X-ray absorption measurements on nanoparticle systems: self-assembled arrays and dispersions (original) (raw)
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Lattice expansion in nonoxidized FePt nanoparticles: X-ray absorption measurements
Physical Review B, 2008
Wet-chemically synthesized FePt nanoparticles were structurally characterized not only in the as-prepared state but also after the removal of organic ligands and reduction in Fe oxides by a soft in situ hydrogen plasma treatment. By the analyses of the extended x-ray absorption fine structures at the Pt L 3 absorption edge, we found an enhanced lattice constant with respect to the bulk material for the oxide-free nanoparticles with clean surfaces. This shows that there exists a lattice expansion in FePt nanoparticles, which is an intrinsic property of the particles, and neither caused by Fe oxides at the surface nor by the organic ligands surrounding the nanoparticles in the as-prepared state. In addition, a first evidence of an inhomogeneous composition within the nanoparticles is given.
Correlation of magnetic moments and local structure of FePt nanoparticles
Journal of Physics: Conference Series, 2009
The influence of structural and compositional changes within FePt nanoparticles on their magnetic properties was studied by means of x-ray absorption spectroscopy in the near-edge regime and its associated magnetic circular dichroism as well as by analysis of the extended x-ray absorption fine structure. The magnetic moments at the Fe sites were found to be a sensitive monitor to changes of the local surrounding: While compositional inhomogeneities in the nanoparticles yield significantly reduced magnetic moments (by 20-30%) with respect to the corresponding bulk material, thermally induced changes in the crystal structure yields strongly enhanced orbital contributions (up to 9% of the spin magnetic moment). Also the break of crystal symmetry at the surface leads to an enhanced orbital magnetism which was confirmed by determination of the ratio of orbital-to-spin magnetic moment for FePt particles with different sizes between 3 and 6 nm in diameter.
X-ray studies of magnetic nanoparticle assemblies
Journal of Applied Physics, 2003
Monodisperse FePt nanoparticles were prepared using high temperature solution phase synthesis. Polymer-mediated layer-by-layer growth leads to precise control of the particle self-assembly. The narrow particle size distribution (σ ≤ 5%) offers the potential for increased data storage density by utilizing a smaller mean particle size and ultimately storage of one bit per individual nanoparticle. We have studied self-assembled multilayers of magnetic FePt nanoparticles. The L1 0 phase of FePt has a very high magnetic anisotropy which allows the magnetization of particles of about 4 nm diameter to be thermally stable at room temperature. Magnetic measurements using vibrating sample magnetometer were combined with X-ray diffraction (XRD) and Near Edge Xray Absorption Fine Structure (NEXAFS) Spectroscopy to study the annealed FePt nanoparticle assemblies and to optimize annealing conditions. NEXAFS spectra showed that a fraction of the iron in the as-deposited particles was oxidized, and this fraction was reduced by annealing in inert or reducing atmospheres. A very thin layer (<0.4 nm) of oxide surrounding the particle is sufficient to explain the observed spectra. Structural analysis using XRD showed that a minimum temperature of 450°C was required to start the formation of the ordered ferromagnetic phase. Annealing for longer times and at
Journal of Magnetism and Magnetic Materials, 2004
The superlattice structures and coalescence of self-assembled FePt nanoparticles were studied by X-ray diffraction and absorption spectroscopy techniques. The as-synthesized superlattice structure was found to be an FCC-like one with 2170.5 well-packed layers. During the annealing, the iron absorption K edge was found shifted to a higher energy as the annealing temperature increase. The shift of this iron K edge might be attributed to the quantum size effect, surface effect or oxidation effect.
Magnetic moment of Fe in oxide-free FePt nanoparticles
Physical Review B, 2007
We present results of x-ray absorption spectroscopy and x-ray magnetic circular dichroism investigations at the L 3,2 edges of Fe in FePt nanoparticles. Plasma treated, oxide-free FePt nanoparticles with a high degree of L1 0 ordering and a mean diameter of about 6 nm were produced by gas phase condensation followed by in situ flight annealing. The presence of the L1 0 phase is observed by high resolution transmission electron microscopy. Compared to chemically disordered A1 FePt particles of similar size and composition, we find a fourfold enhanced orbital magnetic moment L = 0.19 ͑±0.04͒ B , a nearly unchanged effective spin magnetic moment S eff = 2.21 ͑±0.24͒ B and a 2% increase in the white-line ratio of the L 3 and L 2 Fe edges. Nevertheless, the room temperature coercive field amounts to 0 H C =38͑±7͒ mT only. These results are discussed in relation to possible effects of chemical order and surface anisotropy.
