Single Free Layer Research Articles

Ultralow Current Switching of Synthetic‐Antiferromagnetic Magnetic Tunnel Junctions Via Electric‐Field Assisted by Spin–Orbit Torque

AbstractSpintronic devices, especially electric‐field and spin–orbit torque driven magnetic tunnel junctions (MTJs), are promising candidates to replace the current memory and logic components for satisfying future computing demands. Current spin–orbit torque based MTJ devices with a single free layer and spin Hall channel still face high current density for switching. Here, 150‐nm perpendicular MTJs are designed and fabricated with a synthetic‐antiferromagnetic free layer and a bilayered spin Hall channel. The switching behavior is investigated via combination of forces from electric‐field and spin–orbit torque, where the electric field can modulate the exchange coupling of the synthetic‐antiferromagnetic free layer. Through an assistance of spin–orbit torque, bidirectional switching is obtained with switching current density as low as 3 × 103 A cm−2, which is two orders of magnitude lower than that of the current best reported values. These results suggest that electric‐field switching of synthetic‐antiferromagnetic MTJs could be a promising approach for reducing write current density of spintronic devices.

Ultrahigh frequency spin-torque nano-oscillator based on bilayer-skyrmions

A skyrmion in a nanodisk with a single free layer (Single-layer Skyrmion) can be excited by the spin-polarized current to a steady state of circular motion for high-frequency applications. However, limited by the threshold of the driving current owning to the skyrmion Hall effect (SkHE), the maximum frequency of the skyrmion oscillation is usually 1 ∼ 3 GHz. In this work, the dynamics of skyrmions in the nanodisk with two antiferromagnetic-coupled free layers (Bilayer-skyrmions) are investigated. Our simulations show that the maximum frequency of Bilayer-skyrmions can be increased up to above 10 GHz, about one order of magnitude larger than that of Single-layer Skyrmion because of the disappearance of the SkHE in the antiferromagnetic-coupled system. In addition, it is found that the oscillation frequency of Bilayer-skyrmions linearly increases with the current density. Finally, spin-torque nano-oscillators based on multiple pairs of Bilayer-skyrmions are further demonstrated to increase the frequency limit up to 36.5 GHz. Our results may be useful for the design and applications of current-induced skyrmion-based oscillators for ultrahigh-frequency applications.

A Systematic Assessment of W-Doped CoFeB Single Free Layers for Low Power STT-MRAM Applications

Spin-transfer torque magnetoresistive random access memory (STT-MRAM) technology is considered to be the most promising nonvolatile memory (NVM) solution for high-speed and low power applications. Dual MgO-based composite free layers (FL) have driven the development of STT-MRAMs over the past decade, achieving data retention of 10 years at the cost of higher write power consumption. In addition, the need for tunnel magnetoresistance (TMR)-based read schemes limits the flexibility in materials beyond the typical CoFeB/MgO interfaces. In this study, we propose a novel spacerless FL stack comprised of CoFeB alloyed with heavy metals such as tungsten (W) which allows effective modulation of the magnet properties (Ms, Hk) while retaining compatibility with MgO layers. The addition of W results favours a delayed crystallization process, in turn enabling higher thermal budgets up to 180 min at 400 °C. The presence of tungsten reduces the total FL magnetization (Ms) but simultaneously increasing its temperature dependence, thus, enabling a dynamic write current reduction of ~15% at 2 ns pulse widths. Reliable operation is demonstrated with a WER of 1 ppm and endurance >1010 cycles. These results pave the way for alternative designs of STT-MRAMs for low power electronics.

Spin valves with Conetic based synthetic ferrimagnet free layer

The detection of low magnetic field requires hysteresis-free magnetoresistive sensor with high sensitivity. Giant magnetoresistance spin valve (SV) sensors with CoFeB based synthetic antiferromagnetic (SAF) pinned layer and Conetic based synthetic ferrimagnet (SF) free layer are developed in this paper. Low free layer coercivity (HC) of 0.27 Oe is achieved through adapting synthetic antiferromagnetic pinned layer in SV with Conetic single free layer. In SV with SAF pinned layer and SF free layer, the MR ratio, HC, switching field (HSW) and offset field (HO) increase with the thickness of the Conetic alloy layer right above the Cu spacer. The HC, HO, and HSW show oscillatory behavior with the Ru spacer thickness in the SF free layer. Through introducing CoFeB/SF free layer, an increased sensitivity of 0.27%/Oe and a small coercivity of 0.3 Oe are attained, which holds promise for low field sensing applications.

