Thermal and Magnetic-Field Stability of Holmium Single-Atom Magnets

Abstract

We use spin-polarized scanning tunneling microscopy to demonstrate that Ho atoms on magnesium oxide exhibit a coercive field of more than 8 T and magnetic bistability for many minutes, both at 35 K. The first spontaneous magnetization reversal events are recorded at 45 K, for which the metastable state relaxes in an external field of 8 T. The transverse magnetic anisotropy energy is estimated from magnetic field and bias voltage dependent switching rates at 4.3 K. Our measurements constrain the possible ground state of Ho single-atom magnets to either J_{z}=7 or 8, both compatible with magnetic bistability at fields larger than 10 mT.

Unconventional Spin Relaxation Involving Localized Vibrational Modes in Ho Single-Atom Magnets

“…However, typical prefactors for the Orbach process range from 10 −6 to 10 −11 s [6,8,14,32,38], which are orders of magnitude smaller than the τ 0 estimated for Ho. Additionally, STM experiments show that the excited states lie at least 15 meV above the ground state doublet [24,27], which is not compatible with the barrier derived from the fit. In fact, even qualitatively, the suppression of spin reversal at high fields is neither compatible with a conventional Orbach mechanism, which predicts an increase of the relaxation rate due to the Zeeman shift of the levels defining the activation barrier, nor with a Raman process activated by delocalized substrate phonons, whose relaxation rate also increases with increasing Zeeman splitting of the lowest doublet [30].…”

“…As will be shown here, the latter has the additional effect of reducing the coupling of the spin to the phonon modes of the substrate. The validity of this approach was demonstrated by the observation of magnetic remanence up to 40 K in Ho atoms on MgO=Agð100Þ, first with x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) [22], and subsequently confirmed by scanning tunneling microscopy (STM) [24,27]. These single-bond-coordinated atom magnets are ideal systems for investigating the fundamental mechanisms causing magnetization reversal.…”

“…1(c). Ho atoms on MgO=Agð100Þ present a strong out-of-plane magnetic anisotropy, with the ground state being a magnetic doublet [22,24] separated by a few tens of meV from the first excited states [27]. For such an Ising-like system, the intensity of the XMCD peak reflects the average occupation of the lowest magnetic doublet.…”

Quantum state manipulation of single atom magnets using the hyperfine interaction

“…The appeal of single atom magnets (SAMs) and single molecule magnets (SMMs) lies in their promising roles in magnetic data storage [1][2][3][4][5] and as qubits for quantum information processing [6,7]. As classical magnetic bits, SAMs and SMMs have demonstrated magnetic bistability [1,3,4,8], even above liquid nitrogen temperatures [5], readability and writability [1,[9][10][11][12], and self-assembly into periodic arrays [4,8]. For quantum information processing, the precise control over their local environment, intrinsic isolation, and relative ease of on-surface deposition render SAMs and SMMs attractive qubit candidates [13][14][15][16][17][18][19][20].…”

Direct capture and electrostatic repulsion in the self-assembly of rare-earth atom superlattices on graphene

“…Single-atom transistors [1] and single-atom magnets [2][3][4][5] were reported; single-atom catalysts were proposed [6][7][8][9]. These fascinating properties are unique to the respective atom/substrate combination.…”