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This review is an effort in putting together the latest results about room-temperature magnetoresistive (MR) effects in nanoscale/single-molecule electronic devices consisting of one (few) molecule(s) placed in electrical contact between two nanoscale electrodes. Molecules represent powerful building blocks for developing state-of-the-art MR devices, as they bring long spin relaxation timescales, low cost and high tunability of their electrical and magnetic properties via chemical modifications. The capability to control at room temperature and under bespoke electrodes’ magnetization the MR response of a single-molecule (SM) device has been a longstanding quest. Such SM platforms could serve as fundamental tools to understand what the main mechanistic ingredients of MR effects in a molecular device are, leading to their use as building-blocks for miniaturization in spintronic applications. The work carried out so far in this field has identified two key components directly involved in the MR response of a single(few)-molecule(s) device: (i) The molecule|electrode spinterface, defining the interplay between interfacial electrostatics and spin density, has been proven to play a fundamental role in the interpretation of the observed single-molecule junction's MR effects, which is governed by the electrode material and the electrode-molecule chemistry. (ii) Two aspects of the molecular structure have been demonstrated to be involved in the spin-dependent conduction mechanism: (1) the presence of paramagnetic metal centres in the molecular structure and how their orbitals bearing the unpaired electrons couple with the device electrodes, and (2), the degree of chirality within the molecular wire. This contribution will focus on the above points (i-ii) by making use of specific examples in the literature.

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.

Spin‐torque transfer magnetoresistive random‐access memory (STT‐MRAM) is far more energy efficient than field MRAM. This chapter describes the memory operation and the performance of STT‐MRAM. It discusses energy barriers as a function of a magnetic tunnel junction (MTJ) film stack and device structure. The chapter focuses on the switching properties. The discussion starts from a simple uniform magnetization reversal model, called the Macrospin model, in which the magnetization of the entire MTJ free layer magnetization is assumed to precess in unison under spin current. Then, the chapter discusses two STT‐MRAM device reliability issues: tunnel barrier degradation and data retention. It covers an MgO tunnel barrier degradation model and the relation between the thermal energy barrier and the data retention time performance at the chip level. The chapter also discusses the 1 MTJ‐1 transistor MRAM cell design and scaling. It covers the SPICE model for memory chip‐level circuit simulation.

This chapter describes three promising new modes of magnetoresistive random‐access memory (MRAM) cells that are currently under research and development at the time of this manuscript preparation: current‐induced domain‐wall motion mode (CIDM), spin orbit torque (SOT) mode, precession‐toggle mode. All are based on magnetic tunnel junction (MTJ) technology, and all promise very fast access time. The MTJ is switched in a way similar but different from the pure spin‐transfer mechanism. Both CIDM memory cell and SOT memory cell are configured in 1M‐2T (1 MTJ, 2 transistors) cell topology. The precession‐toggle mode MRAM is based on the voltage‐control magnetic anisotropy effect. The chapter compares the properties of advanced switching mode MRAMs against field‐MRAM and spin‐transfer torque‐MRAM. The MRAM cell size has been predominantly determined by the size of the cell transistor and the number of connecting wire per bit cell and wire pitch, not the MTJ.

2D conductive metal-organic frameworks (2D c-MOFs) feature promising applications as chemiresistive sensors, electrode materials, electrocatalysts, and electronic devices. However, exploration of the spin-polarized transport in this emerging materials and development of the relevant spintronics have not yet been implemented. In this work, layer-by-layer assembly was applied to fabricate highly crystalline and oriented thin films of a 2D c-MOF, Cu3 (HHTP)2 , (HHTP: 2,3,6,7,10,11-hexahydroxytriphenylene), with tunable thicknesses on the La0.67 Sr0.33 MnO3 (LSMO) ferromagnetic electrode. The magnetoresistance (MR) of the LSMO/Cu3 (HHTP)2 /Co organic spin valves (OSVs) reaches up to 25 % at 10 K. The MR can be retained with good film thickness adaptability varied from 30 to 100 nm and also at high temperatures (up to 200 K). This work demonstrates the first potential applications of 2D c-MOFs in spintronics.

