32 research outputs found
Numerical calculation of magnetic form factors of complex shape nano-particles coupled with micromagnetic simulations
We investigate the calculation of the magnetic form factors of nano-objects
with complex geometrical shapes and non homogeneous magnetization
distributions. We describe a numerical procedure which allows to calculate the
3D magnetic form factor of nano-objects from realistic magnetization
distributions obtained by micromagnetic calculations. This is illustrated in
the canonical cases of spheres, rods and platelets. This work is a first step
towards a 3D vectorial reconstruction of the magnetization at the nanometric
scale using neutron scattering techniques.Comment: 7 pages, 5 figures. To appear in Physics Procedi
Ordered arrays of magnetic nanowires investigated by polarized small-angle neutron scattering
Polarized small-angle neutron scattering (PSANS) experimental results
obtained on arrays of ferromagnetic Co nanowires ( nm) embedded
in self-organized alumina (AlO) porous matrices are reported. The
triangular array of aligned nanowires is investigated as a function of the
external magnetic field with a view to determine experimentally the real space
magnetization distribution inside the material during the
magnetic hysteresis cycle. The observation of field-dependentSANSintensities
allows us to characterize the influence of magnetostatic fields. The PSANS
experimental data are compared to magnetostatic simulations. These results
evidence that PSANS is a technique able to address real-space magnetization
distributions in nanostructured magnetic systems. We show that beyond
structural information (shape of the objects, two-dimensional organization)
already accessible with nonpolarized SANS, using polarized neutrons as the
incident beam provides information on the magnetic form factor and stray fields
\textgreek{m}0Hd distribution in between nanowires.Comment: 13 pages, 10 figures, submitted to Phys. Rev.
Transverse Magnetic Anisotropy in Mn12-acetate: Direct Determination by Inelastic Neutron Scattering
A high resolution inelastic neutron scattering (INS) study of fully
deuterated Mn-acetate provides the most accurate spin Hamiltonian
parameters for this prototype single molecule magnet so far. The
Mn-clusters deviate from axial symmetry, a non-zero rhombic term in the
model Hamiltonian leading to excellent agreement with observed positions and
intensities of the INS peaks. The following parameter set provides the best
agreement with the experimental data: meV,
meV, meV and
\textit{E} meV. Crystal dislocations are not
the likely cause of the symmetry lowering. Rather, this study lends strong
support to a recently proposed model, which is based on the presence of several
molecular isomers with distinct spin Hamiltonian parameters.Comment: 4 pages, 4 figure
Pressure Dependence of the Magnetic Anisotropy in the "Single-Molecule Magnet" [Mn4O3Br(OAc)3(dbm)3]
The anisotropy splitting in the ground state of the single-molecule magnet
[Mn4O3Br(OAc)3(dbm)3] is studied by inelastic neutron scattering as a function
of hydrostatic pressure. This allows a tuning of the anisotropy and thus the
energy barrier for slow magnetisation relaxation at low temperatures. The value
of the negative axial anisotropy parameter changes from
-0.0627(1) meV at ambient to -0.0603(3) meV at 12 kbar pressure, and in the
same pressure range the height of the energy barrier between up and down spins
is reduced from 1.260(5) meV to 1.213(9) meV. Since the bond is
significantly softer and thus more compressible than the bonds,
pressure induces a tilt of the single ion Mn anisotropy axes, resulting
in the net reduction of the axial cluster anisotropy.Comment: 4 pages, 3 figure
Exchange bias in Co/CoO core-shell nanowires: Role of the antiferromagnetic superparamagnetic fluctuations
The magnetic properties of Co (=15 nm, =130nm) nanowires are reported.
In oxidized wires, we measure large exchange bias fields of the order of 0.1 T
below T ~ 100 K. The onset of the exchange bias, between the ferromagnetic core
and the anti-ferromagnetic CoO shell, is accompanied by a coercivity drop of
0.2 T which leads to a minimum in coercivity at K. Magnetization
relaxation measurements show a temperature dependence of the magnetic viscosity
S which is consistent with a volume distribution of the CoO grains at the
surface. We propose that the superparamagnetic fluctuations of the
anti-ferromagnetic CoO shell play a key role in the flipping of the nanowire
magnetization and explain the coercivity drop. This is supported by
micromagnetic simulations. This behavior is specific to the geometry of a 1D
system which possesses a large shape anisotropy and was not previously observed
in 0D (spheres) or 2D (thin films) systems which have a high degree of symmetry
and low coercivities. This study underlines the importance of the AFM
super-paramagnetic fluctuations in the exchange bias mechanism.Comment: 10 pages, 10 figures, submitted to Phys. Rev.
