Magnetoresistive Devices Research Articles

Investigation on transport property of heterostructure in Bi/Bi2Sr2CaCu2O8+δ composite film

The heterostructure of bismuth (Bi) and Bi2Sr2CaCu2O8+δ (Bi2212) is considered as a potential type of quantum material. The strain-engineering controls the band structure in the composite films, which is applied in the high-temperature quantum spin Hall materials. In this work, Bi/Bi2212 composite film is synthesized using a two-step method to reduce the fabrication cost. The microstructure and transport properties of the composite films are characterized and analyzed. Experimental results demonstrate that the Bi/Bi2212 composite film obtains the high-quality crystallinity and excellent connectivity. The transport property measurement results show that the band structure changes due to the coupled effects of the strain and electric field. The origin is the compensation between the electron and hole charge carriers. The above band gap change leads to a shift in the metal-semiconductor transition temperature. Functionally coupled composite films exhibit new properties in magnetoresistance devices.

Scattering-induced positive unsaturated linear magnetoresistance in LaAlO3/SrTiO3 two-dimensional electron gas system

In this paper, positive and unsaturated linear magnetoresistance (LMR) in an LaAlO3/SrTiO3 two-dimensional electron gas system is reported. LMR appears in this system over a wide range of temperatures and magnetic fields and does not tend to saturate at magnetic field up to 14 T. The spatial fluctuation in mobility is the most likely origin of positive unsaturated LMR because the existence of strong Shubnikov-de Haas oscillations rules out the extreme quantum origin. Moreover, LMR ∝ μ and HC ∝ 1/μ are confirmed and meet the requirements of Δμ/μ < 1 in the classical model proposed by Parish and Littlewood. This suggests that the observed LMR is the classical LMR effect induced by a narrow mobility distribution. These findings are of great interest for the practical application of magnetoresistive devices such as magnetic sensors, magnetometers, and magnetic storage devices.

Evolution of magnetoresistance with temperature in the insulating van der Waals compound Ta2Pd3Te5

Ta2Pd3Te5 has been recently identified as a van der Waals topological crystalline insulator candidate, but its magnetotransport properties are not reported. Here, we study electrical transport properties of Ta2Pd3Te5 single crystals. Ta2Pd3Te5 shows an insulating behavior with a trend of saturation below 20 K. The activity energy determined from the fits to polaron hopping model is Ea ∼ 36.1 meV, close to the value of 42 meV determined from previous scanning tunneling microscopy measurement. The linear Hall resistivity with a negative slope suggests an n-type conductivity for bulk Ta2Pd3Te5. The magnetoresistance (MR) is positive at 2 K, and the dip-like feature at low magnetic fields is attributed to the weak antilocalization effect. On the other hand, a negative low-field MR develops in the temperature range between 5 and 30 K, which is identified as a fingerprint of the weak localization effect. Such a transition from weak antilocalization to weak localization with increasing temperature reveals that the spin–orbit interaction plays an important role in Ta2Pd3Te5. Our results reveal highly enriched physical properties in Ta2Pd3Te5 single crystals, and they can be exploited in future magnetoresistance device design.

Planar Hall effect and large anisotropic magnetoresistance in a topological superconductor candidate Cu0.05PdTe2

We report the observation of a large anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) in a topological superconducting candidate Cu0.05PdTe2. The AMR and PHE data in Cu0.05PdTe2 can be well explained by the semiclassical theory, confirming that the magneto-transport behaviors of the Cu0.05PdTe2 superconductor are related to its topological nature. The AMR ratio in Cu0.05PdTe2 is one order of magnitude larger than those in traditional ferromagnetic metals. The present results suggest that Cu0.05PdTe2 is a promising material in future magnetoresistive devices with low power consumption.