A relatively newer generation of magnetic field sensors has found application in broadband electric current measurement. They are based on the magnetoresistance (MR) effect, remain linear for strong magnetic field stimulus, and can operate at a wide band of frequencies. This chapter explains the study of the effect of sensor size, and then designs a measurement system with small MR sensor head. A single magnetic sensor can be employed for large current measurement when placed in the close vicinity of a conductor. However, the magnetic field is complex in modern substations. Higher sensitivity limits the application of MR sensors in environments with nearby magnetic noise sources. A novel method for a wide range of electric current measurement with tunnel magnetoresistance sensors has been proposed to reduce the effect from an adjacent current‐carrying conductor, which is verified by numerical simulations, finite element analysis of the field, and laboratory experiments in the presence of strong magnetic disturbance.

With the rapid development of magnetic field sensors, non‐contact monitoring by magnetic field measurement can provide an advanced sensing technology for the smart transmission and distribution network. Magnetic fields can store energy and act as the media for transferring energy. Magnetic field based energy storage/conversion may become one of the solutions for energy generation in smart grids. Magnetoresistance (MR) sensors have been applied to magnetic field measurement. Compared with Hall effect sensor, an MR sensor has higher sensitivity. Therefore, MR sensors can response to the speed change of the synchronous machine accurately and quickly. The operation status of the synchronous machine can be monitored by measuring the magnetic field of the synchronous machine. Magnetic field based measurement can directly detect the non‐symmetry of the structure, together with its non‐invasive character, hence will find more applications in future.

Van der Waals (vdW) materials are an indispensable part of functional device technology due to their versatile physical properties and ease of exfoliating to the low-dimensional limit. Among all the compounds investigated so far, the search for magnetic vdW materials has intensified in recent years, fueled by the realization of magnetism in two dimensions (2D). However, metallic magnetic vdW systems are still uncommon. In addition, they rarely host high-mobility charge carriers, which is an essential requirement for high-speed electronic applications. Another shortcoming of 2D magnets is that they are highly air sensitive. Using chemical reasoning, we introduce TaCo2Te2 as an air-stable, high-mobility, magnetic vdW material. It has a layered structure, which consists of Peierls distorted Co chains and a large vdW gap between the layers. We find that the bulk crystals can be easily exfoliated and the obtained thin flakes are robust to ambient conditions after four months of monitoring using an optical microscope. We also observe signatures of canted antiferromagntic behavior at low-temperature. TaCo2Te2 shows a metallic character and a large, non-saturating, anisotropic magnetoresistance. Furthermore, our Hall data and quantum oscillation measurements reveal the presence of both electron- and hole-type carriers and their high mobility.

Spin-resolved electron symmetry filtering is a key mechanism behind giant tunneling magnetoresistance (TMR) in Fe/MgO/Fe and similar magnetic tunnel junctions (MTJs), providing room temperature functionality in modern spin electronics. However, the core process of the electron symmetry filtering breaks down under applied bias, dramatically reducing the TMR above 0.5 V. This strongly hampers the application range of MTJs. To circumvent the problem, resonant tunneling between ferromagnetic electrodes through quantum well states in thin layers has been used so far. This mechanism, however, is mainly effective at low temperatures. Here, a fundamentally different approach is demonstrated, providing a strong TMR boost under applied bias in V/MgO/Fe/MgO/Fe/Co hybrids. This pathway uses spin orbit coupling (SOC) controlled interfacial states in vanadium, which contrary to the V(001) bulk states are allowed to tunnel to Fe(001) at low biases. The experimentally observed strong increase of TMR with bias is modelled using two nonlinear resistances in series, with the low bias conductance of the first (V/MgO/Fe) element being boosted by the SOC-controlled interfacial states, while the conductance of the second (Fe/MgO/Fe) junctions controlled by the relative alignment of the two ferromagnetic layers. These results pave a way to unexplored and fundamentally different spintronic device schemes, with tunneling magnetoresistance uplifted under applied electric bias.

The effect of thickness on distorted double perovskite structured BiFe0.5Cr0.5O3 (BFCO) thin films have been investigated for the first time using soft chemical method over Pt(111)/Ti/SiO2/Si substrate. The X-ray diffraction (XRD) shows the presence of both BFO and BCO phases and confirms the formation of double perovskite-structured BFCO. The morphology of the films changes with the increase in layers signifying the thickness dependence. The magnetization (M) versus magnetizing field (H) curve was well consistent with magnetoresistance (M-R) results and exhibits anisotropic magnetoresistance. Thickness-dependent dielectric behaviors of BFCO thin films were deeply understood from space charge, intrinsic and extrinsic mechanisms. The BiFe0.5Cr0.5O3 crystal systems with 60 nm thin layer showed an improved magnetism and magnetoresistance properties and are applicable to future magnetic application in the field of spintronics as a promising device.

