Understanding the Superior Stability of Single-Molecule Magnets on an Oxide Film

The stability of magnetic information stored in surface adsorbed single-molecule magnets is of critical interest for applications in nanoscale data storage or quantum computing. The present study combines X-ray magnetic circular dichroism, density functional theory and magnetization dynamics calculations to gain deep insight into the substrate dependent relevant magnetization relaxation mechanisms. X-ray magnetic circular dichroism reveals the opening of a butterfly-shaped magnetic hysteresis of DyPc2 molecules on magnesium oxide and a closed loop on the bare silver substrate, while density functional theory shows that the molecules are only weakly adsorbed in both cases of magnesium oxide and silver. The enhanced magnetic stability of DyPc2 on the oxide film, in conjunction with previous experiments on the TbPc2 analogue, points to a general validity of the magnesium oxide induced stabilization effect. Magnetization dynamics calculations reveal that the enhanced magnetic stability of DyPc2 and TbPc2 on the oxide film is due to the suppression of two-phonon Raman relaxation processes. The results suggest that substrates with low phonon density of states are beneficial for the design of spintronics devices based on single-molecule magnets.

Superlattice of Single Atom Magnets on Graphene

Romana Baltic, Harald Brune, Fabio Donati, Jan Gui-Hyon Dreiser, Marina Pivetta, Stefano Rusponi, Aparajita Singha, Christian Wäckerlin

Regular arrays of single atoms with stable magnetization represent the ultimate limit of ultrahigh density storage media. Here we report a self-assembled superlattice of individual and noninteracting Dy atoms on graphene grown on Ir(111), with magnetic hysteresis up to 5.6 T and spin lifetime of 1000 s at 2.5 K. The observed magnetic stability is a consequence of the intrinsic low electron and phonon densities of graphene and the 6-fold symmetry of the adsorption site. Our array of single atom magnets has a density of 115 Tbit/inch(2), defined by the periodicity of the graphene moire pattern.

American Chemical Society2016

Exchange Interaction of Strongly Anisotropic Tripodal Erbium Single-Ion Magnets with Metallic Surfaces

Harald Brune, Fabio Donati, Jan Gui-Hyon Dreiser, Stefano Rusponi, Aparajita Singha, Christian Wäckerlin

We present a comprehensive study of Er(trensal) single-ion magnets deposited in ultrahigh vacuum onto metallic surfaces. X-ray photoelectron spectroscopy reveals that the molecular structure is preserved after sublimation, and that the molecules are physisorbed on Au(111) while they are chemisorbed on a Ni thin film on 0(100) single-crystalline surfaces. X-ray magnetic circular dichroism (XMCD) measurements performed on Au(111) samples covered with molecular monolayers held at temperatures down to 4 K suggest that the easy axes of the strongly anisotropic molecules are randomly oriented. Furthermore XMCD indicates a weak antiferromagnetic exchange coupling between the single-ion magnets and the ferromagnetic Ni/Cu(100) substrate. For the latter case, spin-Hamiltonian fits to the XMCD M(H) suggest a significant structural distortion of the molecules. Scanning tunneling microscopy reveals that the molecules are mobile on Au(111) at room temperature, whereas they are more strongly attached on Ni/Cu(100). X-ray photoelectron spectroscopy results provide evidence for the chemical bonding between Er(trensal) molecules and the Ni substrate. Density functional theory calculations support these findings and, in addition, reveal the most stable adsorption configuration on Ni/Cu(100) as well as the Ni-Er exchange path. Our study suggests that the magnetic moment of Er(trensal) can be stabilized via suppression of quantum tunneling of magnetization by exchange coupling to the Ni surface atoms. Moreover, it opens up pathways toward optical addressing of surface-deposited single-ion magnets.

Amer Chemical Soc2014

Two-dimensional metal-organic networks as templates for the self-assembly of atom and cluster arrays

Giulia Elisabetta Pacchioni

This thesis presents a study of the use of two-dimensional metal-organic systems as templates for the organization of metal atoms and clusters on surfaces. We start with a systematic characterization of the metal-organic porous networks formed on Cu(111) by polyphenyl-dicarbonitrile molecules, and of the temperature dependence of the assembly process, leading to a variety of geometrical structures. Using molecules of two different lengths we observe networks with distinct periodicities, and we reveal a competition between the different interactions governing the assembly. We also study the self-assembly of a single molecule magnet on supported graphene, observing the same disposition as in a layer of the molecular crystal, which explains the high magnetic anisotropy measured for the system. The metal-organic template is used to organize metal atoms and clusters in the network pores, obtaining a regular array of clusters with a narrow size distribution. We demonstrate how this approach can be used to produce clusters of different elements, such as Fe, Co and Er, as well as mixed transition metal - rare earth metal clusters. Otherwise, the metal-organic networks can be used to organize Fe atoms under the molecules, in which case a two-orbital Kondo system with a marked spatial dependence is obtained. After characterizing the magnetic properties of Fe atoms adsorbed on bare Cu(111), we use a combination of scanning tunneling spectroscopy, density functional theory and x-ray absorption and dichroism to study the Kondo effect of the Fe-molecule system, identifying the involved magnetic orbitals and demonstrating that they are both Kondo screened.

EPFL2016