Switching Of Magnetic Tunnel Junctions Research Articles

Spin-transfer switching in magnetic tunnel junctions with synthetic ferri-magnetic free layer

We have fabricated the SyF structure with both high annealing stability and strong interlayer exchange coupling and investigated tunnelling magnetoresistance (TMR) and spin-transfer switching properties of magnetic tunnel junctions (MTJs) with developed SyF free layer. The fabricated SyF with structure of Ta/Ru/CoFe/Ru/CoFeB possessed high annealing stability of 400°C and strong interlayer exchange coupling. Consequently, a large TMR ratio of 122% has been observed after annealing at high temperature of 350°C. In addition, we have successfully observed spin-transfer switching by the net current density of 14 MA/cm2 and the large thermal stability factor of 62.

Inherent spin transfer torque driven switching current fluctuations in magnetic element with in-plane magnetization and comparison to perpendicular design

This paper presents a systematic micromagnetic modeling study on switching current fluctuations in both in-plane and perpendicular spin-transfer-torque (STT) magnetoresistive random access memory devices. For the magnetic tunnel junction (MTJ) with in-plane magnetization, high-order spin wave modes are excited during a STT-driven switching, which leads to an inherently broad switching current distribution. If the MTJ size is not sufficiently small, a stable vortex can be formed over a wide range of current amplitudes. In contrast, the excitation of such high-order spin waves is absent in STT switching of MTJs with perpendicular magnetic anisotropy. Consequently, the fluctuation in switching current amplitude or pulse duration is significantly smaller in comparison.

Spin transfer torque switching of magnetic tunnel junctions using a conductive atomic force microscope

We show that a nonmagnetic conductive atomic force microscopy probe can be used to read and write magnetic bits using current passed between the tip and bit. The bits were patterned using electron beam lithography from a magnetic tunnel junction (MTJ) film with in-plane shape anisotropy using an MgO tunnel barrier. Probes were made having a thick Pt coating and could deliver up to several milliamps, so that MTJ structures were easily switched repeatedly using the spin transfer torque effect.

Electroforming, magnetic and resistive switching in MgO-based tunnel junctions

Magnetic tunnel junctions (MTJs) are under investigation since they offer great potential for applications in magnetic memories. An interesting effect in TJs concerns non-volatile resistive switching of non-magnetic origin. We report magnetic (magnetoresistance) and structural (resistive; R) switching in MgO-based MTJs (barrier thicknesses t = 0.75, 1.35 nm). As-grown MTJs display R-switching only in the thinnest series, while thicker barrier samples need an electroforming step for R-switching to occur. Forming changes the electrical resistance temperature dependence, from tunnel- to metallic-like, revealing the formation of conductive bridges across the barrier which, leading to local high electrical fields and temperatures, are essential for resistive switching.

Landau–Lifshitz–Gilbert study of the effect of pulse width on spin-transfer torque magnetization switching

We studied the effect of a current pulse width on current-induced magnetization switching in magnetic tunnel junctions based on a macrospin model of the free layer. We performed finite temperature Langevin simulations of the Landau–Lifshitz–Gilbert–Slonczewski equation with an additional spin-torque term. By evaluating the switching current density, we obtained the diagram in the plane of the critical current density and the pulse width at 300 K. As the pulse width increased, we observed an adiabatic regime in the shorter pulse widths, an intermediate crossover regime, and a thermally activated regime in long pulse widths. We found that the easy-plane anisotropy field shifts the crossover pulse width to the lower pulse width, suggesting that the reversed region is enhanced by controlling the device shape. Our results are consistent with those of recent experiments over the pulse widths ranging from 10−1 to 105 ns.

Distinction and correlation between magnetization switchings driven by spin transfer torque and applied magnetic field

The dynamics of the magnetization switching in magnetic tunnel junctions (MTJs) designed for spin-transfer-torque (STT) magnetoresistive random access memory (MRAM) was investigated using micromagnetic simulations. It was found that for short pulses (<∼10ns), the switching current threshold does not correlate with the switching field threshold and it is, therefore, essentially insensitive to MTJ shape (aspect ratio) and edge roughness. However, if the magnetization reversal is thermally activated for longer pulses, the switching current and the switching field are directly correlated. For a relatively large MTJ, the distribution is wide due to the excitation of high-order spin wave modes.

Reduction in switching current using a low-saturation magnetization Co–Fe–(Cr, V)–B free layer in MgO-based magnetic tunnel junctions

Magnetic properties, magnetoresistance (MR), and spin-transfer switching of magnetic tunnel junctions having a structure of Co60Fe20B20 3 nm/MgO 1 nm/(Co75Fe25)80−XCr(V)XB20 2 nm (X=0–25) were investigated. Magnetization of the (Co–Fe)–(Cr, V)–B free layer decreased from 1.2 T before substitution to 0.6 T at Cr of 10% (0.8 T at V of 10%). The MR ratio and a resistance-area product (RA) before substitution were, respectively, about 130% and about 2 Ω μm2. The MR ratio decreased to 80% at Cr of 10% and 40% at V of 10%. The RA values were almost independent of the composition. The intrinsic switching current density (Jc0) decreased from 15 to 8 MA/cm2 at Cr of 10% and 12 MA/cm2 for V of 10%. Upon the further increase in Cr and V, stable switching was difficult to observe. In summary, Jc0 decreased to half in the case of Cr, but the effect was small for V.

