Room-temperature Tunneling Magnetoresistance Research Articles

631% room temperature tunnel magnetoresistance with large oscillation effect in CoFe/MgO/CoFe(001) junctions

We demonstrate tunnel magnetoresistance (TMR) ratios of up to 631% at room temperature (RT) using CoFe/MgO/CoFe(001) epitaxial magnetic tunnel junctions (MTJs). The TMR ratio increased up to 1143% at 10 K. The large TMR ratios resulted from fine-tuning of atomic-scale structures of the MTJs, such as crystallographic orientations and MgO interface oxidation by interface insertion of ultrathin CoFe and Mg layers, which are expected to enhance the well-known Δ1 coherent tunneling transport. Interestingly, the TMR oscillation effect, which is not covered by the standard coherent tunneling theory, also became significant. A 0.32-nm period TMR oscillation with increasing MgO thickness dominates the transport in a wide range of MgO thicknesses; the peak-to-valley difference of the TMR oscillation exceeds 140% at RT, which is attributed to the appearance of large oscillatory components in the resistance area product.

Highly Tunable Beyond-Room-Temperature Intrinsic Ferromagnetism in Cr-Doped Topological Crystalline Insulator SnTe Crystals.

The combination of topological phase and intrinsic beyond-room-temperature ferromagnetism is expected to realize the quantum anomalous Hall effect at a high temperature. However, no beyond-room-temperature intrinsic ferromagnetism has been reported in either topological insulator or topological crystalline insulator (TCI) so far. Here, we report Cr-doping in TCI-phase SnTe crystals which possess highly tunable beyond-room-temperature intrinsic ferromagnetism including Tc, magnetic moment, and coercivity by varying Cr contents and crystal thickness. With the increase of the Cr content, the Tc increases by 159 K from 221 to 380 K and the saturation magnetic moments increase by ∼23.6 times from 0.018 to 0.421 μB/f.u. This intrinsic beyond-room-temperature ferromagnetism is fully demonstrated by the anomalous Hall effect and magneto-optical Kerr effect in a single CrxSn1-xTe nanosheet. Moreover, the room-temperature tunneling magnetoresistance effect has been realized by using a CrxSn1-xTe flake, a Fe thin film, and a commercially compatible ultrathin AlOx tunneling barrier. This work indicates a great potential of CrxSn1-xTe crystals in room-temperature magnetoelectronic and spintronic devices.

Boosting Room Temperature Tunnel Magnetoresistance in Hybrid Magnetic Tunnel Junctions Under Electric Bias

AbstractSpin‐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 spin electronics. However, 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 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 modeled 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 are controlled by the relative alignment of the ferromagnetic layers. These results pave a way to unexplored and fundamentally different spintronic device schemes, with tunneling magnetoresistance uplifted under applied electric bias.

Room-temperature tunnel magnetoresistance across biomolecular tunnel junctions based on ferritin

We report exceptionally large tunnel magnetoresistance (TMR) for biomolecular tunnel junctions based on ferritins immobilized between Ni and EGaIn electrodes. Ferritin stores iron in the form of ferrihydrite nanoparticles (NPs) and fulfills the following roles: (a) it dictates the tunnel barrier, (b) it magnetically decouples the NPs from the ferromagnetic (FM) electrode, (c) it stabilizes the NPs, and (d) it acts as a spin filter reducing the complexity of the tunnel junctions since only one FM electrode is required. The mechanism of charge transport is long-range tunneling which results in TMR of 60 ± 10% at 200 K and 25 ± 5% at room temperature. We propose a magnon-assisted transmission to explain the substantially larger TMR switching fields (up to 1 Tesla) than the characteristic coercive fields (a few Gauss) of ferritin ferrihydrite particles at T < 20 K. These results highlight the genuine potential of biomolecular tunnel junctions in designing functional nanoscale spintronic devices.

Structural and magnetotransport properties of ultrathin Co/ZnO and Co/ZnAlO films

We report the structural and magnetotransport properties of ultrathin superparamagnetic Co/ZnO and Co/ZnAlO films deposited by sequentially sputtering Co layers and semiconductor layers. The films consisting of Co nanodots embedded in an amorphous semiconductor matrix exhibit large room temperature tunneling magnetoresistance with a maximum value of over 7%. The single-layer-nanodot structures gradually develop in the films with thicknesses below 8 nm, where tunneling processes existing only between nanodots that lie in a plane are realized. The tunneling magnetoresistance ratio at room temperature is as high as 5% although the thickness of the Co/ZnAlO film is as thin as 3.2 nm. These single-layer-nanodot films having high tunneling magnetoresistance ratios and superparamagnetic behavior can be used in transparent nano-granular in gap sensors.

