F.H. Liu

The spin flop of synthetic antiferromagnetic films

The spin flop of synthetic antiferromagnetic pinned layers (SAF), under a magnetic field has been theoretically predicted and recently reported [J. G. Zhu and Y. Zheng, IEEE Trans. Magn. 34, 1063 (1998); J. G. Zhu, IEEE Trans. Magn. 35, 655 (1999)]. However, no experimental data have yet being reported to confirm the theoretical prediction. This article will provide direct experimental evidence to confirm the spin flop phenomenon in SAF layers. A spin valve, [CoFe/NiFe]/Cu/[CoFe(II)/Ru/CoFe(I)]/IrMn, was used to verify the spin flop in SAF layers. The exchange bias direction of CoFe(I)/IrMn was introduced by a magnetic annealing process at 225 °C with a field strength of Han(10 kOe) and the exchange bias direction was found parallel to the magnetic field. These samples serve as the reference for the remaining experiments. By magnetic annealing the reference samples at 225 °C with lower magnetic fields, we found that the magnetic field threshold for SAF spin flop is about 1 kOe. When the field is further i...

Kerr effect imaging of dynamic processes in magnetic recording heads

Techniques for imaging microscopic dynamic magnetic phenomena in magnetic recording heads are reviewed. Two experimental apparatus which utilize the Kerr magnetooptic effect are described. A scanning magnetooptic photometer uses the principles of confocal optical microscopy in which a focused laser spot serves as a high-resolution ( approximately 0.3 mu m) probe of magnetic activity to very high frequencies (250 MHz). Magnetooptic flash photography uses the technique of stroboscopic imaging with digital image processing to provide instantaneous (10 ns exposure time) images of magnetic phenomena on a microscopic scale by utilizing a pulse laser for illumination. Results from various studies of ferrite, metal-in-gap, and thin-film magnetic recording heads using these apparatus are reviewed along with their methods. >

Greater than 13 Gb/in/sup 2/ spin valve heads

We have designed and fabricated dual spin valve heads with synthetically pinned layers. They have excellent sensitivity. The read-back waveform has a small asymmetry and that is also insensitive to the bias current. The read gap length is of 0.14 /spl mu/m and the write gap length is of 0.2 /spl mu/m. The read heads have a nominal magnetic trackwidth of 0.5 /spl mu/m and a nominal write magnetic trackwidth of 0.8 /spl mu/m. The write heads were conventional inductive heads with their pole geometry defined by focused ion beam (FIB) technology. The dual spin valve heads have achieved an areal read/write density as high as 14.5 Gb/in/sup 2/ at a data transfer rate of 23/spl sim/24 MB/sec. The sensitivity of the readers is as high as 3.6 mVp-p//spl mu/m. The bit aspect ratio is as high as 14.

Fabrication and performance of high moment laminated FeAlN thin film inductive recording heads

Experimental thin film inductive heads using previously developed laminated FeAlN high moment soft magnetic materials have been designed, and fabricated to the wafer level. The heads, with a gap length of 0.2 /spl mu/m and trackwidths varying from 6 to 84 /spl mu/m, were fabricated with a mainly dry process. The dynamic domain patterns of the top magnetic poles were observed with a high speed wide-field Kerr microscope. Closure domains were not present, while multiple easy-axis domains were observed in the head yokes. The head inductances were measured from 1 to 50 MHz with a network analyzer. The electrical and magnetic testing results show that the fabricated heads function well at the wafer level and that laminated FeAlN high moment material is a very promising candidate for future high-density recording head applications. >

High frequency dynamic imaging of domains in thin film heads

A wide-field magnetooptic domain observation system with a time resolution of 10 ns has been developed to study magnetization dynamics in thin-film heads. The instantaneous dynamic response on the top yoke of thin-film recording heads is examined at any chosen instant within the drive current cycle at frequencies up to 20 MHz. Different phase responses from different domain walls in the head are observed and interpreted in terms of hysteretic wall motion, effective field density variation in the head, and wall orientations relative to the flux conduction direction. Two different flux conduction mechanisms associated with two different domain structures in the central region of the head are observed and discussed. Flux conduction in the center of the head by motion of backgap walls and magnetization rotation for domain structures with and without the backgap walls was observed. The domain structure with the backgap walls is probably undesirable because the backgap wall motion may cause a decrease in head efficiency during high-frequency operation and could cause noise during read-back. >

