Antiferromagnetic Tunnel Junction Research Articles

Magnetic Switching Dynamics and Tunnel Magnetoresistance Effect Based on Spin-Splitting Noncollinear Antiferromagnet Mn3Pt

In comparison to ferromagnets, antiferromagnets are believed to have superior advantages for applications in next-generation magnetic storage devices, including fast spin dynamics, vanishing stray fields and robust against external magnetic field, etc. However, unlike ferromagnetic orders, which could be detected through tunneling magnetoresistance effect in magnetic tunnel junctions, the antiferromagnetic order (i.e., Néel vector) cannot be effectively detected by the similar mechanism due to the spin degeneracy of conventional antiferromagnets. Recently discovered spin-splitting noncollinear antiferromagnets, such as Mn3Pt with momentum-dependent spin polarization due to their special magnetic space group, make it possible to achieve remarkable tunneling magnetoresistance effects in noncollinear antiferromagnetic tunnel junctions. Through first-principles calculations, we demonstrate that the tunneling magnetoresistance ratio can reach more than 800% in Mn3Pt/perovskite oxides/Mn3Pt antiferromagnetic tunnel junctions. We also reveal the switching dynamics of Mn3Pt thin film under magnetic fields using atomistic spin dynamic simulation. Our study provides a reliable method for detecting Néel vector of noncollinear antiferromagnets through the tunnel magnetoresistance effect and may pave its way for potential applications in antiferromagnetic memory devices.

Electrically Controlled All-Antiferromagnetic Tunnel Junctions on Silicon with Large Room-Temperature Magnetoresistance.

Antiferromagnetic (AFM) materials are a pathway to spintronic memory and computing devices with unprecedented speed, energy efficiency, and bit density. Realizing this potential requires AFM devices with simultaneous electrical writing and reading of information, which are also compatible with established silicon-based manufacturing. Recent experiments have shown tunneling magnetoresistance (TMR) readout in epitaxial AFM tunnel junctions. However, these TMR structures are not grown using a silicon-compatible deposition process, and controlling their AFM order required external magnetic fields. Here are shown three-terminal AFM tunnel junctions based on the noncollinear antiferromagnet PtMn3, sputter-deposited on silicon. The devices simultaneously exhibit electrical switching using electric currents, and electrical readout by a large room-temperature TMR effect. First-principles calculations explain the TMR in terms of the momentum-resolved spin-dependent tunneling conduction in tunnel junctions with noncollinear AFM electrodes.

Néel Spin Currents in Antiferromagnets.

Ferromagnets are known to support spin-polarized currents that control various spin-dependent transport phenomena useful for spintronics. On the contrary, fully compensated antiferromagnets are expected to support only globally spin-neutral currents. Here, we demonstrate that these globally spin-neutral currents can represent the Néel spin currents, i.e., staggered spin currents flowing through different magnetic sublattices. The Néel spin currents emerge in antiferromagnets with strong intrasublattice coupling (hopping) and drive the spin-dependent transport phenomena such as tunneling magnetoresistance (TMR) and spin-transfer torque (STT) in antiferromagnetic tunnel junctions (AFMTJs). Using RuO_{2} and Fe_{4}GeTe_{2} as representative antiferromagnets, we predict that the Néel spin currents with a strong staggered spin polarization produce a sizable fieldlike STT capable of the deterministic switching of the Néel vector in the associated AFMTJs. Our work uncovers the previously unexplored potential of fully compensated antiferromagnets and paves a new route to realize the efficient writing and reading of information for antiferromagnetic spintronics.

Octupole-driven magnetoresistance in an antiferromagnetic tunnel junction

The tunnelling electric current passing through a magnetic tunnel junction (MTJ) is strongly dependent on the relative orientation of magnetizations in ferromagnetic electrodes sandwiching an insulating barrier, rendering efficient readout of spintronics devices1–5. Thus, tunnelling magnetoresistance (TMR) is considered to be proportional to spin polarization at the interface1 and, to date, has been studied primarily in ferromagnets. Here we report observation of TMR in an all-antiferromagnetic tunnel junction consisting of Mn3Sn/MgO/Mn3Sn (ref. 6). We measured a TMR ratio of around 2% at room temperature, which arises between the parallel and antiparallel configurations of the cluster magnetic octupoles in the chiral antiferromagnetic state. Moreover, we carried out measurements using a Fe/MgO/Mn3Sn MTJ and show that the sign and direction of anisotropic longitudinal spin-polarized current in the antiferromagnet7 can be controlled by octupole direction. Strikingly, the TMR ratio (about 2%) of the all-antiferromagnetic MTJ is much larger than that estimated using the observed spin polarization. Theoretically, we found that the chiral antiferromagnetic MTJ may produce a substantially large TMR ratio as a result of the time-reversal, symmetry-breaking polarization characteristic of cluster magnetic octupoles. Our work lays the foundation for the development of ultrafast and efficient spintronic devices using antiferromagnets8–10.

