FePt Particle Size Research Articles

Hard Magnetic FePt Nanoparticles within Nanostructured Silicon to Improve the Maximum Energy Product

In this work nanostructured silicon, silicon nanotubes (SiNTs) and porous silicon (PSi), with embedded hard magnetic FePt nanoparticles (NPs) is used as platform to create nanomagnet-arrays. The magnetic response of FePt-loaded composite materials is investigated, which have potential in high-performance magnets and as rare earth magnet alternatives. PSi/FePt demonstrates superior hard magnetic behavior with a higher coercivity and remanence compared to SiNTs/FePt. Varying the Fe molar ratio in deposits results in a small coercivity range. FePt-loaded samples consistently show increased coercivity and remanence compared to Co-loaded samples, with PSi exhibiting a stronger effect compared to SiNTs.The fabrication of the PSi samples was performed by anodization of p+ silicon in a 5 wt% HF solution. A current density of 50 mAcm-2 was applied during the etching. The samples offer an average diameter of 50 nm. The SiNTs were produced by using arrays of ZnO nanowires (NWs) as template, followed by Si deposition and finally etching off the ZnO NWs which is described in (1). FePt nanocrystals were formed within the SiNTs by a multistep process, consisting of soaking previously APTES functionalized SiNTs in a solution containing 0.1 mM Citric acid, 0.5 mM of H2PtCl6 6H2O, and depending on the given ratio 0.5-1.5 mM of Fe(NO3)3 9H2O. The samples were dried in a vacuum oven, followed by a heat treatment at 500° C for 1 hr in Ar atmosphere, then the samples were washed several times with water and ethanol. In a similar way FePt NPs were formed in PSi samples, however PSi did not receive the APTES functionalization before the solution soaking.For these samples, the average FePt particle size is 5 (± 2) nm in SiNTs samples and 9 (± 4) nm for the particles inside PSi, however it should be pointed out that the FePt ratios calculated from reactants was different than the ratios determined from EDX experimental data. The calculated stoichiometry of Pt:Fe (based on reactant concentrations) of 1:1, 1:3 and 1:6 was experimentally determined values via EDX to be 10:1, 1.6:1 and 0.5:1 for SiNTs; for the corresponding PSi samples of calculated 1:3 and 1:6 ratios was experimentally 40:1 and 10:1.Magnetization measurements were carried out by VSM in recording the magnetic response dependent on the magnetic field. Samples with different Fe content (Pt:Fe = 1:1, 1:3, 1:6) offer a variation of the coercivity in the range of 5% for both types of matrices (SiNTs, PSi). Furthermore the investigations show that the coercivities of PSi loaded with FePt NPs is about twice the coecivities of SiNTs loaded with FePt NPs. SiNTs/FePt samples with a ratio Pt:Fe 1:6 offer a coercivity of about 240 Oe and PSi/FePt offers a coercivity more than twice as high of about 560 Oe.Comparing the FePt loaded samples with samples loaded with Co NPs, in both sample types an increase of the coercivity is observed for FePt. Also in the case of Co-loading the utilization of PSi as template offers higher coercivities compared to SiNTs as template. From the investigated composite systems the ones consisting of PSi and FePt offer the highest energy product.(1) Huang, X., Gonzalez-Rodriguez, R., Rich, R., Gryczynski, Z., Coffer, J.L., Chem. Comm. 2013, 49, 5760.

FePt: LaSrFeO4 nanocomposite film prepared by magnetron and pulsed laser co-deposition method

The extensive application of a multifunctional composite film of metal nanoparticles embedded in a dielectric matrix is remarkable. Due to the oxidation susceptibility of atomic iron, it is difficult to directly fabricate embedded Fe nanoparticles in an oxide dielectric matrix with a Fe target. Because La0.5Sr0.5FeO3 can be decomposed into LaSrFeO4 and α-Fe, ferromagnetic and optical absorption FePt:LaSrFeO4 composite films were successfully fabricated at room temperature using Pt and La0.5Sr0.5FeO3 as targets via magnetron and pulsed laser co-deposition. Further annealing was performed on the FePt:LaSrFeO4 composite films. FePt is chemically disordered face-centered cubic phase (A1 structure) below 600 oC annealing and chemically ordered face-centered tetragonal phase (L10 structure) above 600 oC annealing. With increasing the annealing temperature, a redshift of the absorption peak from 256 to 285 nm is seen, which can be attributed to the size growth of the LaSrFeO4 and FePt nanoparticles in the composite films. The strongest absorption peak at around 269 nm for the 600 oC annealed sample can be attributed to the high crystallographic quality and relatively tiny grain size of FePt particles with uniform distribution. FePt:LaSrFeO4 composite films are ferromagnetic and optically absorbent, making them suited for multifunctional applications. Furthermore, magnetron and pulsed laser co-deposition techniques have the potential to produce multiferroic oxides, in which the coupling of different ordering processes e. g. ferroelectricity and ferromagnetism provides the potential for a new breed of multifunctional devices.

