APPLICATIONS OF THE MÖSSBAUER SPECTROSCOPY
G. Spina
INFM Firenze, Dip. Fisica Università di Firenze.
E-mail: [email protected]
L. Cianchi, F. Del Giallo, M. Lantieri, P. Moretti
IFAC CNR, Sez. Struttura della Materia e Spettroscopia
E-mail: [email protected]
Isomer+ SOD shift
Electric quadrupole
Magnetic dipole
Hyperfine
transitions
57Fe
Mössbauer
spectra
The figure shows typical
spectra of the 57Fe subject to
static hyperfine fields. In the
magnetic case, the field is
usually time depending, due to
spin fluctuations. Frequencies
in the 106-1012 s-1 range can be
obtained from the spectrum
fits.
As the frequency increases, the
hyperfine structure tends to
disappear more and more. The
relaxation spectra are then
obtained (see the figure at the
Mössbauer spectra of iron
oxides embedded as small
grains in a pink marble.
Contributions of different
oxidation states of the iron
are separately shown.
Moreover, different grain
sizes give different
relaxation spectra of the
Fe3+ ions.
right side).
The Lamb-Mössbauer f-factor (fraction of g-photons recoilless
emitted ) can be obtained from the absorption area A of the
spectrum. In its turn, the mean square displacement (MSD) of the
emitting nucleus in the g-ray direction is given by:
<x2> = -(l2/ 4p2) ln(f), where l is the photon wave-length.
Example of Fe(III) cluster (Fe4). The ion spins in the ground state are
shown. Fe4 Mössbauer spectra between 1.38 and 77 K were collected
which show evident relaxation effects.
The 1.32 K spectrum displays a well-resolved hyperfine structure, since
only the ground state (for which the spin fluctuations are negligible) is
populated. The trend versus T of the mean frequency of the spin
fluctuations is:
-1 = 0-1 exp(-/T) where
0-1 = 2 107+1012 exp(-136/T) (s-1)
  magnetic anisotropo barrier  7 K
In the figure the MSD trends of the 151Eu in the
cuprate EuSr2Cu3O7 (red) and in the
superconducting counterpart EuBa2Cu3O7 (violet)
are reported as functions of the temperature.