306
N. Kojima and A. Okazawa
The
57 Fe Mossbauer spectrum at room temperature shows an asymmetric doublet
with much broadening in order to a magnetic relaxation, where the IS of ~0.4 mm/s
and the QS of 1.3 mm/s. The temperature-independent characteristics of quadrupole
splitting is observed, indicating a large separation between the ground orbital states
and the excited ones. Below about 50 K, the broad doublet becomes a well-resolved
sextet, and then ends up with the sharp spectrum having overall hyperfine splitting
of 48.8 mm s
−1 at 4.2 K. The result indicates slower magnetization relaxation than
the Mössbuaer time scale (τ ~ 10
−8 s). Magnetic hyperfine field, H n , is estimated as
152 T. According to H n = H F + H D + H L , where H F is the Fermi contact roughly
determined by −12.7 × (2S) T [69] and H D is the dipolar contribution, the value of
the orbital contribution term, H L , is estimated as 203 T when H D is assumed to be
zero.
In the precedent paper [70], saturation magnetization of this compound was
reported to reach ~32,500 cm
3 Oe mol
−1 (or 5.82 μ B ) at 1.85 K (for field-oriented
sample), which is larger than the spin-only value (4 μ B ) that would be expected for the
Fe
II ion with S = 2. After this preliminary study, the details of dynamic magnetization
in the SIM compound have been examined by means of ac and dc magnetic property
measurements and ab initio calculations [71], followed by the analysis of
57 Fe Mössbauer relaxation spectra [72]. Low-temperature saturation magnetization for a fixed
sample of [Fe(C(SiMe 3 ) 3 ) 2 ] is of M sat = 3.24 μ B . This value is lower than the spinonly value of the HS Fe
II ion because of random orientation of strong anisotropic
species. Actually, the χ M T value at room temperature is 4.79 cm
3 K mol
−1 (the effective magnetic moment: μ eff = 6.19 μ B ), indicating strong anisotropy with a large g
value of 2.53. At zero applied dc field, [Fe(C(SiMe 3 ) 3 ) 2 ] shows no slow relaxation of
magnetization in ac magnetic susceptibility, indicating no SIM behavior. However,
such a relaxation can be detectable by means of
57 Fe Mössbauer spectroscopy as
shown in Fig. 6.40, and analysed using a relaxation model formalized by Dattagupta
and Blume [73]. Relaxation time constant (τ ) ranges from 2.5 × 10
−7 to 1.2 ×
10
−10 s within the measuring temperature range of 5–295 K. From the analysis of
Arrhenius plots, the physical parameters on quantum tunnelling and Orbach relaxation processes were extracted as τ
−1
QTM = 4.2(3) × 10
6 s
−1 and U eff = 178(9) and
553(68) cm
−1 , respectively. Two obtained U eff values correspond to the energy splitting from the ground M J = ±4 to the first excited M J = ±3 or the second excited
M J = ±2 doublets among the Stark sublevels of
5 D 4 . It should be mentioned that
ac susceptibility measurements (τ < ~10
4 s) cannot detect the magnetic relaxation
for this system due to the fast QTM process (τ QTM = 2.4 × 10
−6 s) at the level
crossing between the ground M J = ±4 states under zero field. Applying a small dc
field induced suppression of the level crossing between the M J doublets, and thus
magnetization dynamics was observable as a nonzero signal in the out-of-phase ac
susceptibility (χ M
). For Cole–Cole plots, the relaxation dynamics can be evaluated by using a generalized Debye model [74, 75]. Their results reveal clearly the
field-induced SIM nature of [Fe(C(SiMe 3 ) 3 ) 2 ].
Following the pioneering compound, some of ca. 30 linear two-coordinate
Fe
II complexes [76] have currently been investigated from the viewpoint of SIM
N. Kojima and A. Okazawa
The
57 Fe Mossbauer spectrum at room temperature shows an asymmetric doublet
with much broadening in order to a magnetic relaxation, where the IS of ~0.4 mm/s
and the QS of 1.3 mm/s. The temperature-independent characteristics of quadrupole
splitting is observed, indicating a large separation between the ground orbital states
and the excited ones. Below about 50 K, the broad doublet becomes a well-resolved
sextet, and then ends up with the sharp spectrum having overall hyperfine splitting
of 48.8 mm s
−1 at 4.2 K. The result indicates slower magnetization relaxation than
the Mössbuaer time scale (τ ~ 10
−8 s). Magnetic hyperfine field, H n , is estimated as
152 T. According to H n = H F + H D + H L , where H F is the Fermi contact roughly
determined by −12.7 × (2S) T [69] and H D is the dipolar contribution, the value of
the orbital contribution term, H L , is estimated as 203 T when H D is assumed to be
zero.
In the precedent paper [70], saturation magnetization of this compound was
reported to reach ~32,500 cm
3 Oe mol
−1 (or 5.82 μ B ) at 1.85 K (for field-oriented
sample), which is larger than the spin-only value (4 μ B ) that would be expected for the
Fe
II ion with S = 2. After this preliminary study, the details of dynamic magnetization
in the SIM compound have been examined by means of ac and dc magnetic property
measurements and ab initio calculations [71], followed by the analysis of
57 Fe Mössbauer relaxation spectra [72]. Low-temperature saturation magnetization for a fixed
sample of [Fe(C(SiMe 3 ) 3 ) 2 ] is of M sat = 3.24 μ B . This value is lower than the spinonly value of the HS Fe
II ion because of random orientation of strong anisotropic
species. Actually, the χ M T value at room temperature is 4.79 cm
3 K mol
−1 (the effective magnetic moment: μ eff = 6.19 μ B ), indicating strong anisotropy with a large g
value of 2.53. At zero applied dc field, [Fe(C(SiMe 3 ) 3 ) 2 ] shows no slow relaxation of
magnetization in ac magnetic susceptibility, indicating no SIM behavior. However,
such a relaxation can be detectable by means of
57 Fe Mössbauer spectroscopy as
shown in Fig. 6.40, and analysed using a relaxation model formalized by Dattagupta
and Blume [73]. Relaxation time constant (τ ) ranges from 2.5 × 10
−7 to 1.2 ×
10
−10 s within the measuring temperature range of 5–295 K. From the analysis of
Arrhenius plots, the physical parameters on quantum tunnelling and Orbach relaxation processes were extracted as τ
−1
QTM = 4.2(3) × 10
6 s
−1 and U eff = 178(9) and
553(68) cm
−1 , respectively. Two obtained U eff values correspond to the energy splitting from the ground M J = ±4 to the first excited M J = ±3 or the second excited
M J = ±2 doublets among the Stark sublevels of
5 D 4 . It should be mentioned that
ac susceptibility measurements (τ < ~10
4 s) cannot detect the magnetic relaxation
for this system due to the fast QTM process (τ QTM = 2.4 × 10
−6 s) at the level
crossing between the ground M J = ±4 states under zero field. Applying a small dc
field induced suppression of the level crossing between the M J doublets, and thus
magnetization dynamics was observable as a nonzero signal in the out-of-phase ac
susceptibility (χ M
). For Cole–Cole plots, the relaxation dynamics can be evaluated by using a generalized Debye model [74, 75]. Their results reveal clearly the
field-induced SIM nature of [Fe(C(SiMe 3 ) 3 ) 2 ].
Following the pioneering compound, some of ca. 30 linear two-coordinate
Fe
II complexes [76] have currently been investigated from the viewpoint of SIM