Showing papers on "Magnetoresistance published in 2020"

Room temperature ferromagnetism in ultra-thin van der Waals crystals of 1T-CrTe 2

Abstract: Although many emerging new phenomena have been unraveled in two dimensional (2D) materials with long-range spin orderings, the usually low critical temperature in van der Waals (vdW) magnetic material has thus far hindered the related practical applications. Here, we show that ferromagnetism can hold above 300 K in a metallic phase of 1T-CrTe2 down to the ultra-thin limit. It thus makes CrTe2 so far the only known exfoliated ultra-thin vdW magnets with intrinsic long-range magnetic ordering above room temperature. An in-plane room-temperature negative anisotropic magnetoresistance (AMR) was obtained in ultra-thin CrTe2 devices, with a sign change in the AMR at lower temperature, with −0.6% and +5% at 300 and 10 K, respectively. Our findings provide insights into magnetism in ultra-thin CrTe2, expanding the vdW crystals toolbox for future room-temperature spintronic applications.

Single-shot all-optical switching of magnetization in Tb/Co multilayer-based electrodes.

Abstract: Ever since the first observation of all-optical switching of magnetization in the ferrimagnetic alloy GdFeCo using femtosecond laser pulses, there has been significant interest in exploiting this process for data-recording applications. In particular, the ultrafast speed of the magnetic reversal can enable the writing speeds associated with magnetic memory devices to be potentially pushed towards THz frequencies. This work reports the development of perpendicular magnetic tunnel junctions incorporating a stack of Tb/Co nanolayers whose magnetization can be all-optically controlled via helicity-independent single-shot switching. Toggling of the magnetization of the Tb/Co electrode was achieved using either 60 femtosecond-long or 5 picosecond-long laser pulses, with incident fluences down to 3.5 mJ/cm2, for Co-rich compositions of the stack either in isolation or coupled to a CoFeB-electrode/MgO-barrier tunnel-junction stack. Successful switching of the CoFeB-[Tb/Co] electrodes was obtained even after annealing at 250 °C. After integration of the [Tb/Co]-based electrodes within perpendicular magnetic tunnel junctions yielded a maximum tunneling magnetoresistance signal of 41% and RxA value of 150 Ωμm2 with current-in-plane measurements and ratios between 28% and 38% in nanopatterned pillars. These results represent a breakthrough for the development of perpendicular magnetic tunnel junctions controllable using single laser pulses, and offer a technologically-viable path towards the realization of hybrid spintronic-photonic systems featuring THz switching speeds.

Tailoring Magnetic Anisotropy in Cr$_2$Ge$_2$Te$_6$ by Electrostatic Gating

Abstract: Electrical control of magnetism of a ferromagnetic semiconductor offers exciting prospects for future spintronic devices for processing and storing information. Here, we report observation of electrically modulated magnetic phase transition and magnetic anisotropy in thin crystal of Cr$_2$Ge$_2$Te$_6$ (CGT), a layered ferromagnetic semiconductor. We show that heavily electron-doped ($\sim$ $10^{14}$ cm$^{-2}$) CGT in an electric double-layer transistor device is found to exhibit hysteresis in magnetoresistance (MR), a clear signature of ferromagnetism, at temperatures up to above 200 K, which is significantly higher than the known Curie temperature of 61 K for an undoped material. Additionally, angle-dependent MR measurements reveal that the magnetic easy axis of this new ground state lies within the layer plane in stark contrast to the case of undoped CGT, whose easy axis points in the out-of-plane direction. We propose that significant doping promotes double-exchange mechanism mediated by free carriers, prevailing over the superexchange mechanism in the insulating state. Our findings highlight that electrostatic gating of this class of materials allows not only charge flow switching but also magnetic phase switching, evidencing their potential for spintronics applications.

Ultrasensitive Magnetic Field Sensors for Biomedical Applications.

Abstract: The development of magnetic field sensors for biomedical applications primarily focuses on equivalent magnetic noise reduction or overall design improvement in order to make them smaller and cheaper while keeping the required values of a limit of detection. One of the cutting-edge topics today is the use of magnetic field sensors for applications such as magnetocardiography, magnetotomography, magnetomyography, magnetoneurography, or their application in point-of-care devices. This introductory review focuses on modern magnetic field sensors suitable for biomedicine applications from a physical point of view and provides an overview of recent studies in this field. Types of magnetic field sensors include direct current superconducting quantum interference devices, search coil, fluxgate, magnetoelectric, giant magneto-impedance, anisotropic/giant/tunneling magnetoresistance, optically pumped, cavity optomechanical, Hall effect, magnetoelastic, spin wave interferometry, and those based on the behavior of nitrogen-vacancy centers in the atomic lattice of diamond.

