Tunneling Anisotropic Magnetoresistance Research Articles

Electrical-Controllable Antiferromagnet-Based Tunnel Junction.

An electrical-controllable antiferromagnet tunnel junction is a key goal in spintronics, holding immense promise for ultradense and ultrastable antiferromagnetic memory with high processing speed for modern information technology. Here, we have advanced toward this goal by achieving an electrical-controllable antiferromagnet-based tunnel junction of Pt/Co/Pt/Co/IrMn/MgO/Pt. The exchange coupling between antiferromagnetic IrMn and Co/Pt perpendicular magnetic multilayers results in the formation of an interfacial exchange bias and exchange spring in IrMn. Encoding information states "0" and "1" is realized through the exchange spring in IrMn, which can be electrically written by spin-orbit torque switching with high cyclability and electrically read by antiferromagnetic tunneling anisotropic magnetoresistance. Combining spin-orbit torque switching of both exchange spring and exchange bias, a 16 Boolean logic operation is successfully demonstrated. With both memory and logic functionalities integrated into our electrically controllable antiferromagnetic-based tunnel junction, we chart the course toward high-performance antiferromagnetic logic-in-memory.

Proposal for All-Electrical Skyrmion Detection in van der Waals Tunnel Junctions.

A major challenge for magnetic skyrmions in atomically thin van der Waals (vdW) materials is reliable skyrmion detection. Here, based on rigorous first-principles calculations, we show that all-electrical skyrmion detection is feasible in two-dimensional vdW magnets via scanning tunneling microscopy (STM) and in planar tunnel junctions. We use the nonequilibrium Green's function method for quantum transport in planar junctions, including self-energy due to electrodes and working conditions, going beyond the standard Tersoff-Hamann approximation. We obtain a very large tunneling anisotropic magnetoresistance (TAMR) around the Fermi energy for a graphite/Fe3GeTe2/germanene/graphite vdW tunnel junction. For atomic-scale skyrmions, the noncollinear magnetoresistance (NCMR) reaches giant values. We trace the origin of the NCMR to spin mixing between spin-up and -down states of pz and dz2 character at the surface atoms. Both TAMR and NCMR are drastically enhanced in tunnel junctions with respect to STM geometry due to orbital symmetry matching at the interface.

Tunnel anisotropic magnetoresistance in magnetic tunnel junctions using FeAlSi

We fabricated magnetic tunnel junctions (MTJs) with FeAlSi free layers and investigated the tunnel magnetoresistance (TMR) properties at low temperature. We observed TMR ratio increase with temperature decrease, and confirmed the TMR ratio of 179.9% at 10 K. The conductance dependence on bias voltage was measured, and a clear peak at low bias voltage similar to Fe/MgO/Fe MTJs was observed. This behavior can be explained by considering the majority band of Fe(001), and Δ5 electrons tunneling in FeAlSi/MgO/CoFeB MTJs at low bias voltage. We also investigated tunnel anisotropic magneto-resistance (TAMR) and clearly observed a TAMR peak similar to Fe/MgO/Fe MTJs, where the TAMR ratio of FeAlSi/MgO/CoFeB MTJs was 1.0% comparable to 1.1% in Fe/MgO/Fe MTJs. We concluded that electron tunneling was caused by the interfacial resonance states originating from spin orbit coupling in the FeAlSi/MgO/CoFeB MTJs.

Tunable range of terahertz oscillations triggered by the spin Hall effect in a biaxial antiferromagnet

We present a theoretical study on the current and frequency windows of self-oscillations induced by the dampinglike spin-orbit torque in a biaxial antiferromagnet. By the linear stability analysis and averaging technique, we analytically formulate the lower and upper thresholds and the frequency of self-oscillations. We find that the self-oscillation range is highly sensitive to the damping and magnetic anisotropies, which have a variety of options in abundant antiferromagnets. Beyond a critical damping, the self-oscillation can arise after the instability of an antiferromagnetic state, with its range widening for a heavier damping. Below it, a spin-flip transition occurs when increasing the current, similar to the spin flip under an increasing magnetic field. Meanwhile, we examine the role of anisotropies. With the spin polarization along the easy axis, the weak easy- and hard-axis anisotropies allow self-oscillations, the range of which is broadened for weaker anisotropies. At the same time, the spin-flip transition is permitted for strong anisotropies. If the spin polarization coincides with the hard axis, the transition types are mainly determined by the easy-axis anisotropy. For a weak easy-axis anisotropy, the self-oscillation can develop with its range expanding for a stronger hard-axis anisotropy. Finally, we envision that (tunneling) anisotropic magnetoresistance may act as an effective means to probe the oscillations.

