The interfacial oxidation level and thermodynamic properties of the MgO-based perpendicular magnetic tunneling junctions are investigated. The symmetry-conserved tunneling effect depends sensitively on the MgO adatom energy during the RF sputtering, as well as the thermal stability of the structure during the post-growth thermal annealing. Two different failure modes of the magnetoresistance are highlighted, involving with the decay of perpendicular magnetic anisotropy and destruction of coherent tunneling channels, respectively. Through the careful control of interfacial oxidation level and proper selection of the heavy metal layers, both perpendicular magnetic anisotropy and tunneling magnetoresistance of the junctions can be increased.

Tetherless sensors have long been positioned to enable next generation applications in biomedical, environmental, and industrial sectors. The main challenge in enabling these advancements is the realization of a device that is compact, robust over time, and highly efficient. This paper presents a tetherless optical tag which utilizes optical energy harvesting to realize scalable self-powered devices. Unlike previous demonstrations of optically coupled sensor nodes, the device presented here amplifies signals and encodes data on the same optical beam that provides its power. This optical interrogation modality results in a highly efficient data link. These optical tags support data rates up to 10 Mb/s with an energy consumption of ~ 3 pJ/bit. As a proof-of-concept application, the optical tag is combined with a spintronic microwave detector based on a magnetic tunnel junction (MTJ). We used this hybrid opto-spintronic system to perform self-powered transduction of RF waves at 1 GHz to optical frequencies at ~ 200 THz, while carrying an audio signal across (see Supplementary Data for audio files).

Antiferromagnets are magnetically ordered materials without a macroscopic magnetization. As a result, they could be of use in the development of memory devices because data cannot be erased by external magnetic fields. However, this also makes it difficult to electrically control their magnetic order (Néel vector). Here, we show that pillars of antiferromagnetic PtMn, which are grown on a heavy-metal layer and have diameters down to 800 nm, can be reversibly switched between different magnetic states by electric currents. The devices are based on materials that are typically used in the magnetic memory industry, and we observe switching down to a current density of ~2 MA cm −2 . Furthermore, by varying the amplitude of the writing current, multilevel memory characteristics can be achieved. Micromagnetic simulations suggest that the different magnetic states may consist of domains separated by domain walls with vortex and anti-vortex textures that move in response to current, modifying the average Néel vector.

In this work, we investigate the current-induced switching in micrometer-scale circular memory bits based on the metallic antiferromagnet PtMn, which is already widely used as part of the pinned layer in in-plane magnetic tunnel junctions manufactured on CMOS. The device shows reversible switching in response to currents applied to the Pt layer, with opposite current polarities achieving opposite switching directions in the PtMn. The switching current density is ~2 MA/cm2. We show that the switching process is essentially unaffected by external fields up to 16 T, and is robust over a wide temperature range. We also investigate the switching process by micromagnetic simulations, which shed light on the current-controlled domain structure of the device and the role of different torque terms in the switching process. Our results pave the way towards practical antiferromagnetic memories integrated on silicon.

We propose a method for switching the Neel vector of an antiferromagnetic thin film by the application of an ultrashort electric field pulse. The electric field induces a reorientation of the antiferromagnetic order parameter due to the voltage-induced modification of the magnetic anisotropy. When the electric field pulse is timed to half the oscillation period of the terahertz antiferromagnetic dynamics, it induces a picosecond-timescale reversal of the Neel vector. Importantly, the electric field required to induce this reversal is as small as approximately 100 mV/nm, comparable to fields used for switching of ferromagnetic tunnel junctions in earlier works. This electric field is determined by the anisotropy of the antiferromagnet, while the much larger exchange field determines the frequency of the resulting dynamics (and hence the switching time). Our results indicate the possibility to switch a 50-nm circular antiferromagnetic element with an energy dissipation of 250 aJ in less than 30 ps and in the absence of any current-induced torque. The electric-field-induced switching of the Neel vector opens an alternative route toward energy-efficient and ultrafast magnetic memories and computing devices based on antiferromagnets.

•Mo capped pMTJs show high TMR and thermal stability.•Above ∼35 nm Hc is size-independent.•The VCMA effect is present even at 20 nm diameters.

