Size distribution and frustrated antiferromagnetic coupling effects on the magnetic behavior of ultrafine akaganéite (β-FeOOH) nanoparticles

The magnetic properties of low dimensional materials of several iron oxyhydroxide phases, such as akaganéite (β-FeOOH) or lepidocrocite (γ-FeO(OH)), remain poorly explored, probably due to their specific preparation as single crystalline phase requires special conditions owing to their structural instability. In the present work, ultrafine akaganéite nanoparticles were prepared by the hydrolysis of FeCl3 solutions at room temperature induced by the presence of NaOH. The resulting product was characterized by several analytical techniques. Structural investigations using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) revealed that the sample was mainly constituted by rather-equiaxial akaganéite nanocrystals with mean diameter of 3.3 ± 0.5 nm. In addition, a small amount of rodlike akaganéite particles with 23 ± 5 nm in length and 5 ± 1 nm in width was also detected. The study of the respective dependences of the dc magnetization and the ac susceptibility on temperature and exciting magnetic field revealed complex magnetic relaxation processes, high coercivity values at low temperature, and exchange bias effect. These results have been tentatively explained considering size distribution effects and the presence of superparamagnetic and spin glass-like contributions arising from the frustration of the antiferromagnetic order owing to surface effects and an insufficient filling of the akaganéite channels with Cl- anions.Financial support from the Mexican Council of Science and Technology (CONACYT) and Universidad Autonoma de Nuevo León under research projects CB12-179486 and PAICYT-CE793-11, respectively, is acknowledged. Spanish funding under the MINECO research project MAT2010-20798-C05-04 and FEDER is also acknowledged.Peer Reviewe

Magnetism and morphology of Co nanocluster superlattices on GdAu2 /Au(111)- (13×13)

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al.We present a comprehensive study of the magnetism and morphology of an ultrahigh density array of Co nanoclusters self-assembled on the single atomic layer GdAu2 on Au(111) template surface. Combining scanning tunneling microscopy, x-ray magnetic circular dichroism, and magneto-optical Kerr effect measurements, we reveal a significant enhancement of the perpendicular magnetic anisotropy energy for noncoalesced single atomic layer nanoclusters compared to Co/Au(111). For coverages well beyond the onset of coalescence, we observe room-temperature in-plane magnetic remanence.We acknowledge funding from the Swiss National Science Foundation, from the Sino-Swiss Science and Technology Cooperation Project No. IZLCZ2 123892, from the Spanish Ministerio de Ciencia e Innovacion (MAT2010-21156-C03-03), from the Gipuzkoako Foru Aldundia, from the European Social Fund within the program JAE-Doc, the Basque Government (IT-621-13 and IT-627-13) and SAIOTEK (S-PE12UN095), as well as from the EU Calipso program for synchrotron access funding. The MBE chamber on DEIMOS was funded by the Agence National de la Recherche with Grant No. ANR-05-NANO-073.Peer Reviewe

Electronic structure of titanylphthalocyanine layers on Ag(111)

We have investigated the electronic structures of axially oxo functionalized titanylphthalocyanine (TiOPc) on Ag(111) by X-ray and ultraviolet photoelectron spectroscopies, two-photon photoemission, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism. Furthermore, we use complementary data of TiOPc on graphite and planar copper phthalocyanine (CuPc) on Ag(111) for a comparative analysis. Both molecules adsorb on Ag(111) in a parallel orientation to the surface, for TiOPc with an oxygen-up configuration. The interaction of nitrogen and carbon atoms with the substrate is similar for both molecules, while the bonding of the titanium atom to Ag(111) in the monolayer is found to be slightly more pronounced than in the CuPc case. Ultraviolet photoemission spectroscopy reveals an occupation of the lowest unoccupied molecular orbital (LUMO) level in monolayer thick TiOPc on Ag(111) related to the interaction of the molecules and the silver substrate. This molecule-metal interaction also causes an upward shift of the Ag(111) Shockley state that is transformed into an unoccupied interface state with energies of 0.23 and 0.33 eV for the TiOPc monolayer and bilayer, respectively, at the Brillouin zone center.The authors acknowledge financial support from the Deutsche Forschungsgemeinschaft through SFB 1083 “Structure and Dynamics of Internal Interfaces”, the Spanish CSIC I-Link programm, the Spanish Ministry of Economy and Competitiveness, MINECO (under Contract No. MAT2016-78293-C6-2-R, and Severo Ochoa No. SEV-2013-0295.), and by the Secretariat for Universities and Research, Knowledge Department of the Generalitat de Catalunya (2014 SGR 715). M. Paradinas thanks the Spanish Government for financial support through PTA2014-09788-I fellowships. ICN2 is funded by the CERCA Programme/Generalitat de Catalunya.Peer Reviewe

Strain dependent light-off temperature in catalysis revealed by planar laser-induced fluorescence

