# Forschungsgruppe Prof. Dr. Samir Lounis

Functional Nanoscale Structure Probe and Simulation Laboratory (Funsilab)

### Recent highlights

**Detections of spin-excitations of non-collinear magnets**

Essential to the physics of non-collinear magnets is the understanding of their spin-excitations. We explain how inelastic scattering, being suitable for investigations of surfaces and thin fims, can detect the collective spin-excitation spectra of non-collinear magnets such as spin-spirals and topologically nontrivial skyrmions lattices.

Also, we propose the usage of spin-polarized electron-energy-loss spectroscopy augmented with a spin analyzer to allow filtering and selection of specific spin-wave modes.

F. J. dos Santos, M. dos Santos Dias, F. S. M. Guimaraes, J. Bouaziz, S. Lounis:

Spin-resolved inelastic electron scattering by spin waves in noncollinear magnets, Phys. Rev. B 97, 024431 (2018)

**A little bit strange bit of three atoms. **

As a result of a collaboration with the experimental teams of Jens Wiebe at the University of Hamburg and of Alexander Khajetoorians at Radboud University, we show that a ferromagnetic particle composed of only three iron atoms deposited on platinum surface (see figure) can serve as a bit for the magnetic storage of information. By particular electronic interactions of the bit with the conductive substrate it is positioned on, the information the bit carries can be processed in an unusual, so called non-collinear, way, which could add new functionality to future elements of information technology.

J. Hermenau, J. Ibañez-Azpiroz, Chr. Hübner, A. Sonntag, B. Baxevanis, K. T. Ton, M. Steinbrecher, A. A. Khajetoorians, M. dos Santos Dias, S. Blügel, R. Wiesendanger, S. Lounis, and J. Wiebe:

A gateway towards non-collinear spin processing using three-atom magnets with strong substrate coupling, Nature Communications 8, 642 (2017)

Press release: A little bit strange bit of three atoms

**Spin-excitations of non-magnetic adatoms **

We demonstrate that even boring non-magnetic atoms can carry spin-excitations analogous of the so-called paramagnons and predict that the corresponding lifetimes can be of similar magnitude that those characterizing magnetic atoms. This results from the satisfaction of the Stoner criterion for magnetism in the dynamical regime, which promotes the usually not-so interesting atoms in the arena of potentially useful nano-objects.

We combine time-dependent density functional theory with many-body perturbation theory to compute inelastic transport spectra detectable with scanning tunnelling microscopy. We also predict the possibility to observe the atomic analog of a magnetic phase transition via a proximity effect, i.e. by approaching an initially non-magnetic atom with a magnetic atom. The inelastic spectrum of Rh adatom on Ag(001) surface is shown on the adjacent figure, where one notices a large resonance at negative bias voltage.

J. Ibanez-Azpiroz, M. dos Santos, B. Schweflinghaus, S. Blügel, S. Lounis:

Tuning paramagnetic spin-excitations of single adatoms, Phys. Rev. Lett. 119, 017203 (2017); Editor’s suggestion

Press release: PRL Editors’ Suggestion: Computing with Paramagnetic Spin Excitations

**Transport in the dynamical regime**

We performed first-time investigations of dynamical transport properties, up to the THz regime and beyond, based on realistic electronic structure of materials. We demonstrated [1], in Fe/W multilayers, the ability to excite, and thus spin-manipulate, different spin wave modes in ferromagnetic trilayers or synthetic antiferromagnets utilizing ac charge currents via spin-orbit torques. We proposed an engineering path to preselect an excitation mode depending on the spacer thickness, the material nature and the location of the applied current. Furthermore, we established that ac magnetoresistances together with the Hall effects could be greatly magnified or dwindled by tuning the frequency of the ac current and the applied magnetic field [2]. Interestingly, the effective conversion from charge to spin currents, described by the spin Hall angle, can be boosted by more than two orders of magnitude.

[1] F. S. M. Guimaraes, S. Lounis, A. T. Costa, R. B. Muniz: Dynamical current-induced ferromagnetic and antiferromagnetic resonances, Phys. Rev. B 92, 220410(R) (2015)

[2] F. Guimaraes, M. dos Santos Dias, J. Bouaziz, A. T. Costa, R. B. Muniz, S. Lounis: Dynamical amplification of magnetoresistances and Hall currents up to the THz regime, Nature Scientific Reports 7, 3686 (2017)

**Chirality-driven orbital magnetic moments **

We discovered that non-collinear magnetic textures generate an orbital magnetic moment that is not driven by spin-orbit coupling. Such an orbital moment is chiral by nature and is related to a finite scalar spin chirality of three neighboring spins: S1.(S2 x S3), which for large magnetic skyrmions converges in the adiabatic limit to the flux of the emergent magnetic field, and thus is proportional to the topological charge N_Sk. Such a chiral orbital moment inherits a topological nature once generated in a topological magnetic object.

