Giant Magnetoresistive Heads Research Articles

Electrical low frequency random telegraph noise in magnetic tunnel junctions

Low frequency random telegraph noise (RTN) of electrical origin is reported in magnetic tunnel junction heads with ultrathin tunnel barriers (<1 nm). The RTN is characterized by abrupt and randomly stepped voltage spikes which modulates 1/f noise property in frequency domain. The appearance of electrical RTN is restricted to limited ranges of bias voltage and temperature. Dependence of RTN statistics on temperature suggests that the dynamic resistance fluctuations could be generally described by a thermal activation model. The extracted activation energy, EA, is found to be 0.3×10−19 J, which is much smaller than reported activation energies for magnetic RTN in early giant magnetoresistive heads.

Detection of nanoscale ESD effect on GMR head signal using the wavelet transform technique: HBM and CDM cases

An electrical signal anomaly is an undesired signal and is difficult to detect by a commercial instrument due to its short duration and unpredictable fault on a signal. Since a GMR recording head is a stack of nanometer thick multilayers, in particular, a magnetic layer and conductor layers, for magnetic insulating spacers, it is very sensitive to electron movements. Visible damage is understandable and protectable but latent failure cannot be measured. It is possibly observed by using frequency-domain apparatus but certainly it is not real-time detection. Therefore, in order to detect a latent failure head affected by ESD in the time domain, current conventional instruments are ineffective. In this study, the wavelet transform technique using the 4th order Daubechies is proposed to detect the glitches on a magnetic recording head signal in the time domain. It is found that the glitches occur when the ESD level of the charged device model (CDM) and human body model (HBM) on giant magnetoresistive (GMR) heads are in ranges of 6–15 V and 40–120 V, respectively. The electrical test parameters and scanning electron microscope (SEM) photo of the recording heads show no visible change in reader sensor. To ensure the results, the GMR damage is observed by SEM when the CDM-ESD and HBM-ESD are 10 V and 130 V, respectively. The glitches in the magnetic response signal of the GMR head are found to increase when the ESD level is increased. Thus, the Daubechies wavelet transform technique can be a novel approach to detect the pre-degradation of a GMR head due to an ESD effect.

Effect of Current-Confined Path on Spin-Torque Noise in CPP-GMR Heads With Current Screen Layer

We investigated the effect of current-confined path on spin-torque noise in current perpendicular to the plane giant magnetoresistive (CPP-GMR) heads with a current screen layer (a nano-oxide layer having current-confined paths). We found that both in measurement and calculation spin-torque noise of heads with a resistance area (RA) product of 1.0 Omega middot mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was smaller than that with RA of 0.25 Omega middot mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . We also found that the calculated spin-torque noise decreased as the exchange stiffness constant in the free layer increased. These behaviors indicate that the magnetization fluctuation in conductive areas can be reduced by the magnetic coupling with the magnetization in nonconductive areas. Accordingly, the current screen structure is effective in suppressing the spin-torque noise. To reduce spin-torque noise in CPP-GMR heads with a current screen layer, it is effective to reduce the area of each metallic nano-hole while keeping the total conductive area constant since the magnetic coupling between the magnetization in conductive and nonconductive areas keeps strong.

Investigation of Thermal Demagnetization Effects in Data Recorded on Advanced Barium Ferrite Recording Media

The thermal stability of data prerecorded onto advanced barium ferrite recording media has been studied. This type of media has been demonstrated to have excellent recording performance for ultranarrow read widths and at high linear densities. However, the reduction in size of the particles used in this type of media, compared with conventional metal particle media, could potentially affect its archival storage capability due to thermal demagnetization effects. The thermal stability of the media has been studied by making time-dependent coercivity measurements, using a standard vibrating sample magnetometer. From these measurements, a value of K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> V/k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T ap 100 was obtained, indicating acceptable archival thermal stability. To determine if data recorded in this media is in fact thermally stable, tapes were also prewritten with pseudorandom encoded data patterns at various linear densities. The tapes were then baked, at temperatures up to 59 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> C and for times up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> seconds. The data was read back after each bake cycle using a nominally 1.0-mum read width giant magnetoresistive read head. No significant degradation in the quality of the read-back data was found, even after several baking cycles at increasing temperatures. This indicates that this media type does indeed have good archival thermal stability. It has also been found that the fundamental recording performance of this media is very acceptable for narrow read widths and at linear densities up to at least 360 kbpi.

