Magnetoresistive Heads Research Articles

A study on HAMR read/write channel models for double layer magnetic recording

Three-dimensional (3D) magnetic recording with multiple recording layers has garnered considerable attention as a next-generation magnetic recording method that uses dual conventional recording layers and a magnetoresistive (MR) head for reading. In this work, to examine signal processing methods for the 3D magnetic read/write (R/W) channel using heat-assisted magnetic recording (HAMR), we have constructed a HAMR R/W channel using a granular media model. Our 3D HAMR R/W channel model, specialized for signal processing development, can account for the Curie temperatures of the recording layers and the temperature distribution of the heat source.

Evaluation of multi-bit domain wall motion by low current density to obtain ultrafast data rate in a compensated ferrimagnetic wire

Architectures based on multi-bit magnetic domain walls (DWs) take advantage of the fast speed, high density, nonvolatility, and flexible design of DWs to process and store data bits. However, controlling multi-bit DWs driven by electric current at an ideal position remains a significant challenge for developing integrated spintronic applications with high reliability and low power consumption. We exhibit the possibility of driving fast and stable multi-bit DWs at low current density without an in-plane external magnetic field in Fe-rich GdFeCo magnetic wires. When an in-plane magnetic field is applied in the wire direction, the front edge accelerates, although the rear edge decelerates, and the recorded data are destroyed. Hence, this method is not practical. Here, the DW speed of the multi-bit DWs is 1500 m/s under a low current density of 29 × 1010 (A/m2). A straight DW shape is required to accurately read the bits of information by the tunneling magnetoresistance head in real DW memory devices. Moreover, we demonstrate that the DW position is related to the DW shape after injecting a pulse current into the magnetic wire. A straight DW shape is exhibited for 3 ns pulse duration width, while the DW shape became rounded for 30 and 50 ns pulse duration widths. Our finding provides a practical concept for multiple-bit-per-cell memory and presents a viable platform for DW memory applications.

A Study of Channel Time-Domain Response on Equalization for Reproducing a Double-Layer Magnetic Recording Medium

To increase the recording density of hard disk drives (HDD), 3-D magnetic recording using microwave-assisted magnetic recording (MAMR) with multiple recording layers is attracting much attention as a next-generation recording method. Developing a signal processing method for 3-D magnetic recording using the multiple conventional recording layers and an magnetoresistive (MR) head reading system is desired. In this article, we evaluate 2-D partial response maximum likelihood (PRML) channels by the bit error rate (BER) performance, where the transfer function of the PR channel is determined by the response of the in-phase or differential-phase bits between layers in a double-layer magnetic recording medium. The results show that a BER of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-4}$ </tex-math></inline-formula> can be achieved by the PRML channel determined by the differential-phase response with the signal-to-noise ratio (SNR) greater than 26.0 dB for system noise.

Tungsten Ditelluride: Synthesis, Structure, and Magnetoresistance Property

AbstractTungsten ditelluride (WTe2) is arising as an attractive material in magnetoresistance (MR) properties filed. The magnetoresistance properties of WTe2 makes it an ideal material for its magnetoresistive heads of computer and magnetic sensor device applications. So far, people have developed various controlled synthesis methods to produce large‐quantity uniform flat WTe2 crystals. Nevertheless, none of the methods are sustainable, some methods use reducing gases, and some use very long heat preservation, and the process is very complicated. Here, a mild and fast self‐flux method is described for WTe2 synthesis that can avoid producing reducing gases. After conducting structural and physical tests, it is found that annealing time is critical for WTe2 structure formation. Long annealing time improves the crystallinity of WTe2 and makes its texture more obvious, increasing the conductivity of WTe2. Therefore, it will lead to the anomalous nonsaturating positive magnetoresistance with different field dependent behaviors, reaching the ultrastrong magnetoresistive value of 1040% at 5 K and 14 T. The results provide a sustainable approach for uniform WTe2 crystal synthesis, which will benefit the large‐quantity production of WTe2 magnetoresistive materials.

