Perpendicular Magnetic Recording Media Research Articles

FePtCu–C granular film for perpendicular magnetic recording media

We worked on a FePtCu–C granular film 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 Fe48Pt42Cu10–C45% (6.8nm) film was deposited on a silicon substrate with a MgO (12nm) interlayer at 490°C. The perpendicular coercivity of the film is 21kOe, with a squareness of 1. Bright-field TEM images shows that the FePt granular film has small and uniform grains of 7.2±1.9nm. 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 330kBT at room temperature, meaning excellent thermal stability. As a result, FePtCu–C granular thin film is a qualified candidate for high-density magnetic recording media.

グラニュラー型垂直磁気記録媒体における磁気的交換結合の抑制—記録層成長初期部への高ガス圧成膜層の挿入—

グラニュラー型垂直磁気記録媒体における磁気的交換結合の抑制—記録層成長初期部への高ガス圧成膜層の挿入—

Electron Spin Resonance In Electrodeposited Cobalt Nanowire Arrays

Arrays of magnetic nanowires electrodeposited into nano-channel templates have attracted a lot of attention and research efforts in recent years. They are a promising system for perpendicular magnetic recording media. A major issue regarding the fabrication of such nanowires is the interplay between the structure and magnetic properties. In this paper template-assisted electrodeposition technique using a three-electrode electrochemical cell is used to produce high density cobalt nanomaterial arrays with cylindrical shapes. The morphology of the samples is investigated by means of Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). The length and average diameter of Cobalt nanowire arrays was found 4–15 µm and ~300 nm, respectively. The structural characteristic of the samples is examined using XRD and EDX, which confirm the hexagonal closely packed cobalt array structures. Magnetic property measurements show the influence of morphology on the magnetic properties of the arrays. Magnetic characterizations were carried out by Magneto-optical Kerr Effect (MOKE) and Electron paramagnetic resonance (EPR). The experimental results suggest close agreement with the resonance field seen in micro-magnetic modelling. We have calculated the resonance field for single nanowires with different length and for an array of seven nanowires. It is shown that the resonance field varies with the length, the interaction strength and also the spacing of the nanowires.

Intergranular Exchange Coupling Field Under Multidomain State for Stacked Perpendicular Recording Media Evaluated by Ferromagnetic Resonance

The intergranular exchange (coupling) field <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex</sub> of single/stacked granular perpendicular magnetic recording media with a switching field (SF) distribution was measured by ferromagnetic resonance (FMR) by sweeping the external field <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DC</sub> through the internal effective field <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> . <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> consists of components corresponding to the anisotropy field <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> , the dipole interaction field, the self-demagnetization field, and the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex</sub> for up and down magnetic moments of magnetic grains classified by their SF. FMR measurements were performed for one kind of weak intergranular exchange coupled medium (medium I) and two kinds of strong intergranular exchange coupled media (medium II (single magnetic layer structure) and III (stacked structure)). These measurements revealed that the increase in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex</sub> with decreasing 4π <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> for media II and III was larger than that for medium I. In addition, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex</sub> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> was determined to be 0.25 (for grains with low SF) and 0.37 (for grains with high SF) for medium II and 0.34 (for both low and high SF) for medium III under <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> =0.3. The variation in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> for the multidomain state was smaller for the strong intergranular exchange coupled media than for the weak intergranular exchange coupled medium. The values normalized by <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> were approximately ± 0.038-0.060, ± 0.008-0.015, ± 0.012-0.030 for media I, II, and III, respectively.

Effect of divalent cation substitution in the magnetoplumbite structured BaFe12O19 system

Synthesis of magnetically ordered barium hexaferrite powders and the adjustment of magnetic properties for perpendicular magnetic recording media are realized through substitution of divalent cation (Ca) in the BaFe12O19 system. The Ca2+ substituted Ba1−xCaxFe12O19 (where x = 0.05, 0.1, 0.15 and 0.2) compounds have been prepared through solid state reaction technique. The powder X-ray diffraction analysis reveals that all the prepared compounds crystallized in magnetoplumbite hexagonal structure and the flat hexagonal platelet morphology of the crystallites was identified through scanning electron microscopy. The formation of magnetoplumbite structured Ba1−xCaxFe12O19 system due to mechanical activation was supported by micro-Raman measurements. Both pure and Ca substituted BaFe12O19 compounds exhibit sharp intense peaks which reveals defect free environment in the crystal lattice. From the room temperature magnetization studies, it was observed that the saturation magnetization (MS) and remanent magnetization (MR) values drastically decreases for the Ba0.95Ca0.05Fe12O19 compound which may be due to the existence of spin canting effect and leads to the reduction of super exchange fields. The increase in MS and MR values for the Ba0.9Ca0.1Fe12O19 and Ba0.85Ca0.15Fe12O19 compounds could be attributed to the enhanced hyperfine fields at 12k and 2b sites due to the strengthening of Fe3+–O–Fe3+ super exchange interactions. A large reduction in the coercivity value from 3,090 to 1,548 Gauss may be attributed to the fall in magneto crystalline anisotropy. The high temperature magnetization studies infer that while increasing substitution level of Ca in the BaFe12O19 system results in decreasing trend in Curie temperature. The room temperature dielectric measurement shows that the incorporation of Ca2+ in the BaFe12O19 system results with increase in the dielectric constant and this case substantiates the space charge polarization. The magnetically ordered BaFe particulate having higher saturation and low coercivity values with superior magnetic and dielectric behaviour exhibiting the possibility for future high-recording-density storage products.

