L10-FePt Nanoparticles Research Articles

Fabrication of Ultrafine L10-FePt-Based Magnetic Patterns Enabled by Single-Step Nanoimprint-Assisted Block Copolymer Self-Assembly.

The bit islands in magnetic patterns play a crucial role in advancing magnetic recording density, but the trade-off issues between miniaturization and scalable production are still challenging. Here we present a two-in-one technique of nanoimprint lithography (NIL)-assisted self-assembly using a specially engineered FePt-containing block copolymer (BCP), offering a simple one-step fabrication for L10-FePt bit-patterned media with high throughput. This method combines top-down NIL with bottom-up self-assembly to precisely control the ultrafine magnetic bits in the nanoscale patterns. The FePt-derived BCPs, designed with an equal atomic ratio of Fe and Pt, ensure the preparation of homogeneously dispersed L10-FePt nanoparticles (NPs). Subsequent BCP pyrolysis results in NPs with enhanced magnetic properties, significantly advancing high-density data storage devices. This strategy offers a novel approach to drive the innovative creation of ordered magnetic metal alloy NPs arrays for practical applications.

Temperature and size dependence of energy barrier for magnetic flips in L10 FePt nanoparticles: A theoretical study

L10 FePt nanoparticle is among the most promising materials for magnetic recording and nanomagnetic applications. In nanoparticles, the superparamagnetic effect, wherein the thermal fluctuation is comparable to the energy barrier for magnetic flips, strongly affects the stability of magnetic recording. The temperature dependence of the magnetic anisotropy constant in L10 FePt nanoparticles is a crucial factor for estimating the relaxation time of magnetic flips in a magnetic nanoparticle. However, comprehensive simulations of the atomic level for energy barrier for magnetic flips in the L10 FePt nanoparticles are lacking. This study entailed a simulation that quantitatively reproduced the size dependence of the Curie temperature of nanoparticles in the experimental studies. Moreover, the surface effect, wherein the magnetization decreases owing to the loss of magnetic exchange interaction pairs on the surface, was also clarified. The temperature and size dependences of energy barrier for the magnetic flips of L10 FePt nanoparticles were elucidated.

Optimizing Co content in intermetallic L10-FeCoPt nanoparticles to enhance the electrocatalytic performance

The L10-FeCoPt nanoparticles with ultra-small size, highly ordered structure, and various Co contents have been successfully obtained by annealing the wet-chemical synthesized nanoparticles. The addition of third-element Co into L10-FePt nanoparticles would increase the grain size, facilitate the disorder-order transition, and enhance the saturation magnetization and coercivity. The better permanent magnet properties and maximum electrocatalytic HER activity of L10-FeCoPt nanoparticles were achieved when added the moderate Co content (4.5 % and 8.5 %). The hydrogen evolution reaction (HER) kinetics and density functional theory revealed that the Co atom would promote the hybridization of Fe-3d and Pt-5d orbitals and increase the hydrogen adsorption free energy to closer to 0. Optimization of Co content to a suitable range would generate higher permanent magnetic properties and suitable surface energy, which facilitated the proton transformation and lowered the Tafel slope to follow the Volmer-Tafel process in HER. This work illustrates that optimizing third-element content in Pt-based magnetic intermetallic is a feasible strategy to operate the electrocatalytic reaction pathway and enhance the electrocatalytic performance.

Regulating the morphology and structures of wet-chemical synthesized L10-FePtMn nanoparticles by applying magnetic fields.

We report an effective strategy to prepare highly ordered L10-FePt nanoparticles (NPs) by magnetic-field-assisted wet-chemical synthesis. The Mn element with high magnetic susceptibility was chosen for third-element addition, which is beneficial to reduce the required magnetic field strength for ordering transition. The evolution of morphology, ordering degree, and magnetic properties of the FePtMn nanomaterials in the synthesis process were systematically investigated. The attachment of spherical FePtMn NPs was facilitated by applying magnetic fields. The particle size was refined only when the magnetic field strength was as high as 6 T. The ordering degree and coercivity of NPs have been improved and reached the peak values when the magnetic field was 2 T, which can be easily achieved by using an ordinary electromagnet. Therefore, this work provides a feasible route for regulating the morphology and structural ordering of L10-phase ternary alloy NPs.

