Synthetic Antiferromagnetic Layer Research Articles

Implementation of Logic Function and Memristive Behavior in Synthetic Antiferromagnet with Strong Immunity to Perpendicular Magnetic Field Interference

Spintronic devices with heavy metal/ferromagnet structures have shown potential applications for in‐memory computing. However, the sensitivity of ferromagnet to external magnetic field raises a challenge for the practical applications of these devices. The usage of synthetic antiferromagnet (SAF) can effectively address this issue due to its good magnetic field immunity. This work demonstrates the implementation of all 16 types of Boolean logic functions via controlling the magnetization states of the SAF layer by using current pulses, and also the stable multistate storage under the interference of perpendicular magnetic field in these devices. The SAF devices also exhibit synaptic plasticity under the stimulation of current pulses. The artificial neural network constructed based on the SAF device can perform handwritten digit recognition tasks with an accuracy rate of 90%. These results showcase the viability and superiority of the SAF device in building logic‐in‐memory architecture for its strong immunity to magnetic field interference.

Co/NM/Pt]n-based seedless synthetic anti-ferromagnetic layer design for preventing MgO crystallinity degradation from Pt diffusion

Co/NM/Pt]n-based seedless synthetic anti-ferromagnetic layer design for preventing MgO crystallinity degradation from Pt diffusion

Sub-terahertz excitations in a synthetic antiferromagnet with perpendicular anisotropy

In this paper, micromagnetic simulations are employed to investigate terahertz (THz) magnetic excitations in a spin torque nano-oscillator (STNO) with a perpendicularly magnetized synthetic antiferromagnetic (SAF) free layer. The magnetization precession of the free layer can be finely tuned into the sub-THz range without the necessity of external magnetic fields. The excited frequency exhibits two distinctive regions, namely region-I and region-II, depending on the applied current strength. In region-I, characterized by relatively small currents, the two ferromagnetic layers are stabilized at two separate precession orbits. The frequency in this region decreases with current strength, exhibiting similar features as the Néel vector change observed in antiferromagnets. In contrast, region-II is defined by currents where the two ferromagnetic layers synchronize into the same precession orbit. The frequency increases with current, correlating with the variation in the net magnetization of the SAF layer. An analytical model is developed through the canonical transformation of Lagrange’s equation, which can describe the frequency dependence on both the applied current and the antiferromagnetic interlayer coupling strengths. The simulations and the analytical model show good agreement, offering a more profound understanding of the magnetic excitation properties in STNOs with ultrathin SAF free layers. These insights are crucial for the design of advanced terahertz spintronic devices.

Heusler-alloy-based magnetoresistive sensor with synthetic antiferromagnet

Heusler alloy has been widely utilized in magnetoresistive sensors to enhance the device performance. In this work, we theoretically investigate the performance of Heusler-alloy-based magnetoresistive sensors with a synthetic antiferromagnet (SAF) layer. The atomistic model combined with the spin accumulation model will be used in this work. The former is used to construct the reader stack and investigate the magnetization dynamics in the system. The latter is employed to describe the spin transport behavior at any position of the structure. We first perform simulations of the exchange bias (EB) phenomenon in the IrMn/ (CFS) system providing a high EB field. Then, a realistic reader stack of IrMn/CFS/Ru/CFS/Ag/CFS is constructed via an atomistic model. Subsequently, the resistance–area product (RA) and magnetoresistance (MR) ratio of the reader can be calculated by using the spin accumulation model. As a result of the spin transport behavior in the Heusler-alloy-based reader stack including SAF structure at 0 K, an enhancement of the MR ratio up to 120 and RA can be observed. This study demonstrates the important role of the Heusler alloy and SAF layer in the development of magnetoresistive sensors for the application of readers in hard disk drives with an areal density beyond 2 Tb in−2.