The effect of an applied magnetic field on the (111) texture of a NiFe layer for perpendicular magnetic recording media

The magnetic properties and the texture of CoPt/Ru/NiFe/Ta films with and without an applied magnetic field were examined during sputter deposition of the NiFe layer. The fcc (111) texture of a magnetically soft NiFe layer was enhanced by the applied magnetic field. The Ru (0002) and the Co-Pt (0002) textures were improved by the enhanced (111) texture of the NiFe layer. Consequently, the perpendicular magnetic anisotropy and coercivity of the Co-Pt layer were increased by the enhanced texturing.

Micromagnetic simulations of sweep-rate dependent coercivity in perpendicular recording media

The results of micromagnetic simulations are presented, which examine the impact of thermal fluctuations on sweep rate dependent coercivities of both single-layer and exchange-coupled-composite (ECC) perpendicular magnetic recording media. M-H loops are calculated at four temperatures and sweep rates spanning five decades with fields applied normal to the plane and at 45°. The impact of interactions between grains is evaluated. The results indicate a significantly weaker sweep-rate dependence for ECC media suggesting more robustness to long-term thermal effects. Fitting the modeled results to Sharrock-like scaling proposed by Feng and Visscher [J. Appl. Phys. 95, 7043 (2004)] is successful only in the case of single-layer media with the field normal to the plane.

Effects of media stray field on electromigration characteristics in current-perpendicular-to-plane giant magnetoresistance spin-valve read sensors

Effects of magnetic stray field retrieved from both longitudinal and perpendicular magnetic recording media (denoted by “media stray field”) on electromigration (EM) characteristics of current-perpendicular-to-plane (CPP) giant magnetoresistance spin-valve (GMR SV) read sensors have been numerically studied to explore the electrical and magnetic stability of the read sensor under real operation. The mean-time-to-failure (MTTF) of the CPP GMR SV read sensors was found to have a strong dependence on the physical parameters of the recording media and recorded information status, such as the pulse width of media stray field, the bit length, and the head moving velocity. According to the numerical calculation results, it was confirmed that in the longitudinal media, the shorter the stray field pulse width (i.e., the sharper the media transition) allows for the longer MTTF of the CPP GMR SV read sensors; while in the perpendicular media, the sharper the media transition gives rise to a shorter MTTF. Interestingly, it was also revealed that the MTTF could be improved by reducing the bit length as well as increasing the head velocity in both longitudinal and perpendicular media. Furthermore, the bit distribution patterns, especially the number of consecutive ‘0’ bits strongly affected the MTTF of GMR SV read sensors. The strong dependences of MTTF on the media stray field during CPP GMR SV sensor operation are thought to be mainly attributed to the thermal cycling (temperature rise and fall) caused by the resistance change due to GMR effects.

The Change in Surface Properties of Magnetic Recording Media Under Pulsed Laser Application

The effects of laser heating on surface properties of magnetic recording media were systematically investigated through novel experiments and analytical simulations. When controlled laser pulses were applied onto the surface of a perpendicular magnetic recording (PMR) media, the measured values of surface roughness, surface free energy, and surface adhesive force were significantly increased with the number of the applied laser pulses. The heat transfer modeling and simulation was performed to evaluate the change in surface temperature of PMR media by the pulsed laser. The resulting temperature was not high enough to affect the carbon film and the underlying magnetic materials, but it could change the properties of the molecularly thin lubricant film on the media surface. Based on the thermal stability of the perfluoropolyether lubricant, it was found that the change of surface properties in experiments could be attributed to the thermal degradation of the lubricant through desorption process.

