Enhanced magnetic spin-spin interactions observed between porphyrazine derivatives on Au(111)

The field of molecular spintronics and quantum information processing seeks to harness the spin properties of individual molecules for technological applications. Controlling the interactions between neighboring molecular spins is paramount for building functional devices. Metal phthalocyanine (Pc) molecules have been extensively studied, but their relatively weak molecule-molecule interactions limit their potential. This limitation stems from the inert C-H groups at the molecule's perimeter. To overcome this, researchers have explored substituting the benzene rings of Pc molecules with more reactive moieties. Tetrakis(1,2,5-thiadiazole)porphyrazines (TTDPzs), synthesized by Stuzhin and colleagues, offer a promising alternative. The thiadiazole groups, containing nitrogen and sulfur atoms, are more reactive than benzene rings, leading to stronger intermolecular interactions via S-N electrostatic forces. Previous work on vanadyl TTDPz (VOTTDPz) showed a ferromagnetic ordering in the α-form of its bulk crystal. This study uses scanning tunneling microscopy/spectroscopy (STM/STS) to investigate the magnetic properties of VOTTDPz films grown on Au(111), focusing on the impact of the enhanced molecule-molecule interactions enabled by the thiadiazole ligand. The researchers aim to understand how the functionalization of the ligand with a more reactive moiety influences the complex spin-spin interactions between the molecules.

The literature review section extensively cites previous studies on the magnetic properties of metal phthalocyanine molecules, highlighting the limited number of reports on spin-spin interactions due to weak molecule-molecule interactions. It then focuses on the synthesis and properties of tetrakis(thiadiazole)porphyrazines (TTDPzs) and their derivatives, emphasizing the enhanced reactivity of the thiadiazole moieties and the resulting stronger intermolecular interactions. The previous work on VOTTDPz, including its synthesis, structural analysis (α and β polymorphs), and the observation of ferromagnetic ordering in the α-form, is critically reviewed. This establishes the context for the present study, which aims to investigate the impact of enhanced molecule-molecule interactions on the magnetic properties of VOTTDPz adsorbed on a surface.

The study employs a combination of experimental and theoretical techniques. VOTTDPz molecules were synthesized using a previously reported method. A sublimation method was used to deposit the molecules onto a clean Au(111) surface in an ultra-high vacuum (UHV) chamber. STM and STS measurements were performed at a sample temperature of -4.7 K. STS spectra were obtained using a lock-in amplifier with a modulation voltage of 1 mV. Density functional theory (DFT) calculations, utilizing the Vienna Ab initio Simulation Package (VASP) code with a plane-wave basis set and projector augmented wave potentials, were employed for structural optimization and spin-resolved local density of states (LDOS) calculations. The Au(111) surface was modeled as a three-atom-thick slab, with the bottom layer fixed at the bulk position during optimization. The Tersoff-Hamann method was used to simulate STM images. The calculations were performed on the Numerical Materials Simulator at NIMS, Japan. The study focused on two distinct arrangements of molecules observed in the STM images, designated Phase I and Phase II.

The study reveals two distinct phases in the VOTTDPz film on Au(111). Phase I consists of flat-lying molecules with the VO group pointing towards the vacuum. STS measurements reveal Kondo resonance features at both the center and ligand positions. Applying an external magnetic field enhances the intensity of the Kondo peak at the ligand position. A spatial variation within a single molecule is observed, showing a crossover from a Kondo peak to a Fano dip. This is attributed to varying coupling strengths between the thiadiazole groups and the substrate, with stronger coupling leading to Fano dips. Phase II exhibits an alternating arrangement of flat-lying (VO-up) and tilted (VO-down) molecules. DFT calculations show that while spin polarization persists in all molecules, the VO-down molecules exhibit either FM or AFM coupling with neighboring molecules, resulting in the appearance or disappearance of Kondo peaks in STS measurements. The appearance and disappearance of the Kondo peak is explained by the competition between Kondo resonance and Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, with the latter being amplified by the close proximity of molecules due to the compact thiadiazole ligand. Preliminary XMCD results suggest weaker magnetization in VOTTDPz compared to VOPc, potentially due to AFM ordering in Phase II.

The findings demonstrate that the enhanced molecule-molecule interaction, introduced by the thiadiazole ligand in VOTTDPz, significantly influences the magnetic properties and spin-spin interactions. The observed site-dependent Kondo resonance features and the contrasting behavior in phases I and II highlight the complex interplay between Kondo screening, RKKY interaction, and the local chemical environment. The ability to manipulate spin-spin interactions through molecular design opens avenues for creating novel molecular spintronic devices. The spatial variation of the Kondo peak, resulting in the crossover from Kondo peak to Fano dip, offers a potential tool for chemical analysis with sub-molecular resolution. The contrasting behavior in phases I and II shows how subtle changes in molecular arrangement can drastically affect magnetic behavior, a crucial aspect for future device design. The observed effects are attributed to the enhanced RKKY interaction due to the smaller molecule-molecule spacing resulting from the compact nature of the thiadiazole ligand.

This study successfully demonstrates the impact of molecular design on the magnetic properties of VOTTDPz molecules adsorbed on Au(111). The observation of distinct phases with varying spin-spin interactions opens new avenues for engineering magnetic properties in molecular systems. Future research should explore more sophisticated methods to control molecular arrangement and explore the potential of this system for applications in molecular spintronics and quantum information processing. The observation of a spatial variation in the Kondo peak suggests its potential use as a high-resolution chemical analysis technique.

The study primarily focuses on two distinct phases observed in the VOTTDPz film. It is possible that other phases or arrangements might exist, potentially exhibiting different magnetic properties. The DFT calculations, while informative, do not fully account for all interactions and complexities of the system. The experimental observations are limited by the resolution of the STM/STS technique. Further studies employing complementary techniques, such as XMCD, are needed to confirm the proposed AFM ordering in Phase II.