Synthetic Free Layer Research Articles

All-electrical manipulation of magnetization in magnetic tunnel junction via spin–orbit torque

Besides spin-transfer torque, spin–orbit torque (SOT) provides us with another electrical way for developing magnetic random access memory (MRAM) based on magnetic tunnel junctions (MTJs). By using a CoFeB/Ta/CoFeB T-type magnetic structure as a synthetic free layer, we have realized a magnetic-field-free MTJ switchable by SOT. In the T-type structure, an in-plane CoFeB layer is coupled to a perpendicular CoFeB layer via a Ta spacer. The spacer layer not only mediates exchange coupling but also generates strong SOT, which drives sharp switching between the low and high resistance states of the MTJ without any applied magnetic fields. This work could provide an efficient way toward realization of field-free SOT-MRAM or logic devices.

Voltage-controlled magnetic tunnel junctions with synthetic ferromagnet free layer sandwiched by asymmetric double MgO barriers

Memory and computing applications utilizing voltage-controlled magnetic random-access memory (MRAM) require perpendicular magnetic tunnel junctions (pMTJs) capable of high thermal stability and large write efficiency at advanced technology nodes. We first discuss the scaling requirements for voltage-controlled MRAM to replace various existing memory applications at an advanced CMOS technology node, including cell size, thermal stability, interfacial perpendicular magnetic anisotropy (PMA), and voltage-controlled magnetic anisotropy (VCMA). For replacing volatile memories including SRAM, eDRAM, and DRAM, we employ the retention relaxation technique along with a DRAM-style refresh scheme in the analysis. To enhance both PMA and VCMA at scaled nodes, we explore pMTJs with asymmetric double MgO barriers sandwiching a synthetic ferromagnet (SyF) free layer. In a thin MgO/SyF free layer/thick MgO/fixed layer MTJ stack, the SyF free layer is realized in various ways: single layer (X = Ta, W, Mo, Ir) and bilayers (X = Ta/W, Mo/Ir) of heavy metals insertions in CoFeB/X/CoFeB and Co2FeAl/X/Co2FeAl structures, and multiple insertions in [CoFeB/X]n/CoFeB structures for n = 1–4. A VCMA coefficient of 36 fJ (V m)−1 is achieved in a MgO/CoFeB/Ta/CoFeB/MgO-based MTJ which is comparable to that of the single MgO barrier MTJ. We find PMA enhancement, although with VCMA reduction for an increasing number n of [CoFeB/X]n repetitions. In addition, we demonstrate large TMR of 78%, large interfacial PMA of 1.45 mJ m−2, and VCMA of 65 fJ (V m)−1 in a MgO/Co2FeAl/W/Co2FeAl/MgO-based MTJ annealed above 400 °C.

Detection of Sub-Nano-Tesla Magnetic Field by Integrated Magnetic Tunnel Junctions with Bottom Synthetic Antiferro-Coupled Free Layer

Arrays of 100×100 magnetic tunnel junctions (MTJs) connected in parallel and series were fabricated. A synthetic antiferro-coupled bottom free layer with a NiFe/Ru/CoFeB structure and MgO tunneling barrier were used to realize a high sensitivity, which is defined as TMR/2Hk, where, TMR is the tunneling magnetoresistance ratio and Hk is the magnetic anisotropy field of the free layer. To obtain a linear response of tunneling resistance against an applied external magnetic field, a double annealing process was carried out. From R–H curve measurements, the sensitivity of the 100×100 integrated MTJs was lower (8%/Oe) than that of a single MTJ (25%/Oe). However, a 1/30 decrease in noise power density was realized in the integrated MTJs. Consequently, a very small magnetic field of 0.29 nT was detected with the integrated MTJs.

Critical switching current and thermal stability of magnetic tunnel junctions with uncompensated CoFeB/Ru/CoFeB synthetic free layers

The critical switching current and thermal stability parameter are investigated for magnetic tunnel junctions with uncompensated synthetic ferrimagnetic free layers. The parameters are obtained by analyzing the experimental results for the thermally activated magnetization switching probability as functions of both a bias current and an applied magnetic field. The analysis is greatly facilitated by the use of an analytical equation for the applied magnetic field dependence of the energy barrier. A figure of merit given by the ratio of the two parameters differs substantially depending on the direction of the magnetization switching.

