VSM Simulation of Perpendicular Recording

We report simulations of the writing process of isolated transitions with a magnetometer for perpendicular recording. The fields of three different head types, a pole head, a shielded pole head and a ring head were used to study the writing of transitions on two perpendicular media, one with a very square loop and the other one with a shallow loop. It is found that the shielded head records the narrowest transitions, followed by the ring and then by the pole head     

Properties of one-sided probe heads on double-layer perpendicular recording media

The perpendicular recording mode is still a candidate for obtaining higher densities. Best recording results on a double-layer (DL) medium must be expected from using a probe head (PH). This contribution provides an analysis of the write-read performance of one-sided PHs that do not use an auxiliary pole on the back of the medium. Analytical as well as numerical models were used to calculate the effects of saturation and medium thickness on the write fields. Experimental verification of the modeling results was obtained by measuring the wavelength response of many W-shaped PHs on DL media. Combining this with the calculated response, we obtained a semi-empirical relation between the pole nulls, pole thickness and the pole-to-backlayer distance. It reveals a PH behaviour very different from a ring head (RH). The pole thickness calculated from the measured pole-null wavelengths showed a very close agreement with the optically determined value. The calculated head efficiencies are very small, as was confirmed experimentally, and need improvement before the PH has an output comparable with a RH.     

Three dimensional simulation of side-writing and crosstalk for thinfilm heads with various pole shapes

Side-fringing fields of thin-film heads are studied with the three-dimensional simulation code TOSCA. Readback signals are calculated using the principle of reciprocity for various pole shapes and off-tracks. Read/write characteristics are evaluated in the presence of side-writing and crosstalk. They depend on the shape of the interface between the pole and the air-bearing surface of the sliders. The side-writing is large when the side angle is small. Crosstalk is large when the side angle is small and the projection of the pole from the track edge is large. A pole edge shape on the air bearing surface of thin-film heads that produces a narrow head field outside of the track and a sharp head field gradient is concluded to be essential to achieve large side-writing and small crosstalk     

Exact field calculations for asymmetrical finite-pole-tip ring heads

An exact solution is given for the fields of an infinitely permeable asymmetrical finite-pole-tip ring head. The solution is obtained by conformal mapping and is expressed in terms of elliptic integrals and elementary functions. Using the method of undetermined coefficients, it is shown how to decompose the Schwarz-Christoffel integral into elementary functions and elliptic integrals. For each given pole geometry, the constants appearing in the Schwarz-Christoffel formula are calculated once. Then the fields at various points in the poles domain are calculated using Newton's method and the formulas for the decomposition of the Schwarz-Christoffel integral.<>     

Total magnetic fields and read characteristics of infinite length pole piece magnetic recording heads

Magnetic recording head fields generated by current distributions of zero total current flux are calculated considering the two topological possibilities for the current elements being either external to infinite permeability pole pieces or contained entirely within the pole pieces. The frequency response for each of these field configurations is determined by application of the fast Fourier transform.     

Finite element analysis of the magnetic field distribution inside a rotating ferromagnetic bar

Finite element analysis techniques are being used widely to predict the field distribution in magnetic structures having complex boundary geometries and nonlinear B-H characteristics. In situations where two or more magnetic fields interact, care must be taken to ensure that the principle of superposition applies before adding the fields directly. The magnetic Reynolds number can be used to predict linearity, and in those cases where superposition is invalid, the finite element algorithm must be applied repeatedly in an incremental fashion to ensure accuracy. This procedure is illustrated by considering the magnetic field distribution inside a ferromagnetic bar rotating in a magnetic flux set up by a two pole structure. Results obtained by linearly superposing direct and generated cross axis fields at a particular speed of rotation are compared with the magnetic field distributions resulting from repeated application of the finite element algorithm for incremental step changes in angular velocity. Comparison of the neutral axis angle/speed characteristic for the linear and incremental models shows a significant difference between the two approaches.     

Perpendicular stand-still recording in Co-Cr films

Some features of perpendicular recording were investigated by means of stand-still recording experiments. For this purpose several radio frequency (RF)-sputtered Co-Cr layers of two different compositions were used as media while a Permalloy single pole (SP) head was used as a write transducer. A magnetoresistive transducer (MRT) was utilized to read the recorded pattern. Further, an analytical model was developed enabling the magnetization induced by the head field in the recording medium to be calculated. Both calculations and experimental results show a sharpening of the transition, due to the demagnetizing field. For a good approximation it is sufficient to consider only the vertical head field component and vertical magnetization in the recording medium. This is a consequence of the well-developed perpendicular anisotropy and negligible in-plane remanence of the Co-Cr layers. In addition the read-out signal is completely determined by the magnetic surface charges. The remanent magnetization in the recording medium and therefore the read-out amplitude is limited by demagnetization and consequently determined by the coercivity of the Co-Cr layer. The medium noise of a dc-erased medium indicates magnetic structures of much larger dimensions than the size of the individual crystallites. This noise appears to be dependent on the saturation magnetization of the Co-Cr medium. Activation of the single pole head by a homogeneous field results in a nonlinear behavior, caused by head saturation effects. This is supported by simple one-dimensional calculations of the head field.     

