Spin-valve Structures Research Articles

Magnetization Manipulation of a Flexible Magnetic Sensor by Controlled Stress Application

Spin-based electronic devices on polymer substrates have been intensively investigated because of several advantages in terms of weight, thickness, and flexibility, compared to rigid substrates. So far, most studies have focused on maintaining the functionality of devices with minimum degradation against mechanical deformation, as induced by stretching and bending of flexible devices. Here, we applied repetitive bending stress on a flexible magnetic layer and a spin-valve structure composed of Ta/NiFe/CoFe/Cu/Ni/IrMn/Ta on a polyimide (PI) substrate. It is found that the anisotropy can be enhanced or weakened depending upon the magnetostrictive properties under stress. In the flat state after bending, due to residual compressive stress, the magnetic anisotropy of the positive magnetostrictive free layer is weakened while that of the pinned layer with negative magnetostriction is enhanced. Thus, the magnetic configuration of the spin-valve is appropriate for use as a sensor. Through the bending process, we design a prototype magnetic sensor cell array and successfully show a sensing capability by detecting magnetic microbeads. This attempt demonstrates that appropriate control of stress, induced by repetitive bending of flexible magnetic layers, can be effectively used to modify the magnetic configurations for the magnetic sensor.

Single‐Shot Multi‐Level All‐Optical Magnetization Switching Mediated by Spin Transport

All-optical ultrafast magnetization switching in magnetic material thin film without the assistance of an applied external magnetic field is explored for future ultrafast and energy-efficient magnetic storage and memories. It is shown that femtosecond (fs) light pulses induce magnetization reversal in a large variety of magnetic materials. However, so far, only GdFeCo-based ferrimagnetic thin films exhibit magnetization switching via a single optical pulse. Here, the single-pulse switching of Co/Pt multilayers within a magnetic spin-valve structure ([Co/Pt]/Cu/GdFeCo) is demonstrated and four possible magnetic configurations of the spin valve can be accessed using a sequence of single fs light pulses. The experimental study reveals that the magnetization final state of the ferromagnetic [Co/Pt] layer is determined by spin-polarized currents generated by the light pulse interactions with the GdFeCo layer. This work provides an approach to deterministically switch ferromagnetic layers and a pathway to engineering materials for opto-magnetic multi-bit recording.

Substantial enhancement of thermal spin polarization in Py/Cu interface

We investigated the temperature dependence of thermally excited spin current properties of ferromagnet Py $({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20})$ by using a lateral spin-valve structure. The spin-dependent Seebeck coefficient at the Py/Cu interface was found to show a significant increase at low temperature while the charge Seebeck coefficient showed the ordinal reduction with decreasing temperature. This produces a crossover between the charge- and spin-dependent Seebeck coefficients, resulting in the thermal spin polarization greater than $100%$ below 125 K. From the first-principles calculation specially developed for the Py/Cu interfaces, the spin-dependent Seebeck coefficient is found to be highly susceptible to the magnetic disorder-induced scattering at the interface. The calculation with considering the temperature dependence of the magnetic disorder provides a consistent description for the significant enhancement of spin-dependent Seebeck coefficient in the Py/Cu interface. Our demonstration shows the importance of interface disorder and paves the way toward the development of conventional ferromagnetic metal for the practical application of thermo-spin energy conversion.

Vortex spin valve on a topological insulator

Spin-valve structures are usually associated with the ability to modify the resistance of electrical currents. We here demonstrate a profoundly different effect of a spin-valve. In combination with a topological insulator and superconducting materials, we show that a spin-valve can be used to toggle quantum vortices in and out of existence. In the antiparallel configuration, the spin-valve causes superconducting vortex nucleation. In the parallel configuration, however, no vortices appear. This switching effect suggests a new way to control quantum vortices.

