Large effect of metal substrate on magnetic anisotropy of Co on hexagonal boron nitride

Romana Baltic, Harald Brune, Fabio Donati, Jan Gui-Hyon Dreiser, Stefano Rusponi, Aparajita Singha, Christian Wäckerlin
IOP PUBLISHING LTD, 2019
Journal paper

Abstract

We combine x-ray absorption spectroscopy (XAS), x-raymagnetic circular dichroism (XMCD) and x-ray magnetic linear dichroism (XMLD) data with first principles density functional theory (DFT) calculations and amultiorbital many-body Hamiltonian approach to understand the electronic and magnetic properties of Co atoms adsorbed on h-BN/Ru(0001) and h-BN/Ir(111). The XAS line shape reveals, for both substrates, an electronic configuration close to 3d(8), corresponding to a spin S = 1 Magnetic field dependent XMCD data show large (14 meV) out-of-plane anisotropy on h-BN/Ru(0001), while it is almost isotropic (tens of mu eV) on h-BN/Ir(111). XMLD data together with both DFT calculations and the results of the multiorbital Hubbard model suggest that the dissimilar magnetic anisotropy originates from different Co adsorption sites, namely atop Non h-BN/Ru(0001) and 6-fold hollow on h-BN/Ir(111).

Official source

https://infoscience.epfl.ch/record/268809?ln=en

About this result

This page is automatically generated and may contain information that is not correct, complete, up-to-date, or relevant to your search query. The same applies to every other page on this website. Please make sure to verify the information with EPFL's official sources.

Related concepts

Related publications

This thesis presents combined experimental and theoretical investigations of nanoscale, surface-supported magnets based on rare earths (RE) to understand and control the magnetic properties down to the scale of single atoms. We present the effects of adatom-substrate interaction on the magnetic properties of isolated single RE atoms using x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and multiplet analysis. Our systematic investigations of Dy, Ho, Er, and Tm adatoms adsorbed on Pt(111), Cu(111), Ag(100), and Ag(111), reveal that the REs can possess two types of 4f occupations, namely divalent 4f^n and trivalent 4f^(n-1). The trivalent state is realized in presence of low promotion energy of the RE and strong hybridization of their external spd shell with the surrounding environment. Notably, none of the REs exhibit magnetic hysteresis, suggesting that magnetic relaxation is faster than about 10 seconds. We also report on the effect of adatom-adatom interaction by studying the size-dependent magnetic properties of Er clusters adsorbed on Cu(111). Combining XMCD, scanning tunneling microscopy (STM), and mean-field nucleation theory we reveal that the adatom-adatom interaction dominates over the adatom-substrate interaction in Er clusters starting from the size of three atoms. Consequently the easy axis of Er changes from in-plane for the single atoms and dimers to out-of-plane for trimers and bigger clusters. In addition, the out-of-plane magnetic anisotropy of 2.9 meV/atom results in magnetic hysteresis at 2.5 K in all clusters starting from trimers. With a magnetic lifetime of approximately 2 minutes at 0.1 T, the Er trimers are one of the smallest metal-supported ferromagnetic clusters observed so far. The investigation of adatom-adatom interaction is further extended by studying 4f-3d heterodimers namely, Ho-Co adsorbed on thin insulating layers of MgO. Their magnetic easy axis is oriented out-of-plane. Using spin-polarized STM we have detected spin-excitations in these heterodimers at ±20 and ±8 meV. We have identified the origin behind these spin-excitations using an effective spin Hamiltonian model, which indicates that, given a ferromagnetic exchange interaction between Ho and Co, the most intense feature at ±20 meV corresponds to a transition in which the spin moment of Co is strongly diminished. This is further accompanied by an overall change of the total magnetic moment of the heterodimer, i.e., ¿J=-1. In contrast, the weaker transition at ±8 meV occurs following a change in the out-of-plane moment of Ho while the total moment of the heterodimer remains intact i.e., ¿J=0. Notably, we observe an effective g factor of 3.1 for the ±20 meV transition, which significantly exceeds the free electron value of 2. In addition, the ferromagnetic coupling between Ho and Co is very unusual compared to the ferrimagnetic exchange interaction known for the bulk 4f-3d compounds, especially those derived from the late lanthanides. Nonetheless, this marks the first evidence of spin-excitations in the smallest 4f containing cluster. The knowledge gained on the fundamental aspects of magnetism in surface-supported REs combined with the continued parallel search for magnetic stability down to single atoms, enabled us to achieve magnetic remanence in single adatoms with lifetimes of the order of 1000 s at 2.5 K. In addition we have achieved significantly enhanced hysteresis and magnetic lifetime in TbPc2.

