25 research outputs found
Magnetic transitions induced by tunnelling electrons in individual adsorbed M-Phthalocyanine molecules (M Fe, Co)
We report on a theoretical study of magnetic transitions induced by
tunnelling electrons in individual adsorbed M-Phthalocyanine (M-Pc) molecules
where M is a metal atom: Fe-Pc on a Cu(110)(21)-O surface and Co-Pc
layers on Pb(111) islands. The magnetic transitions correspond to the change of
orientation of the spin angular momentum of the metal ion with respect to the
surroundings and possibly an applied magnetic field. The adsorbed Fe-Pc system
is studied with a Density Functional Theory (DFT) transport approach showing
that i) the magnetic structure of the Fe atom in the adsorbed Fe-Pc is quite
different from that of the free Fe atom or of other adsorbed Fe systems and ii)
that injection of electrons (holes) into the Fe atom in the adsorbed Fe-Pc
molecule dominantly involves the Fe orbital. These results fully
specify the magnetic structure of the system and the process responsible for
magnetic transitions. The dynamics of the magnetic transitions induced by
tunnelling electrons is treated in a strong-coupling approach. The Fe-Pc
treatment is extended to the Co-Pc case. The present calculations accurately
reproduce the strength of the magnetic transitions as observed by magnetic IETS
(Inelastic Electron Tunnelling Spectroscopy) experiments; in particular, the
dominance of the inelastic current in the conduction of the adsorbed M-Pc
molecule is accounted for
Many-body effects in magnetic inelastic electron tunneling spectroscopy
Magnetic inelastic electron tunneling spectroscopy (IETS) shows sharp
increases in conductance when a new conductance channel associated to a change
in magnetic structure is open. Typically, the magnetic moment carried by an
adsorbate can be changed by collision with a tunneling electron; in this
process the spin of the electron can flip or not. A previous one-electron
theory [Phys. Rev. Lett. {\bf 103}, 176601 (2009)] successfully explained both
the conductance thresholds and the magnitude of the conductance variation. The
elastic spin flip of conduction electrons by a magnetic impurity leads to the
well known Kondo effect. In the present work, we compare the theoretical
predictions for inelastic magnetic tunneling obtained with a one-electron
approach and with a many-body theory including Kondo-like phenomena. We apply
our theories to a singlet-triplet transition model system that contains most of
the characteristics revealed in magnetic IETS. We use two self-consistent
treatments (non-crossing approximation and self-consistent ladder
approximation). We show that, although the one-electron limit is properly
recovered, new intrinsic many-body features appear. In particular, sharp peaks
appear close to the inelastic thresholds; these are not localized exactly at
thresholds and could influence the determination of magnetic structures from
IETS experiments.Analysis of the evolution with temperature reveals that these
many-body features involve an energy scale different from that of the usual
Kondo peaks. Indeed, the many-body features perdure at temperatures much larger
than the one given by the Kondo energy scale of the system.Comment: 10 pages and 6 figure
Theoretical study of the electronic excited states in ultrathin ionic layers supported on metal surfaces: NaCl/Cu(111)
We present a theoretical study of the electronic excited states in ultrathin ionic layers supported on metal surfaces. We have studied 1, 2, 3, and 4 monolayers of NaCl on a Cu(111) surface. Energies, lifetimes, and associated wave functions of the excited states have been obtained with a joint, model potential–wave packet
propagation approach. The excited state with the lowest energy has the character of an image potential state repelled from the surface by the NaCl layer. The next two states present a mixed character of image potential states and NaCl layer states corresponding to the quantization of the conduction band in the finite-size layer. We
discuss the role of the layer thickness in decoupling these states from the metal surface and how it affects their lifetimeS.D.-T. gratefully acknowledges postdoctoral support from the Triangle de la Physique and the Juan de la Cierva program from the Spanish Ministerio de Ciencia e Innovación
Structural and magnetic properties of FeMn (1...6) chains supported on CuN / Cu (100)
Heterogeneous atomic magnetic chains are built by atom manipulation on a
CuN/Cu (100) substrate. Their magnetic properties are studied and
rationalized by a combined scanning tunneling microscopy (STM) and density
functional theory (DFT) work completed by model Hamiltonian studies. The chains
are built using Fe and Mn atoms ontop of the Cu atoms along the N rows of the
CuN surface. Here, we present results for FeMn (=1...6) chains
emphasizing the evolution of the geometrical, electronic, and magnetic
properties with chain size. By fitting our results to a Heisenberg Hamiltonian
we have studied the exchange-coupling matrix elements for different chains.