Magnetostrictive GMR spin valves with composite FeGa/FeCo free layers

We have fabricated strain-sensitive spin valves on flexible substrates by utilizing the large magnetostrictive FeGa alloy to promote the strain sensitivity and the composite free layer of FeGa/FeCo to avoid the drastic reduction of giant magnetoresistance (GMR) ratio. This kind of spin valve (SV-FeGa/FeCo) displays a MR ratio about 5.9%, which is comparable to that of the conventional spin valve (SV-FeCo) with a single FeCo free layer. Different from the previously reported works on magnetostrictive spin valves, the SV-FeGa/FeCo displays an asymmetric strain dependent GMR behavior. Upon increasing the lateral strain, the MR ratio for the ascending branch decreases more quickly than that for the descending branch, which is ascribed to the formation of a spiraling spin structure around the FeGa/FeCo interface under the combined influences of both magnetic field and mechanical strain. A strain sensitivity of GF = 7.2 was achieved at a magnetic bias field of -30 Oe in flexible SV-FeGa/FeCo, which is significantly larger than that of SV-FeCo.

Effects of Different Relevant Layers on Magnetic Properties of Bottom Synthetic IrMn Spin Valves

The effects of different relevant layers, especially the insertion layers (which are between Ta buffer layer and IrMn pinning layer) and free layers, on the magnetic properties of IrMn bottom-pinning spin valves are investigated. Spin valve with a NiFe insertion layer exhibits a higher GMR ratio of ~ 6.0% than that of 2.0% for the spin valve with a Cu insertion layer due to a better pinning strength. The spin valves with a CoFe/NiFe composite free layer have relatively better magnetic properties: a higher GMR ratio compared with the spin valve with a single NiFe free layer and a lower free layer coercivity compared with the spin valve with a single CoFe layer.

Spin-torque nano-oscillator based on a synthetic antiferromagnet free layer and perpendicular to plane polarizer

Coupled free layers systems used as active elements in spin-transfer nano-oscillators (STNOs) can have improved microwave performances in terms of linewidth or tunability. Here, we report a numerical study on the magnetization dynamics of a synthetic antiferromagnet (SAF) driven by a perpendicularly spin-polarized current and an in-plane applied magnetic field. Compared to the single free layer STNO, the current-field state diagrams, derived for strong and weak RKKY exchange coupling strength inside the SAF, show a more complex structure, with new static or dynamic states (chaotic dynamics), and also a larger zone of out-of-plane precession state (OPP) oscillations. The OPP frequency behaviour is generally similar to that of the single free layer STNO except for the zone near the chaotic dynamics and for fields larger than the spin-flop field of the SAF, where several frequency jumps have been observed.

Magnetotransport properties of dual MgO barrier magnetic tunnel junctions consisting of CoFeB/FeNiSiB/CoFeB free layers

We report the transport properties of a dual MgO barrier magnetic tunnel junction (DMTJ) where a FeNiSiB layer was inserted in a CoFeB free layer. Upon post-deposition annealing at 330 °C, the tunneling magnetoresistance (TMR) ratio of the DMTJ with a hybrid CoFeB/FeNiSiB/CoFeB free layer reached 195% which is higher than the TMR ratio of 121% from the DMTJ with the single CoFeB free layer. The bias voltage dependence profile was more symmetric for the hybrid case. Boron depth profiling result suggests that the FeNiSiB layer dragged boron atoms more to it rather than letting them diffuse toward CoFeB/MgO interfaces, resulting in improved MTJ performances.

Theory of Spin Torque Assisted Thermal Switching of Single Free Layer

The spin torque assisted thermal switching of the single free layer was studied theoretically. Based on the rate equation, we derived the theoretical formulas of the most likely and mean switching currents of the sweep current assisted magnetization switching, and found that the value of the exponent $b$ in the switching rate formula significantly affects the estimation of the retention time of magnetic random access memory. Based on the Fokker-Planck approach, we also showed that the value of $b$ should be two, not unity as argued in the previous works.