Effect of Pt content on structure and magnetic properties of Fe100−xPtx films deposited on thermally oxidized Si substrates by rapid thermal annealing

Fe100-xPtx films with Pt contents (x) = 29–65 at.% were deposited directly onto thermally oxidized Si(100) substrate by dc magnetron sputtering. The films were then post-annealed at 700 °C for 3 min by rapid thermal annealing (RTA) at a high heating ramp rate of 100 °C/s. Experimental results show that Fe3Pt film displayed (111) preferred orientation and tended towards in-plane magnetic anisotropy when the Pt content was 29 at.%. When the Pt content was increased to 49 at.%, the FePt film inclined towards (001)-texture and perpendicular magnetic anisotropy. Its out-of-plane coercivity (Hc⊥), saturation magnetization (Ms) and out-of-plane squareness (S⊥) reached 1010 kA/m, 0.47 T and 0.8, respectively. These results reveal its significant potential as perpendicular magnetic recording media for high-density recording. Upon further increasing the Pt content to 65 at.%, the coercivity of the films decreased drastically to below 65 kA/m and tended towards in-plane magnetic anisotropy.

Ferromagnetic resonance analysis of internal effective field of classified grains by switching field for granular perpendicular recording media

The internal effective fields Heff of granular perpendicular magnetic recording media with a switching field (SF) distribution were measured by ferromagnetic resonance (FMR) by sweeping the external field HDC. Heff consists of the anisotropy field Hk, the dipole interaction field, the self-demagnetization field, and the intergranular exchange (coupling) field Hex for up and down magnetic moments of magnetic grains classified by their SF. Prior to the measurement, magnetic grains were classified into two classes: Those with high and low SFs when HDC is swept from +18 kOe to a negative field. FMR measurements were performed in a range for which the same domain structure was retained for typical strong and weak intergranular exchange coupled media (media I and II, respectively). These measurements revealed the following points. (1) Resonances originating from majority-direction moments were observed, whereas those of minority-direction moment were not. (2) The increase in Heff with decreasing 4πM for medium II is larger than that for medium I. (3) Hex can be expressed as Hk − 4πM −Heff when the sum of dipole interaction field and self-demagnetization field is assumed to be the average field 4πM. Hex/Hk was evaluated to be 0.25 (for grains with low SF) and 0.37 (for grains with high SF) for medium I and 0.09 (for both low and high SF) for medium II under M = 0.

Effective Switching Field Distribution Evaluation Using Saturation Field in Minor Loop

A new method for evaluating the switching field distribution (SFD) in perpendicular magnetic recording media is proposed using the saturation magnetic field of a minor loop, Hsminor. In this method, the switching fields of focused reversal magnetic domains are measured with all magnetic moments in other domains saturated in the upward direction. Thus, the proposed SFD evaluation is not affected by variety of domain structure, and reveals tails of distribution. Using the proposed method, the SFD for various granular media was evaluated.

Microstructures and Magnetic Anisotropy of Single-Layered Fe-Pt Alloy Films by In-Situ Annealing

The single-layered Fe-Pt films with thickness of 30 nm are in-situ deposited directly on Si substrate at various substrate temperatures (Ts) of 350 to 590 degrees C. As the Fe-Pt film is sputtered at substrate temperature is 350 degrees C, it shows (111) preferred orientation and tends to in-plane magnetic anisotropy. The L1(0) Fe-Pt film with (001) texture is obtained and exhibited perpendicular magnetic anisotropy as the substrate temperature is increased to 470 degrees C. The perpendicular coercivity (Hc perpendicular), saturation magnetization (Ms) and perpendicular squareness (S perpendicular) of this film are 6.9 kOe, 674 emu/cm3 and 0.89, respectively, which reveal its significant potential as perpendicular magnetic recording media.

Study of grain interactions in perpendicular magnetic recording media using first order reversal curve diagrams

It is demonstrated that, for perpendicular magnetic recording (PMR) media, first order reversal curve analysis can independently measure the grain coercive field distribution (or switching field distribution) and the strength of grain interactions, i.e., the demagnetization (mean-field) factor and the dispersion of grain interaction fields around the mean-field. The coercive field distribution is used to determine the intrinsic anisotropy field distribution of PMR media. The temperature dependence of the demagnetization factor shows that the strength of inter-granular exchange coupling is increasing with increasing medium thickness and it is decreasing with increasing temperature.

Effect of Using Oblique-Incidence Deposited Ru Interlayers Deposited by Evaporation and Sputtering on Recording Characteristics of Perpendicular Media

Oblique-incidence evaporation and sputtering were performed to investigate the effect of reducing the Ru interlayer thickness on the recording characteristics of perpendicular magnetic recording media. Deep gaps were observed at grain boundaries on oblique-incidence evaporated and sputtered films when the incident angle was over 60 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> . Evaporated Ru films have a lower density than sputtered Ru films at the same incident angle. This indicates that the self-shadowing effect predominantly occurs due to suppression of migration of deposited atoms for evaporation films. However, degradation of the crystal orientation is observed in evaporated Ru films due to the present experimental limitations. Examination of the recording characteristics revealed that the Ru interlayer thickness can be reduced by using an oblique-incidence sputtered Ru interlayer and the writing ability can be improved by reducing the spacing loss between the head and the soft magnetic underlayer.

