Superlattice of Single Atom Magnets on Graphene

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.

Magnetic Stability of Single Lanthanide Atoms on Graphene

Romana Baltic

This thesis presents a study of the magnetism of surface supported atoms performed principally with XMCD spectroscopy and multiplet calculations. The objective of the research was twofold: first, to study the underlying interactions and conditions governing the magnetization stability of surface supported atoms, with the aim of achieving long magnetic lifetimes, and second, to assemble single atom magnets in an ordered pattern. Through our study of

4f

lanthanide atoms on supporting substrates, we established that their magnetic stability is governed by their quantum level structure, in particular their ground

J_z

state and the height of the energy barrier for thermally assisted magnetization reversal. These features are ruled through the crystal field interaction with their supporting surface. The adsorption site of adatoms governs the symmetry of the crystal field and, consequently, the coupling between the different

J_z

levels. This in turn enables magnetization reversal either through quantum tunneling between

J_z

states or via scattering with the electrons and phonons of the substrate. To reduce the scattering events, it is necessary to decouple the adatoms from the metal substrate by using decoupling layers. Here we show that a single layer of graphene is sufficient to decouple Dy atoms from the underlying Ir(111) substrate, resulting in a magnetic lifetime of about 1000~s at 2.5~K. In addition, we show that the moiré pattern of the graphene/Ir(111) surface can be used as a template for the self-assembly of these single atom magnets into well ordered superlattices. Further, by studying multiple graphene/metal substrates we show that the interaction of graphene with the supporting substrate greatly influences the magnetization stability of adsorbed atoms. Finally, we show that replacing graphene with an insulating layer does not result in a stable magnetization of adsorbed atoms if its superior decoupling is not accompanied with an adequate crystal field symmetry.

EPFL2018

Growth and Magnetism of Nanostructures Investigated by STM, MOKE, and XMCD

Alberto Cavallin

This thesis presents our studies on nanostructures growth and magnetism, mainly based on scanning tunneling microscopy (STM), magneto-optical Kerr effect (MOKE), and x-ray magnetic circular dichroism (XMCD) measurements. In most of these experiments, nanostructures were grown by molecular beam epitaxy on single crystal substrates under ultra high vacuum conditions. Our combined morphological and magnetic investigations aim at an atomic-level description of the nanostructures magnetic properties. In particular, we are searching for strategies to design the smallest ferromagnet with stable magnetization at room temperature to be used in devices such as magnetic random access memories, storage media, and sensors. A critical factor in pursuing this aim is the determination and control of the magnetization reversal mechanism. Different reversal mechanisms may occur in the nanostructures we considered: either all the spins quasi-coherently rotate or a domain wall is nucleated and propagated along the particle. We prepared monolayer-high Co islands on Pt(111) with compact and ramified shape by adjusting the deposition temperatures and coverages for a sequence of deposition and annealing steps. Combining field-dependent magnetization curves and temperature dependent zero-field susceptibility measurements we identify a size- and shape-dependent transition from quasi-coherent to domain wall propagation reversal regimes. In a second study we investigated a system which according to a recent publication presents many of the characteristics desired in bit patterned magnetic media for ultra-high density storage devices. In particular, it was claimed that Co nanoclusters, growing in registry with the GdAu2/Au(111) moiré periodic substrate, and featuring a density of 52 Tera/in2, present out-of-plane remanent magnetization at T = 300 K. At odds with this claim, no perpendicular magnetic anisotropy was found in our combined MOKE and STM study, which shows an in-plane remanent signal appearing only for coalesced islands. A third study is devoted to FeRh, an alloy material which, thanks to its peculiar magnetic phase diagram, has been proposed coupled to FePt for advanced magnetic recording schemes. We present the first observation of low temperature stabilization of the ferromagnetic (FM) phase in FeRh chemically ordered crystals. Nanocrystals measuring 3.3 nm in diameter have B2 structure with alternating atomic Fe and Rh layers. XMCD demonstrates FM alignment of the Fe and Rh magnetic moments of 3 and 1 μB, respectively. In sharp contrast to FeRh bulk and thin films, which show antiferromagnetic iii Abstract (AFM) order below 300 K, the low-temperature FM 3.3 nm nanocrystals are the hallmark of a size-dependent AFM-FM transition temperature. Finally we present our results towards creation by chemical vapor deposition of a “mille-feuille” structure composed of graphene and magnetic nanoclusters. To realize this objective we optimized the cluster size, composition, and annealing temperature to obtain a compromise between graphene syntesis and cluster coalescence. We demonstrate by STM that annealing to T = 720 K in ethylene stabilizes Ir-seeded clusters against diffusion and coalescence, and by Auger Electron Spectroscopy (AES) that Ni-coated clusters can be covered with up to half of the Carbon contained in a graphene layer.

EPFL2013

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

Romana Baltic, Harald Brune, Katharina Johanna Diller, Fabio Donati, Jan Gui-Hyon Dreiser, Stefano Rusponi, Aparajita Singha

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.

WILEY2019