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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UGC-DAE, CSR, Kolkata Centre
Opportunities in Nanomagnetism
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.
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
Typical DOUBLET for a paramagnet:
Presence of EFG
Sextet with cubic symmetry: No EFG
Sextet without cubic symmetry: Presence of EFG
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.
Mössbauer spectroscopy is one of the most sensitive techniques for studying superparamagnetic relaxation.
XRD pattern of BM 4
TEM picture of BM 60
Figure alongside shows Mössbauer spectra of the samples ball milled for different time.
Figure alongside shows the variation of Hc and Ms with ball milling duration.
Zoom-in on a small cluster in FG1