Spin-Valve Effect in Fe3GeTe2/MoS2/Fe3GeTe2 van der Waals Heterostructures

Abstract: The van der Waals (vdW) materials offer an opportunity to build all-two-dimensional (all-2D) spintronic devices with high-quality interfaces regardless of the lattice mismatch. Here, we report on an all-2D vertical spin valve that combines a typical layered semiconductor MoS2 with vdW ferromagnetic metal Fe3GeTe2 (FGT) flakes. The linear current-voltage curves illustrate that Ohmic contacts are formed in FGT/MoS2 interfaces, while the temperature dependence of the junction resistance further demonstrates that the MoS2 interlayer acts as a conducting layer instead of a tunneling layer. In addition, the magnitude of the magnetoresistance (MR) of 3.1% at 10 K is observed, which is around 8 times larger than that of the reported spin valves based on MoS2 sandwiched by conventional ferromagnetic electrodes. The MR decreasing monotonically with increasing temperature follows the Bloch's law. As the bias current decreases exponentially, the MR increases linearly up to a maximum value of 4.1%. Our results reveal the potential opportunities of vdW heterostructures for developing novel spintronic devices.

Planar Hall effect and anisotropic magnetoresistance in polar-polar interface of LaVO 3 -KTaO 3 with strong spin-orbit coupling.

Abstract: Among the perovskite oxide family, KTaO3 (KTO) has recently attracted considerable interest as a possible system for the realization of the Rashba effect. In this work, we report a novel conducting interface by placing KTO with another insulator, LaVO3 (LVO) and report planar Hall effect (PHE) and anisotropic magnetoresistance (AMR) measurements. This interface exhibits a signature of strong spin-orbit coupling. Our experimental observations of two fold AMR and PHE at low magnetic fields (B) is similar to those obtained for topological systems and can be intuitively understood using a phenomenological theory for a Rashba spin-split system. Our experimental data show a B2 dependence of AMR and PHE at low magnetic fields that could also be explained based on our model. At high fields (~8 T), we see a two fold to four fold transition in the AMR that could not be explained using only Rashba spin-split energy spectra. Two dimensional electron gas (2DEG) at oxide interfaces is promising in modern electronic devices. Here, Wadehra et al. realize 2DEG at a novel interface composed of LaVO3 and KTaO3, where strong spin-orbit coupling and relativistic nature of the electrons in the 2DEG, leading to anisotropic magnetoresistance and planar Hall effect.

Exchange bias switching in an antiferromagnet/ferromagnet bilayer driven by spin–orbit torque

Abstract: The electrical manipulation of magnetization and exchange bias in antiferromagnet/ferromagnet thin films could be of use in the development of the next generation of spintronic devices. Current-controlled magnetization switching can be driven by spin–orbit torques generated in an adjacent heavy-metal layer, but these structures are difficult to integrate with exchange bias switching and tunnelling magnetoresistance measurements. Here, we report the current-induced switching of the exchange bias field in a perpendicularly magnetized IrMn/CoFeB bilayer structure using a spin–orbit torque generated in the antiferromagnetic IrMn layer. By manipulating the current direction and amplitude, independent and repeatable switching of the magnetization and exchange bias field below the blocking temperature can be achieved. The critical current density for the exchange bias switching is found to be larger than that for CoFeB magnetization reversal. X-ray magnetic circular dichroism, polarized neutron reflectometry measurements and micromagnetic simulations show that a small net magnetization within the IrMn interface plays a crucial role in these phenomena. The magnetization and exchange bias field in an IrMn/CoFeB bilayer can be independently switched using a current-controlled spin–orbit torque generated in the antiferromagnetic IrMn layer.

Layered Antiferromagnetism Induces Large Negative Magnetoresistance in the van der Waals Semiconductor CrSBr.

