Complete reversal of the atomic unquenched orbital moment by a single electron

Abstract

The orbital angular moment of magnetic atoms adsorbed on surfaces is often quenched as a result of an anisotropic crystal field. Due to spin-orbit coupling, what remains of the orbital moment typically delineates the orientation of the electron spin. These two effects limit the scope of information processing based on these atoms to essentially only one magnetic degree of freedom: the spin. In this work, we gain independent access to both the spin and orbital degrees of freedom of a single atom, inciting and probing excitations of each moment. By coordinating a single Fe atom atop the nitrogen site of the Cu 2 N lattice, we realize a singleatom system with a large zero-field splitting-the largest reported for Fe atoms on surfaces-and an unquenched uniaxial orbital moment that closely approaches the free-atom value. We demonstrate a full reversal of the orbital moment through a singleelectron tunneling event between the tip and Fe atom, a process that is mediated by a charged virtual state and leaves the spin unchanged. These results, which we corroborate using density functional theory and first-principles multiplet calculations, demonstrate independent control over the spin and orbital degrees of freedom in a single-atom system.

A theoretical review on the single-impurity electron spin resonance on surfaces

“…They argued that the efficient higher spin excitations via one-step excitation processes in STM-ESR pointed towards the involvement of mechanical degrees of freedom as the way to fullfil the fundamental angular momentum conservation. Notice that as recently demonstrated for a single Fe adatom on Cu 2 N atop N [55], such an apparent violation of the total angular momentum can appear in single adatoms due to the coupling of the spin and orbital degrees of freedom.…”

“…Hence, it is not clear whether a similar mechanism could be at work in the case of single magnetic adatoms. Even if the orbital degrees of freedom of the adatom were playing supplying the required angular momentum, it would not explain why the magnetic resonance is visible for bias voltages way below the orbital angular momentum excitation threshold [55]. Moreover, it seems that this mechanism works even in the absence of spin-polarization, in clear contrast to the experiments [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23].…”

Quantum stochastic resonance of individual Fe atoms

“…The Fe atom on the fourfold symmetric N site of copper nitride (Cu 2 N; Fig. 1B) has millisecond spin lifetimes when the magnetic field is applied along the spin's easy axis and it switches spontaneously between two spin states (16)(17)(18)(19)(20). These states can be distinguished with a spin-polarized tip as the tip-atom conductance is spin dependent due to tunneling magnetoresistance (21).…”

“…Fe atoms were placed on the nitrogen adsorption site and the copper adsorption site using vertical atom manipulation ( 34 ) with a Pt/Ir tip. The Fe atom is directly dropped onto the N site of the Cu 2 N surface ( 16 ). For the adsorption on the Cu site, the atom is hopped from the N site toward the Cu site.…”

Trends in the hyperfine interactions of magnetic adatoms on thin insulating layers

“…This coupling is a necessary ingredient in order to encode information in the atomic magnetic moment, but at the same time leads to its destabilization, so information may be lost [2]. Two main approaches have so far been pursued to overcome these difficulties: to employ thin insulating layers between the adatoms and substrate to suppress the interactions with the substrate's conduction electrons [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], or to bring several magnetic atoms together to reduce their quantum mechanical fluctuations [21][22][23][24][25]. Experimental investigations of magnetic adatoms on thin insulating layers have mostly centered on Cu 2 N [3][4][5][6][7][8]20] and MgO [11,12,[14][15][16][17][18], and to a lesser extent on h-BN [9,13,19], usually in connection with scanning tunnelling microscopy (STM) [26].…”