Single Spin Valves Research Articles

A Spin Valve-Based Rhombus-Shaped Micro-Object Implementing a Full Wheatstone Bridge.

Spin valves with a synthetic antiferromagnet were fabricated via magnetron sputtering. It was shown that the fabricated spin valve layers had a perfect microstructure and smooth interfaces, and therefore, an RKKY interaction dominated in the coupling of the ferromagnetic layers separated by a copper spacer. Rhombus-shaped micro-objects were fabricated from a single spin valve film. The thermomagnetic treatment procedure was found to form unidirectional anisotropy in the micro-object such that the values of the exchange bias fields in the rhombus' nonparallel sides were opposite in sign. For the CoFeNi/Ru/CoFeNi synthetic antiferromagnet, we determined the differences between the ferromagnetic layer thicknesses at which the thermomagnetic treatment formed the same exchange bias all over each rhombus' side. We also fabricated a sensor element in which each side of the rhombus was the shoulder of a Wheatstone bridge. After the thermomagnetic treatment procedure, each shoulder worked as an active magnetosensitive element, enabling the device to operate as a full Wheatstone bridge. The sensor output exhibited a step shape, high sensitivity to field changes, and significant magnetic hysteresis. Such characteristics are suitable for switching devices.

Формирование обменного смещения и анизотропия формы в микрообъектах на основе спиновых клапанов

Rhombus-shaped microobjects formed by strips of two micrometers wide were fabricated from the spin valve film. The influence of the shape anisotropy on the layers magnetic moment rotation during the spin valve magnetic reversal is studied. A method for two-stage thermomagnetic treatment in a direction-fixed magnetic field has been found. The method allows to obtain the opposite sign values of the exchange bias fields in the non-parallel rhombus sides. The direction of the formed exchange bias is determined by the deviation of the strip from the uniaxial anisotropy axis and from the magnetic field applied during thermomagnetic treatment. Based on a rhombus-shaped microobject made from a single spin valve film, the device is a Full Wheatstone bridge. Each side of the rhombus is an active magnetically sensitive element.

Magnetic Nanoparticle Detection Using Wheatstone Bridge Giant Magnetoresistance (GMR) Sensor with Double CoFeB Spin-Valve Thin Films

The Wheatstone bridge-giant magnetoresistance (GMR) sensor with single and double spin valve thin film was successfully developed for potential biomolecular detection. The GMR sensor with spin valves structure of [Ta (2nm)/IrMn (10nm)/CoFe (3nm)/Cu (2,2nm)/CoFeB (10nm)/Ta (5nm)] was fabricated using DC Magnetic Sputtering method. The Fe3O4magnetic nanoparticles were synthesized by the co-precipitation method as a magnetic label. The magnetic properties of the Fe3O4nanoparticles measured are the saturation magnetization (Ms) of 77.7 emu/g, remanence magnetization (Mr) of 7.7 emu/g, and coercivity (Hc) of 49 Oe. The X-ray diffraction pattern showed the inverse cubic spinel structure with an average crystal size of about 20.1 nm. Fe3O4magnetic nanoparticles with various concentrations were used to be detected using a GMR sensor. The output voltage of the GMR sensor with the single and double spin-valve increased from 1.7 to 3.9 mV and 2.9 to 5.3 mV with the increase of the Fe3O4concentration from 0 to 20 mg/mL, respectively.

Room-Temperature Operation of a p-Type Molecular Spin Photovoltaic Device on a Transparent Substrate.

The coupling of diverse degrees of freedom opens the door to physical effects that go beyond each of them individually, making multifunctionality a much sought-after attribute for high-performance devices. Here, the multifunctional operation of a single-layer p-type organic device, displaying both spin transport and photovoltaic effect at the room temperature on a transparent substrate, is shown. The generated photovoltage is almost three times larger than the applied bias to the device which facilitates the modulation of the magnetic response of the device with both bias and light. The device shows an increase in power conversion efficiency under magnetic field, an ability to invert the current with magnetic field and under certain conditions it can act as a spin photodetector with zero power consumption in the standby mode. The room-temperature exploitation of the interplay among light, bias, and magnetic field in the single device with a p-type molecule opens a way toward the development of efficient high-performance spin photovoltaic cells.

Dynamical Aspects of Magnetic Switching in a Single Molecule-Based Spin Valve

The dynamics of the current-induced magnetic switching process is theoretically studied in a spin-valve device containing a single magnetic molecule of spin S 1. The analysis is performed by using the real-time diagrammatic technique in the sequential electron tunneling regime. In particular, we show that the magnetic moment of a molecule can be reversed also in the presence of intrinsic spin relaxation processes. Moreover, we discuss how the process of magnetic switching depends on a transport bias voltage as well as on some key parameters of the device.

