Ultra-high Density Magnetic Recording Media Research Articles

Effect of boron addition on the phase-transition temperature of CoPt-B nanoparticles synthesized by sol–gel autocombustion using sago starch as a chelating agent

In this research, we reported the effect of boron addition on the phase-transition temperature of CoPt-B nanoparticles using a sol–gel autocombustion method. Sago starch was used as a chelating agent. From the results, the suitable values of the sago starch concentration, boron concentration, and annealing temperature were determined to be 10% (w/v), 0.20 mol/L, and 500 °C, respectively. All the synthesized samples were characterized by the SEM, TEM, EDS, XRD, and VSM techniques. The XRD patterns of the synthesized CoPt-B nanoparticles demonstrated that the phase-transition temperature of CoPt-B changes from the disordered A1 phase to the ordered L10 phase at 500 °C. It can be concluded that the addition of boron could reduce about 100 °C of the ordering temperature of CoPt-B. Moreover, the VSM data confirmed that the obtained samples exhibited their ferromagnetism properties, indicating that the synthesized CoPt-B nanoparticles can be applied as ultrahigh-density magnetic recording media.

Analysis and Control of the Initial Electrodeposition Stages of Co-Pt Nanodot Arrays

We have fabricated Co-Pt nanodot arrays by combining electrodeposition with electron beam lithography (EBL) for applications in ultra-high density magnetic recording media, such as bit-patterned media (BPM). In this work, we analyzed the initial nucleation and growth of Co-Pt inside nanopores to achieve nanodot arrays with high deposition uniformity, as well as magnetic properties. At −900mV (vs. Ag/AgCl), multiple nuclei of 2.0–3.0nm in size were randomly distributed, even in nanopores with a 10nm diameter, which could result in a lack of uniformity in the magnetic properties. The number of nuclei was then reduced by applying a less negative potential (>−700mV vs. Ag/AgCl) to deposit a single nucleus inside each nanopore. As a result, a single grain of 5.0–10nm in size was successfully deposited inside the nanopore, which could induce uniform magnetic properties in each nanodot. In addition, at less negative potentials, the coercivity of the Co-Pt films increased, which was induced by the epitaxial-like growth of Co-Pt from the Ru substrate. Cross-sectional TEM analysis suggested that Co-Pt deposited with a less negative potential was single crystalline with uniform hcp lattice fringes in the direction perpendicular to the Ru interface, indicating the formation of highly uniform nanodot arrays with high perpendicular magnetic anisotropy.

ChemInform Abstract: Disorder—Order Transformations in Metallic Films

AbstractReview: 60 refs.

Physical properties of FePt nanocomposite doped with Ag atoms: First-principles study

L10 FePt nanocomposite with high magnetocrystalline anisotropy energy has been extensively investigated in the fields of ultra-high density magnetic recording media. However, the order—disorder transition temperature of the nanocomposite is higher than 600 °C, which is a disadvantage for the use of the material due to the sustained growth of FePt grain under the temperature. To address the problem, addition of Ag atoms has been proposed, but the magnetic properties of the doped system are still unclear so far. Here in this paper, we use first-principles method to study the lattice parameters, formation energy, electronic structure, atomic magnetic moment and order—disorder transition temperature of L10 FePt with Ag atom doping. The results show that the formation energy of a Ag atom substituting for a Pt site is 1.309 eV, which is lower than that of substituting for an Fe site 1.346 eV. The formation energy of substituting for the two nearest Pt sites is 2.560 eV lower than that of substituting for the further sites 2.621 eV, which indicates that Ag dopants tend to segregate L10 FePt. The special quasirandom structures (SQSs) for the pure FePt and the FePt doped with two Ag atoms at the stable Pt sites show that the order—disorder transition temperatures are 1377 °C and 600 °C, respectively, suggesting that the transition temperature can be reduced with Ag atom, and therefore the FePt grain growth is suppressed. The saturation magnetizations of the pure FePt and the two Ag atoms doped FePt are 1083 emu/cc and 1062 emu/cc, respectively, indicating that the magnetic property of the doped system is almost unchanged.

