Narrow Switching Field Distribution Research Articles

Bit patterned media optimization at 1 Tdot/in2 by post-annealing

We report on the fabrication of 1 Tdot/in2 bit patterned media with high coercivity (HC) and narrow intrinsic switching field distribution (iSFD) based on nanoimprint from a master pattern formed by e-beam guided block copolymer assembly onto a carbon hard mask and subsequent pattern transfer via etching into a thin CoCrPt perpendicular anisotropy recording layer. We demonstrate that an additional vacuum annealing step after pattern transfer into the CoCrPt layer and after Carbon hard mask removal not only yields recovery from undesired damage of the island edges, but actually transforms the islands into a magnetically more favorable compositional phase with higher HC, lower iSFD/HC, and three-fold increased thermal stability. Energy filtered transmission electron microscopy analysis reveals that the diffusion of Cr from the island cores to the periphery of the islands during post-annealing is responsible for the transformation of the magnetic bits into a more stable state.

Potential of Metallic Glass Thin Films as a Soft Magnetic Underlayer for <inline-formula> <tex-math notation="TeX">${\rm L}1_{0}$ </tex-math></inline-formula> FePt-Based Recording Media

We report on the use of metallic glass (FeHfNbYB) thin film as a soft magnetic underlayer for the fabrication of L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> FePt-based bit patterned recording media (BPM). Preferred oriented phase of L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (111) FePt is obtained either by depositing it in situ on a heated soft magnetic metallic glass/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si or at room temperature (FePt/metallic glass/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si) followed by annealing at 400 °C-450 °C for 20 min. Fabrication of recoding bits of size ~2 μm to 20 nm is demonstrated using electron beam lithography. Magnetic measurements on ~500 nm bit patterns show a narrow switching field (SF) distribution with SF of ~3 kOe. The ability to read/write this BPM is demonstrated with a commercial head in perpendicular geometry in a static tester. Results strongly suggest that the soft magnetic metallic glass thin films can solve the problem of writability in high anisotropy L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> FePt-based recording media.

Influence of bias voltage on the switching behaviour of CoCrPt–SiO2 perpendicular recording media

A systematic investigation of the influence of negative substrate bias on the microstructure and magnetic switching behaviour of CoCrPt : SiO2 perpendicular recording media is performed by varying the negative bias voltage from 0 to 600 V during the deposition of a bottom Ru (or Ru : TiO2SiO2) intermediate layer. The microscopic structures and the surface morphology are studied by a transmission electron microscope and an atomic force microscope. The crystallographic properties are studied by an x-ray diffractometer. Our observation shows that high biasing is at work to favour the formation of low-energy hexagonal close packed (0 0 2) planes, resulting in high crystallinity of Ru. Addition of oxide to the Ru intermediate layer leads to an increase in grain segregation, which results in a small grain diameter, but broad standard deviation and large surface roughness. Utilization of a bias voltage on the Ru oxide intermediate layer successfully improves the crystal texture of CoCrPt : SiO2 and induces high Hc, a narrow intrinsic switching field distribution and low Hcr45/Hcr0, while maintaining isolated grains and small grain size.

Comparison of bit-patterned media fabricated by methods of direct deposition and ion-milling of cobalt/palladium multilayers

In the fabrication of bit-patterned media (BPM), two processes are commonly used, i.e., the pattern transfer by ion-milling into an underlying film of magnetic material, and the direct deposition of the magnetic material onto a pre-patterned substrate. We experimentally compared the switching performance of the BPM based on Co/Pd multilayers fabricated using these methods in terms of their switching field distribution (SFD) and physical characteristics of the bits. Our results show that both methods resulted in a narrow (∼15%) SFD at low areal recording densities of ∼0.15 Tdot/in2. However, at higher densities of up to 0.6 Tdot/in2, the SFD of the ion-milled samples detrimentally broadened to ∼30% while the BPM from the direct-deposition method maintained its narrow SFD up to a high bit density of 0.6 Tdot/in2. Our results suggest that in Co/Pd multilayer systems, the direct-deposition method, which produces more uniform bit sizes and profiles especially at high bit densities, is a more promising approach to achieving high-density BPM.

