High-density Magnetic Recording Media Research Articles

Thermal Modulation of Magnetization Dynamics in Nanometer-Thick L10-FePt Nanogranular and Continuous Films for High-Density Magnetic Recording Media

Thermal modulation of magnetism in magnetic recording media is a novel technique to increase the recording density. L10-FePt is a promising material for the media; however, experimental observation of its magnetization dynamics characterized by the Gilbert damping constant α is technically difficult because the dynamics is in the terahertz (THz) frequency range. A cutting edge technique in THz spintronics, an all-optical pump–probe method, leads us to achieve the detection of THz magnetization dynamics in nanometer-scale thin films with thermal heating. Here, thermal modulation of the magnetization dynamics for L10-FePt nanogranular and continuous films is discussed using the all-optical method. The effective damping constant for the nanogranular film decreased from 0.18 to 0.064 with increasing the temperature, and the thermal modulation decreased the extrinsic contribution, while this increased α above 530 K. This observation regarding the thermal modulation of the magnetic damping is valuable to design future high-density magnetic recording media.

Influence of Bi3+ ions substitution on structural, magnetic, and electrical properties of lead hexaferrite

The Microwave induced sol-gel auto-combustion process was used to make lead hexaferrite, PbFe12-xBixO19 (x=0.2 to 0.8), with Bi3+ as a replacement. X-ray powder diffractometer (XRD), scanning electron microscopy (SEM), vibrating sample Magnetometry (VSM), Fourier transform infrared spectroscopy (FT-IR), and Impedance analysis was used to test the impact of added Bi3+ ions on the structure, morphology, magnetic, and dielectric properties of lead hexaferrite. Single-phase hexagonal ferrite nanoparticles with average crystalline sizes of 43 to 59 nm were achieved with the space group of P63/mmc. The magnetic analysis revealed that when the Bi3+ ion is supplemented in the Pb ferrite,the coercive and saturation magnetization of the material was amplified. The substance's dielectric constant, electric conductivity, and loss tangent were calculated for frequencies of 20 Hz to 10 MHz, indicating that it is appropriate for wide applications like permanent magnets, high-density magnetic recording media.

Analysis of structural, electric and magnetic features of Bi3+ substituted nanocrystalline calcium hexaferrite

Calcium hexaferrite (CaFe12−xBixO19, x=0.2–0.8) was successfully brewed through a microwave-induced sol–gel auto-combustion technique with Bi3+ as a substitute. The impact of added Bi3+ ions on the structure, morphology, magnetic, and dielectric properties of calcium hexaferrite was analyzed using an X-ray powder diffractometer (XRD), Field emission scanning electron microscopy (FE-SEM), vibrating sample Magnetometry (VSM), Fourier transforms infrared spectroscopy (FT-IR), and impedance study. The peak position in the XRD pattern confirms the development of single-phase hexagonal ferrite nanoparticles with a space group of P63/mmc and an average crystalline size of 46–56 nm. Magnetic analysis revealed that when the Bi3+ ion is added to Ca ferrite, the material's coercive force improves, but the saturation magnetization decreases. The dielectric constant, electric conductivity, and loss tangent of the substance were determined for frequencies ranging from 20 Hz to 1 MHz, indicating appropriate for various applications, including permanent magnets and high-density magnetic recording media.

High-resolution alternating magnetic force microscopy using an amorphous FeB-based tip driven by an inverse magnetostrictive effect: Imaging of the high-density magnetic recording media

The improved spatial resolution of the alternating magnetic force microscopy (A-MFM) technique developed in this work is demonstrated using a tip coated with a very thin magnetostrictive Fe-B-based amorphous film. Its magnetization can be periodically reversed using an applied alternating current magnetic field, which leads to alternating mechanical stress at the tip end, thus affecting localized magnetization reversal via an inverse magnetostrictive effect. Spatial resolution improvement is achieved via effective reversible magnetization volume at the sharpened tip end, which promotes a decrease in the total magnetic volume and maintains sufficient sensitivity at a lower coating thickness. A-MFM uses frequency modulation induced by an off-resonant alternating magnetic force between the tip and the sample. Therefore, a magnetic force can be selectively detected near the sample surface, independent of non-magnetic interactions. Due to their smooth surfaces without crystalline grain boundaries, large magnetization, and large magnetostriction compared with those of other magnetic materials, Fe-based amorphous films are suitable for tip coating applications. A 500-kfci perpendicular magnetic recording medium (PMR) was imaged using a 6-nm thick amorphous Fe-B coated tip to produce highly resolved images with minimal detection of the wavelength of the magnetic field of less than 10 nm. Also, 1600-kfci PMR imaging was successfully achieved, with the images showing individual ∼ 15-nm bits.