Spin-neutral tunneling anomalous Hall effect

Anomalous Hall effect (AHE) is a fundamental spin-dependent transport property that is widely used in spintronics. It is generally expected that currents carrying net spin polarization are required to drive the AHE. Here we demonstrate that, in contrast to this common expectation, a spin-neutral tunneling AHE (TAHE), i.e. a TAHE driven by spin-neutral currents, can be realized in an antiferromagnetic (AFM) tunnel junction where an AFM electrode with a non-spin-degenerate Fermi surface and a normal metal electrode are separated by a non-magnetic barrier with strong spin-orbit coupling (SOC). The symmetry mismatch between the AFM electrode and the SOC barrier results in an asymmetric spin-dependent momentum filtering of the spin-neutral longitudinal current generating the transverse Hall current in each electrode. We predict a sizable spin-neutral TAHE in an AFM tunnel junction with a RuO$_{2}$-type AFM electrode and a SnTe-type SOC barrier and show that the Hall currents are reversible by the N\'eel vector switching. With the Hall angle being comparable to that in conventional AHE bulk materials, the predicted spin-neutral TAHE can be used for the N\'eel vector detection in antiferromagnetic spintronics.

Tunneling Magnetoresistance in Noncollinear Antiferromagnetic Tunnel Junctions.

Antiferromagnetic (AFM) spintronics has emerged as a subfield of spintronics driven by the advantages of antiferromagnets producing no stray fields and exhibiting ultrafast magnetization dynamics. The efficient method to detect an AFM order parameter, known as the Néel vector, by electric means is critical to realize concepts of AFM spintronics. Here, we demonstrate that noncollinear AFM metals, such as Mn_{3}Sn, exhibit a momentum dependent spin polarization which can be exploited in AFM tunnel junctions to detect the Néel vector. Using first-principles calculations, we predict a tunneling magnetoresistance (TMR) effect as high as 300% in AFM tunnel junctions with Mn_{3}Sn electrodes, where the junction resistance depends on the relative orientation of their Néel vectors and exhibits four nonvolatile resistance states. We argue that the spin-split band structure and the related TMR effect can also be realized in other noncollinear AFM metals like Mn_{3}Ge, Mn_{3}Ga, Mn_{3}Pt, and Mn_{3}GaN. Our work provides a robust method for detecting the Néel vector in noncollinear antiferromagnets via the TMR effect, which may be useful for their application in AFM spintronic devices.

Spin-neutral currents for spintronics

Electric currents carrying a net spin polarization are widely used in spintronics, whereas globally spin-neutral currents are expected to play no role in spin-dependent phenomena. Here we show that, in contrast to this common expectation, spin-independent conductance in compensated antiferromagnets and normal metals can be efficiently exploited in spintronics, provided their magnetic space group symmetry supports a non-spin-degenerate Fermi surface. Due to their momentum-dependent spin polarization, such antiferromagnets can be used as active elements in antiferromagnetic tunnel junctions (AFMTJs) and produce a giant tunneling magnetoresistance (TMR) effect. Using RuO2 as a representative compensated antiferromagnet exhibiting spin-independent conductance along the [001] direction but a non-spin-degenerate Fermi surface, we design a RuO2/TiO2/RuO2 (001) AFMTJ, where a globally spin-neutral charge current is controlled by the relative orientation of the Néel vectors of the two RuO2 electrodes, resulting in the TMR effect as large as ~500%. These results are expanded to normal metals which can be used as a counter electrode in AFMTJs with a single antiferromagnetic layer or other elements in spintronic devices. Our work uncovers an unexplored potential of the materials with no global spin polarization for utilizing them in spintronics.

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.

Electric-Field-Controlled Antiferromagnetic Spintronic Devices.