Large magnetoresistance in Heusler alloy-based current perpendicular to plane giant magnetoresistance sensors

Increasing the data storage in next-generation hard disk drives requires a reduction in the physical dimensions of read sensors. Tunneling magnetoresistance heads yield high magnetoresistance (MR) ratio but with a high resistance-area product (RA) that is suboptimal for devices. Giant magnetoresistance (GMR) head using different materials is an alternative way to improve reader performance with high MR ratio and low RA. In this paper, we theoretically study the effect of material properties and the layer thickness on RA and MR ratio in a trilayer system via an atomistic model combined with the spin transport model. The GMR stack can be constructed by the atomistic model and the RA and MR ratio can be directly calculated by considering the spin accumulation and spin current from the spin transport model. It is found that the spin valve using the Heusler alloy electrode with high spin polarization exhibits a high MR ratio and RA of 64 which is better than the spin valves using conventional ferromagnets such as Co, NiFe and CoFe. Moreover, we consider the thickness dependence of the change of RA (). Increasing the free layer thickness yields the increase in and MR ratio because of the enhancement of the bulk spin scattering. Additionally, the results show that the depends on the spin diffusion length of the nonmagnetic materials (). The increases from 3 up to 10 when increases from 35 to 1200 nm. This investigation shows the possibility for read head design of HDDs with areal density beyond 2 Tb in−2.

Structural, Raman analysis and exchange bias effects in Mn doped multiferroic Bi0.80La0.10Ca0.10Fe1-xMnxO3 ceramics

In this paper, the multiferroic ceramics Bi0.80La0.10Ca0.10Fe1-xMnxO3 (0.0 ≤x ≤ 0.12) are synthesized via conventional solid-state reaction route and their structural, Raman spectroscopy and magnetic properties are explored. Rietveld analysis carried out for all samples, revealed that the system exhibits coexistence of mainly two crystalline phases with symmetries R3c and Pbnm throughout the whole composition range. The presence of two structural phases is further supported by Raman spectroscopy analysis. The magnetic parameters, that is, magnetization, remanency, coercivity and squareness ratio are found to be enhanced by the Mn incorporation into Bi0.80La0.10Ca0.10FeO3. This is basically due to the structural modification, breaking of the incommensurate spin structure and inter-grain magnetostatic interaction in the Bi0.80La0.10Ca0.10Fe1-xMnxO3 ceramic samples. The exchange bias effect (~734 Oe) is observed for the Mn doped sample only with 6 at. %. Such high value may be extremely useful for technological applications including magnetoresistive read heads and sensors.

Anisotropic and Controllable Gilbert-Bloch Dissipation in Spin Valves.

Spin valves form a key building block in a wide range of spintronic concepts and devices from magnetoresistive read heads to spin-transfer-torque oscillators. We elucidate the dependence of the magnetic damping in the free layer on the angle its equilibrium magnetization makes with that in the fixed layer. The spin pumping-mediated damping is anisotropic and tensorial, with Gilbert- and Bloch-like terms. Our investigation reveals a mechanism for tuning the free layer damping insitu from negligible to a large value via the orientation of fixed layer magnetization, especially when the magnets are electrically insulating. Furthermore, we expect the Bloch contribution that emerges from the longitudinal spin accumulation in the nonmagnetic spacer to play an important role in a wide range of other phenomena in spin valves.

Spintronics, Magnetoresistive Heads, and the Emergence of the Digital World

The development of spintronics has had a dramatic impact on the evolution of the magnetic hard disk drive and, in turn, on the ubiquitous nature of digital data in modern society. The advent of the magnetoresistive read head technology transitioning from the anisotropic magnetoresistance read head to the spintronics-based giant-magnetoresistance read head and the tunneling magnetoresistance read head were major drivers in this remarkable increase in digital storage. We will review the development of magnetoresistive read head technologies and their impact on digital data storage. We will then highlight the current state-of-the-art sensor and potential future technologies.

