Ordered L10 Phase Research Articles

L10-ordered FeNi film grown on Cu-Ni binary buffer layer

An FeNi film with a metastable L10-ordered phase was fabricated by an alternate monatomic layer deposition. An Fe monolayer and a Ni monolayer were alternately grown on a Cu0.6Ni0.4 binary buffer layer. A (001) L10-FeNi superlattice peak, which indicates the formation of a L10 phase, was clearly observed by an X-ray diffraction. A long-range order parameter S of L10-FeNi was estimated to be 0.5 ± 0.1. It was found that the film has a uniaxial magnetic anisotropy along c-axis by magnetization measurement. The uniaxial magnetic anisotropy energy Ku was evaluated to be (5.0 ± 0.1) × 106 erg/cc.

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.

Structural characterization for L1-ordered FePt films with (001) texture by x-ray diffraction

The nanostructure of L10-ordered FePt films with a [001] orientation prepared by both an epitaxial growth and nonepitaxial method was characterized by fitting X-ray diffraction profiles. Good fitting results for both types of films were obtained by considering the structural inter-and intragrain disorders in a calculation model. The distributions of the long range order and film thicknesses were considered to be part of the structural disorder among the grains. The fluctuation of the lattice spacing and the depth of x-ray interference along the surface normal were also taken into account to explain the structural disorder within the grain. The epitaxial FePt films grown on the MgO(100) underlayer were gradually transformed from a disordered A1 phase to an ordered L10 phase with increasing deposition temperature. A thinner film of 5 nm indicated a relatively low long range order with the fluctuation of lattice spacing. This is attributed to a large mismatch between the lattice of the initial growth layer and the MgO underlayer. On the other hand, for nonepitaxial FePt films prepared by SiO2 doping and rapid thermal post-annealing, high [001]-oriented L10 grains without the distribution of long range order were obtained at an initial transformation stage at a low annealing temperature, and the L10 grains grew along the film surface with increasing temperature. The fluctuation of lattice spacing was independent of the crystallite height in the nonepitaxial films, whereas the structural disorders such as the lattice fluctuation and chemical disorder in epitaxial films increased with a decrease in the crystallite height.

X-ray absorption measurements on nanoparticle systems: self-assembled arrays and dispersions

X-ray absorption spectroscopy methods are presented as a useful tool to determine local structure, composition and magnetic moments as well as to estimate the effective anisotropy of substrate supported self-assembled arrays of wet-chemically synthesized FePt nanoparticles. A compositional inhomogeneity within the nanoparticles yields reduced magnetic moments with respect to the corresponding bulk material and may also hinder the formation of the chemically ordered L10 phase in FePt nanoparticles. The latter is indicated by a reduced effective anisotropy, which is one order of magnitude smaller than expected from the known value of the corresponding bulk material.As a new approach, measurements of the x-ray absorption near-edge structure of Fe-oxide nanoparticles in dispersion are presented and ageing effects are discussed on the basis of multiplet calculations.

A phase field study for influence of elastic energy on L10→L12 transient ordering in Ni75Al17Zn8

The influence of the elastic energy on L10→L12 transient ordering transformation was investigated by microscopic phase field method. It is found that there are three stages experienced in atomic ordering: solute clustering+L10 short range ordering → L10 long range ordering → L12 long range ordering. Before the formation of the high ordered L12 phase, it has firstly taken place the transformation from matrix to L10 phase, and then held the L10→ L12 secondary transformation. Elastic energy is proved to take little effect on the stage of short range ordering, but as the elastic energy is multiplied, it obviously shortened the course of the solute clustering, and speeded up the proceeding of the L10 long range ordering transition. Accordingly, the increased elastic energy also strengthens the single crystalline directionality of L10 phase projecting on 2D plane and makes the ordered degree of Al and Zn atoms enhanced. With the temperature elevation, Al’s and Zn’s ordered degree decreased in L10 phase.

Optimization of the Magnetic Properties of FePd Alloys by Severe Plastic Deformation

AbstractA FePd alloy was nanostructured by severe plastic deformation following two different routes: ordered and disordered states were processed by high pressure torsion (HPT). A grain size in a range of 50 to 150 nm is obtained in both cases. Severe plastic deformation induces some significant disordering of the long range ordered L10 phase. However, transmission electron microscopy (TEM) data clearly show that few ordered nanocrystals remain in the deformed state. The deformed materials were annealed to achieve nanostructured long range ordered alloys. The transformation proceeds via a first order transition characterized by the nucleation of numerous ordered domains along grain boundaries. The influence of the annealing conditions (temperature and time) on the coercivity was studied for both routes. It is demonstrated that starting with the disorder state prior to HPT and annealing at low temperature (400 °C) leads to the highest coercivity (about 1.8 kOe).

