Mary Frances Doerner

Antiferromagnetically coupled magnetic media layers for thermally stable high-density recording

We describe a magnetic recording media composed of antiferromagnetically coupled (AFC) magnetic recording layers as an approach to extend areal densities of longitudinal media beyond the predicted superparamagnetic limit. The recording medium is made up of two ferromagnetic layers separated by a nonmagnetic layer whose thickness is tuned to couple the layers antiferromagnetically. For such a structure, the effective areal moment density (Mrt) of the composite structure is the difference between the ferromagnetic layers allowing the effective magnetic thickness to scale independently of the physical thickness of the media. Experimental realizations of AFC media demonstrate that thermally stable, low-Mrt media suitable for high-density recording can be achieved.

Dynamic coercivity measurements in thin film recording media using a contact write/read tester

Thermally activated magnetization reversal processes become manifest in the dependence of the remanent coercivity on the time during which a magnetic field is applied opposite to the initial magnetization direction. They have important consequences for the long term stability and short time writeability of future high density recording media. In this paper, we report on a new experiment using a contact write/read tester to study the time dependence of the remanent coercivity over more than 10 orders of magnitude (from 6 ns to >60 s). Remanence coercivity and signal decay measurements of a CoPtCr recording medium with 5.5 nm thickness are presented.

10 Gbit/in.2 longitudinal media on a glass substrate (invited)

This article reports on the properties of the media prepared on glass substrates which were used in IBM’s 10 Gbit/in.2 demonstration. In order to support a linear density of 315 kbpi and a track density of 33 ktpi, the remanant coercivity Hcr and remanant moment thickness product Mrt of the magnetic layer were 3450 Oe and 0.37 memu/cm2, respectively. The media used a NiAl seed layer, a CrV underlayer, a Co alloy magnetic layer, and a carbon overcoat protection layer. The magnetic film had a grain size of 12 nm as observed by transmission electron microscopy. The preferred orientation (PO) of the magnetic layer was (1010). This PO enables one to sustain high coercivities at low values of Mrt. It is observed that the c-axis in-plane texture of the magnetic layer is critical to achieve a low noise medium. Using a focused-ion-beam (FIB) trimmed giant magnetoresistance head and conventional partial response maximum likelihood channel, the on-track-error rates were measured at the 10−10 level.

5 Gb/in/sup 2/ recording demonstration with conventional AMR dual element heads and thin film disks

We have successfully demonstrated magnetic recording at an areal density of 5 Gb/in/sup 2/ and a data rate of 10 MB/s using narrow track dual element heads with conventional AMR sensors and low noise Co alloy thin film disks. In this work, the target densities of 240 K bpi/spl times/21 K tpi were achieved by a combination of narrow track and low-flying technologies. The write and read head trackwidths were reduced to submicron dimensions, with high moment pole-tips to maintain good writability. At the same time, magnetic spacing was substantially reduced by using low-flying airbearing surface designs. Finally, significant signal-to-noise improvements were attained with the development of high sensitivity AMR read heads and very low noise thin film media. Recording tests showed satisfactory writability in terms of overwrite and hard-transitions from the submicron width write heads. Readback yielded symmetrical signals close to 1 mV//spl mu/m and rolloff measurements yielded 50% densities as high as 7000 fc/mm. Track profile and microprofile measurements showed write and read trackwidths to be around 1.2 /spl mu/m and 0.7 /spl mu/m respectively, with tight side-writing and sidereading characteristics. An overall assessment of the parametric recording results suggested an areal density feasibility of around 5 Gb/in2. This projection was confirmed by error rate testing at 10 MB/s using a PRML channel with digital filter and write precompensation. At low ontrack errors of 10/sup -10/-10/sup -9/ without error correction codes, linear densities of /spl sim/240 K bpi and optimized track pitches of /spl sim/1.2 /spl mu/m were achieved, corresponding to areal densities of /spl ges/5 Gb/in/sup /2.

Microstructure and thermal stability of advanced longitudinal media

Thermal stability will ultimately limit the maximum areal density achievable with conventional longitudinal recording. The key aspects of the media microstructure contributing to thermal stability are the grain size and grain size distribution, alloy composition, alloy segregation, lattice defects and strain. Grain size distributions are created by the random nucleation processes occurring during media deposition. For media on glass substrates, c-axis in-plane preferred orientation can be achieved with either Co (112~0) or (101~0) planes parallel to the substrate surface. Improved squareness, S, is observed with the (112~0) orientation due to stronger crystallographic texture, however, larger changes in coercivity with decreasing magnetic layer thickness are observed compared to (101~0). Continued increases in areal density will require tighter grain size distributions and improved microstructural control of very thin magnetic layers.

