Giant Magnetoresistance Devices Research Articles

Negative spin polarization of Mn2VGa Heusler alloy thin films studied in current-perpendicular-to-plane giant magnetoresistance devices

Negative spin polarization of Mn2VGa Heusler alloy thin films studied in current-perpendicular-to-plane giant magnetoresistance devices

Field-Free Spin-Orbit Torque Switching of Co/Pd Memory Layer in CPP-GMR With Perpendicularly Magnetized SAF Pinned Layer

For the development of high-capacity spin-orbit torque magnetic memory, a giant magneto-resistive (GMR) device with perpendicularly magnetized Co/Pd memory layer was fabricated, and current-induced magnetization reversal of the memory layer was investigated. A synthetic antiferromagnet (SAF) reference layer consisting of [Co/Pt]/Ru/[Co/Pt] multilayer was used for the GMR device to reduce the stray field in the pillar-shaped Co/Pd memory layer. Spin-transfer torque (STT) magnetization reversal of the memory layer as well as the field-assisted spin-orbit torque (SOT) magnetization reversal of the memory layer was confirmed, by applying a current pulse through the GMR or through the bottom electrode, respectively. Furthermore, by injecting SOT and STT simultaneously, deterministic SOT switching of the perpendicularly magnetized Co/Pd memory layer was achieved in the absence of an in-plane assisting magnetic field to break the symmetry. We confirmed that STT assisting current significantly reduced the SOT switching current. Moreover, the assist efficiency defined by Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SOT</sub> / Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">STT</sub> depended on the magnitude of in-plane magnetic field. Higher assist efficiency was obtained at lower in-plane magnetic field.

Structural, electronic and magnetic properties of Fe-doped strontium ruthenates

By employing a combined approach of density-functional theory (DFT) and dynamical mean-field theory (DMFT) calculations, we examine the structural, electronic, and magnetic characteristics of two distinct strontium ruthenates: Sr2RuO4, an unconventional superconductor, and the correlated metal SrRuO3, both at 50% Fe-doping level. In both Sr2Fe0.5Ru0.5O4 and SrFe0.5Ru0.5O3, the original ruthenium (Ru) and the dopant iron (Fe) atoms adopt 3-dimensional and 2-dimensional G-type structures, respectively. The hybridization between Fe-3d and Ru-4d is comparatively weaker than in other double perovskite systems. The interplay between strong correlations and reduced itinerancy results in significant spin splitting at Fe and Ru sites. Consequently, a charge transfer process, along with the super-exchange effect, leads to antiferromagnetically coupled Fe3+ and Ru5+ ions and establishes a semiconducting ferrimagnetic order. Subsequent DMFT calculations demonstrate the persistence of the ferrimagnetic order even at room temperature (300 K). These findings align with prior reports on SrFe0.5Ru0.5O3, thus reinforcing the notion that 3d–4d transition metal oxides hold considerable promise as candidates for high-performance spintronic devices, such as spin-valve sensors and spintronic giant magnetoresistance devices.

Photoinduced Nonvolatile Resistive Switching Behavior in Oxygen-Doped MoS2 for a Neuromorphic Vision System.

Controlling resistance by external fields provides fascinating opportunities for the development of novel devices and circuits, such as temperature-field-induced superconductors, magnetic-field-triggered giant magnetoresistance devices, and electric-field-operated flash memories. In this work, we demonstrate a light-triggered nonvolatile resistive switching behavior in oxygen-doped MoS2. The two-terminal devices exhibit stable light-modulated resistive switching characteristics and optically tunable synaptic properties with an on/off ratio of up to 104. The integrated device with crossbar architecture enables simultaneous image sensing, preprocessing, and storage in a single device, thereby increasing the training efficiency and recognition rate of image recognition tasks. This work presents a novel pathway to develop the next generation of light-controlled memory and artificial vision systems for neuromorphic computing.

Evaluation of spin-transfer-torque efficiency using magnetization reversal against a magnetic field: comparison of FeCr with negative spin polarization and NiFe

We propose and demonstrate an experimental method to evaluate spin-transfer-torque (STT) efficiency in current-perpendicular-to-plane giant magnetoresistance devices, which utilizes STT-induced magnetization reversal against a perpendicular magnetic field. Using this method, we estimated the STT efficiency of FeCr with negative spin polarization, which attracts attention in spintronics applications. In comparison with NiFe with positive spin polarization, the sign of the STT induced by FeCr was opposite, reflecting the negative spin polarization. The STT efficiency of FeCr was approximately half that of NiFe at room temperature and surpassed that of NiFe at 50 K.