Large voltage-induced magnetic anisotropy change in a few atomic layers of iron

In the field of spintronics, researchers have manipulated magnetization using spin-polarized currents. Another option is to use a voltage-induced symmetry change in a ferromagnetic material to cause changes in magnetization or in magnetic anisotropy. However, a significant improvement in efficiency is needed before this approach can be used in memory devices with ultralow power consumption. Here, we show that a relatively small electric field (less than 100 mV nm(-1)) can cause a large change (approximately 40%) in the magnetic anisotropy of a bcc Fe(001)/MgO(001) junction. The effect is tentatively attributed to the change in the relative occupation of 3d orbitals of Fe atoms adjacent to the MgO barrier. Simulations confirm that voltage-controlled magnetization switching in magnetic tunnel junctions is possible using the anisotropy change demonstrated here, which could be of use in the development of low-power logic devices and non-volatile memory cells.

Boron Composition Dependence of Spin-Transfer Switching in Magnetic Tunnel Junctions with CoFeB Free Layers

We investigated tunnel magnetoresistance (TMR) and magnetization switching due to spin-transfer torque in CoFeB/MgO/CoFeB junctions having three different free layer compositions, (Co50Fe50)100-xBx (x = 20, 25, 30), and measured their magnetic properties, including magnetization (Ms) and Gilbert damping constant (α), which are theoretically predicted to contribute to the critical current density (Jc0) for spin-transfer switching. We found that the Jc0 in the magnetic tunnel junctions (MTJs) with x = 30 became smaller than that in the MTJs with x = 20 despite having a lower TMR ratio and a larger α. We showed that the Jc0 for each composition tends to follow the theoretical relationship for the magnetic factor of the free layer.

Micromagnetic study of hotspot and thermal effects on spin-transfer switching in magnetic tunnel junctions

The hotspot and current self-heating effects on the spin-transfer-induced magnetization switching are investigated for low resistance magnetic tunnel junctions. Two kinds of spin torque theories, one for ohmic-like conduction from randomly distributed hotspots and the other for tunnel conduction from an insulator barrier, are combined together in this study by using a parallel resistor model. We find that the spin torque amplitude is locally enhanced in the hotspot region due to the large current density, which leads to a strong reduction of the current switching threshold (Jc), in a way that the enhanced spin torque induces the local magnetization near the hotspot switching first, and then drives the switching spreading through the whole free layer. The current self-heating effect is also studied; the free layer temperature increases only a few degrees at an applied current close to Jc∼1×107A∕cm2, which results in a slight decrease in Jc for the low resistance junctions with hotspots.

Spin-transfer torque switching in magnetic tunnel junctions and spin-transfer torquerandom access memory

We present experimental and numerical results of current-driven magnetizationswitching in magnetic tunnel junctions. The experiments show that, forMgO-based magnetic tunnelling junctions, the tunnelling magnetoresistance ratiois as large as 155% and the intrinsic switching current density is as low as1.1 × 106 A cm−2. The thermal effect and current pulse width on spin-transfer magnetization switchingare explored based on the analytical and numerical calculations. Three distinctswitching modes, thermal activation, dynamic reversal, and precessional process,are identified within the experimental parameter space. The switching currentdistribution, write error, and read disturb are discussed based on device designconsiderations. The challenges and requirements for the successful application ofspin-transfer torque as the write scheme in random access memory are addressed.

Spin-transfer-induced magnetization switching in tunnel junctions with nanocurrent channels

Spin-transfer-induced magnetization switching processes are evidenced by the “snapshots” of magnetic configurations for junctions containing nanocurrent channels (NCCs). The switching behaviors are found to be closely related to the NCC size and its actual location. Both the switching time t and critical current density Jc decrease with the increase of NCC size. Faster switching and lower Jc are observed for NCC located in the center. This work suggests a reasonable design frame to efficiently manipulate the Jc reduction and fast switching in practical spin-transfer-switched devices.

Spin-transfer-induced magnetization switching in magnetic tunnel junctions

Micromagnetic simulations for spin-transfer-induced magnetization dynamics in nanoscale low-resistance magnetic tunnel junctions (MTJs) are performed by using the modified Landau-Lifshitz equation in which a spin-transfer model for MTJs is included. Calculations show that critical switching current density (Jc) is about (4–8)×106A∕cm2, consistent with the experimental data, lower than the values found in metallic spin-valve nanopillars (typically in the order of ∼107A∕cm2). A slight asymmetry of the critical current is observed in the magnetization switching loop, due to the asymmetry of the spin-torque factor. The switching time is related to the applied current, and our calculation results show that a longer switching time, 10–15ns, is needed when the applied current density J is close to Jc, and it decreases down to 1ns for J>2×107A∕cm2.

