Kazuhisa Sato

Fabrication and nanostructure of oriented FePt particles

Thin films of oriented tetragonal FePt particles separated by amorphous alumina have been fabricated by electron beam evaporation. The ordering of the FePt particles without coarsening can be tailored by annealing conditions. The value of coercivity of the annealed film reached as high as 4.4 kOe. The perpendicular magnetic coercivity of the annealed film was slightly larger than in-plane coercivity. Some of the tetragonal FePt particles were found to have {111} twins and stacking faults. From our high-resolution electron microscopy observations, it was determined that central region of the ordered FePt particles tended to have c-axis perpendicular to the film plane.

Ordering of island-like FePt crystallites with orientations

Thin films of oriented island-like crystallites of L10 FePt separated by amorphous (a-)Al2O3 were fabricated. The process took advantage of the overgrowth of α-Fe on Pt “seed” particles epitaxially grown on (100) NaCl and MgO substrates and the ordering reaction between Fe and Pt upon annealing at temperatures higher than 500 °C. The coercivities of the annealed (600 °C for 6 h) a-Al2O3/Fe(1 nm)/Pt (1.5 nm) films on both the NaCl (100) and MgO (100) substrates are higher than 3.3 kOe. Transmission electron microscopy observation showed that the film consisted of oriented L10 FePt island-like crystals with an average size of 12 nm and a separation of 4 nm. It was found that any one of the three 〈100〉 axes of the fcc Pt parent particles could act as the tetragonal c axis of the L10 FePt superstructure and all three-variant ordered crystalline domains of the tetragonal L10 phase could exist in the small 10-nm-size FePt crystallites. The high magnetic coercivity of the film, with well-separated FePt particles...

Structure and magnetic property changes of epitaxially grown L10-FePd isolated nanoparticles on annealing

Isolated 10-nm-sized FePd nanoparticles with the L10-type ordered structure have been fabricated by electron-beam evaporation and postannealing above 773 K, and the structural details have been investigated by transmission electron microscopy. FePd particles were epitaxially grown on a cleaved NaCl(001) substrate and were two-dimensionally dispersed on the substrate. In FePd particles formation, Pd nanoparticles were first deposited as “seed” particles epitaxially on NaCl followed by a successive deposition of Fe particles. All the Fe particles were captured by Pd particles forming Fe/Pd nanocomplex particles with a mutual fixed orientation. Coalescence and growth of the particles were not prominent during annealing, indicating that the alloying and atomic ordering reactions proceeded mostly within each nanoparticle. The negligible coalescence can be attributed to an “anchoring effect” of the seed Pd to the coalescence growth. Moreover, both of these reactions are thought to proceed almost simultaneously ...

Fabrication of oriented L10-FePt and FePd nanoparticles with large coercivity

Oriented L10-FePt and FePd nanoparticles with coercivities as large as 5 kOe (FePt) and 3 kOe (FePd) were fabricated by alternating deposition of Pt(or Pd) and Fe nanoparticles and their annealing at the temperatures between 773 and 873 K on single crystal NaCl and MgO. Atomic ordering reactions and degrees of order towards the L10-structure formation in these nanoparticles were investigated by high resolution electron microscopy and electron diffraction, and magnetic coercivities at low temperature were also measured. The long range order parameter of the FePt nanoparticles was ∼0.56 even after annealing at 873 K for 24 h. The coercivity of the FePt nanoparticles at 100 K was as high as twice the room temperature value. The low degree of order in the L10-structure formation and the thermal effect on magnetization have been found in the present FePt and FePd nanoparticles. These are closely concerned with the origin of the coercivity values which are very small in comparison with those expected from the s...

Long-range order parameter of single L10-FePd nanoparticle determined by nanobeam electron diffraction : Particle size dependence of the order parameter

The long-range order (LRO) parameter (S) of single isolated L10-FePd nanoparticle was determined by quantitative analysis of nanobeam electron diffraction (NBD) intensities and intensity calculations considering the multiple scattering of electrons. The obtained order parameters of the nanoparticles larger than 8nm are distributed around the mean LRO parameter (S¯=0.79) which was determined by selected area electron diffraction intensity analysis, while the parameters slightly decreased gradually as the particle size decreased below about 8nm (S=0.60–0.73). The low degree of order in very small particles is responsible for the coercivity decrease of the L10 nanoparticles in smaller-sized regions. Quantitative NBD intensity analysis is quite useful for the determination of the LRO parameter of individual L10-FePd single crystalline nanoparticle. Experimental conditions required for NBD analysis are presented in detail and the possible experimental errors of the determined LRO parameters are discussed.

