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Massive data growth is accelerating the need for storage. Hard Disk Drive read/write technology continues to advance to support this demand. This Advanced Storage Research Consortium report combines a top-down (system) and bottom-up (micromagnetic) approach to define a reader technology roadmap for 2.4, 4.0 and 6.0 Tbpsi including dimensional and performance targets. We examine the resolution, SNR, stability and manufacturing requirements, challenges, and gaps. We show that the extension of today’s Spin Valve-based reader designs may enable 4Tbpsi, but significant gaps must be closed. For higher Areal Densities, we review future reader designs, challenges, and discuss potential solution paths.

Giant magnetoresistance (GMR) sensors have been widely used in current detection, compass, automobile, and Internet of Things. The linearity of GMR sensors relies on a magnetic annealing process above the blocking temperature of the antiferromagnetic layer, which causes the same magnetization directions in the free and pinned ferromagnetic layers. Here we construct an orthogonal magnetic anisotropic structure by using a relatively low annealing temperature of 145 °C, where the magnetization direction of the free layer is perpendicular to that of the pinned layer so that a linear output can be realized. Quantitative magneto-optical kerr microscope (MOKE) is used to investigate the vector distribution of the magnetic domains, showing that an orthogonal anisotropy is established by the low temperature annealing process. GMR magnetic field sensor with a sensitivity of 0.78 mV/V/Oe and a measurement range of -20 to 20 Oe is demonstrated, which provides an effective method to enhance the linearity of the magnetic sensors with spin valve structures.

Spin valves (SVs) based on 2-D materials have garnered so much attention in the past decade. One of the major interests is the sake of getting a higher magnetoresistance (MR) ratio to facilitate industrial applications. However, miscellaneous effects are mixed in the obtained MR and the contributions of 2-D materials are not clearly distinguished even. In this article, we report the angle-dependent MR in WSe2 SV for different orientations of applied magnetic field. By setting reference samples with different structures, the contributions of anisotropic MR (AMR) and MR of 2-D materials are distinguished. The MR ratios generated by WSe2 show a monotonic decreasing trend with the increasing included angle between the direction of magnetic field and the length axis of bottom electrode Py. The related angular variation in such SV structure can be used for future researches and industrial applications.

In this article, we provide an initial qualitative description of the basic magnetization reversal processes behind temperature and cooling field-dependent exchange bias effect at Co/CoO interface of Fe3O4/Alq3/Co/CoO/Au hybrid spin valve (SV) device structure. The exchange field at Co/CoO interface is observed to be inversely proportional to temperature, indicating enhanced magnetic field sensitivity of Co reverse domain nucleation at the high-temperature region. The exchange field, at constant temperature, is found to be insensitive to change in the cooling field, which may be due to efficient ferromagnetic coupling at the Co/CoO interface of the device. Sudden decrease of exchange field with training cycle number ( $n$ ) has been observed at lower values of $n$ , which may be attributed to rearrangement of Co pinned layer spin configuration toward equilibrium. Interestingly, incorporation of antiferromagnetic CoO layer has not been observed to bring any change in magnetoresistance (MR) behavior of the exchange biased SV device, which shows a two-step switching pattern of MR at 100 K.

Precision healthcare brings many technological challenges for the development of new tools. Among these, the detection of biomagnetic fields in the pico-Tesla range, at room temperature, requires the state-of-the-art sensors with high signal-to-noise ratio. Also, given that biomedical signals are characterized by their low frequency, minimizing the sensor noise has to be addressed simultaneously with the efforts to increase sensor sensitivity. In this paper, we maximize the spin-valve sensors sensitivity by optimizing the distance between the poles of double-layer magnetic flux concentrators (MFCs). An enhancement of the sensitivity from 1.5%/mT (3.8 mV/mT) to 654%/mT (1765 mV/mT) could be observed for a double-layer architecture when a distance from the recessed steep CoZrNb MFC to the NiFe pole is $100~\mu \text{m}$ . The impact on the detectivity was evaluated through noise enabling the detection of magnetic fields of 322 pT/Hz1/2 at 10 Hz.

A novel perpendicular spin-transfer torque magnetic random-access memory spin valve with a memory-cell size below 20 nm × 20 nm and a thermal stability factor Δ of ~77 (10-year retention time) was designed by ferromagnetically coupling a multiple free layer [Co/Pt]n to Co 2 Fe 6 B 2 having interfacial perpendicular magnetic anisotropy (i-PMA) instead of coupling to a conventional double i-PMA free layer (Δ = 33). Thermal stability (Δ) increased with an increase of n in the [Co(0.47nm)/Pt(0.23 nm)]n multiple free layer. In particular, Δ = 80 could be achieved with n = 4 for a 15 nm × 15 nm memory-cell size. However, the tunneling magnetoresistance (TMR) ratio, which should be above 150% to assure a reasonable sensing margin, rapidly decreased from 190% to 98% with an increase in n from 0 to 4. This decrease was associated with W and Pt atomic diffusion into the MgO tunneling barrier. Improvement in the crystallinity of the MgO tunneling barrier increased the TMR ratio to 144% for n = 4.

