AVS2016 Session SP+AS+MI+NS+SS-TuA: Probing Spin-Dependent Phenomena

Tuesday, November 8, 2016 2:20 PM in Room 104A
Tuesday Afternoon

Time Period TuA Sessions | Abstract Timeline | Topic SP Sessions | Time Periods | Topics | AVS2016 Schedule

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2:20 PM Invited SP+AS+MI+NS+SS-TuA-1 Spin Sensing and Magnetic Design at the Single Atom Level
Alexander Khajetoorians (Radboud University, The Netherlands)
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. We 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 by substrate mediated interactions. I will discuss our recent efforts toward realizing tailored chiral magnets [8] and present an outlook on future perspectives toward probing quantum matter at ultra-low temperatures.

[1] A.A.K., et al., PRL, 106, 037205 (2011); [2] A. J. Heinrich, et al., Science, 306, 466 (2004); [3] A.A.K, et al., Nature, 467, 1084 (2010); [4] A.A.K., et al., Nature Physics, 8, 497 (2012) [5] A.A.K., et al., Science, 332, 1062 (2011), [6] A.A.K., et al., Science, 339, 55 (2013), [7] A.A.K., et al., PRL, 111, 126804 (2013). [8] A.A.K., et al. Nature Comm, 7, 10620 (2016).

3:00 PM Invited SP+AS+MI+NS+SS-TuA-3 Electron Spin Resonance of Single Atom and Engineered Spin Structures
Taeyoung Choi, William Paul, Christopher Lutz, Andreas Heinrich (IBM Almaden Research Center)

The scanning tunneling microscope (STM) has been one of the most versatile tools for atomic-scale imaging, manipulation, and tunneling spectroscopy. Inelastic spin excitation and spin-polarized tunneling have been employed to study spin physics of individual atoms and engineered structures, demonstrating nanoscale memory bits [1] and logic gates [2]. However, the energy resolution of the STM is mainly limited by a temperature of a system surrounding the atomic spins (>100 μeV).

Here, we successfully combine electron spin resonance (ESR) and STM, coherently driving spin resonance of individual iron (Fe) atoms on surfaces (MgO/Ag(100)) [3]. A radio-frequency electric field (~20 GHz), applied at the tunneling junction, modulates the spin state of the Fe atoms. The spin resonance signal is detected by a spin-polarized tunneling current. The ESR signals from individual Fe atoms differ by a few GHz (~10μeV) while the ESR linewidth is in the range of only a few MHz (~10neV). Such a high energy resolution enables us to distinguish spin distributions down to single-atom level and to investigate weak magnetic interactions.

When we placed two Fe atoms close together with controlled atom manipulation, we found that the ESR signal from each Fe atom splits into doublet, of which separation depends on the distance between two atoms. Our measurements show r -3.024±0.026 distance-dependent splitting, in excellent agreement of magnetic dipole-dipole interaction. We utilized this precisely measured dipolar interaction to determine the location and magnetic moment of unknown spin centers with sub-angstrom and one hundredth of Bohr magneton precision [4].

Coherent quantum control of individual atoms on surfaces combined with atom manipulation may promise the STM as a new and unique platform for a quantum sensor, investigating spin-labeled molecular structures and a quantum information processor, modeling quantum magnetism.

We gratefully acknowledge financial support from the IBM and Office of Naval Research.

[1] S. Loth, S. Baumann, C.P. Lutz, D.M. Eigler, A.J. Heinrich, Science 335, 196 (2012).

[2] A.A. Khajetoorians, J. Wiebe, B. Chilian, and R. Wiesendanger, Science 332, 1062 (2011).

[3] S. Baumann*, W. Paul*, T. Choi, C.P. Lutz, A. Ardavan, A.J. Heinrich, Science 350, 417 (2015).

[4] T. Choi et al., manuscript in preparation.

3:40 PM BREAK
4:40 PM SP+AS+MI+NS+SS-TuA-8 Controlling Kondo Effect of Magnetic Molecules on Au(111) by Small Molecule Binding
MinHui Chang, Se-Jong Kahng (Korea University, Republic of Korea); YunHee Chang (Korea Advanced Institute of Science and Technology (KAIST), Republic of Korea); Howon Kim, Soon-Hyeong Lee (Korea University, Republic of Korea); Yong-Hyun Kim (KAIST, Republic of Korea)
Controlling and sensing spin states of magnetic molecules at the single molecule level is essential for spintronic molecular device applications. Here, we demonstrate that spin interactions of Co-porphyrin on Au(111) can be controlled by adsorption and desorption of small molecules, and be sensed using scanning tunneling microscopy and spectroscopy (STM and STS). Bare Co-porphyrin showed a clear zero-bias peak, a signature of Kondo effect in STS, whereas Co-porphyrin adsorbed small molecules showed modified zero-bias peaks, with reduced full width half maximum or Kondo temperature. Our density functional theory calculation results explain it with spatial redistribution of unpaired spins in dz2 Orbitals. Our study opens up ways to tune molecular spin interactions by means of chemical binding.
5:00 PM Invited SP+AS+MI+NS+SS-TuA-9 Spin-polarized Scanning Tunneling Microscopy on Surfaces Prepared by Molecular Beam Epitaxy
Arthur Smith (Ohio University Nanoscale and Quantum Phenomena Institute)