アークプラズマ蒸着法による複合磁性ナノ粒子の作製

In this study, magnetic nanoparticles were prepared by the arc plasma deposition (APD). The monodisperse Fe-Pt nanoparticles (5~10 nm) were obtained on the surface of SiO2 nanoparticles. Deposition voltage and shot counts can control particle size of Fe-Pt. The coercivity of Fe-Pt/SiO2 composite particles increased after annealing at 300°C, and the maximum coercivity value reached to 583 kA·m−1. In the selected area diffraction pattern obtained by transmission electron microscopy (TEM), amount of the order L10 phase was increased by annealing at 300°C. The Mn-Bi nanoparticles (5~20 nm) were also prepared by APD. The Mn-Bi was deposited on Fe nanoparticles, which were prepared by hydrogen-plasma-meta-reaction (HPMR) method, and monodisperse Mn-Bi particles were obtained on the Fe nanoparticles. Consequently, the arc plasma deposition is effective process to prepare composite magnetic nanoparticles.

Optical properties of L10 FePt nanoparticles dispersed in a C matrix

Optical properties of L10 FePt nanoparticles dispersed in a C matrix have been studied by using an approach combining spectroscopic ellipsometry and intensity transmission. Dielectric functions of FePt nanoparticles over a wavelength range of 380–1700 nm have been determined based on the Bruggeman effective medium approximation theory. Via analysis using the Drude–Lorentz model, the extracted optical properties provide electronic information of FePt nanoparticles. As compared with the thin film composed of larger grains, decrease of free electron absorption is observed in FePt nanoparticles, and this absorption decrease causes dielectric suppression in the near-IR photon energy range. And it can be due to the increased surface scattering and reduced unscreened plasma frequency in nanoparticles. On the other hand, two interband electronic transitions located at 0.7 and 1.0 eV are derived, which are in good agreement with the reported density of states of L10 FePt. In the present work, they show only a small dependence on FePt particle size. Such a study of optical properties of FePt nanoparticles is important for technological applications and fundamental physics.

Synthesis and characterization of chemically ordered FePt magnetic nano-particles

Monodispersed FePt alloy magnetic nano-particles are prepared by reduction of platinum acetyl acetonate and iron acetyl acetonate salts together in the presence of oleic acid and oleyl amine stabilizers by polyol process. The particle size of FePt is in the range of 2–3 nm confirmed by transmission electron microscopy (TEM). As-synthesized FePt nano-particles are chemically disordered with face centre cubic (fcc) structure where as after vacuum annealing these particles changed to face centre tetragonal (fct) ordered structure confirmed by the X-ray diffraction technique. Magnetic coercivity of 5.247 KOe was observed for fct structure.

FePt-C nanogranular films for perpendicular magnetic recording

We report the study of L10 ordered FePt-C nanogranular perpendicular anisotropy films and a media consisting of an 8 nm thick FePt-C hard magnetic layer covered with a 2 nm thick soft magnetic layer (CoFe and FePt-C). All the films were fabricated on oxidized Si substrates with a (100) textured polycrystalline MgO intermediate layer of 10 nm in thickness. Cosputtering of Fe, Pt, and C on the MgO layer results the formation of L10 ordered FePt nanogranular films with a strong c-axis texture and the average FePt particle size of about 5 nm with a narrow particle size distribution of 1.4 nm in standard deviation. Perpendicular coercivity is controllable between 7 and 12 kOe. The coercivity and the saturation field of the FePt-C granular films can be reduced by 55% and 38%, respectively, with the capping of thin soft magnetic layer. This study demonstrates that the FePt-C system accomplishes suitable nanogranular structures and magnetic properties for ultrahigh-density perpendicular recording media.