Writer Footprint Measurement on Spinstand and Media Transition Curvature Characteristics

The head media integration played a very important role to push up the areal density of perpendicular recording successfully. Besides the success of good sensitivity and high resolution reader at desired feature size of recording bit, the advanced writing process is the key enabler to sustain the areal density growth continuously. The sharper field gradient of writer and the narrower switching field distribution of recording media improve the transition jitter and promote the SNR of recording media. Without reducing grain size, the areal density of perpendicular recording has been increased by a few times. In this work, we investigate the performance of writing process via the measurement of head field footprint in recording media on the Spinstand. The feature size variations of head footprint at different writing current and different flying height (FH) illustrate the effective writing process visually. The transition curvature with transition width value at the trailing edge of writer pole is the key indicator to show the linear density capability related to writing process.

Effect of Composite Designs on Writability and Thermal Stability of Perpendicular Recording Media

The function of two types of magnetic inhomogeneity employed in perpendicular magnetic recording media designs are analyzed. Continuous granular composite (CGC) media have variable lateral exchange coupling (Hex) through the thickness of the magnetic layers, which can reduce coercivity (Hc) and narrow switching field distribution (SFD). Exchange coupled composite (ECC) media have variable crystalline anisotropy field (Hk) and interlayer exchange coupling (J) through the film thickness, which also reduces Hc. Interactions between the Hk, Hex, and J values of combined ECC+CGC media are presented. The optimum anisotropy profile for conventional ECC media is found to be different than previously modeled. A CGC layer included in the ECC media architecture is shown to change the optimal anisotropy profile and interlayer coupling values.

Microwave assisted switching mechanism and its stable switching limit

Microwave assisted switching (MAS) of magnetization has been intensively studied as an alternative technique for ultrahigh density magnetic recording. In this paper, comparison between the Landau–Lifshitz–Gilbert simulation and the analytical model in the rotating frame {Bertotti et al., [Phys. Rev. Lett. 86, 724 (2001)]} reveals that the switching behaviors of MAS can be clearly divided into two groups, that is, stable and unstable switching regions, depending on the frequency and amplitude of the ac field. The stable switching exhibits small switching field and narrow switching field distribution, and perfectly coincides with by the analytical model. Furthermore, in this region, the Sharrock type thermal fluctuation formula can be applicable to the MAS at finite temperature. On the other hand, for the unstable switching, the switching field is rather large and the SDF becomes very broad. Obviously, the former is preferable for the practical application of MAS. The critical frequency of the ac field for the limit of stable switching is almost proportional to the ac field amplitude.

Bit patterned media based on block copolymer directed assembly with narrow magnetic switching field distribution

Electron-beam (E-beam) directed assembly, which combines the long-range phase and placement registration of e-beam lithography with the sharp dot size and spacing uniformity of block copolymer self assembly, is considered highly promising for fabricating templates that meet the tight magnetic specifications required for write synchronization in bit patterned media magnetic recording systems. In our study, we show that this approach also yields a narrower magnetic switching field distribution (SFD) than e-beam patterning or block copolymer self-assembly alone. We demonstrate that the pattern uniformity, i.e., island diameter and placement distributions are also important for achieving tight magnetic SFDs.

Effects of array arrangements in nano-patterned thin film media

In this work, the effect of different arrays arrangements on the magnetic behaviour of patterned thin film media is simulated. The modeled films consist of 80×80 cobalt grains of uniform diameter (20 nm) distributed into two different array arrangement: hexagonal (triangular) or square arrays. In addition to that, for each array arrangement, two cases of anisotropy orientations, random and textured films are considered. For both array arrangements and media orientations, hysteresis loops at different array separation ( d) were simulated. Predictions show that for closely packed films, the shearing effects on the magnetization loop are much larger for the square array arrangement than the hexagonal one. According to these predictions, the bit switching field distribution in interacting 2D systems is much narrower for the hexagonal array arrangement. This result could be very important for high-density magnetic recording where a narrow bit switching field distribution is required.

