L10 FePt Phase Research Articles

Two-phase separation facilitating the ordering of L10-FePt: experimental and thermodynamic investigation on Fe–Pt–Cu phase diagrams

The L10-FePt nanocrystals exhibit unique magnetic and catalytic properties, but suffer from the ordering difficulty. The addition of Cu can facilitate the L10-FePt ordering for some Fe–Pt–Cu alloys, however, the action mechanism is far from being well understood. In this work, a systematic study has been performed to the phase diagrams of the Fe–Pt–Cu system. The isothermal sections at 600 °C, 750 °C and 1000 °C, as well as the partial vertical sections of Fe50Pt50–Cu50Pt50 and Fe50Pt50–Cu, were constructed. An emphasis has been paid to the L10-FePt phase, which has a wide composition range with a solid solubility of Cu up to 40.0 at% at 600 °C. Its order-disorder transition temperature decreases rapidly when deviating from 54 at% Pt, but at different lower rates along (FePt)100-xCux and (Fe50-xCux)Pt50 with the increase of Cu. The ternary miscibility gap which originates from the Fe–Cu side has been studied by both experimental determinations and thermodynamic calculations. The results indicate that, at certain low temperatures, for instance 300 °C, the metastable miscibility gap can expand even up to 45 at% Pt, entering the L10-FePt phase region. An ordering transition mechanism of the L10-FePt phase stimulating by a two-phase separation was thus proposed. For a specially designed disordered Fe–Pt–Cu alloy, a metastable two-phase separation can take place firstly in the annealing process, driving the Cu atoms forming Cu-rich particles, leaving a large number of vacancies in the alloy, and consequently, accelerate the atomic diffusion and stimulate the subsequent L10 ordering transition. These results will provide a new perspective for the composition design of Fe–Pt–Cu catalysts and magnetic recording media.

Highly Coercive L10-Phase Dots Obtained through Low Temperature Annealing for Nano-Logic Magnetic Structures

Nano-logic magnetic structures are of great interest for spintronic applications. While the methods used for developing arrays of magnetic L10-phase dots are, in most cases, based on deposition followed by annealing at high temperatures, usually around 700 °C, we demonstrate here a technique where a much lower annealing temperature (i.e., 400 °C) is needed in order to promote fully the disorder–order phase transformation and achievement of highly coercive L10-phase dots. In order to develop building blocks based on arrays of L10-phase FePt dots for further spintronic applications, an engraving technique using electron beam lithography is employed. This paper describes the fabrication, as well as the morphological and magnetic characterization, of regularly placed FePt dots of various shapes, as pre-requisites for integration into nano-logic devices. As a proof of concept, regular arrays of FePt circular dots were devised and their structural characterization, using X-ray diffraction (XRD) and transmission electron microscopy (TEM), was performed. It has been shown that annealing at only 400 °C for 30 min proved the occurrence of the tetragonal L10 phase. Moreover, structural characterization showed that the disorder–order phase transformation was complete with only the L10 phase detected in high resolution TEM. The magnetic characterization provided more insight into the potential of such arrays of magnetic devices with convenient values of magnetic coercivity, remanent and saturation magnetization. These findings show good potential for developing regular arrays of uniformly shaped magnetic entities with encouraging magnetic performances in view of various applications.

Effects of Ni addition on the order–disorder transitions of Fe–Pt intermetallic compounds: An experimental study on Fe–Ni–Pt phase diagrams

