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: [email protected] http://www.nuc.elf.stuba.sk/bruno.
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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.
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 Matter9 (1997) 2321
click the picture to rotate the 3D-HFD
Miglierini M, Grenèche J-M and Idzikowski B Mater Sci EngA 304-306 (2001) 937
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.
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Miglierini M and Seberíni M phys status solidi (a)189 (2002) 351
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.
M = Mo
M = Nb
M = Ti
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.
Miglierini M, Seberíni M, Grenèche J M Czech J Phys51 (2001) 677
Miglierini M and Seberíni M phys stat. sol. (a)189 (2002) 351
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.
Miglierini M and Grenèche J-M Hyperfine Interact113 (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 ) 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.
 K. Hono, Y. Zhang, A. Inoue and T. Sakurai, Mater. Sci. Eng. A226-228 (1997) 498