The WRITE process in magnetic recording

An extension of an earlier model of the WRITE process in saturation recording that accounts for internal fields due to magnetization divergence in the recording medium is presented. It is shown that the initial condition of the medium determines the internal field distribution during WRITE and may lead to appreciable peak shift and WRITE over modulation. The special case of two successive reversals of magnetization for a dc biased recording medium is considered in detail. System design criteria for optimization of the recording medium parameters, the head pole gap, and the WRITE time constant are presented for a system where the remaining parameters are constrained. Calculation for the initial and quiescent magnetization and internal field and the expected signal trace, in addition to experimental data, are presented for an out-of-contact drum system employing a NiZn ferrite WRITE-READ head and a CoNiP plating.     

Wavelength response of perpendicular magnetic recording

From the measurements of the surface field of a recording medium, it was obtained that an almost ideal step change distribution of magnetization is formed in perpendicular recording for digital signals. The fact represents that the upper limit of usable recording density is determined only by the resolution of a reproducing head. Therefore, the reproduction by a perpendicular head was successfully analyzed by using the reciprocity theorem. By introducing thickness loss of main pole of the head, the satisfactory coincidence was obtained between the calculated and the measured results in the wavelength response even at the densities of more than 150 kMRPI. In reproducing process, the magnetic interaction between the main pole and the medium acts so as to decrease the effective spacing between them.     

A study on the field distribution of thin-film heads

The field distribution of a thin-film inductive head and its relationship to pole configurations are examined. Three components of the field distribution at high frequencies are measured using a newly developed electron beam tomography method. Focused ion beam etching is used for the processing of the pole configuration. A comparison between the measured field distribution and the results of three-dimensional computer simulation shows that a sharp field distribution suitable for high-density recording is produced to promote magnetization at the tip region. Furthermore, the optimum design of the pole configuration clarifies the feasibility of 1-μm track recording     

A graded magnetization single pole head for perpendicular recording

The magnetic field of a graded magnetization single pole head in the presence of a highly permeable magnetic underlayer is analyzed. Vertical field amplitudes and gradients in the region close to the trailing edge of the pole are shown to be higher than those produced by a conventional single pole. For a perpendicularly magnetized medium, output spectral response functions, roll-off curves and peak shift computations indicate improved replay performance. The exact theoretical solution is obtained in the form of an infinite series using Fourier analysis but it is shown that the leading term alone can lead to a simple and accurate approximation. New theoretical results are also presented for a constant potential pole, including a simple approximation to the field and explicit exact formulae for certain output calculations     

Analysis of two-dimensional shielded pole heads using singular expansion functions

We have used a series of singular expansion functions to represent the potential across the gap of a shielded pole head in the presence of a highly permeable underlayer. This method of analysis reduces the size of the system of equations to be solved for the series coefficients to the number of coefficients that will be used. Just two series terms suffice to evaluate the magnetic potential to within 1% of the pole potential at any point on the air-bearing surface for a large range of head dimensions. Here, we express the surface field spectrum analytically. Including just two series terms provides an excellent estimate.     

Method of measurement and analysis of texture in thin films

A method of measurement and analysis of texture in thin films is proposed. In this method, the incidence angle of the X-ray beam is low and is kept constant during the measurements. This requirement allows information to be obtained from a thin surface layer of the specimen. However, it limits the area on the pole figure from which data can be measured. Quantitative analysis of various factors and parameters which affect the X-ray penetration depth and the measured area of the pole figure is proposed. The proposed method was implemented and tested and the results obtained were used to calculate the crystal orientation distribution function.     

