Fcc FePt Research Articles

Evolution of structural and magnetic properties of FePt/C granular films with thermal annealing

We report the structural and magnetic properties of as-deposited and thermally annealed FePt/C granular multilayer films. The as-deposited system exhibits a disordered fcc FePt phase with an average grain size of 3 nm. Thermal annealing at 650 °C results in partial L 1 0 ordering and an associated grain growth to 7 nm. Mössbauer measurements show that there is no non-magnetic component present, suggesting that carbon resides only in the grain boundary region. The ferromagnetic grains are magnetically decoupled.

Synthesis and characterization of FePt nanoparticles and FePt nanoparticle/SiO2-matrix composite films

Superparamagnetic face-centered cubic (fcc) FePt nanoparticles were synthesized using a polyol process. The effect of reaction temperature and molar ratio of Fe(CO)5 to Pt(acac)2 on the structure, composition and morphology of nanoparticles has been investigated. The optimum processing condition has been obtained for producing well-monodisperse fcc-phase FePt nanoparticles with the 2:1 molar ratio of Fe-Pt at 220 °C. In order to circumvent the problem of FePt particle coalescence during high temperature annealing for the L10 ordering, FePt nanoparticle/SiO2-matrix composite films have been fabricated by sol–gel method. The experimental results confirm that the amorphous SiO2 matrix effectively inhibits the grain growth and particle aggregation during 700 °C annealing for 1 h. Well-monodisperse face-centered tetragonal (fct) FePt particles embedded in the SiO2 matrix can be obtained with the long-range chemical order parameter S of ~0.74, indicating partially ordered L10 phase transition in FePt/SiO2 composite films. The FePt/SiO2 system exhibits a hysteretic behavior with smaller coercive field of 1,450 Oe. The incomplete phase transition from cubic deredat height maxsium (A1-disordered phase to tetragonal L10-ordered phase) might be responsible for it.

Microstructures and Magnetism of Different Oxides Separating FePt Grains via Ion-Beam Bombardment and Annealing

The effects of the fabrication methods and different capped oxide (SiO2 and TiO2) layers on the microstructure and magnetism of FePt thin films were studied. Both structural ordering (S ∼0.7) from the fcc FePt phase to the fct FePt phase and magnetic hardening were observed in the annealed FePt/SiO2 thin films with a low substrate rotation speed (Sr = 1 rpm). However, only the annealed FePt/SiO2 thin films prepared with a high Sr (10 rpm) exhibited isolated FePt grains separated by the grain boundary SiO2, as revealed by transmission electron microscopy and magnetometry. Furthermore, similar results in microstructures and magnetic properties were obtained after replacing the capped layer with TiO2. However, an enhanced order parameter (S ∼0.85) and a smaller FePt grain size (∼6.8 nm), which are promising characteristics for ultrahigh-density magnetic recording, were achieved in the annealed FePt/TiO2 thin films; however, the annealed FePt/SiO2 thin films exhibited a larger grain size (∼15 nm). This indicates that TiO2 inhibits the grain growth of FePt more effectively than SiO2.

The Effect of Cobalt Addition on Structural and Magnetic Properties of Electrodeposited Iron-Platinum Nanocrystalline Thin Films

Iron-Platinum (Fe-Pt) and Cobalt-Iron-Platinum (Co-Fe-Pt) nanocrystalline thin films were deposited on brass substrates using a single bath electrodeposition system. The effects of addition of various concentrations of cobalt towards the composition, crystallographic structure, microstructure and magnetic properties of the Fe-Pt alloy films were investigated. Ammonium tartrate and ammonia solution were used as complex forming additives to stabilize the Fe2+ and Co2+ ions and to enhance co-deposition with platinum complexes. The as-synthesized thin films were characterized by energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and alternating gradient magnetometry (AGM). EDS results showed that cobalt was preferentially deposited compared to iron during Co-Fe-Pt film electrodeposition. SEM results showed that the microstructure of the films consisted of spherical granules. XRD showed formation of disordered face-centered cubic (fcc) Fe-Pt phase and all peaks were shifted to slightly higher diffraction angles with increasing cobalt content. The average crystallite size calculated from XRD peak broadening varied from 2.33nm to 6.54nm. The saturation magnetization and coercivity increased with increasing cobalt content.

