Iron(II) octaethyltetraazaporphyrin has been discovered toexist as two polymorphs at roomtemperature,
- and
-Fe(OETAP). This has beenestablished by X-ray powder diffraction and scanningelectronmicroscopy. Both ac and dc SQUID magnetometry show that the
phase is a soft molecular ferromagnetwith
TCurie = 2.8 K. The
phasepossesses the same number of unpaired spins and a similar
gvalue, but itdoes not order above ~1 K. Zero-field splitting is likelycompeting with ferromagnetic coupling in thiscompound.
57Fe Mössbauer spectroscopy hasbeen used to support the local structural similarity betweenthetwo phases and to study the ferromagnetic transition. Theisomorphous manganese analogue of
-Fe(OETAP),
-Mn(OETAP), expected to be a ferromagnet with a higher
TCurie, exhibits many characteristicsofferromagnetism, notably hysteresis at 1.8 K with
Hcoer = 2.5 kG and
Mrem= 4 kemu-G/mol. However, incontrast to ferromagnetic
-Fe(OETAP),
-Mn(OETAP) showsstrongly frequency-dependent ac susceptibilityin the range from 1 to 1000 Hz, indicating that it exists in aspin-glass-like state. This result underscores theneed for frequency-dependent ac magnetometry to fully characterizeferromagnetic molecular materials.