Thin-film Recording Media Research Articles

Micromagnetic Dynamics of Single-Domain Grain in Thin-Film Magnetic Recording Media

This paper reports a micromagnetic study of single-domain-grain dynamics in a thin-film magnetic recording medium which is used in the hard disk drive industry today. The behavior of the magnetization-vector of an individual grain is modeled by a modified Landau-Lifshitz (MLL) equation with the damping torque originating from the coupling between the magnetization and stresses of the intrinsic and extrinsic magnetic anisotropy. An analytic two-parameter formula governing the magnetization-vector reversal as a function of precession frequency is derived from the equation of magnetization energy loss. This equation depends on material-dependent parameters which are deduced from ferromagnetic resonance measurements. The results show a characteristic feature of magnetization reversal, a transient quasi-stationary state. This state is generally a property of multilayered thin-film recording media and can cause shortening of the relaxation time to the picosecond regime.

Development of Ferroelectric Data Storage Test System for High-Density and High-speed Read/Write

AbstractIn this study, we have developed ferroelectric data storage test systems based on scanning nonlinear dielectric microscopy (SNDM) to conduct various experiments concerning read/write capability. Nanodomain formation on ferroelectric recording media was studied using the data storage test system. A nanodomain dot array was successfully written on a single-crystal LiTaO3 recording medium. The diameter of the written dot was as small as 7 nm. Epitaxial-thin-film LiTaO3 recording media were also developed. Nanodomain dots with the diameter of 25 nm were written on the thin-film recording medium. In addition, a non-contact probe-height control technique was adopted to solve the problem of tip abrasion using higher-order nonlinear dielectric response detection method. Finally, a hard-disk-drive (HDD)-type ferroelectric data storage test system was developed for conducting read/write tests under conditions close to those of actual operation. Capabilities of reading at the bit rate of 2 Mbps and writing at the bit rate of 20 Mbps were confirmed using the HDD-type data storage test system.

Thin-film recording media on flexible substrates for tape applications

Co-based alloys were investigated as candidates for sputtered thin-film magnetic tape media. Magnetic media on polymeric substrates having acceptable magnetic properties were obtained by applying NiAl/CrMn underlayers prior to CoCrPt deposition with a nonmagnetic CoCrTa intermediate layer. Typical sputtered tape media deposited on traditional polymeric substrates at room temperature revealed sideband modulation noise and large intergranular exchange coupling. A substantial improvement in surface roughness by utilizing smooth polymeric substrates eliminated this modulation noise and accounted for the signal dependent broad-band noise power. This suggests that transition noise is a dominant component of medium noise in sputtered tape media. Additionally, substrate bias is shown to be effective in producing grain decoupling that is necessary for good signal-to-noise ratio.

Hysteresis modeling of Gd films and AFC thin-film recording media

This paper presents simulations of hysteresis processes in thin film media using 1D and 2D Preisach models. In the 2D version, a vector operator and superposition of angularly distributed models are used. The characteristic density of the material being modeled is reconstructed via a curve-fitting least-squares procedure that determines the parameters of a bivariate normal probability function density or a weighed mixture of normal densities based on major loop data only. The models have been identified for several samples of Gd films annealed at various temperatures and AFC thin-film recording media consisting of a hard and a soft phase antiferromagnetically coupled. The major and minor hysteresis loops calculated for all samples are in good agreement with experimental data.

Thermal effects and in-plane magnetic anisotropy in thin-film recording media

The effect of thermal activation on the in-plane magnetic anisotropy [measured as orientation ratio (OR)] of granular longitudinal magnetic recording media is investigated. Temperature and time dependent studies were made on media with different magnetic layer thicknesses. We find that OR is independent of temperature for a stable medium but shows a large increase with temperature for thermally unstable media. At low temperatures and high field sweep rates, the OR values are found to be the same, independent of the magnetic layer thickness. The unique value when thermal activation is reduced is consistent with the high population of the cobalt c axes along the texturing direction as the origin of anisotropy.

Fast switching and coercivity in thin-film recording media

The effects of finite-switching rates of media on measured coercivities are studied using two methods. The first starts from expressions for the switching times of magnetic media and goes on to derive simple expressions for the coercivities of media subjected to various applied field wave forms. The second uses a well-developed numerical micromagnetic model to investigate both the switching times and coercivities. The variations of coercivity with frequency derived using the two methods show similar results.

Using of organic thin-film recording media for solving electric-discharge diagnostic problems

A new class of thin-film organic inexpensive systems are proposed for obtaining electric-discharge images of flaw fields. These systems are characterized by the insensitivity to visible light, high sensitivity and resolving capacity, and absence of operations of development and fixing of the images. A number of particular examples of using thin-film layers and compositions made from them for recording the electric-discharge images are given.