Effect of Annealing on Magnetic Properties and Domain Structures of FePt Thin Films by Rapid Thermal Annealing Technique

Single-layered FePt films of 30 nm thick were annealed at temperature between 300 and 800 °C for 1–180 sec by a rapid thermal annealing (RTA) with a high heating rate of 100 °C/sec. It is found that both the grain size and magnetic domain size of the FePt film increase with increasing annealing temperature and annealing time. The FePt films exhibited soft magnetic properties and without domain images were observed by magnetic force microscope (MFM) when the films were post-annealed at below 500 °C for 180 sec. The in-plane coercivity (Hc//) and perpendicular coercivity (Hc⊥) of FePt film increases significantly to 7.5 and 6.5 kOe respectively as annealing temperature increases to 600 °C. When the annealing temperature is increased to 700 °C, they are increased to 11.1 and 9.5 kOe, respectively, and the domain structure inclines to isolated domain. However, further increasing the annealing temperature to 800 °C, the Hc// and Hc⊥ values decrease to 9.8 and 8.9 kOe respectively due to largely increase the grain size of FePt and change the domain structure from isolation to continuity. On the other hand, in order to transform the FePt film from disordered γ phase to the ordered L10 phase, the annealing time of over 3 seconds is necessary when the film was post-annealed at 700 °C with a high heating rate of 100 °C/sec by RTA technique.

Carbon nanotubes terminated with hard magnetic FePt nanomagnets

The advancement in carbon nanotube (CNT) technology includes significant interest in their functionalization to modify their chemical and physical properties. In particular, the selective functionalization of the CNT ends opens exciting opportunities to design nanoscale architectures and networks. The realization of hard-magnetically terminated CNT via plasma enhanced chemical vapor deposition from Fe–Pt thin films is reported. Although FePt is rarely used as a catalyst for CNT synthesis the said binary catalyst affords attractive hard magnetic properties when present in the chemically ordered L10 phase.

Cold-working and annealing of the chemically ordered L1 0-phase Fe 50–Pd 50

We performed an experimental investigation of the annealing behavior of equiatomic γ 1-FePd at temperatures below the critical disordering temperature after cold deformation by rolling in the fully L1 0-ordered state. We used scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) to monitor microstructural changes. We used vibrating sample magnetometer (VSM) measurements to determine the evolution of the coercivity of the material in response to cold deformation and annealing. We discuss our findings in the context of previous reports related to studies of recovery and recrystallization during annealing after cold deformation in intermetallic systems that undergo ordering transformations.

Phase equilibria and thermal analysis in the Fe–Mn–Ni system

Abstract Based on the critical review of the experimental data available in the literature, 7 decisive ternary alloys were prepared. New phase transition temperatures and phase equilibria at 800°C in the Fe – Mn – Ni system were measured. An effective method to accurately determine phase transition temperatures from differential thermal analysis curves with overlapped peaks and nonlevel baselines within a narrow temperature range was developed. A massive transformation was observed in one quenched Fe-rich alloy. In order to make the fcc_A1 phase always in the fully disordered state in the ternary system, the effect of the binary reciprocal parameter on the Gibbs energy of ternary ordered L12 phase was considered. It was found that the introduction of only one parameter could describe all reliable experimental data in the Fe – Mn – Ni system satisfactorily. The presently obtained parameters were successfully applied in two practical cases.

FePt Magnetic Nanoparticles and Their Assembly for Future Magnetic Media

Magnetic nanoparticles with extremely high anisotropy such as chemically ordered L10 phase FePt nanoparticles have been considered one of the best candidates for future magnetic recording media with areal density beyond 1 Tbit/in2 for either the nanocomposite-film-type heat-assisted magnetic recording (HAMR) media or the self-organized-magnetic-array (SOMA)-type bit patterned media. However, current preparation methods via phase transformation must overcome many obstacles, including particle agglomeration, twinning, and difficulty of easy axis alignment. In this paper, the effort on and promise of the preparation of monodispersed magnetic nanoparticles with high anisotropy by a gas phase condensation method is reviewed and reported. The focus is to review recent progress on the fabrication of monodispersed highly ordered L10 phase FePt nanoparticles without phase transformation, successfully self-assembled and magnetically aligned magnetic nanoparticles for SOMA and HAMR media. The mechanisms for directly forming L10 phase FePt nanoparticles during nucleation and growth processes and magnetically aligning these particles are analyzed.

