8 Mössbauer Spectroscopy in External Magnetic Fields
403
applied field. In case of the RE 6 Fe 13 X compounds the jumps are too small to be
explained by reversal of one of the ferromagnetically coupled sublattices present
in the above mentioned proposed antiferromagnetic ordered moment arrangements,
indicating that the till then proposed models are too simple. This is confirmed by
57 Fe
Mössbauer high field measurements. Figures 8.24 and 8.25 show two typical spectra
measured 4.2 K at 13.5 T. Comparison with the measurements on the same samples
in zero field (Figs. 8.20 and 8.21) show quite different shape. The sharp structure of
the spectra in zero field is washed out by very flat side wings in the in-field spectra.
Therefore the in-field spectra can by no means be fitted by only 4 (Nd 6 Fe 13 Sn) or 5
(Pr 6 Fe 13 Pd) subspectra according to the different Fe-sites. A further subdivision of
the 4 (5) subspectra was necessary. By restricting the overall number of subspectra
to nine, 4, 2, 2, and 1 spectra are used for the k, l 1 , l 2 , and d site, respectively, keeping
the intensity ratio of 4:4:4:1 for the 4 sites. The fit gives values for the magnitude of
the hyperfine field and the angle θ between measured hyperfine field at the Fe nucleus
and the γ -ray direction. From the difference of hyperfine field and applied field B a ,
by taking into account θ , the internal field B int could be determined. The obtained
mean values for each crystallographic iron site are in good agreement with the values
obtained from the zero-field measurements in case of Nd 6 Fe 13 Sn. The projection of
B int on the B a direction gives 55% of the value of B int . This 55% are also obtained
by comparing the magnetic moment at 13.5 T and 4.2 K to the moment calculated
assuming full alignment of RE
3+ moments and Fe moments of 1.77 μ B . For the
Pr 6 Fe 13 Pd compound at 13.5 T, B int values which are higher than those at zero field
are necessary to get agreement with the degree of saturation found in magnetization.
On the other hand, taking both the RE and Fe moment of the neutron refinement
[62] leads to a saturation moment which is much higher than the one observed. This
together with the found deviations of hyperfine field from complete alignment with
B a indicates a further jump of magnetization at even higher applied fields. The high
number of subspectra for the different Fe-sites needed to get reasonable in-field fits
points to a magnetic structure which is not simply antiferromagnetic but indicates
strong tilting of the spins around the antiferromagnetic alignment of the individual
iron layers.
8.5.2.1 Skutterudites
The mineral skutterudite CoAs 3 has given its name to a large class of substances.
Its structure was first solved by Oftedal in 1928 [64]. Binary skutterudites MPn 3 are
formed by many atoms, with M = Co, Rh, Ir and Pn stands for pnictides (P, As, and
Sb). The structure consists of a three-dimensional array of slightly distorted octahedra
formed by the pnictide atoms, with the M atom in the center. The octahedra are tilted
in such a way, that a rectangular arrangement of Pn atoms form, which connect
the adjacent octahedra. Due to this tilting large cage-like voids are created in the
structure, which can be filled by electropositive atoms A forming the large class of
ternary skutterudites A x M 4 Pn 12 (Fig. 8.26).
403
applied field. In case of the RE 6 Fe 13 X compounds the jumps are too small to be
explained by reversal of one of the ferromagnetically coupled sublattices present
in the above mentioned proposed antiferromagnetic ordered moment arrangements,
indicating that the till then proposed models are too simple. This is confirmed by
57 Fe
Mössbauer high field measurements. Figures 8.24 and 8.25 show two typical spectra
measured 4.2 K at 13.5 T. Comparison with the measurements on the same samples
in zero field (Figs. 8.20 and 8.21) show quite different shape. The sharp structure of
the spectra in zero field is washed out by very flat side wings in the in-field spectra.
Therefore the in-field spectra can by no means be fitted by only 4 (Nd 6 Fe 13 Sn) or 5
(Pr 6 Fe 13 Pd) subspectra according to the different Fe-sites. A further subdivision of
the 4 (5) subspectra was necessary. By restricting the overall number of subspectra
to nine, 4, 2, 2, and 1 spectra are used for the k, l 1 , l 2 , and d site, respectively, keeping
the intensity ratio of 4:4:4:1 for the 4 sites. The fit gives values for the magnitude of
the hyperfine field and the angle θ between measured hyperfine field at the Fe nucleus
and the γ -ray direction. From the difference of hyperfine field and applied field B a ,
by taking into account θ , the internal field B int could be determined. The obtained
mean values for each crystallographic iron site are in good agreement with the values
obtained from the zero-field measurements in case of Nd 6 Fe 13 Sn. The projection of
B int on the B a direction gives 55% of the value of B int . This 55% are also obtained
by comparing the magnetic moment at 13.5 T and 4.2 K to the moment calculated
assuming full alignment of RE
3+ moments and Fe moments of 1.77 μ B . For the
Pr 6 Fe 13 Pd compound at 13.5 T, B int values which are higher than those at zero field
are necessary to get agreement with the degree of saturation found in magnetization.
On the other hand, taking both the RE and Fe moment of the neutron refinement
[62] leads to a saturation moment which is much higher than the one observed. This
together with the found deviations of hyperfine field from complete alignment with
B a indicates a further jump of magnetization at even higher applied fields. The high
number of subspectra for the different Fe-sites needed to get reasonable in-field fits
points to a magnetic structure which is not simply antiferromagnetic but indicates
strong tilting of the spins around the antiferromagnetic alignment of the individual
iron layers.
8.5.2.1 Skutterudites
The mineral skutterudite CoAs 3 has given its name to a large class of substances.
Its structure was first solved by Oftedal in 1928 [64]. Binary skutterudites MPn 3 are
formed by many atoms, with M = Co, Rh, Ir and Pn stands for pnictides (P, As, and
Sb). The structure consists of a three-dimensional array of slightly distorted octahedra
formed by the pnictide atoms, with the M atom in the center. The octahedra are tilted
in such a way, that a rectangular arrangement of Pn atoms form, which connect
the adjacent octahedra. Due to this tilting large cage-like voids are created in the
structure, which can be filled by electropositive atoms A forming the large class of
ternary skutterudites A x M 4 Pn 12 (Fig. 8.26).