Guo Mian

An algorithm for a real time measurement of nonlinear transition shift by a time domain correlation analysis

An algorithm has been developed for a real time measurement of the nonlinear transition shift in high areal density magnetic recording, by a time domain correlation analysis. By measuring SNR and a correlation coefficient of the time domain readback voltages, r[/spl nu/(t), /spl nu/(t+/spl epsiv//2+(M/sub 1/-1/2)T)], written by a pseudorandom sequence, we can determine the nonlinear transition shift in real time. This new algorithm not only simplifies data processing of the readback waveform, but also includes noise effects on the nonlinear transition shift measurement. This algorithm has been implemented by LeCroy Corporation into their 7200-A high-speed digital oscilloscope. The LeCroy oscilloscope measures the nonlinear transition shift in less than 100 ms utilizing this algorithm. >

Effects of a highly permeable keeper layer on transition shifts for thin film media

Utilizing a time domain correlation algorithm, we observed that the readback signal for media with a soft keeper layer can be shifted from their original written locations in the hard under layer. The shift was similar to the hard-easy transition shift. It results from a change of magnetic charge distribution in the keeper layer in the read process. The effect of the keeper layer on transition shifts in the write process was insignificant. >

Noise Correlation of Magnetic Thin Film Media

We have experimentally observed a linear relationship between the correlation coefficient of the noise voltages read on a saturated magnetic thin film rigid disk by a magnetic recording head at different radii, and the radial offset of the head between readings, in both longitudinal and perpendicular media. This linear relationship indicates that for the length of the data stream used in the experiments, the noise voltages from adjacent narrow longitudinal strips that could be denoted pseudo microtracks are uncorrelated. The experiments indicate that the correlation length between the pseudo microtracks is less than 0.5 µm. We have used the linear relationship as the basis for a track following scheme that follows an 11.3 µm wide track with 0.3 µm accuracy.

Micromagnetic transverse correlation length in thin film magnetic recording media, medium noise, and tracking

Abstract We compare the cross correlation coefficient of signal and noise voltages read on magnetic thin film rigid disks by a magnetic recording head at accurately controlled different lateral positions, and deduce from the results a transverse correlation length λ which is a measure of the characteristic lateral dimension of the magnetic microfeatures of the medium. By analyzing correlation data from uniformly magnetized media and from media with saturated and unsaturated written transitions we can determine λ as a function of the mediums state of magnetization. Values of λ inferred from our measurements on typical media are less than 0.5μm, ranging down to several hundred angstroms in some instances. We have applied the short correlation length as the basis for detecting and controlling lateral head displacements.

An in situ measurement of intergranular coupling in magnetic film media

We describe an in situ nondestructive method to measure the intergranular coupling in magnetic thin film recording media based on Stoner–Wohlfarth switching theory [Philos. Trans. Roy. Soc. London A 240, 599 (1948)]. The method uses recording write and read heads and a disk test stand to measure a medium’s remanence curves from saturation and demagnetization. Using this technique, we have measured Henkel plots [Phys. Status. Solidi 7, 919 (1964)] for single layer and multiple layer longitudinal media, as well as a Henkel plot for a perpendicular thin film with a highly permeable underlayer, which cannot be obtained by the conventional vibrating sample magnetometer method.

An interaction matrix for the energy analysis of an n-layered magnetic thin-film system

Abstract Normalized interaction factors have been developed for the analysis of the interaction energy and the magnetic as well as the electric properties of a multi-layered magnetic thin-film system. Mathematical expressions of these factors with respect to dimensions and separations of the films have been developed. Interaction factors and self-demagnetization energies of an n-layered magnetic thin-film system can be expressed by a matrix representation. For a magnetic thin-film system with k-dimensional magnetization, the interaction energy of the system can be described by k (k ≤ 3) symmetric interaction matrices. Diagonal elements of the interaction matrices are zero because they represent the self-energy of the system. The off-diagonal elements of the interaction matrix are nonzero for a system which includes interactions. By using these interaction factors and finding a minimum energy configuration, the direction of magnetization in each film can be numerically determined. The numerical solutions to the electrical property of a coupled MR thin-film system using these interaction factors agree with previous experimental findings.

Determination of a Track's Edge by Differential Power Spectrum

We show experimentally that the transverse edge of recorded data can be determined by spatially differentiating the readback power spectrum with respect to the displacement of the recording head. This was accomplished using straightforward signal processing described here as the differential power spectrum. This technique can be applied to data tracks having pseudorandom data even in systems not possessing guard bands. We describe a multi-element head structure which may be used to exploit this phenomenon to realize a new continuously monitored track following servo system.

Interaction factors of a multi-layered magnetic thin film system

Magnetostatic interaction factors of coupled magnetic thin films are proposed to assist the analysis of the electric and magnetic properties of such systems. The factors, f/sub y/ and f/sub z/, which are diagonal elements of an interaction tensor, are proportional to the interaction energies due to the magnetization in y and z directions. Three-dimensional mathematical expressions of these factors are developed. The interaction energy of the system in each film can be determined numerically using these factors. As a result, the magnetoresistive character of the system can be obtained numerically. The interaction factor decreases rapidly as the distance is increased, with the rate of decrease being faster for thinner films. The numerical solution of the output character of a coupled thin-film read head agrees with previous experimental findings. >