Novel Nanohybrids Derived from the Attachment of FePt Nanoparticles on Carbon Nanotubes

Multiwalled carbon nanotubes (MWCNTs) were used as nanotemplates for the dispersion and stabilization of FePt nanoparticles (NPs). Pre-formed capped FePt NPs were connected to the MWCNTs external surface via covalent binding through organic linkers. Free FePt NPs and MWCNTs-FePt hybrids were annealed in vacuum at 700 °C in order to achieve the L10 ordering of the FePt phase. Both as prepared and annealed samples were characterized and studied using a combination of experimental techniques, such as Raman and Mössbauer spectroscopies, powder X-ray Diffraction (XRD), magnetization and transmittion electron microscopy (TEM) measurements. TEM measurements of the hybrid sample before annealing show that a fine dispersion of NPs along the MWCNTs surface is achieved, while a certain amount of free particles attached to each other in well connected dense assemblies of periodical or non-periodical particle arrangements is also observed. XRD measurements reveal that the FePt phase has the face-centered cubic (fcc) disordered crystal structure in the as prepared samples, which is transformed to the face-centered tetragonal (fct) L10 ordered crystal structure after annealing. An increase in the average particle size is observed after annealing, which is higher for the free NPs sample. Superparamagnetic phenomena due to the small FePt particle size are observed in the Mössbauer spectra of the as prepared samples. Mössbauer and magnetization measurements of the MWCNTs-FePt hybrids sample reveal that the part of the FePt particles attached to the MWCNTs surface shows superparamagnetic phenomena at RT even after the annealing process. The hard magnetic L10 phase characteristics are evident in the magnetization measurements of both samples after annealing, with the coercivity of the hybrid sample over-scaling that of the free NPs sample by a factor of 1.25.

Non-Linear Optical Properties of Zinc Oxide Nanowires

Multiwalled carbon nanotubes (MWCNTs) were used as nanotemplates for the dispersion and stabilization of FePt nanoparticles (NPs). Pre-formed capped FePt NPs were connected to the MWCNTs external surface via covalent binding through organic linkers. Free FePt NPs and MWCNTs-FePt hybrids were annealed in vacuum at 700 degrees C in order to achieve the L1(0) ordering of the FePt phase. Both as prepared and annealed samples were characterized and studied using a combination of experimental techniques, such as Raman and Mössbauer spectroscopies, powder X-ray Diffraction (XRD), magnetization and transmittion electron microscopy (TEM) measurements. TEM measurements of the hybrid sample before annealing show that a fine dispersion of NPs along the MWCNTs surface is achieved, while a certain amount of free particles attached to each other in well connected dense assemblies of periodical or non-periodical particle arrangements is also observed. XRD measurements reveal that the FePt phase has the face-centered cubic (fcc) disordered crystal structure in the as prepared samples, which is transformed to the face-centered tetragonal (fct) L1(0) ordered crystal structure after annealing. An increase in the average particle size is observed after annealing, which is higher for the free NPs sample. Superparamagnetic phenomena due to the small FePt particle size are observed in the Mössbauer spectra of the as prepared samples. Mössbauer and magnetization measurements of the MWCNTs-FePt hybrids sample reveal that the part of the FePt particles attached to the MWCNTs surface shows superparamagnetic phenomena at RT even after the annealing process. The hard magnetic L1(0) phase characteristics are evident in the magnetization measurements of both samples after annealing, with the coercivity of the hybrid sample over-scaling that of the free NPs sample by a factor of 1.25.

Easy control of the size and composition of FePt nanoparticles with Improved synthesis

A new synthesis of FePt nanoparticles with tunable size and composition has been developed. Unlike conventional synthesis methods with which it is difficult to simultaneously control the size and chemical composition of FePt nanoparticles, the new synthesis offers a convenient way to tune FePt nanoparticles with different sizes and compositions. The synthesis involves the simultaneous decomposition of Fe3(CO)12 and reduction of Pt(acac)2 in diethylene glycol. Fe3(CO)12 is a powder (nonvolatile) which is critical in governing the FePt particle size and composition. By varying the amount of surfactants and precursors ratio [Fe3(CO)12∕Pt(acac)2], FePt particles with tunable composition and particle size (2–8nm) can be obtained. After high temperature annealing (∼600°C), Fe50Pt50 nanoparticles are partially transformed to the L10 phase as indicated by the soft and hard components in the hysteresis loops. The coercivity ranges from a few kilo-Oersted to more than 15kOe, depending on the particle size. Dynamic coercivity measurements reveal that the large FePt particles are easier to chemically order than the small FePt particles under the same annealing condition.