Frequency-Domain Approach To Determine Magnetic Address-Sensor Separation Distance Using the Harmonic Ratio Method.

In this work, we describe an approach to determine the distance separating a magnetic address from a scanning magnetoresistive sensor, a critical adjustable parameter for certain bioassay analyses where magnetic nanoparticles are used as labels. Our approach is leveraged from the harmonic ratio method (HRM), a method used in the hard drive industry to control the distance separating a magnetoresistive read head from its data platter with nanometer resolution. At the heart of the HRM is an amplitude comparison of a signal's fundamental frequency to that of its harmonics. When the signal is derived from the magnetic field pattern of a periodic array of magnetic addresses, the harmonic ratio contains the information necessary to determine the separation between the address array and the read head. The elegance of the HRM is that there is no need of additional components to the detection platform to determine a separation distance; the streaming "bit signal" contains all the information needed. In this work, we demonstrate that the tenets governing HRM used in the hard drive industry can be applied to the bioanalytical arena where submicrometer to 100 μm separations are required.

Electromagnetic Interference Effects on Stability of Tunneling Magnetoresistive Read Heads

We used micromagnetic simulations to investigate the effects of electromagnetic interference (EMI) caused by electrostatic discharge on the stability of tunneling magnetoresistive (TMR) read heads based on International Electrotechnical Commission standard 61000-4-2. In simulation, EMI was applied to TMR heads between 0.1 cm and 2 cm from the source, and magnetization fluctuations and induced magnetic noise were evaluated. Although EMI can induce spin-transfer torque in the heads, it causes little change in the magnetization orientation and no noticeable influence on the readback signal or signal-to-noise ratio of the heads.

対向面積の異なる金属筐体から帯電物体を遠ざけたときに生じる静電誘導電圧

A sensitive device, for example, a giant magnetoresistive head contained in an electronic equipment, often malfunctions because of the induced voltage of less than 10V when a charged human body is present near the equipment. Such accidents due to electrostatic induction occur when the charged body moves. The author focuses on the induced voltage of opposite polarity generated on the metal box when the charged body moves away from the box. In experiments, metal boxes of different heights are used to change the facing area between the charged body and the box. In addition, the position of the metal box is changed so that the induced voltages of the box can be compared. The results show that the induced voltages and the electric charges increase as the facing area between the metal box and the charged body increases. The induced voltage of the metal box indicates the value of approximately -370% of the charged body when the facing area between the metal box and the charged body increases. Further, the induced voltages and the charges are slightly dependent of the position of the metal box. The results will be useful for the design of electronic equipment from the standpoint of preventing malfunction and failure of the equipment.

Free Layer Characterization of TMR Head with Multi-Stripe Height by Using Ferro Magnetic Resonance Analyzer (FMRA)

As continued reducing the track width (TW) of magnetic read sensor for gaining areal density in hard disk drive (HDD) industry, stripe height (SH) is needed to be reduced as keeping aspect ratio(TW/SH) for maintain resistance and signal constant. This paper is proposed to characterize the free layer behavior of tunneling magneto resistive (TMR) head with multiple SH by using ferro magnetic resonance analyzer (FMRA) that is able to analyze by layer-level state without applying external field. The results are found that, at shorter SH, effective stiffness field of free layer is increased which is based on decreasing FMR amplitude and increasing FMR frequency. As a result, magnetic read sensor is shown less sensitivity and signal to noise ratio (SNR) with shorter SH. The SH is a correlated function of the effective stiffness field in free layer that is referred to changed shape anisotropy field which has an effect to read ability.

Development of advanced barium ferrite tape media

We developed an advanced particulate magnetic tape using fine barium ferrite (BaFe) particles for magnetic-tape storage systems. The new tape showed a signal-to-noise ratio (SNR) that was 3.5dB higher than that of the commercially available BaFe tape used for the Linear Tape Open generation 6 tape-storage system, at a linear density of 300kfci measured with a giant magnetoresistive head with a reader width of 0.45µm. Such significant increase in SNR was achieved by reducing the magnetic particle volume from 1950 to 1350nm3, while maintaining a sufficiently high thermal stability, improving the perpendicular squareness ratio from 0.66 to 0.83, and improving the surface roughness from 2.5 to 2.0nm when measured by atomic force microscopy and from 2.4 to 0.9nm when measured by optical interferometry. This paper describes the characteristics of the new BaFe particles and media, which are expected to be employed for future high-capacity linear-tape systems.