Structural transformations and magnetic properties of Fe60Pt15B25 and Fe60Pt25B15 nanocomposite alloys

Abstract Structural and magnetic properties of two rapidly solidified and post-annealed Fe60Pt15B25 and Fe60Pt25B15 alloys are compared. The as-quenched Fe60Pt15B25 ribbon was fully amorphous whereas in the Fe60Pt25B15 alloy the amorphous phase coexists with an fcc FePt disordered solid solution. Differential scanning calorimetry curves of both alloys reveal a single exothermal peak with onset temperatures of 873 and 847 K for Fe60Pt15B25 and Fe60Pt25B15, respectively. Magnetically hard, tetragonal ordered L10 FePt and magnetically soft Fe2B nanocrystalline phases were formed due to the annealing of the alloys, as indicated by X-ray diffraction and Mossbauer spectroscopy measurements. Two-phase behavior was detected in the temperature dependence of magnetization of the annealed samples. A magnetic hardening was observed for all annealed ribbons. Magnetic properties of the annealed alloys, studied by hysteresis loop measurements, were related to the differences in the relative fractions of the hard and soft magnetic phases calculated from Mossbauer spectra. The alloy with 25 at% Pt exhibits better hard magnetic properties (Hc=437 kA/m, Mr/Ms=0.74) than the alloy with smaller Pt content (Hc=270 kA/m, Mr/Ms=0.73) mainly due to the larger abundance of the ordered tetragonal FePt phase.

Mechanically and thermally induced structural transformations in Fe-Pt-B alloys

Mechanically and thermally induced structural transformations in the Fe56Pt24B20 and Fe59.5Pt25.5B15 alloys are compared. Both alloys were prepared as ribbons by the melt spinning and were subsequently exposed to high-energy ball milling. Thermal behaviour of the samples was studied by differential scanning calorimetry measurements. The structure of the samples was characterised by Mössbauer spectroscopy and X-ray diffraction. The as-quenched alloys have shown disordered structures with a major contribution of the disordered cubic FePt solid solution. Ball milling of the as-quenched alloys induced a separation of the Fe-Pt and Fe-B regions. The heating of the final powders up to 993 K led to a transformation of the fcc FePt solid solution to the ordered tetragonal FePt phase, and to a formation of the Fe2B phase.

(0 0 1) Exchange-coupled FCC/L1 0 FePt bilayers

Exchange coupling between hard and soft magnetic materials has implications for both permanent magnet and magnetic recording technologies. This paper looks at exchange coupling of FCC FePt epitaxially grown onto (0 0 1) oriented L1 0 FePt deposited on (0 0 1) MgO substrates at elevated temperature. By varying the thickness of the FCC layer there is a relaxation of the single crystal FCC layer that produces a polycrystalline microstructure as evidenced by the development of rings in the electron diffraction pattern. A concurrent decoupling of the layers is apparent from magnetization curves with two distinct switching fields as the FCC layer thickness is increased above 20 nm. The results shown here confirm the importance of the epitaxial relationship between materials of disparate anisotropies in maintaining strong exchange coupling.