Temperature controlled Fe/Au/FeRh spin valves

We demonstrate that FeRh layers can be implemented to the epitaxial Fe/Au/FeRh spin valve structures grown on MgO(001). Owing to the AFM-FM phase transition in the FeRh film, the magnetic structure of our Fe/Au/FeRh system can be temperature controlled. The indirect exchange coupling between Fe and FeRh films mediated by the Au spacer enables to control the relative orientation of the Fe and FeRh magnetizations by the Au spacer thickness between ferromagnetic and non-collinear with nearly orthogonal magnetizations. Moreover, the evolution of magnetic structure of the Fe/Au/FeRh system along with the AFM-FM transition is accompanied by the reversible in-plane rotation of the top Fe-layer magnetization.

Magnetic and magnetotransport properties of epitaxial La0.7Sr0.3MnO3/SrIrO3/La0.7Sr0.3MnO3 spin valves

We report on the fabrication of epitaxial La0.7Sr0.3MnO3/SrIrO3/La0.7Sr0.3MnO3 (LSMO/SIO/LSMO) spin valve structures deposited on (0 0 1) (LaAlO3)0.3(Sr2TaAlO6)0.7 substrates (LSAT) by high pressure sputtering. We present the structural, magnetic and magnetotransport properties of trilayers. Magnetization measurements indicate indirect exchange coupling between LSMO layers across the SrIrO3 layer. An enhancement of saturation magnetic moment was observed for trilayers with one and six unit cell thick SrIrO3 spacing layers. Low field magnetoresistance measurements show standard spin switching behavior and inverse spin switching in epitaxial LSMO/SIO/LSMO spin valve structures depending on the orientation of magnetic field to the plane and magnetic field to the current.

Optically excited spin pumping mediating collective magnetization dynamics in a spin valve structure

We demonstrate spin pumping, i.e. the generation of a pure spin current by precessing magnetization, without application of microwave radiation commonly used in spin pumping experiments. We use femtosecond laser pulses to simultaneously launch the magnetization precession in each of two ferromagnetic layers of a Galfenol-based spin valve and monitor the temporal evolution of the magnetizations. The spin currents generated by the precession cause a dynamic coupling of the two layers. This coupling has dissipative character and is especially efficient when the precession frequencies in the two layers are in resonance, where coupled modes with strongly different decay rates are formed.

Design of Perpendicular Magnetic-Tunneling-Junction Spin-Valve Including [Co/Pt]n Free Layer As Bulk Perpendicular Magnetic Anisotropy

Recently, dynamic random access memory (DRAM) has scaling limit because of its capacitor area. To replace DRAM, the next generation memory devices are being studied. Among them, perpendicular spin transfer torque magnetic random access memory (p-STT MRAM) is considered as a promising candidate because of easy scaling down, low power consumption, high read and write speed and non-volatility. p-STT MRAM has several challenges for commercialization. To keep a high retention time (~10 years), STT-MRAM should have thermal stability greater than 74. Although the thickness of free layer is increased with thermal stability, the thickness of free layer is limited to maintain perpendicular magnetic anisotropy (PMA). One of studies, double MgO based p-MTJ spin-valve structure is issued for its large volume because in case of double MgO based p-MTJ spin-valve, PMA of the free layer is originated from two i-PMA between the CoFeB free layer / MgO tunneling barrier and CoFeB free layer / MgO capping layer. The i-PMA induced from the two interfaces increases the total volume of the magnetic free layer which increases the thermal stability, but it is still limited by the thickness of the individual CoFeB free layers. In our study, therefore, we propose a new structure using interfacial Co2Fe6B2 and [Co/Pt]n multi-layer as bulk PMA to achieve higher thermal stability than that of double MgO based p-MTJ spin-valve structure. The dependence of the magnetic moment and thermal stability on the number of [Co/Pt]n multi-layers was estimated by using vibrating sampling magnetometer (VSM) as shown in Fig.1 b. Magnetic moment t~180μemu/cm2) of i-PMA+[Co/Pt]2 multi-layers is similar to that(~180μemu/cm2) of double MgO based p-MTJ spin-valve structure. Also, magnetic moment (t~366μemu/cm2) of i-PMA+ [Co/Pt]5 multi-layers is about two times more than that of double MgO based p-MTJ spin-valve structure and thermal stability (~181) is two times higher than that (~74) of the double MgO based p-MTJ spin-valve structure. The thermal stability can be modulated by simply changing the number of [Co/Pt]n multi-layers. Obviously, these results show that i-PMA + [Co/Pt]n multilayer as bulk PMA based p-MTJ spin-valve can achieve a high thermal stability. In our presentation, we report the different characteristics of the p-MTJ spin-valve with [Co/Pt]n multi-layers by examining the static magnetization behavior, TMR ratio and MgO crystallinity. In addition, we present how these properties influence mechanism of p-MTJ spin-valve. Acknowledgements This work was supported by a Basic Science Research Program grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (No. 2017R1A2A1A05001285) and the Brain Korea 21 PLUS Program in 2014. Figure 1

