Crucial points in perpendicular recording

Since the introduction of perpendicular recording on a floppy disc by IWASAKI in 1977 and its equivalent design on a rigid disc (SPH-like sensor + double-layer medium) in 1981, many tests have been carried out on different R/W sensors. For each test the main goal was the fci record or the improvement of the magnetic layer. Seen from the recording system point of view, the head and the medium are looked at as a unit through a specification, unchanging with increasing area density. For example, a minimum of 26 dB and 70 % must be achieved for the S/N ratio and the resolution respectively. By considering the noise of the best electronic channel (with a thin film head), and ignoring mechanical and medium noises, the output signal must be at least 250 μv pop. For a 50 Kfci application, however, a sensor does not yet exist. Using a ferrite head with a 1.2 μm gap length to write on FeTbGd, the level of the signal will not be high enough to be used. It is improved with a 0.6 μm gap head but then, the field doesn't allow us to write ! Such problems exist also with thin film heads or SPH like sensors on rigid discs. To improve the R/W process, the trend is to use a double layer medium e.g. CrCo/FeNi. The results show that this direction is not necessarily the best. For example, when erasing or over-writing with the head, some domains appear in the FeNi film which create noise from the track or its edge. Another example is the fact that the optimum parameters for a medium such as CrCo are not always compatible with the characteristics of the head (i.e. Hc, the thickness, the crystallographic orientation, the bit stability compared to the write field, the signal, the noise...).     

Exchange coupling and its applications in magnetic data storage

The continuing scaling of magnetic recording is facing more and more scientific and technological challenges because both the read sensor and recording bit are approaching sub-50 nm regime with the ever increasing areal density in hard disk drives. One of the key and indispensable elements for both high-sensitivity sensors and high-density media is the exchange bias between a ferromagnetic and an antiferromagnetic layer or the exchange coupling between two ferromagnets via a non-magnetic spacer. In the nanometer regime, the exchange coupling between ferromagnet and antiferromagnet or two ferromagnets through a conductive spacer is governed by the intergrain exchange interaction which has its origin in electron spins. Interlayer exchange coupling in multilayer or trilayer essentially originates from the quantum confinement effect. In this paper, we first review the physical origin and various theoretical models of the two types of exchange couplings, followed by a review of the applications of the exchange bias and interlayer exchange coupling in data storage with emphasis on the advanced read sensor and advanced media including perpendicular media and patterned media.     

Considerations for applying solid state sensors to high density magnetic disc recording

To compete with optical recording densities of 10⁷or greater bits per square centimeter, equal magnetic disc areal densities must be achieved. Based on signal to noise considerations, inductive sensing of magnetic transitions written on magnetic media must give way to solid state magnetic sensors. There are fundamental considerations involved with implementing solid state sensors as read heads. These considerations embrace the basic sensor characteristics of size, resolution, sensitivity, signal to noise ratio and frequency response. Other equally important considerations involve packaging and head-wear resulting from head-media proximity demanded by the nature of the field to be sensed from the media. Finally, the solutions to these considerations must be simple and inexpensive.     

Fabrication of Metallic Magnetic Calorimeter X-ray Detector Arrays

Microcalorimeters with metallic magnetic sensors show great promise for use in astronomical X-ray spectroscopy. We describe the design and fabrication of a lithographically patterned magnetic microcalorimeter. A paramagnetic AuEr film is sputter-deposited as the sensor, which is coupled to a low noise SQUID via a meander superconducting pickup loop used as an inductor. This inductor also provides the magnetic field bias to the sensor. The AuEr film is deposited over this meander such that the field created by a large current flowing in the loop magnetizes the sensor material. The use of thin film techniques in the fabrication of these magnetic sensors not only allows strong magnetic coupling between the sensor and the inductor, it also is scalable for array fabrication.      

Signal processing and a groove baseband recording method to achieve a high-density optical disc

We propose a method for recording a multilevel signal onto optical read-only-memory discs. In this method we use signal processing to generate a multilevel recording signal that satisfies the zero-intersymbol interference condition and the zero-dc condition. The resultant multilevel signal is emboss recorded as the position displacement of groove walls. To play back a disc, push-pull detection and an adaptive equalizer are used. We also introduce feedback to reduce the nonlinear characteristics existing in the recording and playback systems. An experimental disc with 0.6-mum track pitch and 0.28-mum/bit density is made. When a digital versatile disc equivalent optical pickup is used to play back this disc, we confirm that a two-dimensional eye pattern of 16 levels is clearly observed.     

