Lieping Zhong

First principles investigation of domain walls and exchange stiffness in ferromagnetic Fe and antiferromagnetic NiMn

We investigate the domain walls in ferromagnetic Fe and antiferromagnetic NiMn with the first principles full-potential linearized augmented plane-wave method including intra-atomic noncollinear magnetism. In both cases, the self-consistent results demonstrate that the magnetization changes continuously from one orientation to another as seen in a Bloch wall. The formation energy of the domain wall (ΔEDW) significantly decreases when the wall thickness increases, which leads to an exchange stiffness of 1.13×10−11u200aJ/m for Fe and 1.43×10−11u200aJ/m for NiMn. The predictions agree with those determined separately for Fe from a phenomenological calculation.

Vertical giant magnetoresistive read heads for 50-Gb/in/sup 2/ magnetic recording

We have designed and fabricated a vertical giant magnetoresistive (VGMR) head for high-density magnetic recording. In this style of recording head, read current flows through the sensor length, which is oriented perpendicular to the air-bearing surface (ABS), and the transducer is fabricated from GMR sandwich material. In the VGMR style of head design, the correct bias is obtained through a combination of intrinsic anisotropy, shape anisotropy, and longitudinal bias field generated by the sense current. The material at the edge of the sensor is stabilized using an edge-hardening process, which is designed to prevent switching of the magnetization at the edge of the sensor in a reverse applied field. Both top and bottom shields are recessed away from the ABS in order to increase the signal decay length and boost sensor output. We tested read heads incorporating the VGMR reader design under conditions of up to 50 Gb/in/sup 2/. We describe additional sensor concepts, including folded and differential VGMR head designs.

Head field angle dependent writing in longitudinal recording

Summary form only given. The writing process in magnetic recording is the switching of media grains by the head magnetic field. As the head moves across the media, the grains in the recording layer experience the writing fields that vary both in magnitude and direction. Fundamentally, in the longitudinal recording, the writing field perpendicular component provides a path for the media magnetization gyromagnetic motion. Therefore it is likely to reduce the required longitudinal field for the grain reversal. The traditional Willium-Comstock model includes only the longitudinal components of the head field. It has predicted that the required deep gap field should be roughly 3 times the media (writing) coercivity. This requirement has been contradicted by the recently published experimental result of Liu et al. (see IEEE Trans. Magn., vol. 37, p. 613 et seq., 2001). Using our micro-magnetic recording model, which combines the writing and reading process at head-media level, the impact of the full 3D writing field on the recording process was examined. The present work focused especially on the effect of field angular dependence. A modified Willium-Comstock model was also developed to include the effects of the perpendicular field component. The corrected calculations predicted an optimum writing field that was reduced and agreed reasonably well with the results obtained from experiment and micromagnetic simulations.

Incomplete switching of media magnetization in perpendicular magnetic recording

Incomplete magnetization switching may take place between two recorded transitions in perpendicular magnetic recording. These write process issues were studied with a micromagnetic model. The relatively poor effective field from the central part of the perpendicular writer is unable to reverse the media grains properly. Consequently, media dc noise will increase as the incomplete switching becomes more severe. An expression for the effective field heff was proposed to easily discover those cases. The incompletely switched magnetization observed in the static footprint could also be partially transferred into the dynamic write process. The calculated media signal-to-noise ratio on a pseudorandom sequence could be reduced by as much as 3dB (compared to the fully switched case). On the other hand, transition quality does not become worse with a smaller heff. This insensitivity to the transition quality was caused by the magnetostatic field from the opposite magnetization polarity on the other side of the rec...

Micromagnetic modeling of adjacent track erasure in longitudinal recording

Adjacent track erasure in longitudinal recording was studied using a micromagnetic model. The gap and fringe fields of two heads designed to work for 100 kTPI were used to write and erase a 31-bit pseudorandom sequence (PRS) in the main track. Comparisons on recording performance were made based on the calculated magnetization profile and the equalized PRS read back signal. It was shown that, for the case of a 3800-Oe fringe field, the quality of the transitions degraded drastically when the transitions were exposed to the fringe field more than 1000 times. On the other hand, a 2000-Oe fringe field left unaffected transitions even when the transitions were exposed to the field more than 100 000 times.

Microtrack base line instability of spin valve heads

The baseline shift in a micro-track profile of spin valve heads was investigated. Experimental and micro-magnetic modeling results were compared and found in close agreement. It was confirmed that the nonuniformity of pinning field and variation of anisotropy were among the potential sources of the formation of multi-states and the baseline instability. It was shown that the resulting instability could be suppressed by applying a strong permanent magnet stabilization field.

First-principles calculations for the structural and magnetic properties of ordered NiFe(001) thin films with and without a Ta overlayer

The first-principles calculations within the local density approximation using the full potential linearized augmented plane wave (FLAPW) method were performed to investigate the structural and magnetic properties of the Ta/NiFe interface for both clean NixFe1−x (001) thin films and with a Ta overlayer. To study the composition dependence, we adopted x=0.5 (L10 structure with either Fe or Ni layers on the surface) and 0.75 (L12 structure with mixed Fe-Ni layers or with Ni on the surface). The equilibrium overlayer/substrate distance and the preferred site position of Ta were obtained by structural optimization employing atomic-force calculations and total energy comparisons for several possible adsorption sites of Ta. By comparing with results for the clean surface of five-layer NixFe1−x (001) films, we found that Ta has a significant detrimental effect on the magnetic properties of NiFe with its induced magnetic moment coupled ferro- or antiferro-magnetically with the substrate depending sensitively on t...

Micromagnetic Modeling of Recording on Discrete Track Perpendicular Media

The SNR performance of DTM is evaluated through micromagnetic simulation. The benefits of recording on DTM are discussed and compared to conventional media. In the micromagnetic model, the thin film media is approximated by the Voronoi-shaped three-dimensional array of grains. The anisotropy axis of each grain, oriented perpendicular to the thin film plane, is chosen to have a Gaussian distribution. The signal to media noise ratio (SMNR) and signal to electronic noise ratio (SENR) are calculated based on the signal and noise at the sampling points. The SMNR and SENR are used to calculate the total SNR.

Noncollinear magnetism and enhancement of magnetocrystalline anisotropy at the Σ3(111) grain boundary in ferromagnetic Fe

Magnetic structures and magnetocrystalline anisotropy of the Σ3[110](111) grain boundary (GB) in ferromagnetic Fe are investigated by the first-principles full-potential linearized augmented plane-wave method including intra-atomic noncollinear magnetism. In breaking the spatial translation symmetry in a crystalline solid, the GB is found to give rise to a magnetic noncollinearity, where the magnetic moments at both sides of the GB orient at an angle of about 10° with respect to each other. Importantly, the presence of the GB enhances the magnetocrystalline anisotropy energy by one order of magnitude from its bulk value and may induce a pinning effect on the magnetization rotation or magnetic domain wall motion.

Micromagnetic simulation of pseudorandom sequences for equalized SNR estimations in longitudinal recording

Summary form only given. The determination of signal to noise (SNR) and signal to distortion (SDR) ratios is important in estimating bit error rate in recording systems. The effects of signed equalization also modify the relative contributions of SNR and SDR. In this paper, we simulate the recording of a 31 bit pseudorandom sequence (PRS) of data by writing with head fields derived from a finite element model (FEM) with an exponential risetime onto a longitudinal recording media. The media is modeled by an array of uniformly magnetized cubes and brought to equilibrium after discrete downtrack translations of the write head until the PRS is completed. The process is repeated 46 times with different easy axis distributions of the cubes (grains).