The Fe–Pt based intermetallic compounds exhibit good chemical stability and unique magnetic properties, where Ni is an important additional element to optimize the magnetic properties or obtain the outstanding catalytic performances of the Fe–Pt based alloys. Knowledge of how Ni addition affects the order–disorder transitions of the Fe–Pt intermetallics is thus necessary; however, the related information is limited. Therefore, in this work, the phase diagrams of the Fe–Ni–Pt system were experimentally investigated, and as a result, the isothermal sections of the Fe–Ni–Pt system at 600 and 900 °C, as well as the vertical sections of Fe80Ni20–Pt80Ni20 and Fe50Pt50–Ni50Pt50 were constructed. Based on these results, the influences of Ni addition on the crystal stabilities and phase transformations of the ordered Fe–Pt intermetallics have been well described. The results show that the L10-FePt and L10-NiPt phases form a ternary continuous solid solution of L10-(Fe,Ni)Pt, whereas Ni can dissolve in the L12-Fe3Pt and L12-FePt3 phases as high as 57.0 at.% and 26.0 at.% at 600 °C, respectively. The selective occupancy of Ni atoms has been predicted, which should depend on the alloy composition. For both the L10-(Fe,Ni)Pt and L12-FePt3 phases, when Pt contents are less than their stoichiometric values, Ni atoms will preferentially occupy the Pt sublattice, forming as many nearest-neighbor Fe−Pt bonds as possible. All these results can correlate the alloy compositions, annealing temperatures and crystal structures to both magnetic and catalytic properties, thus providing a basis for optimizing the Fe–Ni–Pt alloys towards enhanced magnetic or catalytic performances.

Systematic study of nitride matrix of FePt granular films for heat assisted magnetic recording media

A systematic investigation of the magnetic properties and nanostructure of FePt-nitride granular films with nitride of various melting points (Tm) is reported. When Tm of nitride grain boundary materials (GBMs) is varied from 1650 to 3305 °C, the saturation magnetization (Msfilm) changes from 710 to 570 emu/cm3. The Msfilm of FePt-nitride is similar to that of FePt-oxide granular films, almost varies linearly against Tm. At almost the same Tm, FePt-nitride shows a higher Msfilm of around 70 emu/cm3 compared to that of FePt-oxide granular films. According to the torque measurement result of the saturated torque coefficient with fourfold components (L4θsat), FePt-nitride shows a smaller L4θsat than that of FePt-oxide granular films, which suggests that FePt-nitride has a smaller content of the in-plane component than that of FePt-oxide granular films. This is supported by the in-plane x-ray diffraction profile measurement, where FePt-nitride has a smaller integral intensity of (001) diffraction of the L10-FePt phase than FePt-oxide granular films. With further promotion of grain separation, nitrides are more promising GBMs than oxides due to their high Msfilm and smaller content of the in-plane component.

Phase composition and magnetic properties of post-annealed asymmetric Pt/Fe/Pt/Au/Fe thin films

Intermetallic compound formation realized via thermally-induced diffusion intermixing of layered stacks is a prospective route for the formation of thin films with properties promising for practical applications. This can be further promoted by the addition of third elements which could provide acceleration of diffusion and ordering processes during post-annealing. In the present study, we have investigated the structural and magnetic properties of post-annealed Pt/Fe/Pt/Au/Fe thin films which contain an additional asymmetric Au layer and compare the results with Fe/Pt bilayers. It was shown that the introduction of an Au layer reduces both the onset temperature and the annealing time for promoting the formation the hard magnetic L10-FePt phase while the remaining layer stack develops a magnetically soft Pt-rich disordered FePt solid solution. Thus, different diffusion pathways present in asymmetric layer stacks can provide an intriguing root for thin film materials synthesis of exchange coupled soft and hard magnetic phases which could be of particular interest for the creation of graded magnetic nanomaterials.

Redetermination of the Fe–Pt phase diagram by using diffusion couple technique combined with key alloys

Abstract Fe–Pt intermetallic nanocrystals exhibit good chemical stability and unique magnetic and catalytic properties. However, there are still some discrepancies with regard to the order–disorder transitions of Fe–Pt phases, and experimental works reported for the phase equilibria of Fe–Pt alloys at low temperatures are scarce. Therefore, in this work, the phase equilibria and phase transformations of Fe–Pt alloys were systematically studied by using the diffusion couple technique combined with key alloys through electron probe microanalysis, X-ray diffraction and differential scanning calorimetry. As a result, the composition ranges of the ordered L12-Fe3Pt, L10-FePt and L12-FePt3 phases, especially at low temperatures, have been well determined, and the related invariant reactions have been evaluated. Finally, a revised Fe–Pt phase diagram has been proposed.

Operating interfaces to synthesize L10-FePt@Bi-rich nanoparticles by modifying the heating process.

