Shengbin Hu

Manipulation of magnetism by electrical field in a real recording system

We report an electrically controlled magnetism in a real recording system with CoCrPt–TiO2 nanocomposite thin films as recording medium. We show that in a spin-stand test, with a small voltage of 3 V applied across the head-media gap during recording, a 13% reduction of saturation current was observed and the read back waveforms showed sharper transitions. These account for the 3 dB improvement in read back signal-noise-ratio of the written magnetic information. The improved recording performance is mainly attributed to the reduction of anisotropy of the recording medium in the presence of electrical field. Simulations were carried out to understand the magnetization reversal process under applied electric and magnetic fields.We report an electrically controlled magnetism in a real recording system with CoCrPt–TiO2 nanocomposite thin films as recording medium. We show that in a spin-stand test, with a small voltage of 3 V applied across the head-media gap during recording, a 13% reduction of saturation current was observed and the read back waveforms showed sharper transitions. These account for the 3 dB improvement in read back signal-noise-ratio of the written magnetic information. The improved recording performance is mainly attributed to the reduction of anisotropy of the recording medium in the presence of electrical field. Simulations were carried out to understand the magnetization reversal process under applied electric and magnetic fields.

A study of multirow-per-track bit patterned media by spinstand testing and magnetic force microscopy

We propose and demonstrate a concept of multirow-per-track bit patterned media (BPM) recording to overcome the problems encountered with the conventional one-row-per-track design. Focused ion beam was used on continuous granular perpendicular magnetic media to fabricate the prototype structures consisting of birow tracks with sub-100-nm single domain magnetic islands with the two rows of islands to be interleaved along the track direction, as well as an additional nonmagnetic spacer band between adjacent birow tracks for further bit aspect ratio (BAR)≥2 adjustment. Readback from such birow tracks with a two-row-wide read head was performed by dynamic spinstand testing. The proposed concept BPM provides many advantages including higher linear recording density (under the same lithographic limit), therefore enabling a higher data rate and a greater BAR≥2 for better integration with head design and servocontrol, as well as allowing the use of wider thus larger recording heads to improve writing efficiency fo...

Trapping Electron Assisted Magnetic Recording

Moving towards 10 Tb/in2 areal density, finding a proper recording scheme with enough write-ability is the most challenging task of a magnetic recording system. Some recording schemes with enhanced write-ability, such as HAMR, MAMR, graded media, etc., have been proposed to achieve higher recording density. Here we propose a new alternative approach for enhanced writing-trapping electron assisted magnetic recording (TEAMR). In the TEAMR configuration, an electrical bias is applied to the main pole of the write head with the disk media and the other parts of head slider grounded. As the main pole area is very small, the electrostatic force produced by electrical potential is a few orders smaller than the air bearing force at the rear pad. Therefore, it will not affect the flying performance of the head slider. At the nanometer head media spacing, a very strong electrical field is produced in the head media interface. This strong electrical field will cause free electrons to accumulate (be trapped) at the interfacial surfaces of metallic magnetic grains. These trapped electrons are localized in the surface atoms of magnetic grains and will alter the valance-electron band filling of those surface atoms. For many magnetic materials, the extra band-filling electrons reduce the magnetic anisotropy energy and make it easier to be magnetically switched. In this work, the TEAMR effect was proved by the experiment study on Co alloy based commercial disk media. The first principle calculation on L10 ordered FePt crystal shows that the magnetic anisotropy can be reduced to zero with around 0.38 electrons trapped into 1 unit cell of FePt. Further increase in trapped electrons will change the magnetic easy axis from out-of-plane to in-plane, which is considered as a negative magnetic anisotropy. With the magnetic anisotropy reduction at the surface atoms of each grain, micromagnetic simulation result shows that the effective switching field can be reduced to around 11% of anisotropy field for a 1.6 ? 1.6 ? 3.2 nm3 FePt grain. Thus TEAMR can be another good candidate for energy assisted recording requiring very little modification to the current perpendicular magnetic recording system.

Switching Probability Distribution of Bit Islands in Bit Patterned Media

In this paper, we report magnetic force microscopy (MFM) observations of switching probability of individual bit islands in bit patterned media. The switching probability (p) of each island was measured by repeated reversal tests at the same experimental conditions for each switching field (SF). It was found that there are ~60% of islands with for (which is approximately the average remnant coercivity, Hcr of the patterned islands) while the rest of the islands are either switched every time (for magnetically softer islands) or never switched (for magnetically harder islands). The observed statistical behavior of is an indication of thermal fluctuation during switching when magnetostatic energy (due to the applied external field) is comparable to magnetic anisotropy energy. As SF is decreased or increased away from Hcr (11 kOe to 9.5 kOe or 11 kOe to 12.5 kOe), percentage of islands with becomes smaller [narrower switching probability distribution (SPD)], due to less dipolar interaction/ variations among islands [which also lead to less switching field distribution (SFD) broadening]. Our results provide insights on the effects of statistical switching behavior of bit islands on the write errors in bit patterned media recording.