Room-temperature electrically tunable magnetoresistance behavior in graphene nanocrystalline

The electrically tunable magnetoresistance (MR) behavior of graphene-based devices at room temperature is becoming increasingly important in the development of continuously adjustable low-power-consumption magnetoelectronic applications. Herein, we design and construct an in-plane graphene nanocrystalline (GNC) film/n-Si device and observe a nonlinear S-shaped I–V curve, which lead to an electrically tunable MR behavior at near-room temperature. The MR value decreases gradually as the applied current increases, offering a switching ability with maximum high-low MR ratio (MR @1 μA/MR @3 mA) of ∼105%. A high MR (which is linearly related to the magnetic field strength) of ∼62% at 300 K is achieved by regulating the GNC grain size from 4 to 10 nm. The large, electrically tunable linear MR at room temperature in GNC film/n-Si devices have intriguing prospects for the fabrication of low-power-consumption sensitive logic and magnetic random-access memory MR devices.

A crossbar array of magnetoresistive memory devices for in-memory computing.

Implementations of artificial neural networks that borrow analogue techniques could potentially offer low-power alternatives to fully digital approaches1-3. One notable example is in-memory computing based on crossbar arrays of non-volatile memories4-7 that execute, in an analogue manner, multiply-accumulate operations prevalent in artificial neural networks. Various non-volatile memories-including resistive memory8-13, phase-change memory14,15 and flash memory16-19-have been used for such approaches. However, it remains challenging to develop a crossbar array of spin-transfer-torque magnetoresistive random-access memory (MRAM)20-22, despitethe technology's practical advantages such as endurance and large-scale commercialization5. The difficulty stems from the low resistance of MRAM, which would result in large power consumption in a conventional crossbar array that uses current summation for analogue multiply-accumulate operations. Here we report a 64×64 crossbar array based on MRAM cells that overcomes the low-resistance issue with an architecture that uses resistance summation for analogue multiply-accumulate operations. The array is integrated with readout electronics in 28-nanometre complementary metal-oxide-semiconductor technology. Using this array, a two-layer perceptron is implemented to classify 10,000 Modified National Institute of Standards and Technology digits with an accuracy of 93.23 per cent (software baseline: 95.24 per cent). In an emulation of a deeper, eight-layer Visual Geometry Group-8 neural network with measured errors, the classification accuracy improves to 98.86 per cent (software baseline: 99.28 per cent). We also use the array to implement a single layer in a ten-layer neural network to realize face detection with an accuracy of 93.4percent.

Emerging dynamic memristors for neuromorphic reservoir computing.

Reservoir computing (RC), as a brain-inspired neuromorphic computing algorithm, is capable of fast and energy-efficient temporal data analysis and prediction. Hardware implementation of RC systems can significantly reduce the computing time and energy, but it is hindered by current physical devices. Recently, dynamic memristors have proved to be promising for hardware implementation of such systems, benefiting from their fast and low-energy switching, nonlinear dynamics, and short-term memory behavior. In this work, we review striking results that leverage dynamic memristors to enhance the data processing abilities of RC systems based on resistive switching devices and magnetoresistive devices. The critical characteristic parameters of memristors affecting the performance of RC systems, such as reservoir size and decay time, are identified and discussed. Finally, we summarize the challenges this field faces in reliable and accurate task processing, and forecast the future directions of RC systems.

Detection of extremely large magnetoresistance in a ring-shaped array of magnetic quantum dots with very high performance and controllable parameters.

Using a Green's function method, we study the magnetoresistance (MR) effect in a ring-shaped array of magnetic quantum dots (QDs), with or without magnetic leads, while the magnetic QDs play the role of magnetic layers in conventional multilayer MR devices. Due to the multiple electronic interferences in this proposed ring-shaped structure, it exhibits extremely large MR values, typically up to four million percent (3.9 × 106% in an array that includes 14 QDs), as it tends to be infinite for a system with more dots. Our results show that when the magnetic moment of all QDs is parallel (antiparallel), the charge current through the outgoing lead is maximal (minimal). In addition, the Rashba spin-orbit interaction and the bias voltage are two significant factors that can control the MR values. Besides, by adjusting the magnitude of the magnetic moment of the QDs, the MR effect can be optimized. Finally, it is revealed that the onsite energy of the QDs is an effective parameter for modifying the value and sign of the MR percentage. Consequently, this ring-shaped structure, with controllable parameters and impressive MR percentages, can be an efficient preferred alternative for ordinary layered MR-based devices, for design of the next generation of nanoscale spintronic systems.