Spin-torque diode spectrum of ferromagnetically coupled (FeB/CoFe)/Ru/(CoFe/FeB) synthetic free layer

We investigated systematically the spin torque diode spectrum of a ferromagnetically coupled (FeB/CoFe)/Ru/(CoFe/FeB) synthetic free layer in an MgO-based magnetic tunnel junction. In the spectra, we observed single peaks shifted to higher frequency with increasing the in-plane magnetic fields, as expected from the ferromagnetic resonance of the FeB/CoFe adjacent to the MgO tunnel barrier. On the other hand, under the perpendicular fields, we observed several peaks below 6 GHz and around 10 GHz, which were rather insensitive to the field. These behaviors are different from our simple calculation taking account the interlayer coupling, suggesting that the excitation of magnetizations occurs in a complicated manner for the synthetic free layers.

Fabrication of magnetic tunnel junctions with a bottom synthetic antiferro-coupled free layers for high sensitive magnetic field sensor devices

Magnetic tunnel junctions with a Ni80Fe20/Ru/Co40Fe40B20 synthetic antiferro-coupled bottom free layer and an MgO barrier layer have been fabricated. Double annealing process was carried out in order to obtain linearity against magnetic field with hysteresis-free resistance response. The effect of the annealing temperature and NiFe thickness in the free layer on the magnetic field sensor performance was investigated. We have observed a very high sensitivity of 25.3%/Oe while keeping linearity.

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.

Theoretical study on dependence of thermal switching time of synthetic free layer on coupling field

The thermally assisted spin torque switching of a synthetic free layer was studied theoretically. Based on the rate equation, theoretical formulas of the switching probability, the switching current, and the standard deviation of the current were obtained. We showed that the switching time is proportional to the coupling field. We also showed that the standard deviation of the current is independent of the coupling field.

Micromagnetic study of current-pulse-induced magnetization switching in magnetic tunnel junctions with antiferromagnetically and ferromagnetically coupled synthetic free layers

We investigated the dynamics of current-pulse-induced magnetization switching in magnetic tunnel junctions (MTJs) with antiferromagnetically and ferromagnetically coupled synthetic free layers through micromagnetic simulations. We found that a magnetic vortex is formed in thick upper ferromagnetic layers and plays an important role in magnetization switching in both types of synthetic free layers. Furthermore, higher thermal stability is observed in an MTJ with the ferromagnetically coupled free layer at an annealing temperature of 250 °C.

Thermally assisted spin transfer torque switching in synthetic free layers

We studied the magnetization reversal rates of thermally assisted spin transfer torque switching in a ferromagnetically coupled synthetic free layer theoretically. By solving the Fokker-Planck equation, we obtained the analytical expression of the switching probability for both the weak and the strong coupling limit. We found that the thermal stability is proportional to Delta_{0}(1-I/I_{c})^{2}, not Delta_{0}(1-I/I_{c}) argued by Koch et al. [Phys. Rev. Lett. 92, 088302 (2004)], where I and I_{c} are the electric current and the critical current of spin transfer torque switching at absolute zero temperature. The difference in the exponent of (1-I/I_{c}) leads to a significant underestimation of the thermal stability Delta_{0}. We also found that fast switching is achieved by choosing the appropriate direction of the applied field.

Enhancement of Thermal Stability Using Ferromagnetically Coupled Synthetic Free Layers in MgO-Based Magnetic Tunnel Junctions

Very high thermal stability ?0 was demonstrated in MgO-based magnetic tunnel junctions (MTJs) with CoFeB/Ru/CoFeB ferromagnetically coupled synthetic free layers (F-coupled Sy). Samples had a structure of underlayer/PtMn(15)/CoFe(2.5)/Ru(0.85)/CoFeB(3)/MgO(1)/F-coupled Sy/Ta(5)/Ru(7), where F-coupled Sy was CoFeB(2)/Ru(1.5)/CoFeB(dCoFeB = 1-4 (nm unit). The MTJs were elliptical with approximate dimensions of 60 nm × 170 nm. Resistance-field (R - H) loops were measured repeatedly; the H-dependence of switching probability PSW was observed. From the distribution of switching field HC, thermal stability factor ?0 was evaluated based on the Sharrock equation. The ?0 values increased concomitantly with increasing thickness of the upper CoFeB layer dCoFeB. We obtained very large ?0 values of about 248 when dCoFeB = 4 nm. Such a large ?0 originates in enhanced shape magnetic anisotropy in the F-coupled Sy. The shape anisotropy is lost because of the low effective magnetization if the magnetizations are coupled antiferromagnetically. The F-coupled Sy with a thicker upper magnetic layer is suitable for use in nonvolatile memory cells in magnetoresistive random access memory.