Tunneling anisotropic magnetoresistance in MgO-based magnetic tunnel junctions induced by spin-orbit coupling

We performed a first-principles study of the tunneling anisotropic magneto-resistance (TAMR) in Ag(Ir,Pt)$/$MgO$/$Fe junctions. Enhanced TAMR with ideal and skewed fourfold angular dependence is found in-plane and out-of-plane TAMR of the system, respectively, which shows simple barrier thickness dependency with number around 10\% in some junctions. The complex angular dependency of the interfacial resonant states due to the spin-orbit coupling should be responsible to the complex and enhanced TAMR found in these junctions.

Bias-dependent tunneling anisotropic magnetoresistance in antiferromagnetic Pd-doped FeRh-based junctions

We report the bias-dependent tunneling anisotropic magnetoresistance (TAMR) in antiferromagnetic α′-Fe(Rh0.98Pd0.02)/MgO/γ-Fe(Rh0.98Pd0.02) junctions. The TAMR effect is driven by the antiferromagnetic-ferromagnetic phase transition of α′-Fe(Rh0.98Pd0.02) and concomitantly large variation of the density of states (DOS) near the Fermi level. It exhibits polarity reversion behavior with increasing bias voltage, i.e., negative and positive polarities for low and high bias voltages, respectively. Such bias-dependent TAMR is comprehended by first-principle calculations, where a crossing point and subsequent magnitude-reversion emerge between the DOS of antiferromagnetic and ferromagnetic phases of α′-Fe(Rh0.98Pd0.02). Harnessing the tunneling behavior by a feasible bias voltage in an antiferromagnet-based junction is a frontier of great promise in antiferromagnet spintronics.

Single-molecule anisotropic magnetoresistance at room temperature: Influence of molecular structure

Organic molecules featuring weak spin-orbit and hyperfine interactions show promise in the construction of effective spintronic devices. However, the impact of molecular structures on anisotropic magnetoresistance at the single-molecule level remains to be explored. In this work, we report systematic investigations on the construction and magnetoresistance of spintronic single molecule junctions of aliphatic dicarboxylic acids HOOC−(CH2)n−COOH (n=1−4) and conjugated benzenedicarboxylic acids HOOC-(C6H4)n-COOH (n=1-3) binding to Fe electrodes by employing electrochemically assisted jump-to-contact STM break junction technique (JTC-STM-BJ) under a constant of external magnetic field. Results show that single-molecule tunneling anisotropic magnetoresistance (AMR) of alkanedicarboxylic acids is independent of the molecular length and can reach as large as 44% (average), which is about twice that of benzenedicarboxylic acids. The almost same constant βN value but different contact conductance Gc with the applied magnetic field indicate that AMR arise from interface effect and spin injection. Besides Br substituent on the benzene ring can almost double the AMR by changing the spatial distribution of molecular charge density and frontier orbital characteristics. The present work shows that molecular structures play important roles in electron transport through single-molecule spintronic junctions.

Quantum-well tunneling anisotropic magnetoresistance above room temperature

Quantum-well (QW) devices have been extensively investigated in semiconductor structures. More recently, spin-polarized QWs were integrated into magnetic tunnel junctions (MTJs). In this work, we demonstrate the spin-based control of the quantized states in iron $3d$-band QWs, as observed in experiments and theoretical calculations. We find that the magnetization rotation in the Fe QWs significantly shifts the QW quantization levels, which modulate the resonant-tunneling current in MTJs, resulting in a tunneling anisotropic magnetoresistance (TAMR) effect of QWs. This QW-TAMR effect is sizable compared to other types of TAMR effect, and it is present above the room-temperature. In a QW MTJ of Cr/Fe/MgAl$_2$O$_4$/top electrode, where the QW is formed by a mismatch between Cr and Fe in the $d$ band with $\Delta_1$ symmetry, a QW-TAMR ratio of up to 5.4 % was observed at 5 K, which persisted to 1.2 % even at 380K. The magnetic control of QW transport can open new applications for spin-coupled optoelectronic devices, ultra-thin sensors, and memories.