Effects of dimensionality and spatial distribution on the magnetic relaxation of interacting ferromagnetic nanoclusters: A Monte Carlo study

The magnetization dynamics of one- or two-dimensional self-organized assemblies of interacting nanoclusters, which are potential candidates for ultra-high density magnetic recording media, was investigated using Monte Carlo simulations. The study was focused on the temperature variation of the ac-susceptibility as well as the decay over time of the magnetization of cobalt nanoparticles having the same anisotropy axis. Evidence was found of significant dipolar interaction effects on the peak position of the imaginary part of the ac-susceptibility and on the relaxation time due to the common easy axis and the low-dimensionality of the assemblies even for low concentration (less than 10%). With increasing the strength of the dipolar interactions, the peak of the out-of-phase component shifts towards higher or lower temperatures depending on whether the magnetic moments are oriented in the plane assembly or perpendicular to it. A peak shift towards higher (lower) temperatures is clearly linked to an increase (decrease) of the relaxation time. The relaxation time in the presence of dipolar interactions still follows an Arrhenius law with an effective energy barrier which is either larger or smaller than the anisotropy energy barrier. It is also shown that random positions slightly strengthen the influence of dipolar interactions.

FePtAg-C nanogranular film for ultrahigh density magnetic recording media

A FePtAg-C nanogranular film was studied for ultrahigh density perpendicular recording media with the density of 1Tbits/in2 or even higher because of the strong anisotropy at FePt L10-phase. In experiments, a Fe47Pt42Ag11-C45% (6.6nm) film was deposited on a silicon substrate with a MgO (12nm) interlayer at 500°C. The perpendicular coercivity of the film is 34kOe, with a unit squareness. Bright-field TEM images show that the FePtAg-C nanogranular film has small and uniform grains of 6.8±1.6nm. More work of high-resolution TEM imaging shows excellent L10-ordering for this film, consistent with the texture measurement by XRD. The measurement of remnant coercivity proves that it has an energy barrier of 310kBT at room temperature, meaning excellent thermal stability. As a result, FePtAg-C nanogranular film is a qualified candidate for high-density magnetic recording media.

Electrically conductive (Mg0.2Ti0.8)O underlayer to grow FePt-based perpendicular recording media on glass substrates

We have explored a new electrically conductive underlayer material to grow (001) textured L10-FePt films on glass substrates for potential application in ultrahigh density magnetic recording media. We found (Mg0.2Ti0.8)O (MTO) polycrystalline film grows with a strong (001) texture on Cr buffer layer, which induces strong (001) texture of L10-FePt polycrystalline and granular films. Strong perpendicular anisotropy of 3.8 × 107 erg/cm3 and coercivity of 15 kOe of a FePt-C film has been demonstrated using the MTO underlayer on glass substrates.

Phase transition of FePt/Ag nanocomposite thin films: Kinetics, activation volume, and atomic processes

Understanding the L10 phase transformation of FePt nanocrystals in FePt:X (X = nonmagnetic materials) nanocomposite thin films is of particular importance for controlling and designing these films as ultrahigh density magnetic recording media. In this study, by employing temperature- and pressure-dependence of the rate constant of phase transition, the nucleation (En = 0.3 eV) and growth (Eg = 0.9 eV) activation energies of L10 ordered domains, as well as an activation volume ΔV* ≈ 0.96 Ω (Ω = average atomic volume) for L10 ordering transition in FePt:Ag nanocomposite thin films have been successfully determined. These data suggest that the growth of L10 ordered domains is predominantly dependent on the diffusion of Fe atoms in FePt crystals, and that the diffusion of Ag atoms out of FePt lattice prompts the nucleation of the L10 ordered domains. These findings have a direct implication for designing FePt:Ag nanocomposite thin films with high pack fraction of L10-FePt nanocrystals.

