Current-induced Magnetization Switching Research Articles

Spin-orbit torque magnetization switching of a three-terminal perpendicular magnetic tunnel junction

We report on the current-induced magnetization switching of a three-terminal perpendicular magnetic tunnel junction by spin-orbit torque and its read-out using the tunnelling magnetoresistance (TMR) effect. The device is composed of a perpendicular Ta/FeCoB/MgO/FeCoB stack on top of a Ta current line. The magnetization of the bottom FeCoB layer can be switched reproducibly by the injection of current pulses with density 5 × 1011 A/m2 in the Ta layer in the presence of an in-plane bias magnetic field, leading to the full-scale change of the TMR signal. Our work demonstrates the proof of concept of a perpendicular spin-orbit torque magnetic memory cell.

Stochastic Current-Induced Magnetization Switching in a Single Semiconducting Ferromagnetic Layer

We show experimental evidence of magnetization switching in a single (Ga,Mn)(As,P) semiconducting ferromagnetic layer, attributed to a strong reduction of the magnetization and the anisotropy due to current injection. The nucleation of magnetization reversal is found to occur even in the absence of a magnetic field and to be both anisotropic and stochastic. Our findings highlight a new mechanism of magnetization manipulation based on spin accumulation in a semiconductor material.

Determination of Thermal Stability of Magnetic Tunnel Junction Using Time-Resolved Single-Shot Measurement

Time-resolved single-shot measurement has been investigated to characterize switching times of the free layer in magnetic tunneling junctions (MTJs). In this study, a stacked MTJ film is patterned into an elongated, nano-scale device using standard electron beam lithography in conjunction with ion beam etching. Subsequently, current-induced magnetization switching is initiated using a single-shot measurement based on the time-domain transmission method. A current pulse with a duration of 50 ns is utilized to switch the magnetization configuration between parallel and anti-parallel states. After the pulse rises, the transmission profile shows a transition in which resistance varies. That is, the incubation time between pulse onset and resistance change represents the switching time of the free layer. The statistical distribution results of the switching times are obtained. The switching times are strongly dependent on current density and the external magnetic field. By fitting the data points using the Neel-Arrhenius law, the thermal stability factor JCo and intrinsic current density JCo are also determined.

Chemically Engineered Graphene-Based 2D Organic Molecular Magnet

Carbon-based magnetic materials and structures of mesoscopic dimensions may offer unique opportunities for future nanomagnetoelectronic/spintronic devices. To achieve their potential, carbon nanosystems must have controllable magnetic properties. We demonstrate that nitrophenyl functionalized graphene can act as a room-temperature 2D magnet. We report a comprehensive study of low-temperature magnetotransport, vibrating sample magnetometry (VSM), and superconducting quantum interference (SQUID) measurements before and after radical functionalization. Following nitrophenyl (NP) functionalization, epitaxially grown graphene systems can become organic molecular magnets with ferromagnetic and antiferromagnetic ordering that persists at temperatures above 400 K. The field-dependent, surface magnetoelectric properties were studied using scanning probe microscopy (SPM) techniques. The results indicate that the NP-functionalization orientation and degree of coverage directly affect the magnetic properties of the graphene surface. In addition, graphene-based organic magnetic nanostructures were found to demonstrate a pronounced magneto-optical Kerr effect (MOKE). The results were consistent across different characterization techniques and indicate room-temperature magnetic ordering along preferred graphene orientations in the NP-functionalized samples. Chemically isolated graphene nanoribbons (CINs) were observed along the preferred functionality directions. These results pave the way for future magnetoelectronic/spintronic applications based on promising concepts such as current-induced magnetization switching, magnetoelectricity, half-metallicity, and quantum tunneling of magnetization.

Fabrication of Nano-Sized Magnetic Tunnel Junctions Using Lift-Off Process Assisted by Atomic Force Probe Tip

We present a fabrication method for nano-scale magnetic tunnel junctions (MTJs), employing e-beam lithography and lift-off process assisted by the probe tip of atomic force microscope (AFM). It is challenging to fabricate nano-sized MTJs on small substrates because it is difficult to use chemical mechanical planarization (CMP) process. The AFM-assisted lift-off process enables us to fabricate nano-sized MTJs on small substrates (12.5 mm x 12.5 mm) without CMP process. The e-beam patterning has been done using bi-layer resist, the poly methyl methacrylate (PMMA)/ hydrogen silsesquioxane (HSQ). The PMMA/HSQ resist patterns are used for both the etch mask for ion milling and the self-aligned mask for top contact formation after passivation. The self-aligned mask buried inside a passivation oxide layer, is readily lifted-off by the force exerted by the probe tip. The nano-MTJs (160 nm x 90 nm) fabricated by this method show clear current-induced magnetization switching with a reasonable TMR and critical switching current density.

