Spin-valve Heads Research Articles

Nanoscale Magnetic Characterization of Tunneling Magnetoresistance Spin Valve Head by Electron Holography

Nanostructured magnetic materials play an important role in increasing miniaturized devices. For the studies of their magnetic properties and behaviors, nanoscale imaging of magnetic field is indispensible. Here, using electron holography, the magnetization distribution of a TMR spin valve head of commercial design is investigated without and with a magnetic field applied. Characterized is the magnetic flux distribution in complex hetero-nanostructures by averaging the phase images and separating their component magnetic vectors and electric potentials. The magnetic flux densities of the NiFe (shield and 5 nm-free layers) and the CoPt (20 nm-bias layer) are estimated to be 1.0 T and 0.9 T, respectively. The changes in the magnetization distribution of the shield, bias, and free layers are visualized in situ for an applied field of 14 kOe. This study demonstrates the promise of electron holography for characterizing the magnetic properties of hetero-interfaces, nanostructures, and catalysts.

Micromagnetic modelling for current-perpendicular-to-plane giant magnetoresistance read head

A micromagnetic model is performed to find the proper geometrical and magnetic parameters for current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin-valve heads at higher recording densities. The readout signals were studied versus the various height to width ratios for GMR sensors. When the GMR sensor width is reduced to 40 nm, the magnetization of the free layer rotates coherently in response to the excitation fields at various height to width ratios. In the calculated transfer curve of the GMR heads, the geometry and remanent Mrδ of the hard magnetic bias layers are optimized for the good performance characteristics of the head signal output. The imaging effects related to the narrow-gap shields are also investigated for the head design.

Tailor-made nano-structured materials for perpendicular recording media and head—precise control of direct/indirect exchange coupling

Tailor-made nano-structured spin materials obtained by precisely controlled nano-scale fabrication technologies for use in ultra-high density hard disk drives (HDDs), as well as an understanding of their nanomagnetics, are essential from the view point of materials, processes, and physics. Artificial control of the exchange coupling among ferromagnetic layers through the RKKY interaction (indirect) and direct exchange coupling represented as the exchange bias at the ferromagnetic (FM)/antiferromagnetic (AFM) interface are of great interest and have received significant attention to induce new modulated spin structures in conventional simple FM materials. In particular, soft magnetic under layer (SUL) with strong synthetic antiferromagnetic (SAF) coupling between two adjacent soft magnetic layers, exchange coupled stacked media introducing exchange coupling between FM layers and giant exchange anisotropy at the FM/AFM interface have attracted significant attention from the view point of applications. Within the framework of the present paper, we discuss future technical trends for SUL, granular media and the spin-valve head from the viewpoint of direct and/or indirect exchange coupling based on our recent results.

Improved corrosion resistance of spin-valve film

We investigated the corrosion behavior and magnetoresistance of spin-valve film in order to improve the corrosion resistance of the spin-valve head for a tape recording system. The conventional spin-valve head (sub./Ta/NiFe/CoFe/Cu/CoFe/PtMn/Ta) with no diamond-like carbon (DLC) protective layer showed poor corrosion resistance. This is because the CoFe for ferromagnetic layer and Cu for spacer in the spin-valve film exhibited poor corrosion resistance. The corrosion resistance of the CoFe film and Cu film improved with the addition of Ni and Au, respectively. The spin-valve film (sub./Ta/NiFe/CoNiFe/CuAu/CoNiFe/PtMn/Ta) showed higher pitting potential than the conventional spin-valve film by +0.45 V. This presents a significant improvement over the conventional spin-valve film. We also investigated the effect of the composition of ferromagnetic layer and spacer on the magnetoresistance of the spin-valve film. The magnetoresistance of the spin-valve film by substitution of CoNiFe for CoFe in ferromagnetic layer decreased slightly. The magnetoresistance of the spin-valve film decreased as the addition of Au of the spacer increased. The diffusion at CoNiFe/CuAu interface has not been observed in annealing process. The quantitative relation between corrosion resistance and magnetoresistance of spin-valve film, and its ferromagnetic layer and spacer's compositions have been clarified. The output voltage at 50 Oe of the corrosion-resistant spin-valve head with CoNiFe ferromagnetic layer and CuAu spacer was about 50% of that of the conventional spin-valve head.

Recording over 15 ktpi using multichannel heads in a tape system

In magnetic recording systems, it has been reported that sufficient signal-to-noise ratio (SNR) was obtained with a narrow track width, using a spin-valve head and metal-evaporated tape. In order to apply this to a helical-scan tape system, techniques for writing narrow tracks and reading them exactly were required. However, the degree of mechanical accuracy demanded was too high to realize. With this in mind, we have developed multichannel heads which can write and read certain tracks with a single scan, as well as a nontracking technique removing the need for such precise mechanical accuracy. A prototype tape drive employing these techniques was developed, and a track density exceeding 15 ktpi was achieved.

