Superlattice of Single Atom Magnets on Graphene

Abstract

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 moirépattern. KEYWORDS: Single atom magnets, self-assembly, superlattice, rare earth atoms, graphene, XMCD T he fabrication of ordered structures at the nanoscale is a crucial step toward information storage at ultimate length scales.1,2 Realizing highly ordered and monodispersed magnetic structures stands as one of the key challenges for increasing the bit density of magnetic storage devices. The ultimate limit of a single atom per bit guarantees the highest storage density and minimal dipolar coupling among the bits. Single-ion molecular magnets, 3 as well as metal−organic networks, 4 allow the selfassembly of single magnetic atoms in ultradense arrays. The molecular cage defines the spacing between the magnetic cores and can protect them from contamination. However, the coupling with electrons and vibrational modes of the surrounding ligands limits the magnetic stability of the magnetic core presently to temperatures below 20 K in bulk 5 and 8 K for surface supported molecules. 6 The absence of the organic ligand, i.e., having individual atoms adsorbed on the surface, may result in a reduced interaction with the environment and greater magnetic stability. Intense research on the magnetism of single atoms 7−12 has culminated in the achievement of magnetic remanence in Ho atoms randomly adsorbed onto MgO, with magnetic stability up to 40 K. 13 Reading and writing of these atoms has recently been demonstrated.14 Nevertheless, so far the realization of an ensemble of single atoms combining long magnetic lifetimes with spatial order has remained elusive and stands as the next milestone. Here we exploit the selective adsorption of Dy atoms in the periodic moirépattern formed by graphene on lattice mismatched Ir(111) 15 to create a superlattice of single atom magnets with a mean distance of 2.5 nm and negligible mutual magnetic interactions.Ensembles of individual Dy atoms on graphene on Ir(111) are obtained by deposition with an e-beam evaporator ( Figure 1a, middle). The spatial arrangement of these atoms on the graphene moirépattern can be controlled by the sample temperature during deposition, T dep . Deposition below 10 K yields statistical growth with a random distribution of Dy atoms, as demonstrated by scanning tunneling microscopy (STM) in Figure 1a, left. At 40 K, surface diffusion of Dy is activated and, therefore, the atoms can reach the most favorable adsorption site in the moiréunit cell, namely, where the C-...

4f occupancy and magnetism of rare-earth atoms adsorbed on metal substrates

“…Note that no hysteresis opening has been observed in any of these magnetization curves i.e., the lifetime of the magnetic states is shorter than about ten seconds. We ascribe this lack of magnetic stability to the interaction with the conduction electrons and the soft phonon modes of the metal substrate, as mentioned in previous works [14,18].…”

“…Cu (111) Ag (100) Ag (111) Pt (111) Cu (111) Ag (100) Ag ( becomes comparable to E f d + δE c and both divalent and trivalent states have been observed [11,14,17,18]. Table II summarizes the values of E f d + δE c [20,21,36], and the trends in the 4f occupation for surface adsorption.…”

“…Starting with the discovery of giant magnetic anisotropy energy (MAE) in single Co atoms on Pt(111) [1], the understanding of the magnetism of single adatoms has evolved rapidly. The most studied elements having been transition metals (TM) [2][3][4][5][6][7][8][9][10], the attention is now moving to rare-earth (RE) elements [11][12][13][14][15][16], culminating in the discovery of magnetic stability in single Ho atoms on MgO(100) thin films [17] and in single Dy atoms on graphene on Ir(111) [18].…”

“…Multiple examples of trivalent states of different REs have been reported for surface-supported low coordinated atoms [11,14,16,17]. As for the divalent state, only Dy atoms on graphene/Ir(111) have been reported so far [18]. What triggers the different 4f occupancies in the surface-adsorbed REs has remained largely unexplored.…”

“…In addition, the XMCD sum rules [38,39] and multiplet analysis can be used to estimate the corresponding J z values. We chose late lanthanides for their large magnetic moments coming from the parallel alignment of L and S. Additionally, they exhibit larger magnetic anisotropy and have been more intensively studied as model systems for atomic magnetic memories in single ion [40][41][42][43] and single atom magnets [17,18]. Among the series of late lanthanides, we investigated Dy, Ho, Er, and Tm as they have been found both in divalent and trivalent states in bulk compounds [44], hence, they represent ideal candidates to explore the interplay between surface binding, 4f occupancy, and strength of the crystal field.…”