Fabrication of L10 FePtAg nanoparticles and a study of the effect of Ag during the annealing process

L1₀ ferromagnetic phase FePt nanoparticles containing Ag atoms (FePtAg) were synthesized by means of a liquid phase process, followed by annealing. The addition of Ag to FePt nanoparticles permits annealing to be conducted at a lower temperature (350 °C). This is further accompanied by a subsequent transformation in the crystal phase from the FCC superparamagnetic phase to the FCT (L1₀) ferromagnetic phase. The effects of annealing temperature and the Ag atoms inside the nanoparticles on the magnetic properties of the FePt nanoparticles have been studied. Using electron spectroscopy for the chemical analysis (ESCA), Ag atoms in the L1₀ phase FePtAg nanoparticles were found to be localized on the surface region of the annealed nanoparticles. The Ag atoms function to inhibit the oxidation of FePt, causing the particles to become more stable and to have ferromagnetic properties.     

Monolayer deposition of L10 FePt nanoparticles via electrospray route

The deposition monolayers of L1₀ FePt nanoparticles via an electrospraying method and the magnetic properties of the deposited film were studied. FePt nanoparticles in a size of around 2.5 nm in diameter, prepared by a liquid process, were used as a precursor. The size of the deposited particles can be controlled up to 35 nm by controlling the sprayed droplet size that is formed by adjusting the precursor concentration and the precursor flow rate. The droplets were heated in a tubular furnace at a temperature of up to 900 °C to remove all organic compounds and to transform the FePt particles from disordered face centered cubic to an ordered FCT phase. Finally, the particles were deposited in the form of a monolayer film on a silicon substrate by electrostatic force and characterized by scanning electron microscopy. The monolayer of particles was obtained by the high charge on particles obtained during the electrospraying process. The magnetic properties of the monolayer were investigated by magneto-optic Kerr effect measurements. Coercivity up to 650 Oe for a film consisting of 35 nm L1₀ FePt nanoparticles was observed after heat treatment at a temperature of 800 °C.     

Stability Investigation of Colloidal FePt Nanoparticle Systems by Spectrophotometer Analysis

FePt magnetic nanoparticle systems are an excellent candidate for ultrahigh-density magnetic recording. Monodisperse FePt nanoparticles are synthesized by superhydride reduction of FECl2·4H2O and Pt （acac）2 at 263℃ under N2 atmosphere. Transmission electron microscopy （TEM） images show monosize EePt nanoparticles with diameter of 4 nm and a standard deviation of about 10%. The average distance between monodispesre particles is nearly 3 nm, and oleic acid and oleylamine surround the nanoparticles as surfactants. Stability investigation of nanoparticle colloidal solution is done via speetrophotometery analysis. The results for FePt nanoparticles dispersed in hexane indicate that adding surfactants with concentration of 3 × 10^-3 part by volume for centrifugation stage increases the stability of FePt nanoparticles solution with concentration of 16 mg/mL, about 67%.     

Structure and magnetism of near-stoichiometric FePd nanoparticles

We investigate from first principles the energetic order of single crystalline L1₀-ordered and multiply twinned morphologies of FePd nanoparticles close to the stoichiometric composition considering up to 561 atoms. The results are related to previous analogous calculations of FePt and CoPt nanoparticles. We find that compared to the isoelectronic FePt alloy, multiply twinned structures are slightly favored in energy, while the latent tendencies to form a layered antiferromagnetic structure in the L1₀ phase are less pronounced.     