We propose X-ray spectroscopy to characterize the topological nature of complex spin-texture by singling out the chiral contribution to the orbital magnetization. Exploiting the distinct properties of the chirality- and spin-orbit-coupling-driven orbital moments we defined the topological orbital magnetization ratio (TOMR), (M_orb (Sk))/(M_orb (F))-(M_spin (Sk))/(M_spin (F) )≈(M_orb (chiral))/(M_orb (F))∝N_Sk, detectable by driving the sample from a reference ferromagnetic state, for example, into a possibly unknown noncollinear state. If the TOMR is finite, there is a strong signature for the topological character of the magnetic structure.

M. dos Santos Dias, J. Bouaziz, M. Bouhassoune, S. Blügel, S. Lounis: Chirality-driven orbital magnetic moments: fingerprints of topological magnetic structures, Nature Communications 7, 13613 (2016)

M. dos Santos Dias, S. Lounis: Insights into the orbital magnetism of noncollinear magnetic systems, SPIE 10357, 103572A (2017); Invited paper for Proceedings of SPIE, Spintronics X

Press release: Characterization of magnetic nanovortices simplified

**Spin-mixing magnetoresistance**

Probably the most interesting skyrmions to be used in devices are the small ones, i.e. below 5nm in size. We realized a breakthrough by simulating from full ab-initio single magnetic 3 nm skyrmions embedded in Pd(1ML or 2MLs)/Fe/Ir(111) surface. We proposed protocols to detect them by all electrical means via we named tunneling spin-mixing magnetoresistance (TXMR), which is generated by the non-collinearity of the spin-texture.

The non-collinearity of the magnetic moments and the presence of spin-orbit interaction open spin-mixing channels for electronic hybridization that have a non-trivial impact on the perpendicular conductance. TXMR has immediate implications for device concepts since it can reach an efficiency of 20% and can be used within a current perpendicular-to-plane geometry.

D. M. Crum, M. Bouhassoune, J. Bouaziz, B. Schweflinghaus, S. Blügel, S. Lounis: Perpendicular reading of single confined magnetic skyrmions, Nature Communications 6, 8541 (2015)

Press release: New magnetic effect to detect complex spin-textures

**Dzyaloshinskii-Moryia interactions in dimers**

The Dzyaloshinskii-Moriya interaction, which defines the chirality (handedness) of a spin-texture is essential in generating magnetic skyrmions. In collaboration with the University of Hamburg, Radboud University and Max-Planck Institute of Solid State Research in Stuttgart, we explored the atomistic origin of magnetization handedness in a structure containing as few as two iron atoms deposited on a platinum substrate. This magnetic interaction changes as function of inter-adatom distances, switches sign and change direction. This opens the possibility of designing nanostructures hosting skyrmions with the desired handedness.

A. Khajetoorians, M. Steinbrecher, M. Ternes, M. Bouhassoune, M. dos Santos Dias, S. Lounis, J. Wiebe, R. Wiesendanger: Tailoring the chiral magnetic interaction between two individual atoms, Nature Communications 7, 10620 (2016)

Press release: Handshake of atoms: lefties or righties?

**Fluctuations, spin-excitations and stability**

Stabilizing the magnetic signal of single adatoms is crucial toward their successful usage in widespread technological applications such as high-density magnetic data storage devices. Ultimately a single magnetic atom is the smallest magnetic bit of information, thus the great interest in grasping its excitation behavior pertaining to any writing or reading process.

Combining time-dependent density functional theory and many-body perturbation theory [1,2], we showed that current-driven spin-state manipulation leads to rich transport patterns with new many-body features, resulting from the interaction of the electrons and the spin-excitations [1]. We identified electron-hole excitations, spin-orbit interaction and details of hybridization effects to be paramount for shaping the lifetime of an excited magnetic state and its energy.

To establish the orientation of a magnetic moment at the nanoscale, we proposed a recipe based on the excitation energy profile as function of an applied magnetic field and its direction [2], which was used to successfully identify the striking change of the magnetic orientation of an Fe adatom on Pt(111) surface after moving it from an fcc- to an hcp-stacking site [3]. Furthemore, we demonstrated that the quantum mechanical nature of these tiny objects generate large intrinsic zero-point fluctuations, which collapse the magnetic anisotropy potential barrier even at absolute zero temperature, which destabilizes the magnetic moment [4].

We provided practical guidelines for designing magnetically stable nanomagnets with minimal quantum fluctuations. Building clusters made of strongly coupled magnetic atoms, for example, lowers the strength of the fluctuations, which explains that the smallest Fe-nanostructure magnetically stable on Cu(111) contains 5 atoms [5]. Also, the concept of zero-point fluctuations unravels the striking non-observation of magnetic patterns in Mn-nanowires/Ni(110) [6].