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.

Nondeteriorating Verwey Transition in 50 nm Thick Fe3O4 Films by Virtue of Atomically Flattened MgO Substrates: Implications for Magnetoresistive Devices

Nondeteriorating Verwey Transition in 50 nm Thick Fe₃O₄ Films by Virtue of Atomically Flattened MgO Substrates: Implications for Magnetoresistive Devices

Sub-Nanometer Thick Wafer-Size NiO Films with Room-Temperature Ferromagnetic Behavior.

Adding ferromagnetism into semiconductors attracts much attentions due to its potential usage of magnetic spins in novel devices, such as spin field-effect transistors. However, it remains challenging to stabilize their ferromagnetism above room temperature. Here we introduce an atomic chemical-solution strategy to grow wafer-size NiO thin films with controllable thickness down to sub-nanometer scale (0.92 nm) for the first time. Surface lattice defects break the magnetic symmetry of NiO and produce surface ferromagnetic behaviors. Our sub-nanometric NiO thin film exhibits the highest reported room-temperature ferromagnetic behavior with a saturation magnetization of 157 emu/cc and coercivity of 418 Oe. Attributed to wafer size, the easily-transferred NiO thin film is further verified in a magnetoresistance device. Our work provides a sub-nanometric platform to produce wafer-size ferromagnetic NiO thin films as atomic layer magnetic units in future transparent magnetoelectric devices.

Sub‐Nanometer Thick Wafer‐Size NiO Films with Room‐Temperature Ferromagnetic Behavior

AbstractAdding ferromagnetism into semiconductors attracts much attentions due to its potential usage of magnetic spins in novel devices, such as spin field‐effect transistors. However, it remains challenging to stabilize their ferromagnetism above room temperature. Here we introduce an atomic chemical‐solution strategy to grow wafer‐size NiO thin films with controllable thickness down to sub‐nanometer scale (0.92 nm) for the first time. Surface lattice defects break the magnetic symmetry of NiO and produce surface ferromagnetic behaviors. Our sub‐nanometric NiO thin film exhibits the highest reported room‐temperature ferromagnetic behavior with a saturation magnetization of 157 emu/cc and coercivity of 418 Oe. Attributed to wafer size, the easily‐transferred NiO thin film is further verified in a magnetoresistance device. Our work provides a sub‐nanometric platform to produce wafer‐size ferromagnetic NiO thin films as atomic layer magnetic units in future transparent magnetoelectric devices.

Magnetoresistance effect realized in current-in-plane Van der Waals spin valve structure by electrically switchable magnetization

We present a device concept for magnetoresistance (MR) effect in which two Van der Waals (vdW) spin valves are used as the left and right ferromagnetic (FM) leads connected by a bilayer graphene as the channel material of the central scattering region. Unlike conventional current-perpendicular-to-plane magnetic tunnel junction consisting of two FM thin films with a thin insulating barrier sandwiched between them, the FM leads in our proposed current-in-plane MR device are vdW spin valves. This is important because the application of an out-of-plane electric field allows control of the direction and magnitude of the magnetization in vdW spin valves. Moreover, we show that the oscillatory behavior is found in the MR and conductance as the height (depth) of the barrier (well) of the scattering region with the p(n)-doping increase, or the width of the scattering region increase. Remarkably, when the potential barrier is present, the oscillation magnitude of MR is considerable and can be changed from positive to negative value, whereas for the potential well, the oscillation amplitude is relatively small and is always positive. Therefore, we hope that this device configuration with electrically tunable large MR can open up new possibilities for future lower power magneto-electric devices since no current-induced magnetization switching.