Low-temperature molecular beam epitaxy of a ferromagnetic full-Heusler alloy Fe2MnSi on Ge(111)

We demonstrate the epitaxial growth of ferromagnetic full-Heusler alloy Fe2MnSi layers on group-IV semiconductor Ge(111) using molecular beam epitaxy at the growth temperatures of 130 and 200 °C. The Fe2MnSi/Ge(111) layers have an atomic-scale abrupt interface and include the ordered L21 phase. We also show ferromagnetic features with a saturation magnetization of ∼2.2 μB/f.u. and a Curie temperature of ∼210 K, which are nearly comparable to those of bulk Fe2MnSi.

Rapid thermal annealing of FePt nanoparticles

We report a systematic study on rapid thermal annealing (RTA) of FePt nanoparticles. FePt particles with an average size of 8nm were synthesized by a chemical solution method, and then annealed using RTA and conventional furnace annealing (FA). It was observed that FePt nanoparticles can be transformed from disordered A1 phase to ordered L10 phase at 650°C for 10s using RTA, which is much shorter than the time needed for FA. The transmission electron microscopy and x-ray diffraction studies have revealed that the particle agglomeration and grain growth in the RTA treated samples are much less than in the FA treated samples. A linear correlation between the coercivity and the square root of the treatment time t was observed in the RTA treated samples, which implies that the phase transition is related to atomic diffusion of Fe atoms from Fe-rich shells into the Pt-rich cores.

Construction of the Al–Ni–Si phase diagram over the whole composition and temperature ranges: thermodynamic modeling supported by key experiments and first-principles calculations

Abstract An extensive thermodynamic investigation of the Al – Ni – Si system is carried out via an integrated approach of calculation of phase diagrams, first-principles calculations, and key experiments. Eighteen decisive alloys are prepared in order to verify the existence of the previously reported ternary compounds and to provide new phase equilibrium data. Phase compositions, microstructure, and phase transition temperatures are determined using the combined techniques of X-ray diffraction, scanning electron microscopy, energy dispersion X-ray analysis, and differential thermal analysis. The order/disorder transition between disordered bcc_A2 and ordered bcc_B2 phases as well as that between disordered fcc_A1 and ordered L12 phase are described using a two-sublattice model. A self-consistent parameter set is finally obtained by considering the huge amount of experimental data including 13 vertical sections and 5 isothermal sections from both the literature and the present experiments. Almost all of the reliable phase diagram data can be well described by the present modeling. The reliability of the calculated thermodynamic properties for ternary phases is verified through enthalpy measurement employing drop calorimetry and first-principles calculations. The thermodynamic parameters obtained can also successfully predict most of the thermodynamic properties and describe the solidification path for the selected as-cast alloy Al6Ni55Si39.

Fabrication of FePt/M (M = C, Ag, Si) nanoparticulate thin films with perpendicular anisotropy

FePt-based nanoparticulate thin films have been deposited by sputtering onto single crystal MgO [100] substrates at elevated temperatures up to 650?C. The deposited FePt film showed a [001] texture due to lattice matching between FePt and MgO substrate. The effects of an additional element on the magnetic and structural properties have been studied; addition of Ag leads to a lower transformation temperature from the disordered fcc to the ordered L10 phase. Si addition leads to a ternary FePtSi system where Si atoms enter into the FePt lattice interstitially, resulting in changes of lattice parameters and magnetic properties.

Synthesis and Characterization of Bimagnetic Bricklike Nanoparticles

Bimagnetic FePt/CoFe2O4 nanoparticles having a bricklike morphology were synthesized by growing a soft magnetic CoFe2O4 phase on FePt cubic nanoparticle seeds. The size of the soft phase could be controlled by tuning the material ratio of the FePt seeds to the CoFe2O4 component. To obtain magnetic hardening, the as-synthesized bricklike nanoparticles were annealed at elevated temperatures under a reductive atmosphere to convert the disordered face-centered cubic FePt phase into the ordered L10 phase having high magnetic anisotropy. When the particles were annealed, a gradual change was observed in morphology from bricklike particles to spherical polycrystalline nanocomposite particles because of diffusion. Meanwhile, the magnetic energy density was enhanced as a result of the exchange coupling between the hard and soft phases. The enhancement was dependent on the ratio of the volumes of the soft phase and the hard phase.

