8 Mössbauer Spectroscopy in External Magnetic Fields
407
Beside the high importance for applications, these materials are of interest because
of the large variety of possible ground states. Depending on the RE atom features
like superconductivity in LaRu 4 As 12 (T c = 10.3 K) [72], LaOs 4 As 12 (T c = 3.2 K)
[72], PrRu 4 Sb 12 (T c = 1 K) [73], PrRu 4 P 12 (T c = 2.4 K) [74], where for the last
compound a metal insulator transition is present at 60 K [75], long range magnetic
order in EuFe 4 Sb 12 (T mag = 84 K) [76, 77], heavy fermion behaviour in YbFe 4 Sb 12
[78], non-Fermi liquid behaviour in CeRu 4 Sb 12 [79], mixed valence behaviour in
RE(Co,Fe) 4 Sb 12 with RE = Yb and Eu [80, 81], are found. In case of the Fe containing skutterudites also a large variety of magnetic ground states is reported. E.g.
REFe 4 P 12 compounds with RE = Nd, Eu, Ho are ferromagnetic with ordering temperatures 1.9 K, 80 K, and 5 K, respectively [76]. Also SmFe 4 Sb 12 is ferromagnetic
below 45 K. NaFe 4 Sb 12 , and KFe 4 Sb 12 skutterudites are itinerant ferromagnets with
ordering temperatures 85 K for both [82]. TlFe 4 Sb 12 was found to be a weak itinerant ferromagnet [83]. PrFe 4 Sb 12 is antiferromagnetic with Neél temperature of
4.6 K [84]. For PrFe 4 P 12 antiferroquadrupolar interactions play an important role
below 6.2 K [85–87]. AFe 4 Sb 12 compounds with A = Ca, Sr, Ba, Tm, and Yb are
paramagnetic respectively nearly ferromagnetic [83]. As mentioned above LaFe 4 P 12
is superconducting below 4 K [88], whereas LaFe 4 Sb 12 is an enhanced paramagnet
[76, 82]. One important question is, how the Fe-atoms contribute to the magnetic
behaviour. In spite of large amount of theoretical and experimental investigations a
precise knowledge about Fe-moments and their interplay with the filler atoms are
still missing [70]. E.g. the LaFe 4 P 12 compound is as mentioned above superconducting, indicating that Fe has no moment, although from susceptibility measurements a
room temperature effective moment of 1.46 μ B /fu is obtained. Because La
3+ has no
magnetic moment, the measured one has to be attributed to the (Fe 4 Sb 12 ) building
blocks. Band structure calculations on La(Co,Fe) 4 P 12 indicate hybridization of the
La sites with Sb and Fe states resulting in an enhanced effective mass for the two
highest occupied bands [89]. Furthermore a double peak structure of the 3d-DOS
in the proximity of the Fermi energy was obtained, from which the presence of a
non-zero moment on the Fe site was concluded [90]. Newer studies pointed out that
spin fluctuations are important and that this compound seems to be near to a ferromagnetic quantum critical point [91]. The question about the contribution of Fe to the
magnetization is further puzzling, if the Sb compounds are considered which show
effective moments of several μ B depending on the type of filler atom (Table 8.2).
It should be mentioned that in contrast to the Fe skutterudites based on P, in the Sb
based Fe skutterudites the RE sublattice is not always fully occupied. In case of the
Pr 0.73 Fe 4 Sb 12 compound an effective moment of 4.19 μ B /fu is found from magnetic
measurements. Figure 8.27 shows magnetization curves at different temperatures.