A Dual Free Layer Sensor With Side Shields

We use a hybrid finite element code to explore the design space of a dual free layer (or trilayer) magnetic sensor to understand the effect of the stripe height (SH) to track width (TW) ratio on the bias point, as well as the noise. Transfer curves and noise properties are found to be strongly sensitive to the SH/TW ratio compared to a standard single free layer sensor. Side shields can change the eigenmodes of the coupled system and modify the noise cancellation.

Numerical Study on Spin Torque Switching in Thermally Activated Region

We studied the spin torque switching of a single free layer in the thermally activated region by numerically solving the Landau–Lifshitz–Gilbert equation. We found that the temperature dependence of the switching time of the in-plane magnetized system is nonlinear, which means b≠1. Here, b is the exponent of the current term in the switching rate formula and has been widely assumed to be unity. This result enables us to evaluate the thermal stability of spintronics devices.

Dependence of spin-transfer switching characteristics in magnetic tunnel junctions with synthetic free layers on coupling strength

Dependence of spin-transfer switching characteristics on the interlayer exchange coupling strength in the MTJs with synthetic free layers of Co90Fe10/Ru/Co40Fe40B20 with strong coupling strength was investigated. In the MTJ with the relatively weakly coupled synthetic ferrimagnetic free layer, larger thermal stability (Δ0) and lower intrinsic critical current density (Jc0) than those of the MTJ with the single free layer were observed. Meanwhile, in the MTJs with the strongly coupled synthetic ferri- or ferromagnetic free layers, very large Δ0 and high Jc0 were observed probably due to high effective magnetic energy barrier. It was found that the MTJ with the relatively weakly coupled synthetic ferrimagnetic free layer is suitable for the STTRAM application.

Effect of Angular Dependence of Spin-Transfer Torque on Zero-Field Microwave Oscillation in Symmetric Spin-Valves

We numerically investigate effect of the angular dependence of spin-transfer torque (STT) on current-induced precession mode in symmetric spin-valve structures where two ferromagnets are free to move. Two spin-valve structures with NiFe and Co as ferromagnets are studied, which show considerably different angular dependence of STT. We find that the zero-field microwave oscillation occurs in symmetric spin-valve structures. Precession frequency can reach around 10 GHz and decreases with increasing current. In contrast to a spin-valve structure with a single free layer, the blue shift is not observed due to highly nonlinear coupling of two magnetizations. We find that more symmetric angular dependence of STT allows the magnetic excitation at a lower current and provides a more monotonous increase of output power.

Inter-diffusion study in MgO tunneling magneto-resistive (TMR) system by XPS

In this paper, we investigated the elemental inter-diffusion in MgO TMR system, namely, between MgO barrier and free layer (CoFeB, NiFe or their combination) interface and the oxygen diffusion into the capping layers (Ta, Ru, TaN) at elevated temperatures using simple sheet film stack to simplify the results interpretation. Boron, cobalt, iron, and nickel show various diffusion tendencies into the MgO barrier after annealing the sheet film stack. Oxygen has different penetration depth into single CoFeB free layer upon annealing under N 2 + Ar protective atmosphere for different capping layers. Ru and TaN capping layer provide much better O 2 diffusion barrier, compared with Ta capping layer. This could potentially change the boron segregation tendency at free layer and capping layer interface and thus affect the interface crystallization process and lattice matching between the crystallized CoFeB free layer and the MgO(0 0 1) barrier layer. All these effects will impact the overall TMR performance.

Transport Properties of Magnetic Tunnel Junctions Comprising NiFeSiB/CoFeB Hybrid Free Layers

We report on the magneto-transport measurements of MgO magnetic tunnel junctions (MTJs) composed of NiFeSiB/CoFeB as the free layer for two different structures (top-type and bottom-type pinning). The magneto-transport properties of these MTJs were investigated by varying the thickness of the amorphous NiFeSiB layer for a fixed CoFeB thickness. The tunnel magnetoresistance (TMR), measured in both type of structures, exhibit the same or a higher amplitude (up to 230% measured at room temperature in the case of top-type device), comparing to the case of a single CoFeB free layer. These results suggest that hybrids free layers can be used as good candidates for MTJs with reduced saturation magnetization while keeping a high TMR ratio.

