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 ($\phi\approx13$ nm) embedded in self-organized alumina (Al$_{2}$O$_{3}$) 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 $\vec{M}(\vec{r})$ 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$_{12}$-acetate provides the most accurate spin Hamiltonian parameters for this prototype single molecule magnet so far. The Mn$_{12}$-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: $D=-0.0570(1)$ meV, $B_{4}^0=-2.78(7)\cdot 10^{-6}$ meV, $B_{4}^4=-3.2(6)\cdot 10^{-6}$ meV and $\mid$\textit{E}$\mid =6.8(15)\cdot 10^{-4}$ 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 $D_{\rm cluster}$ 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 $\rm Mn-Br$ bond is significantly softer and thus more compressible than the $\rm Mn-O$ bonds, pressure induces a tilt of the single ion Mn$^{3+}$ 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 $\sim100$ 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 systèmes hautement quantique