FePt Phase Research Articles

Evaluation of blocking temperature and its distribution for L10-type FePt granular films

The blocking temperature (T B) and blocking temperature distribution (ΔT B) of magnetic grains were evaluated for L10-type FePt granular films with various grain boundary materials (GBMs). T B and ΔT B were measured from remanent magnetization. The relationship between the degree of order and Curie temperature (T C) for the L10-type FePt phase was separately evaluated using alloy films. According to actual measurements, the obtained mean T B, mean T B/T C and ΔT B for granular films were 620 K, 0.83 K, and 200 K, respectively, for SnO GBM film and 450 K, 0.61 K, and 260 K, respectively, for TiO2 GBM film. Based on these values, mean T B, mean T B/T C, and ΔT B were calculated assuming that τ = 2 × 10−7 s, which is the relaxation time, corresponds to the HAMR writing time. These were 737 K, 0.98 K, and 24 K, respectively, for SnO GBM film and 685 K, 0.93 K, and 72 K, respectively, for TiO2 GBM film. These results for granular films can be useful for designing a hard disk drive with HAMR.

Temperature-Dependent Phase Evolution in FePt-Based Nanocomposite Multiple-Phased Magnetic Alloys

A quaternary Fe-Pt-Nb-B alloy has been fabricated by the melt spinning method with the purpose of the formation of crystallographically coherent multiple magnetic phases, emerging from the same metastable precursor, as well as to investigate the phase interactions and the influence of their coupling on magnetic performances. For this purpose, extended structural and magnetic investigations were undertaken by making use of X-ray diffraction, transmission electron microscopy, and 57Fe Mössbauer spectroscopy, as well as magnetic measurements using SQUID magnetometry. It was documented that intermediate metastable phases formed during primary crystallization, in intermediate stages of annealing, and a growth-dominated mode was encountered for the secondary crystallization stage upon annealing at 700 °C and 800 °C where fcc Fe3Pt and fct Fe2B polycrystalline were formed. The Mössbauer investigations have documented rigorously the hyperfine parameters of each of the observed phases. The fcc A1 FePt phase was shown to exhibit a peculiar ferromagnetic transition, and this transition has been proven to occur gradually between 300 K and 77 K. The magnetic measurements allowed us to identify the annealing at 700 °C as optimal for obtaining good magnetic features. Coercive field dependence shows similarities to the random anisotropy model for samples annealed at 500 °C to 700 °C which are nanocrystalline. These results show good perspectives for use in applications where different magnetic states are required at different operating temperatures.

In-situ investigation of exchange bias property in FePt (L10)/FeCo bilayer with increasing FeCo thickness

Magnetization reversal of soft ferromagnetic (FM) FeCo layer coupled to hard FM FePt (L10) layer in FePt (L10)/FeCo) bilayer has been studied in-situ with increasing thickness of FeCo layer. During FePt(L10) phase formation, an in-plane magnetic saturation field (HMAX) is applied to keep the hard FePt(L10) layer in its remanent state. Magneto-optical Kerr effect measurements reveal the absence of hysteresis loops initially up to 4.8 nm thickness of FeCo layer while loops with significant shift, i.e. exchange bias (EB) opposite to the HMAX direction, were observed on further deposition. The EB is found to decrease with the increasing thickness of the FeCo layer. It is due to the combined effect of both magnetostatic coupling and interfacial exchange interaction, where the contribution of magnetostatic interaction to EB is almost constant, and interfacial exchange coupling exhibits an inverse relation. Furthermore, different nucleation mechanisms during magnetization reversal in the descending and ascending branches of hysteresis loops lead to asymmetry in magnetization reversal at the lower thickness of the FeCo layer. This asymmetry decreases with the decrease in the interfacial coupling as the thickness increases. Azimuthal angle dependence of exchange shift with respect to the direction of HMAX further revealed the unidirectional character of the EB.

Switching Diagram of Core-Shell FePt/Fe Nanocomposites for Bit Patterned Media.

In the current work, a core-shell type exchange coupled composite structure was constructed by micromagnetic simulation with a phase FePt core and an iron shell. Four types of switching loops with magnetic domain structure evolution were demonstrated. Based on the simulation results, a switching type diagram was constructed, which displays various hysteresis loops as a function of core radius and shell thickness. Furthermore, the effects of switching type and composite structure on the coercivity and remanent magnetization were predicted and discussed. This finding indicates that core-shell type FePt/Fe composite structure film has a large advantage in designing exchange-coupled bit patterned media to realize high-density storage devices at the nanoscale.

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.