Investigation on wear and recession of the GMR head in helical-scan tape systems

We have investigated the wear and recession of the shielded spin-valve head in a helical-scan tape system. We report the wear caused by rubbing by metal evaporated tape optimized for the giant magnetoresistive (GMR) head. We have also studied the recession of the GMR head in an accelerated dust environment, taking into consideration the actual operating conditions. The results confirm the feasibility of the practical use of shielded spin-valve GMR heads in helical-scan tape systems at a high recording density.

Stability of GMR heads biased utilizing patterned exchange tabs

We have demonstrated the fabrication of a spin-valve head with a CoFe∕PtMn antiferromagnetic (AFM) exchange-biasing layer for longitudinal stabilization. The application of Golden field annealing means the two AFM layers can be made of the same material with the same TB. The AFM layers may then be chosen to optimize pinning strength, corrosion resistance, etc. The transfer curve based on a pattered head structure with PtMn AFM stabilized free layer is smooth and free of any hysteresis. The head also has good linearity and bias point, and the performance is comparable with the standard abutted permanent magnet junction stabilization. Further optimization is expected to improve the performance, enabling an areal density of >120Gb∕in.2

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.

Stability in In-Stack Biased CPP SV Heads

In this paper, we present a quantitative micromagnetic modeling analysis of head stability in in-stack biased current perpendicular to plane (CPP) spin valve (SV) heads at deep submicron track widths. It is found that sufficient pinning on the in-stack bias layer is the key for stability. The narrower track-width heads have better stability. For in-stack biased bottom SV heads, the conventional structure has serious stability problem due to weakened or reversed bias field at free layer track edge. Controlling CPP edge profile is crucial for stability. We also studied the effect of current field on head performance. Depending on the current direction, current field can either induce instability or assist head stability.

Current Induced Noise in CPP Spin Valves

In this paper, an experimental investigation of current-induced noise in current perpendicular-to-plane spin-valve heads is reported. It is found that the sense current yields excitations of coherent spin waves as well as substantial 1/f-like noise when current exceeds a critical value. The noise is magnetic in nature. The criticality and current directional dependence of the noise excitation clearly show that the spin transfer is the cause.

Spin Transfer Induced Noise in CPP Read Heads

In this paper, spin transfer induced magnetic noise in current-perpendicular-to-plane (CPP) spin valve heads is investigated via micromagnetic modeling with spin transfer torque included. It is found that there exists of a critical sense current density beyond which the spin transfer effect yields a substantial noise with a pronounced (1/f/sup n/) spectral content at low frequencies. This magnetic noise arises from the essentially chaotic magnetization precessions excited by the spin transfer effect that can effectively acts as energy pumping to the spin system. The onset of the noise occurs when the pumping rate becomes equal to the rate of energy dissipation. The critical current density is found to be proportional to the damping constant /spl alpha/ as well as the longitudinal bias field.

Narrow Track Current-Perpendicular-To-Plane Spin Valve GMR Heads

We have investigated the performance of dual spin valve heads in the current-perpendicular-to-plane (CPP) geometry on dynamic electrical tests (DET) and quasi-static tests (QST). Shielded CPP reader and stitched pole writer composite heads were fabricated. A various physical read track width and area of the dual spin valve element were deliberately obtained in the range of 41-175 nm and 0.0034-0.022 /spl mu/m/sup 2/, respectively. Materials used for the CPP dual spin valve are Co/sub 90/Fe/sub 10/, Ni/sub 80/Fe/sub 20/, Cu, Ru, Ta, and Pt/sub 50/Mn/sub 50/, which are common in industrial current-in-plane (CIP) spin valves. The shape and smoothness of QST transfer curve, readback waveform, micro-track, and full-track profiles of the narrow read track CPP heads are essentially just the same as those of CIP spin valve. A track-average-amplitude of 0.6 mV was obtained at the bias current density of element around 100 MA/cm/sup 2/ in the CPP head with the physical read track width of 47 nm. Overall reader resistance of the CPP heads at head gimbals assembly (HGA) level was in the range of 10-35 /spl Omega/ and proved to be well appropriate.

Optimization of PM stabilization for balanced sensitivities and variations of spin valve heads

The control of variations of amplitude and symmetry of synthetic antiferromagnetic spin valve heads becomes increasingly difficult with the shrinking of head sizes. Micromagnetic Monte Carlo simulations were carried out to investigate the correlations between different head parameters and head performance variations. It was found that the head performance variation is strongly correlated with the permanent magnet stabilization strength. In addition, the magnetization orientation of the reference layer in the biasing state varies more from head to head than that of the free layer, when the sizes of the spin valve heads are in the deep-submicron range. The conflict between the head sensitivity and the head performance variation is clearly shown by the modeling results.

Thermally excited ferromagnetic resonance as diagnostic tool for spin valve heads

Thermally excited ferromagnetic resonance (TEFMR) in a spin valve head gives rise to mag-noise while its power spectral density directly reflects the magnetic field distribution in the sensing layer as well as the corresponding micromagnetic states. In this article, we report an experimental study on utilization of TEFMR spectral characteristics for spin valve head diagnosis and characterization. The study suggests that the variation of the micromagnetic states in the PM layer at the junctions is one of the most important factors causing the head-to-head variations of the recording performance, especially the heads from the same wafer.