Fabrication of Current-Perpendicular-to-Plane Junctions Based on Fe3Si/FeSi2 Multilayered Films by Lift-Off

Current-perpendicular-to-plane (CPP) junctions comprising Fe3Si/FeSi2 multilayers were prepared by a lift-off process combined with lithography techniques. The Fe3Si/FeSi2 multilayered films were grown on Si (111) substrates at room temperature by facing-targets direct-current sputtering. From the X-ray diffraction measurements, it was confirmed that the Fe3Si layers were epitaxially grown from the first layer on Si (111) up to the top layer across the FeSi2 layers with the same orientation relationship as that in the first layer. The CPP junctions, wherein ferromagnetic interlayer coupling across the FeSi2 spacer layers was induced, exhibited an obvious change in the electrical resistance for the current injection, which might be due to current-induced magnetization switching.

A note on the Fano factor of spin-torque shot noise

The spin shot noise, which is a consequence of the discreteness of the angular momentum transfer, is present in every spin-torque experiment, such as for instance current-induced magnetization switching or current-induced domain wall motion. In particular at low temperatures, it constitutes a dominant contribution to magnetization noise. We discuss some important properties of the spin-torque shot noise and its implications for magnetization dynamics. We introduce the Fano factor of the non-equilibrium noise. For a spin-torque nano-oscillator, we calculate the voltage dependence of the Fano factor and show that there is an optimal precession angle at which the Fano factor is minimized.

Current-induced synchronized switching of magnetization

We investigate current-induced magnetization switching for a multilayer structure that allows a reduced switching current while maintaining high thermal stability of the magnetization. The structure consists of a perpendicular polarizer, a perpendicular free-layer, and an additional free-layer having in-plane magnetization. When the current runs perpendicular to the structure, the in-plane free-layer undergoes a precession and supplies an internal rf field to the perpendicular free-layer, resulting in a reduced switching current for one current polarity. For the other polarity, the in-plane free-layer almost saturates perpendicular to the plane and acts as another perpendicular polarizer, which also reduces the switching current.

Current-Induced Magnetization Switching in CPP Junctions based on Fe3Si/FeSi2 Multilayered Films

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Crucial role of interfacial alloying on spin-transfer torque in magnetic tunnel junctions

We demonstrate that interfacial disorder in magnetic tunnel junctions (MTJs) has a dramatic effect on the bias behavior of both spin-transfer and field-like spin torques, leading to strong enhancement, sign reversal, and bias asymmetry. The underlying mechanism lies on the impurity-induced resonance energies, which can be selectively controlled with alloying and/or bias. The predictions explain for the first time the experimental results in ``dirty'' junctions and have important implications on exploiting further the alloying effect to optimize the current-induced magnetization switching.

Compact Modeling of Perpendicular-Anisotropy CoFeB/MgO Magnetic Tunnel Junctions

Magnetic tunnel junctions (MTJs) composed of ferromagnetic layers with perpendicular magnetic anisotropy (PMA) are of great interest for achieving high-density nonvolatile memory and logic chips owing to its scalability potential together with high thermal stability. Recent progress has demonstrated a capacity for high-speed performance and low power consumption through current-induced magnetization switching. In this paper, we present a compact model of the CoFeB/MgO PMA MTJ, a system exhibiting the best tunnel magnetoresistance ratio and switching performance. It integrates the physical models of static, dynamic, and stochastic behaviors; many experimental parameters are directly included to improve the agreement of simulation with experimental measurements. Mixed simulation based on the 65-nm technology node of a magnetic flip-flop validates its relevance and efficiency for MTJ/CMOS memory and logic chip design.