A full-bridge magnetoresistive spin valve (SV) sensor scheme for applications of linear magnetic measurement is fabricated and analyzed with the sign of magnetoresistance (MR) can be tuned by the modified the artificial antiferromagnetic (AAF) layer. In the last two decades, SV sensors have shown their crucial roles in the life. The common applications of SV sensors are used in position, angle sensors [1], [2], electronic compasses, and contactless current sensors [3]. To build a sensor, especially for resistive sensors, a Wheatstone-bridge is always a good solution for its advantages of the lessening dc-offset, and temperature errors [4]. In the MR sensor fabrication, the bias direction is also the main factor that decides the sensor architectures. For instance, a half-bridge was realized by two active cells, whereas shielding the other two cells [5], [6] or killing their sense by the roughened surfaces [7]. However, the effective sensitivity and linearity of the sensor are reduced [8]. There are several methods to form a full-bridge that provides a maximum output of the sensor. For example, the current strap was directly fabricated on the surface of the device to induce the local bias magnetic field during the cool-field post-annealing process [9]. Using an appropriate location of MR cells within the gap of the flux guide, a full-bridge could be established [10]. An antiparallel alignment of the pinned direction of the two pairs of MR cells was carried out by a microscopic focused laser heating during cool-field treatment [11]. Another method is that the usage of two deposition processes for two pairs of MR cells in the opposite applied magnetic field [12]. In this work, we propose a method to tune the bias direction of two neighboring meanders in the full-bridge by varying the thickness of Ru in AAF layers of patterned SV cells with a single post-field annealing step. In order to implement the proposed design, the two deposition processes were carried out. First one, two normal magnetoresistance SV cells are the structure of (Si/SiO 2 ) /Ta(50A)/NiFe(30A)/Co(15A)/Cu(24A)/CoFe(25A)/ IrMn(100A)/Ta(50A), whereas, the other two cells are the inverse magnetoresistance with an AAF layer of structure of (Si/SiO 2 ) /Ta(50A)/NiFe(30A)/ Co(15A)/Cu(24A)/[CoFe(15A)/Ru(7A)/CoFe(25A)]/IrMn(100A)/Ta(50A). The normal MR and inverse MR curves of the different SV structures are shown in Fig. 1. The SV films were prepared by RF magnetron sputtering. The conventional lift-off method was used to pattern the MR cells. A tiny permanent magnet was used to adjust the sensor operating point. To estimate an appropriate MR ratio for an expectation sensitivity, the hyperbolic tangent model was introduced by author et al. [13], a required MR correspondences a sensitivity and a saturation field range was given by MR =(2.({dV/V/dB}).$B_{s}$)/ (G_{m} -$(dV/V/dB).$B_{s})(1)$ where $MR$ is the magnetoresistance ratio, dV/V/dB is the sensitivity of the bridge (V/V/Oe), $B_{s}$ is the saturation field, and $G_{m}$ is the flux amplification factor. Suppose, $G_{m}$ \quad =1(no gain flux density), and the $B_{s}$ is about 50 Oe, in order to yield a sensitivity of 1 mV/V/Oe, the MR ratio should be at least 10.5 %. The SV cells were patterned to the dimensions of $200 \mu \mathrm{m}\times 2 \mu \mathrm{m}$. The resistance of individual SV cell is about 10 kv Ω. The cut chip dimension is 2 mm \times 1 mm was attached to a printed circuit (PCB) board using the aluminum wire bonding method. The voltage output of the bridge was amplified by a preamplifier using AD620 from Analog Device, Inc., and connected to a data acquisition device (MyDAQ). The data was recorded using a software coded in the LabVIEW. The performance of the SV full-bridge was verified by setting up the sensor in a sweeping magnetic field of \pm 150 Oe induced by a Helmholtz coil. The response curve of the full bridge to the external fields are shown in Fig. 2. The obtained sensitivity is 0.23 mV/V/Oe with $B_{s}$ is about 100 Oe and a patterned MR ratio of 4.5 % is in agreement with the hyperbolic tangent model in Eq. (1) [13]. Further experiments to increase the resistance of GMR elements using the advanced microfabrication methods are still ongoing. The sensitivity of the bridge can be further enhanced by the patterning SV cells in series [14] or the usage of a flux amplification (flux concentrators) [15].

The near-future ultra-high-density magnetic recording with an assist recording technology requires a substantial evolution of the read-head technology. A read sensor with a small sensor resistance-area product ( $RA$ ), a large magnetoresistance ratio ( $\Delta R/R$ ), and a thin total sensor thickness is required for the low bit error rate and high-spatial resolution of reading. We developed current-perpendicular-to-plane giant magnetoresistance sensors using highly spin-polarized Heusler alloy ferromagnetic layers with the spin polarization of the conducting electron $\beta \sim 0.8$ . A reasonably large $\Delta R/R$ up to 14% was obtained by a practical bottom-pinned spin-valve structure with a conventional metallic spacer layer of an Ag90Sn10 (AgSn) alloy. By replacing the spacer layer with a conductive oxide-based AgSn/InZnO bilayer, a large $\Delta R/R$ up to 32% was realized at RA value of $\sim 0.1~\Omega \cdot \mu \text{m}^{{\mathbf {2}}}$ , which is optimal for the read sensor application. A large voltage output up to 6 mV was obtained for a sensor utilization of ~33% at a bias voltage of 100 mV.

