Suchergebnisse

We performed x-ray magnetic circular dichroism (XMCD) measurements on heterostructures comprising topological insulators (TIs) of the (Bi,Sb)2(Se,Te)3 family and the magnetic insulator EuS. XMCD measurements allow us to investigate element-selective magnetic proximity effects at the very TI/EuS interface. A systematic analysis reveals that there is neither significant induced magnetism within the TI nor an enhancement of the Eu magnetic moment at such interface. The induced magnetic moments in Bi, Sb, Te, and Se sites are lower than the estimated detection limit of the XMCD measurements of ∼10−3μB/at. ; This research was supported by the European Union's Horizon 2020 FET-PROACTIVE project TOCHA under Grant Agreement 824140. The authors acknowledge funding by the European Research Council under Grant Agreement No. 306652 SPINBOUND, by the CERCA Programme/Generalitat de Catalunya 2017 SGR 827, by MINECO (under Contracts No. MAT2016-75952-R, No. MAT2016-78293-C6-2-R, No. PID2019-111773RBI00/AEI/10.13039/501100011033, No. PID2019- 107338RB-C65, and Severo Ochoa No. SEV-2017-0706) and the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI). A. I. F. acknowledges funding from MINECO-Juan de la Cierva fellowship ref. IJCI-2015-25514 and from the European Union's Horizon 2020 People Programme (Marie Skłodowska Curie Actions) H2020-MSCA-IF-2017 under REA Grant Agreement No. 796925. F. B. acknowledges funding from MINECO Ramón y Cajal Program under Contract No. RYC-2015-18523. ; Peer reviewed

Metal halides are a class of layered materials with promising electronic and magnetic properties persisting down to the two-dimensional limit. While most recent studies focused on the trihalide components of this family, the rather unexplored metal dihalides are also van der Waals layered systems with distinctive magnetic properties. Here we show that the dihalide NiBr2 grows epitaxially on a Au(111) substrate and exhibits semiconducting and magnetic behavior starting from a single layer. Through a combination of a low-temperature scanning-tunneling microscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy, and photoemission electron microscopy, we identify two competing layer structures of NiBr2 coexisting at the interface and a stoichiometrically pure layer-by-layer growth beyond. Interestingly, X-ray absorption spectroscopy measurements revealed a magnetically ordered state below 27 K with in-plane magnetic anisotropy and zero-remanence in the single layer of NiBr2/Au(111), which we attribute to a noncollinear magnetic structure. The combination of such two-dimensional magnetic order with the semiconducting behavior down to the 2D limit offers the attractive perspective of using these films as ultrathin crystalline barriers in tunneling junctions and low-dimensional devices. ; D.B. acknowledges funding from the Austrian Science Fund (FWF) under the Erwin Schrödinger fellowship agreement (project number: J4395-N). C.G.-O. and M.P.-D. acknowledge funding of the Ph.D. fellowship from the MPC Foundation. We gratefully acknowledge financial support from Spanish AEI (Grant Nos. PID2019-107338RB-C6, RTI-2018-095303-C53, and the Maria de Maeztu Units of Excellence Programme MDM-2016-0618) and from the European Union (EU) through Horizon 2020 (SUPERTED Grant No. 800923), from Interred POCTEFA V-A Spain/France/Andorra Program (EFA 194/16/TNSI), the Basque Government (GV/EJ) under grants IT-1255-19, and the European Regional Development Fund. P.G. acknowledges funding from MINECO Grant No. FIS2016-78591-C3-2-R and FLAG-ERA Grant No. PCI2019-111908-2. ; Peer reviewed