Current-Induced Magnetization Switching in Fe3Si/FeSi2Artificial Lattices

Current-induced magnetization switching in Fe3Si/FeSi2 artificial lattices, wherein ferromagnetic Fe3Si layers were antiferromagnetically coupled, was studied in a current perpendicular to plane geometry. While the electrical resistance–injected current (R–I) curve exhibited a clear hysteresis loop under zero magnetic field, the hysteresis loop in the curve disappeared under magnetic fields larger than the saturation magnetic field. This implies that the interlayer coupling was altered from antiferromagnetic to ferromagnetic by the current injection under zero magnetic field; on the other hand, under the magnetic fields, the coupling was fixed to be ferromagnetic regardless of the current injection.

Current-Induced Magnetization Switching in Fe3Si/FeSi2 Artificial Lattices

Current-induced magnetization switching in Fe3Si/FeSi2 artificial lattices, wherein ferromagnetic Fe3Si layers were antiferromagnetically coupled, was studied in a current perpendicular to plane geometry. While the electrical resistance–injected current (R–I) curve exhibited a clear hysteresis loop under zero magnetic field, the hysteresis loop in the curve disappeared under magnetic fields larger than the saturation magnetic field. This implies that the interlayer coupling was altered from antiferromagnetic to ferromagnetic by the current injection under zero magnetic field; on the other hand, under the magnetic fields, the coupling was fixed to be ferromagnetic regardless of the current injection.

Reduction of intrinsic critical current density under a magnetic field along the hard axis of a free layer in a magnetic tunnel junction

We investigated the effect of a magnetic field along a hard in-plane axis ${H}_{\mathrm{hard}}$ on a current-induced magnetization switching (CIMS) in magnetic tunnel junctions (MTJs). Since ${H}_{\mathrm{hard}}$ causes the effective field to tilt away from the easy axis, we evaluated the ${H}_{\mathrm{hard}}$ dependence of two contributing factors in CIMS [the intrinsic critical current density (${J}_{c0}$) and the thermal stability factor ($E/{k}_{B}T$)] as functions of the tilting angle (${\ensuremath{\theta}}_{H}$). Both measurements and numerical simulations showed that the presence of ${H}_{\mathrm{hard}}$ can reduce ${J}_{c0}$ by more than the amount estimated by Slonczewski's polarization function $g$($\ensuremath{\theta}$) by an order of magnitude and that $E/{k}_{B}T$ is independent of ${\ensuremath{\theta}}_{H}$. These findings suggest that the effect of ${H}_{\mathrm{hard}}$ mainly appears in the dynamic properties due to the nonconservative force of the spin-transfer torque based on the Slonczewski's model. A simple stability analysis demonstrated that the tilt of the magnetization direction away from the easy axis caused by the presence of ${H}_{\mathrm{hard}}$ induces an imbalance between the spin-transfer and damping torques and that applying a current achieves the further tilted stable state. Achievement of this stable state can be interpreted as the suppression of the effect of the effective demagnetization field (${H}_{d}^{*}$). Therefore the major reduction in ${J}_{c0}$ is due to the suppression of ${H}_{d}^{*}$ caused by the presence of ${H}_{\mathrm{hard}}$.

Single-layer magnetic memory based on Rashba three-terminal quantum dot device

It has been shown that three-terminal quantum dot (TTQD) devices and their variants can generate a rather high spin current in the presence of Rashba spin-orbit coupling (RSOC) at low temperature. The effect of contacting ferromagnetic (FM) electrodes to the TTQD devices is studied theoretically via the Keldysh non-equilibrium Green’s function method. The spin torque and spin current in the system are subsequently calculated. It is found that the spin torque in the middle FM lead can be reversed symmetrically by applying a bias voltage with opposite sign. This suggests that current-induced magnetization switching can be applied for data writing. Moreover, the charge current shows significant modulation when the orientation of the FM moment is varied, a feature which can be utilized for reading data. Based on this, we propose an integrated Magnetic Random Access Memory (MRAM) device made up of Rashba TTQD unit cells. In each cell, the electron transport can be modulated by means of a gate voltage without the need for a transistor — unlike the case of conventional MRAMs. What is more, owing to the combination of RSOC and exchange interaction with the local FM moments, both the writing and reading process can be effected with a single FM layer.