Output Properties of Zero-Speed Sensors Using FeCoV Wire and NiFe/CoFe Multilayer Thin Film

Output properties of magnetic sensors generating pulse voltages are described. The sensors principally consisted of double magnetic layers with different coercive forces. Both of thin-film-based material and wire-based material were used for the double layers. When the magnetization of one of the layers was switched by an external magnetic field, a pulse voltage was induced in a pickup-coil wound around the materials. The magnetic sensor using a twisted FeCoV wire, the conventional material for the Wiegand effect, had the disadvantage of the asymmetric output voltage generated by the alternative magnetic field. It was found that a magnetic wire, whose ends were slightly etched, exhibited symmetric output voltage. The sensor element consisting of a patterned NiFe/CoFe multilayer thin film was also studied. Constant output voltage was obtained from this thin-film sensor using an excitation magnetic field at frequencies down to 0.1 Hz.     

Thermal effect limits in ultrahigh-density magnetic recording

In current longitudinal magnetic recording media, high areal density and low noise are achieved by statistical averaging over several hundred weakly coupled ferromagnetic grains per bit cell. Continued scaling to smaller bit and grain sizes, however, may prompt spontaneous magnetization reversal processes when the stored energy per particle starts competing with thermal energy, thereby limiting the achievable areal density. Charap et al. have predicted this to occur at about 40 Gbits/in². This paper discusses thermal effects in the framework of basic Arrhenius-Neel statistical switching models. It is emphasized that magnetization decay is intimately related to high-speed-switching phenomena. Thickness-, temperature- and bit-density dependent recording experiments reveal the onset of thermal decay at “stability ratios” (K_uV/K_BT)₀ ≃35 ± 2. The stability requirement is grain size dispersion dependent and shifts to about 60 for projected 40 Gbits/in ² conditions and ten-year storage times. Higher anisotropy and coercivity media with reduced grain sizes are logical extensions of the current technology until write field limitations are reached. Future advancements will rely on deviations from traditional scaling. Squarer bits may reduce destabilizing stray fields inside the bit transitions. Perpendicular recording may shift the onset of thermal effects to higher bit densities. Enhanced signal processing may allow signal retrieval with fewer grains per bit. Finally, single grain per bit recording may be envisioned in patterned media, with lithographically defined bits     

Computer simulation of MR response to transverse magnetic fields

The magnetoresistive (MR) response of an MR sensor with shields to a uniform applied field was calculated through a micromagnetic simulation, and the results were compared with those from an MR sensor without shields. A uniform longitudinal field, resulting from boundary pinning by exchange-biased antiferromagnetic films, was applied to the MR films of the two sensors. There are three differences between the MR sensors with and without shields: first, the slope of the MR response with shields near an applied field of zero is smaller than the slope of the response without shields. Second, the response of the shielded MR sensor has no subpeak, while the response of the unshielded MR sensor has a subpeak. Third, the output of the shielded MR sensor hardly decreases in a large field (1000 Oe), while the output of the unshielded MR sensor quickly falls to a small value in a field of 300 Oe. These differences are due to attenuated magnetic fields in the gap between the two shields except near the air bearing surface (ABS)     

Patterning of FePt for magnetic recording

Higher areal density for magnetic recording is needed to provide larger storage capacities on harddisk drives. However, as the recording bit size of traditional magnetic recording materials (such as Co/Cr) approaches 10 nm, the magnetic direction of each recording bit would become unstable at room temperature due to thermal fluctuation. To solve this problem, efforts have been made using two methods: one method is to replace the disk media with new materials possessing higher magnetic anisotropy which would lead to better thermal stability; and the second one is to employ different configurations for the recording layer. FePt with patterned media configuration is a combination of these two methods. In this paper we review some novel and interesting methods of patterning FePt for magnetic recording, including thermal patterning, self-assembly patterning, and lithography patterning.     