Atomic-scale investigation on the origin of in-plane variants in L1-FePt nanoparticles embedded in a single-crystalline MgO matrix

In this work, we present an atomic-scale investigation of L10-FePt particles that are partly or fully embedded in a single-crystalline MgO matrix. Hundreds of particles in different orientations, of different sizes, and with different side contact facets are statistically and quantitatively analyzed. It is found that the presence of side contact facets does not introduce any misorientation in particles with the c axis out-of-plane (OP), even in those of small sizes. In addition, a markedly higher proportion of in-plane (IP) variants is found in smaller particles of sizes less than 10 nm, and most IP variants and the IP part in multi-variants have a large area of side contact facets or are even fully embedded. Those results can be explained by the fact that the transformation strain and interface strain jointly affect the particle orientation, and the competition between total strain energy in the film plane and in lateral planes plays a key role in determining particle orientation. Thus, a suggestion on the L10-FePt nanoparticle film production can be proposed that a moderate area of side contact facets may help keep a perfect OP orientation in the OP particles without increasing the proportion of IP variants. Additionally, our work can provide reference information on the variant orientation preference during a post-annealing process in nanoparticle films on a rough substrate or embedded in a matrix as well as in core–shell material systems.

Influence of Cu on the Improvement of Magnetic Properties and Structure of L10 FePt Nanoparticles.

L10 ordered FePt and FePtCu nanoparticles (NPs) with a good dispersion were successfully fabricated by a simple, green, one-step solid-phase reduction method. Fe (acac)3, Pt (acac)2, and CuO as the precursors were dispersed in NaCl and annealed at different temperatures with an H2-containing atmosphere. As the annealing temperature increased, the chemical order parameter (S), average particle size (D), coercivity (Hc), and saturation magnetization (Ms) of FePt and FePtCu NPs increased and the size distribution range of the particles became wider. The ordered degree, D, Hc, and Ms of FePt NPs were greatly improved by adding 5% Cu. The highest S, D, Hc, and Ms were obtained when FePtCu NPs annealed at 750 °C, which were 0.91, 4.87 nm, 12,200 Oe, and 23.38 emu/g, respectively. The structure and magnetic properties of FePt and FePtCu NPs at different annealing temperatures were investigated and the formation mechanism of FePt and FePtCu NPs were discussed in detail.

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.

Synthesis of super-fine L10-FePt nanoparticles with high ordering degree by two-step sintering under high magnetic field

Super-fine L10-FePt nanoparticles (NPs) with high ordering degree were successfully prepared by a modified two-step sintering method, which includes low-temperature pre-sintering, and the high magnetic field (HMF) assisted post-sintering processes. The particle size of the L10-FePt NPs was obviously refined by lowering the sintering temperature. By applying the HMF during the post-sintering process, the fine size characteristics of L10-FePt NPs were retained, and the ordering degree was significantly improved. The L10-FePt NPs with sizes of about 4.5 nm, ordering degree of 0.940, and coercivity of 22.01 kOe were obtained by this two-step sintering under a magnetic field of 12 T. The mechanism investigation of HMF enhancing the ordering degree indicates that the HMF enhances lattice distortion and magnetization energy (Zeeman energy). The enhanced lattice distortions cause high stress existing in the lattice, which can effectively promote the disordered-order transition. When the magnetic field reaches to 3 T, the Zeeman energy of the NPs is higher than the thermal disturbing energy of the NPs, and the magnetization effect is stronger. Therefore, the HMF (higher than 3 T) can obviously improve the disorder-order transition by lowering the energy barrier and accelerating the orderly diffusions of atoms. The HMF is a promising assistant to synthesize the L10-phase NPs with both of high ordering degree and super-fine size.

Direct Synthesis of L10-FePt Nanoparticles with High Coercivity via Pb Addition for Applications in Permanent Magnets and Catalysts

We report a promising strategy to directly synthesize L10-FePt nanoparticles for applications in permanent magnets and catalysts. The element Pb with lower melting point, smaller surface energy, an...

Dispersible and manipulable magnetic L10-FePt nanoparticles.

Oriented single-domain magnetic nanoparticles with a high remanence ratio Mr/Ms and maximum magnetic energy product (BH)max have attracted immense attention. However, nanoparticles easily agglomerate due to their extremely small size, which impedes the process of orientation. So manipulating the orientation of nanoparticles is still a key challenge. Here, L10-FePt single-domain nanoparticles were successfully synthesized by a chemical method in the liquid phase and nanoparticle-based anisotropic nanocomposites were obtained by dispersing the nanoparticles in liquid epoxy resin under an external magnetic field. The main factors that impact the orientation of L10-FePt single-domain nanoparticles were investigated further. It is found that the dispersibility of nanoparticles has a great impact on the degree of orientation, so do the applied magnetic field and the concentration of nanoparticles. Nanocomposites with homodisperse nanoparticles oriented under a suitable external magnetic field exhibit excellent magnetic performance, such as high coercivity Hc and remanence Mr, which gives the nanocomposites a higher (BH)max than the isotropic samples. The anisotropic nanocomposites show great potential in multifarious permanent magnet applications and fundamental research.