Epitaxially grown L1 0-FePt/(C, SiO 2, and Al 2O 3) granular films

Additive effects of C, SiO 2, and Al 2O 3 on L1 0-FePt epitaxial thin film were studied. Samples were fabricated by co-sputtering of FePt and (C, SiO 2, and Al 2O 3) on MgO (1 0 0) substrate at 973 K. These additives effectively reduce the particle size of FePt. C additive deteriorates the crystal orientation, while Al 2O 3 additive gradually degrades the L1 0 ordering. On the other hand, SiO 2 additive can reduce the particle size while keeping the high c-axis orientation and higher degree of L1 0 ordering.

New Non-Volatile Memory with Magnetic Nano-Dots

We proposed a new non-volatile memory with magnetic nano-dots (MND) dispersed in an insulating film as charge retention layer. We evaluated fundamental characteristics of FePt magnetic nano-dot film which is employed in such new magnetic nano-dot memory. We successfully formed a highly ordered L10 phase face-centered tetragonal structured FePt nano-dot (< 4.2nm) films on Si3N4 (5nm)/SiO2 (10nm)/silicon substrates and SiO2(10nm)/ silicon substrate by using SAND method. The uniformity of FePt particle size is dramatically improved by introducing a Si3N4 buffer layer on SiO2/Si substrate. In addition, it is found that FePt nano-dot film formed on Si3N4 (5nm)/SiO2(10nm)/Si substrate has a large coerce-ivity of ~22 kOe at room temperature.

Structure and magnetic properties of N-doped L10-ordered FePt–C nanocomposite films

As-deposited and annealed N additive FePt–C nanocomposite films fabricated using facing-target sputtering were investigated systemically. Structure analyses reveal that the as-deposited films are composed of ∼3nm amorphous metal granules buried in a-CN matrix, and transform into L10-ordered FePt particles followed by an increase of particle size after being annealed. The N atomic fraction increases, and the size of FePt particles in the 650°C annealed films reduces, with increasing N2 partial pressure (PN). The diffusion of Fe and Pt atoms into the positions of the ordered L10 lattice was improved by the decompounding of Fe–N bonds and the escape of N atoms from the film during the annealing. Magnetic measurements indicate that high-temperature annealing turns the superparamagnetism (at PN&amp;lt;10%) and paramagnetism (at PN⩾10%) of the as-deposited films into ferromagnetism. The coercivity of the 650°C annealed N-doped films decreases with increasing PN due to the particle size reduction.

High Coercivity of Ordered Macroporous FePt Films Synthesized via Colloidal Templates

The preparation of a three-dimensionally (3D) ordered macroporous iron-platinum (FePt) film derived from monodisperse FePt nanoparticles (approximately 3 nm in diameter) and polystyrene latex particles (254 nm in diameter) is described. The prepared film has a hexagonally ordered porous structure and coercivity up to 10 kOe after annealing at a temperature of 600 degrees C. We also found that size of FePt particles was maintained at around 3 nm, even after annealing at a temperature of 600 degrees C.

Structure and magnetic properties of FePt/Ag nano-granular films with perpendicular magnetic anisotropy

［FePt 2 nm/Ag d nm］10 (d=0—11) multilayers were prepared on single crystal MgO(100) substrates kept at room temperature and 250 ℃ by means of magnetosputtering. L10-FePt/Ag granular films with perpendicular magn etic anis otropy and high coercivity have been obtained after vacuum annealing the multila yers deposited on substrates kept at 250 ℃ for 15 min. X-ray diffraction result s indicate that FePt particles have a strong ［001］ orientation. The size of Fe Pt particles decreases with the increase of the thickness of Ag layer. After vac uum annealing the ［FePt 2 nm/Ag 9 nm］10 films at 600 ℃ for 15 min, FePt /Ag granular films are obtained with only 8 nm of particle size and 692 kA/m of coercivity. Isothermal magnetization remanence and dc-demagnetization remanence measurements show that no magnetic exchange coupling between magnetic particles can be observed. Such a kind of films is suitable for future ultra-density perp endicular magnetic recording medium.

Nonmagnetic shell in surfactant-coated FePt nanoparticles

Self-assemblies of chemically-synthesized 3 nm FePt nanoparticles show a dramatic increase of the saturation magnetization from about 210 emu/cm3 in the as-prepared state to about 850 emu/cm3, when annealed above 400 °C for 30 min. This increase correlates directly with the decomposition of the surfactant coating above 400 °C. The low magnetization state is attributed to the formation of a nonmagnetic shell (dead layer) due to oxygen bonds of the polar end group surfactant of oleic acid/oleylamine with Fe. Magnetic analysis in the superparamagnetic regime suggests a 2.7-nm-diameter FePt particle size with core–shell structure of 1.7-nm-diameter FePt magnetic core and ∼0.5 nm nonmagnetic shell.