Resonant tunneling anisotropic magnetoresistance induced by magnetic proximity

We reveal that the interplay between Rashba spin-orbit coupling and proximity-induced magnetization in a two-dimensional electron gas leads to peculiar transport properties and large anisotropy of magnetoresistance. While the related tunneling anisotropic magnetoresistance (TAMR) has been extensively studied before, we predict an effect with a different origin arising from the evolution of a resonant condition with the in-plane rotation of magnetization and having a much larger magnitude. The resonances in the tunneling emerge from a spin-parity-time symmetry of the scattering states. However, such a symmetry is generally absent from the system itself and only appears for certain parameter values. Without resonant behavior in the topological surface states of a proximitized three-dimensional topological insulator (TI), TAMR measurements can readily distinguish them from often misinterpreted trivial Rashba-like states inherent to many TIs.

Experimental observation of large tunneling anisotropic magnetoresistance in a magnetic tunnel junction without heavy metals

Tunneling anisotropic magnetoresistance (TAMR) in the magnetic tunnel junctions (MTJs) driven by spin–orbit coupling (SOC) has attracted much attention for potential spintronic applications based on magnetic manipulation of electric transport. However, it has been believed that heavy metals are indispensable for the large TAMR. Here we experimentally show a TAMR of up to 46% in a La0.7Sr0.3MnO3 (LSMO) based MTJ without heavy metals. This value represents the largest TAMR for MTJs with half-metallic electrodes. We demonstrate that the TAMR ratio is enhanced by over one order of magnitude by tuning interface termination layer, achieving quasilocalized states at the interface Fermi level on the basis of magnetization orientation. These results are also supported by our first-principles density functional calculations. This finding illustrates a crucial role of interface tuning in the TAMR effect, opening a route for engineering the large anisotropic magnetoresistance by interface termination control and manipulating high spin polarization without using heavy metals.

Electronic and magnetic properties of 3d transition-metal adatoms on Mn/W(110)

Using density functional theory, we investigate the electronic and magnetic properties of $3d$ transition-metal adatoms adsorbed on a monolayer of Mn on W(110). Mn/W(110) has a noncollinear cycloidal spin-spiral ground state with an angle of 173$^\circ$ between magnetic moments of adjacent Mn rows. It allows to rotate the spin orientation of an adsorbed magnetic adatom quasi-continuously. Therefore, this surface is ideally suited for manipulating the spin direction of individual atoms and exploring their magnetic properties using scanning tunneling microscopy (STM). The adsorbed V and Cr transition-metal adatoms couple antiferromagnetically to the nearest neighbor Mn atom of Mn monolayer while Mn, Fe, Co, and Ni couple ferromagnetically. The magnetic moments of the $3d$ adatoms are large and show a Hund's rule type of trend with a peak in the middle of the series. We find large spin splitting of the $3d$ transition-metal adatoms, large spin polarization of the local vacuum density of states up to 73\% at the Fermi energy, and significant tunneling anisotropic magnetoresistance enhancement up to 27\%. We conclude that such large values stem from the strong hybridization between the adatoms and the Mn atoms of the monolayer. Furthermore, identification of spin orientations of the adatom using spin-polarized STM is only possible for Co and V adatoms.

Terahertz frequency spectrum analysis with a nanoscale antiferromagnetic tunnel junction

A method to perform spectrum analysis on low power signals between 0.1 and 10 THz is proposed. It utilizes a nanoscale antiferromagnetic tunnel junction (ATJ) that produces an oscillating tunneling anisotropic magnetoresistance, whose frequency is dependent on the magnitude of an evanescent spin current. It is first shown that the ATJ oscillation frequency can be tuned linearly with time. Then, it is shown that the ATJ output is highly dependent on matching conditions that are highly dependent on the dimensions of the dielectric tunneling barrier. Spectrum analysis can be performed by using an appropriately designed ATJ, whose frequency is driven to increase linearly with time, a low pass filter, and a matched filter. This method of THz spectrum analysis, if realized in the experiment, will allow miniaturized electronics to rapidly analyze low power signals with a simple algorithm. It is also found by simulation and analytical theories that for an ATJ with a 0.09μm2 footprint, spectrum analysis can be performed over a 0.25THz bandwidth in just 25 ns on signals that are at the Johnson–Nyquist thermal noise floor.