CPP-GMR heads with a current screen layer for high areal density

We investigated the potential for high-areal-density recording in current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) heads with a current screen layer. The current screen layer is a nano-oxide layer with confined current paths. We fabricated the current screen CPP-GMR heads with a narrow sensor width of 40–50 nm, a high MR ratio of 17%, and low-resistance-area (RA) product of 0.2 Ω μm 2. The fabricated heads showed a sufficiently high signal-to-noise ratio (SNR) of 30–40 dB. No extra noise, such as spin-torque noise, was observed. Linear density of 1360 kFCI from the head with the magnetic read width of 45 nm was obtained. Distribution of sensor resistance due to nano-hole area distribution in the current screen layer can be reduced with low-RA film. Spin-torque noise can be suppressed by reducing the current-induced field and controlling the shape anisotropy. Accordingly, the current screen CPP-GMR head is a promising candidate that has the potential for high-areal-density recording.

Bidirectional recording performance of a perpendicular evaporated Co–CoO tape

It is the first report on the recording performance of a perpendicular metal evaporated (ME) tape measured with a giant magnetoresistive head. To solve the application difficulty of oblique evaporated tape media to linear scan tape systems, a perpendicular evaporated Co–CoO tape was proposed instead. The prepared sample showed perpendicular anisotropy with coercivity of 107.3 kA/m, M r t of 3.9 mA and squareness of 0.25. Identical recording characteristics were obtained for both head-media moving directions, which enables the application of perpendicular evaporated Co–CoO tape to linear scan tape systems. The better carrier-to-noise ratio was also confirmed by comparison with a current advanced product of metal particulate tape, which can realize the higher recording density of linear scan tape systems using ME tape.

Exchange Effect in TM-TbCo/RE-TbCo Double Layer Films

The High saturation magnetization (Ms) media are desired to yield large flux density for high resolution giant magnetoresistive head readout in Heat Assisted Magnetic Recording (HAMR). To ensure adequate stability of small size domains, the media for HAMR should possess large coercivity (Hc). In this work, we study of interface wall energy and the behavior of the magnetization of each layer in exchange coupled doubled-layered (ECDL) films. The Tb17Co83 (90 nm)/Tb30Co70 ECDL films with different Tb30Co70 thickness (20, 35, 50, 70, 90 nm) are made to increase the Ms value. As the thickness of Tb30Co70 film increase from 20 to 90 nm, the coercivity of the films increases from 2.5 to 8 kOe. The enhanced coercivity of ECDL films is due to the formation of domain wall at the interface of the ECDL film. As the Tb30Co70 increases from 20 to 90 nm, the Ms value of the films decreases from 380 to 330 emu/cm3. Due to Co has larger magnetic moment than that of Tb at room temperature, the resulting Ms value of the films decreases with increasing the thickness of Tb30Co70 layer.

Anomalous magnetic responsiveness of giant magnetoresistive heads under specific electromagnetic interference frequencies using quasistatic tester

