Mössbauer study of the hyperfine interactions and spin dynamics inα-iron(II) phthalocyanine

Abstract

The 57Fe Mössbauer spectroscopy on ␣-iron͑II͒ phthalocyanine ͑FePc͒ as a function of temperature ͑1.3 Ͻ T Ͻ 295 K͒ and applied field ͑0 Ͻ B Ͻ 10 T͒ has been used to study the peculiar magnetic properties of this ferromagnetic quasilinear chain type compound. One sextet with an internal hyperfine field B int = 66.2 T was observed at 1.3 K, a very large value for a bivalent iron with S = 1 pointing to the existence of large positive orbital and dipolar contributions in the investigated FePc. Under an applied field, the experimental spectra exhibited two nonequivalent Fe positions, due to spin canting, with the values for the hyperfine fields of the split sextets increasing with increasing field, an indication that unlike most cases, B int in ␣-FePc is positive, i.e., parallel to the magnetic moment of iron. Therefore, the origin of the large hyperfine field is the orbital moment rather than the Fermi's contact interaction. This fact is ascribed to the orbital degeneracy of the ground state of Fe͑II͒ in the present configuration, where an unpaired hole occupies the orbital doublet ͑d xz , d yz ͒. This feature supports and explains the magnetization and susceptibility data as well as the anomalously high hyperfine field observed at 57 Fe nucleus. The relaxational behavior in the ac susceptibility and Mössbauer spectra found in the region 5 -20 K was ascribed to solitonlike motion of domain walls within the magnetic chains, with a singlekink activation energy of 72 K.

Highly unquenched orbital moment in textured Fe-phthalocyanine thin films

“…This fact also explains the Mössbauer spectroscopy ͑MS͒ as a function of field measurements, which found the highest hyperfine field of positive sign ever measured on an Fe͑II͒ ion ͑B hf = 66.2 T͒. 19 Indeed, this requires the existence of a large orbital moment to explain the field dependence of the low temperature MS sextets, in excellent agreement with our result.…”

“…ground state configuration is 3 E, under lower symmetry than D 4h . They mention that this configuration confirms the 3 E proposed earlier from Mössbauer experiments, 19 or from theoretical density functional theory ͑DFT͒ calculations on the FePc isolated molecule. 17 In fact, the empty state configuration given in Ref.…”

“…The electronic ground state has been proposed to be 3 E g , [17][18][19][20] among others. In the following, we show that our experimental results do support this assignment.…”

“…This result is quite unexpected since ␣-FePc had been reiteratively described as an Ising system, with the moment perpendicular to the molecule plane. 19 Of course, the FePc film has a different molecular stacking than ␣-FePc, but the molecule anisotropy would be expected to be similar, if not the same. A quite reasonable possibility is that in the bulk the molecular anisotropy axis is xy, but since our previous work on ␣-FePc was performed on powder samples, where the anisotropy is averaged in all directions, we were misled to propose the molecule z axis as the magnetic easy axis.…”

Constraint density functional calculations for multiplets in a ligand-field applied to Fe-phthalocyanine

“…(There is, however, general consensus that the ground state has S=1.) Meanwhile, recent Mössbauer spectroscopy and X-ray magnetic circular dichroism (XMCD) experiments 8,9 for α-FePc found a 3 E g ground state with a large orbital moment of 0.53 µ B , and a large hyperfine field of 66.2 T that is parallel to the magnetic moment 8 . Magnetization hysteresis experiments, furthermore, indicated that the easy axis of magnetization is parallel to the molecule plane, i.e., a planar magnetic anisotropy 9 .…”