Toward pTesla Detectivities Maintaining Minimum Sensor Footprint With Vertical Packaging of Spin Valves

The reduction of the noise level of magnetoresistive (MR) micrometric sensors is often achieved by increasing the sensor area and/or connecting MR elements in large arrays. This paper introduces an innovative strategy to increase the sensing area without increasing the area occupied by the MR sensors on a wafer, therefore minimizing the sensor footprint. Here, the idea is to deposit vertically spin-valve (SV) films, separated by an insulator. This packed structure is then incorporated into arrays, connected in parallel and series, as a general noise reduction strategy. Using calculations, the number of SVs vertically packed ( $Z$ ), combined in series ( $X$ elements) and parallel ( $Y$ elements) was investigated, so that the optimum geometry, which minimizes the field detectivity, can be obtained. The magnetic field detection of SV sensors in the thermal noise limit can be improved from 39 pT/Hz $^{1/2}$ for $Z = 1$ with a total number of 500 SVs (distributed in plane) to 12 pT/Hz $^{1/2}$ for $Z =10$ with a total number of 5000 SVs. Besides, the device footprint is also demonstrated to decrease from 0.7 mm2 ( $Z = 1$ ) to 0.07 mm2 ( $Z = 10$ ). These limits of detection are competitive with single SV sensors using magnetic flux concentrators, with the advantage of a high spatial resolution with fewer fabrication steps (at least three).

COMPARATIVE STUDY OF SPIN INJECTION AND TRANSPORT IN Alq3 AND Co–PHTHALOCYANINE-BASED ORGANIC SPIN VALVES

Recent experimental reports suggest the formation of a highly spin-polarized interface ("spinterface") between a ferromagnetic (FM) Cobalt ( Co ) electrode and a metal-phthalocyanine (Pc) molecule. Another report shows an almost 60% giant magnetoresistance (GMR) response measured on Co / H 2 Pc -based single molecule spin valves. In this paper, we compare the spin injection and transport properties of organic spin valves with two different organic spacers, namely Tris(8-hydroxyquinolinato) aluminum ( Alq 3) and CoPc sandwiched between half-metallic La 0.7 Sr 0.3 MnO 3 (LSMO) and Co electrodes. Alq 3-based spin valves exhibit clear and reproducible spin valve switching with almost 35% negative GMR at 10 K, in accordance with previous reports. In contrast, cobalt-pthalocyanine ( CoPc )-based spin valves fail to show clear GMR response above noise level despite high expectations based on recent reports. Investigations of electronic, magnetic and magnetotransport properties of electrode/spacer interfaces of LSMO/ CoPc / Co devices offer three plausible explanations for the absence of GMR: (1) CoPc films are strongly chemisorbed on the LSMO surface. This improves the LSMO magnetic properties but also induces local traps at the LSMO interface for spin-polarized charge carriers. (2) At the Co / CoPc interface, diffusion of Co atoms into the organic semiconductor (OS) layer and chemical reactivity between Co and the OS deteriorates the FM properties of Co . This renders the Co / CoPc interface as unsuitable for efficient spin injection. (3) The presence of heavy Co atoms in CoPc leads to large spin–orbit coupling in the spacer. The spin relaxation time in the CoPc layer is therefore considerably smaller compared to Alq 3. Based on these findings, we suggest that the absence of GMR in CoPc -based spin valves is caused by a combined effect of inefficient spin injection from FM contacts and poor spin transport in the CoPc spacer layer.

Temperature driven transition from giant to tunneling magneto-resistance in Fe3O4/Alq3/Co spin Valve: Role of Verwey transition of Fe3O4

We demonstrate interface energy level engineering, exploiting the modification in energy band structure across Verwey transition temperature (TV) of Fe3O4, in a Fe3O4(111)/Alq3/Co spin-valve (SV). I-V characteristics exhibit a transition in conduction mode from carrier injection to tunneling across TV of Fe3O4 electrode. Both giant magneto-resistance (GMR) and tunneling MR (TMR) have been observed in a single SV, below and above TV, respectively. We have achieved room-temperature SV operation in our device. We believe that the tuning of charge gap at Fermi level across TV resulting in a corresponding tuning of conduction mode and a unique cross over from GMR to TMR.