Role of canting and depleted-triplet minima in superconducting spin valve structures

The trilayer and pentalayer spin valve structures are revisited to determine the behavior of pair correlations and Josephson current when the magnetic layers are canted at arbitrary angle. The two systems display markedly different behaviors in the center magnetic layer. While the trilayer generates a triplet component that is weakly affected by canting, the pentalayer tunes in singlet pair correlations depending heavily on canting. We also show that a minimum with depleted $m=\pm1$ triplet components, rather than a $0-\pi$ transition, may be observed in the current profile $I_c(d_F)$ of a trilayer spin valve. The depleted-triplet minimum (DTM) is directly attributable to a decrease of $m=\pm1$ triplet correlations with increased thickness of the central ferromagnet, accompanied by a hidden, simultaneous sign change of the Gor'kov functions contributed from the left and right superconductors. We introduce a toy model for superconducting-magnetic proximity systems to better illuminate the behavior of individual components of the Gor'kov function and compare with a full numerical calculation.

Observation of Anomalous Spin Torque Generated by a Ferromagnet

In this work we report observation of in-plane current induced out-of-plane magnetic field driven torque in spin valve structure. Since ferromagnet has high spin orbit coupling it is expected to be the source of spin-orbit-torque as it possesses anomalous-Hall-effect (AHE: equivalent to spin Hall effect in heavy metal). So we have carried out spin-torque ferromagnetic resonance (ST-FMR) experiment in a spin valve (consists of a fixed magnet and a free magnet which are separated by Cu spacer), passing in-plane radio frequency current and measuring DC voltage. Our experimental results eventually indicate that spin torque exerted on the free magnet is not caused due to the spin current injection by the fixed magnet owing to its AHE, but it is originated from in-plane current driven out-of plane effective magnetic field. This is new class of spin torque which is completely different from Slonczewski-spin transfer torque and Rashba like field like torque. The effective out-of plane magnetic field depends on the direction of current (in-plane) and magnetization (in-plane) of the pinned layer. One possible mechanism behind this unconventional torque could be intefacial spin-scattering which is also origin of current in-plane GMR effect. Most importantly this effective out-of plane torque can be useful to switch out-of plane magnetic bits in spintronic memory application.

Current-induced synchronized magnetization reversal of two-body Stoner particles with dipolar interaction**Project supported by the National Natural Science Foundation of China (Grant No. 11274236) and the Deutsche Forschungsgemeinschaft via SFB 689.

We investigate magnetization reversal of two-body uniaxial Stoner particles, by injecting spin-polarized current through a spin-valve structure. The two-body Stoner particles perform synchronized dynamics and can act as an information bit in computer technology. In the presence of magnetic dipole–dipole interaction (DDI) between the two particles, the critical switching current Ic for reversing the two dipoles is analytically obtained and numerically verified in two typical geometric configurations. The Ic bifurcates at a critical DDI strength, where Ic can decrease to about 70% of the usual value without DDI. Moreover, we also numerically investigate the magnetic hysteresis loop, magnetization self-precession, reversal time and synchronization stability phase diagram for the two-body system in the synchronized dynamics regime.