[1] B. Schweflinghaus, M. dos Santos, Dias, A. T. Costa, S. Lounis, Renormalization of electron self-energies via their interaction with spin excitations: A first-principles investigation, Phys. Rev. B 89, 235439 (2014).

[2] M. dos Santos Dias, B. Schweflinghaus, S. Lounis, Relativistic dynamical spin excitations of magnetic adatoms, Phys. Rev. B 91, 075405 (2015).

[3] A. A. Khajetoorians, T. Schlenk, B. Schweflinghaus, M. dos Santos Dias, M. Steinbrecher, M. Bouhassoune, S. Lounis, J. Wiebe, R. Wiesendanger, Spin excitations of individual Fe atoms on Pt(111): impact of the site-dependent giant substrate polarization, Phys. Rev. Lett. 111, 157204 (2013).

[4] J. Ibanez-Azpiroz, M. dos Santos Dias, S. Blügel, S. Lounis, Zero-point spin-fluctuations of single adatoms, Nanoletters 16, 4305 (2016).

[5] A. A. Khajetoorians, B. Baxevanis, C. Hübner, T. Schlenk, S. Krause, T. O. Wehling, S. Lounis, A. Lichtenstein, D. Pfannkuche, J. Wiebe, R. Wiesendanger, Current-driven spin dynamics of artificially constructed quantum magnets, Science 339, 55 (2013).

[6] S. Holzberger, T. Schuh, S. Blügel, S. Lounis, W. Wulfhekel, Parity effect in the ground state localization of antiferromagnetic chains coupled to a ferromagnet, Phys. Rev. Lett. 110, 157206 (2013).

Press release: Atomic bits despite zero-point energy?

**Signature of the Kondo effect in STM from first-principles**

Here we address the signature of strong correlations such as the Kondo effect in the transport measurements based on scanning tunnelling microscopy (STM). For this we interfaced the Korringa-Kohn-Rostoker Green function method based on density functional theory with both quantum Monte Carlo and exact diagonalization impurity solvers. As concrete examples, we study Co and Mn adatoms on the Cu(111) surface, which are expected to represent the opposite limits of Kondo physics and local moment behaviour.

Theoretical STM spectra are computed as a function of STM tip position relative to each adatom. Because of the multiorbital nature of the adatoms, the STM spectra are shown to consist of a complicated superposition of orbital contributions, with different orbital symmetries, self-energies, and Kondo temperatures. For a Mn adatom, which is close to half-filling, the STM spectra are featureless near the Fermi level. On the other hand, the quasiparticle peak for a Co adatom gives rise to strongly position-dependent Fano line shapes.

H. T. Dang, M. dos Santos Dias, A. Liebsch, S. Lounis: Strong correlation effects in theoretical STM studies of magnetic adatoms, Phys. Rev. B 93, 115123 (2016)

**Amplification of spin-filtered Friedel oscillations via confinement**

We investigate charge density oscillations produced after scattering at an oxygen impurity embedded in the surface of a ferromagnetic thin film of Fe grown on W(001). We demonstrate that by changing the thickness of the Fe films, we control quantum well states confined to two dimensions that manifest as multiple flat energy contours in one spin channel. The flatness leads to a focusing of the electronic beams while the number of contours increases with the film thickness. This provides the possibility of tuning the strength of the induced charge oscillations which allow to detect the oxygen impurity at large distances (≈50 nm).

M. Bouhassoune, B. Zimmermann, Ph. Mavropoulos, D. Wortmann, P. H. Dederichs, S. Blügel, S. Lounis: Quantum well states and amplified spin-dependent Friedel oscillations in thin films, Nature Communications 5, 5558 (2014)

Press release: Giant charge density disturbances discovered in nanomaterials

**Interplay between the Kondo effect and the RKKY interactions**

In collaboration with the groups of Martin Wenderoth and of Thomas Pruschke, we report on the experimental and theoretical investigation of iron dimers buried below a Cu(100) surface by means of low-temperature scanning tunnelling spectroscopy combined with density functional theory and numerical renormalization group calculations. The Kondo effect, in particular the width of the Abrikosov–Suhl resonance, is strongly altered or even suppressed due to magnetic coupling between the impurities. It oscillates as a function of dimer separation revealing that it is related to indirect exchange interactions mediated by the conduction electrons.

H. Prüser, P. E. Dargel, M. Bouhassoune, R. G. Ulbrich, T. Pruschke, S. Lounis, M. Wenderoth: Interplay between Kondo effect and Ruderman-Kittel-Kasuya-Yosida interaction, Nature Communications 5, 5417 (2014)

Press release: New insights into the Kondo effect.