Spin-polarized scanning tunneling microscopy (SP-STM) has proven to be a powerful in-situ technique for obtaining detailed information about spin structures at surfaces down to atomic scale.1 It has been applied extensively to investigate pristine ferromagnetic and antiferromagnetic (aFM) transition metal surfaces, with many great results in the case of model systems such as nano-sized magnetic islands and single magnetic monolayers.2 This has led to fascinating discoveries of nanoscale magnetic domains, domain walls, spin spirals, spin skyrmions, and much more.3,4 Although not simple in practice, SP-STM can in principle also yield unprecedented spin characterization on a broad spectrum of material surfaces, including practical, real world systems. For example, it could be applied to investigate surfaces of intermetallic compounds, superconductors, complex magnetic oxides, and magnetic semiconductors.

We are applying SP-STM to study various magnetic systems grown in-situ by molecular beam epitaxy, including transition metal nitrides,5 magnetic-doped nitride semiconductors, and several bi-metallic magnetic systems. I will present our recent work using STM and SP-STM, beginning with a discussion of manganese nitrides, including our work on aFM θ-phase MnN and ferrimagnetic ε-phase Mn4N. The θ-phase films are very complex due to the expectation of canted spins within each atomic layer with four unique canting angles, while the ε-phase films contain two types of spins (MnI and MnII) with equally complex spin arrangements.

A second material we are working on is the chromium nitride system in which we investigate its electronic and spin properties in a low-temperature SP-STM system. Spectroscopy results to date suggest a d-wave resonance on the surface and a Kondo signature for nanoscale iron islands grown on atomically-smooth CrN surfaces.

I will also present results for Mn δ-doped semiconducting gallium nitride surfaces in which we find atomic layer ferromagnetism within a unique and stable √3 x √3 - R30° MnGaN surface reconstruction. Spectroscopy clearly reveals spin-polarized and spin-split Mn states, as predicted by first principles theory calculations. SP-STM measurements map out ferromagnetic domains at room temperature, and the additional presence of magnetic rim states seen at the edges of ferromagnetic islands, as well as magnetic hysteresis, give further interest to this intriguing system.

1 R. Wiesendanger, Rev. Mod. Phys. 81, 1495 (2009).

2 M. Bode et al., Phys. Rev. Lett. 92, 67201 (2004).

3 P. Ferriani et al., Phys. Rev. Lett. 101, 027201 (2008).

4 S. Loth et al. Science 335, 196 (2012).

5 K.K. Wang and A.R. Smith, Nano Lett. 12, 5443 (2012).

5:40 PM SP+AS+MI+NS+SS-TuA-11 The Use of Scanning Probe Techniques to Study the Behaviour of Second Phase Particles in Beryllium and Their Role in Localised Corrosion
Christopher Mallinson, John Watts (University of Surrey, UK)

Scanning Kelvin probe force microscopy (SKPFM) has been employed to examine the galvanic activity of a wide range of second phase particles in S-65 beryllium that are believed to have a role in the localised corrosion of the metal. SKPFM and AFM analysis has been combined with additional surface and bulk analysis techniques of scanning electron microscopy, energy dispersive x-ray spectroscopy and Auger electron spectroscopy to provide a detailed overview of the link between the bulk and surface composition of particles and their Volta potential or surface contact potential.

Initial results appear to show that all second phase particles are more noble than the beryllium matrix with the greatest potential difference observed for AlFeBe4 and alumina or carbide like particles. The more negative Volta potential indicates that the particles should act as local cathodes when the metal is exposed to an aqueous environment.

The initial investigation, which is being performed in-air, will be expanded to determine the effect of increasingly higher humidity environments on the behaviour of the particles. It is hoped that this will provide a greater understanding about the onset of pitting corrosion in beryllium.

6:00 PM SP+AS+MI+NS+SS-TuA-12 Many-body Interaction induced Spin-split States of Single Vacancy in Graphite
Wonhee Ko (Samsung Advanced Institute of Technology, Republic of Korea); HyoWon Kim, Yeonchoo Cho (Samsung Advanced Institute of Technology); Young Kuk (Seoul National University, Korea, Republic of Korea); SungWoo Hwang (Samsung Advanced Institute of Technology)
Although carbon atoms have no magnetic states, it has been known that defects in graphene or graphite can have magnetic states induced by many-body interaction. By utilizing ultra-low-temperature scanning tunneling microscopy, we observed the spin-split states of single vacancy in graphite, which is a hallmark of magnetic states. Evolution of the spin splitting in the magnetic field did not follow the Zeeman effect of single electron states, and can be explained only when we consider electron-electron interaction. Quantitative analysis showed that the strength of the electron-electron interaction is in the range of 1~3 meV. Our observation implies that the simplest defect in graphite like single vacancy can behave as magnetic, which would be an important ingredient for development of carbon-based spintronic devices.
Time Period TuA Sessions | Abstract Timeline | Topic SP Sessions | Time Periods | Topics | AVS2016 Schedule