Magnetoresistive sensors using a tunnel magnetoresistance (TMR) spin valve structure with CoFe and CoFeB free layer (FL) are investigated. Lateral dimension, thickness, and magnetic properties of the circular-shaped FL energetically favor the nucleation of a magnetic vortex. In the vortex configuration, the sensors show the expected hysteresis-free transfer curve. Distinct differences between CoFe and CoFeB in the vortex typical parameters are observed. The experimental results are compared to micromagnetic simulations. Influence of saturation magnetization ${M}_{s}$ on the vortex transfer curve is studied in order to reproduce the experimental data. However, only by adjusting ${M}_{s}$ iteratively the experimental data cannot be fully reproduced: The vortex annihilation ( ${H}_{\mathrm {an}}$ ) in CoFeB occurs at smaller fields than simulated. On the other hand, the change in TMR signal at ${H}_{\mathrm {an}}$ is significantly smaller in the experiment for CoFe. Discrepancies are most pronounced at ${H}_{\mathrm {an}}$ . This investigation focuses on the magnetic and electric properties at the edge of the FL structure. Reduction in ${H}_{\mathrm {an}}$ can be explained for 1.1 $\mu \text{m}$ FL diameters by introducing a magnetically disturbed edge. Reduced change in TMR signal at ${H}_{\mathrm {an}}$ can be understood by introducing an electrically inactive edge area. The analysis shows that for CoFe and CoFeB different edge inhomogeneity effects are present or edge inhomogeneities have different impacts.

The deviation of the pinning direction from the easy axis was created in spin valves by thermomagnetic treatment. The angle between the uniaxial and the unidirectional magnetic anisotropy axes is shown to depend on the value of the field applied during the thermomagnetic treatment. Anhysteretic dependence of magnetoresistance on the magnetic field is obtained for the spin valves with crossed anisotropy configuration. The interval within which the magnetic field dependence of magnetoresistance is linear depends on the spacer thickness.

Nowadays, giant magnetoresistance (GMR)-based sensor technology is widely used in numerous industrial applications. However, an inexpensive and efficient method for compensating the nonlinear output of GMR spin valve sensors is still under the industrial prospect, which limits the application of the GMR sensors. By applying the B-Spline modeling, excellent results can be achieved; nevertheless, the sensor has to be calibrated in the whole temperature range. In this paper, a simple mathematical operation-based calibration method for GMR spin valve-type current sensors applying B-Spline modeling is introduced. Instead to measure dozens of temperatures and per temperature several hundreds of points, this method allows a calibration for each further sensor applying B-Spline modeling with only two temperatures and per temperature only 15 points. In comparison with the original calibration measurement over temperature for B-Spline modeling, the new calibration with minimized efforts shows outstanding measurement accuracy of ±0.6% at a nominal current of 50 A in a temperature range from −20 °C to 80 °C.

This paper reports on the development and the fabrication of a novel atomic force microscopy cantilever with integrated magnetoresistive sensors at its tip for scanning magnetoresistance applications. A 2-D scan over an area of $100\times 100~\mu \text{m}^{2}$ simultaneously generates topographic and magnetic maps with a spatial resolution better than 1 $\mu \text{m}$ . The magnetic field is detected using two spin valve sensors with a field sensitivity of 61.33 $\mu \text{V}$ /Oe. The device operation is demonstrated with a ferromagnetic test feature of $1\times 20~\mu \text{m}^{2}$ . The results demonstrate that this scanning technique is capable of accurately quantifying the absolute magnitude of magnetic stray fields.

The exchange-coupled hard/soft magnetic material, proposed in 1991 by Kneller et al.[1], is widely regarded as an excellent next-generation permanent magnetic material. A perfect composite material is expected to hold a large energy product because of the combination of the large remanence of the soft phase and the high coercivity of the hard phase. A giant energy product as high as 120 MG Oe was thus predicted by Skomski and Coey in 1993 [2]. However, experimental values are much smaller [3-4], where easy axis deviations are found to be an important factor leading to the outstanding discrepancy. In the meantime, the magnetic reversal mechanism also varies with β, the angle between the easy axis and the applied field. In this work, hysteresis loops and energy products have been calculated for hard/soft multilayers with various β based on a self-contained micromagnetic model, focusing on the angular dependent magnetic reversal mechanism.

A 3-D magnetic field sensor for geomagnetic field measurement was designed, fabricated, and characterized in this paper. The device comprises spin-valve giant magnetoresistance sheet films of Si–SiO2/Ta/NiFe/CoFe/Cu/CoFe/IrMn/Ta to form a Wheatstone bridge using separate chips and a magnetic flux guide. The reduction of hysteresis and control of operation point were solved using the technique applying both the ac and dc magnetic biasing. The outputs from each axis were obtained using a computer-based driving and detection systems to retrieve the $x$ -, $y$ -, and $z$ -field components. The sensitivities were found to be 0.25, 0.15, and 0.07 mV/V/Oe for $x$ -, $y$ -, and $z$ -components, respectively. The proposed concept can be realized as a miniature 3-D magnetic field sensor of low cost and low power.

Exchange bias (EB) refers to the magnetic hysteresis loop shifting away from zero field due to the exchange interaction between a ferromagnet (FM) and an antiferromagnet (AF) across their common interface [1]. Nowadays the FM/AF EB structure with in-plane anisotropy has been widely adopted in hard disk drives and magnetic random access memories based on spin valves and magnetic tunnel junctions (MTJs). Compared with in-plane anisotropy, perpendicular magnetic anisotropy (PMA) is more desirable for spintronic devices owing to the better downsizing scalability and higher thermal stability for the encoded information. The recent discovery of appreciable interfacial PMA in the CoFeB/MgO films and further demonstration of high performance perpendicular MTJs represent significant steps towards next-generation spintronic devices [2]. However, there is no study on the perpendicular exchange bias (PEB) effect of the perpendicular CoFeB/MgO films generated by AF pinning materials, and all perpendicular CoFeB/MgO based MTJs are pseudo spin valve type. In this work, we have explored the exchange bias of the CoFeB/MgO films pinned by an antiferromagnetic IrMn layer along the perpendicular direction.