The helical-scan magnetic tape recorder as a digital communication channel

The properties of a magnetic tape recorder are viewed in terms of a digital magnetic recording/playback channel which exhibits fading (reduction of playback-signal level) and nonlinear behavior. A method is presented whereby channel nonlinearity may be quantified in a format useful for signal and receiver design. Measurements show this nonlinearity to be relatively small for symmetric two-level signals. Deep fades (dropouts) are the most significant source of errors in digital tape recordings. Fading is considered as multiplicative noise on an essentially linear channel, and measurements are made of the fade probability distribution and an associated additional time dispersion. While the fading process appears to occur relatively slowly (compared with the bit period), neither its probability distribution nor its associated dispersion encourage the use of a receiver which is able to adapt to the changing channel characteristics. Finally an attempt is made to ascribe the fading process to repeatable variations in head-tape separation, and a corresponding probability distribution for this separation is obtained. A helical-scan video recorder was used throughout the measurements because of its low cost and its potential as a high-density storage facility.     

The effect of MR head geometry on playback pulse shape and spectra

The playback properties and spectra of a shielded magnetoresistive head are examined utilizing a Fourier series method. Semi-analytic solutions are given for the magnetic scalar potential and spectra in terms of coefficients which have been numerically determined for common geometries. Voltage pulses are generated by convolving the potential with the derivative of a tanh longitudinal magnetization transition. Expressions for the PW₅₀ versus flying height are obtained by fitting the widths of the theoretically determined playback pulses. It is found that the playback properties of a shielded magnetoresistive head depend strongly on the geometry     

Fabrication of magnetic nanodot arrays for patterned magnetic recording media

Fabrication processes of arrayed magnetic nanodots for the use of patterned magnetic recording media were reviewed. One candidate for the patterned media is ordered assemble of magnetic nanoparticles, and the other is patterned magnetic thin films fabricated using various micro/nano scale machining processes. For the formation of patterned masks and molds, lithography processes as well as self-organized pattern formation are utilized. For the deposition processes of magnetic dots, electrochemical deposition processes were widely used. These fabrication processes are reviewed mainly from recent reports. The recording systems for the patterned media including probe-type-recording are also overviewed.     

An experimental digital VCR for consumer use

An experimental digital VCR (DVCR hereinafter) was developed. The DVCR has two new technologies for reduction of tape consumption. One is a bit rate reduction technology of a component video signal down to around 25 Mbps and the other is a high density recording technology. The bit rate reduction technology is based on an 8×8/2×4×8 two dimensional DCT (Discrete Cosine Transform) and a VLC (Variable Length Coding) that completes over 5 macro blocks. Editing, trick plays and invisible error concealments also have been realized by this bit rate reduction while keeping the playback picture quality very high. The high density recording technology is based on ME tape and an ATF (Automatic Track Finding) system. A track pitch of 10 μm and a bit length on tape of 0.25 μm have been realized. The possibility of a higher linear recording density has been confirmed through theoretical analysis, simulations and experiments     

Recording Physics, Design Considerations, and Fabrication of Nanoscale Bit-Patterned Media

Recording physics, design considerations, and fabrication of bit-patterned magnetic medium for next generation data storage systems is presented. (Co/Pd)$_N$ magnetic multilayers are evaluated as candidates for bit-patterned medium recording layer materials for their high and easily tunable magnetic anisotropy. The optimized patterned multilayers used in this study had coercivities in excess of 12–14 kOe. Bit patterning was accomplished using ion-beam proximity printing, a high-throughput direct write lithography where a large array of ion beamlets shaped by a stencil mask is used to write an arbitrary device pattern. It is found that the nature of magnetization reversal strongly depends on bit edge imperfections and is likely to contribute to switching field distribution.      

Graded bit patterned magnetic arrays fabricated via angled low-energy He ion irradiation

A bit patterned magnetic array based on Co/Pd magnetic multilayers with a binary perpendicular magnetic anisotropy distribution was fabricated. The binary anisotropy distribution was attained through angled helium ion irradiation of a bit edge using hydrogen silsesquioxane (HSQ) resist as an ion stopping layer to protect the rest of the bit. The viability of this technique was explored numerically and evaluated through magnetic measurements of the prepared bit patterned magnetic array. The resulting graded bit patterned magnetic array showed a 35% reduction in coercivity and a 9% narrowing of the standard deviation of the switching field.     