Abstract: The recent discovery of magnetism within the family of exfoliatable van der Waals (vdW) compounds has attracted considerable interest in these materials for both fundamental research and technological applications. However, current vdW magnets are limited by their extreme sensitivity to air, low ordering temperatures, and poor charge transport properties. Here the magnetic and electronic properties of CrSBr are reported, an air-stable vdW antiferromagnetic semiconductor that readily cleaves perpendicular to the stacking axis. Below its Neel temperature, TN = 132 ± 1 K, CrSBr adopts an A-type antiferromagnetic structure with each individual layer ferromagnetically ordered internally and the layers coupled antiferromagnetically along the stacking direction. Scanning tunneling spectroscopy and photoluminescence (PL) reveal that the electronic gap is ΔE = 1.5 ± 0.2 eV with a corresponding PL peak centered at 1.25 ± 0.07 eV. Using magnetotransport measurements, strong coupling between magnetic order and transport properties in CrSBr is demonstrated, leading to a large negative magnetoresistance response that is unique among vdW materials. These findings establish CrSBr as a promising material platform for increasing the applicability of vdW magnets to the field of spin-based electronics.

Layered Antiferromagnetism Induces Large Negative Magnetoresistance in the van der Waals Semiconductor CrSBr

Abstract: The recent discovery of magnetism within the family of exfoliatable van der Waals (vdW) compounds has attracted considerable interest in these materials for both fundamental research and technological applications. However current vdW magnets are limited by their extreme sensitivity to air, low ordering temperatures, and poor charge transport properties. Here we report the magnetic and electronic properties of CrSBr, an air-stable vdW antiferromagnetic semiconductor that readily cleaves perpendicular to the stacking axis. Below its Neel temperature, $T_N = 132 \pm 1$ K, CrSBr adopts an A-type antiferromagnetic structure with each individual layer ferromagnetically ordered internally and the layers coupled antiferromagnetically along the stacking direction. Scanning tunneling spectroscopy and photoluminescence (PL) reveal that the electronic gap is $\Delta_E = 1.5 \pm 0.2$ eV with a corresponding PL peak centered at $1.25 \pm 0.07$ eV. Using magnetotransport measurements, we demonstrate strong coupling between magnetic order and transport properties in CrSBr, leading to a large negative magnetoresistance response that is unique amongst vdW materials. These findings establish CrSBr as a promising material platform for increasing the applicability of vdW magnets to the field of spin-based electronics.

Symmetry breaking and skyrmionic transport in twisted bilayer graphene

Abstract: Motivated by recent low-temperature magnetoresistance measurements in twisted bilayer graphene aligned with hexagonal boron nitride substrate, we perform a systematic study of possible symmetry breaking orders in this device at a filling of two electrons per moir\'e unit cell. We find that the surprising nonmonotonic dependence of the resistance on an out-of-plane magnetic field is difficult to reconcile with particle-hole charge carriers from the low-energy bands in symmetry broken phases. We invoke the nonzero Chern numbers of the twisted bilayer graphene flat bands to argue that skyrmion textures provide an alternative for the dominant charge carriers. Via an effective field theory for the spin degrees of freedom, we show that the effect of spin Zeeman splitting on the skyrmion excitations provides a possible explanation for the nonmonotonic magnetoresistance. We suggest several experimental tests, including the functional dependence of the activation gap on the magnetic field, for our proposed correlated insulating states at different integer fillings. We also discuss possible exotic phases and quantum phase transitions that can arise via skyrmion pairing on doping such an insulator.

Multiple Superconducting Phases and Unusual Enhancement of the Upper Critical Field in UTe2

Abstract: We performed AC calorimetry and magnetoresistance measurements under pressure for H ∥ a-axis (easy-magnetization axis) in the novel heavy-fermion superconductor UTe2. Thanks to the thermodynamic in...

Thermoelectric properties of a semicrystalline polymer doped beyond the insulator-to-metal transition by electrolyte gating

Abstract: Conducting polymer thin films containing inherent structural disorder exhibit complicated electronic, transport, and thermoelectric properties. The unconventional power-law relation between the Seebeck coefficient (S) and the electrical conductivity (σ) is one of the typical consequences of this disorder, where no maximum of the thermoelectric power factor (P = S2σ) has been observed upon doping, unlike conventional systems. Here, it is demonstrated that a thiophene-based semicrystalline polymer exhibits a clear maximum of P through wide-range carrier doping by the electrolyte gating technique. The maximum value appears around the macroscopic insulator-to-metal transition upon doping, which is firmly confirmed by the temperature dependence of σ and magnetoresistance measurements. The effect of disorder on charge transport is suppressed in the metallic state, resulting in the conventional S-σ relation described by the Mott equation. The present results provide a physical background for controlling the performance of conducting polymers toward the application to thermoelectric devices.