On-chip full bridge bipolar linear spin valve sensors through modified synthetic antiferromagnetic layers

Realizing the rising demand for high-sensitivity magneto-resistive (MR) sensors in the industry, an attempt was made towards the fabrication of a spin valve (SV) based bipolar sensor on chip in the form of a full Wheatstone bridge with push–pull configuration. For a sensor with a push–pull configuration, the opposite arms are configured with opposite sense layers. To achieve this, either odd or even layers of synthetic antiferromagnetic (SAF) structure were introduced in the conventional spin valve structure, through which the magnetization orientation of the sensing layer was regulated by either negative or positive sensitivity to the applied field direction respectively. A detailed theoretical simulation was carried out to achieve the stable antiparallel magnetic state around zero fields by varying individual layer thicknesses in the case of single and double-layer SAF structures. With optimized conditions, experimental data were presented, and a bipolar device with a sensitivity of the order of 0.7 mV/V/Oe in the linear field range of ± 15 Oe was fabricated. The performance of the device was characterized by a wider temperature range and it was observed that the device has high thermal stability in the range of - 40 °C to 125 °C with a thermal coefficient of the voltage around 12 μV/V/∘C. Angular performance of the fabricated sensor with minimized orthogonality error was presented and it was observed that the sensor can measure the angle with an accuracy of +/-2 ° .

Easy-plane dominant stochastic magnetic tunnel junction with synthetic antiferromagnetic layers

Easy-plane dominant stochastic magnetic tunnel junction with synthetic antiferromagnetic layers

Implementation of a full Wheatstone-bridge GMR sensor by utilizing spin–orbit torque induced magnetization switching in synthetic antiferromagnetic layer

A giant magnetoresistance (GMR) sensor with a Wheatstone bridge structure and an out-of-plane linear response was developed. The spin-valve structure consists of a synthetic antiferromagnetic [(Co/Pt)n/Ru/(Pt/Co)n] reference layer with perpendicular magnetic anisotropy, a Cu spacer layer, and a Co-free layer with in-plane easy magnetization. By utilizing the spin–orbit torque induced magnetization switching in the synthetic antiferromagnetic layer, the magnetization of the reference layers in the adjacent bridge arms is set to the opposite direction, achieving a GMR sensor with a full Wheatstone bridge structure. The sensor exhibits linear response to the out-of-plane magnetic field with adjustable dynamic ranges from hundreds to thousands of Oe, depending on the thickness of the Co-free layer. A similar Wheatstone bridge sensor consisting of magnetic tunnel junctions was also proposed. The sensor with out-of-plane linear response may have promising applications in three-dimensional magnetic field detection and current sensing field.

Impact of Reference-Layer Stray Field on the Write-Error Rate of Perpendicular Spin-Transfer-Torque Random-Access Memory

A finite-temperature micromagnetic study of magnetization switching and write-error rates in a perpendicular magnetic tunnel junction with and without synthetic antiferromagnetic layer (SAF) is presented. In the absence of SAF, magnetization switching is induced by domain-wall nucleation and propagation. Although the various modes of domain-wall propagation are observed to delay switching, it does not show an appreciable impact on the overall write-error-rate slopes. In the presence of the nonuniform stray field from the SAF assembly, the domain-wall-based switching modes turn on more complex magnetization dynamics that impedes the switching process. In cases where the SAF layers fail to balance each other contributing to a stronger stray field, incoherent switching modes give rise to metastable states with significantly longer lifetimes, and a dramatic change in the write-error slopes is observed. Simulation results are compared to recent experimental findings from time-domain measurements of spin-transfer-torque switching and measurements of anomalous write-error rates. These results directly prove the long-predicted relation of the SAF stray field to write-error anomaly in perpendicular spin-transfer-torque magnetic random-access memory and could be useful to solve the anomalous write-error problems.

Control of interface anisotropy for spin transfer torque in perpendicular magnetic tunnel junctions for cryogenic temperature operation

The possibility of higher electrical efficiency in computing by operating at low temperatures raises the need for non-volatile memory cells optimized for cryogenic operation. We report a study on low temperature spin transfer torque switching of magnetic tunnel junctions with 20 to 100 nm in diameter with thermal stability adapted to low temperature operation. The evolution of magnetic and electrical properties are characterized for four different stacks from 300 to 10 K comprising insertions of Mg, Ru and permalloy (Py) in the storage layer to reduce its effective anisotropy. Two figures of merit are used to compare different devices and stacks, Δ/Ic and Δ/Esw, normalizing the thermal stability Δ by the critical current or switching energy. Devices with a Py insertion layer show a higher FOM (3.78 kBTop/μA) and switching energy Esw below 655 fJ for 100 ns pulses at Top = 10 K. A procedure to optimize the reference layer stray field was also implemented to achieve full compensation using a synthetic antiferromagnetic layer for 20 nm diameter devices.