Direct Synthesis of Water-Dispersible FePt Nanoparticles Capped with L-Cysteine

A direct synthetic route of FePt nanoparticles (NPs) has been developed. Reduction of platinum acethylacetonate (Pt(acac)2) and thermal decomposition of iron carbonyl (Fe(CO)5) in the presence of L-cysteine produced fcc FePt NPs with a mean diameter of 2.9 nm. Capping of the FePt NPs with L-cysteine is accomplished by a thiolate linkage between the amino acid and the NP surfaces, which renders them water-dispersible rather than in organic solvents. SQUID magnetization study demonstrated that the FePt NPs exhibited superparamagnetic behavior at 300 K. For the structural model of the FePt NPs, the Fe K-edge and Pt L(III)-edge XAFS results suggest that most of the Pt atoms are preferentially located in the core region, the Fe atoms are preferentially located in the shell region.

FePt nanoparticles as an Fe reservoir for controlled Fe release and tumor inhibition.

Chemically disordered face centered cubic (fcc) FePt nanoparticles (NPs) show the controlled release of Fe in low pH solution. The released Fe catalyzes H(2)O(2) decomposition into reactive oxygen species within cells, causing fast oxidation and deterioration of cellular membranes. Functionalized with luteinizing hormone-releasing hormone (LHRH) peptide via phospholipid, the fcc-FePt NPs can bind preferentially to the human ovarian cancer cell line (A2780) that overexpresses LHRH receptors and exhibit high toxicity to these tumor cells. In contrast, the fcc-FePt NPs pre-etched in the low pH (4.8) buffer solution show nonappreciable cytotoxicity. The work demonstrates that fcc-FePt NPs may function as a new type of agent for controlled cancer therapy.

Increase in the Curie temperature and magnetic anisotropy in FePd/Pt–iron oxide core-shell nanoparticles

Partially oxidized fcc FePd and FePt nanoparticles with mean diameters of 5 and 3 nm, respectively, were synthesized by a reverse micelle polyol process. In situ measurements of magnetic and structural properties during annealing showed a large increase in the magnetocrystalline anisotropy and in the Curie temperature of the nanoparticles due to (i) a phase transition from A1 to L10 and (ii) a simultaneous phase separation between a metallic core and an iron oxide shell. These occurred at 675 K in the FePd nanoparticles and at above 850 K for the FePt. The Curie temperature of the nanoparticles was found to be about 850 K, an increase of more than 100 K from the bulk L10 phase. The ferromagnetic resonance results are discussed and compared with a phenomenological model that makes it possible to estimate the magnetocrystalline anisotropy as 1.6×105 and 1.5×106 J m−3 in FePd and FePt, respectively. Exchange coupling between the core and the shell explains both the high magnetocrystalline anisotropy of the core and the high Curie temperature of the shell.

Preparation of ultrafine FePt nanoparticles by chemical reduction in PAMAM-OH template

Using dendrimer templates, FePt nanoparticles with narrow size-distribution were prepared by a simple chemical reduction method at room temperature. In this approach, PAMAM-OH dendrimers act as templates in which metal ions are coordinated in, and NaBH 4 acts as reductant. The FePt nanoparticles were synthesized in aqueous solution. The as-made FePt nanoparticles are spherical with an average size of ∼3 nm and have the chemically disordered face-center-cubic (fcc) structure. The FePt nanoparticles can be transformed to the face-center-tetragonal (fct) phase after annealing at 700 °C for 1 h. The as-prepared fcc FePt nanoparticles are found to be superparamagnetic at room temperature. After annealing in a reducing atmosphere, the FePt nanoparticles transform to ferromagnetic and show high covercivity. The structural and magnetic properties of the as-made and annealed FePt nanoparticles were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and alternating gradient magnetometry (AGM).

STUDY THE POLYOL PROCESS OF PREPARING THE Ru DOPED FePt NANOPARTICLES

The structure of Ru doped FePt nanoparticles using polyol process was studied. The particle size grown is around 5 nm, and a shell structure might be formed. By selecting the time and temperature of adding the Ru precursors into solution, three different processes to synthesize the FePtRu particles were studied resulting in different growing mechanics. The possible models during the reaction process are also discussed. The phase transition temperature for the as-grown FCC FePt nanoparticle to transform into L 10 FePt nanoparticle is about 823 K which is about the same as the one without doping Ru atoms. From the XAS study of each element, the possible scenario is that: although Ru atoms with the size close to the Pt , they do not totally replace the Pt sites in the FePt alloy. Instead, most of Ru formed a shell outside the FePt nanoparticles and Fe atoms are replaced.