Most investigations of all spin logic (ASL) have been based on the monodomain approximation of magnetization switching. The sensitive and stochastic nature of spin transfer torque (STT) switching creates pressing requirements for analyzing ASL from a micromagnetic perspective also. Hence, in this paper, ASL is studied from a micromagnetic perspective by developing a simulation framework that can capture diffusive spin transport. The spin transport model is calibrated from the existing experimental results on nonlocal spin valve measurements. Then, using this framework, STT switching is studied for input–output nanomagnets with in-plane and perpendicular magnetic anisotropy for ASL devices (ASLDs). Precisely defined architecture and design constraints for such input–output nanomagnets are presented. Further, a previously proposed switching mechanism by driving the nanomagnet into a metastable state is also incorporated into micromagnetic framework. Using the aforementioned mechanism, the critical current required for STT switching can be reduced to as low as 20 $\mu $ A. Moreover, the graphene channel ASLD using tunnel contact at the injector is demonstrated for copy and invert logic operations.

This paper investigates real-time monitoring and detection of 10.5 nm magnetic nanoparticles (MNPs) (Fe3O4) flowing in a nitrocellulose (NC) membrane over two spin valve sensors. The fabricated device gives information on the concentration of MNP flowing through the NC membrane of a typical lateral flow test, and gives relevant dynamic characteristics, such as flow rate and MNP distribution on capillary-driven systems. Sensor output varies linearly with MNP concentration. Experiments show average flow rates varying from 0.2 to 0.3 mm/s depending on MNP concentration. Signal shape [ ${V}$ ( ${t}$ )] gives information on MNP spatial distribution at the particle front flowing over the sensor.

This paper deals with the dependence of NiMn-based spin valves’ functionality on temperature and time and with the factors influencing their thermal stability. As will be shown, the NiMn-based systems can be used for more than 660 h at 250 °C without degradation, but also an application up to 400 °C for short time is possible. The investigations reveal the dependence of the system’s thermal stability on the layer-stack and its structure. The exchange bias turns out to be an indicator for a spin valve’s thermal stability.

In the pursuit of point-of-care systems that present high sensitivity and fast turnaround time, lateral flow is considered a remarkable promising class of in vitro diagnostics, and popularized by the pregnancy tests. A variety of lateral flow tests have emerged able to provide cardiac panels in emergency conditions, infectious disease screening, drug abuse, among others. However, most commercial tests only provide qualitative results, limiting their applicability for quantification thresholds of most biomarkers. In this paper, we propose a novel detection architecture for quantification of biomolecules labeled with magnetic nanoparticles employing two spin valve sensors. The main goal of this paper is to demonstrate that this architecture can detect and monitor MNPs flowing in a LFT strip for detection and quantification of different solutions varying in MNP concentration. In addition, we present preliminary analytical results that support our strategy for achieving biomolecular quantification using the proposed solution.

Spin valves (SVs) with synthetic-antiferromagnet (SAF) pinned layers and synthetic-ferrimagnet (SF) free-layers deposited by ion beam deposition are optimized for incorporation in nanometric sensors. The results on combined SAF-SF structures indicate a reduced saturation and offset fields when compared with the simple top-pinned or SAF structure. Therefore, SAF-SF SV display sensitivities of ~0.025%/Oe (200 nm sensor), ~0.1%/Oe (500 nm sensor), and ~0.2%/Oe (1 μm sensor), which correspond to an improvement of 2x, 4.5x, and 7x, respectively, when compared with all other SV stacks tested. The results are relevant for geometries where nanometric SVs are incorporated within very narrow gaps of magnetic flux concentrators leading to superior and competitive gains in sensitivity. These geometries have the unique feature of submicrometric spatial resolution, and have high impact on surface scanning applications.

Nanocontact spin-torque oscillators (NC-STOs) act as intrinsically nanoscale and highly current and magnetic field tunable, ultrawide band microwave signal generators. However, their low output power and high phase noise remain critical obstacles toward actual applications. Mutual synchronization of multiple NCs is one possibility to overcome these shortcomings. This letter presents a detailed study of the mutual synchronization in a NC-STO with two NCs. In particular, the effect of repeated measurements on the synchronization behavior is explored. Repeated measurements at high drive currents are shown to significantly degrade the performance of the devices with the most striking consequence being that the devices can no longer be synchronized. Ferromagnetic resonance measurements reveal a decrease in the saturation magnetization and an increase in the damping coefficient in annealed NiFe films, consistent with Cu diffusion into the NiFe from the adjacent Cu layers. This increase in damping will act to sever the spin wave-mediated communication channel between the NCs necessary for synchronization. These results highlight an important consideration when studying the synchronization behavior of multi-NC devices where Joule heating is expected to scale unfavorably with the number of NCs.