Dry Reforming of Methane for Hydrogen Production Using Bimetallic Catalysts of Pt‐Fe Supported on γ‐Alumina

AbstractBiogas is a combustible composed mostly by methane (CH4) and carbon dioxide (CO2). Methane, the main component of biogas, is the responsible for the characteristics of the fuel itself. Greenhouse gases (GHG), including methane, along with global warming and depletion of fossil fuels have become issues of global concern. In the present work, Pt−Fe catalysts supported on γ‐Al2O3 varying the metal load were synthesized, characterized, and applied in the hydrogen production through dry reforming of methane. The synthesized catalysts were extensively characterized by analytical techniques such as X‐Ray diffraction (XRD), thermal gravimetric analysis coupled with differential thermal analysis (DTA/TGA), infrared spectroscopy with Fourier transform (FT‐IR), scanning electron microscopy (SEM), N2 physisorption, X‐ray photoelectron spectrometer (XPS) and high–resolution transmission electron microscopy (HR‐TEM). The results from the catalytic tests exhibited that methane conversion reached values up to 95 % for all studied catalysts. The maximum H2 selectivity was 68 % (973 K) and 55 % (1173 K), for the bimetallic catalysts 0.15 PFAc (0.5 % Pt‐0.15 % Fe wt/Al2O3) and 0.5 PFAc (0.5 % Pt–0.5 % Fe wt/Al2O3) respectively. XPS and HR‐TEM results confirmed the iron‐platinum phase (FePt). Catalyst characteristics such as specific surface area, average crystallite size and morphology showed to play a role in the methane reforming performance.

Magneto-optical properties of two – layer film systems based on Fe and Pt

The paper shows the experimental results of the study of magneto - optical properties and phase composition of thin films and multilayers based on Fe and Pt. Samples were obtained in a high-vacuum chamber (10–8 Pa) by layer-by-layer deposition from pots at room temperature. It has been shown that FCC solid solution of FePt is formed already in the process of deposition on the substrate (sital plates). Depending on the concentration of the atoms of the components in the unannealed samples, three phases can be formed: (i) s.s. Fe(Pt); (ii) Fe3Pt; (iii) FePt. The first sign of the beginning of ordering in the FCC phase of FePt should be considered the appearance of superreflections in the form of lines (001) and (002) during heat treatment. Depending on the total thickness of the multilayer or individual layers, the annealing temperature at which extrareflexes appear can vary in the range of 300–570 K. All the samples obtained have a low coercivity (0.25 - 0.4 mT). Magneto-optical studies have shown that an increase in the content of the non-magnetic component decreases the main magnetic characteristics. However, In [Fe(3)/Pt(3)]n/S multilayers with the number of repeatable elements from 2 to 8 an increase in the main magnetic parameters is observed compared with bilayer films at the same effective thickness.

Improving the ordering and coercivity of L10-FePt nanoparticles by introducing PtAg metastable phase

Third element doping can accelerate ordering transformation and directly synthesize L10-FePt nanoparticles (NPs). However, this accelerating effect is limited due to the extremely low migration rate of Fe and Pt atoms. In this work, PtAg metastable phase is first introduced into chemical synthesis. Compared with the normal direct synthesis, the metastable phase method shows a much higher efficiency on promoting ordering transformation, and the FePt NPs present good dispersity, ultra-high ordering and coercivity up to 8600 Oe. The PtAg metastable phase is easy to be decomposed to Ag and Pt. The obtained Ag atoms migrate to surface area of the NPs because of their higher self-diffusion coefficients and the smaller surface energy. Simultaneously, the Fe atoms migrate into the NPs, and form the FePt phase. During the ordering diffusion of atoms, the mobility of Ag is much higher than that of Fe, which can generate a lot of vacancies in the lattice. These vacancies can obviously improve the migration rate of Fe and Pt atoms in the lattice, and powerfully promote ordering diffusion. Therefore, the introduced metastable phase can construct an optimal condition for the disorder-order transformation, and synthesize FePt NPs with highly ordering degree and good dispersity.

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.

The effect of Cu additions in FePt–BN–SiO2 heat-assisted magnetic recording media

Structural and chemical order impact magnetic properties of solids, which are governed by spin–orbit coupling and exchange interaction. The ordered L10 phase of FePt is a key material to heat-assisted magnetic recording; to enable high storage density, a solid understanding is needed of structural and chemical disorder at small length scales, as well as associated modifications of the electronic band structure. Here, we investigate the effect of boron and copper additions (≲6 mol% Cu) on structural and magnetic properties of L10 FePt granular media. Two copper-driven mechanisms, although competing, can lead to improvements in both structural and magnetic properties. In particular, the Cu substitution on the Fe-site leads to a degradation of magnetic properties due to the delocalized electron orbitals originating from a larger Cu d-orbital occupancy. At the same time, Cu substitution leads to an enhanced crystallographic order and consequently magneto-crystalline anisotropy, which offsets the former effect to a large extent. Our study is based on magnetometry, x-ray absorption spectroscopy, ab-initio calculations and a phenomenological theory of disordered FePt granular media. We do not observe a sizable modification to Fe moments and electronic configuration; Cu reveals two different resonances associated with the presence and absence of Cu–B bonds that vary with total Cu concentration.

