NANOMAGNETISM: An Evaluation Through Mцssbauer Spectroscopy

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NANOMAGNETISM An Evaluation Through Mössbauer
Spectroscopy
Dipankar Das UGC-DAE, CSR, Kolkata Centre
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Opportunities in Nanomagnetism
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Mössbauer spectroscopy and its Sensitivity
Mössbauer spectroscopy is a technique in which
interaction between the electromagnetic moment of
the nuclear charge and electromagnetic field
produced by the extra-nuclear electrons are
studied. This interaction gives
splitting/shifting of the nuclear energy levels.
For the most common Mössbauer isotope, 57Fe, the
linewidth is 5x10-9ev. Compared to the Mössbauer
gamma-ray energy of 14.4keV this gives a
resolution of 1 in 1012, or the equivalent of a
small speck of dust on the back of an elephant or
one sheet of paper in the distance between the
Sun and the Earth. This exceptional resolution is
necessary to detect the surface magnetism in
nanoparticles.
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Hyperfine Parameters

• Chemical Isomer Shift (IS) (?) Arises out of the
  interaction between nuclear charge density and
  the surrounding s electron charge cloud. IS can
  give information about the spin state as well as
  the co-ordination number.

Quadrupole Splitting (QS) (?) Arises due to
interaction between the electric quadrupole
moment of the nucleus and EFG created by the
electrons. QS can give information about the
charge symmetry around the nucleus.
Hyperfine field (Hint) It gives the internal
magnetic field of a magnetic material
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Typical SINGLET for a Paramagnet No EFG
Typical DOUBLET for a paramagnet Presence of EFG
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Sextet with cubic symmetry No EFG
Sextet without cubic symmetry Presence of EFG
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Superparamagnetism
For a magnetic particle the magnetic energy with
uniaxial anisotropy is given by For particles
with nanometric dimensions Superparamagnetic
relaxation is the spontaneous fluctuations of the
magnetization direction such that it alternately
is near ?00 and ?1800. The superparamagnetic
relaxation time t is given by
where t0 is of the order of 10-10-10-13 s, kB is
the Boltzmanns constant and T is the temperature.
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• Mössbauer spectroscopy is one of the most
  sensitive techniques for studying
  superparamagnetic relaxation.
• The relaxation time should be of the order of
  10-7-10-10 s.
• For t ? 5?10-8 s, the 57Fe Mossbauer spectra
  consist of six relatively sharp lines.
• For 10-9 ? t ? 10-8 s, the spectra contain very
  broad lines and the magnetic hyperfine splitting
  is more or less collapsed.
• Very fast relaxation (t ? 10-10) results in
  spectra without magnetic hyperfine splitting,
  i.e. with one or two absorption lines, depending
  on the quadrupole interaction.

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Fe-MgO nanocomposites prepared by high-energy
ball milling
Motivation

• Nanocomposite magnets comprising a transition
  group element (Fe, Co, Ni, etc.) embedded in a
  non-magnetic matrix have been drawing increasing
  attention for their excellent magnetic properties
  and widespread technological applications.
• MgO was chosen as the matrix as it can easily
  form OH radicals on its surface, which can have
  immense biological applications.
• The microstructural properties of nanocomposites
  and nanocrystalline materials (NCM) in general
  largely depend on the atomic structure of the
  grain boundaries (interfacial regions/interfaces)
  because a substantial fraction of atoms are
  located at the grain boundaries.
• Mössbauer spectra of the nanocomposite samples
  were fitted based on the hypothesis that the
  grain boundaries possess a different atomic
  arrangement than that of the bulk crystalline
  iron, giving rise to a distinct sub-spectrum.

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TEM picture of BM 24
XRD pattern of BM 4
Sl No Ball Mlng Duration (hrs) Particle Size (nm)
1 4 40
2 12 29
3 24 21
4 36 18
5 60 17
TEM picture of BM 60
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Figure alongside shows Mössbauer spectra of the
samples ball milled for different time.

• Initially, all the spectra were fitted with one
  crystalline site for Fe atoms.
• This fitting scheme showed an increase of the
  average linewidth of the sextets with increase in
  milling time.
• The FWHM increases steadily upto 24 hrs followed
  by a sharp rise at 36 hrs and 60 hrs.
• The spectra of the samples BM36 and BM60 could
  be fitted with a discrete sextet and an
  additional component with distribution of
  hyperfine parameters.
• The surface area of acicular particles (BM 36
  and BM 60) being more than that of spherical
  ones, the number of atoms residing at the grain
  boundaries is more in the acicular particles.
• The higher fraction of grain boundary phase in
  these two samples made it possible to fit an
  additional component in the Mössbauer spectra of
  them.

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The FWHM vs Ball Milling Duration variation shown
alongside
Figure alongside shows the variation of Hc and Ms
with ball milling duration.
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Observations

• Initially coercivity increases with ball milling
  duration. We see a sharp increase when the sample
  is milled from 4 hrs to 12 hrs.
• High-energy milling introduces defects in the
  samples. These defects act as pinning centers for
  the domain walls increasing the rotational
  barriers.

• This increasing trend almost saturates after 24
  hrs of milling as the concentration of the
  defects also tend to saturate.