In recent years, the field of antiferromagnetic spintronics has been substantially advanced. Electric-field control is a promising approach for achieving ultralow power spintronic devices via suppressing Joule heating. Here, cutting-edge research, including electric-field modulation of antiferromagnetic spintronic devices using strain, ionic liquids, dielectric materials, and electrochemical ionic migration, is comprehensively reviewed. Various emergent topics such as the Néel spin-orbit torque, chiral spintronics, topological antiferromagnetic spintronics, anisotropic magnetoresistance, memory devices, 2D magnetism, and magneto-ionic modulation with respect to antiferromagnets are examined. In conclusion, the possibility of realizing high-quality room-temperature antiferromagnetic tunnel junctions, antiferromagnetic spin logic devices, and artificial antiferromagnetic neurons is highlighted. It is expected that this work provides an appropriate and forward-looking perspective that will promote the rapid development of this field.

Large Magnetoresistance in an Electric-Field-Controlled Antiferromagnetic Tunnel Junction

Large magnetoresistance effect controlled by electric field rather than magnetic field or electric current is a preferable routine for designing low power consumption magnetoresistance-based spintronic devices. Here we propose an electric-field controlled antiferromagnetic (AFM) tunnel junction with structure of piezoelectric substrate/Mn3Pt/SrTiO3/Pt operating by the magnetic phase transition (MPT) of antiferromagnet Mn3Pt through its magneto-volume effect. The transport properties of the proposed AFM tunnel junction have been investigated by employing first-principles calculations. Our results show that a magnetoresistance over hundreds of percent is achievable when Mn3Pt undergoes MPT from a collinear AFM state to a non-collinear AFM state. Band structure analysis based on density functional calculations shows that the large TMR can be attributed to the joint effect of significant different Fermi surface of Mn3Pt at two AFM phases and the band symmetry filtering effect of the SrTiO3 tunnel barrier. In addition, other than single-crystalline tunnel barrier, we also discuss the robustness of the proposed magnetoresistance effect by considering amorphous AlOx barrier. Our results may open perspective way for effectively electrical writing and reading of the AFM state and its application in energy efficient magnetic memory devices.

Simulation of Strain-Assisted Switching in Synthetic Antiferromagnetic Free Layer-Based Magnetic Tunnel Junction

We perform macrospin simulations on a magnetic tunnel junction structure with perpendicular magnetic anisotropy, in which the conventional free layer is substituted with synthetic antiferromagnetic free layers to improve the thermal stability. In order to reduce the switching current Pb(Zr0.2Ti0.8)O3 material with (001) surface orientation is adopted, which is adjacent to the second free layer so that it applies biaxial stress to the interface under external voltage. We solve the Landau–Lifshitz–Gilbert–Slonczewski equation incorporating thermal and strain fields to describe the transient dynamics of magnetizations in both the free layers. The simulation results show a substantial reduction in switching current and significantly enhanced switching probability by modulating current and strain pulses.

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.

Resonant magnetoresistance in double-barrier antiferromagnetic tunnel junction

Resonant magnetoresistance in double-barrier antiferromagnetic tunnel junction

Terahertz-Frequency Signal Source Based on an Antiferromagnetic Tunnel Junction

We propose a novel type of terahertz-frequency (TF) signal source based on an antiferromagnetic tunnel junction (ATJ), where the generated ac signal is extracted through the variations of the tunneling anisotropic magnetoresistance (TAMR) of an ATJ. The signal source comprises a layered structure consisting of a current-driven platinum (Pt) layer and a layer of an antiferromagnet (AFM) separated by an MgO spacer from an additional Pt electrode. A dc electric current flowing in the first Pt layer due to the spin Hall effect creates a perpendicular spin current that, being injected in the AFM layer, tilts the magnetizations of the AFM sublattices and, therefore, causes TF rotation of these magnetizations in a large internal exchange magnetic field of the AFM. This rotation, through the TAMR effect, causes the TF variation of the total resistance of the layered structure. We evaluate the output power and efficiency of the above-described Pt/AFM/MgO/Pt TF signal source, and demonstrate that optimization of the source's geometrical parameters allows one to obtain the output power exceeding 1 $\mu$ W at the frequency of 0.5 THz with the efficiency not smaller than 1%. Although the efficiency of the TAMR method of the TF signal extraction is reduced with the increase of the generation frequency, we believe that this method could be used for the generation of signals with frequencies of up to 10 THz due to its relative simplicity and convenience.

