Nanocrystalline alloys:
II. Hyperfine Interactions
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
Mössbauer spectroscopy provides unique opportunity to study disordered (e.g.,
amorphous, nanocrystalline) systems by the help of distributions of hyperfine parameters
(quadrupole splitting and/or magnetic fields). They provide information on short-range
order arrangement which is not accessible by other methods. For example XRD sees
amorphous arrangement like a broad structure-less peak.
On the other hand, distributions of hyperfine interactions identify the probability of
occurrence of regions of the resonant atoms with alike hyperfine parameters, i.e. similar
behaviour (magnetic order). They can be obtained from the Mössbauer spectra by their
deconvolution using suitable fitting programs. The most frequently studied distributions
comprise distributions of hyperfine magnetic fields denoted as P(H) or P(B) and
distributions of quadrupole splitting P(D) or P(QS). For the sake of better illustration they
can be eventually presented as 3D mappings in which the investigated distributions are
plotted with respect to the studied parameter. The latter can be the temperature of
measurement, temperature of annealing, composition, etc.
The following slides show selected examples of distributions of hyperfine interactions
and describe how they can be interpreted.
Temperature of Measurement
Fe87.5Zr6.5B6
Amorphous Fe87.5Zr6.5B6 metallic glass is weakly magnetic at
room temperature (300 K) as seen from the corresponding
Mössbauer spectrum which is neither a sextet nor a doublet in
shape. Distribution of hyperfine magnetic fields P(H) shows
prevailing low H-values.
An increase of temperature induces a magnetic transition
from magnetic to paramagnetic state. Consequently, at 348 K
the system is paramagnetic and the doublet-like spectrum is
described by distributions of quadrupole splitting P(D).
On the other hand, a decrease in temperature to 77 K
strengthens the magnetic interactions giving rise to
distribution of hyperfine magnetic fields P(H) shifted towards
higher H-values.
Using temperature Mössbauer effect measurements we can
determine the magnetic ordering temperature (Curie
temperature) of the investigated system.
Miglierini M and Grenèche J-M J Phys Condens Matter 9 (1997) 2321
Hyperfine Field Distribution - HFD
P(H)
HFD
77 K
3D-HFD
Fe80Mo7Cu1B12 440oC/1h
300 K
360 K
540 K
5%
click the picture to rotate the 3D-HFD
-5
0
5
velocity (m m /s)
0
20
40
H (T)
Miglierini M, Grenèche J-M and Idzikowski B Mater Sci Eng A 304-306 (2001) 937
Mössbauer spectra (295 K) as function of annealing
Fe80Mo7Cu1B12
am
o rp h o u sphase
p h ase
amorphous
in terfacezone
zo n e
interface
P(H)
o
P(H)
P(H)
410 C /1h
5%
o
440 C /1h
410 410
0
470
10
20
H (T)
30 580
o
470
t a ( C)
580
20
30
40
H (T)
o
470 C /1h
The temperature of annealing affects the amount of
nanocrystals formed during heat treatment (i.e. crystallization)
of the amorphous precursor. The corresponding Mössbauer
spectra reflect the presence of crystallites, the residual
amorphous matrix as well as the interface zone (see Part I.).
The former are characterized by single values of hyperfine
fields (vertical lines) whereas the latter by P(H) distributions.
These can be eventually plotted as 3D mappings showing the
evolution of hyperfine magnetic fields.
o
520 C /1h
-5
0
5
velocity (m m /s)
0 10 20 30 40
H (T)
Miglierini M, Greneche J M Hyperfine Interact 120/121 (1999) 297
Effect of Nanocrystalline Grain Formation
1.00
1.00
0.95
295 K
TM S
0.90
-5
0
5
velocity (m m /s)
as-quenched
0
10 20
B (T)
30
amorphous annealing
CEMS
295 K
TM S
P(B) (a.u.)
295 K
1.02
1.00
P(B) (a.u.)
1.02
relative emission
295 K
relative transmission
CEMS
1.04
P(B) (a.u.)
Miglierini M and Seberíni M phys status solidi (a) 189 (2002) 351
P(B) (a.u.)
relative transmission
relative emission
Fe80Nb7Cu1B12
1.00
0.98
0.96
0.94
-5
0
5
velocity (m m /s)
nanocrystalline
0
10 20
B (T)
30
470 oC/1h
Formation of bcc-Fe after annealing (blue) causes depletion of the amorphous phase to Fe and,
consequently the chemical short-range order of the latter is changed. Regions with higher
hyperfine magnetic fields are created in the amorphous residual matrix as demonstrated by a
shift of the distributed values (green) to the right. The same effect is observed in the bulk
(TMS) as well as on the surface (CEMS) of the nanocrystalline ribbons.
Mössbauer spectra (295 K) as function of Composition
Fe80M7Cu1B12
amorphous
nanocrystalline
P(D) (a.u.)
M = Mo
5%
5%
P(H) (a.u.)
M = Nb
0
1
D (mm/s)
2
P(H) (a.u.)
