The future of nanoscale spin-based technologies hinges on a fundamental understanding and dynamic control of atomic-scale magnets. The role of the substrate conduction electrons on the dynamics of supported atomic magnets is still a question of interest lacking experimental insight. We characterized the temperature-dependent dynamical response of artificially constructed magnets, composed of a few exchange-coupled atomic spins adsorbed on a metallic substrate, to spin-polarized currents driven and read out by a magnetic scanning tunneling microscope tip. The dynamics, reflected by two-state spin noise, is quantified by a model that considers the interplay between quantum tunneling and sequential spin transitions driven by electron spin-flip processes and accounts for an observed spin-transfer torque effect.
We present a generalization of the recently developed dualfermion approach introduced for correlated lattices to nonequilibrium problems. In its local limit, the approach has been used to devise an efficient impurity solver, the superperturbation solver for the Anderson impurity model (AIM). Here we show that the general dual perturbation theory can be formulated on the Keldysh contour. Starting from a reference Hamiltonian system, in which the timedependent solution is found by exact diagonalization, we make a dual perturbation expansion in order to account for the relaxation effects from the fermionic bath. Simple test results for closed as well as open quantum systems in a fermionic bath are presented.
A cluster of a few magnetic atoms on the surface of a nonmagnetic substrate is one suitable realization of a bit for spin-based information technology. The prevalent approach to achieve magnetic stability is decoupling the cluster spin from substrate conduction electrons in order to suppress destabilizing spin-flips. However, this route entails less flexibility in tailoring the coupling between the bits needed for spin-processing. Here, we use a spin-resolved scanning tunneling microscope to write, read, and store spin information for hours in clusters of three atoms strongly coupled to a substrate featuring a cloud of non-collinearly polarized host atoms, a so-called non-collinear giant moment cluster. The giant moment cluster can be driven into a Kondo screened state by simply moving one of its atoms to a different site. Using the exceptional atomic tunability of the non-collinear substrate mediated Dzyaloshinskii–Moriya interaction, we propose a logical scheme for a four-state memory.
Highly symmetric magnetic environments have been suggested to stabilize the magnetic information stored in magnetic adatoms on a surface. Utilized as memory devices such systems are subjected to electron tunneling and external magnetic fields. We analyze theoretically how such perturbations affect the switching probability of a single quantum spin for two characteristic symmetries encountered in recent experiments and suggest a third one that exhibits robust protection against surface-induced spin flips. Further we illuminate how the switching of an adatom spin exhibits characteristic behavior with respect to low energy excitations from which the symmetry of the system can be inferred. Recently, single magnetic atoms on surfaces, or so-called magnetic adatoms, have gained a lot of interest for spin-based information storage and processing [1][2][3]. These concepts are mostly based on strong magnetic anisotropy energy [4,5], which reduces spin degeneracy at zero magnetic field, thereby defining preferential spatial orientations of the spin. While magnetic anisotropy introduces an energy cost for magnetization reversal, countless studies have illustrated that in the presence of strong magnetic anisotropy, individual magnetic adatoms still exhibit rather short lifetimes [6][7][8] owing to the interplay of the hybridization of the moment bearing orbitals and the underlying substrate. Such observations question the role of both tunneling electrons as well as substrate electrons in dynamical processes of the atomic spin [5,[9][10][11][12][13]. In order to enhance the dynamic stability of such adatoms, strong magnetic coupling between individual spins can be utilized to protect the total spin from fluctuations [2,14].A different approach, that stabilizes a single magnetic moment of an adatom, was utilized by a particular choice of both spin and underlying substrate symmetry [3]. In particular for a threefold symmetric system with the net magnetic moment S = 8 of a holmium adatom, it is possible to protect the spin, in the absence of perturbations, from single-electroninduced spin reversal. It is yet unknown how perturbations, like current-based read out or static magnetic fields which break the symmetry of the system, effect the symmetry caused stability of the spin in such quantum systems.Here we approach this question with a focus on the prospects of using a specific symmetry to protect a spin from switching. We extract the effective switching rate between the high-spin ground states via all possible spin paths, as experimentally manifested in two-state telegraph noise, utilizing a master equation approach. With comparative analysis we show how a single spin on two-, three-and fourfold symmetric substrates [15] responds to temperature, external magnetic field, as well as inelastic tunneling electrons. Higher symmetries are not part of our discussion since they are difficult to realize experimentally. We find that the threeand fourfold symmetric systems are both protected against single-electron-induced ground state s...
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