Giant amplification of tunnel magnetoresistance in a molecular junction: Molecular spin-valve transistor

Amplification of tunnel magnetoresistance by gate field in a molecular junction is the most important requirement for the development of a molecular spin valve transistor. Herein, we predict a giant amplification of tunnel magnetoresistance in a single molecular spin valve junction, which consists of Ru-bis-terpyridine molecule as a spacer between two ferromagnetic nickel contacts. Based on the first-principles quantum transport approach, we show that a modest change in the gate field that is experimentally accessible can lead to a substantial amplification (320%) of tunnel magnetoresistance. The origin of such large amplification is attributed to the spin dependent modification of orbitals at the molecule-lead interface and the resultant Stark effect induced shift in channel position with respect to the Fermi energy.

The Respective Effects of the Free Layer Thickness and Width on the Downtrack and Crosstrack Responses of Read Heads

Miniaturization is very important in magnetic recording. Here, we have micromagnetically evaluated the respective effects of the free layer (FL) thickness and width on the downtrack and crosstrack responses for single spin valves (SSVs) and differential dual spin valves (DDSVs). Our micromagnetic results show that the linear resolution scales down with the free layer thickness for DDSVs, as is the case for SSVs found by the reciprocity theory. We have also found that the crosstrack resolution scales down with free layer width in both SSVs and DDSVs (correspondingly, the side reading increases, which needs to be mitigated). The corresponding scalings of the signal amplitudes with FL thickness and width in SSVs and DDSVs are also discussed in terms of the shape anisotropy of the free layers. Finally, we have found that side shields can provide bias fields for the free layers and, meanwhile, the improvement in track density and the reduction in side reading are not compromised; this bias point of view of side shields is an additional appreciation of the usual roles played by side shields.

Influence of Spin Polarizer on the Magnetoresistance, Switching Property, and Interlayer Interactions in Co/Pd Single Spin Valves

In order to investigate the effect of polarization and saturation magnetization ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</sub> ) 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 M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> 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 Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">65</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">35</sub> (10 Å and 15 Å) SPL. However, GMR value as a function of thickness decreases for all the other films with different <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> . Moreover, we have clarified that GMR is correlated to the fcc (111) orientation of Co/Pd multialyers. The maximum value of GMR with largest <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> 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.

Field Detection in Spin Valve Sensors Using CoFeB/Ru Synthetic-Antiferromagnetic Multilayers as Magnetic Flux Concentrators

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.

Stochastic domain-wall depinning under current in FePt spin valves and single layers

In this paper, we report on the current-induced depinning of domain walls from structural defects in nanowires tailored in both FePt//MgO and FePt/Pt/FePt//MgO epilayers with high perpendicular magnetic anisotropy. In these systems, we show that the nature of domain wall depinning is stochastic. Our results indicate that there are two source of stochasticity: thermal activation and domain wall configuration degeneracy. We show that the depinning rate can be influenced with a strong efficiency by an applied dc current, whose effect on both thermal and configurational stochasticity is exactly similar to that of an additional magnetic field. Interestingly, Joule heating is found to cancel the bipolar effect of the current for quite low current densities. Contrarily to what was expected, the spin-transfer efficiency measured in single layers and spin valves are found to be similar. Results from micromagnetic simulations are shown to reproduce the observed statistical trends.

Hysteretic giant magnetoresistance curves induced by interlayer magnetostatic coupling in [Pd/Co]/Cu/Co/Cu/[Co/Pd] dual spin valves

The giant magnetoresistance (GMR) characteristics of hybrid perpendicular/Cu/in-plane magnetic anisotropy [Pd/Co]/Cu/Co single spin valves and [Pd/Co]/Cu/Co/Cu/[Co/Pd] dual spin valves with applied field perpendicular and parallel to the film plane are studied. Linear and nonhysteretic GMR behaviors are observed for the two spin valves in perpendicular-to-plane applied fields due to the coherent rotation of the Co layer driven by the in-plane shape anisotropy. The GMR value of the dual spin valve is 1.8 times as large as that of the single spin valve. The correlations between the magnetic configuration of the devices and the resistance evolvement are analyzed. In-plane GMR curves with maximum applied field of 30 kOe deviate from the linear behavior and can be well described by the Stoner–Wohlfarth model including high order anisotropy terms. Moreover, obvious hysteresis behavior is observed in the in-plane GMR curve of the dual spin valve. This phenomenon results from the hindered rotation of the Co/Pd moments due to the magnetostatic coupling between the top and bottom Co/Pd multilayers in the dual spin valve.

Current-induced dynamics in noncollinear dual spin valves

Spin-transfer torque and current induced spin dynamics in spin-valve nanopillars with the free magnetic layer located between two magnetic films of fixed magnetic moments is considered theoretically. The spin-transfer torque in the limit of diffusive spin transport is calculated as a function of magnetic configuration. It is shown that non-collinear magnetic configuration of the outermost magnetic layers has a strong influence on the spin torque and spin dynamics of the central free layer. Employing macrospin simulations we make some predictions on the free layer spin dynamics in spin valves composed of various magnetic layers. We also present a formula for critical current in non-collinear magnetic configurations, which shows that the magnitude of critical current can be several times smaller than that in typical single spin valves.

