Statistical study of zig-zag transition boundaries in longitudinaldigital magnetic recording

In this paper, we study the statistics of zig-zag transition walls in digital magnetic recording and their relationship to transition noise defining quantities. We provide analytic results that link the statistics of zig-zag transitions to media/recording parameters. The basis of our study is the triangle zig-zag transition (TZ-ZT) model due to its well-defined triangle zig-zag shape and its cross-track stability. The results we derive here, however, are of a general nature, and given the right interpretation, apply to other zig-zag models as well, as we show in the paper. We also provide an interpretation of the cross-track correlation width, linking this quantity to the statistics of magnetized clusters in thin-film magnetic media. The paper concludes by showing how these results can be used in media noise modeling     

Modelling the asymmetry of the recorded transition zone in digitalmagnetic recording

A vector model of the recorded transition zone for digital magnetic recording is used to incorporate asymmetric transition profiles, non-constant recording contours and both longitudinal and perpendicular components of magnetization into a channel model. These factors all contribute to the asymmetry of the recording zone and hence the asymmetry of replay pulses and the associated nonlinear channel phase response. The use of the model for evaluation of channel spectra is examined. In particular it is found that good fits to experimental results for a typical recording channel are provided by a linear recording contour in conjunction with an arctangent transition profile. This model is useful for inclusion in a full digital magnetic recording system model since analytic expressions are available for both the replay pulse and its spectrum     

Transition shape analysis from medium noise measurements

We present a technique to determine the transition shape in digital magnetic recording. The method involves analyzing both temporal and spectral transition noise measurements. This combined method simultaneously determines the transition parameter and the cross-track correlation width along with the transition shape.     

Observation of Recording Pole Instability in Perpendicular Recording

Down- and cross-track distributions of medium magnetization are determined by the recording pole field profile and the switching dynamics. Earlier studies of the write pole footprint in perpendicular magnetic recording revealed asymmetrical media saturation caused by magnetic pole nonuniformity. In this paper, we report variable footprint shapes indicating recording pole instability. We extend our spin-stand “footprint” measurements to also evaluate recording pole instability.     

The influence of short-range magnetic correlations on the superconducting transition temperature

Taking into account correlations between well-defined localized magnetic moments in a dilute magnetic system, we derive an expression for the super-conducting transition temperature which is different from Bennemann's theory. In particular it is shown that the influence of short-range magnetic correlations in the vicinity of a magnetic phase transition on the superconducting transition temperature is twofold: (1) There are large corrections to the Abrikosov-Gor'kov theory. (2) For a certain choice of parameters the phase diagram in the temperature versus concentration plane shows a break (where the system is normally conducting) between two superconducting regions.     

Properties of side-shielded read heads in longitudinal and perpendicular recording

Side-shielded (SS) read heads were fabricated, and their magnetic track widths were calculated and measured. The measurements in longitudinal recording show that SS heads exhibit sharper profiles compared with side-unshielded heads. To examine the effect of side shielding, we studied the dependence of the magnetic read width on write density using calculations and experiments. The calculations indicate that the SS head can reduce the skirt of the microtrack profile even at low densities, while the side-unshielded head cannot. This result was qualitatively found in an experiment. We also studied the SS effect in perpendicular recording and found better performance. The calculations predict that SS can strongly reduce the skirt of the microtrack profile even in perpendicular recording. We observed a sharper profile in an SS head compared with a side-unshielded one.     

Perpendicular magnetic recording--Its basics and potential for the future

In this tutorial paper, perpendicular and longitudinal magnetic recording are compared in terms of their fundamental magnetization transition sharpness and magnetostatic energy per bit. The superiority of perpendicular recording to longitudinal recording at high densites is demonstrated. Perpendicular recording with ring-shaped heads is also discussed. Performance and structural features are compared for several different head configurations.     

Thermo-magnetic recording on a Co/Pt perpendicular film by a sharp metallic probe

We studied a method of heat-assisted magnetic recording (HAMR). The recording medium is a strongly coupled Co/Pt multilayered thin film, suitable for perpendicular recording. Field emission current from a scanning tunneling microscope (STM) tip is used as the heating source. Pulse voltages of 5?V were applied to the film. Experimental results show that an average mark size of 100-120?nm was achieved, with the minimum being 45?nm. Models of domain stability and dynamic domain formation are built to quantitatively explain the experimental results. They agree well with experiments. The models give us future directions for achieving small marks for ultra-high recording density.     