Microstructure and Magnetic Properties of Nano-Island CoPt Thin Films

Abstract The separated CoPt nano-size islands were fabricated on amorphous glass substrates in this work. The microstructures and magnetic properties of CoPt thin films with thicknesses between 1 and 20 nm after annealing at 700 °C for 30 minutes were investigated. The morphology of CoPt thin film would change from a well-distributed and discontinuous nano-size CoPt islands into a continuous one as increasing the film thickness from 1 to 20 nm. The formation mechanism of the CoPt islands may be due to the surface energy difference between the glass substrate and CoPt alloy. This nano-size CoPt thin film may be a good candidate for ultra-high density magnetic recording media.

Effects of Os Inserted Layers on the Microstructures and Magnetic Properties of the FePt Films

The microstructure and magnetic properties of multilayer [Os(t)/FePt(x)]n films on a glass substrate with a 10 nm Os buffer layer by ion beam sputtering have been studied as a function of the annealing temperatures between 300 and 800 degrees C. Here, t = 0.2, 1 or 5 nm and x varied from 10, 20, 25, 50, to 100 nm with its associated n value of 10, 5, 4, 2, and 1, respectively. No diffusion evidence was found in samples with a thin Os layer and t > or = 1 nm. The average grain size of the multilayer films can be well controlled by both annealing temperature and thickness of the FePt layer by a very thin Os space layer with t > or = 1 nm. The enhancement of H(c) can be understood from the fact that for a FePt film with an Os spacer layers, the increasing number of Os layer will inhibit the grain growth of FePt grains and enriches the grain boundary. We have experimentally demonstrated that even with a very thin 1 nm Os spacer layers, the [Os(t)/FePt(x)]n multilayer films can exhibit good hard magnetic properties and are attractive candidates for ultrahigh density magnetic recording media.

The effect of reflux process on the size and uniformity of FePt nanoparticles

FePt nanoparticles attract great research interest for possible application to ultrahigh density magnetic recording media. In this paper, FePt magnetic nanoparticles were synthesized by superhydride reduction of FeCl 2 and Pt(acac)2 at high temperature. Adding superhydride (LiBEt3H) to the phenyl ether solution of FeCl2 and Pt(acac)2 in the presence of oleic acid, oleylamine, and 1,2-hexadecanediol at 200 °C, followed by refluxing at 250 °C, led to monodisperse 3 nm FePt nanoparticles. The effect of reflux process on the size and uniformity of FePt nanoparticles has been investigated. TEM images showed that the size of FePt nanoparticles increase to 4 nm with reflux process and the standard deviation of FePt nanoparticles increase to 10 % which lead to improve the uniformity of FePt nanoparticles. The results of EDS analysis, showed that the Fe shell around FePt nanoparticles increase with increasing reflux time and the composition of FePt nanoparticles gives Fe68Pt32 after 10 min reflux..

SYNTHESIS OF COBALT NANOTUBULES AND CHARACTERIZATION OF MAGNETIZATION DYNAMICS

Magnetic nanowire arrays formed by templated electrodeposition are a promising way of making ultra-high density magnetic recording media. We have used both Poly Carbonate Track Etched (PCTE) and Anodic Aluminum Oxide (AAO) membranes to obtain Cobalt nanowires. Also by suitably adjusting the electrodeposition process, hollow Cobalt nanotubules were formed. The hollow cylindrical structure gives a unique capability to tailor their magnetization. The morphology of the samples was investigated using SEM and TEM. Magnetic characterization done using SQUID shows the influence of morphology and that the nanotubules have rather different switching behaviour than solid nanowires. Finite element micromagnetic modeling was performed of a single nanowire, a single nanotubule and also arrays of nanotubules. The calculated static M-H curves were found to match closely the SQUID measurements. The coercivity increases with length while the inner and outer diameters greatly affect rotational switching of the magnetization.

