Possibilities of perpendicular magnetic recording

The potential of perpendicular magnetic recording using a single-pole head and a double-layered medium has been investigated theoretically by computer analysis and compared with that of longitudinal magnetic recording. In conventional longitudinal recording, a recording demagnetizing loss due to the change of magnetization mode from semicircular to circular shapes occurs with increasing recording level at high bit density. In perpendicular magnetic recording, the perpendicular magnetization mode is maintained regardless of recording level even at an extremely high bit density of 571 kFRPI. This indicates that the perpendicular recording medium has a very high recording resolution, where a single bit size approaches several diameters of the microcrystalline particles of the Co-Cr layer. An ultrahigh density at which the recording area for 1 bit will reach 1 μ² at present and 500 Å₂ in future should be possible     

Rare earth - transition metal alloys: Another way for perpendicular recording

Many investigators have chosen CoCr alloys or other Co-based alloys to study perpendicular magnetic recording. Although the perpendicular magnetic anisotropy of Co alloys is quite convenient for this use, the demagnetizing fields are high because of high saturation magnetization and then the squareness of the perpendicular M-H loops is poor. To study another kind of perpendicular anisotropic alloy, we tested sputtered amorphous Fe1-x-yTbxGdy(X = y) thin films as a perpendicular recording medium. Several thicknesses with and without a FeNi underlayer were tested. Although a conventional ring-type head was used, with a low trackwidth (20 μm) and a coil with 2 × 13 turns, 162 KFci could be recorded and read and a 1,600 μ Vpp output signal was read at 10 KFci.     

Magnetic recording theories: Accomplishments and unresolved problems

The published theoretical treatments on the magnetic recording process are reviewed with particular emphasis on the validity of the models and the assumptions on which they are based. It is concluded that the existing theories fairly well describe the geometrical aspects of recording-transducer to medium spacing and gap lengths-but are deficient in describing analytically the magnetic state of the recording medium before and after demagnetization. Calculated magnetization distributions by harmonic analysis for different recording media are in support of the fact that the usually assumed linear and arctangent magnetization transitions are only rough approximations of the magnetic state of a recording medium between regions of opposite magnetization. Additional shortcomings of our theoretical understanding are indicated by the assumptions of uniform magnetization through the recording medium thickness, neglecting the perpendicular component of the magnetization, and not taking into account finite track widths and magnetostatic interactions between adjacent transitions.     

Multilayer films of CoCrTa for perpendicular recording media

Multilayer structures of alternate magnetic and non-magnetic CoCrTa alloys have been prepared in which the magnetic properties can be widely varied by changing the magnetic layer and interlayer thicknesses, In this way, one example has shown that enhanced perpendicular anisotropy can be achieved as demonstrated by an increase in perpendicular coercivity Hc(perp) and a decrease in longitudinal coercivity Hc(&dlor.). This condition is desirable for application in perpendicular recording media. In addition, a second example has shown that by using much thinner layers (≃15 nm) it is possible to produce a structure in which the magnetization is longitudinal with a very low coereivity of 1.5 Oe. Such soft magnetic films with high Ms and low Hc(&dlor.) can be used as underlayers for perpendicular recording media in order to enhance the efficiency of pole heads.     

Perpendicular magnetic recording performance of double-layer media

Perpendicular magnetic recording performance for double-layer (Co-Cr,Ni-Fe) media was analyzed, with a simple magnetization model, in comparison with single-layer media. The calculated output voltage from a ring-shaped reproducing head shows good agreement with experimental data. It is shown theoretically and empirically that the Co-Cr layer thickness can be reduced without sacrificing the output amplitude, if the Ni-Fe layer is placed underneath it to perform as a "flux sink" to extinguish the rear surface charge on the Co-Cr layer.     

Perpendicular magnetic recording

This paper describes the recent studies for the magnetic head, the medium and the recording properties on a new perpendicular magnetic recording system. The complemental features between the perpendicular and the longitudinal recording are discussed to establish an efficient magnetic recording system. Superior response in the amplitude and the peak shift characteristics for a digital signal proves that the perpendicular magnetization mode is basically free from the recording demagnetization in high densities and the maximum density has been limited merely by the resolution of the reproducing head. Significant improvement for the recording and the reproducing sensitivities of a perpendicular head has been made by using a composite anisotropy medium composed by double layers of Fe-Ni and Co-Cr thin films.     

Dynamical interpretation of magnetic recording process

The demagnetization effect in magnetic recording must be evaluated, not as static self-demagnetization, but as a dynamic demagnetizing field at an instant when the head field is applied to the medium. From this fact it becomes necessary to obtain a self-consistent magnetization distribution in the medium. A method of calculation and its results are described. The relation between the longitudinal and the vector magnetization is clarified. The experimental results of the recording demagnetization in sinusoidal recording and the pulse width and the peak shift in digital recording are interpreted as the new phenomena that is related to the dynamical behavior of the demagnetizing field in the recording process.     

