Magnetic tunnel junctions (MTJs) with conventional bulk ferromagnets separated by a nonmagnetic insulating layer are key building blocks in spintronics for magnetic sensors and memory. A radically different approach of using atomically-thin van der Waals (vdW) materials in MTJs is expected to boost their figure of merit, the tunneling magnetoresistance (TMR), while relaxing the lattice-matching requirements from the epitaxial growth and supporting high-quality integration of dissimilar materials with atomically-sharp interfaces. We report TMR up to 192% at 10 K in all-vdW Fe3GeTe2/GaSe/Fe3GeTe2 MTJs. Remarkably, instead of the usual insulating spacer, this large TMR is realized with a vdW semiconductor GaSe. Integration of two-dimensional ferromagnets in semiconductor-based vdW junctions offers gate-tunability, bias dependence, magnetic proximity effects, and spin-dependent optical-selection rules. We demonstrate that not just the magnitude, but also the TMR sign is tuned by the applied bias or the semiconductor thickness, enabling modulation of highly spin-polarized carriers in vdW semiconductors.
Van der Waals (vdW) ferromagnetic materials are rapidly establishing themselves as effective building blocks for next generation spintronic devices. When layered with non-magnetic vdW materials, such as graphene and/or topological insulators, vdW heterostructures can be assembled (with no requirement for lattice matching) to provide otherwise unattainable device structures and functionalities. We report a hitherto rarely seen antisymmetric magnetoresistance (MR) effect in van der Waals heterostructured Fe3GeTe2/graphite/Fe3GeTe2 devices. Unlike conventional giant magnetoresistance (GMR) which …
Spin-dependent transport in a full van der Waals (vdW) giant magnetoresistance (GMR) junctions with the structure of Fe3GeTe2/XTe2/Fe3GeTe2 (X = Pt, Pd) has been investigated by using first-principles calculations. The ballistic conductance, magnetoresistance (MR) and resistance-area product (RA) have been calculated in a current-perpendicular-to-plane (CPP) geometry. A giant magnetoresistance of around 2000% and RA less than 0.3 Ω μm2 have been found in the proposed vdW CPP GMR. In addition, the spin-orbit coupling effect on transport and anisotropy magnetoresistance (AMR) has also been inves…
We report the observation of a very large negative magnetoresistance effect in a van der Waals tunnel junction incorporating a thin magnetic semiconductor, CrI3, as the active layer. At constant voltage bias, current increases by nearly one million percent upon application of a 2 Tesla field. The effect arises from a change between antiparallel to parallel alignment of spins across the different CrI3 layers. Our results elucidate the nature of the magnetic state in ultrathin CrI3 and present new opportunities for spintronics based on two-dimensional materials.
The evolution of information technology has been driven by the discovery of new forms of large magnetoresistance (MR), such as giant magnetoresistance (GMR) and tunnelling magnetoresistance (TMR) in magnetic multilayers. Recently, new types of MR have been observed in much simpler bilayers consisting of ferromagnetic (FM)/nonmagnetic (NM) thin films; however, the magnitude of MR in these materials is very small (0.01 ~ 1%). Here, we demonstrate that NM/FM bilayers consisting of a NM InAs quantum well conductive channel and an insulating FM (Ga,Fe)Sb layer exhibit giant proximity magnetoresista…
Strain Tuning of Orbital-Driven Giant Magnetoresistance in van der Waals ferrimagnet Mn$_3$Si$_2$Te$_6$
Institute for Solid State Physics, The University of Tokyo（东京大学固体离子体研究所）；The University of Tokyo（东京大学）；Department of Physics and Astronomy, The University of Manchester（曼彻斯特大学物理与天文学系）；The University of Manchester（曼彻斯特大学）；Department of Physics, Institute of Science Tokyo（东京科学大学物理系）；Institute of Science Tokyo（东京科学大学）
Recently, van der Waals (vdW) magnetic heterostructures have received increasing research attention in spintronics. However, the lack of room-temperature magnetic order of vdW material has largely impedes its development in practical spintronics devices. Inspired by the recently discovered vdW ferromagnet Fe3GaTe2, which has been shown to have magnetic order above room temperature and sizable perpendicular magnetic anisotropy, we investigate the basic electronic structure and magnetic properties of Fe3GaTe2 as well as tunneling magnetoresistance effect in magnetic tunnel junctions (MTJs) with …
Charge transfer plays a key role at the interfaces of heterostructures, which can affect electronic structures and ultimately the physical properties of the materials. However, charge transfer is difficult to manipulate externally once the interface formed. Here, we report electrically tunable charge transfer in Fe3GeTe2/Cr2Ge2Te6/Fe3GeTe2 all-magnetic van der Waals heterostructures, which can be exploited to selectively modify the magnetic properties of the top or bottom Fe3GeTe2 electrodes. The directional charge transfer from metallic Fe3GeTe2 to semiconducting Cr2Ge2Te6 remarkably modifies…
We examined what interactions control the sign and strength of the interlayer coupling in van der Waals ferromagnets such as Fe3-xGeTe2, Cr2Ge2Te6, CrI3 and VI3, to find that high-spin orbital interaction across the van der Waals gaps are a key to understanding their ferromagnetism. Interlayer ferromagnetic coupling in Fe3-xGeTe2 Cr2Ge2Te6, and CrI3 is governed by the high-spin two-orbital two-electron destabilization, but that in VI3 by the high-spin four-orbital two-electron stabilization. These interactions explain a number of seemingly puzzling observations in van der Waals fer-romagnets.