Reduction of critical current in magnetic tunnel junctions with CoFeB/Ru/CoFeB synthetic free layer

Reduction of the critical current density (Jc) in magnetic tunnel junctions (MTJs) can be achieved by replacing the standard Co40Fe40B20 free layer with a synthetic antiferromagnet. Patterned MTJs prepared by ion-beam assisted deposition (nanopillars, sizes down to 60 nm × 80 nm) with 2 nm CoFeB free layer and Co40Fe40B20/Ru (tRu)/ Co40Fe40B20 as a synthetic free layer (SyF) were studied. We have measured critical current density of CIMS in thermally activated switching regime (long current pulses). Values of switching current densities for standard MTJs with SyF were of the order 106 A/cm2, whilst MTJs with standard free layer demonstrated up to four times higher values of Jc.

Thermal stability and spin-transfer switchings in MgO-based magnetic tunnel junctions with ferromagnetically and antiferromagnetically coupled synthetic free layers

We prepared MgO-based magnetic tunnel junctions having a CoFeB/Ru/CoFeB synthetic free layer in which magnetizations of the CoFeB layers were ferromagnetically coupled (F-coupled Sy) or antiferromagnetically coupled (AF-coupled Sy). We studied spin-transfer switchings to evaluate their thermal stability (Δ0=KV/kBT) and intrinsic switching current density (JC0). Although the free layers of two types showed nearly equal JC0, the Δ0 of F-coupled Sy was observed to be twice that of AF-coupled Sy. This difference is attributable to the shape magnetic anisotropy of the free-layer cells. Results show that F-coupled Sy is superior to AF-coupled Sy for memory applications.

Finite size effects on spin-torque driven ferromagnetic resonance in spin valves with a Co∕Ni synthetic free layer

Spin-torque driven ferromagnetic resonance (ST-FMR) is used to study magnetic excitations in Co∕Ni synthetic layers confined in nanojunctions. Field swept ST-FMR measurements were conducted with a magnetic field applied perpendicular to the layer surface. The resonance lines were measured under low amplitude excitation in a linear response regime. The resulting resonance fields were compared with those obtained using conventional rf field driven FMR on extended films with the same Co∕Ni layer structure. A lower resonance field is found in confined structures. The effect of both dipolar fields acting on the Co∕Ni layer emanating from other magnetic layers in the device and finite size effects on the spin wave spectrum are discussed.

Broad-band noise spectroscopy of giant magnetoresistive read heads

This paper describes wide-band (1 MHz-10 GHz) thermal magnetic fluctuation (TMF) noise measurements conducted on magnetoresistive devices such as giant magnetoresistive (GMR) and tunnel junction magnetoresistive (TMR) heads. The paper discusses the instrumentation, as well as the corrections necessary to separate the TMF noise from the total noise as collected. It shows that the thermal electrical noise component of this total noise is not white, but approximately follows the small-signal resistance of a saturated sensor versus frequency. The paper gives examples of noise imaging using magnetic field sweeps and electrical bias sweeps, and interprets the results. As another application example, it shows that the signal-to-noise ratio of a GMR head does not improve if one replaces the free layer by a synthetic (double) free layer with the same net moment. It presents examples of multimode TMF spectra, together with the causes of these extra resonances: spatial standing spin waves in the free layer and pinned layer resonances. It shows how one can calculate the stiffening field, damping, and free layer dimensions (magnetically active height and width) from the TMF spectra. These parameters can be obtained in a nondestructive fashion. They are difficult to measure in other ways.

Extension of the semiclassical theory of current-perpendicular-to-plane giant magnetoresistance including spin flip to any multilayered magnetic structures