Ion-Beam Bombarded SiO2 Layer Effects on the Microstructure and Magnetism in FePt/SiO2 Bilayers

We have shown that the structural and magnetic properties of FePt thin films were affected strongly by capped SiO2 layers prepared by ion-beam bombardment followed by post-annealing. Compared to the single fcc FePt phase in the as-deposited FePt/SiO2 bilayer (0% O2/Ar), annealing at 550 °C produced an ordered L 10 FePt phase with enhanced coercivity (∼14 kOe). Increasing the %O2/Ar during deposition of the top SiO2 layer resulted in smaller ordered FePt grains separated by grain boundaries of SiO2. We find that the (001) diffraction peak is broadened considerably with larger SiO2 deposition %O2/Ar and annealing, likely due to the induced strain. Our results indicate that FePt/SiO2 films deposited with lower %O2/Ar, the oxygen atoms created by the ion-beam bombardment act effectively to inhibit the FePt grain growth, whereas the excess oxygen atoms present during film deposition with higher %O2/Ar may induce a local strain on the FePt crystallites by occupying the interstitial sites in the FePt lattice.

Fabrication of core-shell Ti-containing silica encapsulating FePt particles as a spherical nanocatalyst

The encapsulation of fcc FePt nanoparticles (NPs) having a mean diameter of ca. 6 nm with the Ti-SiO2 thick shell gives a new nanocomposite (FePt@Ti-SiO2), which serves as an efficient nanocatalyst for the liquid-phase selective epoxidation. Recovery from the reaction mixture was readily attained by applying an external permanent magnet, and the spent catalyst could be recycled without any appreciable loss in activity.

L1<sub>0</sub> FePt-Filled Carbon Nanotubes by Wet Chemical Synthesis

This paper reports on the synthesis of iron-platinum (FePt) alloy clusters within Multi Wall Carbon Nanotubes using a novel wet chemical method that allows the filling of the nanotube cavity keeping clear their external wall. In the present case, the procedure permits the filling of the nanotubes with precursor salts of Pt and Fe and, by direct heating at 700°C of the Fe and Pt mixed salts, magnetic hard, face-centred-tetragonal (fct) crystallographic (L10) phase FePt alloy nanoparticles are obtained without the need to pass through the formation of the magnetic soft, face-centred-cubic (fcc) FePt alloy phase. By a suitable choice of the concentration of the starting material solution, we obtained particles of two different sizes into the nanotubes with magnetic blocked and super-paramagnetic states at room temperature. Morphological, structural and magnetic properties of the filled nanotubes have been performed by magnetic measurements, X-ray diffraction and transmission electron microscopy in normal and high resolution mode

Granular L10 FePt–X (X=C, TiO2, Ta2O5) (001) nanocomposite films with small grain size for high density magnetic recording

Fe Pt – X (X=C, TiO2, Ta2O5) nanocomposite films were deposited on MgO∕CrRu∕glass substrates at 350°C by magnetron cosputtering. The comparison investigations on the magnetic properties and microstructure of FePt–X films with various dopants were conducted. All FePt–X films showed (001) preferred orientation and oxide dopants promoted the formation of magnetically soft fcc FePt phase. With 15vol% C doping, FePt–C film with columnar grains of 7.5nm was obtained and the out-of-plane coercivity measured at room temperature was as high as 14.4kOe. The increase in carbon volume fraction to 20% caused the formation of two-layer structure, whereas for the 20vol% TiO2 and Ta2O5 doping, the columnar structure of the FePt films remained and the corresponding grain sizes were 5 and 10nm, respectively. Ta2O5 doping showed better grain isolation than the others. The out-of-plane coercivities of FePt–TiO2 and FePt–Ta2O5 films were 7.5 and 8.8kOe, respectively.

