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

TL;DR

This study demonstrates how the magnetic and optical properties of iron phthalocyanine thin films and nanowires can be precisely controlled through growth techniques, revealing their potential for advanced spintronic applications.

Contribution

It introduces a method to tune ferromagnetic properties of FePc by switching growth strategies, and models their magnetic behavior with a Heisenberg framework applicable to one-dimensional ferromagnetic chains.

Findings

High coercivities over 1 T achieved.

Exchange constants modulated between 15 and 29 K.

Magnetic behavior well-described by soliton and super-Curie-Weiss models.

Abstract

Iron phthalocyanine (FePc) is a molecular semiconductor whose building blocks are one-dimensional ferromagnetic chains. We show that its optical and magnetic properties are controlled by the growth strategy, obtaining extremely high coercivities of over 1 T and modulating the exchange constant between 15 and 29 K through tuning the crystal phase by switching from organic molecular beam deposition, producing continuous thin films of nanocrystals with controlled orientations, to organic vapour phase deposition, producing ultralong nanowires. Magnetisation measurements are analysed using a suite of concepts and simply stated formulas with broad applicability to all one-dimensional ferromagnetic chains. They show that FePc is best described by a Heisenberg model with a preference for the moments to lie in the molecular planes, where the chain Hamiltonian is very similar to that for the classic inorganic magnet CsNiF3, but with ferromagnetic rather than antiferromagnetic interchain interactions. The data at large magnetic fields are well-described by the soliton picture, where the dominant (and topologically non-trivial) degrees of freedom are moving one-dimensional magnetic domain walls, which was successful for CsNiF3, and at low temperatures and fields by the super-Curie-Weiss law of 1/(T^2+theta^2) characteristic of nearly one-dimensional xy and Heisenberg ferromagnets. The ability to control the molecular orientation and ferromagnetism of FePc systems, and produce them on flexible substrates as thin films or nanowires, taken together with excellent transistor characteristics reported previously for nanowires of copper and cobalt analogues, makes them potentially useful for magneto-optical and spintronic devices.