Nonvolatile voltage-controlled magnetization reversal in a spin-valve multiferroic heterostructure

Pure voltage-controlled magnetism, rather than a spin current or magnetic field, is the goal for next-generation ultralow power consumption spintronic devices. To advance toward this goal, we report a voltage-controlled nonvolatile 90° magnetization rotation and voltage-assisted 180° magnetization reversal in a spin-valve multiferroic heterostructure. Here, a spin valve with a synthetic antiferromagnetic structure was grown on a (110)-cut Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) substrate, in which only the magnetic moment of the free layer can be manipulated by an electric field (E-field) via the strain-mediated magnetoelectric coupling effect. As a result of selecting a specified PMN-PT substrate with defect dipoles, nonvolatile and stable magnetization switching was achieved by using voltage impulses. Accordingly, a giant, reversible and nonvolatile magnetoresistance modulation was achieved without the assistance of a magnetic field. In addition, by adopting a small voltage impulse, the critical magnetic field required for complete 180° magnetization reversal of the free layer can be tremendously reduced. A magnetoresistance ratio as large as that obtained by a magnetic field or spin current under normal conditions is achieved. These results indicate that E-field-assisted energy-efficient in-plane magnetization switching is a feasible strategy. This work is significant to the development of ultralow-power magnetoresistive memory and spintronic devices.

Non-reciprocal magnetoresistance, directional inhomogeneity and mixed symmetry Hall devices

Phenomenology similar to the non-reciprocal charge transport violating Onsager's reciprocity relations can develop in directionally inhomogeneous conducting films with non-uniform Hall coefficient along the current trajectory. The effect is demonstrated in ferromagnetic CoPd films and analyzed in comparison with the unidirectional magnetoresistance phenomena. We suggest to use an engineered inhomogeneity for spintronics applications and present the concept of mixed symmetry Hall devices in which transverse to current Hall signal is measured in a longitudinal contacts arrangement. Magnetization reversal and memory detection are demonstrated in the three-terminal and the partitioned normal metal–ferromagnet (NM-FM) device designs. Multi-bit memory is realized in the partitioned FM-NM-FM structure. The relative amplitude of the antisymmetric signal in the engineered ferromagnetic devices is few percent, which is 10–103 times higher than in their unidirectional magnetoresistance analogues.

Spin-orbit dependence of anisotropic current-induced spin polarization

Studies of the current-induced spin polarization (CISP) have been recently reinvigorated due to the discoveries of CISP in some burgeoning materials such as oxide interfaces, van der Waals, and topological quantum materials. Here, we investigate the CISP in two-dimensional systems for different types of spin-orbit coupling (SOC) using the Boltzmann transport theory. We find an anisotropic response of CISP to the current direction which strongly depends on the type of SOC. We demonstrate that the CISP is nonlinear with respect to the SOC magnitude, depends on the Fermi energy, and exhibits two different transport regimes for low or high carrier density. Finally, we propose a magnetoresistance device which can exploit the predicted CISP anisotropy.

First-principles disordered local-moment study on temperature dependence of spin polarization in Co2Fe(Ga0.5Ge0.5) Heusler alloy

Magnetoresistance (MR) devices fabricated with half-metallic Co-based Heusler alloys (Co2YZ) have a large MR ratio but are subject to substantial temperature degradation. Reduction of the spin polarization in the bulk electrode at finite temperatures is one possible reason for the reduction in the MR ratio. In this study, we investigated the temperature dependence of the spin polarization of Co2Fe(Ga0.5Ge0.5) (CFGG) using density-functional theory and the disordered local-moment method. We found that the reduction in the spin polarization at finite temperatures is smaller in CFGG compared to the well known Co2MnSi (CMS) half-metal, which is attributed to the higher Curie temperature of CFGG than CMS. On the other hand, it was also found that modulation of the Fermi level position in the half-metallic gap of CFGG within the rigid band model by electron and hole doping can improve the temperature dependence of the spin polarization. However, off-stoichiometric Co-rich and Fe-rich CFGG corresponding to electron and hole doping show a reduction in spin polarization compared to that of stoichiometric CFGG at 0 K. Therefore, we propose that modulation of the Fermi level position through varying the Y and Z site compositions of Co2YZ will be necessary to improve the temperature dependence of the spin polarization.

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.