Spin-switching effect and giant magnetoresistance in quantum structure of monolayer MoS&lt;sub&gt;2&lt;/sub&gt; nanoribbons with ferromagnetic electrode

Spintronics is a new type of electronics based on electron spin rather than charge as the information carrier, which can be stored and calculated by regulating and manipulating the spin. The discovery and application of the giant magnetoresistance effect opens the door to the application of electron spin properties. Realizing on-demand control of spin degree of freedom for spin-based devices is essential. The two-dimensional novel material, monolayer transition metal dichalcogenide (TMD) (MoS&lt;sub&gt;2&lt;/sub&gt; is a typical example from the family of TMD materials), has become an excellent platform for studying spintronics due to its novel physical properties, such as direct band gap and strong spin-orbit coupling. Obtaining high spin polarization and achieving controllability of degrees of freedom are fundamental problems in spintronics. In this paper, we construct the monolayer zigzag MoS&lt;sub&gt;2&lt;/sub&gt; nanoribbon quantum structure of electrically controlled ferromagnetic electrode to solve this problem. Based on the non-equilibrium Green’s function method, the regulation of the magnetic exchange field and electrostatic barrier on the spin transport in parallel configuration and anti-parallel configuration are studied. It is found that in the parallel structure, spin transport is obviously related to the magnetic exchange field, and 100% spin filtering can occur near the Fermi energy level to obtain pure spin current. When an additional electric field is applied to the middle region, the spin filtering effect is more significant. Therefore, the spin switching effect can be achieved by regulating the incident energy. In addition, it is also found that within a specific energy range, electrons in the parallel configuration are excited to participate in transport, while electrons in the anti-parallel structure are significantly inhibited. Consequently, a noticeable giant magnetoresistance effect can be obtained in this quantum structure. Moreover, it can be seen that the magnetic exchange field strength can effectively modulate the giant magnetoresistance effect. These results provide valuable theoretical references for the development of giant magnetoresistance devices and spin filters based on monolayer zigzag MoS&lt;sub&gt;2&lt;/sub&gt; nanoribbons.

Non-volatile memory storage in tri-layer structures using the intrinsically ferromagnetic semiconductors GdN and DyN

We report on the potential use of the intrinsic ferromagnetic rare earth nitride (REN) semiconductors as ferromagnetic electrodes in tunnelling magnetoresistance and giant magnetoresistance device structures for non-volatile memory storage devices. Non-volatile memory elements utilising magnetic materials have been an industry standard for decades. However, the typical metallic ferromagnets and dilute magnetic semiconductors used lack the ability to independently tune the magnetic and electronic properties. In this regard, the rare earth nitride series offer an ultimately tuneable group of materials. Here we have fabricated two tri-layer structures using intrinsically ferromagnetic rare earth nitride semiconductors as the ferromagnetic layers. We have demonstrated both a non-volatile magnetic tunnel junction (MTJ) and an in-plane conduction device using GdN and DyN as the ferromagnetic layers, with a maximum difference in resistive states of ∼1.2% at zero-field. GdN and DyN layers were shown to be sufficiently decoupled and individual magnetic transitions were observed for each ferromagnetic layer.

Proximity-induced diversified magnetic states and electrically controllable spin polarization in bilayer graphene: Towards layered spintronics

Compared to monolayer graphene, electrons in Bernal-stacked bilayer graphene (BLG) have an additional layer degree of freedom, offering a platform for developing layered spintronics with the help of proximity-induced magnetism. Based on an effective phenomenological model, we systematically study the effect of this magnetism on the spin-dependent band structure near the Fermi energy and identify the magnetic phases induced in BLG by proximity with magnets. We show that spin polarization can develop in BLG due to this proximity effect. This spin polarization depends strongly on the layer distribution of magnetism, and can always be controlled by gate voltage which shifts spin-dependent band edges and modifies the total band gap. We further show that the band spin polarization can be modified by the proximity-induced staggered sublattice potential. By taking full advantage of layer-dependent magnetism in BLG, we propose that spintronic devices such as a spin filter, a giant magnetoresistance device, and a spin diode can operate under fully electric control, which is easier than the common magnetic field control.