A facile strategy to operate interfaces when synthesizing L10-FePt@Bi-rich nanoparticles (NPs) has been proposed. Two interfaces are indispensable to obtain the high ordering L10-FePt structure. One is the mismatched interfaces between the initial γ-PtBi2 nuclei and the disordered fcc-FePt phase. The other is the in situ grown coherent interfaces between the L10-FePt and Bi-rich phases. Increasing the heating rates when the temperature rises from 120 °C to 310 °C benefits the formation of mismatched interfaces and improves the uniformity of phases and composition in NPs. Reducing the heating rate at higher temperature ensures sufficient time for Bi to diffuse across the coherent interface, which facilitates the disorder-order transition of L10-FePt NPs. This study provides a new perspective on operating interfaces during the wet-chemical synthesis process.

Effect of substrate temperature on plasma-enhanced self-assembling formation of high-density FePt nanodots

We formed FePt magnetic nanodots (NDs) by exposing an ultrathin bilayer metal stack on ∼3.0 nm SiO2/Si(100) substrates to a remote H2 plasma (H2-RP) and studied the effect of external heating during the exposure to H2-RP on the formation and magnetic properties of NDs. The ultrathin bilayer with a uniform surface coverage drastically changed to NDs with an areal density as high as ∼3.5 × 1011 cm−2 by exposing to H2-RP with external heating. We also found that NDs formed by the exposure to H2-RP at 400 °C exhibited a perpendicular anisotropy with a perpendicular coercivity of ∼1.5 kOe, reflecting the magneto-crystalline anisotropy of (001)-oriented L10 phase FePt.

Self-diffusion of Fe and Pt in L10-Ordered FePt: Molecular Dynamics simulation

Vacancy-mediated lattice diffusion coefficients of Fe and Pt atoms in the chemically ordered L10-FePt phase at temperatures between 1300 and 1600 K were evaluated by means of Molecular Dynamics (MD) simulations. Due to the anisotropic structure of the L10-ordered FePt phase, Fe and Pt diffusion fluxes and the resulting self-diffusion coefficients were considered along and perpendicular to the [001] crystallographic direction. In view of a very low vacancy concentration in real FePt single crystals, steady state conditions of the simulated process were approximated by specifically scaling the self-diffusion coefficients estimated for higher vacancy concentrations to the equilibrium vacancy concentration. This procedure involved the calculation of vacancy formation energies which appeared temperature dependent. The validity of this approach was thoroughly tested and the final results were analyzed and compared to the relevant literature data. The evaluated temperature dependent Fe and Pt self-diffusion coefficients showed Arrhenius behavior, however, their values were much lower than the reported experimental ones. Apart from the inevitable effect of the applied quasi-empirical potentials, the discrepancies might originate from the fact that while the MD simulations addressed a single crystal of FePt defected exclusively with vacancies and antisites, the existence of fast diffusion paths along linear and planar defects cannot be excluded in real materials.

Formation of ordered L10-FePt phase in FePt–Ag thin films

In this work, the influence of an Ag layer, which is located at different positions of the layer stack (top, intermediate, bottom), on the structural and magnetic properties of ordered L10 phase in Fe50Pt50 films was investigated. It was found that the position of an additional Ag layer affects the stress state of the as-deposited FePt films. Mechanical stress is induced during deposition and post-annealing process due to the difference in thermal expansion coefficients and lattice mismatch of FePt, Ag, and substrate. The initial stress state affects the ordering process during subsequent heat treatment. The higher level of initial compressive stress (− 7.8 GPa) in FePt/Ag/FePt films results in L10 phase formation at a temperature of 700 °C which is about 100 °C lower than in films with Ag bottom (− 3.6 GPa) and top (+ 0.86 GPa) layers. A more pronounced (001) oriented growth of the L10 phase was observed in stacks with Ag top layer as compared to Ag bottom and intermediate layers. The highest values of coercivity were obtained for films with Ag intermediate and bottom layers, which is due to the distribution of Ag along FePt grain boundaries that result in strong exchange decoupling of FePt grains. Furthermore, the formation of the ordered L10 phase at comparatively low temperatures of about 370 °C was observed in a free-standing FePt/Ag (30 nm)/FePt film.