The magnetic Weyl fermion originates from the time reversal symmetry (TRS)-breaking in magnetic crystalline structures, where the topology and magnetism entangle with each other. Therefore, the magnetic Weyl fermion is expected to be effectively tuned by the magnetic field and electrical field, which holds promise for future topologically protected electronics. However, the electrical field control of the magnetic Weyl fermion has rarely been reported, which is prevented by the limited number of identified magnetic Weyl solids. Here, the electric field control of the magnetic Weyl fermion is demonstrated in an epitaxial SrRuO3 (111) thin film. The magnetic Weyl fermion in the SrRuO3 films is indicated by the chiral anomaly induced magnetotransport, and is verified by the observed Weyl nodes in the electronic structures characterized by the angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. Through the ionic-liquid gating experiment, the effective manipulation of the Weyl fermion by electric field is demonstrated, in terms of the sign-change of the ordinary Hall effect, the nonmonotonic tuning of the anomalous Hall effect, and the observation of the linear magnetoresistance under proper gating voltages. The work may stimulate the searching and tuning of Weyl fermions in other magnetic materials, which are promising in energy-efficient electronics.

The discovery of magnetism in 2D materials offers new opportunities for exploring novel quantum states and developing spintronic devices. In this work, using field-effect transistors with solid ion conductors as the gate dielectric (SIC-FETs), we have observed a significant enhancement of ferromagnetism associated with magnetic easy-axis switching in few-layered Cr2 Ge2 Te6 . The easy axis of the magnetization, inferred from the anisotropic magnetoresistance, can be uniformly tuned from the out-of-plane direction to an in-plane direction by electric field in the few-layered Cr2 Ge2 Te6 . Additionally, the Curie temperature, obtained from both the Hall resistance and magnetoresistance measurements, increases from 65 to 180 K in the few-layered sample by electric gating. Moreover, the surface of the sample is fully exposed in the SIC-FET device configuration, making further heterostructure-engineering possible. This work offers an excellent platform for realizing electrically controlled quantum phenomena in a single device.

Layered iron chalcogenides (FeX, X = S, Se, Te) provide excellent platforms to study intertwined phase transitions, superconductivity, and magnetism. However, layered iron dichalcogenides (FeX2 , X = S, Se, Te) are rarely reported and their intrinsic properties are still unknown. Here, phase-pure layered iron diselenide (FeSe2 ) nanocrystals are epitaxially grown on mica by the sublimed-salt-assisted chemical vapor deposition method at atmospheric pressure. The layered atomic structure of FeSe2 is confirmed by X-ray diffraction and atomic-resolution scanning transmission electron microscopy. Electrical transport shows that the layered FeSe2 is a metal with high conductivity and a phase transition at ≈11 K. The phase transition manifests itself as a kink in the temperature-dependent resistivity, as well as anomalous magnetoresistance (MR) appearing around the phase-transition temperature. The MR changes from negative to positive, accompanied by large hysteresis near the phase-transition temperature upon cooling. The negative MR and hysteresis might originate from magnetic field suppression scattering of spin fluctuations and competition of magnetic interactions induced by the phase transition, respectively. Layered iron dichalcogenide will be potential candidate to explore novel quantum phenomena and other applications.

Spin‐torque memristors are proposed in 2009, and can provide fast, low‐power, and infinite memristive behavior for neuromorphic computing and large‐density non‐volatile memory. However, the strict requirements of combining high magnetoresistance, stable domain wall pinning and current‐induced switching in a single device pose difficulties in physical implementation. Here, a nanoscale spin‐torque memristor based on a perpendicular‐anisotropy magnetic tunnel junction with a CoFeB/W/CoFeB composite free layer structure is experimentally demonstrated. Its tunneling magnetoresistance is higher than 200%, and memristive behavior can be realized by spin‐transfer torque switching. Memristive states are retained by strong domain wall pinning effects in the free layer. Experiments and simulations suggest that nanoscale vertical chiral spin textures can form around clusters of W atoms under the combined effect of opposite Dzyaloshinskii–Moriya interactions and the Ruderman–Kittel–Kasuya–Yosida interaction between the two CoFeB free layers. Energy fluctuation caused by these textures may be the main reason for the strong pinning effect. With the experimentally demonstrated memristive behavior and spike‐timing‐dependent plasticity, a spiking neural network to perform handwritten pattern recognition in an unsupervised manner is simulated. Due to advantages such as long endurance and high speed, the spin‐torque memristors are competitive in the future applications for neuromorphic computing., Nanoscale spin memristors are realized based on magnetic tunnel junctions with large tunneling magnetoresistance ratio. Strong domain wall pinning effects in the free layer with an atom‐thick tungsten insertion is shown to be the origin of the memristive behavior. Spike‐timing‐dependent plasticity of the device is studied. A spiking neural network is built, and handwritten pattern recognition is numerically demonstrated.

