Spin-torque Diode Research Articles

Array of resonant-type spin-torque diodes as a broadband rectifier: Numerical studies

In this paper, we propose a new approach for constructing broadband rectifiers from resonant-type in-plane spin-torque diodes (STDs). The intrinsic bandwidth of such STDs constrained by the ferromagnetic resonance linewidth of the corresponding magnetic tunnel junction’s is relatively narrow, which is a significant limitation. To address this issue, we propose the implementation of an array of these STDs, each with a distinct resonance frequency. By ensuring that the frequency difference between adjacent STDs is smaller than their individual bandwidths, we achieve a device capable of rectifying signals over a much wider frequency range. To verify the effectiveness of our approach, we have performed analytical calculations based on the Landau–Lifshitz–Gilbert equation with the Slonczewski term, complemented by macrospin modeling and full-scale micromagnetic simulations to account for thermal fluctuations and local magnetization inhomogeneities. Our results demonstrate that an STD array can effectively function as a broadband energy harvester, maintaining near-constant rectification efficiency across a broad frequency range. This research establishes the groundwork for the development of broadband energy harvesters based on STD arrays, with potential applications in various fields requiring efficient energy conversion across a wide frequency spectrum.

High-frequency ultra-sensitive spin-torque diode based on Co-rich magnetic tunnel junction with in-plane magnetized free layer

Spin-torque diodes (STDs) based on magnetic tunnel junctions (MTJs) are newly developing spintronic devices. However, so far, the highly sensitive STDs reported in experiments have a low detection frequency, generally lower than 2 GHz, which limits the application of STDs in HF fields. Here, we develop Co-rich MTJs to enhance in-plane magnetic anisotropy and demonstrate an in-plane magnetized HF ultra-sensitive STD. By selecting the proper field and using an injection locking mechanism, HF microwave detection close to 5 GHz can be realized with a sensitivity greater than 3700 mV mW−1. This result may pave the way to developing spin-torque microwave detectors that are HF and highly sensitive.

Magnetic field sensor based on a low-frequency-tail spintronic diode

Spin-torque diode have shown great potentials and performance in many applicative fields, from microwave detectors to energy harvesters. In this work, we use micromagnetic simulations to study, at room temperature, a state-of-the-art non-resonant low-frequency-tail spin-torque diode in terms of dc output voltage as a function of the amplitude of an in-plane external field applied along different directions. We find that there exists a threshold value of the injected ac current that promotes a linear behavior of the output voltage of field down to the pT range, and we suggest exploiting such a behavior for the design of a magnetic field sensor.

Sub micro-accelerometer based on spintronic technology: A design optimization

Here we propose a spintronic based accelerometer that exploits the coupling of mechanical with spintronic components to develop a compact sub micro-accelerometer in which the external acceleration is obtained via measurement of the voltage variation in a spin torque diode. The solution is based on three spintronic devices, two spin torque diodes and a spin torque nano oscillator, that enables a robust and reliable sensing and read-out mechanisms thanks to voltage differential measurements. The results confirm the potential of mechanical-spintronic accelerometers to become an alternative to current state of the art technology.

Electrical Modeling of Spin-Torque Diodes Used as Radio Frequency Detectors: A Step-by-Step Methodology for Parameter Extraction

In this article here below, we present an electrical model for nanoscale spintronic radio frequency (RF) detectors called spin-torque diodes (STDs). A complete methodology for model parameter extraction is proposed. An equivalent electrical circuit for STD is derived and includes a modeling of the device resistance nonlinearity together with the STD effect. A detailed step-by-step methodology is presented to extract the model parameters using conventional dc measurements, RF scattering parameters ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$</tex-math> </inline-formula> -parameters), continuous wave (CW), and power characterizations. After parameter extraction, a comparison with measurements of a single STD successfully validates the model. Finally, the proposed STD electrical model is used to anticipate the behavior of a two-STD-based RF detector architecture. Simulation results highlight the interest of the proposed modeling approach to investigate suitable RF detector architectures to enhance RF-to-dc conversion efficiency for single or multiband RF detection.