Low-temperature monoclinic layer stacking in atomically thin CrI3 crystals

Abstract: Chromium triiodide, CrI3, is emerging as a promising magnetic two-dimensional semiconductor where spins are ferromagnetically aligned within a single layer. Potential applications in spintronics arise from an antiferromagnetic ordering between adjacent layers that gives rise to spin filtering and a large magnetoresistance in tunnelling devices. This key feature appears only in thin multilayers and it is not inherited from bulk crystals, where instead neighbouring layers share the same ferromagnetic spin orientation. This discrepancy between bulk and thin samples is unexpected, as magnetic ordering between layers arises from exchange interactions that are local in nature and should not depend strongly on thickness. Here we solve this controversy and show through polarization resolved Raman spectroscopy that thin multilayers do not undergo a structural phase transition typical of bulk crystals. As a consequence, a different stacking pattern is present in thin and bulk samples at the temperatures at which magnetism sets in and, according to previous first-principles simulations, this results in a different interlayer magnetic ordering. Our experimental findings provide evidence for the strong interplay between stacking order and magnetism in CrI3, opening interesting perspectives to design the magnetic state of van der Waals multilayers.

Incoherent transport across the strange metal regime of highly overdoped cuprates.

Abstract: Strange metals possess highly unconventional transport characteristics, such as a linear-in-temperature ($T$) resistivity, an inverse Hall angle that varies as $T^2$ and a linear-in-field ($H$) magnetoresistance. Identifying the origin of these collective anomalies has proved profoundly challenging, even in materials such as the hole-doped cuprates that possess a simple band structure. The prevailing dogma is that strange metallicity in the cuprates is tied to a quantum critical point at a doping $p*$ inside the superconducting dome. Here, we study the high-field in-plane magnetoresistance of two superconducting cuprate families at doping levels beyond $p*$. At all dopings, the magnetoresistance exhibits quadrature scaling and becomes linear at high $H/T$ ratios. Moreover, its magnitude is found to be much larger than predicted by conventional theory and insensitive to both impurity scattering and magnetic field orientation. These observations, coupled with analysis of the zero-field and Hall resistivities, suggest that despite having a single band, the cuprate strange metal phase hosts two charge sectors, one containing coherent quasiparticles, the other scale-invariant `Planckian' dissipators.

Probing magnetism in atomically thin semiconducting PtSe2.

Abstract: Atomic-scale disorder in two-dimensional transition metal dichalcogenides is often accompanied by local magnetic moments, which can conceivably induce long-range magnetic ordering into intrinsically non-magnetic materials. Here, we demonstrate the signature of long-range magnetic orderings in defective mono- and bi-layer semiconducting PtSe2 by performing magnetoresistance measurements under both lateral and vertical measurement configurations. As the material is thinned down from bi- to mono-layer thickness, we observe a ferromagnetic-to-antiferromagnetic crossover, a behavior which is opposite to the one observed in the prototypical 2D magnet CrI3. Our first-principles calculations, supported by aberration-corrected transmission electron microscopy imaging of point defects, associate this transition to the interplay between the defect-induced magnetism and the interlayer interactions in PtSe2. Furthermore, we show that graphene can be effectively used to probe the magnetization of adjacent semiconducting PtSe2. Our findings in an ultimately scaled monolayer system lay the foundation for atom-by-atom engineering of magnetism in otherwise non-magnetic 2D materials. Beneficiary defects could be utilized to introduce magnetism into materials that are not intrinsically magnetic. Here, the authors demonstrate long range magnetic order in the air-stable, defective Platinum Diselenide in the ultimate limit of thickness by using proximitized graphene as a probe.

Non-magnetic origin of spin Hall magnetoresistance-like signals in Pt films and epitaxial NiO/Pt bilayers

Abstract: Electrical control of magnetic order in antiferromagnetic insulators (AFIs) using a Pt overlayer as a spin current source has been recently reported, but detecting and understanding the nature of current-induced switching in AFIs remain a challenge. Here, we examine the origin of spin Hall magnetoresistance-like signals measured in a standard Hall bar geometry, which have recently been taken as evidence of current-induced switching of the antiferromagnetic order in Pt/AFI bilayers. We show that transverse voltage signals consistent with both the partial switching and toggle switching of the Neel vector in epitaxial Pt/NiO bilayers on Al2O3 are also present in Pt/Al2O3 in which the AFI is absent. We show that these signals have a thermal origin and arise from (i) transient changes in the current distribution due to nonuniform Joule heating and (ii) irreversible changes due to electromigration at elevated current densities, accompanied by long-term creep. These results suggest that more sophisticated techniques that directly probe the magnetic order are required to reliably exclude transport artifacts and thus infer information about the antiferromagnetic order in such systems.