Accurate approximation of fields and spectral response functionsfor perpendicular recording heads

Solutions for the magnetic potential in rectangular regions with simple boundary conditions are used to approximate the two-dimensional potential in the head face plane of a recording head. This method of approximation can be applied to any recording head with rectangular corners, in the presence of an underlayer. The slot approximation is demonstrated for ring heads and shielded (magnetoresistive) pole heads. The quarter plane approximation is given for application to finite-length heads. Estimates of the field components and the spectral response function are derived from the approximate potential. The simple slot approximation is shown to be at least as accurate as other, often more complicated, methods of approximation. Most significantly, both the slot and the quarter-plane approximations are easy to apply and need no prior knowledge of the head field of the particular geometry under consideration     

Theoretical study of vector magnetization distribution using rotational magnetization model

In magnetic recording, the recording fields are essentially vectorial, and the magnetization process in the recording medium has to be analyzed using vector magnetization. From this fact, a vector magnetization distribution in the recording medium must be evaluated by both magnitude and direction of magnetization. This paper describes the vector magnetization distributions obtained by a new method, using reversible and irreversible rotational magnetization model of single domain acicular particles with uniaxial anisotropy. Calculations are done self-consistently at an instant when the head field is applied and after it is removed. Although the results are, at present, limited to the case where the recording medium is standing still, they show quite good agreement with the results of scaled up model experiments, and can clearly explain the demagnetization mechanism in terms of the vector rotation. This new calculation method will, in principle, display its real power in analyzing the dynamical recording process when the recording medium is moving along the head or the head field is changing.     

A technique for measuring pole tip saturation of low inductance heads

Increasing the coercive force of a magnetic recording medium normally improves high density digital performance. However, in rigid disk systems, the head is not in intimate contact with the disk. In addition, the ferrites employed as head materials have much lower saturation magnetization than the metals normally used in other types of heads. Under these conditions, the head field may be inadequate to fully saturate recording media of higher than normal coercive force. In the development of the latest disk products, increasing the coercive force has not improved performance but has increased overwrite modulation. This situation has not been improved by increasing write current amplitude. Pole tip and core saturation of the record head has been suspected as the cause of these observations. This paper describes a method of characterizing saturation effects in low inductance heads such as those used with rigid disks. Evidence of the deterioration of performance due to pole tip and core saturation is shown from isolated pulse measurements on a rigid disk with NiZn and MnZn ferrite heads.     

Numerical Simulations of vector field distributions generated by circular permanent-magnet arrays with side-openings

In the rotary-magnetic refrigerator, a circular permanent-magnet array with side-openings (CPMAS) provides a magnetic field exceeding the remanence of each magnet. Here we show that the magnetic induction in the pole gap as well as inside the magnets of a CPMAS can be calculated using an analytical vectorial formula. The method is based on the representation of the field of a uniformly magnetized magnet by straight current-carrying wires on the surface of the magnet. The Biot-Savart law gives the vector field of a single wire of arbitrary orientation, position, and length; the sum of fields from each wire gives the total field. The magnetization hysteresis loop of the permanent magnet is included by varying the current density; this is then used to calculate modifications to the center field by demagnetization, including remanence inversion under the field generated by the CPMAS itself. The field analysis procedure described here can be used to optimize the structure of permanent-magnet arrays, especially for three-dimensional arrays of restricted or asymmetric geometry, for various applications.     

Irreversible wall motion in inductive recording heads

The effect of signal-induced irreversible domain wall motion on the isolated transition response of recording heads is investigated. An experimental method is presented allowing differentiation between the effects caused by irreversible wall motion and reversible wall motion. The method, utilizing the injection of a small DC current into the head coil, can also be used to determine whether the irreversible motion in a thin-film head occurs in the first pole tip, the second pole tip, or the backgap. The transients observed result from signal-induced irreversible wall motion in the second pole tip. The largest isolated impulse variations occur after saturation of the yoke. In addition, it is found that Barkhausen transitions can occur at or near the top of the isolated impulse (even on its leading edge) as well as after the impulse has decayed, and not only on the trailing edge of the impulse as reported previously. Finally, the authors observe two Barkhausen-noise-free zones, the one at the leading edge being the largest     

Comparison of reproduction performance of a single pole head and a ring head

The reproduction pulse amplitude of a single pole head from double-layer perpendicular media has been analyzed. We derived an analytic expression to relate the pulse amplitude to the head and medium parameters. Experimental data are also presented to verify the calculated results. The reproduction performances of a single pole head and a ring head are theoretically compared and found to be comparable when both are normalized to the same head inductance.     

Analytical Solution of Air-Gap Field in Permanent-Magnet Motors Taking Into Account the Effect of Pole Transition Over Slots

We present an analytical method to study magnetic fields in permanent-magnet brushless motors, taking into consideration the effect of stator slotting. Our attention concentrates particularly on the instantaneous field distribution in the slot regions where the magnet pole transition passes over the slot opening. The accuracy in the flux density vector distribution in such regions plays a critical role in the prediction of the magnetic forces, i.e., the cogging torque and unbalanced magnetic pull. However, the currently available analytical solutions for calculating air-gap fields in permanent magnet motors can estimate only the distribution of the flux density component in the radial direction. Magnetic field and forces computed by the new analytical method agree well with those obtained by the finite-element method. The analytical method provides a useful tool for design and optimization of permanent-magnet motors.