18 research outputs found
Fluence-dependent dynamics of the 5d6s exchange splitting in Gd metal after femtosecond laser excitation
We investigate the fluence-dependent dynamics of the exchange-split 5d6s
valence bands of Gd metal after femtosecond, near-infrared (IR) laser
excitation. Time- and angle-resolved photoelectron spectroscopy (tr-ARPES)
with extreme ultraviolet (XUV) probe pulses is used to simultaneously map the
transient binding energies of the minority and majority spin valence bands.
The decay constant of the exchange splitting increases with fluence. This
reflects the slower response of the occupied majority-spin component, which we
attribute to ElliotāYafet spin-flip scattering in accordance with the
microscopic three-temperature model (M3TM). In contrast, the time constant of
the partly unoccupied minority-spin band stays unaffected by a change in pump
fluence. Here, we introduce as an alternative to superdiffusive spin transport
exchange scattering, which is an ultrafast electronic mechanism explaining the
observed dynamics. Exchange scattering can reduce the spin polarization in the
partially unoccupied minority-spin band and thus its energetic position
without effective demagnetization
Metastability of Free Cobalt and Iron Clusters: A Possible Precursor to Bulk Ferromagnetism
Homonuclear cobalt and iron clusters CoN and FeN measured in a cryogenic molecular beam exist in two states with distinct magnetic moments (Ī¼), polarizabilities, and ionization potentials, indicating distinct valences. The Ī¼ is approximately quantized: Ī¼N ~ 2NĪ¼B in the ground states and Ī¼N* ~ NĪ¼B in the excited states for Co; Ī¼N ~ 3N Ī¼B and Ī¼N * ~ NĪ¼B for Fe. At a large size, the average Ī¼ of the two states converges to the bulk value with diminishing ionization potential differences. The experiments suggest localized ferromagnetism in the two states and that itinerant ferromagnetism emerges from their superposition
Magnetism and exchange interaction of small rare-earth clusters; Tb as a representative
Here we follow, both experimentally and theoretically, the development of
magnetism in Tb clusters from the atomic limit, adding one atom at a time. The
exchange interaction is, surprisingly, observed to drastically increase
compared to that of bulk, and to exhibit irregular oscillations as a function
of the interatomic distance. From electronic structure theory we find that the
theoretical magnetic moments oscillate with cluster size in exact agreement
with experimental data. Unlike the bulk, the oscillation is not caused by the
RKKY mechanism. Instead, the inter-atomic exchange is shown to be driven by a
competition between wave-function overlap of the 5d shell and the on-site
exchange interaction, which leads to a competition between ferromagnetic
double-exchange and antiferromagnetic super-exchange. This understanding opens
up new ways to tune the magnetic properties of rare-earth based magnets with
nano-sized building blocks
Non-classical dipoles in cold niobium clusters
Electric deflections of niobium clusters in molecular beams show that they
have permanent electric dipole moments at cryogenic temperatures but not higher
temperatures, indicating that they are ferroelectric. Detailed analysis shows
that the deflections cannot be explained in terms of a rotating classical
dipole, as claimed by Anderson et al. The shapes of the deflected beam profiles
and their field and temperature dependences indicates that the clusters can
exist in two states, one with a dipole and the other without. Cluster with
dipoles occupy lower energy states. Excitations from the lower states to the
higher states can be induced by low fluence laser excitation. This causes the
dipole to vanish.Comment: 25 pages, 7 figure
Manipulating Multiple Order Parameters via Oxygen Vacancies: The case of Eu0.5Ba0.5TiO3-{\delta}
Controlling functionalities, such as magnetism or ferroelectricity, by means
of oxygen vacancies (VO) is a key issue for the future development of
transition metal oxides. Progress in this field is currently addressed through
VO variations and their impact on mainly one order parameter. Here we reveal a
new mechanism for tuning both magnetism and ferroelectricity simultaneously by
using VO. Combined experimental and density-functional theory studies of
Eu0.5Ba0.5TiO3-{\delta}, we demonstrate that oxygen vacancies create Ti3+ 3d1
defect states, mediating the ferromagnetic coupling between the localized Eu
4f7 spins, and increase an off-center displacement of Ti ions, enhancing the
ferroelectric Curie temperature. The dual function of Ti sites also promises a
magnetoelectric coupling in the Eu0.5Ba0.5TiO3-{\delta}.Comment: Accepted by Physical Review B, 201
The Lantern Vol. 75, No. 2, Spring 2008
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Competing Interface and Bulk Effect-Driven Magnetoelectric Coupling in Vertically Aligned Nanocomposites.