Tunneling anisotropic magnetoresistance of Pb and Bi adatoms and dimers on Mn/W(110): A first-principles study

We show that Pb and Bi adatoms and dimers have a large tunneling anisotropic magnetoresistance (TAMR) of up to 60% when adsorbed on a magnetic transition-metal surface due to strong spin-orbit coupling and the hybridization of 6p orbitals with 3d states of the magnetic layer. Using density functional theory, we have explored the TAMR effect of Pb and Bi adatoms and dimers adsorbed on a Mn monolayer on W(110). This surface exhibits a noncollinear cycloidal spin spiral ground state with an angle of 173$^\circ$ between neighboring spins which allows to rotate the spin quantization axis of an adatom or dimer quasi-continuously and is ideally suited to explore the angular dependence of TAMR using scanning tunneling microscopy (STM). We find that the induced magnetic moments of Pb and Bi adatoms and dimers are small, however, the spin-polarization of the local density of states (LDOS) is still very large. The TAMR obtained from the anisotropy of the vacuum LDOS is up to 50-60 % for adatoms. For dimers the TAMR depends sensitively on the dimer orientation with respect to the crystallographic directions of the surface due to the formation of bonds between the adatoms with the Mn surface atoms and the symmetry of the spin-orbit coupling induced mixing. Dimers oriented along the spin spiral direction of the Mn monolayer display the largest TAMR of 60 % which is due to hybrid 6p-3d states of the dimers and the Mn layer

Tunneling Anisotropic Magnetoresistance in Ferroelectric Tunnel Junctions

Using a simple quantum-mechanical model, we explore a tunneling anisotropic magnetoresistance (TAMR) effect in ferroelectric tunnel junctions (FTJs) with a ferromagnetic electrode and a ferroelectric barrier layer, which spontaneous polarization gives rise to the Rashba and Dresselhaus spin-orbit coupling (SOC). For realistic parameters of the model, we predict sizable TAMR measurable experimentally. For asymmetric FTJs, which electrodes have different work functions, the built-in electric field affects the SOC parameters and leads to TAMR dependent on ferroelectric polarization direction. The SOC change with polarization switching affects tunneling conductance, revealing a new mechanism of tunneling electroresistance (TER). These results demonstrate new functionalities of FTJs which can be explored experimentally and used in electronic devices.

Large nonvolatile control of interfacial magnetic anisotropy in CoPt by a ferroelectric ZnO-based tunneling barrier

The electric control of magnetic anisotropy has important applications for nonvolatile memory and information processing. By first-principles calculations, we show a large nonvolatile control of magnetic anisotropy in ferromagnetic/ferroelectric CoPt/ZnO interface. Using the switched electric polarization of ZnO, the density-of-states and magnetic anisotropy at the CoPt surface show a large change. Due to a strong Co/Pt orbitals hybridization and a large spin-orbit coupling, a large control of magnetic anisotropy was found. We experimentally measured the change of effective anisotropy by tunneling resistance measurements in CoPt/Mg-doped ZnO/Co junctions. Additionally, we corroborate the origin of the control of magnetic anisotropy by observations on tunneling anisotropic magnetoresistance.

Quantum simulation of ferromagnetic and anti-ferromagnetic tunneling anisotropic magnetoresistance in a single-molecule-magnet dimer tunnel-junction

In this work, we simulate the tunneling anisotropic magnetoresistance (TAMR) in a single-molecule-magnet (SMM) dimer tunnel-junction with metal and ferromagnetic (FM) electrodes. The non-collinear polarization of electrode with respect to the uniaxial anisotropy-axis of magnet results in both the FM and anti-ferromagnetic (AFM) TAMR respectively for the FM and AFM inter-molecule couplings. In terms of the spin coherent state representation of electron spin the non-collinear tunneling is able to be analyzed with the usual rate equation approach in a sequential tunneling regime. The ferromagnetic TAMR varies with the non-collinear angle and the tunneling magnetoresistance (TMR) is just a special case of the angle θ = π. With the FM dimer we obtain the higher TMR up to 400% and the high polarization rate (79%) of spin current as well. The angle dependence of TAMR for the AFM dimer is also presented along with the spin current.