Upper Limit of Carbon Concentration in Ferromagnetic L1₀-Ordered FePt-C for Tb/in² Data Storage Density Heat-Assisted Magnetic Recording Media

In high-density magnetic recording media, magnetically isolated grains are required to increase the signal-to-noise ratio (SNR). Carbon can be used to isolate FePt grains enabling their grain size smaller than 4.3 nm. Carbon atoms segregate to the boundaries during growth and provide an exchange-breaking layer, however, some other carbon atoms remain dissolved in the magnetic alloy. To identify the upper limit of carbon concentration in $L 1_{0}$ -ordered (Fe0.5Pt0.5)100– x C x , first-principles calculations are performed based on the density functional theory (DFT). The Brillouin function and Callen–Callen empirical relation determine the temperature-dependent magnetization and magneto-crystalline anisotropy energy enabling the determination of magnetic properties and Curie temperature required by 4 Tb/in2 heat-assisted magnetic recording (HAMR) media and beyond. The calculated magnetization ( $M_{s}$ ) of $L 1_{0}$ -ordered (Fe0.5Pt0.5)100– x C x decreases to 770 emu/cm3 at $x =20$ from 1030 emu/cm3 at $x = 0$ at 300 K, and the magnetocrystalline anisotropy constant ( $K_{u}$ ) to 2.05 MJ/m3 at $x =20$ from 15.48 MJ/m3 at 300 K. It is striking to find that the Curie temperature ( $T_{C}$ ) increases to 728 K at $x =20$ from 719 K at $x =0$ . Regardless of carbon concentration, the magnetic anisotropy direction is the out-of-plane. Combining $M_{s}$ and $K_{u}$ at 300 K with $T_{C}$ , the $M_{s}$ – $K_{u}$ –C concentration relation is plotted to guide the design of $L 1_{0}$ -ordered Fe-Pt film for Tb/in2 recording media. It is found that the upper limit of carbon concentration is determined to be about 12 at.% to retain $M_{s} \ge800$ emu/cm3, $T_{C} \ge430$ K, and $K_{u} \ge 5$ MJ/m3, which are necessary to achieve areal densities of 4 Tb/in2 and beyond.

Microstructure and Properties of Electrodeposited Nanocrystalline Ni-Co-Fe Coatings.

Materials based on Ni-Co-Fe alloys, due to their excellent magnetic properties, attract great attention in nanotechnology, especially as candidates for high-density magnetic recording media and other applications from spintronic to consumer electronics. In this study, Ni-Co-Fe nanocrystalline coatings were electrodeposited from citrate-sulfate baths with the Ni2+:Co2+:Fe2+ ion concentration ratios equal to 15:1:1, 15:2:1, and 15:4:1. The effect of the composition of the bath on the morphology, microstructure, chemical composition, microhardness, and magnetic properties of the coatings was examined. Scanning (SEM) and transmission (TEM) electron microscopy, X-ray diffractometry (XRD), and energy dispersive X-ray spectroscopy (EDS) were used to study surface morphology, microstructure, chemical, and phase composition. Isothermal cross-sections of the Ni-Co-Fe ternary equilibrium system for the temperature of 50 °C and 600 °C were generated using the FactSage package. Magnetic properties were analyzed by a superconducting quantum interference device magnetometer (SQUID). All the coatings were composed of a single phase being face-centered cubic (fcc) solid solution. They were characterized by a smooth surface with globular morphology and a nanocrystalline structure of grain diameter below 30 nm. It was determined that Ni-Co-Fe coatings exhibit high hardness above 4.2 GPa. The measurements of hysteresis loops showed a significant value of magnetization saturation and small coercivity. The microstructure and properties of the obtained nanocrystalline coatings are interesting in terms of their future use in micromechanical devices (MEMS).