The development of multilayered materials with engineered magnetic properties compels a deep knowledge of physical properties at the atomic scale. The magnetic anisotropy is a key property in these materials. This work accounts for the magnetic anisotropy energy and its correlation with atomic properties of Co/Pd multilayers with the number of Co/Pd repetitions. Magnetometry measurements confirm stronger perpendicular magnetic anisotropy energies as the number of repetitions increases up to 40. However, the intrinsic anisotropy, related to the Co–Pd orbital hybridization and spin–orbit coupling, saturates at 15 repetitions. This finding is supported by x-ray magnetic circular dichroism analysis that reveals a direct correlation of the atomic Co and Pd orbital magnetic moments and the effective anisotropy of the system. The proximity effect that accounts for the Pd induced magnetization, along with the increasing Co moment, provides a suitable mechanism for the observed anisotropy energy layer dependence. ; This work received funding from the MINECO-AEI FIS2016-76058, UE FEDER "Una manera de hacer Europa"; the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant No. 734801, and the Basque Country Grant Nos. IT1162-19 and PIBA 2018-11. UPV/EHU authors are thankful for the technical and human support provided by the laser facility, x-ray service, electron microscopy, and magnetic measurement units of SGIker UPV/EHU. We acknowledge the access to BL29 at ALBA synchrotron light facility via official Proposal No. 2015091407. M.V. acknowledges funding via MINECO [Grant No. FIS2016-78591-C3-2-R (AEI/FEDER, UE)]. IMDEA Nanociencia acknowledges the support from the "Severo Ochoa" Program for Centers of Excellence in R&D (MINECO, Grant No. SEV-2016-0686). R.M. acknowledges enlightening discussions with Dr. Arturo Ponce-Pedraza (UTSA) on TEM. ; Peer reviewed

One-atom-thick rare-earth/noble metal (RE-NM) compounds are attractive materials to investigate two-dimensional magnetism, since they are easy to synthesize into a common RE-NM2 structure with high crystal perfection. Here we perform a comparative study of the GdAu2, HoAu2, and YbAu2 monolayer compounds grown on Au(111). We find the same atomic lattice quality and moiré superlattice periodicity in the three cases, but different electronic properties and magnetism. The YbAu2 monolayer reveals the characteristic electronic signatures of a mixed-valence configuration in the Yb atom. In contrast, GdAu2 and HoAu2 show the trivalent character of the rare-earth and ferromagnetic transitions below 22 K. Yet, the GdAu2 monolayer has an in-plane magnetic easy-axis, versus the out-of-plane one in HoAu2. The electronic bands of the two trivalent compounds are very similar, while the divalent YbAu2 monolayer exhibits different band features. In the latter, a strong 4f–5d hybridization is manifested in neatly resolved avoided crossings near the Fermi level. First principles theory points to a residual presence of empty 4f states, explaining the fluctuating valence of Yb in the YbAu2 monolayer. ; This work was supported in part by the Spanish MINECO (MAT-2017-88374-P, MAT2016-78293-C6, FIS2016-75862-P, PGC2018-098613-B-C21, SEV-2017-0706 and SEV-2016-0686), the Spanish Research Agency (PID2019-107338RB-C65), Basque Government Project IT-1255-19, and University of the Basque Country (UPV/EHU) grant GIU18/138 and the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI). The experiments at Elettra were supported by the EU Calipso project and computational resources were provided by the DIPC computing center. L. F. acknowledges financial support from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement MagicFACE No 797109. A. K. K., M. J., P. M. S. and P. M. acknowledge the project EUROFEL-ROADMAP ESFRI. ; Peer reviewed

Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to take advantage of the lanthanides properties in thermally robust nanomaterials. In this work, the electronic and magnetic properties of Dy-carboxylate metal–organic networks on Cu(111) based on an eightfold coordination between Dy and ditopic linkers are inspected. This surface science study based on scanning probe microscopy and X-ray magnetic circular dichroism, complemented with density functional theory and multiplet calculations, reveals that the magnetic anisotropy landscape of the system is complex. Surface-supported metal–organic coordination is able to induce a change in the orientation of the easy magnetization axis of the Dy coordinative centers as compared to isolated Dy atoms and Dy clusters, and significantly increases the magnetic anisotropy. Surprisingly, Dy atoms coordinated in the metallosupramolecular networks display a nearly in-plane easy magnetization axis despite the out-of-plane symmetry axis of the coordinative molecular lattice. Multiplet calculations highlight the decisive role of the metal–organic coordination, revealing that the tilted orientation is the result of a very delicate balance between the interaction of Dy with O atoms and the precise geometry of the crystal field. This study opens new avenues to tailor the magnetic anisotropy and magnetic moments of lanthanide elements on surfaces. ; The ALBA synchrotron is acknowledged for providing beam time at BOREAS beamline (proposal number 2015091454). This project has received funding from the European Research Council (ERC, grant 766555) and Marie Sklodowska-Curie Actions (MSCA, project 894924) under the European Union's Horizon 2020 research and innovation programme. This work has been financed by the Spanish Ministerio de Economía, Industria y Competitividad (projects FIS2016-78591-C3-1-R, RTI2018-097895-B-C42, MAT2016-78293-C6-2-R, MAT2017-85089-C2-1-R, and PID2019-107338RB-C65); the Comunidad de Madrid (Projects S2013/MIT-2850, P2018/NMT4321, and S2018/NMT-4367); the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI); and "Severo Ochoa" Programme for Centres of Excellence in R&D (grants SEV-2016-0686, and SEV-2017-0706).

