertheless, because the excellent magnetic behaviour of nanocrystalline alloys
depends strongly on the amount and size of the crystalline grains, the process of
crystallization should be known.
[1] K. Suzuki, A. Makino, A. Inoue, T. Masumoto, J. Appl. Phys. 70 (1991) 6232.
[2] G. Herzer, Phys. Scr. T49 (1993) 307.
The following slide shows a comparison of some magnetic parameters (magnetic
permitivity me versus saturation magnetization Bs) for different types of magnetic
materials used for, e.g. the production of cores of magnetic circuits. The main three
types of compositions which yield nanocrystalline alloys are also listed.
Nanocrystalline Alloys - Features
• nanocrystalline alloys
– good soft magnetic properties
– thermal stabilization of the structure as
compared to amorphous alloys
nc-FINEMET
NANOPERM
Co-am
HITPERM
• 1988: FINEMET: FeCuNbSiB
• Yoshizawa Y, Oguma A, Yamauchi K
J Appl Phys 64 (1988) 6044
• 1988: NANOPERM: FeMB(Cu)
where M = Zr, Mo, Ti, Nb, Hf, …
• Suzuki K, Kataoka N, Inoue A, et al.
Mater Trans JIM 31 (1990) 743
• 1998: HITPERM: FeCoZrB(Cu)
• Willard M A, Laughlin D E, McHenry M E, et al.
J Appl Phys 84 (1998) 6773
Fe-am
Fe-Co
ferrites
Si steel
A. Makino, A. Inoue and T. Masumoto
Mater Trans JIM 36 (1995) 924
Possible Applications of Nanocrystalline Alloys
core
ribbons
magnetic shielding
transformer
sensors
Preparation of Nanocrystalline Alloys
tube
• production of an amorphous precursor
melt
induction
coil
melt-spun
ribbon
– mixing of appropriate amounts of pure elements
with subsequent melting
quenching
– rapid quenching of the melt ( ~106 K/min)
wheel
 method of planar flow casting
– result: ribbon up to several cm wide
planar
and typically about 20 mm thick
flow
– check of composition (OES ICP)
casting
and amorphicity (XRD)
• (nano)crystallization
amorphous ribbon
– check of crystallization behaviour by DSC (onset
of crystallization, first crystallization peak)
– choice of temperature of annealing
– annealing (in vacuum) for typically 1 hour
at the selected temperature
– characterization of the resulting structural and magnetic properties
Structures from a Melt
Starting material
(melt)
Conditions (quenching rate, composition, …)
crystalline
quasicrystalline
amorphous
annealing
nanocrystalline
• Ordered structure
– periodicity
– long range order
• Disordered structure
– short range order
– no translation symmetry
Characterization of Nanocrystalline Alloys
heat flow (a.u.)
• structural characterization
– DSC (differential scanning calorimetry)
• evolution of structure with temperature
– XRD (X-ray diffraction)
• crystalline phases, relative fraction
of crystallites and amorphous rest
TEM
XRD
DSC
400
500
600
700
800
tem p eratu re (°C )
35
40
45
50
55
o
2 ( )
– TEM (transmission electron microscopy)
• including HREM (high resolution TEM)
and XTEM (cross-sectional TEM)
• type and size of (nano)crystals
– STM (scanning tunnelling microscopy)
• including AFM (atom force microscopy)
• surface features
XTEM
• structural ordering of phases
• magnetic properties
– magnetic measurements
• 57Fe Mössbauer spectroscopy (TMS + CEMS)
– simultaneous information on both structural
arrangement and magnetic behaviour
(hyperfine interactions)
ED
specific magnetization (Am 2/kg)
– ED (electron diffraction)
STM
120
t =440 o C
a
100
as-quenched
80
t = 250 o C
a
60
t = 350 o C
a
40
20
0
200
250
300
temperature (K)
Miglierini M et al. J Appl Phys 85 (1999) 1014
Mössbauer spectrometry is a very sensitive tool for the study of both structural
arrangement and hyperfine interactions (magnetic ordering) in nanocrystalline alloys
[3]. Tthe FINEMET-type alloys, which are very frequently studied because their
macroscopic properties are beneficial for practical applications [4] exhibit rather
complicated Mössbauer spectra. They consist of several sextets of narrow lines
ascribed to different crystallographic positions in the Fe-Si lattice which are
superimposed upon a broadened signal which belongs to the amorphous rest of the
original precursor [5]. Evaluation of such spectra is pretty complicated and,
unfortunately, prevents from acquiring more detail information related to such
phenomena as for example interfacial regions [6].