Preparation, Microstructure, and Magnetic Properties of Electrodeposited Nanocrystalline L10 FePt Films

In this work, we prepared the nanocrystalline near-equiatomic FePt thin films by electrodeposition and explored the effects of annealing temperature and annealing time on morphology, structure, and magnetic properties of FePt film. The fcc-FePt phase starts to form when annealing at 400 °C under the 15% H2 gas mixture. With increasing annealing temperature from 400 to 800 °C, the FePt film experiences a gradual phase transformation from the soft magnetic fcc phase to the permanent magnetic fct L10 phase. The transition degree of the L10 phase is achieved to be 0.94 and coercivity reaches a maximum of 1.5 T for the sample annealed at 800 °C for 4 h. A shoulder behavior in the M(H) curves is found in the electrodeposited FePt films. Through fitting the M(H) curves by the model based on the transition degree of the L10 FePt phase, the shoulder behavior is quantitatively confirmed to originate from the coexistence of the soft magnetic fcc phase and the permanent magnetic L10 phase. The magnetic exchange coupling between the soft magnetic and permanent magnetic nanocrystalline phases is negligible in the electrodeposited FePt films due to that the substrate Ag diffusion into the FePt film separates the FePt nanocrystalline phases.

Thermally-induced phase transitions in Pt/Tb/Fe trilayers

Structural phase formation induced by thermally-activated diffusion processes and intermixing of layered structures is a promising reaction pathway for the synthesis of unexpected (metastable) thin film materials. Furthermore, the sequence of structural phase transitions during annealing can substantially differ from those predicted by corresponding bulk phase diagrams due to the underlying grain boundary diffusion mechanisms as well as surface and interface effects. In this study, phase transitions in Pt/Tb/Fe trilayers occurring during post-annealing up to 620°C were investigated by various techniques including x-ray diffraction, transmission electron microscopy, secondary neutral mass spectrometry, Auger electron spectroscopy depth profiling, and magnetic properties measurements. At an annealing temperature of 215°C pronounced Pt and Tb diffusion intermixing is observed leading to the formation of the binary Pt2Tb phase. Further annealing up to 280 °C is accompanied by the appearance of the chemically disordered A1-FePt phase and significant Tb segregation to the outer surface forming an oxide layer. The chemically ordered L10-FePt phase starts to form in parallel to the disappearance of the Pt2Tb phase at 450°C. The final phase product at 620°C is characterized by the coexistence of two remaining phases, L10-FePt and TbO2. Structural phase transitions have a significant impact on the films magnetic properties – heat treatment leads to an enhancement of the coercive field. This fact is associated with the formation of the hard magnetic L10-FePt phase, which might be accompanied by grain isolation induced by partially Tb/Tb-O grain boundary filling.

Structure and properties of nanoporous FePt fabricated by dealloying a melt-spun Fe60Pt20B20 alloy and subsequent annealing

A nanoporous FePt alloy has been fabricated by dealloying a melt-spun Fe60Pt20B20 alloy composed of nanoscale amorphous and face-centered-cubic FePt (fcc-FePt) phases in H2SO4 aqueous solution. The nanoporous alloy consists of single fcc-FePt phase with an Fe/Pt atomic ratio of about 55.3/44.7, and possesses a uniform interpenetrating ligament-channel structure with average ligament and pore sizes of 27 nm and 12 nm, respectively. The nanoporous fcc-FePt alloy shows soft magnetic characteristics with a saturation magnetization of 37.9 emu/g and better electrocatalytic activity for methanol oxidation than commercial Pt/C in acidic environment. The phase transformation from disordered fcc-FePt into ordered face-centered-tetragonal FePt (L10-FePt) in the nanoporous alloy has been realized after annealing at 823–943 K for 600 s. The volume fraction of the L10-FePt phase in the alloy increases with the rise of annealing temperature, which results in the enhancements of coercivity and saturation magnetization from 0.14 kOe and 38.5 emu/g to 8.42 kOe and 51.4 emu/g, respectively. The ligament size of the samples is increased after annealing.