High efficiency giant magnetoresistive device based on two-dimensional MXene (Mn2NO2)

High efficiency giant magnetoresistive device based on two-dimensional MXene (Mn2NO2)

Epitaxial Mn[formula omitted]VAl films with [formula omitted]-ordered structure for all-Heusler stacks

The structural and magnetic properties of epitaxial Mn2VAl films grown by molecular beam epitaxy are investigated. Epitaxial Mn2VAl films with a relatively high L21-ordering of ∼0.7 are obtained on MgO(001) substrates at a growth temperature of 350 °C. The saturation magnetic moment at 300 K for the epitaxial Mn2VAl films is ∼1.2 μB/f.u., which is almost equivalent to the highest value for high-temperature-grown thin-film samples reported previously. Due to the low-temperature synthesis of L21-Mn2VAl, an epitaxial all-Heusler L21-Mn2VAl/L21-Fe2VAl/L21-Mn2VAl trilayer with sharp heterointerfaces is obtained. This study presents the possibility of all-Heusler current-perpendicular-to-plane giant magnetoresistive devices with high performance.

Microstructure control for magnetic thin films with high functionality

In most magnetic devices with thin films, the magnetic properties are governed by not only the intrinsic properties but also the microstructure. To realize high performance in magnetic devices, understanding the correlation between magnetic properties and microstructure is important. In this study, the development of ferromagnetic thin films for high-density recording media and large output current perpendicular to plane giant magnetoresistance devices are reviewed. Based on the experimental data, the research direction for next-generation high-areal-density magnetic recording is discussed.

Effect of off-stoichiometric composition on half-metallic character of Co2Fe(Ga,Ge) investigated using saturation magnetization and giant magnetoresistance effect

We investigated the Ge-composition (γ) dependence of the saturation magnetization of Co2Fe(Ga, Ge) (CFGG) thin films and the magnetoresistance (MR) ratio of CFGG-based current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices together with first-principles calculations of the electronic states of CFGG. Theoretical calculations showed that spin polarization is highest at the stoichiometric composition γ = 0.56 in Co2Fe1.03Ga0.41Ge γ and that it decreases in off-stoichiometric CFGG, mainly due to the formation of CoFe antisites for Ge-deficient compositions and FeCo antisites for Ge-rich compositions, where CoFe (FeCo) indicates that Co (Fe) atoms replace the Fe (Co) sites. The saturation magnetic moment (μ s) per formula unit decreased monotonically as γ increased from 0.24 to 1.54 in Co2Fe1.03Ga0.41Ge γ . The μ s was closest to the Slater–Pauling value predicted for half-metallic CFGG at the stoichiometric composition γ = 0.56, indicating that stoichiometric CFGG has a half-metallic nature. This is consistent with the result for the theoretical spin polarization. In contrast, the MR ratio of CFGG-based CPP-GMR devices increased monotonically as γ increased from 0.24 to 1.10 and reached an MR ratio of 87.9% at the Ge-rich composition γ = 1.10. Then, the MR ratio decreased rapidly as γ increased from 1.10 to 1.48. Possible origins for the slight difference between the Ge composition at which the highest MR ratio was obtained (γ = 1.10) and that at which the highest spin polarization was obtained (γ = 0.56) are improved atomic arrangements in a Ge-rich CFGG film and the reduction of effective Ge composition due to Ge diffusion in the GMR stacks.

Study on FeCr thin film for a spintronic material with negative spin polarization

Negative spin polarization materials generate a spin current whose spin direction is opposite to the magnetization direction. This characteristic is technologically important because it increases the freedom in the design of spintronic devices. In this paper, we studied FeCr from the viewpoints of band structure, atomic magnetic moments, and local atomic arrangement to explore its potentiality as a negative spin polarization material. First-principles calculations on the disordered FeCr alloy predicted a high negative spin polarization, which was attributed to the band matching of the minority spin. We experimentally fabricated Fe1-xCrx thin films in the Cr concentration (x) range from 0.2 to 0.4 by sputter deposition and demonstrated inverse magnetoresistance using current-perpendicular-to-plane giant magnetoresistance (GMR) devices, which provided evidence of negative spin polarization. The spin polarization of Fe0.7Cr0.3 estimated from the GMR device measurements was −0.28 and fell below the calculated value. Synchrotron X-ray magnetic circular dichroism measurements showed a ferrimagnetic configuration of the Fe and Cr magnetic moments. The Fe moment showed a large moment over a wide Cr concentration range, whereas the Cr moment decreased with the Cr concentration, which agreed with the calculation result. Synchrotron Mössbauer spectroscopy measurements indicated the inhomogeneity of the FeCr composition even in the as-deposited state. Fe-rich regions were formed after annealing, reflecting the phase separation nature of the Fe-Cr system. Inhomogeneity was not considered in the calculation and could be the origin of the smaller negative spin polarization observed in the experiment.