Growth of Sr2FeMoO6 Based Tri-layer Structure for Room Temperature Magnetoresistive Applications

Tunnel magnetoresistance in Sr2FeMoO6 based tri-layered structure was studied at room temperature. The Sr2FeMoO6/SrTiO3/Sr2FeMoO6 tri-layered structure was grown by pulse laser deposition on STO buffered Si(100) substrate. The X-ray diffraction and Micro Raman studies confirm the polycrystalline phase formation of SFMO thin films without any impurity phases. The single layer SFMO thin film shows the good ferromagnetic behavior with saturation magnetization of ∼1.48 μB/ f.u. at room temperature. The high value of tunneling magnetoresistance of ∼7% of tri-layer structure at room temperature was attributed to spin dependent tunneling through uniform STO barrier layer. The room temperature tunneling magnetoresistance in SFMO tri-layer structure open the future prospect for their possible integration to room temperature spintronic devices.

Flexible spintronic devices on Kapton

Magnetic tunnel junctions and nano-sized domain-wall conduits have been fabricated on the flexible substrate Kapton. Despite the delicate nature of tunneling barriers and zig-zag shaped nanowires, the devices show an outstanding integrity and robustness upon mechanical bending. High values of bending angle (r = 5 mm) have been achieved without degradation of the device performance, reaching room-temperature tunneling magnetoresistance ratios of 12% in bended Co/Al2O3/NiFe junctions. In addition, a suitable route to pattern high-quality nanostructures directly on the polyimide surface is established. These results demonstrate that Kapton is a promising platform for low-cost, flexible spintronic applications involving tunnel junction elements and nanostructurization.

Effect of Seed Layer on Room Temperature Tunnel Magnetoresistance of MgO Barriers Formed by Radical Oxidation in IrMn-Based Magnetic Tunnel Junctions

NiFe-seeded magnetic tunnel junctions (MTJs) of IrMn/CoFe/MgO/CoFeB were successfully formed by radically oxidizing a thin Mg layer. Room temperature (RT) tunnel magnetoresistance (TMR) of up to 211±10% was obtained and found to be strongly dependent on the thickness of the NiFe seed layer. High resolution transmission microscopy (HRTEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and magneto-optic Kerr effect (MOKE) analyses performed on NiFe/IrMn bilayer systems revealed that the IrMn(111)-fcc texture, grain size, surface roughness (rms), and exchange-biasing field (Hex) were strongly affected by the thickness of the NiFe seed layer. A critical NiFe thickness (tc ≈12 Å) was found: For tNiFe≤tc, the IrMn showed a very poor (111)-fcc texture with reduced grain size, very smooth surface, and reduced Hex. For tNiFe > tc, the IrMn showed a complete opposite behavior: much enhanced (111)-fcc texture with larger grain size, rougher surface, and larger Hex. For MTJ-based IrMn systems, a striking behavior is reported: larger TMRs and lower tunnel junction resistance (RA) products are obtained for tNiFe ≤tc while lower TMRs and larger RAs are obtained for tNiFe > tc.

Effect of Seed Layer on Room Temperature Tunnel Magnetoresistance of MgO Barriers Formed by Radical Oxidation in IrMn-Based Magnetic Tunnel Junctions

NiFe-seeded magnetic tunnel junctions (MTJs) of IrMn/CoFe/MgO/CoFeB were successfully formed by radically oxidizing a thin Mg layer. Room temperature (RT) tunnel magnetoresistance (TMR) of up to 211±10% was obtained and found to be strongly dependent on the thickness of the NiFe seed layer. High resolution transmission microscopy (HRTEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and magneto-optic Kerr effect (MOKE) analyses performed on NiFe/IrMn bilayer systems revealed that the IrMn(111)-fcc texture, grain size, surface roughness (rms), and exchange-biasing field (Hex) were strongly affected by the thickness of the NiFe seed layer. A critical NiFe thickness (tc ≈12 Å) was found: For tNiFe≤tc, the IrMn showed a very poor (111)-fcc texture with reduced grain size, very smooth surface, and reduced Hex. For tNiFe > tc, the IrMn showed a complete opposite behavior: much enhanced (111)-fcc texture with larger grain size, rougher surface, and larger Hex. For MTJ-based IrMn systems, a striking behavior is reported: larger TMRs and lower tunnel junction resistance (RA) products are obtained for tNiFe ≤tc while lower TMRs and larger RAs are obtained for tNiFe > tc.