Perpendicular magnetic recording -- Evolution and future

This paper reviews research results for the head, medium and recording properties of a perpendicular recording system. Superior bit density characteristics obtained so far indicate that perpendicular recording is basically free from demagnetization in the high density region and that it will establish a new field of recording technology in the future. The prediction is explained in the context of complementary profiles of longitudinal and perpendicular recording. At the very beginning of magnetic recording, a perpendicular-type head was considered but abandoned because a suitable medium did not exist. Progress in material science has enabled us to develop a perpendicular recording medium which is very well suited for ultra high density recording. As so often happens, history has repeated itself through another study of perpendicular recording.     

Direct Solution of Medium Field and Cross-Track Characteristics of Read Sensors

This paper describes a direct calculation method for medium fields in perpendicular recording systems. The method accounts for the effect of the zig-zag transition in the written medium pattern which can be obtained using micromagnetic simulations. The described method is useful for studying the cross-track characteristics of read sensors. The method is able to deal with a recording medium layer with nonunity relative permeability. The computed medium field in the gap region under the read head is compared with that obtained using the finite-element method. The purpose of this study is to develop a fast and effective tool for producing read-back signals based on computer modeling and simulation.     

Amorphous transition metal films

The structure and magnetic properties of amorphous transition elements is reviewed. The methods of obtaining suitable samples will be discussed, the structural investigations so far carried out will be described, and the results compared with those obtained from model building. The experimental measurements on the effect of structural disorder on the magnetic moment and Curie temperature of the transition elements will be reviewed in the light of current theoretical progress.     

Laminated antiferromagnetically coupled media - optimization and extendibility

Lamination of multiple isolated magnetic layers has been shown to be an effective method to significantly increase signal-to-noise ratio in longitudinal media. These laminated media, however, are accompanied by low overwrite and wide magnetic pulse width, mainly as a result of poor writing of the bit transitions in the magnetic layer further away from the head and an offset in the transition position in the multiple magnetic layers resulting from head field spacing loss. We have demonstrated that the transition writing and transition alignment in the multiple magnetic layers of the laminated antiferromagnetically coupled (AFC) media can be optimized by adjusting the magnetic anisotropy of the relevant magnetic layers to compensate for the reduction of the head field magnitude with spacing. Such optimization results in significant improvements in media recording performance, leading to successful application of this medium technology. In this paper, we will highlight some of these improvements and discuss our approaches to further improve the recording performance by reducing the thicknesses of the magnetic layers and the lamination spacer layer in the laminated AFC film stack and by introducing additional elements in the magnetic layer.     

Gd5Ge4的磁结构相变和磁玻璃行为的研究

通过对温度诱导和磁场诱导的磁化强度的测量和分析，证明了Gd5Ge4在低温范围存在从反铁磁到铁磁的一级结构相变，有典型的相分离特征。为了进一步说明Gd5Ge4的磁结构相变，对不同低温的磁场诱导的磁化强度进行了两次循环测量，结果显示Gd5Ge4在低温下磁化行为具有不可逆性和对温度的依赖性。同时对磁化强度的研究揭示了Gd5Ge4在低温时存在磁玻璃态。     

Aliovalent-ion and magnetic field induced phase transition in multiferroic biFe(1-x)Ti(x)O3 system

Multiferroic compounds with general formula BiFe(1-x)Ti(x)O3 (x = 0.1, 0.2, 0.3 and 0.35) have been synthesized by conventional solid state reaction method. The effect of Ti substitution on ferroelectric and magnetic properties is studied. From X-ray diffraction (XRD) analysis, a rhombohedral to orthorhombic phase transition for x > 0.3 was observed. From SQUID measurements, a magnetic field induced phase transition has been observed in the BiFe(1-x)T(x)O3 system for x = 0.3. An anomaly in dielectric constant and dielectric loss in the vicinity of antiferromagnetic Néel temperature (T(N)) and a small enhancement in magnetization have been observed. Magnetization measurements above room temperature showed no systematic variation in antiferromagnetic Néel temperatures on Ti substitution. Further it is seen that this system shows the coupling between electric and magnetic dipoles exhibiting magnetoelectric (ME) effect at room temperature and possess high dielectric constant.     

Electrical control of the ferromagnetic phase transition in cobalt at room temperature

Electrical control of magnetic properties is crucial for device applications in the field of spintronics. Although the magnetic coercivity or anisotropy has been successfully controlled electrically in metals as well as in semiconductors, the electrical control of Curie temperature has been realized only in semiconductors at low temperature. Here, we demonstrate the room-temperature electrical control of the ferromagnetic phase transition in cobalt, one of the most representative transition-metal ferromagnets. Solid-state field effect devices consisting of a ultrathin cobalt film covered by a dielectric layer and a gate electrode were fabricated. We prove that the Curie temperature of cobalt can be changed by up to 12 K by applying a gate electric field of about ±2 MV cm(-1). The two-dimensionality of the cobalt film may be relevant to our observations. The demonstrated electric field effect in the ferromagnetic metal at room temperature is a significant step towards realizing future low-power magnetic applications.     