Microstructural segregation in high anisotropy [Co/Pt]n superlattice films

We show microstructural segregation in oxide containing [Co/Pt]n superlattice films with strong perpendicular magnetic anisotropy for application in advanced high-density magnetic recording media. By adding optimized mixtures of oxides into the Co and Pt layers of the superlattice, films with columnar grains, ~8nm grain pitch (GP), σGP/<GP>~17% and controlled intergranular exchange are demonstrated. The magnetic, microstructural and crystal properties of these films were investigated as a function of Pt thickness and oxide content.

Advances in magnetic films of epsilon-iron oxide toward next-generation high-density recording media.

Iron oxide magnets, which are composed of the common elements iron and oxygen, are called ferrite magnets. They have diverse applications because they are chemically stable and inexpensive. Epsilon-iron oxide (ε-Fe2O3) is a polymorph that shows an extremely large coercive field as a magnetic oxide. It maintains its ferromagnetic ordering even when downsized to a single nano-sized scale (i.e.,<10 nm). Due to these characteristics, ε-Fe2O3 is highly expected to be used for high-density magnetic recording media in the big data era. Here, we describe the recent developments of magnetic films composed of metal-substituted ε-iron oxide, ε-MxFe2-xO3 (M: substitution metal), toward the next-generation of magnetic media.

Synthesis and Characterization of Ni-Ti Partially Substituted Hexaferrites for High-Density Magnetic Recording Applications

Synthesis and Characterization of Ni-Ti Partially Substituted Hexaferrites for High-Density Magnetic Recording Applications

Structural, magnetic and electrical properties of CoSi ferrites synthesized by sol-gel self-propagating method

CoFe2O4 is mainly used in magnetic strain sensor, magnetic resonance imaging, supercapacitor, cryogen, high density magnetic recording medium and magneto-optical devices and so on. In this study, SixCo1-xFe2O4 (where x = 0, 0.05, 0.10, 0.15, 0.30, 0.40, 0.50) ferrites were prepared by sol-gel self-propagating method, and the effects of different amount of silicon substitution on the properties of cobalt ferrite were studied. The X-ray diffraction spectrums demonstrate that the secondary phase Fe2O3, Fe2·56Si0·44O4 and FeO appear when x ≥ 0.15. The SEM images of the samples show that with the increase of silicon ion content, the porosity of cobalt ferrite increases and the sintering density decreases. The substitution of silicon significantly changed the electromagnetic properties of Co ferrite samples. Fortunately, the coercive force (Hc) increased. Moreover, the dielectric constant raised initially when 0 ≤ x ≤ 0.30 and then reduced when 0.30 <x ≤ 0.50 with the increasing Si substitution at low frequency, which achieves the maximum at x = 0.30.

A New Highly Anisotropic Rh-Based Heusler Compound for Magnetic Recording.

The development of high-density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat-assisted magnetic recording was developed, rapidly heating the media to the Curie temperature Tc before writing, followed by rapid cooling. Requirements are a suitable Tc , coupled with anisotropic thermal conductivity and hard magnetic properties. Here, Rh2 CoSb is introduced as a new hard magnet with potential for thin-film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJ m-3 is combined with a saturation magnetization of μ0 Ms = 0.52 T at 2 K (2.2 MJ m-3 and 0.44 T at room temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare-earth-free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 μB on Co, which is hybridized with neighboring Rh atoms with a large spin-orbit interaction. Moreover, the pronounced temperature dependence of the anisotropy that follows from its Tc of 450 K, together with a thermal conductivity of 20 W m-1 K-1 , make Rh2 CoSb a candidate for the development of heat-assisted writing with a recording density in excess of 10Tb in.-2 .