Thermal-vacancy-assisted phase transition in FePt thin films

Understanding the ordering transition from A1 to L10 structure in FePt thin films is of great significance for developing L10-FePt films as ultrahigh density magnetic recording media. Here, the L10-ordering transition of FePt films has been investigated based on activation volume measurements. A large activation volume ΔV∗=10–11 Å3=(0.75–0.8) Ω, where Ω is average atomic volume of FePt, is determined for atomic diffusions in the L10-ordering transition, indicating a thermal-vacancy-assisted phase transition. This transition is suggested to be predominantly dependent on the diffusion of Fe atoms. These findings have direct implications for yielding L10-FePt films at low temperatures and optimizing their microstructures.

Effects of Ag layers on the microstructure and magnetic properties of CoPt/BN multilayer films

Ag/[BN/CoPt]5/Ag and [BN/Ag/CoPt]5/Ag thin films were deposited on glass substrates by magnetron sputtering and then annealed in vacuum at 600°C for 30 min. The structures and magnetic properties of CoPt/BN multilayer films were investigated as a function of Ag layer thickness. It was found that the face-centered tetragonal (fct) (001) texture of CoPt was improved greatly by introducing the Ag toplayer or sublayer together with an Ag underlayer. Good (001)-oriented growth, low intergrain interactions as well as high perpendicular anisotropy can be obtained in the Ag(3 nm)/[BN(2.5 nm)/CoPt(3 nm)]5/Ag(7 nm) and [BN(2.5 nm)/Ag(2 nm)/CoPt(3 nm)]5/Ag(10 nm) films, which become potential candidates for ultrahigh density magnetic recording media.

Light-Induced Covalent Immobilization of Monolayers of Magnetic Nanoparticles on Hydrogen-Terminated Silicon

Specifically tailored ω-alkenyl-1-carboxylic acids were synthesized for use as surfactants in the single-step preparation of manganese ferrite (MnFe2O4) nanoparticles (NPs). Monodisperse manganese ferrite NPs terminated with ω-alkenyl moieties were prepared via a one-pot reaction at high temperature without the need of ligand exchange. Using this approach, simple adjustment of the rate of heating allowed precise tuning of the size of the nanoparticles, which were characterized in bulk form by transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD). These surfactant-coated magnetic nanoparticles were then deposited onto hydrogen-terminated silicon(111) wafers and covalently anchored to the surface by UV-initiated covalent bonding. Analysis by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) confirmed that the UV treatment led to covalent immobilization of the NPs on the silicon surface with a consistent packing density across the surface. The magnetic properties of the stable, surface-bound nanoparticle arrays were characterized using a superconducting quantum interference device (SQUID) magnetometer. The materials and methods described here are being developed for use in bit-patterned ultrahigh density magnetic recording media and nanoscale biomagnetic sensing.

Effect of Thicknesses on the Microstructure and Magnetic Properties of CoPt Thin Films

The microstructures and magnetic properties of CoPt thin films with thicknesses between 1 and 20 nm deposited on amorphous glass substrate and post-annealing at 600°C for 30 min were investigated. The morphology of CoPt thin film would change from a discontinuous nano-size CoPt islands into a continuous film gradually as the film thickness was increased from 1 to 20 nm. The formation mechanism of the CoPt islands may be due to the surface energy difference between the glass substrate and CoPt alloy. Each CoPt island could be a single domain particle. This discontinuous nano-island CoPt recording film may increase the recording density and enhance the signal to noise ratio while comparing with the continuous film. The as-deposited 5 nm CoPt film revealed the separated islands morphology after annealing at 600°C for 30 min. This nano-size CoPt thin film may be a candidate for ultra-high density magnetic recording media due to its discontinuous islanded nanostructure.