Ferromagnetic resonance (FMR) and magnetization-switching-induced spin-transfer torque (STT) in spin valve had been investigated by 3-D micromagnetic simulations. The simulation results show that three resonance peaks attributing to various resonance modes were found. FMR frequencies can be controlled by the perpendicular anisotropy strength. At the same time, the critical current density of STT-induced magnetization switching can be obviously reduced by tuning the perpendicular anisotropy constant too.

This paper investigated the output characteristics of a two-dimensional spin valve chip. The chip was placed in a series of rotating magnetic fields. The output relation curve of the two-dimensional spin valve chip transformed from a circle-like shape to a diamond-like shape with increasing magnetic field intensity because of the influence of the field perpendicular to the sensing axis. The experimental results fitted well with the simulation results. The output characteristics of the two-dimensional spin valve chip can be used to measure the direction of the magnetic fields.

The application of an electrical forward bias across a ferromagnet/semiconductor Schottky barrier can result in a non-equilibrium spin accumulation in the semiconductor. This mechanism is referred to as spin extraction. Like spin injection, spin extraction can also serve to generate the electron spin polarization, which is necessary for the operation of a local spin valve, i.e., a magnetoresistance device consisting of a ferromagnetic/non-magnetic/ferromagnetic hybrid structure, which exhibits an electrical resistance that depends on the relative orientation of the magnetizations of the two ferromagnetic electrodes. In such a spin extraction spin valve (SESV), a spin-polarized drift current is generated in a non-magnetic channel by spin extraction at a ferromagnetic electrode. The spin polarization influences the output current through a subsequent ferromagnetic electrode. We show how spin valve devices relying on multiple spin extraction events can be used for magneto-logic gate operation. Furthermore, we investigate the potential use of these structures as sources of highly spin-polarized drift currents.

Several materials (Ta, Cu, Al, Ru) are investigated for the gap layer (GL) in a differential dual spin valve (DDSV) as a function of GL thickness in terms of the differential effect and interlayer coupling. High field measurements in current in the plane geometry show that similar GMR effect can be obtained for two spin valves (SVs) with Ta, Ru, and Cu GLs, while for Al GL, GMR ratio of the upper SV decreases as Al thickness increases. For Ta GL, the two free layers (FLs) may switch either simultaneously or separately, depending on which FL switches first due to the weak ferromagnetic (FM) Neel coupling between the FLs. For Ru GL, the switching behavior depends on the thickness of Ru, due to the oscillation of interlayer exchange coupling. Cu GL causes additional GMR resulted from the FL/GL/FL structure. Detailed investigation on thickness dependence of Ru GL shows that FM interlayer coupling is achieved with Ru thickness of 1.6, 2.4, and 4 nm. The good differential effect can be obtained till Ru = 8 nm, above which MR of the upper SV decreases. Patterned DDSV sensors with Ru GL = 2.4 nm shows an overall FM coupling between the FLs, implying FM exchange coupling is larger than the antiferromagnetic magnetostatic coupling at edges.

Head-to-head domain walls (DW) are formed and detected in U-shaped spin valve nanostrips by a Bitter Method on scanning electron microscope. We find that optimization of the washing technique is critical to proper deposition of magnetic nanoparticles (MNPs). The DW position is monitored electrically with giant magnetoresistance (GMR) similar to a potentiometer. DWs are repeatedly pinned and depinned from a site near the device center using a stage with two axis field. We first confirm the distribution of depinning events from a random sample of devices and obtain a standard deviation of 2.2 Oe. We then obtain an experimental signal of 8 Oe shift in depinning field from just 7 MNPs of 35 nm diameter.

In this paper, the authors have proposed a possibility of a magnetic field biasing tunable inductive device. To confirm a new scheme, a tunable coplanar waveguide (CPW) resonator with a combination of soft magnetic thin film and hard magnetic film has been fabricated and evaluated. The magnetic pole of hard magnetic film can be controlled by magnetization using a pulsed magnetic field. A bias magnetic field is applied in the soft magnetic film from the magnetic pole of the hard magnet film. Consequently, permeability of the soft magnetic film can be controlled by changing amplitude of the pulsed magnetic field in the magnetizing for the hard magnetic film. A 20 mm long coplanar wave guide resonator has been fabricated using FeSiO/SiO2 granular multilayer film and FeCoSm amorphous hard magnetic film. From the experimental results, in case of using 0.2 mu m thick soft granular film, by changing amplitude of the 1 ms width current-pulse for magnetizing pulsed magnetic field for hard magnetic film, the maximum inductance change was up to 18%, and maximum change of the resonant frequency was 9.6%. The control energy for one time tuning was small enough (5.4 mu Wh)., Article, IEEE TRANSACTIONS ON MAGNETICS. 49(3):978-981 (2013)

A theoretical model is proposed to investigate the electromigration (EM) behavior in giant magnetoresistance spin valve (GMR SV) read sensors under pulsed direct current (dc) magnetic field during sensor operation. It was found that the externally applied magnetic field gives rise to a more severe Joule heating and consequently raises the device temperature leading to EM acceleration. Black's equation was modified based on Green's function method and the Boltzmann equation to more precisely interpret the EM mean time to failure (MTTF) of GMR SV read sensors under pulsed dc magnetic field. The physical validity of this proposed model was confirmed by the comparisons with experimental results.

We microscopically derive the spin-transfer torque due to diffusive spin current arising from nonequilibrium spin accumulation as realized in systems with nonlocal spin valve structure. Spin relaxation effects in spin transport is taken into account by introducing magnetic impurities.