Intermediate layer thickness dependence on switching field distribution in perpendicular recording media

The effect of intermediate layer (IL) thickness on crystallographic texture and magnetic properties of CoCrPtSiO 2 granular perpendicular recording media was investigated with switching field distribution (SFD) as the focus. Even though the c-axis orientation of the Co-based recording layer (RL) broadens with the reduction of IL thickness, the SFD becomes narrower. This result demonstrates that the intrinsic SFD is not directly dependent on c-axis orientation of the recording layer but instead dependent on the magnitude of exchange coupling. It is thus possible to have a medium with thin IL and narrow SFD. This is desirable for bit-patterned media (BPM), where highly exchange-coupled grains are required.

Spin-transfer induced switching in nanomagnetoresistive devices composed of Co/Pt multilayers with perpendicular magnetic anisotropy

We demonstrate spin-transfer switching of nanomagnetic devices composed of Co/Pt multilayer electrodes with perpendicular magnetic anisotropy. In order to obtain highly spin-polarized current, we have proposed the adoption of thick Co adjacent layers. The film stack has been optimized so that two different Co/Pt multilayer electrodes switch separately, with each having narrow switching field distribution. Detrimental effects of surface roughness have been observed and minimized to achieve well-defined switching behavior. Diverse nanopillar devices fabricated by e-beam lithography and subsequent processes show well-defined single step switching in both perpendicular field and injected current applications. The critical current density obtained is comparable to that of Co/Ni electrode based devices, indicating enhanced spin-transfer efficiency.

Antiferromagnetically coupled patterned media

Magnetostatic interactions in patterned media are responsible for increasing the switching field distribution and reducing the thermal stability. Micromagnetic simulations indicate that a reduced saturation magnetization (Ms) and increased anisotropy constant (Ku) lead to better thermal stability. However, writability will be a concern with this approach. Patterned media with an antiferromagnetically coupled (AFC) recording media structure is studied to obtain high thermal stability even with a larger Ms and larger Ku. With the AFC structure, the reduced remanence magnetization (Mr) leads to a reduced magnetostatic interaction, even though the Ms is larger. Magnetic force microscopy measurements indicate that the AFC media show a narrower switching field distribution.

Epitaxially Grown FCC/BCC/HCP Multi-Underlayers for High Performance Perpendicular Media

In this study, a new underlayer structure consisting of FCC NiW, BCC Cr and HCP Ru layers is investigated as a promising underlayer that offers good perpendicular magnetic and recording properties. The FCC (111) planes of NiW layer enhances hetero-epitaxial growth of the subsequent BCC Cr (110), HCP Ru and Co (0002) layers in the direction perpendicular to the film plane. After the NiW-Cr-Ru layer structure is optimized, the Co (0002) <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> axis dispersion is greatly reduced and ~ 3 degree of XRD rocking width is achieved. As a result, excellent magnetic properties such as high coercivity, high nucleation field and narrow switching field distribution (SFD) are obtained. In addition, low media jitter and high signal-to-media noise (SNR <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> ) can be achieved. The SNR <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> is also highly dependent on head-to-SUL spacing which is altered by varying NiW or Cr layer thickness without affecting crystallography. Optimized head-to-SUL spacing is needed to gain head writeability as well as resolution.

Designing magnetics of capped perpendicular media with minor-loop analysis

A novel method of analyzing the switching field distribution (SFD) and magnetic correlation length of perpendicular magnetic recording media that uses major and minor magnetization loops is proposed. By applying the analysis to a series of capped perpendicular media, we found that a thick capping layer with a low saturation magnetization effectively reduced SFD without rapidly increasing the magnetic correlation length. Transmission electron microscope observation suggests that the SFD is narrowed by the increased uniformity of intergranular exchange coupling via the thick capping layer. Evaluations of recording characteristics demonstrated a close correlation between narrower SFDs and improved recording performance. Reducing exchange coupling dispersion is a clear solution for improving the performance of recording media.

Fabrication challenges for patterned recording media

Lithographically patterned recording media are one of the approaches to achieving Tb/in 2 and beyond recording densities. This will require fabrication of sub-10 nm discrete magnetic islands covering a full disk with tight spacing and size distributions and a narrow switching field distribution. To become an economically successful technology, this will need to be achieved with high throughput and low cost. The technology to fabricate such patterned media will need to be developed, and may require innovative solutions such as self-assembly and nanoimprinting, along with improved magnetic thin films for achieving high anisotropy and narrow switching field distributions.

