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.

The robustness of the gapless topological surface state hosted by a 3D topological insulator against perturbations of magnetic origin has been the focus of recent investigations. We present a comprehensive study of the magnetic properties of Fe impurities on a prototypical 3D topological insulator Bi2Se3 using local low temperature scanning tunneling microscopy and integral x-ray magnetic circular dichroism techniques. Single Fe adatoms on the Bi2Se3 surface, in the coverage range ≈ 1% are heavily relaxed into the surface and exhibit a magnetic easy axis within the surface-plane, contrary to what was assumed in recent investigations on the opening of a gap. Using ab initio approaches, we demonstrate that an in-plane easy axis arises from the combination of the crystal field and dynamic hybridization effects.Topological insulators (TI) have demanded heavy interest from the scientific community as a new class of materials illuminating fascinating yet exotic physics and offering large potential for applications in the field of spintronics [1]. TI host a gapless topological surface state (TSS) which exhibits a Dirac-cone like dispersion. However, unlike in the case of graphene, the Dirac cone is located in the center of the Brillouin-zone and the spin and momentum degrees of freedom are locked. The latter so-called topological quality of the TSS is protected by time-reversal symmetry which leads to a variety of interesting effects. For example, these materials may be a forum for Majorana fermions [2], a topological magnetoelectric effect [3], and a quantized anomalous Hall effect [4].The locking of both spin and momentum degrees of freedom leads to the suppression of 180 • elastic backscattering of TSS electrons in the absence of spin-flip processes. The robustness of these "topologically protected" processes, when introducing impurities which break time reversal symmetry, is of critical importance for spinbased transport in such materials. It has been suggested that the interaction between magnetic impurities and the topological state can cause an opening of an energy gap at the Dirac point (DP), provided that the magnetic order is oriented normal to the surface plane [5][6][7]. Nevertheless, the stability of the TSS against local magnetic perturbations is currently under heavy debate [8][9][10], since little is known about the fundamental interface effects of magnetic entities with TI surfaces as well as the static magnetic properties of impurities in these materials.In this work we present a comprehensive study of the magnetic properties of iron impurities on a prototypical TI Bi 2 Se 3 using local scanning tunneling microscopy (STM) and integral x-ray magnetic circular dichroism (XMCD) techniques. We show that the multiplet structure visible in x-ray absorption spectra reflect a high-spin state of the adsorbed Fe impurities, which are confirmed to bind at hollow sites of the surface lattice, under the influence of trigonal crystal fields given in hollow site positions of the Bi 2 Se 3 surface. In contrary to rec...

We demonstrate using inelastic scanning tunneling spectroscopy and simulations based on density functional theory that the amplitude and sign of the magnetic anisotropy energy for a single Fe atom adsorbed onto the Pt(111) surface can be manipulated by modifying the adatom binding site. Since the magnitude of the measured anisotropy is remarkably small, up to an order of magnitude smaller than previously reported, electron-hole excitations are weak and thus the spin excitation exhibits long lived precessional lifetimes compared to the values found for the same adatom on noble metal surfaces.

A combined experimental and theoretical study of doping individual Fe atoms into Bi2Se3 is presented. It is shown through a scanning tunneling microscopy study that single Fe atoms initially located at hollow sites on top of the surface (adatoms) can be incorporated into subsurface layers by thermally-activated diffusion. Angle-resolved photoemission spectroscopy in combination with ab-initio calculations suggest that the doping behavior changes from electron donation for the Fe adatom to neutral or electron acceptance for Fe incorporated into substitutional Bi sites. According to first principles calculations within density functional theory, these Fe substitutional impurities retain a large magnetic moment thus presenting an alternative scheme for magnetically doping the topological surface state. For both types of Fe doping, we see no indication of a gap at the Dirac point.

The recently proposed theoretical concept of a Hund's metal is regarded as a key to explain the exotic magnetic and electronic behavior occuring in the strongly correlated electron systems of multiorbital metallic materials. However, a tuning of the abundance of parameters, that determine these systems, is experimentally challenging. Here, we investigate the smallest possible realization of a Hund's metal, a Hund's impurity, realized by a single magnetic impurity strongly hybridized to a metallic substrate. We experimentally control all relevant parameters including magnetic anisotropy and hybridization by hydrogenation with the tip of a scanning tunneling microscope and thereby tune it through a regime from emergent magnetic moments into a multi-orbital Kondo state. Our comparison of the measured temperature and magnetic field dependent spectral functions to advanced many-body theories will give relevant input for their application to non-Fermi liquid transport, complex magnetic order, or unconventional superconductivity.1 arXiv:1604.03854v1 [cond-mat.str-el] Apr 2016Recent examples of exotic phases of matter, including unconventional superconductivity in iron pnictides and chalcogenides [1][2][3] as well as non-Fermi liquid behavior in ruthenates [4][5][6], depend subtly on the complex interplay of magnetic moments and delocalized electron states taking place in transition metal d-shells. All these materials combine sizable Coulomb interactions and hybridization, which are comparable in their strength. In such cases, it is generally unclear, to which extent local magnetic moments exist, how they can be described using quantum impurity models [7], and how far electronic correlation effects such as Kondo screening [8,9] modify material properties, particularly magnetism, as a function of temperature and magnetic field. The recent concept of a Hund's metal [2,10,11] has been introduced in order to describe exactly this regime, where charge fluctuations in the orbitals are not negligible due to the presence of strong hybridization, but where local magnetic moments can still survive.The fundamental constituent of such a Hund's metal is a magnetic impurity strongly coupled to the electron states of a metallic host, which we coin Hund's impurity. This concept is described in the following for the particular case of a 3d transition metal atom that gets adsorbed (adatom) onto a metallic substrate (Fig. 1). If the atom is still in the gas phase an integer number of electrons is filled into the five 3d orbitals according to Hund's first rule: [12,13] The orbitals are first filled up by electrons having the same spin, before being filled with the remaining electrons of opposite spin. This is driven by the intraatomic exchange energy, or so-called Hund's rule exchange J Hund , which has to be paid if one of the electron spins is flipped. If the 3d transition metal atom is adsorbed onto the metallic substrate, electrons can hop on or off of these orbitals into the bath of substrate conduction electrons, which has ...