Recording Performance Analysis of Perpendicular Magnetic Recording Media With Anisotropy Graded Oxide and Cap Layers

We have experimentally and analytically studied the writeability and recording characteristics of perpendicular magnetic recording media in which the anisotropy (Ku) of the granular oxides and the magnetization (Ms) and lateral exchange coupling (Aex) of the cap layer were varied. The analytical results are obtained with a 9-spin model, which consists of 3 grains with 3 layers each, representing dual oxide media with a capping layer. The 9-spin model estimates bit error rate (BER) using an error-pattern-correcting code (EPCC) based iterative channel model. The experimental results qualitatively agree well with the analytical results of the 9-spin model. High Ku gradient oxides and high Ms cap layer improve writeability and BER, whereas high Aex in the cap layer degrades switching field distribution and off-track capability. Media switching field and switching field distribution strongly depend on spatial distribution of the external field. Both experimental and analytical results indicate that an appropriate design of Ku graded media with an optimum cap layer can effectively reduce jitter and switching field distribution for given head fields, resulting in higher recording density.

Magnetic properties and microstructures of single-layered Fe 100 − x Pt x films deposited onto heated substrates

The single-layered Fe 100 − x Pt x films of 30 nm thick with Pt contents (x) of 35–57 at.% are deposited on heated Si (100) substrate at a temperature (Ts) of 620 °C by magnetron co-sputtering. When the Pt content in the Fe–Pt alloy film is 35 at.%, the value of in-plane coercivity (Hc //) is close to perpendicular coercivity (Hc ⊥) and both values are about 800 kA/m. The FePt films exhibit perpendicular magnetic anisotropy when the Pt content increases to the values of between 45 and 51 at.%. The perpendicular coercivity, saturation magnetization (Ms) and perpendicular squareness (S ⊥) for Fe 54Pt 46 film are as high as 1113 kA/m, 0.594 Wb/m 2 and 0.96, respectively. These magnetic properties reveal its significant potential as perpendicular magnetic recording media. Upon further increasing the Pt content to 57 at.%, the coercivity of the Fe–Pt film decreases drastically to below 230 kA/m and tends to be closer to in-plane magnetic anisotropy.

Direction detectable static magnetic field imaging by frequency-modulated magnetic force microscopy with an AC magnetic field driven soft magnetic tip

Direction detectable static magnetic field imaging, which directly distinguishes the up and down direction of static perpendicular magnetic field from a sample surface and the polarity of magnetic charges on the surface, was demonstrated for CoCrPt-SiO2 perpendicular magnetic recording media based on a frequency-modulated magnetic force microscopy (FM-MFM), which uses a frequency modulation of the cantilever oscillation induced by an alternating force from the tip-sample magnetic interaction. In this study, to generate the alternating force, we used a NiFe soft magnetic tip driven by the ac magnetic field of a soft ferrite core and imaged the direction and the amplitude of the static magnetic field from the recorded bits. This method enables measurement of the static magnetic field near a sample surface, which is masked by short range forces of the surface. The present method will be effective in analyzing the microscopic magnetic domain structure of hard magnetic samples.

Oblique-incidence sputtering of Ru intermediate layer for decoupling of intergranular exchange in perpendicular recording media

During the Ru deposition process for granular type perpendicular magnetic recording media, both a reduction in the Ru intermediate layer thickness and lowering of sputtering gas pressure were successfully achieved by focusing on a self-shadowing effect. Oblique-incidence sputtering with a 60° incident angle under an Ar gas pressure of 0.6 Pa yielded (1) columnar Ru grains with a growth direction of 30° from the film normal, (2) c-plane sheet texture by epitaxial growth on the Pt underlayer, and (3) a flat envelope of the surface and a deep gap at grain boundaries. This change in the Ru structure significantly contributes to reducing exchange coupling among magnetic grains, especially in the initial growth region in an overlying granular medium.

Tuning perpendicular magnetic anisotropy and coercivity of L1-FePt nanocomposite film by interfacial manipulation

Based on the interfacial anisotropy manipulation of multilayer structure and the orientation manipulation of surfactant Au atoms, Fe/Pt/Au multilayers were designed to achieve tunable perpendicular magnetic anisotropy (PMA) of L10-FePt nanocomposite films. In the mean time, the ordering degree of the Fe/Pt/Au multilayers was modified by adjusting the defect concentration in the film, which can be controlled by using different multilayer structures and by the diffusion of Au atoms. This makes the coercivity (HC) of L10-FePt nanocomposite films able to be tailored. Thus a L10-FePt nanocomposite film with high PMA and tunable HC was constructed that provides important experimental data for preparing writable FePt perpendicular magnetic recording media.