New Journal of Physics, 2009
Recently, magnetite nanoparticles have attracted much attention, due to their technological potential based on different optic, magnetic and catalytic sections. In particular, the magnetic properties of hybrid nanocrystals can be tailored by the combination of complementary magnetic materials like for example magnetite and FePt. In order to analyse the related magnetic and structural properties of the resulting bi-component systems, we present x-ray absorption and x-ray magnetic circular dichroism studies at the Fe L 2,3 edges simultaneously performed in total electron yield and transmission mode, done at room and low temperatures. This provides in particular the separation of volume-and surface-related properties. The investigated system was made up of FePt/FeO x hybrid nanocrystals, which could be uniquely tuned in size and volume ratios. These measurements have been combined with magnetometry and high-resolution transmission electron microscopy studies. The separation between surface and bulk has been done by a deconvolution of the absorption spectra in terms of a linear superposition of reference spectra. With this universally applicable technique we are able to experimentally determine that 4 Author to whom any correspondence should be addressed.
Inhomogeneous alloying in FePt nanoparticles as a reason for reduced magnetic moments
Journal of Physics: Condensed Matter, 2009
The reduced magnetic moments of oxide-free FePt nanoparticles are discussed in terms of lattice expansion and local deviation from the averaged composition. By analyses of the extended x-ray absorption fine structure of FePt nanoparticles and bulk material measured both at the Fe K and Pt L 3 absorption edge, the composition within the single nanoparticles is found to be inhomogeneous, i.e. Pt is in a Pt-rich environment and, consequently, Fe is in an Fe-rich environment. The standard Fourier transformation-based analysis is complemented by a wavelet transformation method clearly visualizing the difference in the local composition. The dependence of the magnetic properties, i.e. the element-specific magnetic moments on the composition in chemically disordered Fe x Pt 1−x alloys, is studied by fully relativistic SPR-KKR band structure calculations supported by experimental results determined from the x-ray magnetic circular dichroism of 50 nm thick films and bulk material.
Thermal treatment effects in the self-assembly of FePt nanoparticle arrays
J. Magn. Magn. Mater. 320(21) pp. 2665-2671 (2008)
This work concentrates on the influence of synthetic mechanisms of FePt nanoparticles on their self-arrangement and some structural and magnetic properties as studied by means of different electron microscopy techniques and SQUID magnetometry. High reflux points associated with long boiling durations seem adequate to increase iron precursor's decomposition yield and facilitate the simultaneous cubic-to-tetragonal FePt transformation, in single-phase FePt nanoparticles. Nevertheless, such conditions also result in the loss of long-range arrangement and the appearance of coalescence effects. A core-shell structure comprising of FePt and Fe 3 O 4 is favoured under mild thermal conditions during synthesis, seems to confront the undesirable atomic diffusion. Additionally, particle isolation due to the surfactant coating leads in an hcp-symmetry self-assembly.
Microstructures and magnetic alignment of L1 FePt nanoparticles
Journal of applied …, 2007
Chemically ordered FePt nanoparticles were obtained by high temperature annealing a mixture of FePt particles with NaCl. After the NaCl was removed with de-ionized water, the transformed FePt nanoparticles were redispersed in cyclohexanone. X-ray diffraction patterns clearly show the L1 0 phase. Scherrer analysis indicates that the average particle size is about 8 nm, which is close to the transmission electron microscopy ͑TEM͒ statistical results. The coercivity ranges from 16 kOe to more than 34 kOe from room temperature down to 10 K. High resolution TEM images reveal that most of the FePt particles were fully transformed into the L1 0 phase, except for a small fraction of particles which were partially chemically ordered. Nano-energy dispersive spectroscopy measurements on the individual particles show that the composition of the fully transformed particles is close to 50/ 50, while the composition of the partially transformed particles is far from equiatomic. TEM images and electron diffraction patterns indicate c-axis alignment for a monolayer of L1 0 FePt particles formed by drying a dilute dispersion on copper grids under a magnetic field. For thick samples dried under a magnetic field, the degree of easy axis alignment is not as high as predicted due to strong interactions between particles.
Structural studies of L1[sub 0] FePt nanoparticles
Applied Physics Letters, 2002
We have studied the lattice parameter changes of L1 0 FePt nanoparticles annealed to near equilibrium as a function of composition by x-ray diffraction. We have found that the ͑111͒ diffraction peak shifts linearly with composition, however, the c parameter mostly changes in the Pt rich compositions and the a parameter mostly changes in the Fe rich compositions with respect to the equiatomic composition. This causes the tetragonality of the L1 0 structure to be maximized near the Fe 50%/Pt 50% composition. The magnetic properties were measured at room temperature and at 5 K and are correlated to the structural changes occurring as a function of composition.