Understanding how specific atom sites on metal surfaces lower the energy barrier for chemical reactions is vital in catalysis. Studies on simplified model systems have shown that atoms arranged as steps on the surface play an important role in catalytic reactions, but a direct comparison of how the light-off temperature is affected by the atom orientation on the step has not yet been possible due to methodological constraints. Here we report in situ spatially resolved measurements of the CO production over a cylindrical-shaped Pd catalyst and show that the light-off temperature at different parts of the crystal depends on the step orientation of the two types of steps (named A and B). Our finding is supported by density functional theory calculations, revealing that the steps, in contrast to what has been previously reported in the literature, are not directly involved in the reaction onset but have the role of releasing stress.The authors thank the Knut and Alice Wallenberg Foundation, the Swedish Research Council, the Swedish Foundation for Strategic Research, and the Crafoord Foundation. Support by the MAX IV staff is gratefully acknowledged. The calculations were performed at C3SE through a SNIC grant. J.E.O. acknowledges support from the Spanish Ministry of Economy (MAT2013-46593-C6-4-P) and the Basque Government (IT621-13).Peer Reviewe

Identification and manipulation of defects in black phosphorus

We identify and manipulate commonly occurring defects in black phosphorus, combining scanning tunneling microscopy experiments with density functional theory calculations. A ubiquitous defect, imaged at negative bias as a bright dumbbell extending over several nanometers, is shown to arise from a substitutional Sn impurity in the second sublayer. Another frequently observed defect type is identified as arising from an interstitial Sn atom; this defect can be switched to a more stable configuration consisting of a Sn substitutional defect + P adatom, by application of an electrical pulse via the STM tip. DFT calculations show that this pulse-induced structural transition switches the system from a non-magnetic configuration to a magnetic one. We introduce States Projected Onto Individual Layers (SPOIL) quantities which provide information about atom-wise and orbital-wise contributions to bias-dependent features observed in STM images.We acknowledge the Indo-French Centre for the Promotion of Advanced Scientific Research (CEFIPRA) for funding our collaboration. We are especially grateful for assistance provided by the Late Ms. A. Sathidevi. We are grateful to the ANR and MOST (DEFINE2D Project No. ANR-20-CE09-0023, MOST 110-2923-M-002-010) for financial support and the TUE-CMS, JNCASR, and the Sheikh Saqr Laboratory, JNCASR, for computational resources. D.S. acknowledges support from the NPDF of the Science and Engineering Research Board (SERB), Government of India.Peer reviewe

Noncollinear magnetic order in two-dimensional NiBr2 films grown on Au(111)

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 reviewe

Ba+2 ion trapping using organic submonolayer for ultra-low background neutrinoless double beta detector

If neutrinos are their own antiparticles the otherwise-forbidden nuclear reaction known as neutrinoless double beta decay can occur. The very long lifetime expected for these exceptional events makes its detection a daunting task. In order to conduct an almost background-free experiment, the NEXT collaboration is investigating novel synthetic molecular sensors that may capture the Ba dication produced in the decay of certain Xe isotopes in a high-pressure gas experiment. The use of such molecular detectors immobilized on surfaces must be explored in the ultra-dry environment of a xenon gas chamber. Here, using a combination of highly sensitive surface science techniques in ultra-high vacuum, we demonstrate the possibility of employing the so-called Fluorescent Bicolor Indicator as the molecular component of the sensor. We unravel the ion capture process for these molecular indicators immobilized on a surface and explain the origin of the emission fluorescence shift associated to the ion trapping.This material is based upon work supported by the following agencies and institutions: the European Research Council (ERC) under ERC-2020-SyG 951281; the MCIN/AEI/10.13039/501100011033 of Spain and ERDF A way of making Europe under grants PID2020-114252GB-I00, PID2019-107338RB-C63, PID2019-104772GB-I00, PID2019-111281GB-I00, and RTI2018-095979, the Severo Ochoa Program grant CEX2018-000867-S; the Basque Government (GV/EJ) under grants IT-1553-22, IT-1591-22. The NEXT Collaboration acknowledges support from the following agencies and institutions: the European Union’s Framework Programme for Research and Innovation Horizon 2020 (2014-2020) under Grant Agreement No. 957202-HIDDEN; the MCIN/AEI of Spain and ERDF A way of making Europe under grants RTI2018-095979 and PID2021-125475NB, the Severo Ochoa Program grant CEX2018-000867-S and the Ramón y Cajal program grant RYC-2015-18820; the Generalitat Valenciana of Spain under grants PROMETEO/2021/087 and CIDEGENT/2019/049; the Department of Education of the Basque Government of Spain under the predoctoral training program non-doctoral research personnel; the Portuguese FCT under project UID/FIS/04559/2020 to fund the activities of LIBPhys-UC; the Pazy Foundation (Israel) under grants 877040 and 877041; the US Department of Energy under contracts number DE-AC02-06CH11357 (Argonne National Laboratory), DE-AC02-07CH11359 (Fermi National Accelerator Laboratory), DE-FG02-13ER42020 (Texas A&M), DE-SC0019054 (Texas Arlington) and DE-SC0019223 (Texas Arlington); the US National Science Foundation under award number NSF CHE 2004111; the Robert A Welch Foundation under award number Y-2031-20200401. Finally, we are grateful to the Laboratorio Subterráneo de Canfranc for hosting and supporting the NEXT experiment.Peer reviewe