(Keynote) Status and Challenges in Spin-Transfer Torque MRAM Technology

Spin-transfer torque magnetic random access memory (STT-MRAM) has been considered as a promising candidate for next generation non-volatile memory due to a combination of fast speed, high endurance, excellent scalability, and ease of integration with the standard CMOS processes. While feasibility demonstrations used magnetic tunnel junction (MTJ) devices with an in-plane magnetization, significant advances in materials allowed implementation of the perpendicular MTJ design, which provides the best long-term scalability for STT-MRAM beyond 20 nm. Despite significant progress, there are many remaining hurdles in the productization of STT-MRAM. They are stemming from a set of requirements imposed on the MTJ performance: simultaneously low writing current/voltage and high thermal stability, high magneto-resistance at low RA (resistance and area product) value, low error rates and high endurance cycles. We will discuss the inter-dependencies of these parameters, and show how they are affected by varying magnetic properties of the MTJ stack layers. In particular, we study impact of the free layer anisotropy, magnetic moment, thickness, and Gilbert damping on writing current and thermal stability. In this presentation, we report our progress in meeting these requirements and our development approaches for future challenges. ----------------------------------------------------------------------------------------------------------------------------------------------- Mohamad Krounbi has over 25 years of experience in magnetic recording technology and recording head process and design innovations. While at IBM from 1978 to 1997, he played a leading role in the development and production of the magneto resistive heads from MR to GMR and TMR generations of products. Mohamad contributed to major breakthroughs in design, process, and integration of heads and media in the HDD industry. He held executive positions at IBM, Western Digital, and Hitachi Global Storage Technology (HGST). Mohamad holds over 50 US patents and co-authored numerous scientific publications in magnetic recording and fundamental materials and polymer research. He received numerous outstanding innovation and contribution awards while at IBM and was elected to the IBM academy of technology in 1995. At HGST he was awarded the President’s technology award for the development and launch of the first perpendicular magnetic recording product. In 2010, Mohamad joined Grandis, Inc. as the CTO and Senior Vice President of technology development and operations. Mohamad is currently the CTO and Senior Vice President of Samsung’s New Memory Technology Laboratory in the US ( NMT Lab). Mohamad has a BS in Physics from Hagazian College in Beirut Lebanon and a MS in Physics from San Jose State University.

Effect of stripe height on the critical current density of spin-torque noise in a tunneling magnetoresistive read head with a low resistance area product below 1.0 Ω μm2

To understand the spin-torque effect on the noise in tunneling magnetoresistive (TMR) read heads, the GHz range noise spectra of TMR read heads with a narrow track width (w = 36 nm), and various stripe heights (h) are investigated as a function of the external magnetic field (Hex) and dc bias current density (j). The strong noise peak intensity depends on both Hex and j, indicating that the spin-torque affects the thermal mag-noise under a positive (negative) j for a positive (negative) Hex, regardless of h in the TMR heads. Due to the increased shape anisotropy, the critical current density (jc), where the non-thermal fluctuation noise originates from the spin-torque, increases markedly as the head dimension is reduced, and the maximum value of jc is approximately +1.5 × 1012 A/m2 for a head with w = 36 nm and h = 15 nm. These results demonstrate that the non-thermal fluctuation noise originating from the spin-torque in the TMR head can be suppressed in the current density range below 1012 A/m2, as the head dimension is reduced and the shape anisotropy is increased.