Exchange‐coupled L10‐FePt/Fe composite patterns with perpendicular magnetization

L1₀‐FePt and exchange‐coupled L1₀‐FePt/Fe composite films are grown epitaxially on MgO(001) single crystal substrates and are subsequently large‐area patterned utilizing an electron beam lithography process with Ar⁺ ion etching. The patterning process of the continuous film system leads to a different demagnetization behavior resulting in an increase of the out‐of‐plane coercivity of the patterned samples. Due to exchange‐coupling between L1₀‐FePt and Fe the coercivity of the L1₀‐FePt/Fe composite patterns is reduced by 52% as compared to the coercivity of L1₀‐FePt patterns. From the analysis of the temperature dependence of the coercivity it follows that the dots include regions with reduced anisotropy. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of GePt buffer layer on magnetic properties and microstructure of FePt films

We have explored the interlayer diffusion effect of Ge/FePt, GePt/FePt bilayer on the formation of ordered L1₀ FePt phase. In Ge/FePt bilayer, the Ge₃Pt₂ compound was formed during post annealing at 400^oC for 1.0 h. Diffusion between Ge and FePt layer suppres the formation of ordered L1₀ FePt phase. With Ge₂Pt₃ underlayer, the FePt film was ordered at 400 °C and the in-plane coercivity was 9.3 kOe. The ordering temperature was reduced about 50 °C compared to the single layer FePt film.     

Quasi-Hexagonal Ordered Arrays of FePt Nanoparticles Prepared by a Micellar Method

Hexagonally ordered arrays of magnetic FePt nanoparticles on Si substrates are prepared by a self assembly of diblock copolymer PS-b-P2VP in toluene, a dip coating process and finally plasma treatment. The as-treated FePt nanoparticles are covered by an oxide layer that can be removed by a 40 s Ar＋ sputtering. The effects of the sequence of adding salts on the composition distribution are revealed by x-ray photoelectron spectroscopy measurements. No particle agglomeration is observed after 600℃annealing for the present ordered array of FePt nanoparticles, which exhibits advantages in patterning FePt nanoparticles by a micellar method. Moreover, magnetic properties of the annealed FePt nanoparticles at room temperature are investigated by a vibrating sample magnetometer.     

Magnetic viscosity effects in epitaxial L10 FePt thin films and exchange spring Fe–FePt bilayers

We have studied the thermal stability of L1₀ FePt thin films and Fe–FePt exchange-spring (ES) bilayers grown on (1 0 0) MgO by RF sputtering. The viscosity curves showed both for FePt films and bilayers a clear logarithmic decay of magnetization. Moreover, it was possible to evaluate the viscosity coefficient S for different applied reverse fields and the activation volumes at the coercivity. The latter values were then related to structural, magnetic and morphological measurements performed on the samples.     

Superparamagnetic FePt nanoparticles as excellent MRI contrast agents

Chemically disordered face-centered cubic (fcc) FePt nanoparticles (NPs) with a mean diameter of 9 nm were synthesized via pyrolysis of iron(III) ethoxide and platinum(II) acetylacetonate. The surface ligands of these NPs were then exchanged from oleic acid to tetramethylammonium hydroxide (TMAOH) to measure the longitudinal (T₁) and transverse (T₂) proton relaxation times of aqueous dispersion of FePt NPs. Magnetic resonance relaxometry reveals that TMAOH-capped FePt NPs have a higher T₂-shortening effect than conventional superparamagnetic iron oxide NPs, indicating that fcc-phase FePt NPs might be superior negative contrast agents for magnetic resonance imaging.     

The role of short exchange length in the magnetization processes of L10-ordered FePt perpendicular media

A three-dimensional micromagnetic model with non-uniform grain size distribution has been built up to study the magnetization process in FePt L1₀ perpendicular media. A 3D model of a single FePt magnetic grain is also set up for comparison. The high magneto-crystalline anisotropy K_u results in a short exchange length l_ex in FePt nanograins. Therefore a magnetic grain is divided into smaller grids on the order of l_ex. The simulated perpendicular and longitudinal loops are consistent with experiments, and it is explained why the measured perpendicular H_c is relatively smaller compared with the saturation field of the longitudinal loop in the FePt perpendicular medium.     