The texture ordering in L10-FePt\u2013Ag nanocomposites

L10-FePt nanoparticles with different Ag percentages were prepared. The presence of 6% Ag in co-sputtering process with deposition rate of 1 {dot{A}}/text{s} at 500 °C led to the formation of FePt nanoparticles with 5–7 nm size in L10-fct phase. Moreover, Ag atoms laying in boundaries of these nanoparticles in these circumstances caused c axis to orient and (001) peak intensity in XRD spectrum to increase. This is the result of the indirect exchange interactions of Fe magnetic atoms through Ag atoms in these boundaries. Therefore, where L10-FePt grains have the most boundaries with each other, the order index would be maximum.

Direct synthesis of L10-FePt nanoparticles from single-source bimetallic complex and their electrocatalytic applications in oxygen reduction and hydrogen evolution reactions

L10-FePt nanoparticles (NPs) with high chemical ordering represent effective electrocatalysts to reduce the cost and enhance their catalytic performance in fuel cells. A molecular strategy of preparing highly ordered FePt NPs was used by direct pyrolysis of a Fe, Pt-containing bimetallic complex. The resultant L10-FePt NPs had very high crystallinity as reflected by the obvious diffraction patterns, clear lattice fringes and characteristic X-ray diffraction peaks, etc. Besides, the strong ferromagnetism with room temperature coercivity of 27 kOe further confirmed the face-centered tetragonal (fct) phase in good agreement with the ordered nanostructures. The FePt NPs can be used as electrocatalysts to catalyze oxygen reduction reaction (ORR) in an O2-saturated 0.1 M HClO4 solution and hydrogen evolution reaction (HER) in the 0.5 M H2SO4 electrolyte with much better performance than commercial Pt/C, and showed quite high stability after 10,000 cycles. The strategy utilizing organometallic precursors to prepare metal alloy NPs was demonstrated to be a reliable approach for improving the catalytic efficiency in fuel cells.

Anisotropy field and dynamic coercivity of L10 FePt nanoparticles close to the Curie temperature

An investigation of the intrinsic and dynamic coercivity of L10 FePt nanoparticles close to but below the Curie temperature is presented using Langevin dynamics simulations based on an atomistic spin and also a Landau–Lifshitz–Bloch (LLB) macrospin model. The anisotropy field in the critical temperature regime is best defined as the minimum reverse field along the easy axis resulting in switching by spontaneous thermal fluctuations of the magnetization. The temperature dependence of the anisotropy field for the classical spin model is then given by the relation HK(T)≃HK(0)(1−T∕Tc)1−β, where β=0.366 is the critical exponent of the 3D Heisenberg model involved in the temperature dependence of the magnetization. The dynamic coercivity in the thermo-activated regime exhibits a logarithmic time dependence. No transition from elliptical to linear magnetic reversal is observed for 3nm FePt nanoparticles, when the temperature is increased beyond the Curie point, which is a finite size effect.

Comparing the structural and magnetic properties of FePt/MgO/Si and FePt/Ag/Si granular nanolayers

The properties of L10-FePt nanoparticles can be improved in the presence of MgO and Ag interlayers in direct sputtering and annealing method, respectively. Such properties are crystal and compound ordering, nanostructure and crystal orientation. In this work, FePt nanoparticles in ferromagnetic L10-fct phase were synthesized using sputtering method on Ag and MgO layers. According to XRD analyses, the impact of the presence of these two kinds of interlayer on crystal structure and its orientation has been investigated. Furthermore, the effect of the presence of 10% Ag on these properties has been studied and their granular layer nanostructures were characterized through the FE-SEM analysis. The results show that the presence of Ag as nanocompound and interlayer is desirable on declining the transition temperature and controlling the size during annealing. The presence of MgO as a sublayer in direct synthesis leads to the formation of 10 nm monosize smaller nanoparticles. According to VSM analysis, MgO and Ag sublayers have increased the magnetic coercivity of the FePt nanoparticles by 3.4 times and 3.7 times, respectively.

Model of ballistic-diffusive thermal transport in HAMR media

A model is presented that considers the grain size and temperature dependence of the thermal conductivity and specific heat of L10 FePt nanoparticles and is applied to study the temperature distribution within the top recording layer in heat assisted magnetic recording. Ballistic effects on the thermal conductivity are included assuming diffuse scattering of the electron heat carriers at interfaces and grain boundaries. The magnetic contribution to the specific heat during laser heating close to the Curie temperature is determined using Monte Carlo simulations based on the effective classical spin Hamiltonian of chemically ordered L10 FePt. Following the application of a nanosecond laser pulse, the perpendicular heat flow through a FePt/MgO/heat-sink stack is calculated using the finite element method. The temperature drop across the thickness of the FePt layer is shown to be significant for grains of small in-plane size. Size induced grain temperature variations are small due to the temperature dependence of the specific heat of FePt, so the resultant contribution to transition jitter is expected to be limited in extent.