Robust spin transfer torque in antiferromagnetic tunnel junctions

We theoretically study the current-induced spin torque in antiferromagnetic tunnel junctions, composed of two semi-infinite antiferromagnetic layers separated by a tunnel barrier, in both clean and disordered regimes. We find that the torque enabling the electrical manipulation of the N\'eel antiferromagnetic order parameter is out of plane $\sim {\bf n}\times{\bf p}$, while the torque competing with the antiferromagnetic exchange is in-plane $\sim {\bf n}\times({\bf p}\times{\bf n})$. Here, ${\bf { p}}$ and ${\bf { n}}$ are the N\'eel order parameter direction of the reference and free layers, respectively. Their bias dependence shows similar behavior as in ferromagnetic tunnel junctions, the in-plane torque being mostly linear in bias while the out-of-plane torque is quadratic. Most importantly, we find that the spin transfer torque in antiferromagnetic tunnel junctions is much more robust against disorder than in antiferromagnetic metallic spin-valves due to the tunneling nature of spin transport.

Magnetic and magnetotransport behavior ofRFe5Al7(R=GdandDy): Observation of reentrant inverse-magnetocaloric phenomenon and asymmetric magnetoresistance behavior

We have compared and contrasted magnetic, magnetocaloric, and magnetoresistive properties of Gd and Dy members of the rare-earth $(R)$ series $R\mathrm{F}{\mathrm{e}}_{5}\mathrm{A}{\mathrm{l}}_{7}$, crystallizing in $\mathrm{ThM}{\mathrm{n}}_{12}$ structure, known to order antiferromagnetically. Among other observations, we would like to emphasize the following findings: (i) There are multiple sign crossovers in the temperature $(T)$ dependence of isothermal entropy change $(\mathrm{\ensuremath{\Delta}}S)$ in the case of Dy compound. In addition to nil $\mathrm{\ensuremath{\Delta}}S$ at the magnetic compensation point known for two-magnetic-sublattice systems, there is an additional sign crossover at low temperatures, as though there is a ``reentrant inverse-magnetocaloric'' phenomenon. Corresponding sign reversals could also be observed in the magnetoresistance data. (ii) The plots of magnetoresistance versus magnetic field are found to be highly asymmetric with the reversal of the direction of magnetic field $(H)$ well below ${T}_{N}$ for both compounds, similar to those known for antiferromagnetic tunnel junctions. We attribute these to subtle changes in spin orientations of $R$ and Fe moments induced by $T$ and $H$.

Spin-dependent transport in antiferromagnetic tunnel junctions

We investigate the behaviour of spin transfer torque (STT) and tunnelling magnetoresistance (TMR) in epitaxial antiferromagnetic-based tunnel junctions using tight binding calculations in the framework of the Keldysh formalism. We find that the STT out-of-plane component exhibits a staggered spatial distribution similar to its in-plane component. This behaviour is specific to the use of a tunnel barrier and significantly differs from the out-of-plane torques reported in previous works using a metallic spacer. Additionally, we show that unlike conventional ferromagnetic-based tunnel junctions, the TMR can increase with applied bias and reach values comparable to typical magnetoresistances found for usual spin valves.

Rotation of the Pinned Direction in Artificial Antiferromagnetic Tunnel Junctions by Field Annealing

The effect of field annealing on the magnetic tunnel junctions (MTJs) with an artificial antiferromagnetic layer is investigated. Samples, which were grown under the same condition, are annealed with low (200-Oe) and high (1-kOe) magnetic fields. The rotation of the pinned direction is observed for the sample annealed at high magnetic field without decrease of tunneling magnetoresistance by varying the applied field direction. This result provides a method to adjust the pinned direction of the MTJ, which may be critical in switching a magnetic RAM (MRAM) cell.

Artificial antiferromagnetic tunnel junction sensors based on Co/Ru/Co sandwiches

A novel method is used for pinning the magnetization of the magnetically hard subsystem in micron-size magnetic tunnel junctions: the so-called artificial antiferromagnetic structure. The latter uses the strong antiparallel exchange coupling between two Co layers through a Ru spacer layer to ensure a high rigidity of the hard subsystem magnetization. The tunnel barriers were formed by sputter etching previously deposited Al layers in a rf Ar/O2 plasma. Wafers, 3 in. in diameter, were patterned into arrays of square junctions with lateral sizes of 20 and 50 μm. All junctions of a given size show resistances reproducible within several percents. The tunnel magnetoresistance (TMR) is found to be independent of the junction size and TMR ratios of 14%–16% are achieved at room temperature.