EPFL2017

Magnetocrystalline anisotropy energy of Co and Fe adatoms on the (111) surfaces of Pd and Rh

Harald Brune, Markus Etzkorn, Pietro Gambardella, Anne Lehnert, Géraud Moulas, Stefano Rusponi

We performed a combined theoretical and experimental investigation of the orbital magnetism and magnetocrystalline anisotropy of isolated Co and Fe adatoms on Pd(111) and Rh(111). Theoretical calculations of the spin and orbital moments are based on ab initio spin-polarized density-functional theory (DFT) including a self-consistent treatment of spin-orbit coupling. The calculations use a slab model to represent the adsorbate/substrate complex and allow for a complete structural relaxation leading to a strong inward displacement of the adatom and modest vertical and lateral relaxations in the substrate atoms. Compared to an idealized geometry where the atoms are kept on bulk lattice positions up to the surface, relaxation leads to a much stronger adatom/ligand hybridization. This is also reflected in the results for orbital moments and magnetocrystalline anisotropy energy (MAE). The enhanced hybridization leads to strong quenching of the adatom orbital moments but also to the formation of large induced spin and orbital moments in the substrate. As a consequence, we find that the substrate contribution to the MAE is much more important than estimated before on the basis of studies using an idealized geometry. We also find the surprising result that the MAE strongly depends on the adsorption site. The magnitude and even the sign of the MAE change for adatoms on face-centered cubic with respect to the ones on hexagonal close-packed hollow sites on the (111) surface. The dependence of the MAE on the combination of adatom and substrate has been analyzed in terms of the electronic structure, leading to a sound physical picture of the origin of the MAE. A fundamental problem, however, is the correct prediction of the size of the orbital moments of the adatoms. We suggest that this problem can be solved only via post-DFT corrections introducing an orbital dependence of the exchange potential. The theoretical results are compared to site-averaged, element-specific x-ray magnetic circular dichroism (XMCD) measurements. Low-temperature XMCD spectra and magnetization curves reveal weak out-of-plane anisotropy for Fe adatoms on both substrates. Interestingly, Co adatoms on Rh(111) present in-plane anisotropy with MAE of about -0.6 meV, contrary to the known out-of-plane anisotropy of Co on Pd(111) and Pt(111). The orbital to spin magnetic-moment ratio measured by XMCD shows that the Co adatoms present much stronger orbital magnetization components compared to Fe. The connection between orbital moments and MAE is discussed at the theoretical level including the contribution of the induced substrate magnetization.

2010

Magnetic anisotropy from single atoms to large monodomain islands of Co/]Pt(111)

Harald Brune, Pietro Gambardella, Stefano Rusponi, Nicolas Weiss

By studying the magnetic behavior of self-assembled Co islands on a single-crystal metal surface, Pt(111), we show how the magnetic anisotropy evolves from isolated atoms to monolayer islands and films. Single Co adatoms are found to have a giant magnetocrystalline anisotropy energy of E-a = 9.3 +/- 1.6 meV/atom arising from the combination of partly preserved orbital moments (m(L) = 1.1 mu(B)) and strong spin-orbit coupling induced by the Pt substrate. Combined scanning tunneling microscopy and X-ray magnetic circular dichroism experiments performed for differently sized small two-dimensional Co islands establish a clear connection between E-a and m(L), both quantities decrease sharply with the lateral coordination of the magnetic atoms. In accordance with this, Kerr magnetometry experiments, again performed in conjunction with scanning tunneling microscopy, reveal that the anisotropy energy of large two-dimensional Co islands is almost entirely determined by the relatively low number of perimeter atoms, having E-a = 0.9 +/- 0.1 meV/atom. These results confirm theoretical predictions and are of fundamental value the understanding of how the magnetic anisotropy develops in finite-size magnetic particles. Identification of the role of perimeter and surface atoms opens up new opportunities for engineering the anisotropy and the moment of a magnetic nanostructure. (C) 2004 Academie des sciences. Published by Elsevier SAS. All rights reserved.

2005

Magnetocrystalline anisotropy energy of Co and Fe adatoms on the (111) surfaces of Pd and Rh

Harald Brune, Markus Etzkorn, Pietro Gambardella, Anne Lehnert, Géraud Moulas, Stefano Rusponi

2010

EPFL2017