Bump characterization with MR/GMR head as transducer

With a magneto-resistive (MR) head as transducer, this paper report a method and experimental results of fast in-situ characterization of laser textured bumps over disk surface. The thermally induced change of the resistance of MR head is used to conduct fast analysis of the uniformity of bump width, bump pitch interval, and bump height in the laser textured landing zone of a magnetic disk. Two modes of the tester setup are discussed. One mode is for the fast detection of the above mentioned uniformity within one disk revolution. Another mode is for mapping the selected bumps or all the bumps in a selected sector or a sub-zone of the textured landing zone. The setup can be used in both on-line and off-line analysis of the laser bump uniformity in media manufacturing process.

Patterned media and energy assisted recording studied by drag tester

We developed a static drag tester for characterization and read/write of patterned and continuous magnetic disk media. To enable write synchronization for patterned media application, a scheme for write synchronization was developed and results for synchronized writing of patterned bits using a read-while-write approach are presented. In addition, a method for characterization of energy assisted media by drag tester was also explored.

Scanning carrier current method for in-situ measurement of flying height variation

A novel writing process based in-situ flying height measurement method, scanning carrier current method, is proposed. An erasure current oscillating in the frequency range of 200/spl sim/800 kHz is injected to the write head during the period of interest. Later, its effect on previously written data is detected to infer the flying height during the period of interest. The maximum scanning carrier current method provides a good solution to measure flying height variation in a large range during dynamic transient process. When the slider is in steady flying status, a lower scanning carrier current method can reduce the noise caused by medias MrT fluctuation, whilst a biased scanning carrier current method is suitable to reduce the noise caused by medias coercivity fluctuation in the flying height testing process.

A comparative study of write field distribution of trailing-edge shielded and unshielded perpendicular write heads by quantitative magnetic force microscopy

Three-dimensional write field distributions of trailing-edge shielded and unshielded perpendicular write heads with applied direct current write currents were studied by magnetic force microscopy (MFM) measurements. A quantitative MFM approach of two times integrating measured frequency shift at a series of tip scan heights over the perpendicular direction was proposed to measure the magnetic field strength and field gradient. Results indicate shielded heads in addition to smaller saturation write currents, exhibited more than two to three times higher write field gradients over a wide range of moderate field strength over unshielded heads, thus, producing sharper bit transition and improved signal to noise ratio as theoretically predicted.

In-situ flying height testing technology

As the technology goes to sub-10 nm flying height, there exists the high likelihood of slider disk interaction, and it becomes even more important to characterize the dynamic slider-lube-disk interaction. Due to the use of special glass disk, it is difficult for the commercial optical FH tester to test this interaction. The proper way to study this kind of interaction issue is the in-situ flying height testing technology. In this work, the triple harmonic method is selected to test FH in-situ, and the testing hardware had been setup. The accuracy of the developed in-situ FH test setup is within 2% of magnetic spacing between head and medium. The experimental results indicate that the nonlinearity of R/W process does not influence the FH testing much, and the testing setup also has good off-track capability of FH testing.

Tribo-Charge of Diamond-Like Carbon (DLC) Contact at Slider Disk Interface and Tribo Current Flow Through Disk DLC

This paper reports an experimental investigation on tribo-charging of diamond-like carbon (DLC) at the slider-disk interface and tribo-current flow through disk DLC. Measurements of the tribo-current are in three regimes-slider flying and touch down during disk deceleration, slider dragging on disk surface, and slider takeoff and flying during disk acceleration. Comparisons are made for tribo-charge currents at different disk initial linear velocities. It is observed that the tribo-current appears earlier with larger amplitude for a disk with larger initial velocities. During disk constant velocity phase, tribo-current ripple was observed while the slider is dragging on the disk surface due to intermittent slider/disk interactions. Despite having the same constant velocity, the equilibrium level of the ripple is different and depends on the disk initial velocity. Additional tribo-voltage and acoustic emission measurements were performed and compared with the tribo-current measurement to provide more insight to the tribo-charging process.