Altres ajuts: This work was supported in part by the Basque Government Project IT-1255-19, and University of the Basque Country (UPV/EHU) grant GIU18/138 and the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI). ; One-atom-thick rare-earth/noble metal (RE-NM) compounds are attractive materials to investigate two-dimensional magnetism, since they are easy to synthesize into a common RE-NM2 structure with high crystal perfection. Here we perform a comparative study of the GdAu2, HoAu2, and YbAu2 monolayer compounds grown on Au(111). We find the same atomic lattice quality and moiré superlattice periodicity in the three cases, but different electronic properties and magnetism. The YbAu2 monolayer reveals the characteristic electronic signatures of a mixed-valence configuration in the Yb atom. In contrast, GdAu2 and HoAu2 show the trivalent character of the rare-earth and ferromagnetic transitions below 22 K. Yet, the GdAu2 monolayer has an in-plane magnetic easy-axis, versus the out-of-plane one in HoAu2. The electronic bands of the two trivalent compounds are very similar, while the divalent YbAu2 monolayer exhibits different band features. In the latter, a strong 4f-5d hybridization is manifested in neatly resolved avoided crossings near the Fermi level. First principles theory points to a residual presence of empty 4f states, explaining the fluctuating valence of Yb in the YbAu2 monolayer.

Nanometer-thick epitaxial Co films intercalated between graphene (Gr) and a heavy metal (HM) substrate are promising systems for the development of spin–orbitronic devices due to their large perpendicular magnetic anisotropy (PMA). A combination of theoretical modeling and experiments reveals the origin of the PMA and explains its behavior as a function of the Co thickness. High quality epitaxial Gr/Con/HM(111) (HM = Pt,Ir) heterostructures are grown by intercalation below graphene, which acts as a surfactant that kinetically stabilizes the pseudomorphic growth of highly perfect Co face-centered tetragonal (fct) films, with a reduced number of stacking faults as the only structural defect observable by high-resolution scanning transmission electron microscopy (STEM). Magneto-optic Kerr effect (MOKE) measurements show that such heterostructures present PMA up to large Co critical thicknesses of about 4 nm (20 ML) and 2 nm (10 ML) for Pt and Ir substrates, respectively. X-ray magnetic circular dichroism (XMCD) measurements show an inverse power law of the anisotropy of the orbital moment with Co thickness, reflecting its interfacial nature, that changes sign at about the same critical values. First principles calculations show that, regardless of the presence of graphene, ideal Co fct films on HM buffers do not sustain PMAs beyond around 6 mLs due to the in-plane contribution of the inner bulk-like Co layers. The large experimental critical thicknesses sustaining PMA can only be retrieved by the inclusion of structural defects that promote a local hcp stacking such as twin boundaries or stacking faults. Remarkably, a layer resolved analysis of the orbital momentum anisotropy reproduces its interfacial nature, and reveals that the Gr/Co interface contribution is comparable to that of the Co/Pt(Ir). ; Financial support from MINECO (Grant Nos. RTI2018-097895-B-C41, RTI2018-097895-B-C42 and RTI2018-097895-B-C43 (FUN-SOC), PID2019-103910GB-I00, FIS2016-78591-C3-1-R and FIS2016-78591-C3-2-R (SKYTRON), PGC2018-098613-B-C21 (SpOrQuMat), PCI2019-111908-2 and PCI2019-111867-2 (FLAGERA 3 grant SOgraphMEM)], from Regional Government of Madrid (grant number P2018/NMT-4321 (NANOMAGCOST-CM)) and from Gobierno Vasco-UPV/EHU (grant numbers GIU18/138 and IT-1246-19). We acknowledge experiments at ALBA BL29 via proposal no. 2019023333. IMDEA-Nanociencia acknowledges support from the "Severo Ochoa" Program for Centres of Excellence in R&D (MINECO, Grant SEV-2016-0686). ; Peer reviewed