A new approach to micromagnetic simulation of thermal magnetic fluctuation noise in magnetoresistive read sensors

Thermal magnetic fluctuation noise forms the ultimate application limit of small magnetoresistive devices for magnetic data storage. The noise analysis of these devices becomes increasingly important for high-density recording. Such noise analyses by micromagnetic simulation are, however, computationally very intensive and require enormous amounts of simulation time. This paper presents a faster micromagnetic method to arrive at the noise and small signal dynamics of these sensors. It uses, for every cell in the simulation, a behavioral analog (i.e., an "analog computer" model) described by the same equations as the basic magnetic simulation cell (the Landau-Lifshitz-Gilbert equation, Slonczewski's spin torque addition and the equipartition principle). The sensor, as a network of exchange and demagnetization coupled cells, is then solved with a network simulator (such as PSpice). This process gives a drastic reduction in computing time and yet leads to high resolution spectra with very little residual uncertainty. The paper further presents a large number of simulation results for uniform sensors as well as for sensors with a nonuniform magnetic bias and a nonuniform electrical bias. It addresses the spatial distribution of the noise (standing spin waves in the noise) and the correlation of the noise in various parts of the sensors. Finally, as a further example of this method, the paper shows the effect of spin torque transfer on the noise and the small signal dynamics of a current perpendicular to plane giant magnetoresistive (CPP GMR) sensor.     

Study of peak shift in thin recording surfaces

One of the most severe limitations on high-density digital recording is imposed by peak shift, which is defined as the outward displacement of the readback signal peaks corresponding to a recorded bit pattern of two successive ones followed and preceded by a number of zeros. Results of an experimental study of peak shift in thin metallic media are represented along with a correlation of the measured percent peak shift with the magnetic properties and the thickness of the media. It was found that the percent peak shift at a specified bit density varies proportionally with the thickness of the coercivity ratio for coercivities larger than 250 Oe. For lower coercivities, the remanent magnetization of the recording surface becomes increasingly significant in adversely affecting the peak shift, with a corresponding decrease in the importance of thickness. By superposing isolated pulses it was possible to predict the percent peak shift up to extremely large bit densities.     

Differential-mode vibrational noise cancellation structure for Metglas/Pb(Zr,Ti)O₃ fiber magnetoelectric laminates

A differential structure which has the ability to reject external vibrational noise for Metglas/Pb(Zr,Ti)O(3) (PZT) fiber-based magnetoelectric (ME) heterostructures has been studied. This type of ME structure functions better than conventional sensors as a magnetic sensor when used in an environment in which vibrational isolation is impractical. Sensors fabricated with this differential mode structure can attenuate external vibrational noise by about 10 to 20 dB at different frequencies, while simultaneously having a doubled ME voltage coefficient. Interestingly, in addition to offering a means of mitigating vibrational noise, this ME structure offers the potential to be a hybrid sensor, separating magnetic and acoustical signals.     

An experimental, multilevel, high density disk recording system

Communication technology has been applied to a magnetic disk recording channel to achieve up to a fourfold increase in linear bit density as compared to conventional binary recording. Among the techniques incorporated were digital data transmission by Class IV Partial-Response signaling (Interleaved NRZI), recording channel pre-emphasis, equalization and filtering, and periodic amplitude sampling of the data signal. The magnetic recording channel was linearized using very high frequency a.c. bias, which also served simultaneously to erase old data. This enabled multilevel recording and the addition of a pilot tone for timing recovery. System block diagrams are presented together with a discussion of the optimization procedure and attained system performance.     

Ferrite-Piezoelectric Multilayers for Magnetic Field Sensors

A magnetic field sensor based on magnetoelectric effects in a ferrite-piezoelectric layered sample is proposed. Such sensors are passive, provide direct conversion of magnetic fields into an electrical signal, and allow measurements of both ac and dc magnetic fields. A multilayer sample of nickel zinc ferrite-lead zirconate titanate has been used to characterize the sensor response to ac and dc fields, field orientations, frequency, and temperature. The sample shows a linear response for dc fields up to a maximum of 1750 Oe. The sensor output is temperature independent over 273–337 K, but is dependent on frequency of the ac excitation field. Operating at electromechanical resonance for the element enhances the sensor sensitivity by an order of magnitude. For ac magnetic field sensors, the output varies linearly with amplitude.