Spin current and spin transfer torque in ferromagnet/superconductor spin valves

Using fully self consistent methods, we study spin transport in realistic, fabricable experimental spin valve systems consisting of two magnetic layers, a superconducting layer, and a spacer normal layer between the ferromagnets. Our methods ensure that the proper relations between spin current gradients and spin transfer torques are satisfied. We present results as a function of geometrical parameters, interfacial barrier values, misalignment angle between the ferromagnets, and bias voltage. Our main results are for the spin current and spin accumulation as functions of position within the spin valve structure. We see precession of the spin current about the exchange fields within the ferromagnets, and penetration of the spin current into the superconductor for biases greater than the critical bias, defined in the text. The spin accumulation exhibits oscillating behavior in the normal metal, with a strong dependence on the physical parameters both as to the structure and formation of the peaks. We also study the bias dependence of the spatially averaged spin transfer torque and spin accumulation. We examine the critical bias effect of these quantities, and their dependence on the physical parameters. Our results are predictive of the outcome of future experiments, as they take into account imperfect interfaces and a realistic geometry.

Biomolecule detection using wheatstone bridge giant magnetoresistance (GMR) sensors based on CoFeB spin-valve thin film

A potential wheatstone bridge giant magnetoresistance (GMR) biosensor have been successfully developed for biomolecule detection. [IrMn(10 nm)/CoFe(3 nm)/Cu(2.2 nm)/CoFeB(10 nm)] spin-valve structure has been chosen as the magnetic sensing surface, showing a magnetoresistance (MR) of 6% fabricated by DC magnetron sputtering method. The Fe3O4 magnetic nanoparticles used as biomolecular labels (nanotags) was synthesized by co-precipitation method, exhibiting soft magnetic behavior with saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) is 77.2 emu/g, 7.8 emu/g and 51 Oe, respectively. The X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) images showed that Fe3O4 was well crystallized and grew in their inverse spinel structure, highly uniform morphology with an average grain size was about 20 nm. Fe3O4 was coated with polyethylene-glycol (PEG)-4000 for surface functionalization. Detection of biomolecule such as formalin, gelatin from bovine-skin and porcine-skin were dispersed in ethanol at room temperature. Induction would cause a shift in output voltage with a minimum delta output voltage (ΔV) 4.937 mV (10%) for formalin detection, 2.268 mV (7%) for bovine-skin gelatin and 2.943 mV (7%) for porcine-skin gelatin detection. The ΔV of the wheatstone bridge in real-time measurement decrease by increase in biomolecules concentration. The change of ΔV with various concentration of biomolecule indicates that the spin-valve thin film with wheatstone-bridge circuit is potential as a biosensor.

Reduction of shunt current in buffer-free IrMn based spin-valve structures

The presence of thick buffer layers in magnetic sensor devices decreases sensor sensitivity due to shunt currents. With this motivation, we produced IrMn-based spin-valve multilayers without using buffer layer. We also studied the effects of post-annealing and IrMn thickness on exchange bias field (HEB) and blocking temperature (TB) of the system. Magnetization measurements indicate that both HEB and TB values are significantly enhanced with post-annealing of IrMn layer. In addition, we report that IrMn thickness of the system strongly influences the magnetization and transport characteristics of the spin-valve structures. We found that the minimum thickness of IrMn layer is 6 nm in order to achieve the lowest shunt current and high blocking temperature (>300 K). We also investigated the training of exchange bias to check the long-term durability of IrMn-based spin-valve structures for device applications.

