Magnetic moment of Fe in oxide-free FePt nanoparticles (original) (raw)
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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.
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.
Physical Review B, 2004
S eff averaged over the element-specific contributions of Fe and Pt has been measured for 3-nm Fe x Pt 1Ϫx nanoparticles at room temperature using the multifrequency electron paramagnetic resonance method for different concentrations of Fe. From a detailed g-factor analysis we determine that the ratio decreases from L eff / S eff ϭ0.049 for xϭ0.43 to L eff / S eff ϭ0.016 for xϭ0.70 which is much smaller than the bulk iron value (L Fe / S Fe ϭ0.045). The observed concentration dependence is much stronger than the one calculated for Fe x Pt 1Ϫx bulk samples and reveals likely changes of the confined electronic structure of the nanoparticle system. The ratio L eff / S eff takes the lowest value at the concentration (x ϭ0.70) where the magnetic anisotropy energy vanishes in bulk alloys. For xϾ0.72 a phase transition from a fcc to the Fe bcc structure occurs resulting in the increased bulk ratio again.
MAGNETISM AT THE NANOSCALE: THE CASE OF FePt
Modern Physics Letters B, 2007
FexPt 1−x nanoparticles prepared by organometallic synthesis and gas-phase condensation were structurally and magnetically characterised. The effective spin magnetic moments at both the Fe and Pt sites are reduced with respect to the moments in the corresponding bulk material by up to 20% and further decrease with decreasing particle size at the Fe sites. The ratio of orbital-to-effective-spin magnetic moment µ l /µ eff S at the Fe sites increases from 2.1% for 6 nm particles to 3.4% for 3.4 nm particles due to the break of symmetry at the surface. A lowering of the crystal symmetry after the transformation to the chemically ordered L1 0 state yields a µ l /µ eff S ≈ 9% and is accompanied by an enhancement of the coercive field at 15 K from (36 ± 5) mT to (292 ± 8) mT indicating an increase of the anisotropy.
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.
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.
Order–disorder transformation in FePt nanoparticles studied by ferromagnetic resonance
Applied Surface Science, 2007
We have investigated the ferromagnetic resonance (FMR) response of as-made and temperature annealed FePt magnetic nanoparticles. The asmade nanoparticles, which have been fabricated by a chemical route, crystallize in the low magnetic anisotropy fcc phase and have a diameter in the range of 2-4 nm. The annealing of the particles at high temperatures (T A ¼ 550, 650 and 750 C) in an inert Ar atmosphere produces a partial transformation to the high magnetocrystalline anisotropy L1 0 phase, with a significant increase in particle size and size distribution. FMR measurements at X-band (9.5 GHz) and Q-band (34 GHz) show a single relatively narrow line for the as-synthesized particles and a structure of two superimposed lines for the three annealed samples. The origin of this line shape has been attributed to the presence of the disordered fcc phase. Assuming that the system consists of a collection of identical particles with a random distribution of easy axes, we have been able to estimate a mean value for the magnetic anisotropy constant of the particles in the fcc phase, K $ 2 Â 10 6 erg/cm 3 . The measured line shape in the annealed samples can be explained if we consider that the magnetic anisotropy of the particles has a gaussian distribution with a relatively broad width. # 2007 Published by Elsevier B.V.
Magnetic Properties of FePt Nanoparticles Annealed With NaCl
IEEE Transactions on Magnetics, 2006
FePt nanoparticles have the potential to extend the data density of magnetic recording beyond 1 Tbit/in 2. The phase transformation from low anisotropy, face-centered cubic to face-centered tetragonal with high anisotropy is essential and complex. The samples measured in this paper were annealed in a mixture with fine ground (20 m) NaCl. The switching field distributions and activation volumes of two different sizes of particles are presented, 4 and 15 nm. The samples exhibit complex structural behaviour, demonstrating how difficult it is to obtain uniform nanoparticles. We find that the 4-nm particles reverse coherently whereas the 15 nm do not due to inhomogeneous crystallinity.
Phase Transformation and Magnetic Hardening in Isolated FePt Nanoparticles
Nanotechnology, …, 2009
Isolated monodisperse L1 0 FePt nanoparticles coated by carbon were obtained by adding enough surfactants that decomposed into carbon after the chemical synthesis and postannealing of the A1 FePt nanoparticles. The effect of isolation between FePt nanoparticles on the phase transition temperature and magnetic properties has been studied systematically by thermal, magnetic, and structural characterizations and analyses. It was found that the A1 to L1 0 phase transition temperature is dependent sensitively on the amount of isolation medium. The transition temperature shift reaches 150-200 • C from nonisolated particle assemblies to completely isolated particles, which may be attributed to the high activation energy of the phase transformation for the isolated particles.
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.