Perpendicular Anisotropy Controlled by Seed and Capping Layers of Heusler-Alloy Films

Half-metallic Heusler alloys typically have in-plane magnetic anisotropy, which can be converted to perpendicular by attaching MgO or heavy metal, e.g., Pt, layers as similarly applied for conventional ferromagnets. Recently we have found body-centered cubic (bcc) seed layers, e.g., V and W, to induce perpendicular anisotropy in Heusler-alloy films above, however, they show small giant magnetoresistive (GMR) ratios in spin-valve structures to date. This is partially because of the large resistivity of the seed layer and the nonmagnetic layer in the spin-valve. In this study, we have systematically investigated nonmagnetic overlayers and have found that a Ag layer best maintains the perpendicular anisotropy. The corresponding GMR devices have then been fabricated and characterized, achieving the GMR ratio of ~0.03% at room temperature. Such bcc seed layers can offer an alternative method for perpendicularly magnetized GMR junctions for applications.

Analysis of current-in-plane giant magnetoresistance using Co2FeAl0.5Si0.5 half-metallic Heusler alloy

There has been renewed interest in current-in-plane giant magnetoresistance (CIP-GMR) devices for high-sensitivity magnetic sensors. However, further improvement in magnetoresistance (MR) ratio is necessary to achieve sufficient magnetic field sensitivity. Use of a half-metallic Co-based Heusler alloy ferromagnetic (FM) layer has been demonstrated to be effective in enhancing GMR in the configuration with current perpendicular to the plane; however, only small MR ratios are obtained in the CIP configuration. To understand the origin of the disappointingly low MR in the CIP configuration when using Heusler alloy FM layers, we investigated the magnetotransport properties of CIP-GMR devices using half-metallic Co2FeAl0.5Si0.5 (CFAS) Heusler alloy and conventional CoFe alloy as the FM layers in combination with Ag or Cu as a nonmagnetic (NM) spacer layer. Regardless of the high lattice and electronic band matching at the CFAS/Ag interface, CFAS/Ag CIP spin valves (SVs) show a MR ratio of only 1.2% at room temperature, which is much smaller than those of reference CoFe/Cu and CoFe/Ag SVs (21.6% and 8.4%, respectively). Current density distribution simulations suggest that large current shunting occurs in the Ag layer due to the significant resistivity gap between CFAS and Ag, which limits the generation of highly spin-polarized current from the CFAS layer, resulting in the very small MR ratios. To enhance the MR ratio in CIP-GMR using half-metallic materials, resistivity matching between FM layers and the NM layer is required, in addition to the high electronic band match that has been considered, as a key factor to obtain a high MR ratio in CIP-GMR devices.

Two-terminal current-in-plane giant magnetoresistance devices driven by the spin–orbit torque

Spin valve effect driven by current-induced spin–orbit torques can help to realize electrically controllable and miniaturized two terminal spintronic devices. Here, we show that in a perpendicularly magnetized Fe1−x1Tbx1/Pt/Fe1−x2Tbx2 spin-valve multilayer, spin currents from dual surfaces of the inserted Pt layer can be used to generate spin torques that can separately switch the adjacent ferrimagnetic Fe1−xTbx layers. In a two-terminal device, we further show that the accompanied parallel and anti-parallel magnetization configurations between the two Fe1−xTbx layers can lead to the observation of the current-in-plane giant magnetoresistance (CIP-GMR). Our results demonstrate that the current-induced spin–orbit torques can be implemented into two-terminal spin-torque devices, which can be electrically read out by the CIP-GMR. The present study could be important for designing miniaturized spintronic devices.