For the shorter chains, , we have included spin-orbit effects in the
DFT calculations, extracting the magnetic anisotropy energy. Our results are
also fitted to a simple anisotropic spin Hamiltonian and we have extracted
values for the longitudinal-anisotropy and transversal-anisotropy
constants. These parameters together with the values for allow us to
compute the magnetic excitation energies of the system and to compare them with
the experimental data.Comment: 10 pages 8 figure
Quenching of magnetic excitations in single adsorbates at surfaces: Mn on CuN/Cu(100)
The lifetimes of spin excitations of Mn adsorbates on CuN/Cu(100) are
computed from first-principles. The theory is based on a strong-coupling
T-matrix approach that evaluates the decay of a spin excitation due to
electron-hole pair creation. Using a previously developed theory [Phys. Rev.
Lett. {\bf 103}, 176601 (2009) and Phys. Rev. B {\bf 81}, 165423 (2010)], we
compute the excitation rates by a tunneling current for all the Mn spin states.
A rate equation approach permits us to simulate the experimental results by
Loth and co-workers [Nat. Phys. {\bf 6}, 340 (2010)] for large tunnelling
currents, taking into account the finite population of excited states. Our
simulations give us insight into the spin dynamics, in particular in the way
polarized electrons can reveal the existence of an excited state population. In
addition, it reveals that the excitation process occurs in a way very different
from the deexcitation one. Indeed, while excitation by tunnelling electrons
proceeds via the s and p electrons of the adsorbate, deexcitation mainly
involves the d electrons
Correlation-mediated processes for electron-induced switching between Néel States of Fe antiferromagnetic chains
The controlled switching between two quasistable Néel states in adsorbed antiferromagnetic Fe chains has recently been achieved by Loth et al. [Science 335, 196 (2012)] using tunneling electrons from an STM tip. In order to rationalize their data, we evaluate the rate of tunneling electron-induced switching between the Néel states. Good agreement is found with the experiment, permitting us to identify three switching mechanisms: (i) low STM voltage direct electron-induced transitions, (ii) intermediate STM voltage switching via spin-wave-like excitation, and (iii) high STM voltage transitions mediated by domain-wall formation. Spin correlations in the antiferromagnetic chains are the switching driving force, leading to a marked chain-size dependence
Excitation of local magnetic moments by tunnelling electrons
The advent of milli-kelvin scanning tunneling microscopes (STM) with inbuilt
magnetic fields has opened access to the study of magnetic phenomena with
atomic resolution at surfaces. In the case of single atoms adsorbed on a
surface, the existence of different magnetic energy levels localized on the
adsorbate is due to the breaking of the rotational invariance of the adsorbate
spin by the interaction with its environment, leading to energy terms in the
meV range. These structures were revealed by STM experiments in IBM Almaden in
the early 2000's for atomic adsorbates on CuN surfaces. The experiments
consisted in the study of the changes in conductance caused by inelastic
tunnelling of electrons (IETS, Inelastic Electron Tunnelling Spectroscopy).
Manganese and Iron adatoms were shown to have different magnetic anisotropies
induced by the substrate. More experiments by other groups followed up, showing
that magnetic excitations could be detected in a variety of systems: e.g.
complex organic molecules showed that their magnetic anistropy was dependent on
the molecular environment, piles of magnetic molecules showed that they
interact via intermolecular exchange interaction, spin waves were excited on
ferromagnetic surfaces and in Mn chains, and magnetic impurities have been
analyzed on semiconductors. These experiments brought up some intriguing
questions: the efficiency of magnetic excitations was very high, the
excitations could or could not involve spin flip of the exciting electron and
singular-like behavior was sometimes found at the excitation thresholds. These
facts called for extended theoretical analysis; perturbation theories,
sudden-approximation approaches and a strong coupling scheme successfully
explained most of the magnetic inelastic processes. In addition, many-body
approaches were also used to decipher the interplay between inelasComment: Review article to appear in Progress of Surface Scienc