A novel technique to detect effects of electromagnetic interference by electrostatic discharge simulator to test parameters of tunneling magnetoresistive read heads

Electrostatic discharge (ESD) has been a significant problem in the manufacturing processes of the magnetic recording head technologies for many years. Besides direct discharge damage, ESD can also generate electromagnetic interference (EMI) which could possibly cause failure in magnetic read heads. The aims of this work are to measure the EMI from ESD simulator based on the standard IEC 61000-4-2 and to investigate the effects of EMI on tunneling magnetoresistive (TMR) read heads. The discharge current and the EMI generated by ESD simulator are measured in the experiment. Also, the EMI is applied to the TMR read heads at various amplitudes and distances in order to evaluate the changes of read head parameters including the bit error rate, resistance, read back signal amplitude, and asymmetry parameter of the head. The results show that the discharge current waveform is consistent with the IEC standard current waveform. In addition, it is found that the EMI is insufficient to cause a permanent change of the read head parameters at distances above 2 cm indicating the minimal impact on the head performance. Further experiments are proposed to carry out more detailed studies of the EMI effects on head parameters in order to improve the methods to prevent the degradation of parameters which can be a latent failure in the magnetic read heads.

Control over electronic components by their magnetic field

We show the results of our studies related to control over objects by the magnetic field they generate. We propose a model to compute the current and its magnetic field in a conductor with a rectangular defect. We describe the developed and produced anisotropic magnetoresistive gradiometer head to measure surface magnetic fields in order to control the functionality of electronic components by the magnetic field they generate. We describe the operation of a testbed with a test circuit in the plotter.

Baseline Popping Detection and Correction Algorithms for Perpendicular Magnetic Recording System

Hard disk drives (HDDs) employ the magneto-resistive (MR) head to sense the change in magnetic flux via the transitions of magnetization pattern, resulting in a readback signal. Thus, head instability plays an important role on the reliability of HDDs because it can deteriorate the system performance considerably. Baseline popping (BLP) is one of the crucial problems caused by the head instability, whose effect can distort the readback signal to the extent of causing a sector read failure. This paper proposes three BLP detection and correction algorithms for a perpendicular magnetic recording (PMR) system. Specifically, to suppress the BLP effect experienced in the readback signal, the first algorithm is based on an averaging filter and a threshold detector; the second one relies on the estimated BLP signal obtained from a linear curve fitting technique; and finally the third one uses two sequence detectors running in parallel. Experimental results indicate that the third algorithm performs better than the other schemes because it can detect and correct the BLP better than the others, especially when the peak BLP amplitude is large.

Magnetic properties of patterned arrays of exchange-biased IrMn/Co square dots

Finite size effects on the exchange-bias properties of patterned antiferromagnetic/ferromagnetic (IrMn/Co) nanodots were investigated experimentally and by numerical simulations. Both the dot lateral dimension (200 × 200 nm2–50 × 50 nm2) and ferromagnetic layer thickness (2–25 nm) were varied. Two different behaviours were observed: single-shifted loops with coherent reversal for thin Co layers and double-shifted loops via a multidomain state during magnetization reversal for thicker Co. Atomistic simulations including magnetostatic interactions confirmed the existence of these two different regimes. MOKE measurements, focused on few dots, allowed a quantitative evaluation of dot-to-dot exchange-bias variability, which becomes particularly large in the regime of switching via multidomain state formation. This variability issue is of particular importance in the context of MRAM development and of magnetoresistive heads for hard-disk drives.

Effect of Thermal Conditions on Bit Error Rate for Barium-Ferrite Particulate Media

In this study, the thermal stability of user data written on perpendicularly oriented barium-ferrite media was evaluated in terms of the bit error rate. To determine whether recorded data can be read back after long-term preservation, we recorded a pseudorandom data pattern on barium-ferrite media at a linear density of 500 kbpi and baked the media at 65°C for an extended period. After baking, the recorded data were read back using a 0.45m-wide giant magnetoresistive read head. The bit error rate was calculated using the GPR4ML-AR model. It was found that the bit error rate was stable even after one year at 65°C. This result indicates that perpendicularly oriented barium-ferrite media has high thermal stability.