Determination of interface layer effects on magnetic properties of nanocomposite magnets and exchange coupling between magnetic entities

The interlayer plays a crucial role in improving the properties of many extremely important magnetic materials and devices by regulating the magnetic exchange coupling, while it is still not fully understood in a quantitative way. The main obstacle is that there is still no effective way to characterize quantitatively the ferromagnetic exchange coupling (FEC) strength after inserting the interlayer. In this paper, a rationally designed hard/interlayer/soft layered film is used to study the interlayer effects. It is demonstrated that the magnetic properties of nanocomposite magnets can be effectively enhanced through the modification of interface. Here a new concept, using the nucleation field Hns of soft phase as a “detector” to directly and quantitatively characterize/measure the FEC strength across interface and interlayer, is put forward. It is applied to address the dependence of FEC strength on thickness of nonmagnetic Cr and rare earth-rich phase interlayers. The detailed, distinguishing and typical decay behavior and length of FEC strength for these interlayers are successfully resolved for the first time. The new conception and experimental results get also strongly supported by the theoretical calculation. The nature or mechanism of exchange coupling between ferromagnetic grains or layers after adding the nonmagnetic interlayer is identified by using the hard/interlayer/soft layered film as a model system. Our results demonstrate the validity for characterizing/measuring quantitatively the FEC strength across different interlayer materials via strategy established in this work. It opens up new perspectives to gain insight into the interlayer property, and mechanism comprehension and high-performance design for magnetic materials and devices, such as permanent magnets, high-density magnetic recording media, advanced spin-orbit torque devices, etc.

Development of High Density Magnetic Recording Media

1．はじめに

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.

Magnetic properties of microwave-plasma (thermal) chemical vapour deposited Co-filled (Fe-filled) multiwall carbon nanotubes: comparative study for magnetic device applications

‘Co-filled’ and ‘Fe-filled’ multiwall carbon nanotubes (MWCNTs) were grown using microwave-plasma chemical vapour deposition (MPCVD) and thermal chemical vapour deposition (TCVD) methods respectively, and their structural and magnetic properties were studied for magnetic device applications. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show the average tube length ≈80–500 μm with outer (inner) diameter ≈20–50 (≈10–20) nm for MWCNTs prepared by both methods. The diffraction peaks of both x-ray diffraction patterns show the interlayer distance, d002 ≈ 3.36 Å, which is comparable to the graphite structure (d002 = 3.35 Å). The graphitic crystallite sizes (La) of MPCVD (TCVD) synthesized MWCNTs are ≈24.78 nm (≈22.13 nm) as obtained from the intensity ratio of (ID/IG) D-peak, the disordered structure of graphite and G-peak, the C−C bond in graphitic structure of Raman spectra. The magnetization of ‘Fe-filled’ TCVD grown MWCNTs is much higher than ‘Co-filled’ MPCVD grown MWCNTs due to the formation of higher content of Fe–C and/or Fe-oxides in the MWCNT structures. The higher magnetic coercivity ≈2900 Oe and formation of isolated single-domain Fe-nanoparticles in ‘Fe-filled’ TCVD grown MWCNTs, as found from SEM / TEM micrographs, makes the ferromagnetic MWCNTs a promising material for the high-density magnetic recording media.

Influence of anisotropic dipolar interaction on the spin dynamics of Ni80Fe20 nanodot arrays arranged in honeycomb and octagonal lattices

Ultrafast spin dynamics in ferromagnetic nanodot arrays with dot diameter 100 nm and thickness 20 nm arranged in honeycomb and octagonal lattice symmetries are studied to explore the tunability of the collective magnetization dynamics. By varying the inter-dot separation between 30 nm and 300 nm drastic variation in the precessional dynamics from strongly collective to completely isolated regime has been observed by using all-optical time-resolved magneto-optical Kerr microscope. Micromagnetic simulation is exploited to gain insights about the resonant mode profiles and magnetic coupling between the nanodots. A significant spectral and spatial variation in the resonant mode with increasing dipolar interaction is demonstrated with increasing inter-dot separation. The spins driven by effective field inside single nanodots are prone to precess independently, generating two self-standing centre and edge modes in the array that are influenced by the relative orientation between the inter-dot coupling direction and bias magnetic field. The anisotropic behavior of dipolar field is rigorously investigated here. Splitting of the centre mode in case of octagonal lattice is experimentally observed here as a consequence of the anisotropic dipolar field between the nanodot pairs coupled horizontally and vertically, which is not found in the honeycomb lattice. In addition, proper understanding of the modification of dynamic mode profile by neighboring dipolar interaction built up here, is imperative for further control of the dynamic dipolar interaction and the corresponding collective excitation in magnonic crystals. The usage of nanodot lattices with complex basis structures can be advantageous for the designing of high density magnetic recording media, spin-wave filter and logic devices.