Exchange-coupled nanocomposites with novel microstructure and enhanced remanence by a new approach

Exchange-coupled nanocomposites with hard/soft magnetic phases are promising for the next generation of permanent magnets. Chemical methods have an advantage in controlling the nanoscale size of both phases. The nanocomposites obtained by the chemical method generally consist of a hard phase core and a soft phase shell. However, the soft-phase shell is easily oxidized leading to small enhancement of remanence. Here, a novel microstructure of Fe@FePt nanocomposites with Fe soft phase core and FePt hard phase shell has been synthesized by replacement reaction, in which the size of core and shell can be controlled below 10 nm by adjusting the ratio of Fe nanoparticles to PtCl4. Excellent exchange-coupling (single-phase-like demagnetization curves) between soft-hard phases was observed due to the precise size control of both phases, and substantial enhancements both in remanence (32 %) and saturation magnetization (81 %) were obtained in optimal nanocompistes. This work provides an alternative routine to prepare heterostructure materials with various applications.

Magnetic Phase Coexistence and Hard-Soft Exchange Coupling in FePt Nanocomposite Magnets.

With the aim of demonstrating phase coexistence of two magnetic phases in an intermediate annealing regime and obtaining highly coercive FePt nanocomposite magnets, two alloys of slightly off-equiatomic composition of a binary Fe-Pt system were prepared by dynamic rotation switching and ball milling. The alloys, with a composition Fe53Pt47 and Fe55Pt45, were subsequently annealed at 400 °C and 550 °C and structurally and magnetically characterized by means of X-ray diffraction, 57Fe Mössbauer spectrometry and Superconducting Quantum Interference Device (SQUID) magnetometry measurements. Gradual disorder–order phase transformation and temperature-dependent evolution of the phase structure were monitored using X-ray diffraction of synchrotron radiation. It was shown that for annealing temperatures as low as 400 °C, a predominant, highly ordered L10 phase is formed in both alloys, coexisting with a cubic L12 soft magnetic FePt phase. The coexistence of the two phases is evidenced through all the investigating techniques that we employed. SQUID magnetometry hysteresis loops of samples annealed at 400 °C exhibit inflection points that witness the coexistence of the soft and hard magnetic phases and high values of coercivity and remanence are obtained. For the samples annealed at 500 °C, the hysteresis loops are continuous, without inflection points, witnessing complete exchange coupling of the hard and soft magnetic phases and further enhancement of the coercive field. Maximum energy products comparable with values of current permanent magnets are found for both samples for annealing temperatures as low as 500 °C. These findings demonstrate an interesting method to obtain rare earth-free permanent nanocomposite magnets with hard–soft exchange-coupled magnetic phases.

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.

Spin transfer and proximity effects in case of FePt (L1) nanoparticles coated with P3HT

Nanocomposites based on half-metallic FePt (L10) magnetic nanoparticles coated with the semiconducting conjugated polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) show a significant reduction in the magnetic coercivity. This study adopts a physical approach based on chemical potential equalization at the interface. The underlying charge/spin transfer mechanism unveils an imbalance: only spin-down polarized electrons are allowed to be transferred from the semiconductor to the half-metal (spin-down) conduction band, while spin-up states remain blocked at the interface. This process determines an excess of spin-up states on the P3HT side, and due to a RKKY mechanism, this effective spin system becomes ferromagnetic polarized. Due to this proximity effect, the conjugate polymer becomes exchange coupled to the hard magnetic FePt (L10) phase, thus reducing the coercivity of the half-metal. These processes make this type of composite suitable for magnetic recording applications.

Microstructure and magnetic properties of nanocrystalline Fe-Pt-based ribbons

The structural and magnetic properties of FePt – magnetic nanocomposites prepared by isothermal annealing of melt-spun Fe52Pt28Nb2B18 and Fe51Pt27Nb2B20 ribbons are reported. As it was evidenced the as-cast samples consists of L10-FePt hard magnetic grains dispersed in a predominant soft magnetic matrix composed of A1 FePt, Fe2B and boron-rich (FeB)PtNb residual phases. The applied annealing procedure reflects in temperature-induced structural and magnetic transition of dominated A1 FePt phase into L10-FePt dependent on Fe-Pt composition. As it was shown the Fe52Pt28Nb2B18 ribbon annealed at 700 °C exhibit promising hard magnetic properties at room temperature manifested as reasonably high remanence to saturation ratio (Mr/Ms = 0.68); coercivity (Hc = 0.92 T) and (BH)max = 83 kJ/m3 parameter. Strong exchange coupling between both FePt – based hard and soft magnetic phases was demonstrated by a smooth demagnetizing curve.