Interlayer magnetostatic coupling induced Co layer coercivity enhancement and exchange bias in [Pd/Co]/Cu/Co spin valves

Interlayer stray field coupling in hybrid perpendicular/Cu/in-plane magnetic anisotropy [Pd/Co]/Cu/Co single spin valves and [Pd/Co]/Cu/Co/Cu/[Co/Pd] dual spin valves was investigated. Enhancements of Co layer coercivity were observed after ac or dc demagnetization of the samples. This effect originates from the pinning of Co domain walls by the in-plane stray field from Co/Pd stripe domains, and the enhancements are more efficient as the Co/Pd domain size decreases. Moreover, the magnetostatic interaction between the net in-plane Co/Pd remanent moments and the Co moments leads to exchange bias in the Co layer of the dual spin valve after the dc demagnetization.

Reduction in critical current for spin transfer switching in perpendicular anisotropy spin valves using an in-plane spin polarizer

We describe a strategy to reduce spin transfer switching (STS) currents in CoFe/Pd-based perpendicular anisotropy single spin valves (SSVs) by the insertion of an in-plane spin polarizer, thus creating a modified-dual spin valve (m-DSV). For SSV devices, concurrent STS of both magnetic layers was observed for positive currents, making the parallel-to-antiparallel (P→AP) transition impossible. In m-DSV devices, we observed a 60% reduction in the energy barrier for AP→P transitions and a 40% reduction in JcAP→P with 10 ns STS current pulses compared to SSV devices. Furthermore, the m-DSV structure enabled the soft layer to switch independently from the hard layer via STS.

Microwave spin-torque excitation in a template-synthesized nanomagnet

A microwave-compatible lithography-free process is shown to allow the electrical contact of a single nanometric spin valve grown by template synthesis. The complex spin dynamics of a single nanomagnet is revealed by resonant microwave current excitations, detected electrically in the ${V}_{\text{dc}}$ and second-harmonic voltages. The uniform modes are attributed to spin transfer torque effects, whereas the effect of oersted field may play a role in exciting the nonuniform modes.

Magnetoresistance and Switching Properties of Co–Fe/Pd-Based Perpendicular Anisotropy Single- and Dual-Spin Valves

Switching characteristics and magnetoresistance of spin valves with perpendicular anisotropy based on Co-Fe/Pd multilayers deposited by ultrahigh vacuum (UHV) sputtering have been studied. In unpatterned thin films without exchange bias, high current-in-plane giant magnetoresistance (CIP-GMR) of 9.7% and 15.2% in single-spin valves (SSVs) and dual-spin valves (DSVs) was measured, a significant improvement over previous work. We describe the effects of a Ta seed layer and postdeposition annealing on the perpendicular anisotropy and magnetoresistance of Co-Fe/Pd spin valves, which can be attributed to improvements in the fcc (111) orientation of Pd and the formation of Co-Pd alloys at the Co-Fe/Pd interfaces, respectively. We also show that the coercivity of the layers can be tuned by varying the Co-Fe alloy composition in the multilayers, and describe the minor loops of perpendicular DSVs that exhibit four distinct resistance states, which is a potential structure for multistate storage devices.

The optimised spin-valve magnetotransistor

The paper presents a technique for the realization of a spin-valve transistor. Its semiconductor emitter (a field emitters array) and collector are realized on a (1 0 0) Si wafer and a metallic base is made up of several thin layers, alternatively magnetic and non-magnetic. This structure has the advantage of a higher Schottky barrier between the base and the emitter, compared to an emitter realized on monocrystalline Si wafer. For a single spin-valve (either inferior or superior), GMR ratios of 12% have been obtained; for a symmetric dual structure, GMR ratios were higher than 20%. The fabrication of the spin-valve transistor includes several original technological operations: (1) realization of multi-layer magnetic nanostructures with a high magneto-resistive ratio and a low number of defects at the interface; the base is realized on porous silicon substrate and is made of a nanostructured thin ferromagnetic (FM) layer alternating with thin non-ferromagnetic (NFM) ones, leading to very high variations of their electrical resistance under an external magnetic field. The width of each layer varies from 2 to 5 nm. (2) The emitter, an array made out of 3000 field emission vertical nanoemitters, is realized inside the drain of a MOSFET by reactive ion etching (RIE) and phosphor ionic implantation. The emission current increases in this case due to the current thermally generated in the depletion layer underneath the emitters and due to the electron current from the inversion layer underneath the MOSFET gate.