Nanoelliptic Ring-Shaped Magnetic Tunnel Junction and Its Application in MRAM Design With Spin-Polarized Current Switching

Nanoring magnetic tunnel junction (NR-MTJs) with outer diameter of 90 nm, and nanoelliptical ring (NER) -shaped MTJ with major outer diameter of 120 nm, and minor outer diameter of 70 nm were fabricated. For both geometrical structures, the ring width was kept at around 30 nm. The magnetic field driven and current-induced magnetization switching of the NR-MTJs and NER-MTJs were investigated. Compared with the NR-MTJs, the NER-MTJs show a sharp magnetization switching and lower coercivity under the sweep of an external magnetic field, which indicates the higher uniaxial anisotropy of NER-MTJs. In addition, the current-induced magnetization switching was achieved in both MTJs which indicated a lower critical current density in sandwiched-type and spin-valve-type NER-MTJs. This reduction of critical current could be resulting from different distribution of current-induced Oersted field in the NR-MTJs and NER-MTJs, which play an assisted role in spin transfer torque switching. This work proposes the NER-MTJs with the low current density, and small stray field, as data storage bits of magnetic random access memory (MRAM), could be a promising candidate for further reducing the power consumption, scaling memory cell size and increasing density of MRAM.

Instability of storage and temperature increment in nanopillars due to human body model electrostatic discharge

Abstract MRAM relevant to current induced magnetization switching (CIMS) is studied due to thermal increment caused by CIMS. In this paper, the instability of storage and the thermal increment caused by the transient current from the HBM ESD in nanopillars of MRAM are studied. We determine the voltage which can cause erroneous switching in MRAM by inducing CIMS. The finite element method is used to calculate the temperature increase caused by the discharge. Results indicate that this voltage is not sufficient to cause permanent physical or magnetic damage to MRAMs but only affects the reliability of the stored information.

Temperature-Dependent Current-Induced Magnetization Switching in Fe3Si/FeSi2/Fe3Si Trilayered Films

Fe3Si/FeSi2/Fe3Si trilayered films were grown on Si(111) substrates at a substrate temperature of 300 °C by facing-targets direct-current sputtering, and current-induced magnetization switching in current-perpendicular-to-plane geometry was studied for the films wherein an antiferromagnetic interlayer coupling perpendicular to the plane was probably formed at room temperature. The appearance of a hysteresis loop in the electrical resistance–injection current curve well coincided with that of a hysteresis loop in the magnetization curve perpendicular to the plane. In addition, the hysteresis loop in the electrical resistance–injection current curve disappeared under large magnetic fields. The origin of the change in the electrical resistance for the injection current might be the change in the interlayer coupling.

Temperature Increase Due to Joule Heating in a Nanostructured MgO-Based Magnetic Tunnel Junction Over a Wide Current-Pulse Range

The temperature increase due to Joule heating in a nanopillar of a magnetic tunnel junction sandwiched by top and bottom electrodes was calculated by the finite element method. The results for the critical condition for the current-induced magnetization switching measured over a wide current-pulse range were taken from the literature. At long pulse widths, the temperature increase was solely dependent on the magnitude of the critical current density. However, no saturation in the temperature increase occurred for short pulse widths. In this case, the temperature increase additionally depended on the pulse width, so that a broad maximum occurred in the pulse width (or the critical current density) dependence of the temperature increase. The original results for the critical condition were corrected by accounting for the temperature increase and these corrected results, together with the Slonczewski equation, were used to extract an accurate value for the thermal stability factor.

Perpendicular magnetization of Co2FeAl full-Heusler alloy films induced by MgO interface

The perpendicular magnetization of Co2FeAl (CFA) full-Heusler alloy films was achieved in the structures of CFA/MgO and MgO/CFA with the perpendicular magnetic anisotropy energy density (KU) of 2–3×106 erg/cm3, which can be used as the perpendicular ferromagnetic electrodes of MgO-based magnetic tunnel junctions (MTJs) with high thermal stability at sub-50-nm dimension. The CFA thickness dependence of KU was investigated at different annealing temperatures, indicating that the perpendicular anisotropy of CFA is contributed by the interfacial anisotropy between CFA and MgO. This letter will open up a way for obtaining perpendicular magnetization of Co-based full-Heusler alloys, which is promising for further reduction in the critical current of current induced magnetization switching in MgO-based MTJ nanopillars with perpendicular full-Heusler alloy electrodes.