Structural and magnetic properties of FePt-Tb alloy thin films

We have studied structural and magnetic properties of ${(\mathrm{FePt})}_{1\ensuremath{-}x}\mathrm{T}{\mathrm{b}}_{x}$ thin films grown at elevated temperatures on MgO(100) substrates. The incorporation of Tb into the chemically ordered $\mathrm{L}{1}_{0}$ FePt lattice gives rise to considerable changes in the structural and magnetic properties, depending on the Tb content and the deposition temperature. When grown at $770{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, pronounced $L{1}_{0}$ ordering of the initial FePt phase is present. It vanishes gradually with Tb addition, thereby forming additional crystalline Tb-Pt phases at higher Tb content. These structural changes are accompanied by a strong reduction of the perpendicular magnetic anisotropy with increasing Tb content, finally resulting in a soft magnetic material with in-plane easy-axis magnetization. Extended x-ray absorption fine-structure measurements were performed to examine the possible incorporation of Tb into the FePt lattice. Based on the number of Pt first neighbors, the transformation from the initial tetragonal $L{1}_{0}$ structure to the chemically disordered fcc ($A$1) phase with increasing Tb content could be firmly concluded. In a further sample series, FePt was grown at a lower temperature of 530 \ifmmode^\circ\else\textdegree\fi{}C, which leads to reduced initial $L{1}_{0}$ chemical ordering. Adding Tb results in strong elastic stress in the lattice, eventually causing full amorphization of the film. Interestingly, in this case, a spin-reorientation transition from in-plane to out-of-plane magnetic easy axis is found at low temperatures, which is associated with an anisotropic chemical short-range order present in the amorphous phase. Furthermore, x-ray magnetic circular dichroism studies at the $\mathrm{Fe}\phantom{\rule{0.28em}{0ex}}{L}_{3,2}$ and Tb ${M}_{5,4}$ edges for all samples reveal strong antiferromagnetic coupling between Fe and Tb, resulting in a reduction of the net magnetization of the film.

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.

RETRACTED: Cryogenic ball milling synthesis of Bi2MoO6/FePt and Bi2MoO6/Pt composites and the comparison of their photocatalytic characteristics

Abstract Bi2MoO6 and FePt particles were synthesized by hydrothermal and wet chemistry methods, respectively. Bi2MoO6/FePt composites were synthesized by milling, and their mass ratio was controlled. The photocatalytic performance was evaluated by degrading an organic dye. When FePt mass is 10%, the composite presented the best photocatalytic performance. The photocatalytic mechanism investigation indicated that the highest photocatalytic performance was attributed to the prevented electron/hole recombination because of the existence of FePt nanoparticles in Bi2MoO6. The stability testing showed that Bi2MoO6/FePt-2 had relatively high reaction stability over reactions. The magnetic separation can be applied due to the presence of the magnetic phase of FePt in Bi2MoO6/FePt composites.

Superior Magnetic Performance in FePt L10 Nanomaterials.

The discovery of the high maximum energy product of 59 MGOe for NdFeB magnets is a breakthrough in the development of permanent magnets with a tremendous impact in many fields of technology. This value is still the world record, for 40 years. This work reports on a reliable and robust route to realize nearly perfectly ordered L10 -phase FePt nanoparticles, leading to an unprecedented energy product of 80 MGOe at room temperature. Furthermore, with a 3 nm Au coverage, the magnetic polarization of these nanomagnets can be enhanced by 25% exceeding 1.8 T. This exceptional magnetization and anisotropy is confirmed by using multiple imaging and spectroscopic methods, which reveal highly consistent results. Due to the unprecedented huge energy product, this material can be envisaged as a new advanced basic magnetic component in modern micro and nanosized devices.

Elaboration of Nanomagnet Arrays: Organization and Magnetic Properties of Mass-Selected FePt Nanoparticles Deposited on Epitaxially Grown Graphene on Ir(111).

The moiré pattern created by the epitaxy of a graphene sheet on an iridium substrate can be used as a template for the growth of 2D atomic or cluster arrays. We observed for the first time a coherent organization of hard magnetic preformed FePt nanoparticles on the 2D lattice of graphene on Ir(111). Nanoparticles of 2nm diameter have been mass selected in a gas phase and deposited with low energy on the hexagonal moiré pattern. Their morphology and organization have been investigated using grazing incidence small angle x-ray scattering, while their magnetic properties have been studied by x-ray magnetic circular dichroism, both pointing to a FePt cluster-graphene surface specific interaction. The spatial coherence of the nanoparticles is preserved upon annealing up to 700 °C where the hard magnetic phase of FePt is obtained.