Transmission electron microscopy study on FeSi2 nanoparticles synthesized by electron-beam evaporation

We have synthesized epitaxially grown iron disilicide (FeSi2) nanoparticles using an electron-beam evaporation technique and characterized them by transmission electron microscopy (TEM). An Fe film was deposited on a Si(100) substrate, followed by thermal annealing at 1073K for 2h. It was found that epitaxially grown nanoparticles with an average size of ∼10nm were formed just beneath the Si surface, suggesting that the deposited Fe atoms diffuse into the substrate. Every single phase of nanoparticles was examined in detail by TEM observation, nanobeam electron diffraction, and energy-dispersive x-ray spectroscopy. Plan-view and cross-sectional TEM observations revealed that these nanoparticles consist of α-, β-, and γ-FeSi2. It was found that the morphology of nanoparticles is closely related to the phases. The α and β phases consist of angled hemisphere and asymmetric triangle-shaped nanoparticles, respectively, while the γ phase consists of hemispherical or columnar-shaped nanoparticles. These particle...

Artificially produced rare-earth free cosmic magnet

Chemically ordered hard magnetic L10-FeNi phase of higher grade than cosmic meteorites is produced artificially. Present alloy design shortens the formation time from hundreds of millions of years for natural meteorites to less than 300 hours. Electron diffraction detects four-fold 110 superlattice reflections and a high chemical order parameter (S  0.8) for the developed L10-FeNi phase. The magnetic field of more than 3.5 kOe is required for the switching of magnetization. Experimental results along with computer simulation suggest that the ordered phase is formed due to three factors related to the amorphous state: high diffusion rates of the constituent elements at lower temperatures when crystallizing, a large driving force for precipitation of the L10 phase, and the possible presence of L10 clusters. Present results can resolve mineral exhaustion issues in the development of next-generation hard magnetic materials because the alloys are free from rare-earth elements, and the technique is well suited for mass production.

Determination of order parameter of L10-FePd nanoparticles by electron diffraction

Long-range order (LRO) parameters of two-dimensional dispersed single-crystalline 10-nm-sized FePd nanoparticles with the L10 structure have been determined accurately by electron diffraction in transmission electron microscopes (TEMs) under accelerating voltages of 300kV and 1kV. Diffraction patterns by exciting hh0 systematic reflections effectively reduced the numbers of diffracted beams and simplified the thickness dependence of intensity ratio I110∕I220 for 110 and 220 reflections. Mean thickness of the nanoparticles was estimated to be 7.8nm by electron holography. The relation between the intensity ratio and the order parameter was calculated on the basis of multiple-scattering intensity calculation. By comparing the relation and experimentally obtained intensity ratios, the order parameters of 0.65 and 0.79 were obtained using 300-kV TEM for FePd nanoparticles after annealing at 873K for 3.6 and 36ks, respectively. Also, the order parameter of 0.82 was obtained using 1-MV TEM for the same specimen...

Fabrication of oriented L10-FeCuPd and composite bcc-Fe∕L10-FeCuPd nanoparticles: Alloy composition dependence of magnetic properties

Oriented and well-isolated L10-FeCuPd ternary alloy nanoparticles have been fabricated by electron-beam evaporation followed by postdeposition annealing. A single L10 phase was formed in the FeCuPd nanoparticles with (Fe+Cu) content lower than 48 at. %. A strong preferential c-axis orientation along the film normal direction was achieved by Cu addition, which leads to a strong perpendicular magnetic anisotropy. Also, a lowering of the ordering temperature by 50 K compared to the binary L10-FePd nanoparticles was achieved by Cu addition. By contrast, composite particles composed of the bcc Fe and the L10-FeCuPd were formed when the (Fe+Cu) content was higher than 52 at. %. Coexistence of the bcc Fe and the L10-FeCuPd was confirmed by high-resolution transmission electron microscopy and nanobeam electron diffraction. It was found that perpendicular magnetic anisotropy of the L10-FeCuPd nanoparticles on the NaCl substrate is sensitive to the alloy composition.

Fabrication of exchange-coupled α-Fe∕L10-FePd nanocomposite isolated particles

Oriented nanocomposite particles of α-Fe and ordered L10-FePd have been fabricated by electron-beam deposition and successive postdeposition annealing at temperatures higher than 773K. High-resolution transmission electron microscopy, nanobeam electron diffraction, and nanobeam elemental analysis indicated that two regions composed of α-Fe and L10-FePd coexisted in each nanoparticle separated by a diffuse boundary. Magnetic hysteresis measurements using a superconducting quantum interference device magnetometer showed a gradual increase in the saturation magnetization with increasing volume ratio of the α-Fe region, whereas the coercivity decreased with the volume of α-Fe. There were no obvious steps in each magnetization curve. These results indicated the existence of an exchange coupling between the α-Fe and L10-FePd crystallites connected coherently with each other within the nanoparticles. The effects of the α-Fe region size on hard magnetic properties are also discussed.