Influence of domain wall interactions on nanosecond switching in magnetic tunnel junctions

We have obtained microscopic evidence of the influence of domain wall stray fields on the nanosecond magnetization switching in magnetic trilayer systems. The nucleation barrier initiating the magnetic switching of the soft magnetic ${\mathrm{Fe}}_{20}{\mathrm{Ni}}_{80}$ layer in magnetic tunnel junctionlike $\mathrm{FeNi}∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}∕\mathrm{Co}$ trilayers is considerably lowered by stray fields generated by domain walls present in the hard magnetic Co layer. This internal bias field can significantly increase the local switching speed of the soft layer. The effect is made visible using nanosecond time- and layer-resolved magnetic domain imaging and confirmed by micromagnetic simulations.

Quasi-static and dynamic switching of exchange biased micron-sized TMR junctions

We study the quasi-static and dynamical switching of magnetic tunnel junction patterned in micron-sized cells with integrated field pulse line. The tunnel junctions are CoFe/AlO/CoFe with an exchange biasing layer of MnIr. Quasi-static characterizations have been used to determine anisotropy, coercive as well as exchange bias fields. Dynamic switching measurements are done by applying fast-rising magnetic field pulses (178 ps–10 ns) along the hard axis of the junction with a quasi-static easy-axis applied field. We identify the field conditions leading to no-switching, to direct-writing and to toggle switching. We identify these field conditions up to the precessional limit, and construct the experimental dynamical astroïd. The magnetization trajectories leading to direct-writing and to toggle switching are well described by macrospin simulations.

Thermal activation effect on spin transfer switching in magnetic tunnel junctions

A calculation of the intrinsic spin transfer switching current Ic0 is realized in the magnetic tunnel junctions (MTJs). Ic0 is determined by the distribution of Ic in repeated measurements, assuming the linear decrease of the thermal stability by increasing the spin-polarized current. The spin transfer torque is found to be proportional to η(I)I with the applied current I and I-dependent spin polarization η(I). Even in case that I is limited less than Ic due to the tunnel barrier breakdown, Ic0 is calculated with an assisting external magnetic field and making better use of the magnetic phase diagram. In our MTJs, the average intrinsic switching current density was 6.4MA∕cm2.

Current-induced butterfly shaped domains and magnetization switching in magnetic tunnel junctions

Patterned magnetic tunnel junctions (MTJs) with the layer structure of Ta (5 nm)/Ni79Fe21 (5 nm)/Cu (20 nm)/Ni79Fe21 (5 nm)/Ir22Mn78 (10 nm)/Co75Fe25 (4 nm)/Al (0.8 nm)-oxide/Co75Fe25 (4 nm)/Ni79Fe21 (20 nm)/Ta (5 nm) were fabricated using magnetron sputtering deposition and lithography. High tunnelling magnetoresistance ratios of 22 and 50% were obtained at room temperature before and after annealing, respectively. The evolution of leaf shaped images was observed via Lorentz transmission electron microscopy (LTEM) on the MTJs, which were deposited on a patterned and carbon-coated transmission electron microscopy grid. These leaf-like LTEM images correspond to a butterfly shaped domain structure that was confirmed by a micromagnetics simulation. When a large DC current or bias voltage was applied across the MTJ, the butterfly-like vortex domain structures could be induced to form in the free layer of the MTJ, resulting in a significant decrease of magnetization in the free layer. The existence of these butterfly shaped domains could be one of the major causes of the bias voltage dependence of the TMR ratio.

Current-induced magnetization switching in magnetic tunnel junctions

Current-induced magnetization switching (CIMS) in low-resistance magnetic tunnel junctions was shown at average critical current densities (Jc=1.33×106 A/cm2). When large vertical currents pass through the junctions, spin-transfer torque, and vortex fields can rotate the magnetization of the free layer from the initial parallel state to a vortex state, resulting in 10.8% CIMS resistance change at zero-bias current, which is about half of the resistance change (22%) induced when switching is created by an external field. A micromagnetic simulation including the spin-transfer torque and the vortex field correctly predicts the critical negative-current-inducing switching from the parallel state into the vortex state, but fails to explain the reverse switching from the vortex state into the parallel state at an approximately symmetric positive critical current. Lead fields were analyzed and found to be not the cause of the observed switching. The very small dependence of the switching currents on an external magnetic field suggests the existence of hot-spots where local current densities may be much larger.

Switching stability of magnetic tunnel junctions with an artificial antiferromagnet

Avoiding fatigue in the switching of magnetic tunnel junctions is crucial for their long-term use in nonvolatile magnetic memories. We compare the switching stability of two types of junctions with different soft layers: Fe or Ni81Fe19, both with Co dusting at the barrier interface. The magnetically hard electrode is a Co/Cu/Co artificial antiferromagnet. While the tunneling magnetoresistance (TMR) remains unchanged after 104 cycles in a 4 kA/m rotating field, it decreases by more than 45% due to uniaxial switching. Fringing fields of domain walls in the soft layer and an intrinsic instability of Co/Cu/Co are identified as the main reasons. Magnetization reversal by two perpendicular switching pulses avoids this magnetic degradation and maintains a full TMR signal.