270 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
Excitation of spin waves by tunneling electrons in ferromagnetic and antiferromagnetic spin-1/2 Heisenberg chains
14 páginas, 14 figuras.-- PACS number(s): 68.37.Ef, 72.25.−b, 73.23.−b, 75.30.DsExcitation of finite chains of magnetic atoms adsorbed on a surface by tunneling electrons from a scanning tunneling microscope tip is studied using a Heisenberg Hamiltonian description of the magnetic couplings along the chain and a strong coupling approach to inelastic tunneling. The excitation probability of the magnetic levels is very high and the excitation spectra in chains of different lengths are very similar. The excitations in finite chains can be considered as spin waves quantized in the finite object. The energy and momentum spectra of the spin waves excited in the idealized infinite chain by tunneling electrons are determined from the results on the finite chains. Both ferromagnetic and antiferromagnetic couplings are considered, leading to very different results. In particular, in the antiferromagnetic case, excitations linked to the entanglement of the chain ground state are evidenced.Peer reviewe
Magnetic reversal of a quantum nanoferromagnet
When the external magnetic field applied to a ferromagnetically coupled atomic chain is reversed suddenly, the magnetization of the chain switches, due to the reversal of all the atomic magnetic moments in the chain. The quantum processes underlying the magnetization switching and the time required for the switching are analyzed for model magnetic chains adsorbed on a surface at 0 K. The sudden field reversal brings the chain into an excited state that relaxes towards the system ground state via interactions with the substrate electrons. Different mechanisms are outlined, ranging from the global stepwise rotation of the chain macrospin induced by spin-flip collisions with substrate electrons in the pure Heisenberg chain (Néel-Brown process) to a correlation-mediated direct switching process in the presence of strong magnetic anisotropies in short chains (the global spin of the chain reverses in a single electron interaction). The processes for magnetization switching induced by electrons tunneling from a scanning tunneling microscope tip are also analyzed. © 2013 American Physical Society.Peer Reviewe
Extremely long-lived magnetic excitations in supported Fe chains
We report on a theoretical study of the lifetime of the first excited state
of spin chains made of an odd number of Fe atoms on Cu2N/Cu(100). Yan et al
(Nat. Nanotech. 10, 40 (2015)) recently observed very long lifetimes in the
case of Fe3 chains. We consider the decay of the first excited state induced by
electron-hole pair creation in the substrate. For a finite magnetic field, the
two lowest-lying states in the chain have a quasi-N\'eel state structure. Decay
from one state to the other strongly depends on the degree of entanglement of
the local spins in the chain. The entanglement in the chain accounts for the
long lifetimes that increase exponentially with chain length. Despite their
apparently very different properties, the behaviour of odd and even chains is
governed by the same kind of phenomena, in particular entanglement effects. The
present results account quite well for the lifetimes recently measured by Yan
et al on Fe3Comment: 21 page
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
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