Influence of DC-biasing on the performance of graphene spin valve

Generating and controlling the spin valve signal are key factors in ‘spintronics’, which aims to utilize the spin degree of electrons. For this purpose, spintronic devices are constructed that can detect the spin signal. Here we investigate the effect of direct current (DC) on the magnetoresistance (MR) of graphene spin valve. The DC input not only decreases the magnitude of MR but also distorts the spin valve signal at higher DC inputs. Also, low temperature measurements revealed higher MR for the device, while the magnitude is noticed to decrease at higher temperatures. Furthermore, the spin polarization associated with NiFe electrodes is continuously increased at low DC bias and low temperatures. We also demonstrate the ohmic behavior of graphene spin valve by showing linear current-voltage (I-V) characteristics of the junction. Our findings may contribute significantly in modulating and controlling the spin transport properties of vertical spin valve structures.

Double MgO-based Perpendicular Magnetic-Tunnel-Junction Spin-valve Structure with a Top Co2Fe6B2 Free Layer using a Single SyAF [Co/Pt]n Layer

A new perpendicular spin-transfer-torque magnetic-tunnel-junction (p-MTJ) spin-valve was developed to achieve a high tunneling magnetoresistance (TMR) ratio. It had a double MgO-based spin-valve structure with a top Co2Fe6B2 free layer and incorporated a single SyAF [Co(0.4 nm)/Pt(0.3 nm)]3 layer and a new buffer layer of Co(0.6)/Pt(0.3)/Co(0.4). It had a TMR ratio of 180% and anisotropy exchange field (Hex) of 3.44 kOe after ex-situ annealing of 350 °C for 30 min under a vacuum below 10−6 torr and a perpendicular magnetic field of 3 tesla, thereby ensuring a memory margin and avoiding read disturbance failures. Its high level of performance was due to the face-center-cubic crystallinity of the MgO tunneling barrier being significantly improved by decreasing its surface roughness (i.e., peak-to-valley length of 1.4 nm).

Advanced CPP-GMR Spin-Valve Sensors for Narrow Reader Applications

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.

Magnetization precession by short-wavelength magnon excitations and spin-transfer torque

The Bloch-Bloembergen equation is extended to study the magnetization precession in a spin-valve structure by taking into account the short-wavelength magnon excitation and spin-transfer torque. A stable magnetization precession is achieved when the transverse relaxation time ${T}_{2}$ is longer than the longitudinal relaxation time ${T}_{1}$. A wide current-density window of stable magnetization precession is found for large ${T}_{2}$ and it broadens with increasing ${T}_{2}$ for fixed ${T}_{1}$. Our simulation results also reveal that short-wavelength magnon excitation can reduce the amplitude of the magnetization precession, which is different from the prediction of the macro-spin Landau-Lifshitz-Gilbert-Slonczewski equation.

Spin Absorption Effect at Ferromagnet/Ge Schottky-Tunnel Contacts

We study the influence of the junction size in ferromagnet (FM)/semiconductor (SC) contacts on four-terminal nonlocal spin signals in SC-based lateral spin-valve (LSV) structures. When we use FM/Ge Schottky-tunnel junctions with relatively low resistance-area products, the magnitude of the nonlocal spin signal depends clearly on the junction size, indicating the presence of the spin absorption effect at the spin-injector contact. The temperature-dependent spin signal can also be affected by the spin absorption effect. For SC spintronic applications with a low parasitic resistance, we should consider the influence of the spin absorption on the spin-transport signals in SC-based device structures.

Spin transfer torque magnetization reversal in a hard/soft composite structures

Current induced magnetization manipulation in a spin valve structure where the free layer is a magnetic hard/soft composite structure is studied using micromagnetic simulations. In this structure where the hard layers has strong perpendicular magnetic anisotropy, a domain wall can be nucleated in the soft layer due to the spin transfer torque effect. Depending on the magnetic properties of the layers and the current intensity the domain wall can induce the free layer reversal or be pinned by the hard layer. For these non-uniform magnetic configurations both bulk and interface spin transfer torques need to be considered. The potential reduction of the critical current observed in this geometry is of potential technological interest.