A piezoelectric, strain-controlled antiferromagnetic memory insensitive to magnetic fields.

Spintronic devices based on antiferromagnetic (AFM) materials hold the promise of fast switching speeds and robustness against magnetic fields1-3. Different device concepts have been predicted4,5 and experimentally demonstrated, such as low-temperature AFM tunnel junctions that operate as spin-valves6, or room-temperature AFM memory, for which either thermal heating in combination with magnetic fields7 or Néel spin-orbit torque8 is used for the information writing process. On the other hand, piezoelectric materials were employed to control magnetism by electric fields in multiferroic heterostructures9-12, which suppresses Joule heating caused by switching currents and may enable low-energy-consuming electronic devices. Here, we combine the two material classes to explore changes in the resistance of the high-Néel-temperature antiferromagnet MnPt induced by piezoelectric strain. We find two non-volatile resistance states at room temperature and zero electric field that are stable in magnetic fields up to 60 T. Furthermore, the strain-induced resistance switching process is insensitive to magnetic fields. Integration in a tunnel junction can further amplify the electroresistance. The tunnelling anisotropic magnetoresistance reaches ~11.2% at room temperature. Overall, we demonstrate a piezoelectric, strain-controlled AFM memory that is fully operational in strong magnetic fields and has the potential for low-energy and high-density memory applications.

Tunneling Anisotropic Magnetoresistance in L10-MnGa Based Antiferromagnetic Perpendicular Tunnel Junction**Supported by the National Basic Research Program of China under Grant No 2015CB921500, the National Natural Science Foundation of China under Grant Nos 61334006 and 11774339, the Key Research Project of Frontier Science of the Chinese Academy of Sciences under Grant No QYZDY-SSW-JSC015, and the Key Research Program of the Chinese

We report on the tunneling anisotropic magnetoresistance in antiferromagnetic perpendicular tunnel junction consisting of L10-MnGa/FeMn/AlOx/Pt grown on GaAs (001) substrates by molecular-beam epitaxy. The temperature-dependent perpendicular exchange bias effect reveals an exchange coupling between ferromagnetic L10-MnGa and antiferromagnetic FeMn. The rotation of antiferromagnetic spins in FeMn can be driven by perpendicularly magnetized L10-MnGa due to the exchange-spring effect at the interface and leads to room-temperature tunneling anisotropic magnetoresistance ratio of 0.86%. We also find that the tunneling anisotropic magnetoresistance strongly depends on temperature and angle. These results have broadened the material selection range for high performance antiferromagnetic spintronic devices.

Large tunneling anisotropic magnetoresistance mediated by surface states

We investigated the tunneling anisotropic magnetoresistance (TAMR) in thick hcp Co films at cryogenic temperatures using scanning tunneling microscopy. At around -350 mV, a strong TAMR up to 30\% is found with a characteristic voltage dependence and a reversal of sign. With the help of \textit{ab initio} calculations the TAMR can be traced back to a spin-polarized occupied surface states that experience a strong spin-orbit interaction leading to a magnetization direction depending hybridization with bulk states.

Tunneling anisotropic magnetoresistance in fully epitaxial magnetic tunnel junctions with different barriers

We report the tunneling anisotropic magnetoresistance (TAMR) in fully epitaxial Fe/Barrier/Fe (001) magnetic tunnel junctions (MTJs) where the Barrier is annealed MgO, MgAlOx, MgO-MgAlOx, or as-grown MgO/MgAlOx. The TAMR was measured as the magnetization of Fe electrodes rotated from in-plane to out-of-plane. The angular dependence of TAMR for all samples exhibited superposed behavior of twofold and fourfold symmetries. The proportion of fourfold symmetry is larger in MTJs with MgO and MgO-MgAlOx than that in MTJs with MgAlOx and MgO/MgAlOx barriers. By characterizing inelastic electron tunneling spectroscopy in the antiparallel state and parallel conductance of the MTJs, we revealed diverse minority interfacial resonant states (IRSs) and different contributions from Δ1 and Δ5 symmetry states to the conductance in the MTJs. Our results illustrate that the minority IRS dominated by Δ5 symmetry can mix with majority Δ1 states and give rise to the enhanced fourfold symmetric angular dependence in MTJs with MgO and MgO-MgAlOx barriers.