Ultra-clean sputtering process for magnetic thin films on hard disk drives

A specialized multi-sputtering system is developed, which enables one to realize 10 −12 Torr order of the ultimate pressure, and to supply the ultra-clean processing gases. The extremely clean sputtering process (XC process) was realized by the reduction of out-gassing rate from the sputtering chamber and the purification of process gases at use point. The impurity level in the XC process is four orders less than that in the process normally used. Spin valve multilayers, Ta/Ni–Fe/Co/Cu/Co/Mn–Ir/Ta, with ultra-thin thickness were fabricated by using this system. Thin-film recording media, Cr/CoCrTa, were also fabricated by applying the ultra-clean sputtering process. The relation between the cleanness during film deposition process and their magnetic properties was discussed in connection with the changes of their microstructure. As results we found that: (1) the purification of the sputtering atmosphere facilitates the growth of grains, and reduces defects in the film; (2) the reduction of the grain boundaries, impurities, and defects in the XC processed spin valves results in the reduction of saturated resistivity and the enlargement of the magnetoresistance (MR) ratio; (3) the reduction of impurities, especially oxygen in the sputtering atmosphere, enhances the Cr segregation in a magnetic layer of thin-film media; (4) the formation of Cr segregated grain boundary structure reduces intergranular exchange coupling and results in the high coercivity of thin-film media.

Magnetic materials and structures for thin-film recording media (invited)

There have been three distinct stages in the development of magnetic materials and processes for thin-film media. The first era focused on designs for use with inductive heads. Vacuum sputter deposition techniques were developed to achieve high Hc, with high Ms materials epitaxially grown on Cr underlayers. The second phase of thin-film media development demanded media optimized for the use with MR heads. Noise reduction through the decoupling of grains by chemical segregation of Cr at the grain boundaries proved effective. The introduction of giant magnetoresistance heads required low noise and high Hc but added the necessity of the reduction in the average grain size and grain size distribution width. The next and perhaps final phase of thin-film media development will be focused on the minimization of the magnetization thermal decay by the use of materials and designs that produce high KuV/kBT ratios.

Spatial effects at dynamic switching in thin-film recording media

Measurements of coercivity as a function of time have been performed using a nondestructive spin-stand technique. Two disks were measured using nine different test conditions, including various writing gap lengths and head to medium velocities. No spatial effects were found, but it was noticed that the results depended on subtleties of the measurement procedure.

Micromagnetic recording model of pole-tip saturation effects

Auxiliary write-head fields, either from simple analytic approximations or from finite-element computations, are used to record transitions on a Landau-Lifschitz-Gilbert model of thin-film recording media. A micromagnetic model of a magnetoresistive element is used to read back the transitions. Focus is on the impact of pole-tip saturation on the quality of the recorded track and subsequent playback. It is shown that with increasing write current, the quality of the recorded transitions degrades much more rapidly than would be expected from the analytical expressions for the write-head field. Specifically, head saturation causes the transition parameter to increase. At high write currents, poor write field gradients at the track edges result in a strong increase in transition parameter with read width percentage of the recorded track.

Micromagnetic study of recording process on longitudinal thin-film media

Through a micromagnetic computer siumlation, we have examined the configuration of magnetic transitions in thin-film recording media. We found that the perpendicular component Hz of head field significantly affects the bending of the magnetization transition. Specifically, the bending is dominated by the ratio between the in-plane component of the head field Hxy and the head-field angle, which is given by /spl phi/=atan(Hz/Hxy). This indicates that the reduction of Hz will help enable a straight magnetization transition and higher areal density.

Rotational magnetizations of particulate and thin film recording media

Magnetizations (M/sub x/, along the direction of applied field) as functions of rotational angle (/spl theta/) for particulate and thin film recording media in various applied fields (H) were studied. M/sub k/(/spl theta/) was analyzed through the Fourier components (C/sub n/) and the corresponding phases (/spl theta//sub n/). The /spl theta//sub n/(H) curves for the increasing- and decreasing-field processes of the particulate recording medium are observed to be separated for H lower than specific values. However, this feature is not evident for the thin film recording medium. The distinction is related with the different microstructures of these two types of recording medium.