In the framework of the Valet and Fert theory of current perpendicular to plane (CPP) giant magnetoresistance (GMR) in metallic multilayers [Valet and Fert, Phys. Rev. B 48, 7099 (1993)], the calculation of the CPP resistance and magnetoresistance has been generalized to any multilayered stacks including spin valves with synthetic free layers, laminated free and pinned layers, and dual spin valves. The theory takes into account bulk and interfacial spin-dependent scattering as well as spin flip in all layers. It also considers the effect of scattering at lateral edges of submicron multilayered pillars which can be viewed as a current in plane effect intruding on CPP transport. This latter effect plays a role when the diameter of the pillar becomes of the order of the elastic mean-free paths (i.e., below ∼30 nm). Based on the theory, a code has been developed to compute the CPP resistance and CPP magnetoresistance from the transport parameters of each material involved in the stack (spin-dependent resistivities, spin-dependent interfacial resistances, and spin-diffusion length in each layers). As examples, we compare the results of the calculations with various already published CPP experiments. In particular, we interpret experiments previously carried out on (NiFe/Cu/Co/Cu)N and (Co 6 nm/Cu/Co 1 nm/Cu)N multilayers in which the order of the layers in the stack had been shown to affect the CPP resistance and magnetoresistance, a property which could not be explained in a simple two-channel serial resistance model. We also investigate the influence of the thickness of the various layers and underline the key role of the spin diffusion length in these thickness variations. Unexpected predictions are made with this theory such as the existence of a maximum in CPP–MR as a function of the thickness of the antiferromagnetic pinned layer. This type of calculations should allow a faster optimization of CPP–GMR in metallic multilayers.

Synthetic ferrimagnet free layer tunnel junction for magnetic random access memories

In this work a synthetic ferrimagnet structure as a spin dependent tunnel junction free layer is proposed as a magnetic random access memories cell. The CoFe/Ru/CoFe and CoFe/NiFe/Ru/NiFe/CoFe systems show antiferromagnetically coupling range >1000 Oe and rotate rigidly anti-aligned with coercive fields below 30–40 Oe. Tunnel junctions were patterned to 4 μm×2 μm rectangles and ellipses using standard CoFe/Ni and synthetic ferrimagnet free layers. Identical signal levels are measured (17.8% and 16.3% as-deposited). Switching fields are dominated by material anisotropy for the synthetic ferrimagnet structure. Both free layer types show reversal by domain wall motion. Less switching dispersion is found for synthetic ferrimagnet free layer (≈10 Oe), with magnetization reversal occurring mostly in sharp single steps.

ＴＭＲ素子の磁化反転磁界抑制の検討

Two types of laminated structures for free layers were investigated to avoid the increase in the switching field of the free layer caused by the reduction of the size of the TMR element. Laminated free layers of NiFe(5)/Ru(0.7)/NiFe(3) and NiFe(4)/Ta(3)/NiFe(4) showed a lower switching field than a single free layer of NiFe(8). The low switching field of the synthetic free layer is due to the decrease in the net moment, which results in a decrease of the demagnetizing field. In the case of the NiFe(4)/Ta(3)/NiFe(4) free layer, there is a possibility that magnetostatic coupling plays an important role in the decrease in the demagnetizing field.

Spin valve sensors with synthetic free and pinned layers

Unshielded spin-valve sensors (6×2 μm2) with both synthetic ferrimagnet free (SF) and synthetic antiferromagnet pinned (SAF) layers are demonstrated, exhibiting high sensitivities, good thermal stability and reduced free layer effective magnetic thickness (teff). The free layer teff can be reduced to ∼10 Å while maintaining its physical thickness near 60 Å, without significant magnetoresistive signal loss (MR∼6%). After current biasing, the sensors show a linearized response, demonstrating a coherent switching of the synthetic free layer. Sensor sensitivity (S) increases with decreasing free layer effective magnetic thickness (S≈0.5%/Oe at teff=10 Å) as expected from the decrease of the free layer demagnetizing field. The coupling field (Hf) between the free and pinned layers increases with decreasing teff (Hf∝Ms2/(teffMefff)). The sensors are thermally stable upon anneals up to 300 °C.

Spin valves with synthetic ferrimagnet and antiferromagnet free and pinned layers

Spin valve structures with both synthetic ferrimagnet F/sub a//Re/F/sub b/ free layers and synthetic antiferromagnet F/sub 1//Re/F/sub 2//AF pinned layers are demonstrated. Physical thicknesses for the free layer of /spl ap/50 /spl Aring/ can be maintained while decreasing the effective magnetic thickness (t/sub eff/) to 8-10 /spl Aring/, without significant signal loss. Inverted structures with the synthetic free layer on top present magnetoresistance (MR) values up to 7% for t/sub eff//spl sim/17 /spl Aring/. The synthetic pinned layer gives high unidirectional exchange H/sub ex/ (/spl sim/1250 Oe) and saturation fields H/sub s/ (/spl sim/3-4 KOe) with no significant change after several hours anneal at 300/spl deg/C. MR decreases linearly as the temperature increases ((1/MR(RT))x dMR(T)/dT/spl ap/-0.0017//spl deg/C).