High-coercivity FePt nanoparticle assemblies embedded in silica thin films

The ability to process assemblies using thin film techniques in a scalable fashion would be a keyto transmuting the assemblies into manufacturable devices. Here, we embed FePt nanoparticleassemblies into a silica thin film by sol–gel processing. Annealing the thin film composite at650 °C transforms the chemically disordered fcc FePt phase into the fct phase, yielding magnetic coercivityvalues Hc>630 mT. The positional order of the particles is retained due to the protection offered by thesilica host. Such films with assemblies of high-coercivity magnetic particles areattractive for realizing new types of ultra-high-density data storage devices andmagneto-composites.

Structure and magnetic properties of magnetron-sputtered FePt/Au superlattice films

Using dc magnetron sputtering, FePt/Au multilayer films were prepared, and the effects of Au thickness and annealing temperature on structure and magnetic properties of the FePt/Au films were investigated. The as-deposited FePt/Au multilayer films with a thicker Au layer show a superlattice structure. The interatomic layer spacing d(2 2 0) in face centered cubic (fcc) FePt lattice increases with increase in Au thickness. Accordingly, the strain energy stored in the fcc FePt lattice increases. After annealing at 300 °C, ordered FePt films were obtained, and the in-plane and out-of-plane coercivities of the films were found to increase with the increase in the Au layer thickness. When the Au layer thickness is 3.5 nm, the in-plane and out-of-plane coercivities of the films can reach 2.08 kOe and 1.56 kOe, respectively. The strain energy stored in fcc FePt lattice is responsible for the lower temperature ordering of FePt phase. For the films annealed at high temperature, the thicker Au layer in the films restrained the ordering of the FePt phase, which leads to the decrease in the coercivities of the films with increasing Au thickness.

Thickness Dependence of (001) Texture Evolution and Magnetic Properties of Sputter-Deposited FePt:MgO Nanocomposite Films

We investigate the effect of the number of bilayers (total film thickness) on the (001) texture development, L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ordering reaction and magnetic properties in multilayered [FePt/MgO] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> films that were sputter deposited on thermally oxidized Si substrates. We obtained nearly perfectly L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (001) oriented nanocomposite films when the initial multilayer structure was [FePt 2.8 nm/MgO 3.2 nm] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> and the total film thickness was 36 nm. When the film thickness was thicker than about 36 nm or the film was overaged, the grain orientation of the film developed in a random direction. The evolution of the (001) texture was favorable when the total film thickness was thin enough for the strain generated during the ordering transformation to relax in the plane normal direction, thus satisfying the plane stress condition. In addition, the disordered fcc FePt (200) texture grown on (200)-textured MgO in the as-deposited state also affected the L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (001) texture evolution.

Regular Arrangement of Gas Phase Prepared In-Flight Annealed FePt Nanoparticles on S Layers

To achieve a regular quadratic particle arrangement, gas phase prepared FePt nanoparticles which were annealed in flight in order to obtain the highly anisotropic tetragonal L10 structure are deposited onto the regular 2-D bacterial surface-protein layer (S layer) of Bacillus sphaericus NCTC 9602 which exhibits a four-fold lattice symmetry with a lattice constant of 12.5 nm. The degree of regularity is studied by the statistical evaluation of the particles positions observed by transmission electron microscopy. Although the obtained regularity for the annealed hard magnetic particles is less than for previously studied disordered fcc FePt nanoparticles, a template-directed transfer of both lattice symmetry and periodicity from the 2-D protein crystal to the particle arrangement is observed. In addition, particle agglomeration which has been shown to have a strong effect on the arrangement's regularity, is clearly reduced for the annealed particles because of the mutual magnetic repulsion of adjacent particles.