Unexpected two-fold symmetric superconductivity in few-layer NbSe$_2$

Abstract: Two-dimensional transition metal dichalcogenides (TMDs) have been attracting significant interest due to a range of properties, such as layer-dependent inversion symmetry, valley-contrasted Berry curvatures, and strong spin-orbit coupling (SOC). Of particular interest is niobium diselenide (NbSe2), whose superconducting state in few-layer samples is profoundly affected by an unusual type of SOC called Ising SOC. Combined with the reduced dimensionality, the latter stabilizes the superconducting state against magnetic fields up to ~35 T and could lead to other exotic properties such as nodal and crystalline topological superconductivity. Here, we report transport measurements of few-layer NbSe$_2$ under in-plane external magnetic fields, revealing an unexpected two-fold rotational symmetry of the superconducting state. In contrast to the three-fold symmetry of the lattice, we observe that the magnetoresistance and critical field exhibit a two-fold oscillation with respect to an applied in-plane magnetic field. We find similar two-fold oscillations deep inside the superconducting state in differential conductance measurements on NbSe$_2$/CrBr$_3$ superconductor-magnet junctions. In both cases, the anisotropy vanishes in the normal state, demonstrating that it is an intrinsic property of the superconducting phase. We attribute the behavior to the mixing between two closely competing pairing instabilities, namely, the conventional s-wave instability typical of bulk NbSe$_2$ and an unconventional d- or p-wave channel that emerges in few-layer NbSe2. Our results thus demonstrate the unconventional character of the pairing interaction in a few-layer TMD, opening a new avenue to search for exotic superconductivity in this family of 2D materials.

Observation of Large Unidirectional Rashba Magnetoresistance in Ge(111)

Abstract: Relating magnetotransport properties to specific spin textures at surfaces or interfaces is an intense field of research nowadays. Here, we investigate the variation of the electrical resistance of Ge(111) grown epitaxially on semi-insulating Si(111) under the application of an external magnetic field. We find a magnetoresistance term that is linear in current density j and magnetic field B, hence, odd in j and B, corresponding to a unidirectional magnetoresistance. At 15 K, for I=10 μA (or j=0.33 A m^{-1}) and B=1 T, it represents 0.5% of the zero field resistance, a much higher value compared to previous reports on unidirectional magnetoresistance (UMR). We ascribe the origin of this magnetoresistance to the interplay between the externally applied magnetic field and the pseudomagnetic field generated by the current applied in the spin-splitted subsurface states of Ge(111). This unidirectional magnetoresistance is independent of the current direction with respect to the Ge crystal axes. It progressively vanishes, either using a negative gate voltage due to carrier activation into the bulk (without spin-splitted bands), or by increasing the temperature due to the Rashba energy splitting of the subsurface states lower than ∼58k_{B}. We believe that UMR could be used as a powerful probe of the spin-orbit interaction in a wide range of materials.

First-principles theory of proximity spin-orbit torque on a two-dimensional magnet: Current-driven antiferromagnet-to-ferromagnet reversible transition in bilayer CrI 3

Abstract: The recently discovered two-dimensional (2D) magnetic insulator CrI₃ is an intriguing case for basic research and spintronic applications since it is a ferromagnet in the bulk, but an antiferromagnet in bilayer form, with its magnetic ordering amenable to external manipulations. Using first-principles quantum transport approach, we predict that injecting unpolarized charge current parallel to the interface of bilayer-CrI₃/monolayer-TaSe₂ van der Waals heterostructure will induce spin-orbit torque (SOT) and thereby driven dynamics of magnetization on the first monolayer of CrI₃ in direct contact with TaSe₂. By combining calculated complex angular dependence of SOT with the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization, we demonstrate that current pulses can switch the direction of magnetization on the first monolayer to become parallel to that of the second monolayer, thereby converting CrI₃ from antiferromagnet to ferromagnet while not requiring any external magnetic field. We explain the mechanism of this reversible current-driven nonequilibrium phase transition by showing that first monolayer of CrI₃ carries current due to evanescent wavefunctions injected by metallic transition metal dichalcogenide TaSe₂, while concurrently acquiring strong spin-orbit coupling (SOC) via such proximity effect, whereas the second monolayer of CrI₃ remains insulating. The transition can be detected by passing vertical read current through the vdW heterostructure, encapsulated by bilayer of hexagonal boron nitride and sandwiched between graphite electrodes, where we find tunneling magnetoresistance of $\backslashsimeq 240$%.