Effect of Heating and Cooling Rates in Annealing for Preparation of L10-FePt Nanoparticles on Si Substrate

In order to obtain highly ordered L10-FePt nanoparticles for hard disk drive applications, the L10-phase transformation of chemically synthesized FePt nanoparticles deposited on a naturally oxidized Si substrate was investigated using rapid thermal annealing. The heating and cooling rates during annealing were changed logarithmically with a constant annealing temperature (800°C) and holding time (10 min). Almost completely ordered L10-FePt nanoparticles were confirmed by grazing incidence X-ray diffraction measurements, irrespective of the heating and cooling rates, and the amount of the silicide changed in response to both. Nearly pure L10-FePt was obtained when rapid heating (more than 780 K/min) and rapid cooling (more than 290 K/min) were applied. L10-FePt degraded into Fe3Si and PtSi when the cooling rate was lower than 7.8 K/min. Rapid heating as well as rapid cooling of FePt nanoparticles can provide a facile route for the high-throughput production of L10-FePt-based high-density magnetic recording media.

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.

Magnetization Switching of L10-FePt Nanoparticles by Combination Effect From Multishot Circularly Polarized Laser Pulses

All-optical magnetic switching observed on some materials provides a potential recording option due to its ultrashort recording time and low-power consumption. In most studies, this phenomenon is attributed to the inverse Faraday effect that a circularly polarized laser will induce an optomagnetic field or magnetic circular dichroism that one magnetization orientation will absorb more energy than another. L10-FePt is a promising candidate for high-density magnetic recording due to its high perpendicular anisotropy, whose magnetization is fixed out of plane either up or down. However, the mechanism about all-optical switching on FePt is still under debate. In this paper, we show that it benefits from both inverse Faraday effect and magnetic circular dichroism. We use an accumulative model to demonstrate the results of an ensemble of single-domain L10-FePt nanoparticles after multishot circularly polarized femtosecond laser pulses. The simulation results agree with the observation results of Lambert et al. and demonstrate the possibility of reaching deterministic all-optical magnetic recording by optimizing the laser fluence and the optomagnetic field strength.

Lithographic patterning of ferromagnetic FePt nanoparticles from a single-source bimetallic precursor containing hemiphasmidic structure for magnetic data recording media

Patterning of L10 FePt nanoparticles (NPs) with high coercivity offers a promising route to develop bit-patterned media (BPM) for the next generation magnetic data recording system, but the synthesis of monodisperse FePt NPs and mass production of their nanopatterns has been a long-standing challenge. Here, highly efficient nanoimprint lithography was applied for large-scale universal patterning, which was achieved by imprinting the solution of a single-source bimetallic precursor. The rigid coplanar metallic cores and the surrounding flexible tails in the bimetallic complex permit the spontaneous molecular arrangements to form the highly ordered negative morphology replicated from the soft template. In-situ pyrolysis study was then investigated by one-pot pyrolysis of the precursor under an Ar/H2 atmosphere, and the resultant NPs were fully characterized to identify the phase, morphology and magnetic properties. Finally, highly-ordered patterns on certain substrates were preserved perfectly after pyrolysis and could be potentially utilized in magnetic data recording media.

High magnetic coercivity of FePt–Ag/MgO granular nanolayers

L10-FePt ferromagnetic nanoparticles have a hight coercivity of Tesla order. Thus, these nanoparticles, with size of 10 to 15 nm and uniform surface distribution, are suitable in magnetic data storage technology with density of more than 1GB. In order to improve structural and magnetic properties of FePt nanoparticles, some elements and combinations have been added to compound. In this research, we show that due to the presence of the Ag, the phase transition temperature of FePt from fcc to L10-fct phase decreases. The presence of Ag as an additive in FePt–Ag nanocomposite, increases the magnetic coercivity. This nanocomposite, with 10% Ag, was deposited by magnetron sputtering on the MgO heat layer. VSM results of 10 nm nanoparticles show that coercivity has increased up to 1.4 T. XRD and FESEM results confirm that the size of the L10-FePt nanoparticles are 10 nm and their surface distribution are uniform. Ag gradually form nano scale clusters with separate lattice and FePt–Ag nanocomposite appears. The result of this process is emptiness of Ag position in FePt-fcc lattice. So, the mobility of Fe and Pt atoms in this lattice increases and it can be possible for them to move in lower temperature. This mechanism explain the effect of Ag on decreasing the transition temperature to fct-L10 phase, and hight coercivity of FePt nanoparticles.