Understanding of amplitude symmetry and its variation of synthetic spin valve heads for high density recording

In synthetic antiferromagnetic spin valve (SAF–SV) recording heads, as SV sensor dimensions approach the nanometer scale and individual layer thickness shrinks to only a few angstroms, it becomes more challenging to achieve a unity amplitude symmetry (LFA–SYM) and a small variation of LFA–SYM. A better understanding of LFA–SYM, good control of its variation, and an effective measurement of SAF–SV canting angles has become more important. In this study, high field transfer curve (HF–Xfer) was measured to determine head magnetic polarity, bias point, and output linearity. Rotational field transfer curve (RF–Xfer) was measured to determine the canting angles of reference layer and pinned layer. Free layer canting angle, its dependence on magnetic field, and the angle between the reference layer and free layer were consequently determined based on HF–Xfer and RF–Xfer data. A good linear correlation between LFA–SYM and free layer canting angle was observed, indicating that the large variation of LFA–SYM was mainly due to a wide range of free layer canting.

Side-reading free spin-valve read sensor

A novel scheme essentially eliminates side-reading while maintaining on-track magnetic sensitivity for spin-valve read heads. By directly patterning a magnetic layer that is antiparallelly coupled with the free layer beyond the track edges, the magnetic torque generated by signal field in the wing regions of the free layer can be counterbalanced by the torque generated by the patterned layer via the antiparallel coupling. The magnetic read width of the sensor becomes essentially the same as the physical track width for track widths as low as 50 nm. With this scheme, the on-track magnetic sensitivity exhibits virtually no reduction as track width reduces from 200 to 50 nm. In addition, the scheme enables recessing the contact leads from the track edges to reduce shield-to-shield widening at the track edges.

Quasi-static and dynamic analysis of spin valve tape heads with synthetic free and pinned layers versus heads with a conventional free layer and a synthetic pinned layer

A comparative analysis of tape heads with two spin-valve designs is presented: spin valves with synthetic free and pinned layers (SF-SAF/AF) versus spin valves with a conventional free layer and a synthetic pinned layer (SAF/AF). The SF layer, corresponding to a smaller free layer's effective magnetic thickness (t/sub eff,SF/ /spl sim/ 28 /spl Aring/) and lower magnetic moment (M/sub eff,SF/ /spl sim/ 398 emu/cc), is more sensitive to the field flux from the medium and has a higher sensitivity (/spl sim/ 3.4 mV//spl mu/m) than the conventional NiFe/CoFe free layer, with D50 /spl sim/ 130-138 kfci, in comparison with D50 /spl sim/ 99 kfci for SAF/AF spin-valve heads. Thick media limitations become visible on the heads' performance above /spl sim/ 112 kfci and the heads are shown to be mostly time-instability free.

Recording characteristics on thin metal evaporated media in a helical-scan tape system with a spin-valve head

The recording characteristics of Co-CoO metal evaporated (ME) tapes with a giant magnetoresistive head have been investigated. The reproduced spin-valve head output was 7.6 times that of an anisotropic magnetoresistive head at a recording wavelength of 0.4 /spl mu/m. We also studied the relation between the carrier-to-media noise (C/N/sub media/) ratio and remanent magnetization (Mr) magnetic layer thickness (t) in the ME tape. The media noise level was reduced with decrease in Mr /spl middot/ t by the increase of the oxygen flow rate, whereas the C/N/sub media/ ratio of the tape with a magnetic layer thickness of 33 nm was maximized at around 10 mA. We thus confirmed the feasibility of an areal density of 3.8 Gb/in/sup 2/ in a helical-scan tape drive system.

Spin-valve and tunnel-valve structures with in situ in-stack bias

The use of in-stack longitudinal magnetic stabilization for spin-valve and tunnel-valve recording head sensors has been investigated. An in-stack ferromagnetic layer pinned with an IrMn antiferromagnet is used to magnetostatically stabilize the free layer by flux closure. The use of IrMn with lower blocking temperature than PtMn allows the bias layer to be set independently from the PtMn-pinned reference layer in the spin-valve or tunnel-valve. A Ta spacer 10-30 /spl Aring/ in thickness is used to separate the free layer from the bias layer resulting in low coupling fields. IrMn delivers up to 0.34 erg/cm/sup 2/ of pinning strength, resulting in stable unshielded sensor operation for device sizes below 0.2 /spl mu/m.

Thermal noise in linear damped systems

It is frequently assumed that estimating the noise, or thermal fluctuations, in a damped linear system necessarily depends on knowing the precise magnitude of the damping or energy dissipative component of the system. This paper proves that this notion is incorrect by considering the noise power of a resistor. It then shows that the magnetization fluctuation noise power of a spin-valve giant magnetoresistive head, a subject of considerable current interest, can be analyzed in a particularly simple and straightforward manner without knowing the exact value of the gyromagnetic damping factor.