Evidence of a magnetic transition in atomically thin Cr2TiC2Tx MXene

Abstract: Two-dimensional (2D) transition metal carbides and nitrides known as MXenes have shown attractive functionalities such as high electronic conductivity, a wide range of optical properties, versatile transition metal and surface chemistry, and solution processability. Although extensively studied computationally, the magnetic properties of this large family of 2D materials await experimental exploration. 2D magnetic materials have recently attracted significant interest as model systems to understand low-dimensional magnetism and for potential spintronic applications. Here, we report on synthesis of Cr2TiC2Tx MXene and a detailed study of its magnetic as well as electronic properties. Using a combination of magnetometry, synchrotron X-ray linear dichroism, and field- and angular-dependent magnetoresistance measurements, we find clear evidence of a magnetic transition in Cr2TiC2Tx at approximately 30 K, which is not present in its bulk layered carbide counterpart (Cr2TiAlC2 MAX phase). This work presents the first experimental evidence of a magnetic transition in a MXene material and provides an exciting opportunity to explore magnetism in this large family of 2D materials.

Liquid metal circuit based magnetoresistive strain sensor with discriminating magnetic and mechanical sensitivity

Abstract: This work reported a stretchable, magneto-sensitive and discriminative strain sensor consisting of liquid metal (LM) as the electronic circuit and carbonyl iron particles (CIPs)/polydimethylsiloxane (PDMS) composite as the packaging material (LME-MRE). Due to the good elasticity of PDMS matrix and unique magnetic interactions among CIPs, the LME-MRE sensor was both sensitive to the external mechanical and magnetic stimuli. Moreover, owing to the fluidic nature of liquid metal, LME-MRE sensor also exhibited good electrically healing capability. The relative resistance variation ( Δ R / R 0 ) of LME-MRE sensor reached as high as 1038 % under 40 % compressive strain. The electrical properties of LME-MRE sensor also remarkably changed under cyclic stretching and bending. Moreover, a 7.4 % decrement of resistance was achieved under applying a 300 mT magnetic field to LME-MRE sensor. Importantly, the magnetic and mechanical stimuli could be discriminated by LME-MRE sensor and a possible mechanism was proposed to describe the mechanic-electric-magnetic sensing characteristics. Finally, a LME-MRE sensor array is developed for the detection of both compressive force and magnetic field, demonstrating a broad promising in future intelligent devices, like artificial electronic skins and soft robotics.

Antisymmetric Magnetoresistance in a van der Waals Antiferromagnetic/Ferromagnetic Layered MnPS3/Fe3GeTe2 Stacking Heterostructure.

Abstract: The presence of two-dimensional (2D) layer-stacking heterostructures that can efficiently tune the interface properties by stacking desirable materials provides a platform to investigate some physical phenomena, such as the proximity effect and magnetic exchange coupling. Here, we report the observation of antisymmetric magnetoresistance in a van der Waals (vdW) antiferromagnetic/ferromagnetic (AFM/FM) heterostructure of MnPS3/Fe3GeTe2 when the temperature is below the Neel temperature of MnPS3. Distinguished from two resistance states in conventional giant magnetoresistance, the magnetoresistance in the MnPS3/Fe3GeTe2 heterostructure exhibits three states, of high, intermediate, and low resistance. This antisymmetric magnetoresistance spike is determined by an unsynchronized magnetic switching between the AFM/FM interface layer and the bulk of Fe3GeTe2 during magnetization reversal. Our work highlights that the artificial vdW stacking structure holds potential to explore some physical phenomena and spintronic device applications.

Magnetic exchange induced Weyl state in a semimetal EuCd2Sb2

Abstract: Magnetic Weyl semimetals (WSMs) bearing long-time seeking are still very rare. We have identified herein that EuCd2Sb2, a semimetal belonging to the type IV magnetic space group, hosts a magnetic exchange induced Weyl state via performing high magnetic field magnetotransport measurements and ab initio calculations. In the A-type antiferromagnetic structure, the external field larger than 3.2 T can align all Eu spins to be fully polarized along the c-axis and consequently drive EuCd2Sb2 into a spin polarized state. Magnetotransport measurements up to ∼55–60 T showed striking Shubnikov-de Hass oscillations associated with a nontrivial Berry phase. The ab initio calculations unveiled a phase transition of EuCd2Sb2 from a small gap antiferromagnetic topological insulator to a spin polarized WSM in which the Weyl points emerge along the Γ-Z path. Fermi arcs on (100) and (010) surfaces are also predicted. Meanwhile, the observed large anomalous Hall effect indicates the existence of Weyl points around the Fermi level. The results pave a way toward the realization of various topological states in a single material through the magnetic exchange manipulation.