In order to benefit from its diagnostic potential, it is useful to investigate such
materials whose Mössbauer spectra are reasonably simple. This is the situation for
example in NANOPERM-type alloys which crystallize into bcc-Fe, the latter being a
calibration material for Mössbauer spectrometry. Thus, here we concentrate on the FeMo-Cu-B system which belongs to the NANOPERM family.
[3] H. Bremers, O. Hupe, C. E. Hofmeister, O. Michele and J. Hesse: J. Phys.: Condens. Matter
17 (2005) 3197.
[4] T. Liu, Z. X. Xu and R. Z. Ma, J. Magn. Magn. Mat. 152 (1996) 365.
[5] T. Pradell, N. Clavaguera, J. Zhu and M. T. Clavaguera-Mora: J. Phys.: Condens. Matter 7
(1995) 4129.
[6] J. M. Grenèche and A. Slawska-Waniewska, J. Magn. Magn. Mat. 215-216 (2000) 264.
Structural Arrangement and Mössbauer Spectra
Mössbauer spectra of an ordered structure (crystallites) exhibit narrow lines which
lead to single values of the spectral parameters. Due to non-unique positions of
resonant atoms in a disordered structure the spectral lines are broad and, consequently,
distributions P() and P(B) of the spectral parameters must be considered.
crystalline
(disordered structure)
hyperfine parameters
P()
(ordered structure)
amorphous
non-magnetic


P(B)
0
B
AM CR
B
0
1
2
 (mm/s)
AM CR
magnetic
10 20 30
B (T)
1.00
FINEMET
Fe73.5Nb3Cu1Si13.5B9
P(H)
relative transmission
Mössbauer Spectra of Nanocrystalline Alloys (295 K)
0.95
-5
0
5
0
10
velocity (mm/s)
20
30
Fe-Si
H (T)
1.00
NANOPERM
Fe80Mo7Cu1B12
P(H)
relative transmission
Miglierini M J Phys Condens Matter 6 (1994) 1431
0.95
-5
0
velocity (mm/s)
5
0
10
20
H (T)
30
bcc- Fe
Miglierini M and Grenèche J-M J Phys Condens Matter 9 (1997) 2303
Fe on A site
Fe on D site
Si on D site
Annealing of the Amorphous Precursor
• DSC  continuous heating (temperature ramp of 10 K/min)
• choice of annealing temperatures (B-M, A = as-quenched) => sample preparation
• onset of crystallization identified at Tx1
Tx1  460oC
Fe76Mo8Cu1B15
Miglierini M et al. phys stat sol (b) 243 (2006) 57
B C D EFG
heat power
A
H
M
L
Tx1
300
400
diffusion-like precrystallization effects
J K
I
RT
structural relaxation
normal grain-growth-like formation of
a-Fe nanocrystallites in amorphous matrix
diffusion controlled grain-growth of
already created a-Fe nanocrystallites
500
o
temperature ( C)
600
700
diffusion controlled nucleation and
growth-like precipitation of g-Fe(Mo)
TEM and XRD
• Tx1 = 450 oC
550
Fe76Mo8Cu1B15
oC
650 oC
Miglierini M et al.