Dipolar interactions in magnetic nanowires aggregates
We investigate the role of dipolar interactions on the magnetic properties of
nanowires aggregates. Micromagnetic simulations show that dipolar interactions
between wires are not detrimental to the high coercivity properties of magnetic
nanowires composites even in very dense aggregates. This is confirmed by
experimental magnetization measurements and Henkel plots which show that the
dipolar interactions are small. Indeed, we show that misalignment of the
nanowires in aggregates leads to a coercivity reduction of only 30%. Direct
dipolar interactions between nanowires, even as close as 2 nm, have small
effects (maximum coercivity reduction of ~15%) and are very sensitive to the
detailed geometrical arrangement of wires. These results strenghten the
potential of magnetic composite materials based on elongated single domain
particles for the fabrication of permanent magnetic materials.Comment: 7 pages, 8 figures, submitted to Journal of Applied Physic
Butterfly Hysteresis and Slow Relaxation of the Magnetization in (Et4N)3Fe2F9: Manifestations of a Single-Molecule Magnet
(Et4N)3Fe2F9 exhibits a butterfly--shaped hysteresis below 5 K when the
magnetic field is parallel to the threefold axis, in accordance with a very
slow magnetization relaxation in the timescale of minutes. This is attributed
to an energy barrier Delta=2.40 K resulting from the S=5 dimer ground state of
[Fe2F9]^{3-} and a negative axial anisotropy. The relaxation partly occurs via
thermally assisted quantum tunneling. These features of a single-molecule
magnet are observable at temperatures comparable to the barrier height, due to
an extremely inefficient energy exchange between the spin system and the
phonons. The butterfly shape of the hysteresis arises from a phonon avalanche
effect.Comment: 18 pages, 5 eps figures, latex (elsart
High temperature structural and magnetic properties of cobalt nanowires
We present in this paper the structural and magnetic properties of high
aspect ratio Co nanoparticles (~10) at high temperatures (up to 623 K) using in
situ X ray diffraction (XRD) and SQUID characterizations. We show that the
anisotropic shapes, the structural and texture properties are preserved up to
500 K. The coercivity can be modelled by u0Hc=2(Kmc+Kshape)/Ms with Kmc the
magnetocrystalline anisotropy constant, Kshape the shape anisotropy constant
and Ms the saturation magnetization. Hc decreases linearly when the temperature
is increased due to the loss of the Co magnetocrystalline anisotropy
contribution. At 500K, 50% of the room temperature coercivity is preserved
corresponding to the shape anisotropy contribution only. We show that the
coercivity drop is reversible in the range 300 - 500 K in good agreement with
the absence of particle alteration. Above 525 K, the magnetic properties are
irreversibly altered either by sintering or by oxidation.Comment: 8 pages, 7 figures, submitted to Journal of Solid State Chemistr
Contribution de la diffusion des neutrons à l'étude des aimants moléculaires
soutenue le 16 janvier 2009This "habilitation" manuscript deals with inelastic neutron scattering (INS) studies of magnetic molecular magnets and focuses on their magnetic properties at low temperature and low energies. Several molecular magnets (Mn12, V15, Ni12, Mn4, etc.) are reviewed. INS is shown to be a perfectly suited spectroscopic tool to (a) probe magnetic energy levels in such systems and (b) provide key information to understand the quantum tunnel effect of the magnetization in molecular spin clusters.Ce manuscrit retrace un certain nombre d'études des propriétés magnétiques d'aimants moléculaires (Mn12, V15, Ni12, Mn4, etc.) par diffusion inélastique des neutrons. L'apport spécifique des neutrons est mis en évidence notamment en ce qui concerne la spectroscopie des niveaux d'énergie magnétique et les implications concernant l'effet tunnel de l'aimantation dans ces systÚmes hautement quantique