Geometric effects in the magnetic recording process

A simplified view of magnetic recording is presented in which the effect of head-medium geometry is clearly exhibited. The model is applied to contact recording with a ring head on thick particulate media. Demagnetization fields are neglected and identical particles with angularly invariant switching fields are considered with various orientation distributions in the recording plane. For all orientation distributions symmetric about the longitudinal direction the longitudinal magnetization component vanishes at the head surface and increases with depth into the medium. Conversely, the vertical component is maximum at the head surface and decreases with depth. The resulting recording spectra therefore have greater magnitudes for vertically well-oriented particles and isotropic distributions of particles with biaxial symmetry. At short wavelengths these distributions give approximately 5 dB more output than longitudinally well-oriented media. The depth variation of longitudinal magnetization also provides one explanation for the peak in the short wavelength input-output curve measured on longitudinal media. These conclusions are expected to be dependent on the assumed angular variation of the particle nucleation field.     

Correlation between magnetic and recording properties in thin surfaces

Correlation between the magnetic properties and the recording performance of magnetic recording surfaces is a subject under very active investigation by several researchers. In this paper we report the results of a study conducted in which each magnetic parameter and the thickness of a recording surface was varied independently, and their effect on recording performance was individually determined. The width at half the amplitude of the pulse readback from an isolated magnetization transition on a recording surface varies proportionally to the one-half power of the thickness, and inversely proportionally to the one-half power of the coercivity; it is essentially independent of the remanent magnetization of the recording surface for coercivities greater than 150 Oe, but become increasingly more dependent on the remanent magnetization for coercivities below 150 Oe. The pulse amplitude varies proportionally to the one-half power of the coercivity, the remanence, and the thickness of the recording surface.     

Theoretical study on stress-induced demagnetization in magnetic recording media

A simple and fundamental model of stress-induced demagnetization, which has scarcely been analyzed theoretically, is presented. Stress-induced demagnetization in a magnetic recording medium results from the magnetization reversal of individual magnetic particles dispersed in the magnetic layer. The magnetoelastic energy due to the application of three-dimensional stresses and the magnetostatic energy due to the demagnetizing field must be taken into account for an investigation of the magnetization reversal of each particle. It is found that the magnetization reversal can take place even during small stresses if a sufficient demagnetizing field exists. The sign of the magnetostrictive coefficient λ100has a significant relation to the susceptibility of magnetization reversal.     

Thermomagnetic recording: Physics and materials

Thermomagnetic recording is the production of a remanent magnetization in a material by cooling it from a critical temperature in a small magnetic field. The most prominent applications for thermomagnetic recording presently under consideration are high density information storage and tape copying; other applications, e.g., display and magnetic printing, can also be envisioned. In this review of the field, we discuss the proposed applications of thermomagnetic recording, the physical principles of the various magnetic phenomena that can be used for this purpose, and the degree to which specific materials meet the requirements for thermomagnetic recording.     

Magnetic properties and longitudinal recording performance of corrosion-resistant alloy films

This paper describes the magnetic properties, recording performance and corrosion resistance of sputtered CoCr alloy films and CoCrTa alloy films. The saturation magnetization of CoCr was 525 emu/cc and not affected by substitution of a small amount of Ta. CoCrTa films exhibit greater coercive force values than CoCr films. The maximum coercive force of CoCrTa was 1400 Oe at a thickness of 400A, whereas at a similar thickness the coercive force of CoCr was 900 Oe. The coercive force decreases linearly with temperature (25°C. to 125°C.) at a rate of 3.16 Oe/°C. for CoCrTa and 1.87 Oe/°C. for CoCr. Longitudinal recording performance at -3 dB signal level was 8386 flux reversals/cm (21,300 fci) and 11,063 flux reversals/cm (28,100 fci) for CoCr and CoCrTa respectively. The alloys exhibited corrosion resistance at 80% relative humidity and 65° C. for a two-week period equivalent to at least six years under ambient disc drive conditions.     

Saturation magnetic recording process

This review of the saturation magnetic recording process is tutorial in approach. The emphasis throughout is on the development of intuitive understanding as well as the development of a first-order analysis (good to 10-20 percent) of the process. By means of treating two different functional approximations to the magnetization distribution in the medium, linear and arctangent, it is shown that the peak output voltage and half-amplitude pulse width of a transition are nearly independent of the distribution. Thus good estimates of these important system parameters may be made without reliance upon a specific model. It is made apparent that head-to-medium spacing and effective-medium thickness are of dominant importance in system density considerations.     

