Session T18: Invited Session: Keithley Award Symposium

Spin-Polarized Electron Probes of Nanomagnetism

The development of spin-polarized electron sources and electron spin analyzers for free electrons has enabled various useful spectroscopic and microscopic measurements of magnetic nanostructures, thin films and surfaces. The evolution of GaAs spin-polarized electron sources has made the production of intense, highly polarized electron beams relatively easy and routine, while several spin analysis techniques have greatly simplified what was once a difficult measurement. This talk will review some of these developments and describe how they have been applied to specific measurements of nanoscale magnetic properties in various structures. In particular, recent measurements using Scanning Electron Microscopy with Polarization Analysis (SEMPA) to investigate magnetoelectric coupling in multiferroic heterostructures will be described.   

Mapping the spin texture of topological insulators with spin, energy, momentum and time resolution

The helical spin texture of surface electrons in topological insulator has attracted a great deal of interest in the past few years. Although this texture was predicted with the discovery of topological insulators and experimentally confirmed in in few points in the momentum space, its full experimental verification has been non trivial because of the low efficiency of spin resolved experiments. In this talk I will present new results on a, Bi2Se3 topological insulator, obtained by using an innovative ultra-high efficiency spin-resolved photoemission instrument, which provide a complete mapping of the spin texture of these electrons both in momentum and time space. I will show that the spin texture of photoelectrons can be fully manipulated by light and how this manipulation evolves as a function of time, paving the way of use of these materials for spintronics applications.   

Putting a new spin on unoccupied electronic states

Inverse photoemission provides experimental information on the unoccupied electronic states, which is complementary to that obtained by photoemission about the occupied states. The first experimental demonstration of inverse photoemission in the vacuum ultraviolet energy range in 1977 [1] was followed by an important add-on in 1982, the use of spin-polarized electrons [2]. This pioneering experiment opened the way to reveal the spin character of unoccupied electron states in ferromagnets [3]. In this contribution, I will describe the technical development of spin-resolved inverse photoemission with respect to efficiency as well as energy, momentum and spin resolution since the beginning until today [4]. I will give a review about important results obtained by this technique. For about three decades, exchange-split electron states of majority and minority spin character at ferromagnetic surfaces and in ultrathin films were the topics of interest. Since recently, spin textures in momentum space caused by spin-orbit interaction in Rashba systems and topological insulators offer a new field of application for spin-resolved inverse photoemission [5]. I will present a selection of examples, from small and giant Rashba splittings to rotating spins with chiral texture, influenced by the specific symmetry of the system and the orbital character of the respective states. [1] V. Dose, Appl. Phys. 14, 117 (1977) \newline [2] J. Unguris et al., Phys. Rev. Lett. 49, 1047 (1982) \newline [3] M. Donath, Surf. Sci. Rep. 20, 251 (1994) \newline [4] M. Budke et al., Rev. Sci. Instrum. 78, 083903 (2007); S.D. Stolwijk et al., Rev. Sci. Instrum. 85, 013306 (2014) \newline [5] S.N.P. Wissing et al., New J. Phys. 15, 105001 (2013); S.D. Stolwijk et al$.$, Phys. Rev. Lett. 111, 176402 (2013); S.N.P. Wissing et al., Phys. Rev. Lett. 113, 116402 (2014)   

Imaging chiral spin textures with spin-polarized low energy electron microscopy

Chirality in magnetic materials is fundamentally interesting holds potential for logic and memory applications [1-4]. Using spin-polarized low-energy electron microscopy, we recently observed chiral N\'{e}el walls in thin films [5]. We developed ways to tailor the Dzyaloshinskii-Moriya interaction, which drives the chirality, by interface engineering [6], and we found that N\'{e}el- and Bloch- chirality type can be tuned in the presence of uniaxial strain. This work was done in collaboration with G. Chen, A.T.N'diaye, T.P.Ma, A.Mascaraque, C.Won, Z.Q.Qiu, Y.Z.Wu. \\[4pt] [1] M. Bode~et al., \textit{Nature}~\textbf{447}, 190 (2007).\\[0pt] [2] X.Z. Yu, et al.,~\textit{Nature}~\textbf{465}, 901 (2010). \\[0pt] [3] A. Fert et al., \textit{Nature Nanotechnol}. \textbf{8}, 152 (2013).\\[0pt] [4] N. Nagaosa et al. \textit{Nature Nanotechnol}. \textbf{8}, 899 (2013).\\[0pt] [5] G. Chen, et al. \textit{Phys. Rev. Lett}. \textbf{110}, 177204 (2013).\\[0pt] [6] G. Chen, et al. \textit{Nat. Commun}. \textbf{4}, 2671 (2013).   

Spin sensing and magnetic design at the single atom level

Unraveling many of the current dilemmas in nanoscience hinges on the advancement of techniques which can probe the spin degrees of freedom with high spatial, energy, and ultimately high temporal resolution. With the development of sub-Kelvin high-magnetic field STM, two complementary methods, namely spin-polarized scanning tunneling spectroscopy (SP-STS) [1] and inelastic STS (ISTS) [2-3], can address single spins at the atomic scale with unprecedented precession. While SP-STS reads out the projection of the impurity magnetization, ISTS detects the excitations of this magnetization as a function of an external magnetic field. They are thus the analogs of magnetometry and spin resonance measurements pushed to the single atom limit. I have recently demonstrated that it is possible to reliably combine single atom magnetometry with an atom-by-atom bottom-up fabrication to realize complex atomic-scale magnets with tailored properties [4-6] on metallic surfaces [1,7]. I will discuss the current state of the art of this growing field as it pertains to single spin information storage, and how the functionality of coupled magnetic adatoms can be tailored on surfaces. Finally, I will present an outlook on future perspectives in the field of single atom magnetism and the promising application of single spin detection to broader scopes in nanoscience as a whole. \\[4pt] [1] A.A.K., et al., \textit{PRL}, 106, 037205 (2011);\\[0pt] [2] A. J. Heinrich, et al., \textit{Science}, 306, 466 (2004);\\[0pt] [3] A.A.K, et al., \textit{Nature}, 467, 1084 (2010);\\[0pt] [4] A.A.K., et al., \textit{Nature Physics}, 8, 497 (2012);\\[0pt] [5] A.A.K., et al., \textit{Science}, 332, 1062 (2011);\\[0pt] [6] A.A.K., et al., \textit{Science}, 339, 55 (2013);\\[0pt] [7] A.A.K., et al., \textit{PRL}, 111, 126804 (2013).