Exchange interaction effect of TbCo/FePt bilayers

Interlayer exchange coupling in dc-magnetron sputtered Tb_29.6Co_70.4/FePt bilayers with different annealing temperatures of the FePt film have been investigated. The dependence of ordering degree on perpendicular magnetic properties of the FePt film was studied. The Tb_29.6Co_70.4/FePt film has high perpendicular coercivity and high saturated magnetization about 7.5 kOe, and 302 emu/cm³, respectively as the substrate temperature is 500 °C and annealing at 500 °C for 30 min. It also shows a strong exchange coupling between this FePt layer and Tb_29.6Co_70.4 layer. We also examined the interface wall energy in the exchange coupled Tb_29.6Co_70.4/FePt double layers.     

Control of crystallographic orientation in L10 FePt films by using Cr and CrW underlayers

The microstructure and magnetic properties of FePt films grown on Cr and CrW underlayers were investigated. The FePt films that deposited on Cr underlayer show (2 0 0) orientation and low coercivity because of the diffusion between FePt and Cr underlayer. The misfit between FePt magnetic layer and underlayer increases by small addition of W element in Cr underlayer or using a thin Mo intermediate layer, which is favorable for the formation of (0 0 1) orientation and the transformation of FePt from fcc to fct phase. A good FePt (0 0 1) texture was obtained in the films with Cr₈₅W₁₅ underlayer with substrate temperature of 400 °C. The FePt films deposited on Mo/Cr underlayer exhibit larger coercivity than that of the films grown on Pt/Cr₈₅W₁₅ because 5 nm Mo intermediate layer depressed the diffusion of Cr into magnetic layer.     

Comparative evaluation of performance of percolated perpendicular media using FePt–MgO/Pt/Cr trilayer and FePt–MgO/MgO bilayer

(Fe₄₈Pt₅₂)_100−x–(MgO)_x films were used to examine the performance of a perpendicular percolated medium. Two underlayers, Pt(0 0 1)/Cr(0 0 2) and MgO(0 0 2), were used for comparison. The (Fe₄₈Pt₅₂)_100−x–(MgO)_x film with the MgO underlayer exhibits a strong preference to segregate at FePt grain boundaries. The microstructure with small closely packed MgO particles (2–4 nm) dispersed uniformly in the L1₀ FePt matrix was achieved in the Pt/Cr underlayered sample. Structural data reveal that the precipitate is crystallographically coherent with the surrounding L1₀ FePt phase and preserves good lattice alignment. Magnetic results indicate significant pinning behavior for those introduced non-magnetic columns with an enhanced coercivity of about 70%—much greater than that of the MgO underlayered samples. Percolated perpendicular medium can be realized in the FePt system and a Pt(0 0 1)/Cr(0 0 2) underlayer promotes the formation of pinning sites within the FePt grains.     

Magnetic properties and microstructure of x/FePt (x=Ge,GePt) bilayer

We have investigated the effect of Ge, GePt underlayers on the formation of ordered L1₀ FePt films. With Ge underlayer, the Ge₃Pt₂ compound was formed during post-annealing at 400 °C for 1 h. Interlayer diffusion of Ge and FePt layer suppress the formation of ordered L1₀ FePt phase. With Ge₂Pt₃ underlayer, the FePt film was ordered at 350 °C and the in-plane coercivity was 5.1 kOe. The ordering temperature was reduced to about 50 °C compared to the single-layer FePt film.     

Microstructural and magnetic studies of L10 FePt–SiO2 nano-composite thin film with columnar structure for perpendicular magnetic recording

(Fe₅₀Pt₅₀)_100−x-(SiO₂)_x films (x=0–30 vol%) were grown on a textured Pt(0 0 1)/CrRu(0 0 2) bilayer at 420 °C using glass substrates. FePt(0 0 1) preferred orientation was obtained in the films. Interconnected microstructure with an average grain size of about 30 nm is observed in the binary FePt film. As SiO₂ is incorporated, it precipitates as particles are dispersed at FePt grain boundaries. When the content of SiO₂ is increased to 13 vol%, columnar FePt with (0 0 1) texture separated by SiO₂ is attained. The FePt columns have a length/radius ratio of 2:1. Additionally, the mean grain size is reduced to about 13 nm. The development of this well-isolated columnar structure leads to an enhancement in coercivity by about 44% from 210 to 315 kA/m. As the SiO₂ content exceeds 20 vol%, a significant ordering reduction is found accompanied by a transformation of preferred orientation from (0 0 1) to (2 0 0) and the columnar structure disappears, resulting in a drastic degradation in magnetism. The results of our study suggest that isolated columnar, grain refined, (0 0 1)-textured FePt film can be achieved via the fine control of SiO₂ content. This may provide useful information for the design of FePt perpendicular recording media.     

