Granular Recording Media Research Articles

Thermal stability and magnetic anisotropy dispersion in high-density hard-disk media

Abstract A method to correct the anisotropy field distribution for granular recording media, measured by switching methods (isothermal remanent magnetization (IRM) and direct-current demagnetisation (DCD) processes were used as an example) for the effects of thermal relaxation, that supposes an a priori knowledge of the grain size distribution, is presented. For a hard-disk medium, it is shown that the anisotropy field distribution is much narrower (about one order of magnitude) with respect to the grain size distribution. The anisotropy field value estimated from the dependence of the remanence coercivity on the field application time is found in agreement (to within less than 10 %) with the most probable value of the anisotropy field distribution.

Manipulating exchange bias using all-optical helicity-dependent switching

Deterministic all-optical control of magnetization without an applied magnetic field has been reported for different materials such as ferrimagnetic and ferromagnetic thin films and granular recording media. These findings have challenged the understanding of all-optical helicity-dependent switching of magnetization and opened many potential applications for future magnetic information, memory and storage technologies. Here we demonstrate optical control of an antiferromagnetic layer through the exchange bias interaction using the helicity of a femtosecond pulsed laser on IrMn/[Co/Pt]xN antiferromagnetic/ ferromagnetic heterostructures. We show controlled switching of the sign of the exchange bias field without any applied field, only by changing the helicity of the light, and quantify the influence of the laser fluence and the number of light pulses on the exchange bias control. We also present the combined effect of laser pulses and applied magnetic field. This study opens applications in spintronic devices where the exchange bias phenomenon is routinely used to fix the magnetization orientation of a magnetic layer in one direction.

Estimation and Detection on Uniform Hexagonal Array Models

Magnetic recording at the highest areal density on conventional granular recording media uses $\sim 10$ grains to carry each bit of customer data. Efforts to reduce this number have focused on improving the resolution of the writing and reading processes and, in particular, on improving uniformity of the recording medium and on incorporating knowledge of the recording medium statistics into the detection process. This paper considers the extreme case of data detection on a perfect hexagonal array of grains. This paper looks at two aspects: 1) the estimation of the angle and positioning of the array with respect to the bit-steam and 2) the use of that information to assist data detection. At aggressive densities with very few grains per bit (GPB), the estimation proceeds well and the extra information is helpful in detection. With more GPB, the gains become small.

A femtosecond twist to magnetism

Imagine being able to write magnetic information efficiently at THz data rates, creating such content using only the helicity of light. A consortium of researchers from institutes in San Diego, Nancy, Tsukuba and Kaiserslautern recently showed the feasibility of this process, using femtosecond laser pulses. In a recent issue of Science, Lambert et al.1 demonstrate all-optical helicity-dependent switching (AO-HDS) in ferromagnetic thin films with perpendicular magnetic anisotropy, [Co/Pt]n multilayers, as well as in granular recording media. Firing trains of femtosecond laser pulses while moving the laser spot across the specimen caused traces with up or down magnetization to be written when using left- or right-circularly polarized light, respectively, independently of the original magnetic state (see Figure 1).

Microstructure and Crystalline Texture Quality of $L1_{0}$ Ordered Fe-Pt-Ag-C Alloy Recording Media

Fe-Pt-Ag-SiO2 and Fe-Pt-Ag-C granular recording media with an L10 ordered structure were fabricated using an in-line sputtering system with full-surface alloy targets. To understand the degree of ordering, the x-ray diffraction intensity ratio of a superlattice peak to a fundamental peak was measured for various samples. Some samples showed large intensity ratios despite having low anisotropy energy. This can be explained by the simple theory that the relative intensity at a low diffraction angle increases for samples with weaker preferred orientation. That is, the intensity ratio is affected not only by the degree of ordering but also by the crystalline texture quality. The larger intensity ratio of Fe-Pt-Ag-C media than that of Fe-Pt-Ag-SiO2 media was interpreted as the degraded Fe-Pt(001) texture of Fe-Pt-Ag-C media. Thus, good Fe-Pt(001) texture needs to be achieved without deteriorating the well-isolated granular structure in Fe-Pt-Ag-C media.

Channel Modeling and Capacity Bounds for Two-Dimensional Magnetic Recording

Two-dimensional magnetic recording (TDMR) differs from traditional track recording in that the bit-size-to-grain-size ratio is drastically reduced so that the channel bits are roughly of the same sizes as those of the magnetic media grains. This is envisioned to be achieved by shingled writing, a write-process in which the corner-writer partially overwrites the previously written adjacent track, effectively writing very closely spaced narrow tracks with no guard bands. Since the tracks are very narrow, they can be read by an array of read-elements whose spacing is equal to the narrow track pitch, creating a two-dimensional (2-D) array of readback signals (the two dimensions being the cross-track and the down-track dimensions). In the absence of an array of read elements, the same 2-D readback can be obtained by progressive scans of a single read element that reads one narrow track at a time and stores the readback signal in a 2-D array. In this paper, we present simple magnetic grain media models and shingled-write process models that capture the essence of TDMR. We assume that the granular recording medium consists of randomly shaped tiles (each tile represents a grain), randomly covering the medium plane. We then derive a suitable 2-D read/write process model. Using proper information-theoretic inequalities and bounding techniques, we derive methods to bound the capacities of TDMR channels. The results reveal that information capacities in excess of 0.6 user bits per grain are possible to attain over TDMR channels.