Formation of Phases in the FePt/Au/FePt Films and their Magnetic Properties

The effect of annealing atmosphere (vacuum, hydrogen) and intermediate Au layer thickness on the formation of a magnetically hard L10 phase and the magnetic properties of Fe50Pt50(15 nm)/Au(7.5, 30 nm)/Fe50Pt50(15 nm) films deposited by magnetron sputtering onto SiO2(100 nm)/Si(001) substrates was studied. Samples were heat-treated in a hydrogen atmosphere at 100 kPa. The disordered A1-FePt phase formed in the as-deposited films. The ordered L10-FePt phase appeared in the film with an intermediate Au (7.5 nm) layer during vacuum annealing at 650°C. Increase in the intermediate Au layer thickness to 30 nm reduces the ordering temperature to 600°C. This is promoted by higher compressive stresses in the as-deposited Fe50Pt50 layers with a thicker Au interlayer. In hydrogen annealing, the A1 → L10-FePt phase transition in the Fe50Pt50(15 nm)/Au(7.5, 30 nm)/Fe50Pt50(15 nm) films starts at 500°C regardless of the intermediate Au layer thickness. The Au and FePt axial (111) textures are observed in the films annealed in hydrogen. This is more evident in the films with a thicker Au (30 nm) interlayer. The film with an intermediate Au (30 nm) layer shows 27.3 kOe coercivity after vacuum annealing at 900°C. The same coercivity can be achieved by hydrogen annealing at 700°C, which is 200°C lower than after vacuum annealing due to the introduction of hydrogen into voids of the L10-FePt lattice and increase of stresses.

Effect of initial stress/strain state on the L10 phase formation of FePt in FePt/Au/FePt trilayers

The influence of the initial stress/strain state in FePt/Au/FePt thin films on the chemical L10-FePt formation after post-annealing is studied. It is shown that the level of stress/strain depends on the Au interlayer thickness revealing a correspondence in L10 ordering-onset temperature. Compressive strain present in the as-deposited films promotes lowering of the L10-FePt ordering temperature. A reduction of the onset temperature for ordering by 100 °C is observed in films having a 30-nm-thick intermediate Au layer compared to samples revealing lower compressive strain values. During annealing, the strain state caused by the difference in thermal expansion coefficients of FePt, Au, and substrate is relaxed during A1 → L10-FePt phase transformation. It is well known that the coercivity of FePt films rises with increasing fraction of the ordered L10-FePt phase. Furthermore, the insolubility of Au in the L10-FePt lattice results in Au diffusion predominantly along grain boundaries leading to magnetic FePt grain isolation, which in turn gives rise to enhanced coercivity. At an annealing temperature of 900 °C, coercivities as high as 27.5 kOe are achieved for FePt samples with an initial 30-nm-thick Au interlayer.

Phase transformations in Pt/Fe bilayers during post annealing probed by resistometry

X-ray diffraction (XRD), secondary neutral mass spectrometry (SNMS) depth profiling, and electrical resistivity measurements were used to follow the phase transformations in Pt/Fe bi-layered thin films during annealing. Initially, the electrical resistivity increases linearly with temperature up to 150 °C due to the contribution of phonon scattering of the metallic Pt and Fe bilayer. Further increase of the annealing temperature leads to a steeper linear increase, which is associated with the initial formation of the chemically disordered A1-phase followed by the formation of the chemically ordered L10-FePt phase, as confirmed by XRD and SNMS studies. Finally, at about 620 °C the single L10-FePt phase has formed throughout the film. Moreover, the electrical resistivity contains also the magnetic contribution to the total resistivity. In this case, the loss in magnetic order is indicated by a change in temperature dependence of the resistivity at about 310 °C, representing the Curie temperature of the initially formed A1-FePt alloy, while the finally formed L10-FePt alloy reveals a higher magnetic transition temperature of about 410 °C.In this study, it has been demonstrated that resistometry in combination with structural and chemical analysis provides valuable information on diffusion processes, structural phase formations and its stability range, as well as on the magnetic transition temperature.

