173 research outputs found
Proximity effecs and curie temperature enhancement in Co/EuS and Fe/EuS multilayers
Two identical Co/EuS and Fe/EuS multilayers of six periods each and with
individual layers of about 4 nm thick are grown by e-beam evaporation under
ultrahigh vacuum conditions. The films show polycrystalline structure with a
grain size limited by the individual layer thickness. Both multilayers consist
of almost continuous layers with some roughness. The surface peak-to-peak
roughness is about 4â5 nm. Magnetization measurements and calculations of the
loops based on a StonerâWohlfarth-like model allow us to determine the direct
antiferromagnetic exchange coupling constant between the 3d metal and EuS at 5
K. Both samples show strong enhancement of the Curie temperature of EuS up to
at least 50 K with a EuS magnetization tail, which persists up to about 100 K.
The J = 7/2 character of the EuS layers is shown to be responsible for the
large Curie temperature enhancement
Direct evidence for significant spin-polarization of EuS in Co/EuS multilayers at room temperature
The new era of spintronics promises the development of nanodevices, where the
electron spin will be used to store information and charge currents will be
replaced by spin currents. For this, ferromagnetic semiconductors at room
temperature are needed. We report on significant room-temperature spin
polarization of EuS in Co/EuS multilayers recorded by x-ray magnetic circular
dichroism (XMCD). The films were found to contain a mixture of divalent and
trivalent europium, but only Eu11 is responsible for the ferromagnetic
behavior of EuS. The magnetic XMCD signal of Eu at room temperature could
unambiguously be assigned to magnetic ordering of EuS and was found to be only
one order of magnitude smaller than that at 2.5 K. The room temperature
magnetic moment of EuS is as large as the one of bulk ferromagnetic Ni. Our
findings pave the path for fabrication of roomâtemperature spintronic devices
using spin polarized EuS layers
Paramagnetic gold in a highly disordered Au-Ni-O alloy
Magnetic materials are usually classified into a distinct category such as diamagnets, paramagnets or ferromagnets. The enormous progress in materials science allows one nowadays, however, to change the magnetic nature of an element in a material. Gold, in bulk form, is traditionally a diamagnet. But in a ferromagnetic environment, it can adopt an induced ferromagnetic moment. Moreover, the growth of gold under certain conditions may lead to a spontaneous ferromagnetic or paramagnetic response. Here, we report on paramagnetic gold in a highly disordered AuâNiâO alloy and focus on the unusual magnetic response. Such materials are mainly considered for plasmonic applications. Thin films containing Au, Ni and NiO are fabricated by co-deposition of Ni and Au in a medium vacuum of 2âĂâ10â2 mbar. As a result, Au is in a fully disordered state forming in some cases isolated nanocrystallites of up to 4ânm in diameter as revealed by high resolution transmission electron microscopy. The disorder and the environment, which is rich in oxygen, lead to remarkable magnetic properties of Au: an induced ferromagnetic and a paramagnetic state. This can be proven by measuring the x-ray magnetic circular dichroism. Our experiments show a way to establish and monitor Au paramagnetism in alloys
Layering and temperature-dependent magnetization and anisotropy of naturally produced Ni/NiO multilayers
Ni/NiO multilayers were grown by magnetron sputtering at room temperature,
with the aid of the natural oxidation procedure. That is, at the end of the
deposition of each single Ni layer, air is let to flow into the vacuum chamber
through a leak valve. Then, a very thin NiO layer (~1.2nm) is formed.