Processing effects on the tribological characteristics of reactively sputtered chromium oxide (Cr2O3) overcoat films

The mechanical and tribological characteristics of thin chromium oxide films are investigated by controlling the process parameters in reactive deposition and subsequent annealing. From this, the correlation between the deposition process, film structure, and properties is established. Substrate heating and oxygen partial pressure are found to be the critical parameters that alter the oxygen concentration and crystallization of the films which, in turn, affect their stress, hardness, and wear resistance. The chromium oxide film deposited at 150 °C in pure argon and annealed at 300 °C shows a hardness of 25 GPa, which is near the bulk hardness of Cr2O3, and exhibits a good wear resistance with adhesive wear being the dominant wear mechanism. Reactive deposition at 25 °C or with excessive oxygen leads to films with nonstoichiometric composition, which hinders their crystallization upon low‐temperature annealing. As a result, the hardness is reduced, and the wear resistance deteriorates several orders of mag...

Composition effects in high density CoPtCr media

The optimization of the CoPtCr alloy for use in high density longitudinal magnetic recording is discussed. Low noise media can be obtained with a combination of high Cr concentration (22 at.%) and high deposition temperature. The results are consistent with a Cr segregation mechanism that provides reduction of exchange coupling without the need for a voided microstructure. The Curie temperature decreases significantly at 25 at.% Cr preventing further increase in the Cr concentration. Pt is added to the alloy to increase coercivity with little effect on other recording properties. >

Effects of stacking faults on magnetic viscosity in thin film magnetic recording media

There is much interest in crystallographic defects in thin film magnetic recording media and their role in influencing recording performance such as media noise and thermal loss. In this article we report a correlation between the magnetic viscosity in CoPtCr thin film media, which is the origin of thermal loss effects, and the concentration of local fcc-like regions. The concentration of these defects (the type and density of stacking faults) was varied by growth on different underlayers (Cr and CrTa/Cr) and was measured with grazing incidence x-ray scattering using synchrotron radiation. We show that a substantial percentage of local fcc regions in an otherwise hcp cobalt alloy film leads to significant magnetic viscosity effects at quite modest magnetic fields. We find that the activation volume is reduced for a sample with a higher percentage of fcc-like regions and suggest that this can be understood in terms of the effect of weak links acting to stabilize local micromagnetic configurations.

Materials Issues in Magnetic-Disk Performance

The continued exponential growth in areal density for longitudinal magnetic-recording devices places ever more stringent demands on disk performance. The design of materials and processes must provide the required advances in technology. The magnetic properties are controlled through the choice of underlayers, magnetic alloys, and the deposition processes that control crystallographic orientation and magnetic isolation between grains. The requirement of lower head-disk spacing places increasing stress on the tribological performance of the disks, controlled by a very thin overcoat and lubricant layer. This article reviews the various materials issues relevant to magnetic-disk technology. The major obstacle for achieving high areal density in thin-film media is transition noise. This noise arises from the zig-zag transition boundaries that occur due to cooperative switching of the magnetic grains. Both exchange coupling between grains and magnetostatic interactions cause magnetic-cluster sizes larger than the grain size. The goal is to magnetically isolate the grains and keep the grain size small. As the dimensions of the bit cell shrink, smaller grain size is required to obtain enough grains per bit cell to maintain the required signal-to-noise ratio (SNR). In the 10–40-Gbits/in. 2 areal-density range, the issue of thermal stability of small ( In addition to good SNR, a narrow transition width is needed in order to pack the transitions closer together. The objective is to minimize interactions between transitions to reduce nonlinear amplitude loss and superlinear noise.

Micromagnetic and experimental studies of CoPtCr polycrystalline thin film media with bicrystal microstructure

Polycrystalline CoPtCr/CrV (or Cr) thin films with different Cr concentrations were prepared by sputter deposition on NiP-plated Al substrates. TEM images revealed the existence of bicrystal clusters. High field torque measurement was used to determine the intrinsic anisotropy constant. The M-H loop and torque hysteresis measurement coupled with micromagnetic modeling permitted determination of intergranular interactions. The cross-track correlation length was evaluated from micromagnetic noise calculation and compared well with the result from measured noise spectra. The physical implication of a bicrystal structure is discussed in general.