Highly sensitive spin-torque diodes based on in-plane magnetized magnetic tunnel junctions

We investigate the highly sensitive spin torque diode (STD) effect in a magnetic tunnel junction (MTJ) with an in-plane polarizer and an in-plane free layer. Under injection locking mechanisms, a high rectification voltage of 12 mV is obtained with an input radio frequency power of 1 μW under direct current bias current and a weak magnetic field, corresponding to a high sensitivity of 12 000 mV/mW. In addition, we use the nonlinear rectification characteristics of STD to mimic a neuron with a ReLU-like activation function to perform the recognition of handwritten digits in the Mixed National Institute of Standards and Technology database, where a produced accuracy of up to 93.53% is obtained. These findings suggest that the MTJ with in-plane magnetized electrodes holds promising potential in developing high sensitivity STDs for Internet of Things applications and neuromorphic computing.

Neural-like population coding based on spin-torque diode

Neuroscience studies have shown that population coding in biological systems can carry out resilient information processing with ensemble of neurons. Such strategy is valuable for the future development of electronics, particularly as the downscaling of transistors is reaching atomic limits and causing problems of large device-to-device variability and even device failure. In this work, we propose that nanoscale spin-torque diode (STD) based on a magnetic tunnel junction can be used to implement population coding. We also demonstrate that a basis set obtained from a single STD by time multiplexing can realize the generation of cursive letters. Furthermore, different activation functions of an artificial neural network have been acquired. In addition, high recognition rates of the Mix National Institute of Standards and Technology handwritten digits up to 94.88% are achieved using an output function constructed from the experimental data. Our work may provide inspiration for designing neuromorphic computing systems.

Ultralow-current density spin-torque diodes with high sensitivity

Spin-torque diodes (STDs) offer the possibility of using spin torque to generate rectification voltage with promising applications in microwave detecting, energy harvesting, and neuromorphic computing. Here, we demonstrate a highly sensitive STD with ultralow current density based on a magnetic tunnel junction with perpendicular magnetic anisotropy. At zero magnetic field, a high sensitivity exceeding 3785 V/W is obtained with a low current of −20 μA, corresponding to a current density of ∼105 A/cm2, which is one order lower than the previously reported. When a weak external magnetic field is applied, the sensitivity can be further increased by five times to 20 000 V/W. Furthermore, we construct an artificial neural network with STD neurons to perform recognition of handwritten digits in the Mixed National Institute of Standards and Technology database, where a produced accuracy of up to 94.92% is obtained. Our work provides a route to develop low-power consumption high-sensitivity STDs for Internet of Things applications and neuromorphic computing.

Spin torque oscillator based on magnetic tunnel junction with MgO cap layer for radio-frequency-oriented neuromorphic computing

Recently, it has been proposed that spin torque oscillators (STOs) and spin torque diodes could be used as artificial neurons and synapses to directly process microwave signals, which could lower latency and power consumption greatly. However, one critical challenge is to make the microwave emission frequency of the STO stay constant with a varying input current. In this work, we study the microwave emission characteristics of STOs based on magnetic tunnel junction with MgO cap layer. By applying a small magnetic field, we realize the invariability of the microwave emission frequency of the STO, making it qualified to act as artificial neuron. Furthermore, we have simulated an artificial neural network using STO neuron to recognize the handwritten digits in the Mixed National Institute of Standards and Technology database, and obtained a high accuracy of 92.28%. Our work paves the way for the development of radio-frequency-oriented neuromorphic computing systems.

Memory and Communication-in-Logic Using Vortex and Precessional Oscillations in a Magnetic Tunnel Junction

Wearable and implantable devices (WIDs) come with several separate blocks such as pre-processing units, memory, and data transmission blocks. On the other hand, the multi-functional nature of spintronics devices such as memory, in-memory-computing, microwave frequency generation, and so on are paving paths for further advancement. Hence, in this paper, we present the concept of memory and communication-in-logic (MCL) using a magnetic tunnel junction (MTJ). In addition to reducing footprint area, this method also saves time and power consumption required for data transfer between the in-memory-computing and the communication blocks. Here, MTJ is presented as a memory device as well as an oscillator for communication purposes. Vortex-based spin-torque nano oscillators (V-STNO) and precessional STNOs (P-STNO) generate microwave frequency range (a few hundred MHz to a few GHz) wherein the frequency readout technique using the spin-torque diode is implemented for memory read function. In this work, a 300 nm nanodisk V-STNO generates 296 MHz and 312 MHz frequency for two states of chirality (a characteristic of magnetic vortex), respectively. These different frequencies can be sensed for a bit ‘0’/ ‘1’ read out through which the data from WIDs can be transmitted in a more energy and area-efficient way. The output power emission is 3.22 and 1.76 μW for bit ‘1’ and ‘0’, respectively for V-STNO which is three orders of magnitude larger than that of P-STNO. Finally, we demonstrate that V-STNO can transmit data up to 10 m in the air medium which is much longer than P-STNO (0.24 m).