Controlling domain wall and field-free spin–orbit torque switching in synthetic antiferromagnets

Perpendicular magnetization switching driven by spin–orbit torques plays an increasingly important role for spintronic devices toward practical applications but is also hindered by the well-known technical challenge that an external in-plane magnetic field is required for deterministic switching. Here, we show that the deterministic switching can be achieved in synthetic antiferromagnets through the flexible domain control in the absence of external magnetic fields. Specifically, we have observed that the domain wall (DW) distorts under an applied electric current in contrast to the conventional rigid DW motion in a single ferromagnet. More importantly, the distorted DWs can be precisely controlled under zero magnetic field, leading to the deterministic switching. Our results indicate that the critical technical challenge may be addressed by employing a synthetic antiferromagnetic layer through the DW motion dominated field-free switching.

Angle-dependent switching in a magnetic tunnel junction containing a synthetic antiferromagnet

The angle dependence of field-induced switching was investigated in magnetic tunnel junctions with in-plane magnetization and a pinned synthetic antiferromagnet reference layer. The 60 × 90 nm2 elliptical nanopillars had sharp single switches when the field was applied along the major axis of the ellipse, but even with small (20°) deviations, reversal occurred through an intermediate state. The results are interpreted with a model that includes the external applied field and the effective fields due to shape anisotropy and the fringe field of the synthetic antiferromagnet and used to extract the magnetization direction at various points in the magnetoresistance loop. The implications for faster spintronic probabilistic computing devices are discussed.

Enhancement of current to spin-current conversion and spin torque efficiencies in a synthetic antiferromagnetic layer based on a Pt/Ir/Pt spacer layer

We investigated the current-induced spin-orbit torque (SOT) originating from the spin Hall effect in stack systems with perpendicularly magnetized Co/Pt/Ir/Pt/Co synthetic antiferromagnetic (AFM) structures which exhibit nearly compensated magnetization and interlayer exchange coupling of ${J}_{\mathrm{ex}}=0.11\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{m}}^{2}$. The results were compared with ferromagnetic stack systems with perpendicularly magnetized Pt/Co and (Ir/Pt)-multilayer/Co structures. The magnetizations of the two Co layers in the Co/Pt/Ir/Pt/Co synthetic AFMs can be switched between two antiparallel states simultaneously by SOT. By this switching mechanism, the current to spin-current conversion and spin-torque efficiencies in a synthetic AFM layer stack based on a Pt/Ir/Pt spacer layer are about twice as high as those of a ferromagnetic stack using a Pt heavy metal electrode. The efficient switching of compensated synthetic AFMs would advance magnetic memory devices with high density, high speed, and low power consumption. We expect the Pt/Ir/Pt spacer layer paves the way to AFM spintronics based on multilayer systems.

High-efficiency array-level MRAM parameters extraction with the device-in-series test structure

The precise extraction of magnetic tunnel junction parameters at device level is important for understanding the weak point and its root cause in the stack design, which allows for future developments. The related variability is also vital for a reliable memory technology. Current test methods, however, are limited either to the material level or low efficiency. In this work, a device-in-series structure is proposed that directly monitors the statistical properties of the devices. This allows for a massively parallel measurement and, in this way, permits an accurate, high-efficiency testing with the device-to-device variability embedded intrinsically. Based on this method, we studied the temperature dependence of spin-transfer torque magnetoresistive random access memory’s retention from 12 to 300 K, using a statistical domain wall switching model. The synthetic antiferromagnetic layers are more immune to the temperature change, compared with the free layer. The magnetoresistance is found to be a convex function of the temperature below 100 K, which contrasts the single-device measurements. The results show that as the temperature decreases, the domain wall shrinks and the zero-field energy barrier still increases.