Room-temperature magnetoelectric (ME) coupling is developed in artificial multilayers and nanocomposites composed of magnetostrictive and electrostrictive materials. While the coupling mechanisms and strengths in multilayers are widely studied, they are largely unexplored in vertically aligned nanocomposites (VANs), even though theory has predicted that VANs exhibit much larger ME coupling coefficients than multilayer structures. Here, strong transverse and longitudinal ME coupling in epitaxial BaTiO3:CoFe2O4 VANs measured by both optical second harmonic generation and piezoresponse force microscopy under magnetic fields is reported. Phase field simulations have shown that the ME coupling strength strongly depends on the vertical interfacial area which is ultimately controlled by pillar size. The ME coupling in VANs is determined by the competition between the vertical interface coupling effect and the bulk volume conservation effect. The revealed mechanisms shed light on the physical insights of vertical interface coupling in VANs in general, which can be applied to a variety of nanocomposites with different functionalities beyond the studied ME coupling effect.The work at Los Alamos National Laboratory was supported by the NNSA's Laboratory Directed Research and Development Program and was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is managed by Triad National Security, LLC for the U.S. Department of Energy's NNSA, under contract 89233218CNA000001. Angularādependent magnetization studies (L.C.) were partially supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences, and Engineering Division
Electric dipole moments, cluster metallicity, and the magnetism of rare earth clusters
One of the fundamental properties of bulk metals is the cancellation of electric
fields. The free charges inside of a metal will move until they find an arrangement where
the internal electric field is zero. This implies that the electric dipole moment of a metal
particle should be exactly zero, because an electric dipole moment requires a net separation
of charge and thus a nonzero internal electric field.
This thesis is an experimental study to see if this property continues to hold for tiny sub-
nanometer metal particles called clusters (2 - 200 atom, R < 1 nm). We have measured the
electric dipole moments of metal clusters made from 15 pure elements using a molecular
beam electric deflection technique. We find that the observed dipole moments vary a great
deal across the periodic table. Alkali metals have zero dipole moments, while transition
metals and lanthanides all have dipole moments which are highly size dependent. In most
cases, the measured dipole moments are independent of temperature (T = 20 - 50 K), and
when there is a strong temperature dependence this suggests that there is a new state of
matter present. Our interpretation of these results are that those clusters which have a non-
zero dipole moment are non-metallic, in the sense that their electrons must be localized
and prevented from moving to screen the internal field associated with a permanent dipole
moment.
This interpretation gives insight to several related phenomena and applications. We
briefly discuss an example cluster system RhN where the measured electric dipole moments
appear to be correlated with a the N2O reactivity.
Finally, we discuss a series of magnetic deflection experiments on lanthanide clusters
(Pr, Ho, Tb, and Tm). The magnetic response of these clusters is very complex and highly
sensitive to size and temperature. We find that PrN (which is non-magnetic in the bulk) becomes magnetic in clusters and TmN clusters have magnetic moments lower than the atomic value as well as the bulk saturation value implying that the magnetic order in the cluster involves non-collinear or antiferromagnetic order. HoN and TbN show very similar size dependent trends suggesting that these clusters have similar structures.Ph.D.Committee Chair: de Heer, Walter A.; Committee Member: Chou, Mei-Yin; Committee Member: El-Sayed, Mostafa; Committee Member: First, Phillip; Committee Member: Whetten, Rober