Amphiphilic bimetallic polymer as single-source precursors for the one-pot synthesis of L10-phase FePt nanoparticles

Amphiphilic polymers have attracted extensive research attention in constructing various nanostructures by self-assembly. Here we designed and synthesized two amphiphilic bimetallic polymers with different length of the tails P1 and P2, in which Fe,Pt-containing conjugated complex acted as the hydrophobic block and hydrophilic poly(ethylene glycol) (PEG) was bonded to the bimetallic core as the flexible tails. P1 and P2 were used as the single-source precursors to prepare FePt nanoparticles (NPs) by one-pot pyrolysis. The resultant NPs were fully characterized and had a chemically ordered face-centered tetragonal (fct) phase with high crystallinity. The size of NPs pyrolyzed from P1 and P2 was 24.7 and 8.2 nm with the relative coercivity of 9.6 and 1.3 kOe, respectively. The difference was preliminarily explained by the discrepancy of their degrees of crystallinity, and also analyzed by the precursors’ structural effect. The amphiphilic design showed a good potential in preparing monodisperse ferromagnetic FePt NPs, and the possible favorable properties of self-assembly might provide a bright venue for future magnetic recording media.

Magnetic Anisotropy in Thin Films of FePt Detected by the Ferromagnetic Resonance Method

FePt thin films are made via the heat treatment of structures containing Pt (001) and Fe (001) epitaxial layers. X-ray spectral analysis and field-emission scanning electron microscopy show that the samples contain the cubic L12–Fe3Pt phase and the tetragonal L10–FePt phase. The magnetic anisotropy of the thin films is studied by ferromagnetic resonance and magnetometric methods. The contributions from the cubic and tetragonal phases to the ferromagnetic-resonance spectrum of the sample are distinguished. The key characteristics of thin films which determine their practical applicability are determined: the Gilbert damping parameter G ~ 0.4, the first and second crystal anisotropy constants K1⊥ = –4.08 × 106 erg/cm3, K2⊥ = 1.34 × 106 erg/cm3, K2|| = –9.76 × 104 erg/cm3.

A Controllability Investigation of Magnetic Properties for FePt Alloy Nanocomposite Thin Films.

An appropriate writing field is very important for magnetic storage application of L10 FePt nanocomposite thin films. However, the applications of pure L10 FePt are limited due to its large coercivity. In this paper, the ratios of L10 and non-L10 phase FePt alloy nanoparticles in FePt/MgO (100) nanocomposite thin films were successfully tuned by pulsed laser deposition method. By adjusting the pulsed laser energy density from 3 to 7 J/cm2, the ordering parameter initially increased, and then decreased. The highest ordering parameter of 0.9 was obtained at the pulsed laser energy density of 5 J/cm2. At this maximum value, the sample had the least amount of the soft magnetic phase of almost 0%, as analyzed by a magnetic susceptibility study. The saturation magnetization decreased with the increase in the content of soft magnetic phase. Therefore, the magnetic properties of FePt nanocomposite thin films can be controlled, which would be beneficial for the magnetic applications of these thin films.

Halide Ion-Mediated Synthesis of L10-FePt Nanoparticles with Tunable Magnetic Properties.

L10-FePt nanoparticles (NPs) have great potential in areas of advanced magnetic and catalytic applications. Here, we present a facile control route for synthesis of hard magnetic L10-FePt NPs in which halide ions (Cl-, Br-, or I-) were added to the synthetic process to promote the phase transformation. It is confirmed that the strong ionic binding force between halide ions and Fe3+ or Pt2+ ions could facilitate the formation of L10-FePt phase due to favoring growth of FePt NPs in a more thermodynamically stable way, which enables the formation of an ordered structure. L10-FePt NPs with the highest coercivity of 8.64 kOe and saturation magnetization of 64.21 emu/g at room temperature can be directly obtained by controlling the amount of the halide ions. In comparison with conventional solution phase reduction methods, the halide ion-assisted method shows enhanced capability to tune the growth of hard magnetic bimetallic NPs, particularly Pt-based bimetallic NPs.