Morphological and magnetic properties of TiO2/Fe50Co50 composite films

Nanocomposites of FeCo and TiO2 with wide range of metal volume fractions (MVFs) were prepared by co-sputtering. High resolution transmission electron microscopy analysis reveals that the microstructure of the nanocomposites depends on the MVF which determines the particle size and separation. FeCo nanoparticles are amorphous at lower MVF whereas crystallites are present at higher MVF. Likewise, the magnetic characteristics of these films depend on the MVF. At low MVF, composite films exhibit superparamagnetism whereas at high MVF, coalescence of crystalline nanoparticles results in the opening up of hysteresis loop. These composite films show a considerable room temperature tunnel magnetoresistance and being proportional to the square of the normalized magnetization (M/Ms)2.

Atomic-scale spectroscopic imaging of CoFeB/Mg–B–O/CoFeB magnetic tunnel junctions

Atomic-scale electron spectroscopic imaging on sputtered magnetic tunnel junctions (MTJs) with a thin, <2 nm, MgO layer and B-alloyed electrodes reveals B diffusion into the MgO, resulting in a Mg–B–O tunnel barrier. This ∼2 nm thick interfacial layer forms due to oxidation of CoFeB during radio frequency sputtering of MgO and subsequent B diffusion into MgO during annealing. We measure a room-temperature tunneling magnetoresistance (TMR) of ∼200% in IrMn/CoFeB/Mg–B–O/CoFeB MTJs after annealing, demonstrating that thin Mg–B–O barriers can produce relatively high TMR.

Bulrush-Like Double Perovskite: Synthesis, Tunneling Magnetoresistance, and Magnetocaloric Effects

The recent observation of room temperature tunneling magnetoresistance (TMR) in half-metallic A2FeMoO6 (A = Ca, Sr, Ba) double perovskites, and their importance to the emerging field of spintronics has led to considerable effort being dedicated to detailed investigations of the physical and chemical properties of these materials. This article will present an review of our recent investigations covering the synthesis, structures, magnetic and transport properties of "bulrush-like" A2FeMoO6 (A = Sr, Ba). Utilizing the high shape anisotropy as well as the reactivity of A2FeMoO6 to water and a sonochemical technique, we managed to manipulate the properties of grain boundary barriers, and thus put forward a new approach for the enhancement of room temperature TMR. The magnetocaloric effects of A2FeMoO6 double perovskites will also be discussed.

Spin-polarized tunneling with Au impurity layers

We have inserted nonmagnetic impurity layers of Au into sputtered AlOx-based magnetic tunnel junctions (F/I/F) and Meservey–Tedrow junctions (S/I/F) in order to study their effect on the tunneling magnetoresistance (TMR) and spin polarization (TSP). Both room temperature TMR and the TSP at 250mK decay exponentially as an interfacial Au layer is introduced between the barrier and one Co electrode, with 1∕e decay lengths λTMR=11±3A and λTSP=14±2A. We also inserted a 1A thick Au layer at a variable distance from the barrier/Co interface and find that both the TMR and TSP recover to the undoped value with the shorter exponential length scales of λTMR=7±4A and λTSP=6±2A.

Large Room-Temperature Tunneling Magnetoresistance of “Bulrush-Like” Double Perovskite Ba$_{2}$FeMoO$_{6}$

We conducted magnetic and transport measurements on two Ba2 FeMoO6 double perovskite samples prepared by wet chemistry and solid-state reaction methods, respectively. The one synthesized by the latter is in the form of isotropic grains. The bulrush-like Ba2FeMoO6 sample fabricated by wet chemistry is high in aspect ratio and Fe/Mo ordering. Utilizing the high shape anisotropy of the sample, we obtained a material with electric anisotropy where the long axis of the sample is almost parallel to the plane of the pellet. With carrier transport perpendicular to the plane, there was an appreciable increase in room-temperature tunneling magnetoresistance (TMR) response (from 22% to 31% at 10 kOe) while the concentration of Fe/Mo disorder remained unchanged. Experimental results indicate that the TMR value could be greatly enhanced by adjusting the nature and amount of grain boundary (GB) insulating barriers