Effect of dipole interaction on microwave assisted magnetization switching

Microwave assisted switching (MAS) of magnetization has attracted much attention as an alternative technique for future ultrahigh density magnetic recording. In this study, to elucidate the effect of magnetostatic interparticle interaction on MAS, we have calculated the switching behavior for a row of three uniaxial magnetic particles as a simplest case. When a dc field is applied only to one particle of the row in an ac field, MAS selectively occurs in the particle and not in the neighboring particles. This behavior may be very effective to realize narrow track recording without adjacent track erasure problem by downsizing a dc field source below an ac source. It has been also found that when both ac and dc fields are applied to all the particles, coherent magnetization precession is cooperatively induced in all the particles due to dipole-dipole interaction among neighboring particles, resulting in significant reduction of the switching field. This coherent magnetization precession is not only effective to reduce the switching field but also to suppress the effect of magnetic anisotropy dispersion, suggesting the effectiveness of MAS in granular recording media.

Increases in effective head field gradients in exchange spring media

Finite head field gradients in combination with switching field distribution of individual grains lead to transition jitter in granular recording media. We studied the effective head field gradients in various recording media structures using finite element micromagnetics. Inhomogeneous reversal was strongly induced in exchange spring structures that had a hard layer at the bottom. Due to the inhomogeneous reversal mode the effective head field gradient increases. As a result the transition jitter that is caused by an anisotropy distribution was reduced by more than 50% in exchange spring media compared to single phase media.

Message from the Conference Chair

This message introduces the special issue on the 17th annual Magnetic Recording Conference (TMRC), which was held August 7-9, 2006 in Pittsburgh, PA. The issue includes 28 invited papers that cover such subjects as advanced perpendicular writer and reader designs, granular recording media, coupled recording media, advanced channels, and mechanical and systems integration topics.

High-density perpendicular recording media with large grain separation

The possibility of 300-500 Gbit/in/sup 2/ perpendicular recording using granular recording media has been investigated through micromagnetic simulation based on the Langevin equation. Writability and thermal stability in 10 years were obtained changing media parameters such as the grain size D, the grain separation d, and the thickness of the recording layer t/sub mag/ for proper combination of the grain saturation magnetization M/sub s-grain/ and the grain perpendicular anisotropy energy K/sub u-grain/. It was found that high-density recording is realized under the large grain separation, the large grain saturation magnetization, and the large grain anisotropy energy. The read/write calculation using ordered medium with D of 4.2 nm, d of 2.3 nm, t/sub mag/ of 12.0 nm, M/sub s-grain/ of 1313 emu/cm/sup 3/, and K/sub u-grain/ of 7.0 Merg/cm/sup 3/ confirmed the possibility of 1303 kFCI and 1954 kFCI perpendicular recording, leading to 325 and 488 Gbit/in/sup 2/ with 250 kTPI (track pitch of 102 nm).

The ultimate speed of magnetic switching in granular recording media.

In magnetic memory devices, logical bits are recorded by selectively setting the magnetization vector of individual magnetic domains either 'up' or 'down'. In such devices, the fastest and most efficient recording method involves precessional switching: when a magnetic field B(p) is applied as a write pulse over a period tau, the magnetization vector precesses about the field until B(p)tau reaches the threshold value at which switching occurs. Increasing the amplitude of the write pulse B(p) might therefore substantially shorten the required switching time tau and allow for faster magnetic recording. Here we use very short pulses of a very high magnetic field to show that under these extreme conditions, precessional switching in magnetic media supporting high bit densities no longer takes place at well-defined field strengths; instead, switching occurs randomly within a wide range of magnetic fields. We attribute this behaviour to a momentary collapse of the ferromagnetic order of the spins under the load of the short and high-field pulse, thus establishing an ultimate limit to the speed of deterministic switching and magnetic recording.

Thermal stability and magnetic anisotropy dispersion in high-density hard-disk media

Abstract A method to correct the anisotropy field distribution for granular recording media, measured by switching methods (isothermal remanent magnetization (IRM) and direct-current demagnetisation (DCD) processes were used as an example) for the effects of thermal relaxation, that supposes an a priori knowledge of the grain size distribution, is presented. For a hard-disk medium, it is shown that the anisotropy field distribution is much narrower (about one order of magnitude) with respect to the grain size distribution. The anisotropy field value estimated from the dependence of the remanence coercivity on the field application time is found in agreement (to within less than 10%) with the most probable value of the anisotropy field distribution.

Coercivity and fluctuation field in granular recording media

Fluctuation field as a function of applied field and coercivity data derived from magnetic measurements on two well-characterized CoPt/B (1 1 1) textured films is presented and compared with the predictions of a simple model based on Arrhenius–Néel type of thermal activation of Stoner–Wohlfarth particles that takes into account the specifics of the easy-axis distribution in hand. The model gives a good qualitative description of the results.

Collective thermal fluctuations in nominally "independent" granular recording media: application to thermal decay

We present initial results of a study of a system of fine magnetic grains with resonance interactions. For the case of magnetostatic and exchange interactions a general criterion for collective magnetization motion of a pair of grains is derived. Simulations demonstrate a dynamic cluster structure in both longitudinal and perpendicular recording media. These clusters may significantly reduce thermal degradation of recorded patterns. An indirect interaction between grains through an antiferromagnetic substrate is also discussed.

Micromagnetic study of CoPt–SiO2 granular films by spin-wave Brillouin scattering

A spin-wave Brillouin scattering study of a CoPt–SiO2 granular magnetic recording medium was made. This film contains ferromagnetic CoPt particles in a SiO2 matrix, and has an extremely low medium noise property due to little exchange coupling between magnetic grains. Spin waves of both the propagating surface mode and standing wave mode were found to be excited in granular magnetic films with various microstructures. A possible origin of the spin wave is a magnetostatic coupling between regularly ordered CoPt grains, as reported for artificially patterned magnetic thin films. This result shows two promising features of the CoPt–SiO2 granular film for high density recording medium: It is an ordered media obtained in a self-organizing manner, and it is less influenced by the thermal fluctuation effect, which is a serious problem for current high density magnetic recording.