Magnetized Magnetic Tunnel Junction Research Articles

Ar+ ion irradiation of magnetic tunnel junction multilayers: impact on the magnetic and electrical properties

The impact of 400 keV Ar+ irradiation on the magnetic and electrical properties of in-plane magnetized magnetic tunnel junction (MTJ) stacks was investigated by ferromagnetic resonance, vibrating sample magnetometry and current-in-plane tunneling techniques. The ion fluences ranged from to . Below , the anisotropy of the Ta-capped FeCoB free layer was weakly modulated, following a decrease in the saturation magnetization. The tunnel magnetoresistance (TMR), along with the exchange-bias and the interlayer exchange coupling providing a stable magnetic configuration to the reference layer, decreased continuously. Above , a strong decrease in the saturation magnetization was accompanied by a loss of the magnetic coupling and of the TMR. We show there is an ion-fluence window where the modulation of magnetic anisotropy can occur while preserving a large TMR and stable magnetic configuration of the MTJ, and demonstrate that the layers surrounding the free layer play a decisive role in determining the trend of the magnetic anisotropy modulation resulting from the irradiation. Our results provide guidance for the tailoring of MTJ parameters via ion irradiation, which we propose as a potentially suitable technique for setting the magnetic easy-cone state in MTJ for attaining field-free, fast, and non-stochastic magnetization switching.

Field-free switching of magnetic tunnel junctions driven by spin–orbit torques at sub-ns timescales

We report time-resolved measurements of magnetization switching by spin–orbit torques in the absence of an external magnetic field in perpendicularly magnetized magnetic tunnel junctions (MTJs). Field-free switching is enabled by the dipolar field of an in-plane magnetized layer integrated above the MTJ stack, the orientation of which determines the switching polarity. Real-time single-shot measurements provide direct evidence of magnetization reversal and switching distributions. Close to the critical switching voltage, we observe stochastic reversal events due to a finite incubation delay preceding the magnetization reversal. Upon increasing the pulse amplitude to twice the critical voltage, the reversal becomes quasi-deterministic, leading to reliable bipolar switching at sub-ns timescales in zero external field. We further investigate the switching probability as a function of dc bias of the MTJ and external magnetic field, providing insight into the parameters that determine the critical switching voltage.

A Spin–Orbit‐Torque Memristive Device

AbstractMemristors, demonstrated by solid‐state devices with continuously tunable resistance, have emerged as a new paradigm for self‐adaptive networks that require synapse‐like functions (artificial synapse, for example). Spin‐based memristors offer advantages over other types of memristors because of their significant endurance and high energy efficiency. Yet it remains a challenge to build dense and functional spintronic memristors with structures and materials that are compatible with existing ferromagnetic devices. Here, a memristive device based upon Ta/CoFeB/MgO heterostructures is demonstrated, which are commonly used in out‐of‐plane magnetized magnetic tunnel junctions (MTJ). To achieve the memristive function, a domain wall (DW) is driven back and forth in a continuous manner in the CoFeB layer by applying in‐plane positive or negative current pulses along the Ta layer, utilizing the spin–orbit torque (SOT) that the current exerts on the CoFeB magnetization. Hence, the magnetization and consequently the anomalous Hall effect (AHE) resistance are modulated in an analog manner, being controlled by the pulsed current characteristics including amplitude, duration, and repetition number. The quasi‐continuous AHE resistance variation is explained by the SOT‐induced DW creep motion. These results pave the way for developing SOT‐based energy‐efficient neuromorphic systems.

Electric-field assisted spin torque nano-oscillator and binary frequency shift keying modulation

Electric-controlled magnetization precession introduces technologically relevant possibility for developing spin torque nano-oscillators (STNO) with potential applications in microwave emission. Using the perpendicularly magnetized magnetic tunnel junction (MTJ), we show that the magnetization oscillation frequency can be tuned by the co-action of electric field and spin polarized current. The dynamical phase diagram of MTJ-based STNO is analytically predicted through coordinate transformation from the laboratory frame to the rotation frame, by which the nonstationary out-of-plane magnetization precession process is therefore transformed into the stationary process in the rotation frame. Furthermore, using this STNO as a microwave source, we numerically demonstrate that the bit signal can be transmitted by a binary frequency shift keying (BFSK) modulation technique. The BFSK scheme shows good modulation features with no transient state.

Effect of Geometrical Modulation on pMTJ Magnetization Reversal

Spin-transfer torque-magnetic random access memory (STT-MRAM) is one of the most prominent spintronic devices for replacing conventional electronic memory devices due to its extraordinary performance as it delivers fast operational speed, non-volatility, and a high-memory density. At advanced technology nodes, manufacturing process would invariably lead to bit on bit variation of STT-MRAM cell at wafer level. Here, we show that in a perpendicularly magnetized magnetic tunnel junction (MTJ), the in-plane demagnetizing tensor greatly influences the magnetization switching process of the free layer (FL). The geometrical asymmetricity was introduced to induce a varying in-plane demagnetizing field. This impacts the stability of the MTJ and requires a larger anisotropy to sustain an out-of-plane magnetization state. Interestingly, a faster switching of the FL comprising the MTJ is observed. This resultant rapid switching is ascribed to the nucleation of a reversal embryo at the defect.

Magnetoresistance and Structure of MgO-Based Magnetic Tunnel Junctions with Perpendicularly Magnetized L10-FePt

Perpendicularly magnetized magnetic tunnel junctions (MTJs) were fabricated by depositing thin L10-ordered FePt films on MgO(001) substrates using a UHV sputtering system, and the dependence of structural, magnetic, and magnetotransport properties of the junctions on the thickness of the FePt layers was investigated. A full epitaxial structure of MgO(001) sub./Cr/Pt/FePt/MgO/CoPt/Ta was observed. The tunnel magnetoresistance (TMR) ratio was measured to be 6% at room temperature, and magnetization switching was clearly observed in the thin FePt layer. Transmission electron microscopy (TEM) observations revealed that the interface between FePt, MgO, and CoPt layer has strain due to lattice mismatch, which might be a reason for the low TMR ratio.

Structural, magnetic, and magnetotransport properties of FePt/MgO/CoPt perpendicularly magnetized tunnel junctions

Perpendicularly magnetized magnetic tunnel junctions (MTJs) were fabricated by depositing thin L10-ordered FePt films on MgO(001) substrates using a UHV sputtering system, and the dependence of structural, magnetic, magnetotransport properties of the junctions on the thickness of the FePt layers was investigated. A full epitaxial structure was observed when the thickness of the L10-ordered FePt film was 4 nm. The tunnel magnetoresistance (TMR) ratio was measured to be 6% at room temperature, and magnetization switching was clearly observed in the thin FePt layer.