In this paper, we describe a fully-functional 1 Gb standalone spin-transfer torque magnetoresistive random access memory (STT-MRAM) integrated on 28 nm CMOS and based on perpendicular magnetic tunnel junctions (pMTJ’s). Electrical short flows were used to guide the pMTJ stack development. We demonstrate reliable operation of the 1 Gb devices, including well-behaved STT write distributions, an endurance cycling lifetime up to at least 2×1011 cycles, and data retention of 10 years at 85°C. Testing results at -35°C to 110°C for the 1 Gb devices indicate good capability for industrial temperature range applications.

Magnetization switching utilizing the spm-orbit torque of heavy metals is a promising alternative to spin-transfer torque for a faster and more energy-efficient write mechanism for magnetic random-access memory. We report spm-orbit-torque switching m 20-nm-diameter Co20Fe60B20-MgO-based perpendicular magnetic tunnel junctions with a thermal stability factor of similar to 47. Conductive atomic force microscopy was used to measure the tunnel magnetoresistance before and after current pulses through the heavy metal underlayer, and magnetostatic shifts m the minor loops provided evidence of spm-orbit-torque switching. Comparison of estimated critical current densities and write energies suggests that spm-orbit torque can be used as an effective switching mechanism for small and thermally stable perpendicular magnetic tunnel junctions.

Dense arrays of pillars, with diameters of 64 and 25 nm, were made from a perpendicular CoFeB magnetic tunnel junction thin film stack using block copolymer lithography. While the soft layer and hard layer in the 64 nm pillars reverse at different fields, the reversal of the two layers in the 25 nm pillars could not be distinguished, attributed to the strong interlayer magnetostatic coupling. First-order reversal curves were used to identify the steps that occur during switching, and the thermal stability and effective switching volume were determined from scan rate dependent hysteresis measurements.

Voltage controlled magnetic anisotropy (VCMA) currently attracts considerable attention as a novel method to control and manipulate magnetic moments in high-speed and low-power spintronic applications based on magnetic tunnel junctions (MTJs). In our experiments, we use ferromagnetic resonance (FMR) to study and quantify VCMA in out-of-plane magnetized CoFeB/MgO/CoFeB MTJ pillars. FMR is excited by applying a microwave current and detected via a small rectified voltage which develops across MTJ at resonance. The VCMA effective field can be extracted from the measured resonance field and was found to vary as a function of electrical bias applied to MTJ. At low applied biases, we observe a linear shift of the VCMA field as a function of the applied voltage which is consistent with the VCMA picture based on the bias-induced electron migration across the MgO/CoFeB interface. At higher biases, both positive and negative, we observe a deviation from the linear behavior which may indicate a saturation of the VCMA effect. These results are important for the design of MTJ-based applications.

We report that synthetic antiferromagnets (SAFs) can be efficiently switched by spin-orbit torques (SOTs) and the switching scheme does not obey the usual SOT switching rule. We show that both the positive and negative spin Hall angle (SHA)-like switching can be observed in Pt/SAF structures with only positive SHA, depending on the strength of applied in-plane fields. A new switching mechanism directly arising from the asymmetric domain expansion is proposed to explain the anomalous switching behaviors. Contrary to the macrospin-based switching model that the SOT switching direction is determined by the sign of SHA, the new switching mechanism suggests that the SOT switching direction is dominated by the field-modulated domain wall motion and can be reversed even with the same sign of SHA. The new switching mechanism is further confirmed by the domain wall motion measurements. The anomalous switching behaviors provide important insights for understanding SOT switching mechanisms and also offer novel features for applications.

We present a study on perpendicular magnetic tunnel junctions with W as buffer and capping layers. A tunneling magnetoresistance of 138% and an interfacial magnetic anisotropy of 1.67 erg/cm2 were obtained in optimally annealed samples. However, after extended annealing at 420 °C, junctions with W layers showed extremely small resistance due to interdiffusion of W into the MgO barrier. In contrast, in Ta-based junctions, the MgO barrier remained structurally stable despite disappearance of magnetoresistance after extended annealing due to loss of perpendicular magnetic anisotropy. Compared with conventional tunnel junctions with in-plane magnetic anisotropy, the evolution of tunneling conductance suggests that the relatively low magnetoresistance in perpendicular tunnel junctions is related to the lack of highly polarized Δ1 conducting channel developed in the initial stage of annealing.