SAF exhibits better resistance to roughness edge than single-free layer

Switching behavior and spin configuration of single-free layer and SAF-free layer with rough edge and under stray field are studied with micromagnetic simulation using Landau–Lifshize–Gilbert equation. From the simulation results, the R–H loops of single free layer in MTJs have some kinks. However, junctions with SAF-free layers exhibit kink-free R–H loops. Because of the enclosed magnetic flux of the SAF-free layer, unbalanced SAF-free layer shows kink-free switching, even with pronounced fabrication defects at the memory element boundaries.

Composite free layer for high density magnetic random access memory with lower spin transfer current

A magnetic tunnel junction (MTJ) structure with a composite free layer consisting of a nanocurrent-channel (NCC) layer sandwiched by two CoFe layers was proposed and investigated. The NCC layer increased the local spin current density inside the free layer and thus enhanced the writing capability for MTJ devices. In comparison with the conventional MTJ design with a single free layer, the intrinsic critical switching current density was reduced from 2.4×107to8.5×106A∕cm2 by using the composite free layer. On the other hand, the thermal stability factor of the composite free layer, KuV∕kBT, is around 149, which is almost the same as the value (159) for the MTJ device with a single free layer. The MTJ structure with the composite free layer is a candidate to solve the scaling problem for high density magnetic random access memory.

Current-Induced Magnetization Switching in Two Types of Nanopillar with Dual Fixed Layers

We study current-induced magnetization switching in nanofabricated magnetic multilayers with two different types of dual fixed layer structures. The first type is a single free layer with antiparallel aligned dual fixed layers. The second type is a synthetic antiferromagnetic (Syn.AF) free layer with parallel aligned dual fixed layers. In the former case, the switching current density (Jc) is reduced by a factor of more than two, as expected from our estimation. On the other hand, in the latter case, the dual structure does not reduce the Jc, which suggests the contributions of spin-dependent reflections and scatterings at various interfaces inside the free layer are significant. In some situations, they can reduce the total efficiency of the spin-transfer torque.

Activation barriers of submicron magnetoresistive random access memory cells with single and synthetic antiferromagnet free layers

Thermally activated magnetization reversals of submicron sized magnetic tunnel junctions with NiFe single and NiFe∕Ru∕NiFe synthetic antiferromagnet (SAF) free layers are investigated by varying the pulse durations and current amplitudes in switching pulse measurements. The measured data show good agreement with the switching probability predicted by the Arrhenius-Neel theory and switching behaviors with a single activation energy barrier. Estimated activation barriers for magnetization reversals are higher for magnetic tunnel junctions (MTJs) with SAF free layers than MTJs with single free layers. It is believed that the high activation barrier is achieved by the larger magnetic volume of SAF layers.

Magnetoresistance and magnetization switching characteristics of magnetic tunnel junctions with amorphous CoFeSiB single and synthetic antiferromagnet free layers

To obtain low switching field (Hsw) we introduced amorphous ferromagnetic Co70.5Fe4.5Si15B10 single and synthetic antiferromagnet (SAF) free layers in magnetic tunnel junctions (MTJs). The switching characteristics for MTJs with structures Si∕SiO2∕Ta 45/Ru 9.5/IrMn 10/CoFe 7∕AlOx∕CoFeSiB 7 or CoFeSiB (t)∕Ru 1.0/CoFeSiB (7−t)∕Ru 60 (in nanometer) were investigated and compared to MTJs with Co75Fe25 and Ni80Fe20 free layers. CoFeSiB showed a lower saturation magnetization of 560emu∕cm3 and a higher anisotropy constant of 2800erg∕cm3 than CoFe and NiFe, respectively. An exchange coupling energy (Jex) of −0.003erg∕cm2 was observed by inserting a 1.0nm Ru layer in between CoFeSiB layers. In the CoFeSiB single and SAF free layer MTJs, it was found that the size dependence of the Hsw originated from the lower Jex experimentally and by micromagnetic simulation based on the Landau-Lisfschitz-Gilbert equation. The CoFeSiB SAF structures showed lower Hsw than that of NiFe, CoFe, and CoFeSiB single structures. The CoFeSiB SAF structures were proved to be beneficial for the switching characteristics such as reducing the coercivity and increasing the sensitivity in micrometer- to submicrometer-sized elements.