Thermally robust synthetic antiferromagnetic fixed layers containing FeCoB for use in STT-MRAM devices

In this work we measure the bilinear and biquadratic interlayer exchange coupling, J1 and J2, for several different synthetic antiferromagnetic structures containing FeCoB both before and after annealing at 350°C, with Ru spacer layers ranging in thicknesses from 0.4 to 1 nm. We show that when FeCoB is on top of the Ru spacer layer, in a Co/Ru/FeCoB trilayer structure, it is able to maintain antiferromagnetic coupling after annealing at 350°C, for Ru spacer layer thicknesses of 1 nm or less. This is in contrast to coupling in FeCoB/Ru/FeCoB trilayer structures that becomes strongly ferromagnetically coupled after annealing above 300°C. This difference is thought to be caused by boron diffusion into the spacer layer, which in turn enhances the diffusion of magnetic atoms into the spacer layer. We also show that coupling in a FeCoB/Co/Ru/FeCo/FeCoB structure remains AFC after annealing at 350°C, for Ru spacer layer thicknesses of 0.8 nm or less. This is thought to be caused by the Co and FeCo layers acting as diffusion barriers and reducing the diffusion of boron into the Ru spacer layer during the annealing process. We also show that, in a Co/Ru/Co trilayer structure, J1 can be increased in magnitude by replacing the top Co layer with FeCo, for spacer layer thicknesses of 0.5 nm or less. After these samples have been annealed at 350°C, the replacement of the top Co layer with FeCo was found to result in a two fold increase in the magnitude of J1, for Ru spacer layer thicknesses of 0.5 nm or less.

Synthetic antiferromagnetic layer based on Pt/Ru/Pt spacer layer with 1.05 nm interlayer exchange oscillation period for spin–orbit torque devices

We investigated interlayer exchange coupling through Pt/Ru/Pt and Pt/Ru multilayers as candidates of nonmagnetic spacer layers in the synthetic antiferromagnetic (AF) layer, which is available for studying AF spintronics using current-induced spin–orbit torque (SOT) switching originating from the spin Hall effect. The AF interlayer exchange coupling with the oscillation period of Λ2 ∼ 1.05 nm was observed even for the face-centered cubic (fcc) Pt (tPt)/hexagonal Ru/fcc Pt (tPt) nonmagnetic spacer layer structures in the wide range of both Pt and total nonmagnetic spacer layer thicknesses (0 ≤ tPt ≤ 0.8 nm, 1.0 ≤ ttotal ≤ 2.3 nm), which would be useful for the systematic investigation of the SOT on the AF structure. Moreover, we observed the disappearance of the one oscillation period (Λ1 ∼ 1.65 nm) in the case of Pt(111)/Ru(0001) and Pt(111)/Ru(0001)/Pt(111) spacer layers, whereas the existence of two oscillation periods of AF interlayer exchange coupling (Λ1 ∼ 1.65 nm and Λ2 ∼ 1.05 nm) in the case of Ru spacer layer was observed. We expect that the Pt/Ru/Pt spacer layer with the oscillation period of Λ2 ∼ 1.05 nm will pave a way to the AF spintronics based on the multilayer systems.

The heavy ions irradiation effects on advanced spin transfer torque materials

We investigate the heavy ions irradiation effect on the spin transfer torque film stacks with perpendicular magnetic anisotropy. Samples are exposed to 175 MeV Cl ion and 235 MeV Ge ion at the fluence of 1x105 ions/cm2 and 1x106 ions/cm2. Measurements of magnetization vs. magnetic field are performed on the film stacks before and after irradiation. The results exhibit that magnetic properties of free layer (CoFeB) in the spin transfer torque film keeps unchanged while magnetic properties of the synthetic antiferromagnet layer (Co/Pt) and the ferromagnetic exchange coupling between reference layer (CoFeB) and synthetic antiferromagnet layer through W layer have changed significantly when the fluence of the two kinds of ions reach 1x106 ions/cm2. We analysis the irradiation damage of the film stacks by SRIM simulation and give a reasonable explanation for this phenomenon. Thus, our work demonstrates that perpendicular spin transfer torque MRAM technology itself, the memory element construction, is subject to damage from heavy ions irradiation.