Junction size dependence of the heat controlled magnetic anisotropy in magnetic tunnel junctions

We investigated the heat controlled magnetic anisotropy (HCMA) in magnetic tunnel junctions with various junction sizes. We evaluated the HCMA from perpendicular magnetic anisotropy under a direct current voltage measured by the spin-torque diode technique. The maximum HCMA magnitude of 5.4 μJ (Wm)−1 was observed, and the HCMA increased with increasing diameter. Our results can be explained by a simple heat dissipation model and suggest that the in-plane heat current affects HCMA.

Easy-cone state in spin-torque diode under combined action of magnetostatics and perpendicular anisotropy

Spin-torque diodes (STDs) with interfacial perpendicular magnetic anisotropy (IPMA) in the free layer (FL) demonstrate outstanding microwave signal rectification performances. Large sensitivity values in such systems are usually associated with an easy-cone (EC) magnetic state, when the magnetization in the FL is tilted from the normal to the plane of the film. Here, we theoretically investigate the phase diagram for the EC state in an infinite FL of the magnetic tunnel junction (MTJ) considering both IPMA (of the first and of the second order) and magnetostatic interaction. We show that the increase of the magnetostatic field leads to the EC state phase expansion. For elliptical MTJ nanopillars we investigate the influence of the orientation of the nanopillar ellipticity on the obtained phase diagrams. And finally, we consider the dynamic properties and rectification efficiency of the STD under microwave current injection. Our results clarify the role of magnetostatic interaction for microwave rectification with the IPMA-based STDs and suggest approaches to the EC state effective rectification phase extension through the parameters optimization.

Magnetostatically Induced Easy-Cone Magnetic State Tuning by Perpendicular Magnetic Anisotropy in an Unbiased Spin-Torque Diode

We report the magnetostatically induced formation of an easy-cone magnetic state in free layers of magnetic tunneling junction (MTJ) with only first-order perpendicular magnetic anisotropy. By means of micromagnetic modeling, we study easy-cone angle dependence on the first-order anisotropy and its impact on the microwave sensitivity of the unbiased spin-torque diode. Considered magnetization tilt can be effectively engineered by choosing the ratio of the shape and first-order surface anisotropy, defined by the free layer's thickness, in-plane MTJ shape, and interlayer magnetostatic interactions. Through micromagnetic modeling over a broad range of input powers and free-layer thicknesses, we demonstrate a significant role of nonlinearity and inhomogeneous magnetization dynamics in achieving high sensitivity levels. Our modeling for typical state-of-the-art parameters of MTJ shows a possibility to reach a record-breaking unbiased sensitivity of 1100 mV/mW and 4650 mV/mW after impedance matching, which is far beyond the parameters of commercial Schottky diodes. These results will be very helpful for the development of efficient spintronic ambient energy harvesters.

RF signal detector and energy harvester based on a spin-torque diode with perpendicular magnetic anisotropy

We demonstrate theoretically that in a spintronic diode (SD), having a free magnetic layer with perpendicular magnetic anisotropy of the first and second order and no external bias magnetic field, the out-of-plane regime of magnetization precession can be excited by sufficiently large (exceeding a certain threshold) RF signals with the frequencies ≲250 MHz. We also show that such a device can operate as a broadband energy harvester capable of converting incident RF power into a DC power with a conversion efficiency of ∼5%. The developed analytical theory of the bias-free SD operation can be used for the optimization of high-efficiency RF detectors and energy harvesters based on SDs.

Measurement of Microwave Signal Frequency by a Pair of Spin-Torque Microwave Diodes

A single resonance-type spin-torque microwave diode (STMD) can be used as a quadratic detector to determine the frequency f of an external microwave signal when the power P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rf</sub> of this signal is known. However, unless the signal's frequency f exactly coincides with the detector's resonance frequency f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">res</sub> , there exists a finite frequency determination error Δf. In this letter, we show theoretically and experimentally that if a pair of independent STMDs with different resonance frequencies connected in parallel is used for the signal frequency determination, knowledge of the signal power is not necessary, and the frequency determination error can be substantially reduced to Δf ≈ 30 MHz at the frequency of f ≈ 5.0 GHz.