Domain structures and dynamics near the backgap closure of thin-film heads

Statically unstable spikelike reversed magnetic domains emanating from the backgap closure of thin‐film heads have been observed with high‐frequency (1–20 MHz) and high‐current amplitude (20–80 mA p‐p) drive fields by using a wide‐field magneto‐optic domain observation system with a 10‐nsec exposure time. The spikelike domains are repeatedly nucleated and annihilated within the drive cycle and do not remain after removal of the excitation. The formation of spikelike domains is due to the magnetization rotation back to the nearest easy directions after fanning out near the backgap closure to carry the spreading flux out of (or converging flux into) the backgap closure. The spikelike domains block the flux flow into and out of the backgap closure at excitation frequencies above 10 MHz due to the slow domain annihilation process. It is speculated that occasional failure of these spikelike domains to annihilate after a write pulse could lead to noise when they suddenly collapse during read out.

Dynamic domain instability and popcorn noise in thin‐film heads

The effects of dynamic domain instability on popcorn noise probabilities were studied by correlating the measured popcorn noise probabilities with the instantaneous observations of dynamic domain states of thin‐film heads during and after writing. Heads with large peak popcorn noise probabilities at intermediate values of write current amplitude were observed to exhibit asymmetric domain patterns and occasional popping wall motion in the yoke structure during writing. The occurrences of dynamic domain instability were imaged by comparing many instantaneous domain patterns at 10 μs after a specific write current excitation. Delayed‐relaxation Barkhausen wall jumps near the backgap closure were observed occasionally in several noisy heads. Quantitative correlation between the probabilities of popcorn noise and dynamic domain instability was also obtained in a very noisy head. Dynamic domain instabilities in the yoke are thus responsible for the peak popcorn noise probabilities in thin‐film heads.

Dynamic response of domain walls on the air‐bearing surface of thin‐film heads

The domain configurations on the air‐bearing surface (ABS) of inductive thin‐film recording heads were studied. It was found that, instead of being a single domain structure, the ABS of a thin‐film head usually has multidomains. The direction of the domain walls is neither parallel nor perpendicular to the gap plane. The magnetization was found to be in the plane of the ABS along the track width, with the magnetizations on the two sides of the domain walls either ‘‘head‐to‐head’’ or ‘‘tail‐to‐tail.’’ The domain walls are slanted in order to spread the magnetic charges along the wall over a larger region, thereby reducing magnetostatic energy in this configuration. The responses of the domain walls are not all in phase, and they are generally out of phase with the rotational process along the gap edge. The magnetization configuration on the ABS and in the throat and the sloped region were investigated in one head and correlated with the domain walls on the ABS.

Characterization of high density spin valve recording heads by novel magnetic force microscope

High area density magnetic recording heads with giant-magnetoresistance (GMR) read sensors and focus-ion-beam (FIB) trimmed inductive writers were studied with a special magnetic force microscope (MFM). The MFM was equipped with a magnetoresistance sensitivity mapping (MSM) component that directly measures the magnetoresistance of the reader sensor as a function of position, and a High Frequency MFM (HFMFM) capability that maps 3D high frequency writing field distribution. Good agreement between the MSM measurement and the spin stand tests were obtained. MFM measurement offers unique high spatial resolutions not readily achievable with any other measurement techniques. It is the ideal tool for current and future high area density heads.

A heating model including effects on magnetoresistance (MR) in MR and giant MR heads

An analytic model is presented that describes current induced heating effects in giant magnetoresistive (GMR) heads. Unlike previous transmission line models, the model includes temperature effects on the measured GMR response of the head in addition to changes in the temperature and the resistance. The response of the head will decrease with temperature in part due to the negative thermal coefficient for the GMR. The model predicts that the measured GMR response does not decrease as much as indicated by the thermal coefficient since the signal is enhanced by the change in temperature due to the GMR response to the applied field.