Analysis of the Magnetic Force Acting on Magnetic Toner in Magnetography with Longitudinal Recording

The magnetic force acting on magnetic toner from the magnetic latent image recorded by longitudinal recording is theoretically analyzed. Heretofore analysis using sinusoidal approximation of magnetization has been reported wherein the force acting on toner can be several hundred times the force of gravity and it is not much smaller than the force used in electrostatic printing. This paper employs arctangent approximation of magnetization for digital longitudinal recording. A new approximate equation of the force attracting toner is presented which is applied to the magnetic printer using metal thin film recording medium and dry toner. Typically, the recording medium comprises Co-Ni-P plated metal thin film with the thickness of 1 micrometer. The toner comprises soft magnetic toner with the diameter of 10 micrometers. The resulting force acting on toner can be about three orders larger than the force of gravity. It is almost the same as the force used in electrophotography.

Analysis of track-edge noise in thin-film recording media

Track-edge noise has been analyzed to clarify track-edge noise mechanisms. Edge noise arises from two different regions: the bit edge where the magnetization direction is opposite to the previously dc-erased direction and the magnetic transition edge. The edge noise at low density is mainly due to the magnetic distribution at the bit edge. The transition edge noise increases with increasing recording density, and it becomes dominant at high-recording density for an oriented medium. On the other hand, the transition edge noise is almost constant with increasing recording density for an isotropic medium.

Effects of Grain-Size Distribution on Time Dependence of the Remanent Magnetization in Thin Film Recording Media

In this paper we present a model incorporated with a Gamma distribution of the grain sizes for the time dependence of magnetization. Performances of writing process and long-term information storage are evaluated in terms of the grain-size distributtion, applied reverse field, frequency constant and temperature. Qualitative agreement is obtained between numerical calculation and experiment for the time decay of the magnetization in thin film recording media.

Analysis of Noise in a Perpendicular Magnetic Recording Medium with Three-Dimensional Simulation

Interparticle interactions in a thin-film recording medium affect the recording characteristics and medium noise. The relation between the exchange coupling of particles and the noise in a perpendicular magnetic recording medium was analyzed with a three-dimensional simulation program. The dc-erased medium noise increases significantly with stronger exchange coupling. However, the change in the noise of a high-density recorded medium resulting from a change in the exchange coupling is not as large. In addition, the exchange coupling increases the playback voltage, because it helps to align the direction of magnetization in the recorded bit area. As a result, the exchange coupling increases the signal-to-noise ratio.

Noise time series simulation for a thin film recording medium

A previously developed simulation of a thin film recording medium has been extended to include evaluation of noise characteristics. The recording medium has been taken through the dc demagnetisation curve and noise characteristics at various remanent states computed for three degrees of inter-granular exchange coupling. The IRM curves were also computed, to allow the computation of the δM( H) curves. The remanence curves and noise curves were in good agreement with experiment. It was found that the peak in the noise curves for all three degrees of exchange coupling occurred at approximately the same field as the maximum slope in the δM( H) curves.

Intergranular magnetic coupling in CoNiCrPt and CoCrTaPt thin-film media characterized by the temperature dependence of the rotational hysteresis loss

In order to characterize intergranular magnetic coupling in (Co 86Cr 12Ta 2) 100− x Pt x ( x = 0 and 9) and (Co 62.5Ni 30Cr 7.5) 100− x Pt x ( x = 0 and 14) thin-film recording media, the temperature dependence of the rotational hysteresis loss has been examined. In all media, the coercive force H c( T) gradually decreases to about half the value attained at 25°C by increasing the measuring temperature T to about 300°C without a change in the microstructure. In CoNiCr, CoNiCrPt and CoCrTaPt media, the decrease in H c( T) with increasing T corresponds directly to the decrease in the magnetocrystalline anisotropy field of the grains, H k grain. In contrast, in CoCrTa media, H c( T)/ H k grain( T) gradually decreases with increasing T. This decrease in H c( T)/ H k grain( T) is suggested to be caused mainly by the increase in 4 πM s( T)/ H k grain( T). This means that the intergranular magnetostatic coupling strongly affects the magnetic properties of the CoCrTa media examined here due to the low intergranular exchange coupling.

Reciprocity in magnetic force microscopy

A theoretical treatment for contrast formation in the magnetic force microscope is given which relies on calculation of the force acting on the sample, rather than the more usual method which calculates the force acting on the microscope tip. The equivalence of this reciprocal force method is demonstrated by calculating the theoretical image for longitudinal step and arctangent magnetization transitions in thin-film recording media. The reciprocal force approach leads naturally to unambiguous definitions for the resolution of the magnetic force microscope and it is shown that conventional resolution measures, such as impulse response, line-spread function and step response, used in many other forms of microscopy, may be readily applied to the magnetic force microscope.