Ordered Porphyrin Arrays on Fe(001): An Enabling Technology for Future Spintronics ^†

Abstract

We give evidence of the formation of an ordered array of tetra-phenyl porphyrins (TPP) when these molecules are deposited on top of oxygen-passivated Fe(001), namely the Fe(001)-p(1 × 1)O surface. We also prove that they are magnetically coupled with the substrate. The ordered molecular packing, together with the magnetic coupling, are fundamental conditions for application in organic spintronic devices. The system is studied by means of spin-resolved photoemission spectroscopies and scanning tunneling microscopy.

1. Introduction

Nowadays, a lot of effort in spintronic research is devoted to the construction of devices where magnetic moments are spatially located in an ordered array, so that it is possible to address them individually to encode information. During the last decade, studies have focused on the possibility of replacing standard inorganic silicon-based electronics with a new class of hybrid organic/inorganic devices [1], thanks to the outstanding achievements in the chemical synthesis of molecules allowing for a fine-tuning of their properties. Within this framework, metal tetra-phenyl porphyrins (MTPPs) can play a key role, due to their flat structure that allows a direct interaction with the substrate underneath. Moreover, it has been demonstrated that an isolated MTPP can carry a magnetic moment associated with a total not-null spin [2]. In this frame, we have focused our attention on the study of the interaction of different MTPPs with a magnetic substrate. Molecules are deposited onto the oxygen-passivated Fe surface, where O atoms sit in the hollow sites of Fe(001), forming a p(1 × 1)O superstructure. The presence of this ultra-thin oxygen layer leads to the formation of well-ordered arrays [3] and, on the other hand, enhances the surface magnetic moment, with respect to the bare iron surface [4]. Here, we investigate the magnetic coupling with the substrate of a single MTPP layer (where M is Co (CoTPP), Ni (NiTPP) or Zn (ZnTPP)) by means of spin-resolved photoemission spectroscopy (SR–PES) and inverse photoemission spectroscopy (SR–IPES), while we exploit scanning tunneling microscopy (STM) to investigate the film structure.

2. Materials and Methods

Porphyrin molecules were provided by Sigma Aldrich and used without further purification. The cleaning of the substrate and the deposition of the molecular film were performed in dedicated ultrahigh vacuum chambers (pressure in the 10⁻¹⁰ mbar range). The preparation and the analysis of the system were carried out in situ without exposure to the atmosphere. The substrate was a thick iron film (500 nm) deposited on MgO. After a standard cleaning by means of sputtering and annealing cycles, the surface was exposed to 30 L of oxygen (1 langmuir (L) = 1.33⋅10⁻⁶ mbar⋅s) with the sample kept at 450 °C. The sample was then heated up to 700 °C to remove excess oxygen. The molecules were deposited by effusion from a Knudsen cell with the sample kept at room temperature. The sample was magnetized through a coil along one of the in-plane 〈100〉_Fe directions and the spin-resolved spectra were acquired in remanence.

3. Results and Discussion

Figure 1a displays the STM image of a single CoTPP layer. Similar results (not shown) have been obtained also for NiTPP and ZnTPP. The image clearly shows the ordered structure of the molecular array. By comparison with results on the bare substrate, it is possible to understand that the TPP molecules arrange themselves by forming a commensurate (5 × 5) superlattice. SR-PES and SR-IPES spectra are reported in panel (b). The characteristic substrate lineshape, where majority and minority spin features are clearly distinguishable, becomes progressively less intense as the CoTPP film thickness is increased, up to one monolayer (ML) of coverage, while new structures related to the molecule electronic states appear. A fine analysis of their profile reveals that an energy splitting between the majority and minority spin population is present for CoTPP [5]. A similar study has been carried out for NiTPP and ZnTPP, where no magnetic coupling is expected from theoretical predictions [2]. In these two cases, experimental data do not allow to detect any difference between the majority and minority spin populations.