Recording and reversal properties of nanofabricated magnetic islands

Patterned media are a promising approach for future high density magnetic recording. In order to reliably address individual bits, a recording head will be required to reverse a selected island without disturbing neighboring islands. This leads to a requirement for islands with a narrow switching field distribution. We review the switching and recording properties of patterned granular CoPtCr-based films, and present results on the reversal properties of topographically patterned Co/Pd multilayer films, with particular emphasis on switching field distributions. Finally, the use of islands with multiple storage layers is demonstrated as a potential route to even higher storage density without the need for smaller, lithographically defined islands.

Magnetization reversal of micropattern Fe bar array: Combination of vector and Bragg magneto-optical Kerr effect measurements

We report on a magneto-optical Kerr effect (MOKE) investigation of alternating wide and narrow Fe bars, forming a two-dimensional periodic array. The magnetization reversal was studied by regular longitudinal Vector-MOKE in specular geometry as well as in Bragg-MOKE geometry, using the diffraction spots from the grating for hysteresis measurements. With Vector-MOKE a two-step switching process for the wide and narrow Fe bars is observed, indicative for a narrow switching field distribution in this array. In addition, hysteresis loops were determined as a function of the diffraction order for the hard and easy axis direction. The loops at different orders of diffraction can qualitatively and quantitatively be described by Fourier transformations of micromagnetic simulations.

Magnetic and recording characteristics of perpendicular magnetic media with different anisotropy orientation dispersions

A systematic investigation of the magnetic and recording characteristics of CoPtCrO media with different Co grain magnetic anisotropy orientation dispersion (AOD) is performed. AOD is characterized by Δθ50 of the x-ray diffraction Co peak. AOD was systematically varied between 3.3° and 10.5° by varying the growth condition of Ru layer. With increasing AOD, more grain-boundary segregation occurs, resulting in reduced exchange coupling. Micromagnetic simulation and experimentally measured time dependence of coercivity indicate that magnetic switching of well-isolated Co particles is affected by exchange coupling strength as well as Co AOD. Higher coercivity, narrower switching field distribution, and good thermal stability are achieved with lower Co AOD. Reduced Co AOD leads to a substantial decrease of both transition jitter and dc noise.

Highly in-plane oriented CoCrPtB longitudinal media for 130-Gb/in/sup 2/ recording

A recording density of 130 Gb/in/sup 2/ was achieved using thermally stable conventional CoCrPtB longitudinal media. The high in-plane orientation ratio (OR) of the media resulted in an excellent recording performance due to a narrow switching field distribution as well as a high thermal stability caused by narrow energy barrier distribution. The low noise is attributed to the fully isolated fine grains with a narrow size distribution. A good in-plane c axis crystallographic texture was achieved by using an optimum multilayered structure of underlayer and magnetic layers. A detailed study of high OR media is reported by characterizing the magnetic, microstructural, and read/write properties.

Junction-like magnetoresistance of intergranular tunneling in field-aligned chromium dioxide powders

The magnetic and magnetotransport properties of field-aligned half-metallic ${\mathrm{CrO}}_{2}$ powders have been studied. Needle-shaped nanoparticles of ${\mathrm{CrO}}_{2}$ have been aligned in a strong magnetic field. The aligned powder sample shows a strong anisotropy along the alignment direction. The conduction mechanism of the aligned ${\mathrm{CrO}}_{2}$ powder sample has been examined and is consistent with the intergranular spin-dependent tunneling. Negative tunneling magnetoresistance of about 41% is achieved in a small field in the vicinity of the coercive field at 5 K. The magnetoresistance (MR) versus field curve shows two well-separated narrow peaks at the coercive fields and resembles that of a magnetic tunnel junction. This junctionlike MR results from the narrow switching field distribution of the aligned powders. Our results suggest that the aligned magnetic ${\mathrm{CrO}}_{2}$ particles may find novel applications in spin-transport structures and devices.