Physical properties and thermal stability of Fe5GeTe2 single crystals

Abstract: The magnetic and transport properties of Fe-deficient Fe5GeTe2 single crystals (Fe5-xGeTe2 with x~0.3) were studied and the impact of thermal processing was explored. Quenching crystals from the growth temperature has been previously shown to produce a metastable state that undergoes a strongly hysteretic first-order transition upon cooling below ~100K. The first-order transition impacts the magnetic properties, yielding an enhancement in the Curie temperature T_C from 270 to 310K. In the present work, T_HT ~550K has been identified as the temperature above which metastable crystals are obtained via quenching. Diffraction experiments reveal a structural change at this temperature, and significant stacking disorder occurs when samples are slowly cooled through this temperature range. The transport properties are demonstrated to be similar regardless of the crystal's thermal history. The scattering of charge carriers appears to be dominated by moments fluctuating on the Fe(1) sublattice, which remain dynamic down to 100-120K. Maxima in the magnetoresistance and anomalous Hall resistance are observed near 120K. The Hall and Seebeck coefficients are also impacted by magnetic ordering on the Fe(1) sublattice. The data suggest that both electrons and holes contribute to conduction above 120K, but that electrons dominate at lower temperature when all of the Fe sublattices are magnetically ordered. This study demonstrates a strong coupling of the magnetism and transport properties in Fe5-xGeTe2 and complements the previous results that demonstrated strong magnetoelastic coupling as the Fe(1) moments order. The published version of this manuscript is DOI:10.1103/PhysRevMaterials.3.104401 (2019)

Significant tunneling magnetoresistance and excellent spin filtering effect in CrI3-based van der Waals magnetic tunnel junctions.

Abstract: van der Waals (vdW) heterojunctions stacked by different two-dimensional (2D) layered materials not only exhibit the complementary effect of short plates, but also harbor novel physical phenomena. In particular, the emergence of 2D magnetic vdW materials has provided novel opportunities for the application of these materials in spintronics. However, to the best of our knowledge, to date, the spin-related transport mechanism in magnetic tunnel junctions (MTJs) based on these 2D vdW magnetic materials and the effect of pinning layers on their transport properties have not been elucidated by the non-equilibrium state theory. Herein, based on first-principles calculations, we report the spin-polarized quantum transport properties of sandwich-type vdW magnetic tunnel junctions (CrI3/h-BN/n·CrI3) comprising monolayer CrI3, a hexagonal boron nitride (h-BN) spacer layer, and n-layer CrI3 (n = 1, 2, 3, and 4). Considering the inter-layer antiferromagnetic coupling in n-layer CrI3, a few layers of CrI3 can be regarded as its own natural pinning layers. Especially, when n is equal to 3, an almost fully spin-polarized current and large tunnel magnetoresistance ratio (3600%) are obtained in the equilibrium state. Excitingly, due to different numbers of pinning layers in MTJs, the transport properties of these MTJs at positive bias voltages exhibit an interesting odd–even effect within a limited thickness of these pinning layers. Moreover, an almost perfect spin filtering effect and remarkable negative differential resistance (NDR) were observed in the MTJs where n was odd (n = 1 and 3). The observed non-equilibrium quantum transport phenomenon is explained by spin-dependent transmission coefficient at different bias voltages. Our results provide effective guidance for the experimental studies of the MTJs based on 2D magnetic vdW materials.

Magnetically Tunable Near-Field Radiative Heat Transfer in Hyperbolic Metamaterials

Abstract: Active control of radiative heat transfer still remains open due to its exciting application potential. In view of this, we present a theoretical demonstration of dynamically tunable near-field radiative heat transfer (NFRHT) between two multilayer hyperbolic metamaterials (consisting of alternating layers of a magneto-optical (MO) material and a dielectric) using an external magnetic field. We show that magnetization-induced hyperbolic modes play a significant role in radiative heat transfer and allow a highly tunable heat flux. Moreover, the hybridization of intrinsic and magnetization-induced hyperbolic modes significantly enhances the radiative heat transfer. In particular, we find that polarization conversion due to nonzero off-diagonal elements enables the spectral heat transfer coefficient for the TE polarization to be as large as that for the TM polarization, which is vastly different from the case of two closely spaced bulk MO plates. In addition, to quantitatively characterize the thermal modulation, we show a negative thermal magnetoresistance effect in this system, which approaches $\ensuremath{-}59.6\mathrm{%}$ when the magnetic field intensity reaches 10 T. Our findings may be beneficial for active noncontact thermal management at the nanoscale, and facilitate a deeper understanding of the mechanisms of MO hyperbolic metamaterials in terms of NFRHT.