Systematic investigation of the effect of layer thickness on the linear sensing characteristics of asymmetric structured CoFe/Rh/CoFe/Cu/CoFe fully epitaxial CIP-GMR based magnetic sensors

Giant magnetoresistance (GMR) devices known for their widespread application as magnetic field sensors face challenges owing to their nonlinear resistance response with the magnetic field. Here, we have developed CoFe(3 nm)/Rh(tRh nm)/CoFe(tmidCoFe nm)/bcc Cu(1.6 nm)/CoFe(ttopCoFe nm)/MgO(2 nm) based current in plane (CIP)-GMR devices and demonstrated linear MR curves with tunable characteristics. The MR ratio and saturating magnetic field were tunable between 3.9% and 23.5%, and 0.013 T and 1.5 T. The numerical simulation modeling confirmed biquadratic coupling between the bottom and middle CoFe layers through Rh and antiferromagnetic coupling between the middle and top CoFe layers through a Cu spacer, providing the linear MR. The fine-tuning of thicknesses of the middle and top CoFe layers provides supplementary routes for optimizing the sensitivity and operation field range of the CIP-GMR devices to specific magnetic sensor applications in the low and medium magnetic field range. The above results demonstrate that CIP-GMR devices with the asymmetric structure demonstrated in this study are promising for the next generation linear magnetic sensors.

Magnetic Coupling in Y3Fe5O12/Gd3Fe5O12 Heterostructures

Ferrimagnetic Y$_3$Fe$_5$O$_{12}$ (YIG) is the prototypical material for studying magnonic properties due to its exceptionally low damping. By substituting the yttrium with other rare earth elements that have a net magnetic moment, we can introduce an additional spin degree of freedom. Here, we study the magnetic coupling in epitaxial Y$_3$Fe$_5$O$_{12}$/Gd$_3$Fe$_5$O$_{12}$ (YIG/GIG) heterostructures grown by pulsed laser deposition. From bulk sensitive magnetometry and surface sensitive spin Seebeck effect (SSE) and spin Hall magnetoresistance (SMR) measurements, we determine the alignment of the heterostructure magnetization through temperature and external magnetic field. The ferromagnetic coupling between the Fe sublattices of YIG and GIG dominates the overall behavior of the heterostructures. Due to the temperature dependent gadolinium moment, a magnetic compensation point of the total bilayer system can be identified. This compensation point shifts to lower temperatures with increasing thickness of YIG due the parallel alignment of the iron moments. We show that we can control the magnetic properties of the heterostructures by tuning the thickness of the individual layers, opening up a large playground for magnonic devices based on coupled magnetic insulators. These devices could potentially control the magnon transport analogously to electron transport in giant magnetoresistive devices.

Design of a Giant Magnetoresistance Device with High Spin Filter Efficiency

Molecular spintronics may boost the development of nanoscale devices with improved performance and enhanced functionality, which is very promising for the next generation of electronic devices. Here, we design a new type of spintronics device based on the novel 2D material C3N. Using nonequilibrium Green’s function in combination with the density functional theory, we systematically study the spin-dependent transport properties in four different magnetic configurations of the C3N junction, and the perfect spin filter efficiency and giant magnetoresistance effects are achieved. The calculated results clearly demonstrate that the transport properties of the C3N junction are sensitive to the magnetic configuration of the electrodes. The calculated spin-resolved transmission spectra of the proposed ferromagnetic coupling C3N junctions exhibit a robust spin filtering effect, where the conductance is mainly governed by the spin-down electrons. Under the small bias voltage, the transport properties are mainly determined by the spin-down channel and exhibit a large spin polarization. The conductance of three antiferromagnetic coupling states is nearly zero at the Fermi level. These theoretical results indicate that the novel 2D C3N monolayer is promising for molecular spintronics devices in the near future.

Evaluation of edge domains in giant magnetoresistive junctions

We demonstrate that the spin-Seebeck effect can be used to estimate the volume of edge domains formed in a giant magnetoresistive (GMR) device. The thermal gradient induced by Joule heating can be harnessed by the addition of a ferromagnetically insulating channel of Fe2O3 on the sides of the GMR pillar. This generates a spin wave in Fe2O3, which couples with the free-layer edge magnetization and controls the reversal of the ferromagnetic layers in one direction only, increasing the current density from ( 1.1 ± 0.1 ) ×107 A/cm2 to ( 7.0 ± 0.5 ) × 10 7 A/cm2. By simple assumption, we estimate the effect of the edge domain on magnetization reversal to be 10%–15% by spin-transfer torque.