Antiferromagnetic interlayer exchange coupling and large spin Hall effect in multilayer systems with Pt/Ir/Pt and Pt/Ir layers

We investigated Pt/Ir/Pt and Pt/Ir multilayers as candidates of nonmagnetic spacer layers in synthetic antiferromagnetic (AF) layers, which are available for the systematic study on AF spintronics. In these systems, we observed (i) AF interlayer exchange coupling in Pt/Ir/Pt and Pt/Ir nonmagnetic spacer layers sandwiched by Co layers and (ii) large spin Hall conductivity in Pt/Ir multilayer heavy-metal systems which is essential to achieve low power consumption spin-orbit torque switching. We found that total nonmagnetic spacer layer thickness $[{t}_{\mathrm{total}}={t}_{\mathrm{Pt}}(\mathrm{Pt}\phantom{\rule{0.16em}{0ex}}\mathrm{thickness})+{t}_{\mathrm{Ir}}(\mathrm{Ir}\phantom{\rule{0.16em}{0ex}}\mathrm{thickness})]$ range in which AF interlayer exchange coupling is observed is wide in Co/nonmagnetic spacer layer/Co with Pt/Ir/Pt and Pt/Ir nonmagnetic spacer layers. Moreover, the large spin Hall angle of ${\ensuremath{\theta}}_{\mathrm{SH}}=10.3%$ and low resistivity of ${\ensuremath{\rho}}_{xx}=35\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$ in Pt/Ir multilayer heavy metal are observed. These results indicate that Pt/Ir/Pt and Pt/Ir are nonmagnetic spacer layers allowing us to achieve the AF interlayer exchange coupling and generation of large spin-orbit torque via spin Hall effect in synthetic AF coupling layer system.

Thermal stability of SOT-MTJ thin films tuning by multiple interlayer couplings

We investigated the thermal stability of spin-orbit torque magnetic tunnel junction (SOT-MTJ) thin films by tuning multiple interlayer couplings among synthetic anti-ferromagnetic layer (SAF), reference layer and free layer. We find that the bridge layer (here we use W) can influence the thermal stability of perpendicular magnetic anisotropy (PMA) in all the magnetic layers through the interlayer couplings. Increasing the thickness of bridge layer decouples the SAF and reference layer, resulting in higher PMA of SAF layer and lower PMA of reference layer. The reference layer further adjusts the PMA of free layer by orange-peel coupling via the tunneling barrier layer MgO such that we find another way to optimize the anisotropy without changing ferromagnetic layers. This work is helpful for engineering the interlayer coupling of SOT-MTJ thin films to achieve SOT devices with high thermal stability.

Asymmetric magnetization reversal of the Heusler alloy Co2FeSi as free layer in an CoFeB/MgO/Co2FeSi magnetic tunnel junction

We report the in-plane magnetization reversal behavior of the Co2FeSi Heusler alloy free layer in a bottom-pinned magnetic tunnel junction film with 149% TMR. Using magneto-optic indicator film imaging, we visualize the magnetic domain dynamics of this buried layer integrated within a full magnetic tunnel junction stack. The magnetic domain dynamics reveal anisotropic magnetization reversal within Co2FeSi under applied in-plane magnetic fields. While the reversal behavior under in-plane fields applied perpendicular to the in-plane exchange bias direction indicates smooth, coherent rotation away from the easy axis, asymmetric magnetic reversal behavior is observed along the easy axis. Reversed domains propagate from the interior of the film laterally outward toward the edges as the free layer magnetization switches into parallel alignment with the pinned synthetic antiferromagnetic reference layer (IrMn/CoFe/Ru/CoFeB). On the other hand, domains propagate from the film edges inward for the free layer transition into antiparallel alignment with the reference layer. These results have important implications for Heusler magnetic tunnel junction device performance.

Effects of synthetic antiferromagnetic coupling on back-hopping of spin-transfer torque devices

A synthetic antiferromagnetic (SAF) layer is a key component in spin-transfer torque magneto-resistive random-access memory devices. This study reveals that slight fluctuations in SAF coupling at the margin of the reference layer and hard layer (i.e., concurrent reversal) can lead to write errors in the form of back-hopping (BH). It appears that variable BH behavior can be attributed to competition between antiparallel (AP) → parallel (P) and P → AP transitions associated with SAF coupling. Our conclusions are supported by careful analysis of switching phase diagrams and measurements of self-heating and voltage-controlled magnetic anisotropy. We also observed that one form of coupling provided higher perpendicular magnetic anisotropic energy and thermal stability, which is likely due to the Dzyaloshinskii–Moriya interaction (DMI) effect. Thus, minimizing variations in DMI by optimizing SAF coupling is crucial for minimizing write error rates.