Spin–orbit magnetic state readout in scaled ferromagnetic/heavy metal nanostructures

Abstract: The efficient detection of a magnetic state at nanoscale dimensions is important for the development of spin-logic devices. Magnetoresistance effects can be used to detect magnetic states, but they do not generate an electromotive force (that is, a voltage) or a current that can be used to drive a circuit element for logic device applications. Here we report a favourable scaling law for the detection of an in-plane magnetic state of a magnet by using the inverse spin Hall effect in cobalt–iron/platinum (CoFe/Pt) nanostructured devices. By reducing the dimensions of the device, we obtain a large spin Hall signal of 0.3 Ω at room temperature and quantify an effective spin-to-charge conversion rate for the ferromagnetic/heavy metal system. We predict that this spin–orbit detection of magnetic states could be used to drive spin-logic circuits. A favourable scaling law for the magnetic state readout of CoFe/Pt nanostructure devices allows large spin Hall signals of 0.3 Ω at room temperature to be obtained, which could be useful in the development of spin-logic devices.

Interfacial Spin-Orbit Coupling: A Platform for Superconducting Spintronics

Abstract: Spin-orbit coupling (SOC) is a key interaction in spintronics, allowing electrical control of spin or magnetization and, vice versa, magnetic control of electrical current. However, recent advances have revealed much broader implications of SOC that is also central to the design of topological states with potential applications from low-energy dissipation and faster magnetization switching to high tolerance of disorder. SOC and the resulting emergent interfacial spin-orbit fields are simply realized in junctions through structural inversion asymmetry, while the anisotropy in magnetoresistance (MR) allows their experimental detection. Surprisingly, we demonstrate that an all-epitaxial ferromagnet/$\mathrm{Mg}\mathrm{O}$/metal junction with a single ferromagnetic region and only negligible MR anisotropy undergoes a remarkable transformation below the superconducting transition temperature of the metal. The superconducting junction has a MR anisotropy 3 orders of magnitude higher and could enable novel applications in superconducting spintronics. In contrast to common realizations of MR effects that require a finite applied magnetic field, our system is designed to have two stable zero-field states with mutually orthogonal magnetizations: in plane and out of plane. This bistable magnetic anisotropy allows us to rule out orbital and vortex effects due to an applied magnetic field and identify the SOC origin of the observed MR. Such MR reaches approximately $20\mathrm{%}$ without an applied magnetic field and could be further increased for large magnetic fields that support vortices. Our findings call for a revisit of the role of SOC, even when it seems negligible in the normal state, and suggest an alternative platform for superconducting spintronics.

Spin-Momentum-Locking Inhomogeneities as a Source of Bilinear Magnetoresistance in Topological Insulators.

Abstract: A new mechanism of bilinear magnetoresistance (BMR) is proposed and studied theoretically within the minimal model describing surface electronic states in topological insulators. The BMR appears as a consequence of the second-order response to electric field, and depends linearly on both magnetic field and current (electric field). The mechanism is based on the interplay of current-induced spin polarization and scattering processes due to inhomogeneities of spin-momentum locking, that unavoidably appear as a result of structural defects in topological insulators. The proposed mechanism leads to the BMR even if the electronic band structure is isotropic (e.g., absence of hexagonal warping), and is shown to be dominant at lower Fermi energies.

Tuning magnetic confinement of spin-triplet superconductivity

Abstract: Electrical magnetoresistance and tunnel diode oscillator measurements were performed under external magnetic fields up to 41 T applied along the crystallographic b axis (hard axis) of UTe2 as a function of temperature and applied pressures up to 18.8 kbar. In this work, we track the field-induced first-order transition between superconducting and magnetic field-polarized phases as a function of applied pressure, showing suppression of the transition with increasing pressure until the demise of superconductivity near 16 kbar and the appearance of a pressure-induced ferromagnetic-like ground state that is distinct from the field-polarized phase and stable at zero field. Together with evidence for the evolution of a second superconducting phase and its upper critical field with pressure, we examine the confinement of superconductivity by two orthogonal magnetic phases and the implications for understanding the boundaries of triplet superconductivity.