Novel torque magnetometry for uniaxial anisotropy constants of thin films and its application to FePt granular thin films

A technique based on torque magnetometry is proposed for precise determination of the uniaxial magnetic anisotropy constant, Ku, of perpendicularly magnetized thin films, in particular for Ku larger than 106 J/m3, by electrical detection of the anomalous Hall effect and exact determination of the magnetic field direction. This technique can measure the Ku value of several-nanometer-thick polycrystalline films. Further, the Ku value of FePt–C granular films, which are expected to be useful for high-density magnetic recording media, is evaluated. The intrinsic Ku value of 4-nm-thick FePt–C thin films fabricated at 450 °C is approximately 3 MJ/m3 for carbon contents up to 34 vol % C.

Influence of nickel substitution on crystal structure and magnetic properties of strontium ferrite preparation via sol-gel auto-combustion route

In this research, SrFe[Formula: see text]Ni[Formula: see text]O[Formula: see text] (x [Formula: see text] 0 − 1) hexagonal ferrites were prepared by sol-gel auto-combustion method. Effect of Ni substitution on structural, morphological and magnetic properties of nanoparticles was investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM), respectively. The XRD results confirmed that all samples with x [Formula: see text] 0.5 have single phase M-type strontium ferrite structure, whereas for the SrFe[Formula: see text]Ni[Formula: see text]O[Formula: see text] samples with x [Formula: see text] 0.5, the spinel NiFe2O4 phase has also appeared. The lattice parameters and crystallite sizes of the powders were concluded from the XRD data and Williamson–Hall method. Magnetic analyses showed that the coercivity of powders decreased from 5672 Oe to 639 Oe while the saturation magnetization increased from 74 emu/g to 81 emu/g with nickel substitution. The results of this study suggest that the strontium hexaferrites doped with Ni are suitable for applications in high density magnetic recording media as well as microwave devices because of their promising magnetic properties.

Relationship between microstructural and magnetic properties of PrCo-based films prepared by the vacuum evaporation method

In this work, Ce2Ni7 type structural PrCo-based films were deposited on Si(100) substrate by ultra-high (UHV) vacuum evaporation process. The structural and magnetic properties of these films have been performed using X-ray diffraction (XRD), atomic force microscopy (AFM), vibrating sample magnetometer (VSM) and magnetic force microscopy (MFM) techniques. Two effects on structural and magnetic properties of PrCo films have been investigated: the effect of the annealing temperature (Ta) and the effect of the variation of the magnetic X-layer thickness. The as deposited PrCo films have a magnetic coercivity (Hc) of about 40–100 Oe. But after annealing at 600 °C, Hc has increased hight about 9.5 kOe for PrCo(X = 20 nm) and 10.2 kOe for PrCo(X = 50 nm) were observed. The magnetic properties were affected by the thickness due to the morphology, also the relationship between the intergrain exchange coupling (IEC), the size and quantity of the PrCo grains. The hight extrinsic properties of Hc = 10.2 kOe, maximum energy product (BH)max of 5.12 MGOe and remanence ratio Mr/Ms = 0.53 are reported for the PrCo(X = 50 nm) films. These properties are highly desirable for extremely high-density magnetic recording media applications.

Microstructures and Magnetic Properties of CoPt-C Nanogranular Films

CoPt-C films were fabricated on silicon substrate by dc reactive magnetron sputtering followed by vacuum annealing. The effects of C additions and annealing temperature on the microstructure and magnetic properties were investigated. The as-deposited films had flat, compact surfaces and face-centered cubic structure, which transforms into the face-centered tetragonal structure after thermal annealing at 700°C for 1 hour. The grain size of CoPt increased with the annealing temperature but decreased with increasing C content. No carbide appearing, the C content exists in amorphous state in the nanocomposite films, and it is homogeneous distributed between the CoPt nanograins, which have the benefit to restrain grain growth and obtain isolated CoPt particles with uniform size. The fct-CoPt films annealed at 700°C exhibited high in-plane coercivity, up to 4200 Oe at room temperature and better square degrees. In the CoPt-C granular films, the best doping amount is about 35 at.% C. These CoPt-C films with novel embedded structure and moderate coercivity have shown promise for high density magnetic recording medium.