Since 2006, fully scalable matrix-based magnetoresistive biochips have been proposed. This integration was initially achieved with thin film switching devices and moved to complementary metal-oxide-semiconductor (CMOS) switching devices and electronics. In this paper, a new microfabrication process is proposed to integrate magnetoresistive sensors on a small CMOS chip (4 mm2). This chip includes a current generator, multiplexers, and a diode in series with a spin valve as matrix element. In this configuration, it is shown that the fabricated spin-valves have similar magnetic characteristics when compared to standalone spin valves. This validates the successfulness of the developed microfabrication process. The noise of each matrix element is further characterized and compared to the noise of a standalone spin valve and a portable electronic platform designed to perform biological assays. Although the noise is still higher, the spin valve integrated on the CMOS chip enables an increase in density and compactness of the measuring electronics.

We systematically measured the spin torque diode spectra of a magnetic tunnel junction nanoelement as a function of external magnetic field and field angle using a homemade quasi-omnidirectional broadband electrical spectrometer. The electrical spectroscopies reveal a significant higher-order spin wave modes excited in the free layer, which is substantiated by an analytical model.

In order to investigate the effect of polarization and saturation magnetization (MS) in Co/Pd multilayers-based pseudo-spin valve (PSV) structures with perpendicular magnetic anisotropy (PMA), we studied different spin polarizer layers (SPL). The PMA could be achieved by tuning the thickness and Ms of the SPL. Sharp switching of magnetization for both soft layer (SL) and hard layer (HL) were observed for all studied structures with thin SPL, indicating higher PMA in the film structures. However, interesting behavior was observed for thicker SPLs where a tail in hysteresis loop was observed. For further understanding of the interactions in multilayers we have carried out first-order reversal curve (FORC) measurements. The FORC contours pointed out the correlation between exchange interactions with magnetostatic interaction and minor loop shifts in hysteresis curves. Giant magnetoresistance (GMR) was observed for the PSV films with thick Co65Fe35 (10 Å and 15 Å) SPL. However, GMR value as a function of thickness decreases for all the other films with different Ms. Moreover, we have clarified that GMR is correlated to the fcc (111) orientation of Co/Pd multialyers. The maximum value of GMR with largest Ms shows the minimum value of full width at half maximum (FWHM) for fcc (111) peak, indicating that the large GMR as well as high PMA occurs for the samples deposited in good fcc (111) texture.

Two micromagnetic models, Model A with the polycrystalline microstructure and Model B with just regular mesh, are set up to simulate the hysteresis loops of Co/Pt multilayers with perpendicular anisotropy. In both models, a defect region on a scale of 100 nm is introduced in any Co film with only two atomic layers, which plays a key role in this simulation. The magnetic anisotropy constant K of cells in the defect region is weakened compare to the K out of the defect. The simulated M-H loops express the characteristics of the experiments well. The loops of a series of [Co/Pt]n multilayers with different repetition period also agree with experiment well. Moreover, the movement of reversed domain wall during magnetization reversal is observed in this simulation.

This work compares the performance of spin valve sensors comprising magnetic flux concentrators (MFCs) composed of Co93Zr3Nb4 (CZN) or (Co70Fe30)80B20-based synthetic-antiferromagnet (SAF) multilayer stacks. In addition, the influence of a tapered MFC tip is also studied. When compared to CZN films, SAFs have the disadvantage of lower magnetic susceptibility (χ = 196 for SAF vs. χ = 753 for CZN), which affects negatively the field gain in gap (10 for SAF vs. 43 for CZN). However, from the overall noise spectrum, one can conclude that the magnetic field detections for sensors incorporating CoFeB/Ru multilayers as MFCs are close to the ones obtained with CZN, being mainly determined by the sensor intrinsic properties instead. For low frequencies, field detection levels at 10 Hz improved from ~ 61 nT/Hz0.5 for single spin valve sensors down to ~ 1.8 nT/Hz0.5 when CZN concentrators with a steep-profile are used.

For future magnetic recording, one challenge is that the shield to shield spacing (SSS) of readers cannot be scaled down to achieve the linear density requirement. A differential dual spin valve (DDSV) has been proposed to improve the linear resolution as it does not rely on the magnetic shields to diminish the interference from adjacent bits. The side reading becomes more and more challenging as the track width shrinks to below SSS to accommodate high track density, in particular for a DDSV reader in which no magnetic shield or larger SSS is used. A self-biased DDSV structure is proposed to replace the conventional abutted junction stabilization scheme. In the self-biased DDSV readers, two side shields are put at the both sides of the sensor across the track direction in the replacement of the hard bias (HB). The stray fields from the two free layers in the DDSV sensor bias each other to stabilize the domain structure of the free layers. Preliminary analysis shows that the self-bias DDSV is also robust against the spin torque-induced magnetic instability. Simulation results indicate that the self-biased DDSV reader has much better reading sensitivity for the opposing fields generated by magnetic transitions than HB-stabilized sensor. It is found that for DDSV readers, the mag-noise is less significant due to a larger signal field and thicker free layer. Current perpendicular to the plane (CPP)-DDSV sensors have been fabricated and show very good differential effect in the real operating mode. The magnetoresistance performance of individual spin valve in CPP-DDSV sensors has been obtained through measurements with applied field and pinning field parallel to the easy axis of the free layer.