Spin-valve head with corrosion resistance for tape recording system

This paper reports on a corrosion-resistant spin-valve head (Ta/NiFe/CoNiFe/CuAu/CoNiFe/PtMn/Ta) for a tape recording system. The spin-valve film has a +0.4 V higher electrochemical potential than the conventional spin-valve film (Ta/NiFe/CoFe/Cu/CoFe/PtMn/Ta). The spin-valve head with CoNiFe magnetic layer and CuAu spacer exhibits good corrosion resistance. An investigation of the recording characteristics of the corrosion-resistant spin-valve head with metal evaporated (ME) tapes showed that: 1) the spin-valve head had about 64% of the output voltage of an isolated pulse of the conventional spin-valve head; 2) the carrier-to-media noise (C/N/sub media/) ratio at a wavelength of 0.4 /spl mu/m of the spin-valve head is the same as that of the conventional spin-valve head; and 3) the C/N/sub media/ ratio of thin ME tape with magnetic layer 33 nm thick was maximized at around M/sub r/t=10 mA, where M/sub r/ is the remanent magnetization and t is the magnetic layer thickness. These results indicate that the corrosion-resistant spin-valve head (Ta/NiFe/CoNiFe/CuAu/CoNiFe/PtMn/Ta) for a tape recording system provides high recording density.     

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.     

Changes in recording tape magnetization produced by stress

When mechanical forces are applied to single domain particles that are used in magnetic recording tapes, the resultant stress anisotropy energy alters the magnetized state, producing reversible and irreversible changes. We use four different experimental techniques to measure these changes and analyze the results in terms of the values for the saturation magnetostriction constants (λs) and the applied stress (σx). The reversible changes in magnetization (Mx) are roughly proportional tolambda_{s}sigma_{x}; the greatest irreversible or loss component occurs when the product of σxand λsalong the magnetizing axis (x) is negative, increasing when major easy axes make large angles withx.     

A computer simulation of unbiased sine wave recording

A computer simulation is described which uses the approximation that the output voltage derives from longitudinal tape magnetization acquired solely from the longitudinal head field. Using a simple noninteractingM_{r}-Hmodel, the remanent state of 200 points per half-wavelength of tape is calculated by following each point's field history as it passes the record head. We discuss the amplitude and phase angle of the magnetization sinusoids obtained by harmonic analysis. At short wavelengths (wavelength less than coating thickness) we find, for regular γ-Fe2O3tape, that although the peak output is limited by record head field gradient effects, the over-recording phenomenon is attributable to the field shape. The process of "optimizing" the record current is thus shown to be explicable in terms of the record head field geometry. These conclusions hold equally for "partial penetration" recording of high density digital waveforms.     

Chromium dioxide particles for magnetic recording

Chromium dioxide was introduced in the later 1960s as the first magnetic material capable of high density recording. Since that time a continuous improvement of material properties has been made possible. The most recent improvement concerns particles with coercivities well above 70 kA/m which can be prepared by a modified hydrothermal process without using expensive Ir doping. Although CrO₂ has a somewhat higher specific magnetization, the static magnetic properties of CrO₂ and cobalt-modified iron oxides are comparable. There are, however, some significant differences in recording performance. CrO ₂ pigments show better print-through to noise ratios than Co-modified iron oxides of the same mean magnetic volume. There are no problems with erasability, which becomes increasingly difficult to control for high-coercivity cobalt-modified iron oxides. Finally, CrO ₂ is the only pigment material which allows thermomagnetic duplication     

Read performance of a single-pole head with trapezoidal cross section for perpendicular recording

The read performance of a single-pole head with trapezoidal cross section for perpendicular recording is investigated theoretically. With the aid of the reciprocity theorem for magnetic recording the expressions for the response to a single magnetization reversal and to harmonic magnetization on a tape with a soft magnetic backlayer are obtained. Numerical results are presented for different bevellings of the cross section.     

Statistical data analysis of magnetic recording noise mechanisms

Statistical data analysis using empirical eigenfunctions, known as the Karhunen-Loeve (K-L) expansion, is applied to characterize noise mechanisms in magnetic recording. Given any original data set and hence its correlation (covariance) matrix, an empirical orthogonal set of eigenfunctions can be obtained. The original data set can be expressed as an orthonormal expansion of these eigenfunctions. This feature of the K-L expansion can be used to study dominant noise profiles extracted from a large number of magnetization transition data. Two simple models of magnetization transitions are first utilized to investigate the validity of this expansion. Noises induced by transition center shifting (jitter), transition width fluctuation, amplitude modulation, and combined effects are respectively identified by the first several most important eigenfunctions in the expansion. Eigenfunction expansions of transition data obtained from experiments and numerical simulations are also obtained