The giant magnetoresistive (GMR) heads have been used in the computer industry for decade. Recently, the anomalous performance caused by cell phones or external electromagnetic interference (EMI) is reported [V. Kraz and A. Wallash, J. Electrost. 54, 39 (2002)]; [Kruesubthaworn et al., J. Magn. Magn. Mater. 316, e142 (2007)] This prompts an experimental study of an anomalous magnetic disturbance to the heads under ascending and descending frequency variations. The rf generator with predetermined output is set for 30–1000MHz swept frequency in both directions, with the antenna being horizontal and vertical orientations. Five quasistatic tester (QST) parameters; magnetoresistive (MR) resistance, MR amplitude, asymmetry, Barkhausen noise, and hysteresis are used as markers in the EMI sensitivity study of head gimbal assembly. It is found that the worst change of MR amplitude is 10.2% (marginally over the norm), which occurs at 910MHz during ascending swept frequency and horizontal polarization. The largest variation of hysteresis parameter is 21.8% (1.5 times over the norm) during 940MHz descending swept frequency and horizontal polarization. The remaining parameters have small effects, less than 6.5%. During the EMI exposure, QST transfer curves show significant departure from the frequencies of 500–580, 700–850, and 900–1000MHz. However, the trace separation is returned back to the preexposure condition. The scanning electron microscope evaluation of the GMR head after the exposure appears to be normal. Therefore, these parameter disturbances are not adequate to cause visible damage, but since some parameters are over the manufacturing accepted QST values, it may cause a latently failed head.

Heusler alloy based current-perpendicular-to-the-plane giant magnetoresistance heads for high density magnetic recording

Narrow-track current-perpendicular-to-the-plane giant magnetoresistive heads containing Heusler alloy layer have been fabricated utilizing an abutted junction hard bias design. The head performance has been tested quasistatically and dynamically under high density recording conditions using a perpendicular magnetic recording media.

CPP&amp;#x2013;GMR Heads With a Current Screen Layer for 300 &lt;formula formulatype="inline"&gt;&lt;tex&gt;${\hbox{Gb/in}}^{2}$&lt;/tex&gt; &lt;/formula&gt; Recording

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Current-perpendicular-to-plane (CPP) giant magnetoresistive (GMR) heads with a current screen layer were fabricated, and the recording performance was measured. An output voltage of 1.9 mV and head-amp signal-to-noise ratio (SNR) of about 30 dB were obtained from a 50-nm-wide head with an operating voltage of 120 mV. The MR ratio was 4%–5%, shield gap was 36 nm and resistance was 72 <formula formulatype="inline"><tex>$\Omega$</tex></formula>. With using the thermal fly-height control (TFC), the fabricated head showed a potential to yield a 382 <formula formulatype="inline"><tex>${\hbox{Gb/in}}^{2}$</tex> </formula> recording (1252 kBPI <formula formulatype="inline"><tex>$\times$</tex> </formula> 305 kTPI). The current screen structure reduced the spin torque noise since just a low sensing current of 1–2 mA was required for obtaining a high output. Newly developed CPP–GMR films with a current screen layer showed the MR ratio of 18%–19% with the RA product of 0.2–0.3 <formula formulatype="inline"><tex>$\Omega\cdot\mu{\hbox{m}}^{2}$</tex></formula>. Calculation showed that this film allows us to achieve 30 dB or more in the head-amp SNR when the sensor width was 40 nm or larger. The current screen CPP–GMR head is thus an attractive candidate that has a high potential suitable for an areal density of 500 <formula formulatype="inline"><tex>${\hbox{Gb/in}}^{2}$</tex> </formula> or more. Below 40 nm, an all metal CPP–GMR head with the MR ratio of 10% or more would be the best candidate. </para>

Current status and future of magnetic heads for HDD

AbstractThe rapid growth in consumer electronics application requires higher capacity data storage. For hard disk drive (HDD), many new technologies, such as media, heads, head‐disk interface (HDI), pre‐amp. and so on, have been introduced in order to increase the recording density. This paper focus on magnetic head technologies, such as materials, structures, fabrication process and head performance for HDD. For read heads, tunneling magnetoresistive (TMR) heads have come successively to replace giant magnetoresistive (GMR) heads. Barrier materials have been studied to obtain higher magnetoresisance (MR) ratio with lower resistance area product (RA). AlOx have been used as barrier for 100–120 Gb/in2 TMR heads. MgO barrier shows significantly large MR ratio and is also workable to TMR heads as AlOx one. For longitudinal magnetic recording (LMR) writer, recording performance has been improved with reducing yoke length and coil turns and changing to pole materials with higher saturation magnetization (Bs). Perpendicular magnetic recording (PMR) is introduced for over 120 Gb/in2. Combination between head and medium is much more important in PMR than LMR. The magnetic head is still one of the key component of HDD and is expected to be going to further promote the redording density and performance of HDD in the future. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Analytic Three-Dimensional Model of a Double-Shielded Giant Magnetoresistive Perpendicular Head Using a Singular Function Expansion

