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NANOMAGNETISM: An Evaluation Through Mössbauer Spectroscopy. Dipankar Das UGC-DAE, CSR, Kolkata Centre. Nano-bio Mag-sensors. Ultra-strong Permanent Magnets . Ultra High density media. Challenges in Nanomagnetism. RT magnetic semiconductors. 100% spin- polarized materials.

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slide1

NANOMAGNETISM: An Evaluation Through Mössbauer Spectroscopy

Dipankar Das

UGC-DAE, CSR, Kolkata Centre

slide2

Nano-bio

Mag-sensors

Ultra-strong

Permanent

Magnets

Ultra

High density

media

Challenges in

Nanomagnetism

RT magnetic

semiconductors

100% spin-

polarized

materials

Magnetic

logic

Spin-transistor

with gain

Instant boot-up

computer

Opportunities in Nanomagnetism

slide3

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 elephantor 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.

slide4

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

slide5

Typical SINGLET for a Paramagnet: No EFG

Typical DOUBLET for a paramagnet:

Presence of EFG

slide6

M

N

N

M

Sextet with cubic symmetry: No EFG

Sextet without cubic symmetry: Presence of EFG

slide7

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 θ=00 and θ=1800. The superparamagnetic relaxation time τ is given by

where τ0 is of the order of 10-10-10-13 s, kB is the Boltzmann’s constant and T is the temperature.

slide8

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 τ  510-8 s, the 57Fe Mossbauer spectra consist of six relatively sharp lines.
  • For 10-9  τ  10-8 s, the spectra contain very broad lines and the magnetic hyperfine splitting is more or less collapsed.
  • Very fast relaxation (τ  10-10) results in spectra without magnetic hyperfine splitting, i.e. with one or two absorption lines, depending on the quadrupole interaction.
slide9

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.
slide10

TEM picture of BM 24

XRD pattern of BM 4

TEM picture of BM 60

slide11

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.
slide12

The FWHM vs Ball Milling Duration variation shown alongside

Figure alongside shows the variation of Hc and Ms with ball milling duration.

slide13

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.
slide14

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.
slide15

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.
slide16
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.
slide17

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.
summary
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.
slide20

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.
slide21

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.
slide23

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.
slide24

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.