The compound orders around 5 K [84]. Above 52 K the bending of the magnetization curves disappears. The susceptibility determined from the slope of the M(B a )
curves measured at various temperatures is shown on right side of Fig. 8.27. From
this an effective moment of 4.19 μ B and a paramagnetic Curie temperature θ p = 0.5
K is obtained in good agreement with findings of [76]. Assuming that the Pr moment
is the one of the 3+ ion, the moment of the (Fe 4 Sb 12 ) block can be calculated. With
the assumption that the contributions are simply additive according to
407
Beside the high importance for applications, these materials are of interest because
of the large variety of possible ground states. Depending on the RE atom features
like superconductivity in LaRu 4 As 12 (T c = 10.3 K) [72], LaOs 4 As 12 (T c = 3.2 K)
[72], PrRu 4 Sb 12 (T c = 1 K) [73], PrRu 4 P 12 (T c = 2.4 K) [74], where for the last
compound a metal insulator transition is present at 60 K [75], long range magnetic
order in EuFe 4 Sb 12 (T mag = 84 K) [76, 77], heavy fermion behaviour in YbFe 4 Sb 12
[78], non-Fermi liquid behaviour in CeRu 4 Sb 12 [79], mixed valence behaviour in
RE(Co,Fe) 4 Sb 12 with RE = Yb and Eu [80, 81], are found. In case of the Fe containing skutterudites also a large variety of magnetic ground states is reported. E.g.
REFe 4 P 12 compounds with RE = Nd, Eu, Ho are ferromagnetic with ordering temperatures 1.9 K, 80 K, and 5 K, respectively [76]. Also SmFe 4 Sb 12 is ferromagnetic
below 45 K. NaFe 4 Sb 12 , and KFe 4 Sb 12 skutterudites are itinerant ferromagnets with
ordering temperatures 85 K for both [82]. TlFe 4 Sb 12 was found to be a weak itinerant ferromagnet [83]. PrFe 4 Sb 12 is antiferromagnetic with Neél temperature of
4.6 K [84]. For PrFe 4 P 12 antiferroquadrupolar interactions play an important role
below 6.2 K [85–87]. AFe 4 Sb 12 compounds with A = Ca, Sr, Ba, Tm, and Yb are
paramagnetic respectively nearly ferromagnetic [83]. As mentioned above LaFe 4 P 12
is superconducting below 4 K [88], whereas LaFe 4 Sb 12 is an enhanced paramagnet
[76, 82]. One important question is, how the Fe-atoms contribute to the magnetic
behaviour. In spite of large amount of theoretical and experimental investigations a
precise knowledge about Fe-moments and their interplay with the filler atoms are
still missing [70]. E.g. the LaFe 4 P 12 compound is as mentioned above superconducting, indicating that Fe has no moment, although from susceptibility measurements a
room temperature effective moment of 1.46 μ B /fu is obtained. Because La
3+ has no
magnetic moment, the measured one has to be attributed to the (Fe 4 Sb 12 ) building
blocks. Band structure calculations on La(Co,Fe) 4 P 12 indicate hybridization of the
La sites with Sb and Fe states resulting in an enhanced effective mass for the two
highest occupied bands [89]. Furthermore a double peak structure of the 3d-DOS
in the proximity of the Fermi energy was obtained, from which the presence of a
non-zero moment on the Fe site was concluded [90]. Newer studies pointed out that
spin fluctuations are important and that this compound seems to be near to a ferromagnetic quantum critical point [91]. The question about the contribution of Fe to the
magnetization is further puzzling, if the Sb compounds are considered which show
effective moments of several μ B depending on the type of filler atom (Table 8.2).
It should be mentioned that in contrast to the Fe skutterudites based on P, in the Sb
based Fe skutterudites the RE sublattice is not always fully occupied. In case of the
Pr 0.73 Fe 4 Sb 12 compound an effective moment of 4.19 μ B /fu is found from magnetic
measurements. Figure 8.27 shows magnetization curves at different temperatures.
The compound orders around 5 K [84]. Above 52 K the bending of the magnetization curves disappears. The susceptibility determined from the slope of the M(B a )
curves measured at various temperatures is shown on right side of Fig. 8.27. From
this an effective moment of 4.19 μ B and a paramagnetic Curie temperature θ p = 0.5
K is obtained in good agreement with findings of [76]. Assuming that the Pr moment
is the one of the 3+ ion, the moment of the (Fe 4 Sb 12 ) block can be calculated. With
the assumption that the contributions are simply additive according to