A highly sensitive flexible magnetic sensor based on the anisotropic magnetoresistance effect is fabricated. A limit of detection of 150 nT is observed and excellent deformation stability is achieved after wrapping of the flexible sensor, with bending radii down to 5 mm. The flexible AMR sensor is used to read a magnetic pattern with a thickness of 10 μm that is formed by ferrite magnetic inks.

Electrical transport properties of the YBa2Cu3O7−δ + Sr2FeMoO6−δ superconductor–ferrimagnet composite as a function of the temperature and applied magnetic field are reported. Four different intervals in the temperature dependence of the resistivity have been found between room temperature and 4.2 K. In the temperature range from 300 K down to Tc1 a semiconducting behavior is observed, whereas below Tc1 the first transition into a superconducting state sets in. Upon further cooling down, the semiconducting behavior appears again below Tmin

Recently intensive efforts have been devoted to the emerging field of antiferromagnetic (AFM) spintronics, where ferromagnetic electrodes are substituted by antiferromagnets. This study investigates the anisotropic magnetoresistance (AMR) of epitaxial tetragonal antiferromagnetic bimetallic films: Mn2Au and Mn2Au/Fe bilayers. An anomalous AMR effect with additional peaks is observed. This study theoretically and experimentally demonstrates that the AFM spins of Mn2Au can be viewed and controlled at room temperature, and this is achievable with a notably relatively small magnetic field of 200 mT. Strong hybridization between Au and Mn, and strong modification of the intrinsic quadratic anisotropy of Mn2Au from interfacial biquadratic anisotropy result in an additional anomalous AMR component of 1%. The findings suggest that Mn2Au films can be used in room temperature antiferromagnetic spintronics.

We report the photoinduced ultrafast quasiparticle recombination dynamics of colossal magnetoresistance single crystal La1.2Sr1.8Mn2O7 by using femtosecond time-resolved reflection spectroscopy in a wide range of temperatures. Here, we find the transient reflectivity change Delta R exhibits a competition process of a fast negative component (similar to sub-picosecond) against a slow positive component (similar to nanosecond), as the sample goes from paramagnetic (PM) to ferromagnetic (FM) state with decreasing the temperature. The very fast initial decay of the photoinduced Delta R corresponds to the photoionization of the Jahn-Teller polaronic response, which is unambiguously sensitive to the coupling between electron and lattice in the PM and FM phases in La1.2Sr1.8Mn2O7 single crystal. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

One-dimensional (1D) transition metal oxide (TMO) nanostructures are actively pursued in spintronic devices owing to their nontrivial d electron magnetism and confined electron transport pathways. However, for TMOs, the realization of 1D structures with long-range magnetic order to achieve a sensitive magnetoelectric response near room temperature has been a longstanding challenge. Herein, we exploit a chemical hydric effect to regulate the spin structure of 1D V-V atomic chains in monoclinic VO2 nanowires. Hydrogen treatment introduced V(3+) (3d(2) ) ions into the 1D zigzag V-V chains, triggering the formation of ferromagnetically coupled V(3+) -V(4+) dimers to produce 1D superparamagnetic chains and achieve large room-temperature negative magnetoresistance (-23.9 %, 300 K, 0.5 T). This approach offers new opportunities to regulate the spin structure of 1D nanostructures to control the intrinsic magnetoelectric properties of spintronic materials.

The effect of electron doping on the magnetoelectric and magnetoresistivity properties of cation-substituted antiferromagnetic СexMn1–xS (x ≤ 0.05) semiconductors at temperatures of 77–500 K in magnetic fields of up to 10 kOe was investigated. For all the compositions shifts of the temperature of the dielectric loss maximum toward higher temperature (80–500 K) with increasing magnetic field up to 10 kOe are found. The current dependence of magnetoresistance from the I–V characteristic measured in a magnetic field was established. The increase of the magnetoresistive effect with increase of the cerium ions concentration in the СexMn1–xS is found. The type of current carriers was determined on the basis of the Hall coefficient measurement. For explanation of experimental data the model of charge-orbital ordering, including orbital glass is used.