Room-Temperature Tunnel Magnetoresistance and Spin-Polarized Tunneling through an Organic Semiconductor Barrier

Electron spin-polarized tunneling is observed through an ultrathin layer of the molecular organic semiconductor tris(8-hydroxyquinolinato)aluminum (Alq3). Significant tunnel magnetoresistance (TMR) was measured in a Co/Al2O3/Alq3/NiFe magnetic tunnel junction at room temperature, which increased when cooled to low temperatures. Tunneling characteristics, such as the current-voltage behavior and temperature and bias dependence of the TMR, show the good quality of the organic tunnel barrier. Spin polarization (P) of the tunnel current through the Alq3 layer, directly measured using superconducting Al as the spin detector, shows that minimizing formation of an interfacial dipole layer between the metal electrode and organic barrier significantly improves spin transport.

Origin of large magnetocurrent in three-terminal double-barrier magnetic tunnel junctions

Double-barrier magnetic tunnel junctions (DBMTJs) of composition Co∕AlOx∕Co∕AlOx∕Ni81Fe19 have been fabricated by magnetron sputtering through shadow masks. Two terminal measurements made upon the individual tunnel barriers revealed nonlinear I–V curves and significant room-temperature tunnel magnetoresistance (TMR) in all cases. Measurements were also performed with connections made to all three electrodes. The TMR of a particular tunnel barrier within the DBMTJ can be strongly modified by applying a bias voltage to the other barrier, while the TMR measured across the two barriers in series decreases more slowly with increasing bias voltage than for a single barrier. With zero bias applied between the central Co base electrode and the Co collector electrode, the collector current was measured as electrons were injected from the Ni81Fe19 electrode. For structures grown on Si∕SiO2 substrates, the collector current showed a nonmonotonic dependence upon the emitter-base bias voltage, and collector magnetocurrent values in excess of 100% were observed at nonzero emitter-base bias values. For structures grown on quartz the collector current increased while the magnetocurrent decreased with increasing emitter-base voltage. We suggest that the enhanced TMR and magnetocurrent effects can be explained by substrate leakage and geometrical artifacts rather than by transport of spin-polarized hot electrons across the base layer.

Study of the oxygen migration versus anneal in Co/AlOx/Fe–FeOy/Ti tunnel junctions

The thermal stability of junctions of the type Co 40Å/Al 9Å+plasma oxidation/Fe 25Å+plasma oxidation/Ti 30Å has been studied by X-ray photoelectron spectroscopy. Results suggest that the enhancement of the thermal stability of the room-temperature tunneling magnetoresistance (TMR) of magnetic tunnel junctions (MTJs) with the main structure of Ni80Fe20 70Å/Co80Fe20 30Å/Al 9Å+plasma oxidation/Fe (tFe)+plasma oxidation/Co80Fe20 40Å/Mn76(Fe,Ir)24 250Å (tFe=0–25Å), up to annealing temperatures of 380°C (TMR=39%), is related to the oxygen diffusion from the inserted Fe+FeOy layer to the AlOx barrier. This may yield metallic Fe at the interface with the top-pinned CoFe electrode, and some ferrimagnetic, half-metallic Fe3O4 with strong polarization at the top of the AlOx barrier, enhancing TMR. Oxygen diffusion from the initial FeOy layer to the barrier causes the over-oxidation of the latter, further enhancing barrier quality and TMR. This is relevant for the application of MTJs to magnetic random access memories.

Bias Voltage Dependence of Tunneling Magnetoresistance and Annealing Effect in Spin Dependent Tunnel Junctions

Junctions with Al2O3 and AIN barriers were fabricated through contact shadow masks (0.25mm2-size) and by a self-aligned lithography process (9×2μm2-size). The Al2O3 barrier was formed by plasma oxidation. The AlN barrier was prepared by dc reactive magnetron sputtering. Room temperature tunneling magnetoresistance (TMR) is 1.6% for the AIN junctions and up to 24% for the Al2O3 junctions. The TMR for all junctions decreases with increase of applied bias voltage and drops to half its initial value at a bias voltage between 116 mV and 437 mV. The weakest bias voltage dependence occurs for junctions with higher barrier heights, and thinner barrier thickness. Annealing at 100-200 °C leads to a 20% increase of TMR and a 40% decrease of junction resistance for the 9×2μm2 junctions, leading to a maximum TMR of 27%. Both TMR and junction resistance were increased for the AlN junctions due to annealing.