Thickness dependence of coercivity in FePt/C multilayers

A strong dependence of FePt particle size on the number of layers, N in pseudo multilayer precursors is observed for small N. The relationship between coercivity and the microstructure is discussed.

計算状態図と材料設計への展開 Ｐｈａｓｅ‐ｆｉｅｌｄ法に基づくＦｅＰｔ微粒子規則化に対するサイズ依存性の解析

The FePt nano-particle structure has been considered to be a candidate for the next generation of high-density recording media due to its large magnetocrystalline anisotropy. The large magnetocrystalline anisotropy originates in L10 ordered structure of the FePt phase. However, it has been reported in recent investigation that disordering takes place when the FePt particle size is smaller than a critical size for ordering. In this study, the ordering of FePt nano-particle is modeled using the phase-field method, and the size dependence on the ordering of FePt nano-particle is analyzed based on this model. As a result, the disordering in the FePt particle proceeded rapidly when the particle size is less than the critical size for ordering. This phenomenon is due to an increase of the volume fraction of the disordered region at the vicinity of particle surface, when the particle size decreases.

Granular FePt–Ag thin films with uniform FePt particle size for high-density magnetic recording

The granular FePt–Ag thin films consisting of FePt particles dispersed in a Ag matrix were prepared by co-sputtering from FePt and Ag targets. The FePt particles in as-deposited film are soft magnetically disordered face-centered-cubic γ-FePt phase. The magnetic easy-direction of the as-deposited film is parallel to the film plane. The in-plane coercivities of the as-deposited films were below 700 Oe. The higher coercivity of the film will obtain after annealed treatment by transforming the γ-FePt phase into hard magnetically ordered face-centered-tetragonal (fct) γ 1-FePt phase. The in-plane coercivity of the annealed film was much larger than out-plane coercivity. The particle size of FePt in annealed film was increased with annealing temperature but decreased with increasing Ag content. The (FePt) 70–(Ag) 30 film with film thickness of 10 nm annealed at 600 °C for 15 min and ice–water quenched having an in-plane coercivity around 3.5 kOe. The M s value of this film is about 350 emu cm −3 and the squareness is about 0.8. The FePt particle size in this film was very uniform and the FePt particles were isolated by Ag.

Microstructure and coercivity of granular FePt–AlN thin films

(FePt) 100− x –(AlN) x ( x=0–50 vol%) granular films were prepared by DC and RF magnetron co-sputtering of FePt and AlN targets, then annealed in vacuum at various temperatures. Transmission electron microscopy analysis shows that the structure of the film is granular FePt particles embedded in amorphous AlN matrix and the FePt particle in as-deposited film is face-centered-cubic γ-phase and in the high coercivity annealed film is face-centered-tetragonal γ 1-phase. The particle size of FePt in annealed film was increased with the annealing temperature and film thickness but decreased with increasing AlN content. The magnetic anisotropy of the film was parallel to the film plane. The coercivity of the FePt–AlN film was dependent on the annealing temperature, the AlN volume fraction, and film thickness. It was found that the optimum annealing temperature for obtaining highest coercivity of the film is increased with AlN content. The in-plane coercivity of the film could reach to about 8 kOe for the (FePt) 70–(AlN) 30 film after annealing at 750°C for 30 min.

Magnetic properties and microstructure of FePt–Si3N4 nanocomposite thin films

( Fe 50 Pt 50 ) 100−x –( Si 3 N 4 ) x (x=0–50 vol. %) nanocomposite thin films are prepared by dc and rf magnetron cosputtering of FePt and Si3N4 targets on silicon wafer substrates, then annealed in vacuum at various temperatures. The effects of Si3N4 volume fraction, film thickness, and annealing temperatures on the magnetic properties are investigated. Transmission electron microscopy analysis indicated that structurally the film is an amorphous Si3N4 matrix with spherical FePt particles dispersed in it. The particle size of FePt increases with the annealing temperature but decreases with increasing Si3N4 content. Magnetization measurements indicated that maximum in-plane squareness and coercivity occurs at 30 vol. % of Si3N4 after annealing the film at 750 °C for 30 min. The average particle size of FePt in this film is about 40 nm. Saturation magnetization of the FePt–Si3N4 film is independent of film thickness but inversely proportional to the Si3N4 volume fraction. Variation of the films’ coercivity with film thickness is small. In contrast, the magnetic hardening mechanism and coercivity of the FePt–Si3N4 composite film are dependent on the Si3N4 volume fraction.