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Indirect chiral magnetic exchange through Dzyaloshinskii-Moriya enhanced RKKY interactions in manganese oxide chains on Ir(100)


The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction explains the magnetic properties of numerous indirectly coupled systems, where the wave functions of localized electrons carrying magnetic moments overlap marginally. If spin-orbit-related effects are negligible, conventional RKKY predicts only collinear coupling terms, which give rise to either parallel or antiparallel magnetic configurations. Stronger spin-orbit scattering can induce a Dzyaloshinskii-Moriya type enhancement of the RKKY interaction (DME-RKKY) and chiral exchange terms. So far, the tiny effects of this interaction have been highlighted experimentally in very few cases. Here we report on the direct observation by angle-resolved photoemission spectroscopy (ARPES) performed at the VUV-Photoemission beamline at Elettra and spin-polarized scanning tunneling microscopy (SP-STM) of chiral magnetic order in an array of MnO2chains on Ir(100). Density functional theory (DFT) calculations demonstrate that the strong spin-orbit coupling in Ir leads to a robust DME-RKKY, resulting in a chiral spin spiral with a 120° rotation between adjacent MnOchains.
The structural properties of the MnO2chains on Ir(100) are depicted in Fig. 1(a). Nearest-neighbor Mn atoms (orange) are held by oxygen atoms (red) above missing Ir rows (gray), which present a 3× lateral periodicity (3×1 structural unit cell). ARPES data along theΓ_bar–X_bar direction of the Ir(100) surface Brillouin zone (SBZ), i.e. along the chain direction, are displayed in Fig. 1(b,c) before and after the formation of the array of MnO2chains, respectively. Mn 3dstates can be clearly identified between -1.9 and -2.9 eV, where the signal from the overlapping Ir 5dstates is strongly attenuated at 150 eV photon energy. The dispersion of the Mn 3dstates, highlighted by two sinusoidal dotted lines, suggests the presence of a 2× periodicity, as expected for an anti-ferromagnetic (AFM) system.
 

 

Figure 1.     (a) Structural model of the MnO2 chains on Ir(100). Orange, red and gray balls represent Mn, O and Ir atoms, respectively. Shaded rectangles highlight the 3×1 structural unit cell and the 9×2 magnetic unit cell. Arrows indicate the spin orientation of the chiral spin spiral. (b) Second derivative ARPES data for clean Ir. (c) Second derivative ARPES data for the MnO2/Ir(100) system. The dotted curves guide the eye along the dispersion of Mn3 dstates with 2× periodicity. (d) DFT calculations for the MnO2/Ir(100) system. Pink and green dots represent Mn and Ir states, respectively. The size of the symbols indicates the surface localization of the corresponding state. The dotted lines shown in (c) are stretched here by a factor 1.33 to consider the larger band width in DFT and agree well with surface-localized Mn states (Modified version of Fig. 1 of Nat. Commun. 10, 2610 (2019), http://creativecommons.org/licenses/by/4.0/).
 

This experimental observation is confirmed and detailed by STM measurements. The STM image of Fig. 2(a) is taken on MnO2/Ir(001) with a non-magnetic W tip and shows the 3×1 structural unit cell (black box). Fig. 2(b) displays an SP-STM image scanned with an in-plane sensitive Cr-coated W tip. Comparison of the two images reveals two qualitative differences: (i) The periodicity measured with the magnetic tip along the chains is twice longer and (ii) the contrast observed on different MnO2chains is not constant, but becomes significantly smaller for every third chain. These differences can be quantified by looking at the line profiles taken along adjacent chains (Fig. 2(c)). While the traces from Fig. 2(a) are practically identical, the three colored traces from Fig. 2(b) present characteristic amplitudes and a π phase shift between the green and red trace on one side and the blue trace on the other. These data reveal the formation of a complex magnetic structure with 9×2 unit cell (black box).
DFT explains the experimental observations. The coupling between Mn atoms along the chains is of AFM type, while the DME-RKKY interaction determines the formation of a chiral spin spiral with a 120° spin rotation from chain to chain. Fig. 1(a) shows by arrows the orientation of the spins within the 9×2 magnetic unit cell. Fig. 1(d) reports DFT electronic structure calculations along the Γ-X direction. The sinusoidal curves derived from the ARPES spectra match well with the energy position of surface localized Mn bands with dyz and dx2-y2orbital character. The experimental data display a smaller band width than DFT by a factor of 1.33, probably due to correlation effects. Other flat bands observed in the experiment just below the maxima of the sinusoids are ascribed to Mn states with prevalent dzx and dz2character.

In summary, the present study highlights in a direct way the robust effects of the DME-RKKY interaction, giving rise to chiral spin spirals in theMnO2/Ir(100) system. These findings pave the way towards the observation of novel phenomena, such as chiral spin-liquid states in spin ice systems, or new quasiparticles due to the trapping of single electrons in self-induced skyrmion spin textures.



 

Figure 2.  Atomic resolution scans of MnO2 chains on Ir(001). (a) A 3×1 structural unit cell is observed with a non-magnetic W tip (scale bar: 1 nm). (b) With a Cr-coated W tip the magnetic 9×2 unit cell is resolved. (c) Line profiles drawn along the stripes at the positions indicated by arrows measured with the W (black) and the Cr-coated (colored) probe tip. Spin-resolved line sections differ in periodicity, phase, and amplitude from their spin-averaged counterparts (Fig. 2 of Nat. Commun. 10, 2610 (2019), http://creativecommons.org/licenses/by/4.0/).

 



This research was conducted by the following research team:

Martin Schmitt1, Paolo Moras2, Gustav Bihlmayer3, Ryan Cotsakis1,4, Matthias Vogt1, Jeannette Kemmer1,8, Abderrezak Belabbes5, Polina M. Sheverdyaeva2, Asish K. Kundu6, Carlo Carbone2, Stefan Blügel3, and Matthias Bode1,7

 

Physikalisches Institut, Experimentelle Physik II, Universität Würzburg, Würzburg, Germany
Istituto di Struttura della Materia-CNR (ISM-CNR), 34149 Trieste, Italy
Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich & JARA, Jülich, Germany
University of British Columbia, 2329 West Mall, Vancouver, BC, Canada
Physical Science and Engineering Division, King Abdullah University of Science & Technology, Thuwal, Saudi Arabia.
International Center for Theoretical Physics, Trieste, Italy
Wilhelm Conrad Röntgen-Center for Complex Material Systems, Universität Würzburg, Würzburg, Germany

Contact persons:

Paolo Moras, email: paolo.moras@ism.cnr.it
 


Reference

Martin Schmitt, Paolo Moras, Gustav Bihlmayer, Ryan Cotsakis, Matthias Vogt, Jeannette Kemmer, Abderrezak Belabbes, Polina M. Sheverdyaeva, Asish K. Kundu, Carlo Carbone, Stefan Blügel, and Matthias Bode, "Indirect chiral magnetic exchange through Dzyaloshinskii-Moriya—enhanced RKKY interactions in manganese oxide chains on Ir(100)", Nature Communications 10, 2610 (2019); DOI: 10.1038/s41467-019-10515-3 

 
Last Updated on Friday, 19 July 2019 09:55