A 3-D magnetic field sensor based on the giant magnetoresistive (GMR) spin valve (SV) was designed for improving the energy efficiency in intelligent automating devices. Generally GMR SV sensors are selective only in the plane of the substrate due to fabrication restraints. For this application, a sensor with ultra-thin dimensions is required for the system integration. The approach using micro-assembly for the accomplishment of 3-D field detection is not possible. Therefore, the integration of soft magnetic materials is essential to deflect the magnetic flux for the detection of a magnetic field in the direction perpendicular to the substrate. In this paper the design of 3-D flux guides was presented. Finite element method (FEM) simulations were used to optimize the flux guide design. NiFe, with its high permeability μ, low coercivity Hc and relatively high saturation flux density Bs, was regarded as the suitable material for this application. Electroplating and patterning by means of photolithography promises to be the appropriate process to integrate this material after the deposition of the GMR multilayer. To characterize the magnetic properties measurements with a Vibrating Sample Magnetometer (VSM) as well as a hysteresisgraph were carried out.

In this paper are presented results of galvanomagnetic measurements and micromagnetic simulations performed on low-field magnetic sensors based on the planar Hall effect (PHE). Disc-shaped structures of the type Co/Cu/Ni80Fe20, 4 mm diameter, deposited by magnetron sputtering onto Si substrate, were used to build magnetic sensors. At this stage of study, no uniaxial anisotropy axes were defined during the samples deposition. Two types of applications have been considered: (i) magnetic field measurements and (ii) rotation sensing. In order to obtain a coherent rotation of the magnetization inside the PHE sensor under the action of an applied magnetic field, Happl, DC or AC magnetic biasing fields were used. By these means the magnetic sensitivity and the hysteresis width of the PHE signal can be tuned. Sensitivities between 0.07 and 0.17 μV·A-1·m have been obtained for a driving current of 10 mA. Micromagnetic simulations were used to explain some field angular behavior of these sensors.

Spin-transfer-torque-induced multiple-step magnetization switching in a spin valve with a perpendicular polarizer subjected to an inplane magnetic field has been investigated by macrospin simulation. The single-step, triple-step, and double-step reversals are observed at a certain range of the bias field angle and of the current density. With the larger injected current, the angular range of the triple-step reversal becomes much wider. For a given magnetic anisotropy, there is a critical value for the spin-transfer torque, above which the triple-step magnetization switching disappears, and double-step one appears.

The aim of this study is to present the novel methodology for magnetic markers immunoassay measurements by spin valve giant magnetoresistive (SV-GMR) needle probe. Extremely small size and specific construction of this probe type allow detecting the changes of complex susceptibility directly from the sample. The needle-type probe consists of four SV-GMR elements connected in a Wheatstone bridge and it has sensitivity of 11 μV/μT. The SV-GMR sensing element has dimensions of 40 μm × 75 μm. This paper presents a setup feasible to perform spectroscopic magnetic markers measurement. It includes data related to signal changes caused by the magnetic markers dilution. It shows the influence of particle size changes (120 nm, 1 μm, 3.5 μm, 6.5 μm) on real and imaginary parts of the complex susceptibility of the specimen. While performing measurements, probe sensing elements were placed inside the sample. Monitoring changes of samples susceptibility allows performing homogenous immunoassay measurements in a liquid phase by SV-GMR probe. Based on the measurement results, analytical model of changes of complex susceptibility is derived and described.

The electric-field control of magnetization has many potential applications in magnetic memory storage, sensors, and spintronics. A new multiferroic spin-valve (SV) heterostructure was grown by combining a multiferroic BiFeO3 (BFO) and a Co90Fe10/Cu/Co90Fe10 spin valve on Pt/Ti/SiO2/Si substrate. Through the magnetoelectric coupling between a BFO layer and Co90Fe10 layer, the magnetoresistance of SV could be controlled by electric field applied on the BFO layer. The BFO film exhibits an obvious ferroelectric property with a remnant polarization of 17 μC/cm2 and a coercive field of 400 kV/cm. The exchange bias of 6 Oe and magnetoresistance (MR) of 1.04% were observed in the heterostructure at room temperature. Moreover, an electric-field-induced change of MR was demonstrated when we employed 0.5- and -0.5-V voltage between top and bottom electrode layer. Furthermore, the values of MR of the spin were changed into 0.71% and 0.95%, respectively. This realization of the electric-field-induced change of MR indicates the great potential applications in future functional devices.

In this paper, we study the magnetization canting of the free layer in a synthetic spin valve at ultrahigh density data recording up to 10 Tb/in2. Our micromagnetic simulations show that the canting problem deteriorates dramatically as the sensor size shrinks further to match higher data-recording density. Our systematic investigation on the magnetic field distribution reveals that the increasing net magnetostatic coupling field from the synthetic antiferromagnet (SAF) at smaller sensor dimensions is responsible for the observed effect. Further studies show that the stray field experienced by the free layer was dominated by the reference layer, and both the field profile and magnitude change markedly with the closer proximity of the magnetic charges. In order to restore the sensor to the appropriate operating point, affects of adjusting the thickness and magnetization ratio in the SAF, and tapering angle of the sensor stack, have been investigated and it was found that they were able to relieve the canting problem to certain extent. The findings in this work help to optimize the sensor properties for ultrahigh density data storage in the hard disk.