phys stat sol (b) 243 (2006) 57
470 oC
750 oC
Tx1  450oC
heat power
450 oC
100
200
300
400
500
o
temperature ( C)
600
700
Mössbauer Spectrometry
• evolution of Mössbauer spectra with temperature of annealing ta
• transmission Mössbauer spectra are plotted upside-down to enable 3D mapping
• temperature of measurement 300 K and 77 K
Fe76Mo8Cu1B15
300 K
77 K
Miglierini M et al. phys stat sol (b) 243 (2006) 57
Fitting Model
Miglierini M and Grenèche J-M J Phys Condens Matter 9 (1997) 2303, 2321
Miglierini M and Grenèche J-M Hyperfine Interact 113 (1998) 375
crystalline
Fe80Mo7Cu1B12 440oC/1h
interface
HREM
10nm
P(H)
relative transmission
amorphous
1.00
AM
IF
CR
0.95
295 K
-5
0
velocity (m m /s)
5
0
10
20
30
hyperfine field (T)
40
Transmission Mössbauer Spectrometry (295 K)
550 oC
Miglierini M et al. phys stat sol (b) 243 (2006) 57
510 oC
• bulk
• Tx1 = 450 oC (?)
600 oC
450 oC
Fe76Mo8Cu1B15
heat power
410 oC
100
200
300
400
500
o
temperature ( C)
600
700
Conversion Electron Mössbauer Spectrometry (295 K)
550 oC
Miglierini M et al. Hyperfine Int 165 (2005) 75
510 oC
• surface
• Tx1 = 450 oC
600 oC
450 oC
heat power
Fe76Mo8Cu1B15
410 oC
100
200
300
400
500
o
temperature ( C)
600
700
XRD – Peak Decomposition
550 oC
Miglierini M et al. phys stat sol (b) 243 (2006) 57
510 oC
• Tx1 = 450 oC
40
450 oC
40
45
 (deg)
45
 (deg)
50
600 oC
50
Fe76Mo8Cu1B15
40
40
45
 (deg)
45
 (deg)
50
heat power
410 oC
40
45
 (deg)
50
100
200
300
400
500
o
temperature ( C)
600
700
50
Summary
• structure of nanocrystalline alloys
– (nano)crystallites
– residual amorphous matrix
– interface = surface of crystalline grains + crystal-to-amorphous matrix region
• crystallization
• identification of crystalline
phase
• amount of nanocrystals
50
ACR (%)
– first at the surface
– progress of crystallization
is more rapid at the surface
60
40
30
20
XRD
TMS
CEMS
10
0
450
500
550
o
temperature ( C)
600
Mössbauer spectroscopy contributes to the study of nanocrystalline alloys from
several viewpoints. First, it is possible to identify the structural arrangement from a very
first look at a Mössbauer spectrum (e.g., onset and progress of crystallization).
Crystalline phases are characterized by narrow and usually well separated lines whereas
the amorphous residual phase exhibits broad patterns due to its disordered nature. Signal
from resonant atoms located at the interfacial regions can be also distinguished. The
latter two contributions are described by the help of distributions of hyperfine
parameters through which information on both topological and chemical short-range
order can be derived. The fraction (and/or type) of the crystalline phase(s) can be readily
obtained from the spectral parameters.
Second, magnetic order of the system under the study is also directly followed from
changes of the spectral line shapes, viz. (broadened) doublet vs. sextet. This can be
studied as a function of annealing temperature (i.e., crystalline contents), measuring
temperature, and/or composition. More details can be found in another presentation.
In this presentation, we have shown that the crystallization of amorphous precursors for
the preparation of nanocrystalline alloys proceeds more rapidly on the surface of the
rapidly quenched ribbons than in their bulk. In doing so, we have employed CEMS and
TMS, respectively. The crystalline content was determined also from XRD and the
results coincide well with those from TMS.
The temperature of the onset of crystallization Tx1 determined from DSC is somewhat
higher than that from XRD, TEM and MS due to different regime of annealing
(continuous during DSC and isothermal during the preparation of the samples).