Magnetic interlayer coupling is one of the central phenomena in spintronics. It has been predicted that the sign of interlayer coupling can be manipulated by electric fields, instead of electric currents, thereby offering a promising low energy magnetization switching mechanism. Here we present the experimental demonstration of voltage-controlled interlayer coupling in a new perpendicular magnetic tunnel junction system with a GdOx tunnel barrier, where a large perpendicular magnetic anisotropy and a sizable tunnelling magnetoresistance have been achieved at room temperature. Owing to the interfacial nature of the magnetism, the ability to move oxygen vacancies within the barrier, and a large proximity-induced magnetization of GdOx, both the magnitude and the sign of the interlayer coupling in these junctions can be directly controlled by voltage. These results pave a new path towards achieving energy-efficient magnetization switching by controlling interlayer coupling.

Superparamagnetic perpendicular magnetic tunnel junctions are fabricated and analyzed for use in random number generators. Time-resolved resistance measurements are used as streams of bits in statistical tests for randomness. Voltage control of the thermal stability enables tuning the average speed of random bit generation up to 70 kHz in a 60 nm diameter device. In its most efficient operating mode, the device generates random bits at an energy cost of 600 fJ/bit. A narrow range of magnetic field tunes the probability of a given state from 0 to 1, offering a means of probabilistic computing.

Voltage controlled magnetic anisotropy (VCMA) is a novel method to switch magnetizations in low-power and ultra-fast applications based on magnetic tunnel junctions (MTJs). Here we explore the ferromagnetic resonance (FMR) technique to probe VCMA in situations where other methods cannot be applied. We quantify VCMA in CoFeB/MgO/CoFeB MTJ nanopillars with in-plane magnetizations where our FMR method is unique in providing direct information about VCMA. We observe a quadratic shift of the FMR resonance field when a voltage bias is applied across the MTJ. The VCMA energy corresponding to the quadratic shift varies with an energy factor of 8.2μJ/m2 for 1 V2/nm2. These results are important for understanding magnetodynamics in MTJ-based applications with in-plane magnetizations.

Perpendicular magnetic tunnel junctions (p-MTJs) were patterned into nanopillars using electron-beam lithography to study their scaling and switching behaviour. Magnetoresistance measurements of annealed and unannealed p-MTJ films using scanning probe microscopy showed good agreement with Monte Carlo modeling. p-MTJ pillars demonstrated clear parallel magnetic states, both ‘up’ or both ‘down’ following AC-demagnetization. Significant variability in the resistance of p-MTJ pillars was observed and attributed to edge features generated during patterning or local inhomogeneity in the MgO layer.

The reduction of metals from their oxides through solid electrochemical reactions usually requires a high temperature above 800 °C and a specially designed electrochemical structure. It is demonstrated that, in a simple field‐effect transistor (FET) structure, the redox reaction between Co metal and CoOx is reversible under a small electric field and can be achieved at a moderate temperature below 200 °C. The FETs functioning through the reversible redox reaction show nonvolatile behavior and a high on/off ratio of about 105. Moreover, the FETs show a threshold resistance switching behavior at high resistance states, but with opposite switching directions compared to normal metal/oxide/metal structures. The electric field induced metal–oxide transition may also be used for other energy storage applications.

As traditional complementary metal–oxide semiconductor (CMOS) technology approaches its limit, alternative technologies such as magnetic tunnel junctions (MTJs) are being explored to replace CMOSbased devices for memory and logic applications. MTJs have advantages such as nonvolatility, low power consumption, and high densities [1]. These features have enabled application in technologies such as magnetic random access memory (MRAM), static random access memory (SRAM), and spin-transfer torque MTJs (STT-MTJs).

The effect of a thin Mo dusting layer inserted at the interface of Ta/CoFeB of perpendicular magnetic tunneling junction with MgO barriers was investigated. Unlike thick Mo layers that exhibited a strong (110) crystalline texture, the inserted Mo layer between Ta/CoFeB had little negative influence on the crystallization of CoFe (001), therefore combining the advantages of Mo as a good thermal barrier and Ta as a good boron sink. For optimized Mo dusting thickness, a large tunneling magnetoresistance of 208% was achieved in perpendicular magnetic tunneling junctions with superior thermal stability at 500 °C.