Epitaxial monolayers of the magnetic 2D semiconductor FeBr2 grown on Au(111)

Magnetic two-dimensional (2D) semiconductors have attracted a lot of attention because modern preparation techniques are capable of providing single-crystal films of these materials with precise control of thickness down to the single-layer limit. It opens up a way to study a rich variety of electronic and magnetic phenomena with promising routes toward potential applications. We have investigated the initial stages of epitaxial growth of the magnetic van der Waals semiconductor FeBr2 on a single-crystal Au(111) substrate by means of low-temperature scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), low-energy electron emission microscopy (LEEM), and X-ray photoemission electron microscopy (XPEEM). Magnetic properties of the one- and two-layer thick films were measured via X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). Our findings show a striking difference in the magnetic behavior of the single layer of FeBr2 and its bulk counterpart, which can be attributed to the modifications in the crystal structure due to the interaction with the substrate.C.G.-O. and M.P.-D. acknowledge funding of the Ph.D. fellowship from the MPC Foundation. S.E.H. thanks the whole AG Kuch and in particular J. Gördes for help during the BESSY measurements and also the local IT/electronics workshop/fine mechanics workshop teams for their continuous support. In particular, he is very thankful for the possibility to perform some last STM measurements at the PEARL beamline at SLS thanks Dr. Matthias Muntwiler. J.N. thanks the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for his funding under project 277101999 - CRC 183. S.T. acknowledges financial support by BMBF through project VEKMAG (BMBF 05K19KEA). P.G. acknowledges funding from PID2020-116181RB-C32 and FlagEraSOgraphMEM PCI2019-111908-2 (AEI/FEDER). D.G.O. acknowledges funding by the Spanish MCIN/AEI/10.13039/501100011033 and by the European Union “NextGenerationEU”/PRTR (PID2019-107338RB-C63 and TED2021-132388B–C43). C.R., M.I., C.G.-O., and M.P.-D. acknowledge funding by the European Union’s Horizon 2020 research and innovation program (grant agreement No 800923), the Spanish MCIN/AEI/10.13039/501100011033 (PID2020-114252GB-I00, PID2019-107338RB-C63, TED2021-130292B–C42), the Basque Goverment IT1591-22, and by the IKUR Strategy under the collaboration agreement between Ikerbasque Foundation and MPC on behalf of the Department of Education of the Basque Government. C.R., M.I., C.G.-O., and S.E.H. are very thankful for the help during the XMCD and STM measurements of Samuel Kerschbaumer, Andrea Aguirre Baños, and Amitayush Jha Thakur.Peer reviewe

Self-organized growth of high density magnetic Co nanodot arrays on a Moiré template

3 páginas, 4 figuras.We report the self-organized growth of cobalt nanodot arrays using a Gd-Au Moiré superlattice as a template. After analyzing the influence of the Co flux and the substrate temperature, we obtain the suitable parameters to maximize nanodot density, homogeneity, and individual size. Depending on the growth conditions an areal density of up to 54 Teradots/inch2 can be achieved. Below the limit of lateral coalescence, independent nanodots made of ∼ 1000 Co atoms exhibit room temperature remanent magnetization.This work was financed by the Basque Government (Nanotron, Grant No. IT-257-07), the Spanish MCyI (Grant No. MAT2007-63083), and the Acciones Integradas program.Peer reviewe

Magnetic properties of the HoAu2 surface alloy

Resumen del trabajo presentado a la Reunión conjunta de la sección de materia condensada de la DPG y la División de Materia Condensada de la EPS (Gemeinsame Tagung der Sektion Kondensierte Materie der DPG und der Condensed Matter Division der EPS); Berlin 11 al 16 de marzo de 2018.Magnetic properties of single atoms of Holmium deposited on the surface of different single crystalline substrates were a topic of active investigations in last few years. Hybridization of the Ho orbitals with electronic states of the substrate and symmetry of the crystal field were shown to be crucial in defining Ho ground state. In this work we present results of the experimental investigation of the magnetic properties of HoAu2 surface alloy. A crystal field in the position of Ho atoms in this alloy has the same C3v symmetry as in the case of the Ho adsorbed on the hollow sites of (111) surface of the fcc crystal. However, the coordination is higher which results in a well-defined stable stoichiometry. Samples prepared by deposition of Ho on the pre-heated surface of the Au(111) single crystal in the UHV environment were characterized by means of STM, LEED and ARPES. Its magnetic properties were measured in a synchrotron using XMCD spectroscopy and simulated with a help of the multiplet calculations.Peer Reviewe