Simulated x-ray reflectivity patterns reveal that layering is excellent for
individual Ni-layer thickness larger than 2.5nm, which is attributed to the
intercalation of amorphous NiO between the polycrystalline Ni layers. The
magnetization of the films, measured at temperatures 5â300K, has almost bulk-
like value, whereas the films exhibit a trend to perpendicular magnetic
anisotropy (PMA) with an unusual significant positive interface anisotropy
contribution, which presents a weak temperature dependence. The power-law
behavior of the multilayers indicates a non-negligible contribution of higher
order anisotropies in the uniaxial anisotropy. Bloch-law fittings for the
temperature dependence of the magnetization in the spin-wave regime show that
the magnetization in the multilayers decreases faster as a function of
temperature than the one of bulk Ni. Finally, when the individual Ni-layer
thickness decreases below 2nm, the multilayer stacking vanishes, resulting in
a dramatic decrease of the interface magnetic anisotropy and consequently in a
decrease of the perpendicular magnetic anisotropy
'Theory for the enhanced induced magnetization in coupled magnetic trilayers in the presence of spin fluctuations'
Motivated by recent experiments, the effect of the interlayer exchange
interaction on the magnetic properties of coupled Co/Cu/Ni
trilayers is studied theoretically. Here the Ni film has a lower Curie
temperature than the Co film in case of decoupled layers. We
show that by taking into account magnetic fluctuations the interlayer coupling
induces a strong magnetization for T\gtsim T_{C,\rm Ni} in the Ni film. For
an increasing the resonance-like peak of the longitudinal Ni
susceptibility is shifted to larger temperatures, whereas its maximum value
decreases strongly. A decreasing Ni film thickness enhances the induced Ni
magnetization for T\gtsim T_{C,\rm Ni}. The measurements cannot be explained
properly by a mean field estimate, which yields a ten times smaller effect.
Thus, the observed magnetic properties indicate the strong effect of 2D
magnetic fluctuations in these layered magnetic systems. The calculations are
performed with the help of a Heisenberg Hamiltonian and a Green's function
approach.Comment: 4 pages, 3 figure
Dipolar interaction between two-dimensional magnetic particles
We determine the effective dipolar interaction between single domain
two-dimensional ferromagnetic particles (islands or dots), taking into account
their finite size. The first correction term decays as 1/D^5, where D is the
distance between particles. If the particles are arranged in a regular
two-dimensional array and are magnetized in plane, we show that the correction
term reinforces the antiferromagnetic character of the ground state in a square
lattice, and the ferromagnetic one in a triangular lattice. We also determine
the dipolar spin-wave spectrum and evaluate how the Curie temperature of an
ensemble of magnetic particles scales with the parameters defining the particle
array: height and size of each particle, and interparticle distance. Our
results show that dipolar coupling between particles might induce ferromagnetic
long range order at experimentally relevant temperatures. However, depending on
the size of the particles, such a collective phenomenon may be disguised by
superparamagnetism.Comment: 11 pages, 5 figure
On the temperature dependence of multiple- and single-scattering contributions in magnetic EXAFS
We demonstrate that the temperature dependence of structural as well as magnetic fluctuations can be probed by the use of the Magnetic Extended X-ray Absorption Fine Structure (MEXAFS) spectroscopy. We compare those to the dynamic disorder as probed by the EXAFS. Here we present temperature-dependent MEXAFS investigations carried out at the L-edges of a thin Fe film and a Gd single crystal. By comparing the experimental results to ab initio calculations the single-scattering contributions are separated from multiple-scattering contributions. It is found that the multiple-scattering contributions are enhanced for the MEXAFS compared to the normal EXAFS
Ruthenocuprates RuSr2(Eu,Ce)2Cu2O10: Intrinsic magnetic multilayers
We report ac susceptibility data on RuSr_2(Eu,Ce)_2Cu_2O_(10-y) (Ru-1222, Ce
content x=0.5 and 1.0), RuSr_2GdCu_2O_8 (Ru-1212) and SrRuO_3. Both Ru-1222
(x=0.5, 1.0) sample types exhibit unexpected magnetic dynamics in low magnetic
fields: logarithmic time relaxation, switching behavior, and `inverted'
hysteresis loops. Neither Ru-1212 nor SrRuO_3 exhibit such magnetic dynamics.
The results are interpreted as evidence of the complex magnetic order in
Ru-1222. We propose a specific multilayer model to explain the data, and note
that superconductivity in the ruthenocuprate is compatible with both the
presence and absence of the magnetic dynamics.Comment: 9 pages, 11 figures, Revtex; submitted to Phys.Rev.
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