194
V. Schünemann
general strategy in our laboratory in order to analyze Mössbauer spectra of complex
systems with multiple iron sites or iron phases. In order to reduce the number of
unknown parameters it is advisable to fix those Mössbauer parameters originating
from iron phases which are known or have been investigated in their pure state before.
Pure free unsupervised parameter fitting of Mössbauer spectra from samples with
more than one iron site is at least to our experience in almost all cases not successful,
since the analysis is being very much over parametrized.
4.3.3 Exploration of Iron Ligand Modes by Synchrotron
Based Nuclear Inelastic Scattering (NIS)
Synchrotron based Nuclear Inelastic Scattering (NIS) as well as Nuclear Forward
Scattering (NFS)—the latter of which can be regarded as Mössbauer spectroscopy
in the time domain—are ideal spectroscopic tools to study nanostructures and thin
films of isotopically enriched
57 Fe containing materials since these techniques have a
sensitivity down to a sub monolayer
57 Fe [59]. The two methods play an exceptional
unique role compared to other synchrotron based techniques like X-ray diffraction
and absorption, because not only the structural and dynamic but also the spin dependent magnetic properties of solids, surfaces and nanostructures can be investigated in
one experimental set-up (see Chap. 1 of this book). NIS has been successfully applied
to measure the density of phonon states (pDOS) in iron containing materials. From
the measured DOS elastic and thermodynamic parameters including compressional
and shear velocities, heat capacity, entropy, kinetic energy, zero point energy and
Debye temperature can be obtained. Recently these parameters have been reported
for SCO nanoparticles with diameter of 50 nm [60].
In order to investigate size effects in the SCO behaviour of nanostructures we have
started to investigate
57 Fe enriched SCO microstructures via NFS. For this purpose
we have used an iron(II) containing aminotriazole polymer. The microstructures
have been prepared by pre-patterning with photolithography. A solution of the SCO
compound was drop casted onto the pre-patterned substrates and the resist was lifted
off using acetone. Figure 4.13 shows a micrograph of such a structure together with
NFS data which show a clear beating structure characteristic for a species with a
high quadrupole splitting of ΔE Q = 2.73 mms
−1 indicative for a ferrous high spin
state at 283 K. Cooling the structure to 243 K leads to an NFS pattern, with no
clear beating structure showing the absence of quadrupole splitting indicating that
the major species is now in the ferrous low-spin state. These results clearly indicate
a change of the spin state due to the SCO effect [61].
Figure 4.14 shows NFS data of a [2 × 2] Fe(II) grid-complex with four iron ions
which undergoes SCO [62]. Complementary susceptibility data show that at 80 K all
4 iron(II) ions are in their low-spin (LS) state. Iron(II) LS ions generally have a low
quadrupole splitting which does not lead to quantum beats as visible in Fig. 4.13c.
The minimum of the NFS-signal displayed in Fig. 4.14a occurring at ~40 ns is what
V. Schünemann
general strategy in our laboratory in order to analyze Mössbauer spectra of complex
systems with multiple iron sites or iron phases. In order to reduce the number of
unknown parameters it is advisable to fix those Mössbauer parameters originating
from iron phases which are known or have been investigated in their pure state before.
Pure free unsupervised parameter fitting of Mössbauer spectra from samples with
more than one iron site is at least to our experience in almost all cases not successful,
since the analysis is being very much over parametrized.
4.3.3 Exploration of Iron Ligand Modes by Synchrotron
Based Nuclear Inelastic Scattering (NIS)
Synchrotron based Nuclear Inelastic Scattering (NIS) as well as Nuclear Forward
Scattering (NFS)—the latter of which can be regarded as Mössbauer spectroscopy
in the time domain—are ideal spectroscopic tools to study nanostructures and thin
films of isotopically enriched
57 Fe containing materials since these techniques have a
sensitivity down to a sub monolayer
57 Fe [59]. The two methods play an exceptional
unique role compared to other synchrotron based techniques like X-ray diffraction
and absorption, because not only the structural and dynamic but also the spin dependent magnetic properties of solids, surfaces and nanostructures can be investigated in
one experimental set-up (see Chap. 1 of this book). NIS has been successfully applied
to measure the density of phonon states (pDOS) in iron containing materials. From
the measured DOS elastic and thermodynamic parameters including compressional
and shear velocities, heat capacity, entropy, kinetic energy, zero point energy and
Debye temperature can be obtained. Recently these parameters have been reported
for SCO nanoparticles with diameter of 50 nm [60].
In order to investigate size effects in the SCO behaviour of nanostructures we have
started to investigate
57 Fe enriched SCO microstructures via NFS. For this purpose
we have used an iron(II) containing aminotriazole polymer. The microstructures
have been prepared by pre-patterning with photolithography. A solution of the SCO
compound was drop casted onto the pre-patterned substrates and the resist was lifted
off using acetone. Figure 4.13 shows a micrograph of such a structure together with
NFS data which show a clear beating structure characteristic for a species with a
high quadrupole splitting of ΔE Q = 2.73 mms
−1 indicative for a ferrous high spin
state at 283 K. Cooling the structure to 243 K leads to an NFS pattern, with no
clear beating structure showing the absence of quadrupole splitting indicating that
the major species is now in the ferrous low-spin state. These results clearly indicate
a change of the spin state due to the SCO effect [61].
Figure 4.14 shows NFS data of a [2 × 2] Fe(II) grid-complex with four iron ions
which undergoes SCO [62]. Complementary susceptibility data show that at 80 K all
4 iron(II) ions are in their low-spin (LS) state. Iron(II) LS ions generally have a low
quadrupole splitting which does not lead to quantum beats as visible in Fig. 4.13c.
The minimum of the NFS-signal displayed in Fig. 4.14a occurring at ~40 ns is what