Direct synthesis of L10-(Fe, Co)Pt nanocrystallites from (Fe, Co)–Pt–Zr–B liquid phase by melt-spinning

The ordered L10-FePt phase with a size of 20–200nm was directly formed by rapidly quenching the melt in the compositional range of 2–5at.% Zr and 17–20at.% B, and the alloys exhibit coercivity (Hc) higher than 200kA/m in an as-quenched state. In particular, the melt-spun (Fe0.50Pt0.50)78Zr4B18 alloy exhibits high Hc of 684kA/m. The melting temperature (Tm) remarkably decreases by the addition of Zr and B, e.g., from 1837K for FePt to 1345K for (Fe0.55Pt0.45)78Zr4B18. The L10 phase with Zr and B directly formed by the rapidly quenching shows high thermal stability and maintains up to Tm. The simultaneous addition of Zr and B facilitates the direct formation of the ordered L10 phase with a grain size of 20–200nm by rapidly quenching the melt through the effect of the decreasing Tm and the increasing the stability of the L10 phase by the solution of Zr and B into the phase. The substitution of Fe by Co monotonously decreases Tm from 1345K of (Fe0.50Pt0.50)78Zr4B18 to 1083K of (Co0.50Pt0.50)78Zr4B18. However, Hc drastically decreases from 684kA/m of (Fe0.50Pt0.50)78Zr4B18 to 12kA/m of (Co0.50Pt0.50)78Zr4B18, which has a single fcc structure in an as-quenched state. The decrease in Hc with increasing Co content up to Fe:Co=1:3 is caused by the decreasing of the magnetocrystalline anisotropy, which originates from the decreasing the long-range order parameter of the L10 phase.

Atomic ordering and interlayer diffusion of Co2FeSi films grown on GaAs(001) studied by transmission electron microscopy

The influence of the growth temperature on the atomic ordering and interlayer diffusion of Heusler alloy Co2FeSi films grown on GaAs(001) substrates has been studied using high-resolution transmission electron microscopy. The Co2FeSi∕GaAs(001) films grown below 200°C show a coexistence of the Heusler-type L21 phase and the disordered B2 phase, which can be seen mainly near the interface, due to the low growth temperature. This phase coexistence can affect the spin polarization of the Co2FeSi layer near the interface. On the other hand, the film grown at an elevated temperature of 300°C shows a uniformly atomically ordered L21 phase, indicating that 300°C is approximately the transition temperature to the atomically ordered L21 structure. The elevation of the growth temperature, however, results in the formations of interfacial compounds above 250°C, which could be detrimental to the efficient electrical spin injection.

Structure and Magnetic Properties of Nanocrystalline Pd-Co and Pd-Co-Fe Layers

The structure and magnetic properties of sequentially deposited Pd-Co and Pd-Co- Fe nanocrystals have been investigated by transmission electron microscopy (TEM) and superconducting quantum interference device magnetometer. The fcc structured PdCo alloy formation was found in binary Pd-Co nanostructured samples analysed by in situ TEM annealing. In ternary sample, the addition of Fe to Pd-Co resulted in a complex of Pd and FeCo nanoparticles. In situ TEM annealing of the ternary sample lead to the formation of L10 ordered phase. It was found that the magnetic properties of the nanoparticles were improved by Fe addition to the Pd-Co nanoparticles.

Array formation and size effects in chemically synthesized FePt nanoparticles

Monodisperse FePt nanoparticles (NP’s) have been synthesized by reduction of platinum acetylacetonate [Pt(acac)2] followed by thermal decomposition of iron pentacarbonyl [Fe(CO)5] in the presence of oleic acid (OA) and oleyl amine (OY) as surfactants at a low reaction temperature. Particle size is controlled by the injection temperature of Fe(CO)5. The effect of particle size on the blocking temperature has been investigated for nanoparticles with sizes of 3.0 and 6.0nm. The interparticle spacing is varied by changing the addition time of the OA and OY. Well-aligned mono- and multi-layered hexagonal-closed-packed (hcp) to square lattice nanostructures are formed for 5nm FePt NP’s. Surfactant layer thickness, ⟨L⟩ to metal core radius R (⟨L⟩∕R) ratios of 0.46 and 0.60 are evaluated for the monolayer of hexagonal-closed-packed and square lattice array structures, respectively. For hcp arrays bilayer and trilayer structures are also observed. Subjecting the NP’s to thermal processing at 800°C results in a transformation of the nanoparticles from the disordered fcc phase to the ordered L10 phase.