Low-temperature ordering of L10 FePt phase in FePt thin film with AgCu underlayer

FePt (20 nm) films with AgCu (20 nm) underlayer were prepared on thermally oxidized Si (0 0 1) substrates at room temperature by using dc magnetron sputtering, and the films annealed at different temperature to examine the disorder–order transformation of the FePt films. It is found that the ordered L1₀ FePt phase can form at low annealing temperature. Even after annealing at 300 °C, the in-plane coercivity of 5.2 kOe can be obtained in the film. With increase in annealing temperature, both the ordering degree and coercivity of the films increase. The low-temperature ordering of the films may result from the dynamic stress produced by phase separation in AgCu underlayer and Cu diffusion into FePt phase during annealing.     

Perpendicular magnetic anisotropy of FePt film on Si substrate with SiO2 underlayer and B4C interlayers

FePt multilayer composite films with and without B₄C interlayer have been prepared by magnetron sputtering, respectively, and subsequent annealing in vacuum. It was found that the B₄C layers effectively serve as spacers to separate the FePt layers, enhancing (0 0 1) orientation of FePt alloy. Our results show that highly (0 0 1) oriented FePt/B₄C films have significant potential as perpendicular recording media.     

Field dependence of spin and orbital moments of Fe in L10 FePt magnetic thin films

The field dependence of spin and orbital magnetic moments of Fe in L1₀ FePt magnetic thin films was investigated using X-ray magnetic circular dichroism (XMCD). The spin and orbital moments were calculated using the sum rules; it was found that the spin and orbital moment of Fe in L1₀ FePt films are ∼2.5 and 0.2 μ_B, respectively. The relative XMCD asymmetry at Fe L₃ peak on the dependence of applied field suggested that the majority magnetic moment of L1₀ FePt films resulted from Fe.     

Underlayer diffusion-induced enhancement of coercivity in high anisotropy FePt thin films

The L1₀ ordered FePt films have been prepared at 300 °C with a basic structure of CrRu/MgO/FePt, followed by a post-annealing process at temperatures from 200 to 350 °C. The magnetic properties and the microstructure of the films were investigated. It is found that coercivity of FePt films increases greatly from 3.57 to 9.1 kOe with the increasing annealing temperature from 200 to 350 °C. The loop slope of the M–H curves decreases with the increasing annealing temperature, which is due to the grain isolation induced by MgO underlayer diffusion during the annealing process. The underlayer diffusion could be a useful approach to prepare the FePt-based composite films for high-density recording media.     

Microstructure and magnetic properties of nanocomposite FePt/MgO and FePt/Ag films

An in-plane magnetic anisotropy of FePt film is obtained in the MgO 5 nm/FePt t nm/MgO 5 nm films (where t=5, 10 and 20 nm). Both the in-plane coercivity (H_c∥) and the perpendicular magnetic anisotropy of FePt films are increased when introducing an Ag-capped layer instead of MgO-capped layer. An in-plane coercivity is 3154 Oe for the MgO 5 nm/FePt 10 nm/MgO 5 nm film, and it can be increased to 4846 Oe as a 5 nm Ag-capped layer instead of MgO-capped layer. The transmission electron microscopy (TEM)-energy disperse spectrum (EDS) analysis shows that the Ag mainly distributed at the grain boundary of FePt, that leads the increase of the grain boundary energy, which will enhance coercivity and perpendicular magnetic anisotropy of FePt film.