• Elongated particles have an extra component to
  the demagnetization energy, which is associated
  with the shape anisotropy of the particles.
• We propose that the sharp increase of coercivity
  observed for the samples ball milled for 36 hrs
  and more is due to the combined effect of surface
  and shape anisotropies associated with the
  geometrical shape transformation.

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Summary

• To conclude, stable Fe-MgO nanocomposites with
  sizes varying between 17-40 nm have been prepared
  by ball milling.
• Mössbauer spectra of the samples milled for 36
  hrs and above showed an additional component
  other than the crystalline sextet. This extra
  component was assigned to the grain boundary
  fraction.
• A distribution of hyperfine fields was needed to
  fit the grain boundary fraction which indicated
  its disordered amorphous like structure.
• DC magnetization measurements show that the
  coercivity of the nanocomposites tends to rise
  with milling time with a sharp increase after 36
  hrs of milling which is argued to be due to the
  combined effect of shape and surface anisotropies
  associated with the shape transformation.
• The decrease in the Ms values with increase in
  milling time was ascribed to the percentage of
  magnetic dead layer, which increased with
  increase in milling time.

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Fractal Morphology of Iron Oxide Nanoclusters
Motivation

• Magnetic nanoclusters are of current interest
  because of their various applications in
  technology.
• Nanoclusters prepared by chemical route gives
  fairly good narrow particle size distribution
  (PSD).
• Physical properties of the materials depend
  strongly on the particle morphology as well as on
  PSD.
• Knowledge on PSD and morphology will help in the
  development of the material for newer
  technological applications.

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• Samples were prepared by a non-aqueous
  precipitation route. Starting materials were
  ferric nitrate and stearic acid. The as-prepared
  sample was treated at 350 0C for different times.
  SAXS studies confirm self-affine fractal
  morphology of the samples.

• XRD confirms the formation of ?-Fe2O3.
• Increase of holding time gives particle growth
  and partial transformation from ? ? ? phase.

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• TEM shows a particle size of 80 nm in the as
  prepared sample.

TEM IMAGES

• The fractals disintegrate to smaller sub-
  particles when heated at 350oC for 0.5 hr. The
  average particle size being 8 nm.

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Mössbauer results
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Summary

• ?-Fe2O3 nanoclusters, prepared from the
  homogeneous solution of stearic acid and iron
  (III) nitrate, exhibit self-affine surface
  fractal morphology.
• The fractals disintegrate into smaller discrete
  particles as a function of heat treatment holding
  time due to the induction of high amount of
  strain.
• A fraction of nanoparticles undergoes
  superparamagnetic relaxation as confirmed by
  Mössbauer spectroscopy.

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Mössbauer studies of Yttrium Iron Garnets

• The figure alongside shows the Mössbauer spectra
  recorded at 20 K of a YIG sample of average
  particle size 14 nm.
• The spectra was de-convoluted into 4 sextets 2
  for the octahedral sites, 1 for the tetrahedral
  site and another for the Fe3 atoms located at
  the grain boundaries.
• It was seen that for the sample HT at a higher
  temperature, the surface component decreased
  considerably signifying grain growth.

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MOSSBAUER STUDIES OF MAGNETICALLY ADDRESSABLE
FERROGELS

• Nanocrystalline Fe3O4 and ?-FeOOH in polyvinyl
  alcohol gel matrix were synthesized via a novel
  route, without using any cross-linking agent.
• A moderately high-pressure environment of an
  autoclave instead was used for the synthesis.
• TEM studies showed that particles are mostly
  spherical with average size of 10 nm.
• Mössbauer spectra of the as prepared gels at
  different temperatures showed the presence of
  superparamagnetic particles in them. The gels
  were found to be magnetically ordered at 20K
  giving characteristic six-finger patterns.
• DC magnetization studies of the gel were carried
  out and from the saturation magnetization values
  the weight percentage of magnetite in the gel was
  determined.

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TEM picture of FG1 with inset of size
distribution histogram
Zoom-in on a small cluster in FG1
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Mössbauer spectrum of the gels at (a) RT and (b)
20 K

• Mössbauer spectra of the as-prepared gels did
  not show any appreciable absorption, probably
  because of their low Lamb-Mössbauer factors in
  the gel state.
• On lowering the temperature down to 60K and
  finally to 20K, Mössbauer spectra were observed.

• For room temperature measurements, the samples
  were dried by keeping them at ambient temperature
  in a vacuum desiccator for seven days. The
  samples obtained henceforth showed appreciable
  absorption at room temperature.

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CONCLUSIONS

• Mössbauer spectroscopy has proved to be one of
  the best techniques for studying
  superparamagnetic relation.
• Mössbauer spectroscopy allows us to probe the
  spin dynamics at a characteristic time of 10-8 s
  which is much smaller than conventional DC and AC
  magnetization studies.
• It allows us to get an idea about the
  surface/interface magnetism because the hyperfine
  parameters are affected by the difference in the
  microscopic environments of the bulk and the
  surface.
• Mössbauer spectroscopy can also effeciently
  characterize materials in the gel or dis-orderded
  state.

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THANK YOU