Photothermally modulated magnetic resonance applied to the study of the magnetic phase transition in gadolinium thin films

We explore the photothermally modulated magnetic resonance technique to investigate gadolinium thin films deposited on fused quartz substrate, as a function of thickness and thermal treatment, around the magnetic phase transition temperature. It has been observed that the maximum amplitude of the photothermally modulated magnetic resonance (PM-MR) signal takes place near the phase transition temperature, similarly to the magnetocaloric effect, for which Gd has been the prototype material. The reason is that both depend on the temperature derivative of the magnetization, which maximizes at the phase transition. Besides, there is a narrowing of transition with thermal treatment, confirming that thermal treatment stabilizes the film structure. For frequency scan measurements, the heat diffusion in a two-layer system was considered, and a depth profile study was carried out in order to investigate heterogeneities along the film thickness. From the PM-MR response as a function of the modulation frequency it was possible to estimate the thermal properties of the Gd film. Magnetization, X-ray and electron spin resonance measurements were used to complement the analysis and support the conclusions.     

Superconducting to normal transition of type-II Ta by a rectangular current pulse

The superconducting to normal transition by a rectangular pulse current in type-II Ta has been investigated. The potential drop along the specimen is observed as a function of time. The voltage pattern and the magnitude of the potential drop are obtained as functions of current amplitude, temperature, externally applied magnetic field, the angle between magnetic field and specimen axis, and values. The transition mechanisms are discussed. It is also shown in this experiment that the flux flow or flux creep and flux jump occur. A support to the paramagnetic current flow in the presence of longitudinal magnetic field is given.     

Influence of orientation ratio on reverse erase-edge noise andtrack-edge dipole distribution

The cross-track profile of media noise is measured on a precision spinstand for oriented and nonoriented media. These data are correlated with magnetic force microscopy (MFM) images to determine the location of track-edge noise with high spatial resolution. A significant component of track-edge noise is located in a narrow band at the edge of bits recorded in opposition to the previously saturation-erased direction. This reverse erase-edge noise (REEN) increases as orientation ratio increases. The magnitude and distribution of REEN is consistent with a reverse-dc-erase mechanism. δM data indicate a greater influence of magnetostatic and/or exchange coupling for the oriented media. Together with larger on-track reverse-dc-erase noise and higher supralinear transition noise, these results suggest enhanced collective magnetization reversal for the oriented media relative to the nonoriented media. MFM images also reveal the presence of narrow magnetic-dipolar strips at the track edges. These dipolar strips are generated by cross-track components of the head field. The track-edge dipole moment decreases as orientation ratio increases due to preferential alignment of easy axes along the down-track direction. These dipoles contribute to base line shift and are not a significant source of media noise     

Elastic anisotropy and magnetic phase transition of yttrium garnet ferrite

Temperature dependences of elastic moduli describing elastic anisotropy in the rage from room temperature to 600 °C are shown. These temperature dependences are in agreement with available values of elastic moduli and temperature coefficients of the elastic moduli at room temperature. They are in agreement with modern quasi-harmonic theory of crystal lattices. Near 285 °C critical changes of effective elastic moduli due to magnetic phase transition are observed. A correlation was made between experimental and theoretical critical changes of longitudinal and transverse elastic wave velocities for different crystallographic directions. It is proved that the critical change of ultrasound wave velocities of this crystal is connected with the single-ion magnetostriction.     

Combatting partial erasure and transition jitter in magneticrecording

Practical recording channels deviate significantly from the oft-used model based on linearity and additive Gaussian noise. Among the well-known anomalies are partial erasure and transition jitter. In this paper, we first describe a precompensation technique to combat the partial erasure effect. It utilizes the unique property of the maximum transition run (MTR) code wherein all dibits are isolated. The idea is to increase the spacing between the two transitions that make up each dibit. This tends to mitigate the nonlinear amplitude reduction associated with partial erasure. Next, we consider a recording channel that is linear but subject to transition jitter. We modify the hyperplane-based detector known as 3D-110 by introducing data-dependent threshold terms. The overall complexity of the detector is small, yet this modification provides almost 2 dB of improvement relative to the original 3D-110 structure and the extended partial response maximum likelihood (EPRML) method tuned to additive Gaussian noise