We investigate the spin relaxation of Ho single atom magnets on MgO/Ag(100) as a function of temperature and magnetic field. We find that the spin relaxation is thermally activated at low field, ...while it remains larger than 1000 s up to 30 K and 8 T. This behavior contrasts with that of single molecule magnets and bulk paramagnetic impurities, which relax faster at high field. Combining our results with density functional theory, we rationalize this unconventional behavior by showing that local vibrations activate a two-phonon Raman process with a relaxation rate that peaks near zero field and is suppressed at high field. Our work shows the importance of these excitations in the relaxation of axially coordinated magnetic atoms.
A new high‐field magnet endstation for X‐ray magnetic dichroism experiments has been installed and commissioned at the ESRF soft X‐ray beamline ID32. The magnet consists of two split‐pairs of ...superconducting coils which can generate up to 9 T along the beam and up to 4 T orthogonal to the beam. It is connected to a cluster of ultra‐high‐vacuum chambers that offer a comprehensive set of surface preparation and characterization techniques. The endstation and the beam properties have been designed to provide optimum experimental conditions for X‐ray magnetic linear and circular dichroism experiments in the soft X‐ray range between 400 and 1600 eV photon energy. User operation started in November 2014.
The high‐field magnet endstation for X‐ray magnetic dichroism experiments at the ESRF soft X‐ray beamline ID32 is presented.
Iron phthalocyanines (FePc) adsorbed onto a Ag(110) substrate self-assemble into different monolayer phases going from rectangular to different oblique phases, with increasing molecular density. We ...have investigated the oxygen uptake capability of the different phases and their associated magneto-structural changes. Our study combines scanning tunneling microscopy and spectroscopy (STM/STS), X-ray magnetic circular dichroism (XMCD), and density functional theory (DFT) calculations. STM measurements reveal that the oxygenation reaction of the FePc/Ag(110) generally involves a displacement and a rotation of the molecules, which affects the electronic state of the Fe centers. The oxygen intercalation between FePc and the substrate is greatly obstructed by the steric hindrance in the high-density phases, to the point that a fraction of oblique phase molecules cannot change their position after oxidizing. Depending on the oxidation state and adsoption geometry, the STS spectra show clear differences in the Fe local density of states, which are mirrored in the XAS and XMCD experiments. Particularly, XMCD spectra of the oxidized phases reflect the distribution of FePc species (nonoxygenated, oxygenated-rotated, and oxygenated-unrotated) in the different cases. Sum rule analysis yields the effective spin (m s eff) and orbital (m L) magnetic moments of Fe in the different FePc species. Upon oxygenation, the magnetic moment of FePc molecules increases about an order of magnitude, reaching m TOT ∼ 2.2 μB per Fe atom.
The ID32 Soft X-ray Beamline line is an ESRF upgrade beamline for X-ray absorption (XAS) and Resonant Inelastic X-ray Scattering (RIXS) spectroscopic studies. It was opened to Users in October 2014 ...and provides polarised soft X-rays in the 400–1600 eV energy range. It has two branches: One branch has been designed for X-ray magnetic dichroism experiments, both linear (XMLD) and circular (XMCD), with high sensitivity, reproducibility, flexibility, user friendliness and the capability for fast energy scanning. The X-ray beam is available in two experimental areas, one with a dedicated superconducting high field magnet and a sample preparation facility, the second area being open for User instruments. The other branch is dedicated to very high energy resolution Resonant Inelastic X-ray Scattering experiments with a combined resolving power up to 30 000 around 930 eV. A scattering arm which is continuously variable under UHV vacuum over 100 degrees and a four-circle sample goniometer allows accurate positioning of the sample to enable 3D mapping of q space. Combined with the possibility of polarisation analysis of the outgoing photons the beamline provides a state-of-the-art soft X-ray RIXS facility.
An increasing number of promising applications for future technology is arising from size constraints in nanoparticles (NPs) and from the chemical manipulation of their surfaces. In this work, we ...analyse the finite-size and surface effects on polyacrylic acid-coated Fe3O4 NPs and oleic acid-coated γ-Fe2O3 NPs by studying their magnetization curves at different temperatures. The measured thermal dependence of the saturation magnetization is no longer explained by the typical T3/2 Bloch law, yielding higher values than those expected for its exponent. When incorporated in polymeric matrixes to form magnetic transparent nanocomposites, the oleic acid-coated γ-Fe2O3 NPs also deviate from Bloch's law, but following the opposite trend observed in free coated NPs.
We have analyzed by means of scanning tunneling microscopy and spectroscopy the atomic and electronic structure of monolayers of 1T-VxPt1-xSe2 alloys grown by molecular beam epitaxy on epitaxial ...graphene substrates. We have focused on the composition range (0.1≤x≤0.35) where ferromagnetic behaviour has recently been demonstrated. For low Pt concentration, (x=0.07 and x=0.21), small domains (a few nanometres in diameter) exhibiting the characteristic superstructure of the charge density wave (CDW) state of pristine VSe2 monolayer remain visible on most of the sample surface. Thus alloying preserves the short range order of the CDW phase, although it destroys its long range order. For higher Pt concentration (x≈0.35) a disordered alloy forms. It presents a fully developped gap (a few tens meV in width) at the Fermi level and is thus a disordered insulator. This gap exhibits strong variations at the nanometer scale, reflecting the local fluctuations in the composition. An unexpectedly large interaction of the TMD layer with the graphene substrate sets in for this composition range.
Circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES) has been recently proposed as a sensitive probe of either the spin texture or the orbital momentum texture of helical Dirac ...fermions at the surface of three-dimensional topological insulators. Recent results obtained on Bi2Te3 point rather towards a final state effect. In this Rapid Communication, we study the prototypical topological insulator Bi2Se3 by CD-ARPES as a function of the photon energy and as a function of thickness. Analysis of the photon energy dependence of CD-ARPES, as well as the existence of a large dichroic signal from massive Dirac fermion states in ulthathin films, confirm that the dichroism is predominantly a final state effect.
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