We have synthesized unique colloidal nanoplatelets of the ferromagnetic two-dimensional (2D) van der Waals material CrI3 and have characterized these nanoplatelets structurally, magnetically, and by magnetic circular dichroism spectroscopy. The isolated CrI3 nanoplatelets have lateral dimensions of ~25 nm and ensemble thicknesses of only ~4 nm, corresponding to just a few CrI3 monolayers. Magnetic and magneto-optical measurements demonstrate robust 2D ferromagnetic ordering in these nanoplatelets with Curie temperatures similar to those observed in bulk CrI3, despite the strong spatial confine…
Ferromagnetism in two-dimensional materials presents a promising platform for the development of ultrathin spintronic devices with advanced functionalities. Recently discovered ferromagnetic van der Waals crystals such as CrI$_{3}$, readily isolated two-dimensional crystals, are highly tunable through external fields or structural modifications. However, there remains a challenge because of material instability under air exposure. Here, we report the observation of an air stable and layer dependent ferromagnetic (FM) van der Waals crystal, CrPS$_{4}$, using magneto-optic Kerr effect microscopy…
The thickness dependence of spin-orbit torque and magnetoresistance in ferromagnet/heavy-metal bilayers is studied using the first-principles non-equilibrium Green's function formalism combined with the Anderson disorder model. A systematic expansion in orthogonal vector spherical harmonics is used for the angular dependence of the torque. The damping-like torque in Co/Pt and Co/Au bilayers can be described as a sum of the spin-Hall contribution, which increases with thickness in agreement with the spin-diffusion model, and a comparable interfacial contribution. The magnetoconductance in the p…
Magnetic layered van der Waals crystals are an emerging class of materials giving access to new physical phenomena, as illustrated by the recent observation of 2D ferromagnetism in Cr2Ge2Te6 and CrI3. Of particular interest in semiconductors is the interplay between magnetism and transport, which has remained unexplored. Here we report first magneto-transport measurements on exfoliated CrI3 crystals. We find that tunneling conduction in the direction perpendicular to the crystalline planes exhibits a magnetoresistance as large as 10 000 %. The evolution of the magnetoresistance with magnetic f…
Magnetoresistance Effect in Spin-Polarized Junctions of Ferromagnetically Contacting Multiple Conductive Paths: Applications to Atomic Wires and Carbon Nanotubes
The anomalous Hall effect, observed in conducting ferromagnets with broken time-reversal symmetry, offers the possibility to couple spin and orbital degrees of freedom of electrons in ferromagnets. In addition to charge, the anomalous Hall effect also leads to spin accumulation at the surfaces perpendicular to both the current and magnetization direction. Here we experimentally demonstrate that the spin accumulation, subsequent spin backflow, and spin-charge conversion can give rise to a different type of spin current related magnetoresistance, dubbed here as the anomalous Hall magnetoresistan…
Magnomechanical Coupling in Suspended 2D van der Waals Ferromagnets
Kavli Institute of Nanoscience, Delft University of Technology（代尔夫特理工大学卡弗里纳米科学研究所）；Department of Precision and Microsystems Engineering, Delft University of Technology（代尔夫特理工大学精密与微系统工程系）
We demonstrate spin-valve magnetoresistance with a current-in-plane (CIP) configuration in (Ga,Fe)Sb / InAs (thickness $t_\mathrm{InAs}$ nm) / (Ga,Fe)Sb trilayer heterostructures, where (Ga,Fe)Sb is a ferromagnetic semiconductor (FMS) with high Curie temperature ($T_\mathrm{C}$). An MR curve with an open minor loop is clearly observed at 3.7 K in a sample with $t_\mathrm{InAs}$ = 3 nm, which originates from the parallel - antiparallel magnetization switching of the (Ga,Fe)Sb layers and spin-dependent scattering at the (Ga,Fe)Sb / InAs interfaces. The MR ratio increases (from 0.03 to 1.6%) with…
We report the observation of a unidirectional magnetoresistance (UMR) that originates from the nonequilibrium orbital momentum induced by an electric current in a naturally oxidized Cu/Co bilayer. The orbital-UMR scales with the torque efficiency due to the orbital Rashba-Edelstein effect upon changing the Co thickness and temperature, reflecting their common origin. We attribute the UMR to orbital-dependent electron scattering and orbital-to-spin conversion in the ferromagnetic layer. In contrast to the spin-current induced UMR, the magnon contribution to the orbital-UMR is absent in thin Co …
