Substrate-controlled ferromagnetism in iron phthalocyanine films due to one-dimensional iron chains

Abstract

Using magneto-optical Kerr effect, SQUID magnetometry and magnetic circular dichroism spectroscopy, ferromagnetism was found below 4.5K in iron phthalocyanine thin films grown by molecular beam epitaxy. The stacking of the molecules can be controlled via deposition temperature and substrate choice. The molecules self-assemble such that quasi one-dimensional iron chains form. The chains are limited in length by the grain size or film thickness which is manipulated using judicious growth methods. Magnetic hysteresis loops show distinct features in the saturation magnetization and susceptibility that depend on the thin film structure and/or substrate. We find two regimes for the magnetic behavior: below temperatures of ∼ 25K, intra-chain interactions couple the Fe ions and produce non-traditional paramagnetic behavior, while at low temperatures below ∼ 4.5K interactions between the one-dimensional chains produce hysteresis, magnetic order, and slow magnetization dynamics.

Controlling Ferromagnetic Ground States and Solitons in Thin Films and Nanowires Built from Iron Phthalocyanine Chains

“…These planes are normal to the substrate and correspond to the (001) and (100) spacings, respectively, when using the α‐phase structure derived by Hoshino et al for CuPc . We note that the existing indexed structures for FePc correspond to a high temperature preparation traditionally associated with the β‐phase, and that using the structure of α‐CuPc to interpret the diffraction features of FePc films deposited at room temperature is an approach that has been commonly used by other groups . The TEM pattern is as expected from previous studies of the effect of PTCDA on metal phthalocyanine films, although it has been observed that the templated phase can also display (001) planes in TEM due to the spread in the orientation of the molecular crystals …”

“…Transition metal phthalocyanines (MPcs) are very interesting as a new type of organic magnetic semiconductor that can offer tunability of the magnetic properties . Pcs are mostly planar aromatic molecules, existing in a large number of crystal phases, which can host a spin‐bearing ion in the center.…”

“…The next step is to search for ferromagnetic analogues which are more susceptible to small external magnetic fields and for which electron gases in transport channels (e.g., between source and drain in a transistor) could be spin polarized. Iron phthalocyanine (FePc) has already been shown to exhibit ferromagnetism at low temperature with some tunability of the magnetic properties achievable through altering the crystal structure or grain size, and is characterized by an unusually high orbital moment . Ferromagnetic FePc nanowires with crystal structure identical to that of thin films were recently highlighted, with patterned substrate and inorganic shells promoting vertical growth .…”

Molecular tilting and columnar stacking of Fe phthalocyanine thin films on Au(111)

“…At least two factors may contribute to this: / XLPA T is obtained from a film of about 80 molecules thick, while / STM T is measured directly on the 2nd layer. Moreover, the Au substrate on the STM sample is a Au(111) single crystal, while the "smoothed" gold substrate is used for the thick film and XLPA experiments, even though highly ordered, 13,15 it is not a single crystal, and the overall tilt may be larger.…”

Tunable finite-sized chains to control magnetic relaxation

“…Using Oguchi's method, 25 the inter-chain interaction was estimated to be J < 10 −3 J , demonstrating quasi onedimensional character of FePc thin films.Similar to the powder specimen, thin film samples of FePc also show interesting magnetic properties. In particular, FePc thin films have non-equilibrium magnetic hysteresis loops below 5 K. [26][27][28] The coercivity in films of fixed thickness strongly depends on the deposition temperature, the measurement speed, and measurement temperature. 29 XMCD data of FePc/Au thin films reveals an xy anisotropy in the plane of the molecule.…”