By matching a singular function approximation to the Fourier solution at the air-bearing surface (ABS), we have derived a three-dimensional (3-D) analytic model of a shielded giant magnetoresistive (GMR) head, with side shields, for perpendicular replay. We present an explicit expression for the potential in the ABS and give values for the parameters in that expression for a range of practical head dimensions. Using only a few terms of this singular potential model, we show that the vertical field is accurate to within 1% of the sensor potential in the region of the medium for the majority of head dimensions suitable for magnetic recording.

Investigation of Higher Recording Density Using an Improved Co–CoO Metal Evaporated Tape With a GMR Reproducing Head

To demonstrate the highest areal recording density of tape media, we have developed a metal evaporated tape with higher magnetic anisotropy of 2.5times105 J/m3 and finer magnetic activation volume of 2.7times10-24 m3 of a Co-CoO recording layer deposited on a smoother base film. A capability of an areal recording density of 23.0 Gb/in2 was confirmed using a giant magnetoresistive head as a read head. The smoother surface roughness improved signal-to-noise ratio more than 2 dB

Thermally Excited Low Frequency Magnetic Noise in CPP Structure MR Heads

1/f-type of low frequency noise is widely observed in today's state-of-the-art tunneling-magneto-resistive (TMR) and current-perpendicular-to-plane (CPP), and giant-magneto-resistive (GMR) heads. In this paper, the 1/f-type magnetic noise arising from the thermally excited low frequency edge magnetization switch of the free layer is studied by micromagnetic simulation. It is found that although the free layer dimension is in the single domain regime, the edge magnetization random switching related to the thermal excitation and insufficient hard bias field on sensor edge can be a significant low frequency noise contributor in TMR/CPP GMR heads. A strong local hard bias (HB) field at sensor edge that pins the free layer edge magnetization is found to be more critical than a strong averaged HB field across the sensor to avoid magnetic 1/f-type noise. The simulation study shows that a high HB field gradient from sensor edge to center is preferred to maintain a low noise sensor while still achieving high sensitivity. This paper will also discuss the roles of free layer thickness, sensor geometry, current field, temperature, and air-bearing surface physical defect in the behavior of this edge flipping noise

Improvement in the detection of subsurface fatigue cracks under airframe fasteners using improved rotating giant magneto-resistance magnetometer head

Improved Giant Magneto-Resistance Magnetometers (IGMRM) offer many advantages for the detection of subsurface cracks. This paper presents new opportunities for the detection of fatigue cracks close to fasteners or rivets with an improved rotating head. The latter was developed around an IGMRM and a well designed electromagnetic flux focuser. The capabilities of the system for the first to the third aluminium layers (3 to 4 mm-thick each) flaw detection will be highlighted. Experimental Electro Discharge Machined (EDM) crack sizes under fasteners are several millimetres long and about 100 μm wide. Currently, results show the ability of the system to detect very easily a 7 mm-long machined crack near a rivet under 6 mm of aluminium. The use of IGMRMas afield detector, for improved low-frequency response and sensitivity of the device, is discussed. A 2D finite element modelling result of electromagnetic field distribution for this newly designed rotating probe is presented. Also, the complete system configuration is described.