We develop a theory of the anisotropic magnetoresistance (AMR) effects of arbitrary directions of current and magnetization for ferromagnets. Here, we use the electron scattering theory with the $s$--$s$ and $s$--$d$ scattering processes, where $s$ is the conduction electron state and $d$ is the localized d states. The resistivity due to electron scattering is expressed by the probability density of the d states of the current direction. The d states are numerically obtained by applying the exact diagonalization method to the Hamiltonian of the d states with the exchange field, crystal field, …
We demonstrate that kinetics play an important role in the epitaxial growth of Fe$_3$GeTe$_2$ (FGT) van der Waals (vdW) ferromagnetic films by molecular beam epitaxy. By varying the deposition rate, we control the formation or suppression of an initial tellurium-deficient non-van der Waals phase (Fe$_3$Ge$_2$) prior to realizing epitaxial growth of the vdW FGT phase. Using cross-sectional scanning transmission electron microscopy and scanning tunneling microscopy, we optimize the FGT films to have atomically smooth surfaces and abrupt interfaces with the Ge(111) substrate. The magnetic propert…
Emergent color centers with accessible spins hosted by van der Waals materials have attracted substantial interest in recent years due to their significant potential for implementing transformative quantum sensing technologies. Hexagonal boron nitride (hBN) is naturally relevant in this context due to its remarkable ease of integration into devices consisting of low-dimensional materials. Taking advantage of boron vacancy spin defects in hBN, we report nanoscale quantum imaging of low-dimensional ferromagnetism sustained in Fe3GeTe2/hBN van der Waals heterostructures. Exploiting spin relaxomet…
The intersection of topology and magnetism represents a new playground to discover novel quantum phenomena and device concepts. In this work, we show that a van der Waals compound MnSb$_{1.8}$Bi$_{0.2}$Te$_4$ exhibits a ferromagnetic ground state with a Curie temperature of 26 K, in contrast to the antiferromagnetic order previously found for other members of the Mn(Sb, Bi)$_2$Te$_4$ family. We employ magneto-transport, bulk magnetization and neutron scattering studies to illustrate the magnetic and electrical properties of MnSb$_{1.8}$Bi$_{0.2}$Te$_4$ and report on the observation of an unusu…
The unidirectional magnetoresistance (UMR) is one of the most complex spin-dependent transport phenomena in ferromagnet/non-magnet bilayers, which involves spin injection and accumulation due to the spin Hall effect (SHE) or Rashba-Edelstein effect (REE), spin-dependent scattering, and magnon scattering at the interface or in the bulk of the ferromagnet. While UMR in metallic bilayers has been studied extensively in very recent years, its magnitude is as small as 10$^-$$^5$, which is too small for practical applications. Here, we demonstrate a giant UMR effect in a heterostructure of BiSb topo…
The recent emergence of two-dimensional (2D) van der Waals ferromagnets has provided a new platform for exploring magnetism in the flatland and for designing 2D ferromagnet-based spintronics devices. Despite intensive studies, the anomalous Hall effect (AHE) mechanisms in 2D van der Waals ferromagnets have not been investigated yet. In this paper, we report the AHE mechanisms in quasi-2D van der Waals ferromagnet Fe0.29TaS2 via systematically measuring Fe0.29TaS2 devices with thickness from 14 nm to bulk single crystal. The AHE mechanisms are investigated via the scaling relationship between t…
Material structures containing tetrahedral FeAs bonds, depending on their density and geometrical distribution, can host several competing quantum ground states ranging from superconductivity to ferromagnetism. Here we examine structures of quasi two-dimensional (2D) layers of tetrahedral Fe-As bonds embedded with a regular interval in a semiconductor InAs matrix, which resembles the crystal structure of Fe-based superconductors. Contrary to the case of Fe-based pnictides, these FeAs/InAs superlattices (SLs) exhibit ferromagnetism, whose Curie temperature (Tc) increases rapidly with decreasing…
Dependence of tunnel magnetoresistance on ferromagnetic electrode materials in MgO-barrier magnetic tunnel junctions
研究了MgO势垒磁性隧道结（MTJ）中隧道磁电阻（TMR）比与电极结构的关系。具有Co 40 Fe 40 B20参考层和自由层的MTJ中的TMR比率在Ta=400摄氏度的沉积后退火温度下达到355%。当Co 50 Fe 50或Co 90 Fe 10用于参考层材料时，没有观察到高TMR比。具有高TMR比的关键是具有高度取向的（001）MgO阻挡层/CoFeB晶体电极。到目前为止，在伪自旋阀MTJ中，在Ta = 450 ℃时获得的最高TMR比为450%。
