Mao-Min Chen

Gigabit density recording using dual-element MR/inductive heads on thin-film disks

Inductive-write and magnetoresistive (MR)-read dual-element heads of very narrow trackwidths and narrow gaps have been designed, fabricated, and recording-tested on thin-film media of high coercivity and squareness. Results show excellent writability at modest write currents of 50 mA, large readback amplitudes with good signal-to-noise performance, very high linear-density resolution up to 5000 fc/mm (-6-dB density), and trackwidths close to the design dimensions. These results show that such head-disk systems are capable of recording operations at a target areal density of 1 Gb/in/sup 2/. Another result is that novel techniques for measuring nonlinear transition shifts as well as offtrack and squeeze performance have been developed. >

A Study of Write Margin of Spin Torque Transfer Magnetic Random Access Memory Technology

Key design parameters of 64 Mb STT-MRAM at 90-nm technology node are discussed. A design point was developed with adequate TMR for fast read operation, enough energy barrier for data retention and against read disturbs, a write voltage satisfying the long term reliability against dielectric breakdown and a write bit error rate below 10-9. A direct experimental method was developed to determine the data retention lifetime that avoids the discrepancy in the energy barrier values obtained with spin current- and field-driven switching measurements. Other parameters detrimental to write margins such as backhopping and the existence of a low breakdown population are discussed. At low bit-error regime, new phenomenon emerges, suggestive of a bifurcation of switching modes. The dependence of the bifurcated switching threshold on write pulse width, operating temperature, junction dimensions and external field were studied. These show bifurcated switching to be strongly influenced by thermal fluctuation related to the spatially inhomogeneous free layer magnetization. An external field along easy axis direction assisting switching was shown to be effective for significantly reducing the percentage of MTJs showing bifurcated switching.

Physical properties of spinel iron oxide thin films

This paper describes the complexity of the spinel iron oxides in thin-film configuration. First, the experimental deposition conditions are determined for the parameters of substrate temperature and oxygen flow such that only the “Fe 3 O 4 ” phase is formed. Then a study is made of how the structural (grain size, lattice parameter, texture), magnetic ( M ), and optical (visible and infrared transmission) properties of the films depend on the deposition and postdeposition (air annealing) conditions. The experimental deposition region is defined where the films have the most similar physical properties to bulk Fe 3 O 4 and subsequently, after annealing, to bulk gamma Fe 2 O 3 . Finally, a discussion is presented of a model that accounts for the anomalous, low values of the magnetic moment for the samples deposited at room temperature. The model proposes an overpopulation of the iron tetrahedral A sites.

Magnetic instability of iron oxide thin films

The coercivity H/sub c/ and coercive squareness S* of gamma -Fe/sub 2/O/sub 3/ films made by oxidation of reactively sputtered, Os-doped Fe/sub 3/O/sub 4/, are shown to change by as much as 30 and 50% respectively in one day at room temperature. On initial measurement the films are isotropic in the plane and have S* values of 0.5 to 0.7. Along the direction of remanence an easy axis develops with a 10 to 30% higher H/sub c/ and an S* of 0.85 to 0.95, corresponding to global anisotropy in the range of 5*10/sup 4/ erg/cm/sup 3/. Reversible changes happen so rapidly that large differences can be seen on consecutive vibrating sample magnetometer loops. These films also exhibit very high susceptibility ( chi >10/sup -3/ G/Oe) and rotational hysteresis (W/sub r/>10/sup 5/ erg/cm/sup 3/) at 16 kOe, suggesting random local anisotropy with components in excess of 50 kOe. Rotational hysteresis has a strong dependence on the rate of field rotation. In a stationary field the torque initially decreases logarithmically with time, yielding activation energies varying from 0.6 to 0.16 eV as the temperature increases from 24 to 120 degrees C. All iron oxide films made by oxidation of reactively sputtered Fe/sub 3/O/sub 4/, whether undoped or Co- or Os-doped, show both larger W/sub r/ and global anisotropy reversible at room temperature. >

Effects of deposition and oxidation processes on magnetic and structural properties of iron oxide films

Osmium doped (0.75 at.%) iron oxide films were reactively sputter deposited onto Si substrates in an Ar + O 2 gas environment. The magnetic and structural properties of the as-deposited films depend critically on deposition process parameters such as O 2 flow rate, chamber pressure, substrate temperature, RF power and substrate rotation speed. The deposition process window has been determined for which only the spinel structure was observed. X-ray diffraction indicated films grown at a low O 2 flow rate of 3.0 sccm and a substrate temperature of 230°C had more random orientation and a lattice parameter closer to that of the bulk Fe 3 O 4 value. This condition produced a high saturation moment of 410 emu/cc. Increasing O 2 flow rate induced preferential

Performance study and analysis of dual-element head on thin-film disk for gigabit-density recording

Inductive-write and magnetoresistive-(MR)-read dual-element heads with very narrow tracks and gaps have been designed, fabricated, and tested on thin-film media of high coercivity and squareness. The results not only show excellent writeability at modest write currents but also the existence of a narrow region of optimum write current, limited by the onset of self-erasure by the write head at high write currents. This leads to significant degradations of overwrite, signal amplitude, trackwidth, linear resolution and disk-noise-induced peak-jitters. A peak-jitter approach is shown to be useful in characterizing many aspects of recording performance. A peak-jitter evaluation of signal-to-noise behavior reveals not only satisfactory overall performance but also the dominance of disk noise as well as a concentration of the disk noise at the track edges. Peak-jitter evaluations of offtrack and squeeze behavior clearly demonstrate the narrow-track capabilities of these recording heads for high areal density operation. >

Exchange bias enhancement through interdiffusion of NiFe/FeMn/metal films

The exchange bias field of the NiFe/FeMn system was found to increase from 30-35 Oe up to 75 Oe by annealing a three layer film structure of NiFe(300 AA)/FeMn/M (M=Au,Rh,Pt,Cr,Cu, etc.) at elevated temperature. Auger electron spectroscopy shows that interdiffusion occurred in the multilayer structure between the FeMn and the metal layer. The increase in exchange bias field is attributed to the altered interface of NiFe/FeMn due to the interdiffusion. >

2 Gbits/in/sup 2/ Dual Stripe MR Heads

Dual-stripe MR heads with linear density capability of 200 KBPI and track density capability of 11000 TPI have been manufactured. The write head, with a 15 turn coil and a gap length of 0.3 /spl mu/m, is made of conventionally plated Permalloy for both the leading and trailing poles. The thickness and track width of the trailing pole are 3.5 and 2.1 /spl mu/m, respectively. The MR elements have a 1.4 /spl mu/m track width, 1.1 /spl mu/m stripe height and a 17.5 nm thickness. The shield-to-shield separation is 0.18 /spl mu/m. The two MR elements are mutually biased and patterned-exchange domain stabilized. Recording tests were performed on a thin-film disk with a coercivity of about 3000 Oe and an M/sub r/t of 0.75 memu/cm/sup 2/. The read-back amplitude is as large as 1.02 mV(p-p), and the 50% roll-off density is as high as 160 kfci. Error rate test with a PRML channel at a data rate of 105 Mbits/s showed that a low on track error rate of 10/sup -10/ and an off-track capability of more than 15% of the track pitch could be easily achieved at a linear density of 200 KBPI and a track density of 11000 TPI.