An Iron-Porphyrin Complex with Large Easy-Axis Magnetic Anisotropy on Metal Substrate

Abstract

Easy-axis magnetic anisotropy separates two magnetic states with opposite magnetic moments, and single magnetic atoms and molecules with large easy-axis magnetic anisotropy are highly desired for future applications in high-density data storage and quantum computation. By tuning the metalation reaction between tetra-pyridyl-porphyrin molecules and Fe atoms, we have stabilized the so-called initial complex, an intermediate state of the reaction, on Au(111) substrate, and investigated the magnetic property of this complex at a single-molecule level by low-temperature scanning tunneling microscopy and spectroscopy. As revealed by inelastic electron tunneling spectroscopy in magnetic field, this Fe-porphyrin complex has magnetic anisotropy energy of more than 15 meV with its easy-axis perpendicular to the molecular plane. Two magnetic states with opposite spin directions are discriminated by the dependence of spin-flip excitation energy on magnetic field and are found to have long spin lifetimes. Our density functional theory calculations reveal that the Fe atom in this complex, decoupled from Au substrate by a weak ligand field with elongated Fe-N bonds, has a high-spin state S = 2 and a large orbital angular momentum L = 2, which give rise to easy-axis anisotropy perpendicular to the molecular plane and large magnetic anisotropy energy by spin-orbit coupling. Since the Fe atom is protected by the molecular ligand, the complex can be processed at room or even higher temperatures. The reported system may have potential applications in nonvolatile data storage, and our work demonstrates on-surface metalation reactions can be utilized to synthesize organometallic complexes with large magnetic anisotropy.

Visualizing Intramolecular Distortions as the Origin of Transverse Magnetic Anisotropy

“…In both of these structures, the FeTPyP molecules lie in the aforementioned saddle-shape configuration, as can be inferred by the twofold symmetry of the porphyrin macrocycle. Similar shapes have been observed for many metal-tetra-phenyl-porphyrins (M-TPP) and metal-tetra-pyridyl-porphyrins (M-TPyP) on surfaces [6][7][8][9][10][15][16][17].…”

Robust magnetic anisotropy of a monolayer of hexacoordinate Fe(ii) complexes assembled on Cu(111)

“…Unfortunately, it is not possible to obtain a full set of parameters for the magnetic anisotropy of the molecules leading, for instance, to the 4.2 T signal at 662. 4 GHz. An axial magnetic anisotropy D' EPR = 14.1 cm -1 is found considering a slightly larger rhombicity (E' EPR =1.14 cm -1 i.e.…”

“…1,2 Regarding the change of magnetic anisotropy of such molecules when adsorbed on a surface, little is known. It has been shown recently that for planar molecules such as Fe(II)-phtalocyanin or Fe(II)-porphyrin, not only the magnetic anisotropy but also the spin state can be strongly modified on the surface, depending on the adsorption geometry, 3,4 local distortions, and intermolecular interactions. 5 The origin of this large change in the spin electronic structure of this type of molecules is the relatively flexible and unsaturated coordination sphere of Fe(II).…”

Comparing Cyanophenyl and Pyridyl Ligands in the Formation of Porphyrin-Based Metal–Organic Coordination Networks

“…The observed brightness in the porphyrin core is well in line with previous studies of Fe-substituted tetrapyridyl and tetraphenyl porphyrin networks on Au(111) and Ag(111). 15 , 30 , 51 − 54 The formation of the three maxima can be explained by a combination between the saddle-shape conformation of Zn-TPyP on the Au(111) surface and the replacement of Zn atoms by Fe atoms, where the maxima located at the edges of the porphyrin core ( Figure 3 a) can be assigned to the two upper tilted pyrrole moieties and the central maximum is given by an Fe atom that replaced the Zn atom of Zn-TPyP. Such a molecular appearance has been reported for Fe-TPyP molecules on metal surfaces.…”