Magnetic Properties of Multilayer ${[({\rm FePt})_{\rm x}/{\rm Os}]}_{\rm n}$ Films

Multilayer [(FePt)x/Os]n films (with x being thickness in nm; Os with a fixed thickness 5 nm; n being the number of layers) have been deposited by ion-beam sputtering. Spacer layer was found to have substantial effects on the multilayers to exhibit different magnetic properties and microstructure. The average grain size of the multilayer films can be well controlled by both annealing temperature and thickness of the FePt layer with the Os space layer. The enhancement of Hc can be understood from the fact that for a FePt film with fixed thickness of Os spacer layer, the increasing number of Os spacer layer will inhibit the grain growth of FePt grains and enriches the grain boundary. The layer by layer structure of [(FePt)x/Os]n films can control the hard magnetic behaviors. Therefore, the multilayer [(FePt)x/Os]n films with Os as the spacer exhibit good hard magnetic properties and are attractive candidates for ultrahigh density magnetic recording media.

Fabrication of magnetic nanodot array using electrochemical deposition processes

We investigated fabrication processes of magnetic nanodot arrays for the ultra-high density magnetic recording media by using an electrodeposition. A CoZrNb underlayer was sputter-deposited on a glass disk substrate as a soft magnetic underlayer (SUL). Nano-patterns were formed on the substrate by UV-nanoimprint lithography (UV-NIL) and CoPt was electrodeposited into the nano-patterns. For obtaining uniform CoPt nanodot arrays with high perpendicular coercivities, we applied thin Cu intermediate layer on CoZrNb SUL and minimized its thickness. As a result, we obtained CoPt nanodot arrays with 150-nm diameter, 300-nm pitches, and 20-nm heights, which have uniform structures, on the substrates with the construction of Cu (1–2 nm)/CoZrNb (100 nm)/Cr (5 nm)/glass disk. The perpendicular coercivity of the CoPt nanodot arrays was as high as 5.4 kOe. From these results, we showed that the Cu intermediate layer with even 1–2 nm thick considerably improved the deposition condition on the substrates with CoZrNb SUL to successfully fabricate CoPt nanodot arrays with the diameter and pitches of 80 nm and 160 nm with sufficient uniformity.

Magnetic behavior of chemically synthesized FePt–FeRh nanostructures

We describe here the chemical synthesis and magnetization behavior of FeRh–FePt nanostructures fabricated using a two-step procedure. The first step involved chemical synthesis of FeRh nanoparticles using a polyol co-reduction process accompanied by the second stage of encapsulation of FeRh nanoparticles with FePt. The FeRh–FePt nanoparticles were subsequently annealed in a salt matrix at 600 °C for 3 h. Magnetic measurements were made on films of FeRh–FePt nanostructures cast onto silicon wafers before and after the salt matrix annealing. The coercivity of as-synthesized FeRh–FePt nanostructures was ∼70 Oe at room temperature, while the coercivity of salt-matrix annealed FeRh–FePt nanoparticles was ∼7065 Oe. The temperature dependent magnetization measurement of annealed FeRh–FePt nanostructures indicated antiferromagnetic–ferromagnetic transition that is supported by X-ray diffraction study. The observations point toward the potential realization of FeRh–FePt nanostructures for ultra-high density magnetic recording media.

Self-assembled nano-size FePt islands for ultra-high density magnetic recording media

To find a method to form nano-size FePt alloy for ultra-high density magnetic recording media, this work concentrated on the formation mechanisms of nano-island FePt films on amorphous glass substrates. FePt films of different thicknesses (1–10 nm) were deposited on amorphous glass substrates and post-annealed at 700 °C for 10 and 30 min. The configuration of the film changed during the annealing process due to the surface energy difference between the glass substrate and FePt alloy. Investigation of the microstructures and magnetic properties of the ordered L1 0 FePt films revealed that the 1 nm FePt film annealed at 700 °C for 10 min had perpendicular magnetic anisotropy and good reproducibility of forming well-separated FePt nano-size islands for ultra-high density magnetic recording media.