M = Ti
-5
0
velocity (mm/s)
5
0
10
20
H (T)
-5
0
velocity (mm/s)
5
0
10 20 30
H (T)
The as-quenched (amorphous) alloy exhibits paramagnetic, weak, and stronger magnetic
interactions for M = Mo, Nb, and Ti, respectively. They are described by the P(D) and P(H)
distributions. After annealing, the amorphous residual phase shows an increase in magnetic
interactions. The most pronounced change being observed for M = Mo which was actually
transformed from paramagnetic into ferromagnetic state. This is caused by: (1) change in
composition due to segregation of Fe atoms into bcc crystals, and (2) polarization of the
amorphous rest by ferromagnetic exchange interactions among the bcc-Fe nanocrystals.
Effect of Composition (cont.)
bcc-Fe contents
ACR (%)
80
Fe80M7Cu1B12
CEMS
P(B)
40
410
410
470
Mo
o
ta ( C)
0
80
580 0
10
20
60
B (T)
580 0
10
20
30
B (T)
P(B)
Nb
435
0
435
500
60
470
o
ta ( C)
Nb
40
80
30
P(B)
20
ACR (%)
P(B)
Mo
60
20
ACR (%)
TMS
CEMS
TMS
o
ta ( C)
620 0
10
20
30
500
o
ta ( C)
B (T)
620 0
10
20
30
B (T)
P(B)
P(B)
40
Ti
20
400
450
500
550
Ti
600
o
ta ( C)
420
420
520
o
ta ( C)
Miglierini M, Seberíni M, Grenèche J M Czech J Phys 51 (2001) 677
Miglierini M and Seberíni M phys stat. sol. (a) 189 (2002) 351
620 0
10
20
B (T)
30
520
o
ta ( C)
620 0
10
20
B (T)
30
The structural arrangement of nanocrystalline alloys is reflected in the Mössbauer spectra
through its individual spectral components (see Part I.): (nano)crystallites, amorphous residual
matrix, and interface regions (= surface of crystalline grains + crystal-to-amorphous matrix
region). The latter two are described by distributions of hyperfine interactions. In the previous
slide, they are plotted together as 3D mappings. For the given composition, the evolution of
hyperfine fields can be followed as a function of annealing temperature ta in the bulk (TMS)
and on the surface (CEMS) of the investigated samples. The contents of bcc-Fe crystals is
quantified on the accompanied graphs as a function of ta.
For example in M = Mo, originally weak magnetic regions characterized by a pronounced peak
at low values of hyperfine fields (~5 T) is seen at low ta, e.g. small bcc-Fe contents. With rising
ta (crystalline phase), this peak decreases in intensity (relative fraction) and new ones appear at
higher fields (~10 T). So, even though the overall contribution of the amorphous residual phase
decreases with progressing crystallization (rising ta) its magnetic order is strengthened.
Contrary to M = Mo, in the Ti-containing alloy the trend is completely opposite: originally
quite strong hyperfine fields of the amorphous matrix (peak at ~15 T) diminishes with ta down
to 5-9 T. In the M = Nb, the peak at ~7 T is decomposed into two new ones with smaller and
higher fields. This effect is even more pronounced on the surface of the samples.
It should be noted that the mean hyperfine field of the interface regions (~ 30 T) do not
substantially change with ta because they are closely related to the nanograins whereas the
amorphous residual matrix changes its composition with continuing crystallization as well as
experiences the magnetic exchange interactions among the nanograins.
The examples presented above clearly document the effect of composition upon magnetic order
in the amorphous matrix. Such information is hardly accessible by other techniques.
Topography of Hyperfine Fields
structural arrangement hyperfine interactions
Fe80Mo7Cu1B12
440oC/1h
3.
P(H)
relative transmission
1.
1.00
0.95
2.
-5
0
velocity (mm/s)
5
0
10
20
30
40
hyperfine field (T)
Miglierini M and Grenèche J-M Hyperfine Interact 113 (1998) 375
Using the deconvolution of the Mössbauer spectra of disordered systems we can go even
further. The general scheme is shown on the previous slide:
1. Information on structure as obtained from, e.g. TEM (HREM), XRD, AFM, etc. is used to
suggest a physical model (note that in this figure only Fe atoms are considered).
2. Consequently, a fitting model (see also Part I.) is applied and a distribution of hyperfine
fields is derived from the spectrum (including single values of crystalline components).
3. Eventually, the distribution can be decomposed into Gaussian sub-distributions each
describing certain groups of the Fe resonant atoms with a particular mean hyperfine field value.
If we take into consideration information from other techniques (e.g. atom probe field ion
microscopy - APFIM [1]) about the spatial distribution of particular constituent elements we
can sketch a topography of hyperfine fields with respect to structural arrangement. It should be
stressed that Mössbauer spectroscopy is not able to provide information on particular spatial
location of the resonant atoms. On the other hand, distributions of hyperfine interactions can be
obtained only from Mössbauer spectra and in this sense Mössbauer spectroscopy is an unique
tool for studying especially disordered systems, like for example nanocrystalline alloys.
[1] K. Hono, Y. Zhang, A. Inoue and T. Sakurai, Mater. Sci. Eng. A226-228 (1997) 498