We prepared giant magnetoresistance samples comprising of pseudo spin valve structures and measured the current-perpendicular-to-plane magnetoresistance (MR) with applied magnetic field along the easy and hard axes of the samples. When the magnetic field was applied along the easy axis, high and low resistance states representing magnetization parallel and anti-parallel states were observed. However, when the magnetic field was applied along the hard axis, two stochastic MR curves were obtained. The two distinct MR curves correspond to the different magnetizations reversal processes via two kinds of the magnetization orientation states, fully parallel and anti-parallel, at remanence. In our case, the two distinct magnetization reversal processes occurred randomly.

A micromagnetic model is set up to simulate the magnetoresistive (R-H) loops of a polycrystalline TMR spin valve, in which the pinned layer (PL) is exchange biased to an antiferromagnetic (AFM) layer. In the ferromagnetic free layer (FL) and PL, two phases are introduced: the BCC crystal phase in the grain and the amorphous phase at the grain boundary, which have different magnetic parameters in FL and PL. The cubic easy crystal axes of the anisotropy are uniform for all micromagnetic cells inside a grain, but it is randomly oriented at the grain boundary. It is assumed that, only in part of the grains, the pinning field Hpin from the AFM layer to the PL is not zero; furthermore, Hpin has a wide distribution among crystal grains. In this paper, the R-H loops with 50% and 25% of biased grains in the PL are calculated, and they are consistent with experiments. Moreover, the wide magnitude distribution of Hpin and the weak inter-grain exchange in the PL are the essential factors in our model to simulate the inclined R-H loop of PL.

We systematically investigate the effects of interlayer, inter-ring dipolar coupling and the number of bilayer repeats on the magnetization reversal process of [Co/Pd]/Au(t)/[Co/Pd] pseudo-spin-valve (PSV) nano-rings. We observed that exchange coupling dominates the magnetization reversal process in PSV rings when the Au spacer layer thickness is 1 nm, while magnetostatic coupling plays a more significant role for thicker spacer layer. The magnetization reversal process of the PSV nano-rings as a function of inter-ring spacing provides a way to engineer the switching fields in both the ferromagnetic layers. We observed that the coercivity of both the top and bottom Co/Pd stack increases with increasing inter-ring spacing.

At 10 Tb/in2 areal density and bit aspect ratio (BAR) of 4, the 1 T bit length is only 4 nm. By proportional scaling, it requires the head media spacing (HMS) to be around 2 nm. In such a small spacing, it is extremely hard to squeeze in the medium overcoat and lubricant for having a reliable head disk interface. This small HMS demand is mainly determined by the readback process for the reader to have large signal intensity and high signal resolution. The differential dual spin valve (DDSV) reader incorporates a metallic gap layer between the free layers of two spin valves. The opposite pinned reference layers generate the differential operation and reproduce the pulse signal from the transition of perpendicular magnetizations. With the readback resolution defined by the thicknesses of gap layer and two free layers, the DDSV reader gains more resolution than the conventional reader. Unlike conventional reader also using two shields to define signal resolution, the shields of DDSV reader is only for the elimination of inter-symbol interference. This allows the shield to shield spacing of DDSV reader to be much larger, which has smaller shunting effect and promotes the signal strength. With stronger signal intensity and higher readback resolution, the DDSV reader is able to reproduce good quality signal at much higher HMS than the conventional reader.

Spin injection into magnesium from Permalloy is performed in the lateral spin valve geometry. A large spin valve signal of 3.1 mΩ is observed at room temperature. The injector-detector separation dependence of the spin valve signal is analyzed by using a one-dimensional spin-diffusion model. The analysis yields the spin diffusion length of about 200 nm for the Mg nanowires. The short spin diffusion length of magnesium which has a small spin-orbit interaction can possibly be explained by considering the spin-hot-spot relaxation mechanism proposed by Fabian and Das Sarma.

We report on the growth and magnetic characterization of the ferromagnetic insulator Europium Sulfide (EuS) on (001)-oriented Silicon substrates. The influence of the EuS film thickness on the coercive field and Curie temperature was systematically investigated with regard to a potential application of thin EuS films as spin filter tunnel barriers. In a further step, we fabricated EuS/Si(001) spin valve structures with both ferromagnetic Gd and exchange-biased CoO/Co counter electrodes. An independent magnetic switching of the EuS barrier and the ferromagnetic layers was accomplished by eliminating intermediate magnetic exchange couplings. Our results clearly demonstrate the feasibility to employ thin EuS ferromagnetic insulator films as spin filter tunnel barriers on Silicon(001) in spin valve structures for future magnetotransport experiments.

This paper presents studies of the size effect on magnetic switching and interlayer magnetostatic coupling in synthetic antiferromagnet (SAF)-pinned spin-valve nanorings connected to nano stripes with a nanoconstriction. Micromagnetic simulation has been successfully used to reveal the detailed magnetization reversal process of these nanorings. It was observed that SAF-pinned spin-valve nanorings exhibit a double (onion-vortex-reverse onion) or single (onion-reverse onion) magnetization switching process depending on the ring diameter, which is contrary to narrow single-layer NiFe rings that show a double switching process only. Micromagnetic simulations of the SAF-pinned spin valve nanorings suggest that the magnetostatic coupling between the pinned layers of the SAF and the free layer is dependent on the ring size: it plays a very important role in the magnetization switching of small nanorings (600 nm outer diameter), but only a minor role in the switching of big nanorings (1600 nm outer diameter). An efficient reduction of the magnetostatic interaction results in a small shift of the minor magnetoresistance curve, which is beneficial to the magnetic memory applications.