Dual-band microwave detector based on magnetic tunnel junctions

The spin-torque diode effect has enabled a nanoscale category of microwave detectors, which are characterized by ultrahigh sensitivities and can work at sub-μW input power. Here, we develop such detectors having a dual-band rectification curve centered at the frequencies f1 and f2 and with a rectified voltage of opposite sign (Vf1&amp;gt;0 and Vf2&amp;lt;0). By selecting the proper bias current and field, the sensitivity is larger than 8000 mV/mW. The physics behind this behavior is the simultaneous excitation of different magnetization oscillation modes and the injection locking mechanism. This dual-band microwave detector could find potential applications in the Internet of Things by reducing the size and the power consumption for signal demodulation in a binary frequency shift keying modulation/demodulation scheme.

Detection of the Microwave Emission from a Spin-Torque Oscillator by a Spin Diode

Magnetic tunnel junctions are nanoscale spintronic devices with microwave generation and detection capabilities. Here we use the rectification effect called "spin-diode" in a magnetic tunnel junction to wirelessly detect the microwave emission of another junction in the auto-oscillatory regime. We show that the rectified spin-diode voltage measured at the receiving junction end can be reconstructed from the independently measured auto-oscillation and spin diode spectra in each junction. Finally we adapt the auto-oscillator model to the case of spin-torque oscillator and spin-torque diode and we show that accurately reproduces the experimentally observed features. These results will be useful to design circuits and chips based on spintronic nanodevices communicating through microwaves.

Spin‐Torque Diodes: From Fundamental Research to Applications

AbstractAmong a rich variety of emerging spintronic devices, spin‐torque diodes (STDs) are among the most interesting, from both a fundamental and an applied perspective. The spin‐torque diode effect occurs when a microwave alternating electric current injected into a magnetic tunneling junction (MTJ) is rectified due to the simultaneous actions of tunneling magnetoresistance and spin‐transfer torque. While the sensitivity of STDs observed in the initial research is rather modest (about 1.4 mV mW−1), after only a few years researchers have increased it to 200 kV W−1 and demonstrated rectification at nanowatt input powers, which radically exceeds the capabilities of mainstream Schottky diodes. This impressive progress is based on a deep understanding of the complex STD physics and on recent advances in MTJ fabrication technology. Herein, the experimental pathways toward increasing the sensitivity and extending the frequency range of STDs and theoretical works aimed at explaining the complex nonlinear dynamics are analysed, and a wide variety of possible STD applications are discussed.

Phase estimation of spin-torque oscillator by nonlinear spin-torque diode effect

A theoretical analysis is developed on spin-torque diode effect in nonlinear region. An analytical solution of the diode voltage generated from spin-torque oscillator (STO) by the rectification of an alternating current is derived. The diode voltage is revealed to depend nonlinearly on the phase difference between the oscillator and the alternating current. The validity of the analytical prediction is confirmed by numerical simulation of the Landau–Lifshitz–Gilbert equation. The results indicate that the spin-torque diode effect is useful to evaluate the phase of a STO in forced synchronization state.

High rectification sensitivity of radiofrequency signal through adiabatic stochastic resonance in nanoscale magnetic tunnel junctions

Rectification is an important stage in electronic circuits for any wireless radio frequency power transfer application. Currently, Schottky diodes are widely used as rectifiers; however, they are inefficient at low power levels of microwatts or less (providing maximum sensitivities around 4 mV/μW). Nanoscale magnetic tunnel junctions can serve as alternative rectifiers by utilizing the so-called spin-torque diode effect, demonstrating a much higher rectification sensitivity (200 mV/μW) compared to Schottky diodes. However, for this mechanism to work, the signal frequency must match the ferromagnetic resonance frequency, which typically lies in the gigahertz range. For signals in the megahertz range or lower, Schottky diodes remain the only option for rectification. Here, we demonstrate a mechanism based on thermally activated adiabatic stochastic resonance in magnetic tunnel junctions to produce low frequency (up to tens of megahertz) signal rectification at low input power (submicrowatt), with a sensitivity of up to 35 mV/μW—higher than state-of-the-art Schottky diode rectifiers at this frequency and power range. These findings suggest magnetic tunnel junctions as potential alternatives to Schottky diodes for low frequency and low power applications.