Discharge Current From Coated Wire to GMR Head

The giant magnetoresistive (GMR) head used in magnetic recording is an extremely sensitive device that is sensitive to electrostatic discharge (ESD). A generally available coated wire that is electrified on its surface can increase the potential of the copper core. When the wire is touched to the GMR head terminal, the discharge current can damage the sensor of the GMR head. Four types of wires were measured for three lengths each. Resin-coated wires were electrified from 20 V to more than 1 kV. The peak current discharging to the GMR head ranged from 100 mA to more than 1000 mA. A resin-coated wire as short as 5 cm was found to damage the GMR head. Among the wires examined herein, only the enameled wire is recommended to connect the GMR head because no charge or discharge currents were observed for this wire. When an ionizer is used to neutralize the tribocharge on the surface of the wire, the remaining charge in the copper core depends on the effectiveness of the neutralization process. A wire with an antistatic coating is required in order to reduce the ESD threat to sensitive devices such as the GMR head

Modeling Electrostatic Discharge Affecting GMR Heads

Electrostatic discharge (ESD) damage to giant magnetoresistive (GMR) heads is investigated using a charged device model tester. The sensor of a GMR head is easily affected by ESD during handling in production. Simulation of the picking procedure reveals that elevating such a small-capacitance device above the work surface increases the potential by up to ten times and the discharge current by up to five times. With 20-V initial potential, which could be applied by the picking tool, the elevation of a 5-pF capacitance by just 10 mm increases the potential to 200 V and the peak discharge current to 578 mA. In the case of a 1-pF capacitor, however, the potential and peak current remained low due to the effect of stray capacitance. In the case of a GMR head with 3.7-pF capacitance, a 5-mm lift reduced the initial potential causing ESD damage from 60 V to less than 40 V. Waveforms of discharge reveal that the GMR head can be designed to reduce the ESD risk.

Characteristics of Electrostatic Discharge Induced Damage on Magnetic Tunnel Junctions

We report on the investigation of electrostatic discharge (ESD) sensitivity of current perpendicular to plane (CPP) magnetic tunnel junction (MTJ) heads used in hard disk drives (HDD). Our results show that MTJs have different degradation characteristics and mechanisms from current in-plane (CIP) giant magnetoresistive (GMR) heads when exposed to cumulative ESD current transients. Both MTJ and GMR heads show gradual amplitude degradation as a result of increasing ESD levels, but MTJs are more robust than GMR heads in terms of sensor stack magnetic stability, such as polarity flip. Large ESD voltages could change stability state of the MTJ heads, which shows up as random telegraph noise associated with transfer curve magnetic distortions in low-frequency regime and noise spikes several times above baseline noise in high-frequency regime. This MTJ instability is likely a result of defects in the multilayer reader magnetics and disturbances, as well as degradation of tunnel oxide barrier.

Multichannel Write and GMR Heads for Over 1 TB Tape System

This paper reports on the structure and performance of multichannel write and giant magnetoresistive (GMR) heads for a helical-scan tape system. The four-channel write head was fabricated with four layers stacked and shifted by a track pitch. The eight-channel read head was also stacked four layers, each of which has two spin-valve elements. The write head and read head were assembled into a commercial drive. Four channels are written in one revolution without a guard band and each of the eight channels is read, in half of the track pitch. The feasibility of a recording density over 4.3 Gb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was established, which would make possible a tape system of over 1 TB using an 8 mm cassette

Corrosion-Resistant GMR Head for Over 1-TB Tape System

We have developed a corrosion-resistant giant magnetoresistance (GMR) head using a spin-valve (Ta/NiFe/CoNiFe/CuAu/CoNiFe/PtMn/Ta) film for an over 1-TB helical tape system. The magnetic properties of the corrosion-resistant GMR head were preserved during a Battelle Class II test for 20 days. The sense current shunting in the spacer layer of the spin-valve film (SV) was less for CuAu than for Cu due to the higher resistivity of CuAu. The interlayer-coupling field in the corrosion-resistant SV film was about 25% that in conventional SV films. Therefore, each layer of the corrosion-resistant SV film was designed so as to optimize the bias point. For stripe-heights in the range from 0.7 to 0.4 mum, the asymmetry of the head output was maintained at plusmn20%. The recording characteristics had a signal-to-noise ratio (SNR) of 19.5 dB with a 0.7 mum track width at a 254 kFCI linear density. Using this head, a tape system of over 1 TB per cassette is possible