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Functional Nanoscale Structure Probe and Simulation Laboratory (Funsilab)
Detections of spinexcitations of noncollinear magnets
Essential to the physics of noncollinear magnets is the understanding of their spinexcitations. We explain how inelastic scattering, being suitable for investigations of surfaces and thin fims, can detect the collective spinexcitation spectra of noncollinear magnets such as spinspirals and topologically nontrivial skyrmions lattices.
Also, we propose the usage of spinpolarized electronenergyloss spectroscopy augmented with a spin analyzer to allow filtering and selection of specific spinwave modes.
F. J. dos Santos, M. dos Santos Dias, F. S. M. Guimaraes, J. Bouaziz, S. Lounis:
Spinresolved inelastic electron scattering by spin waves in noncollinear magnets, Phys. Rev. B 97, 024431 (2018)
A little bit strange bit of three atoms.
J. Hermenau, J. IbañezAzpiroz, 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 noncollinear spin processing using threeatom magnets with strong substrate coupling, Nature Communications 8, 642 (2017)
Press release: A little bit strange bit of three atoms
Spinexcitations of nonmagnetic adatoms
We demonstrate that even boring nonmagnetic atoms can carry spinexcitations analogous of the socalled 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 notso interesting atoms in the arena of potentially useful nanoobjects.
We combine timedependent density functional theory with manybody 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 nonmagnetic 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. IbanezAzpiroz, M. dos Santos, B. Schweflinghaus, S. Blügel, S. Lounis:
Tuning paramagnetic spinexcitations 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 firsttime 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 spinmanipulate, different spin wave modes in ferromagnetic trilayers or synthetic antiferromagnets utilizing ac charge currents via spinorbit 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 currentinduced 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)
Chiralitydriven orbital magnetic moments
We discovered that noncollinear magnetic textures generate an orbital magnetic moment that is not driven by spinorbit 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 Xray spectroscopy to characterize the topological nature of complex spintexture by singling out the chiral contribution to the orbital magnetization. Exploiting the distinct properties of the chirality and spinorbitcouplingdriven 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: Chiralitydriven 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
Spinmixing 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 abinitio 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 spinmixing magnetoresistance (TXMR), which is generated by the noncollinearity of the spintexture.
The noncollinearity of the magnetic moments and the presence of spinorbit interaction open spinmixing channels for electronic hybridization that have a nontrivial 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 perpendiculartoplane 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 spintextures
DzyaloshinskiiMoryia interactions in dimers
The DzyaloshinskiiMoriya interaction, which defines the chirality (handedness) of a spintexture is essential in generating magnetic skyrmions. In collaboration with the University of Hamburg, Radboud University and MaxPlanck 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 interadatom 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, spinexcitations and stability
Stabilizing the magnetic signal of single adatoms is crucial toward their successful usage in widespread technological applications such as highdensity 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 timedependent density functional theory and manybody perturbation theory [1,2], we showed that currentdriven spinstate manipulation leads to rich transport patterns with new manybody features, resulting from the interaction of the electrons and the spinexcitations [1]. We identified electronhole excitations, spinorbit 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 hcpstacking site [3]. Furthemore, we demonstrated that the quantum mechanical nature of these tiny objects generate large intrinsic zeropoint 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 Fenanostructure magnetically stable on Cu(111) contains 5 atoms [5]. Also, the concept of zeropoint fluctuations unravels the striking nonobservation of magnetic patterns in Mnnanowires/Ni(110) [6].
[1] B. Schweflinghaus, M. dos Santos, Dias, A. T. Costa, S. Lounis, Renormalization of electron selfenergies via their interaction with spin excitations: A firstprinciples 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 sitedependent giant substrate polarization, Phys. Rev. Lett. 111, 157204 (2013).
[4] J. IbanezAzpiroz, M. dos Santos Dias, S. Blügel, S. Lounis, Zeropoint spinfluctuations 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, Currentdriven 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 zeropoint energy?
Signature of the Kondo effect in STM from firstprinciples
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 KorringaKohnRostoker 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, selfenergies, and Kondo temperatures. For a Mn adatom, which is close to halffilling, the STM spectra are featureless near the Fermi level. On the other hand, the quasiparticle peak for a Co adatom gives rise to strongly positiondependent 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 spinfiltered 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 spindependent 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 lowtemperature 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 RudermanKittelKasuyaYosida interaction, Nature Communications 5, 5417 (2014)
Press release: New insights into the Kondo effect.