Perpendicular Recording Research Articles

Writing temperature and thickness dependence of perpendicular magnetic-dot magnetization reversal probability

Investigation is necessary to figure out magnetization dynamics at high temperatures due to the low power requirement for heat-assisted magnetic recording technology. The Landau-Lifshitz-Gilbert equation was employed to assess the dynamic of magnetization in perpendicular magnetic-dots at elevated temperatures, considering the thermal contribution to the effective field. The simulated perpendicular magnetic-dot has a 50×50 nm2 surface area with thicknesses of 10 nm, 50 nm, and 100 nm. This study reveals that the probability of magnetization reversal and the threshold field in the thermally-assisted magnetic reversal process of a magnetic dot is influenced by the thickness and writing temperature. Below the Curie point, as sample thickness increases, demagnetization energy increases, requiring a larger external magnetic field for magnetization reversal. For the thicker sample, the magnitude of the writing field decreases as the temperature increases, which corresponds to temperature-dependent magnetization. Meanwhile, the optimum writing field is obtained below the Curie point. The magnetic-dot with 100 nm of thickness has an effective writing temperature of 739.7 K with a corresponding writing field is 250 Oe while its reversal probability at zero magnetic field is up to 0.2. Magnetic-dot thickness affects magnetization reversal probability due to domain wall propagation mechanisms during magnetic switching that result from the demagnetization factor. Moreover, as the temperature approaches the Curie temperature, the probability of zero-field magnetization reversal in the absence of an external magnetic field is observed to escalate which has a direct impact on the writing field magnitude. It was found that the effective writing temperature is below the Curie temperature, which allows several hundred Oersted of the writing field.

Electric-Field Control of Perpendicularly Magnetized Ferrimagnetic Order and Giant Magnetoresistance in Multiferroic Heterostructures.

Electrical control of magnetism is highly desirable for energy-efficient spintronic applications. Realizing electric-field-driven perpendicular magnetization switching has been a long-standing goal, which, however, remains a major challenge. Here, electric-field control of perpendicularly magnetized ferrimagnetic order via strain-mediated magnetoelectric coupling is reported. We show that the gate voltages isothermally toggle the dominant magnetic sublattice of the compensated ferrimagnet FeTb at room temperature, showing high reversibility and good endurance under ambient conditions. By implementing this strategy in FeTb/Pt/Co spin valves with giant magnetoresistance (GMR), we demonstrate that the distinct high and low resistance states can be selectively controlled by the gate voltages with assisting magnetic fields. Our results provide a promising route to use ferrimagnets for developing electric-field-controlled, low-power memory and logic devices.

Wedge-shaped HfO2 buffer layer-induced field-free spin-orbit torque switching of HfO2/Pt/Co structure

Field-free spin–orbit torque (SOT) switching of perpendicular magnetization is essential for future spintronic devices. This study demonstrates the field-free switching of perpendicular magnetization in an HfO2/Pt/Co/TaO x structure, which is facilitated by a wedge-shaped HfO2 buffer layer. The field-free switching ratio varies with HfO2 thickness, reaching optimal performance at 25 nm. This phenomenon is attributed to the lateral anisotropy gradient of the Co layer, which is induced by the wedge-shaped HfO2 buffer layer. The thickness gradient of HfO2 along the wedge creates a corresponding lateral anisotropy gradient in the Co layer, correlating with the switching ratio. These findings indicate that field-free SOT switching can be achieved through designing buffer layer, offering a novel approach to innovating spin–orbit device.

Ultrastrong-coupled magnon–polariton in a dynamical inductor based on magnetic-insulator/topological-insulator bilayers

We theoretically study a magnon–polariton in a dynamical inductor consisting of a coil and a bilayer of ferromagnetic insulator (FI) with perpendicular magnetization and a three-dimensional topological insulator (TI). Taking into account the coherent magnetic-dipole (Zeeman) interaction as well as the dissipative spin pumping effect on the FI/TI interface, we formulate a dynamical permeability of the FI/TI bilayer and calculate a corresponding electric admittance of an RLC-circuit. As a result, the spin pumping gives rise to a competitive effect on the modes hybridization, whereas a huge level repulsion due to the coherent coupling, which reaches the ultrastrong coupling regime, is found at room temperature for realistic parameter sets. Our work may open a new avenue for designing on-chip microwave devices based on a magnon–polariton in the RLC-circuit.

Magnetization switching driven by spin-orbit torque of Weyl semimetal WTe<sub>2</sub>

The Wely semimetal WTe<sub>2</sub> exhibits significant spin-orbit coupling characteristics and can generate unconventional spin current with out-of-plane polarization, which has become a hotspot in recent years. Meanwhile, WTe<sub>2</sub> also has high charge-spin conversion efficiency, allowing perpendicular magnetization to be switched deterministically without the assistance of an external magnetic field, which is critical for the high-density integration of low-power magnetic random-access memories. The purpose of this paper is to review the recent advances in the research on spin orbit torque in heterostructures composed of WTe<sub>2</sub> and ferromagnetic layers, focusing on progress of research on the detection and magnetization switching in the spin orbit torque of heterojunctions composed of WTe<sub>2</sub> prepared by different methods (e.g. mechanical exfoliation and chemical vapor deposition) and ferromagnetic layers such as conventional magnets (e.g, FeNi and CoFeB, etc.) and two-dimensional magnets (e.g. Fe<sub>3</sub>GeTe<sub>2</sub>, etc.). Finally, the prospect of related research is discussed.

Crystal orientation regulation of spin-orbit torque efficiency and magnetization switching in SrRuO<sub>3</sub> thin films

Spintronic devices utilize the spin property of electrons for the storage, transmission, and processing of information, and they possess inherent advantages such as low power consumption and non-volatility, thus attracting widespread attention from both academia and industry. Spin-orbit torque (SOT) is an efficient method of manipulating magnetic moments through using electric current for writing, controlling the spin-orbit coupling (SOC) effect within materials to achieve the mutual conversion between charge current and spin current. Enhancing the efficiency of charge-spin conversion is a critical issue in the field of spintronics. Strontium ruthenate (SRO) in transition metal oxides (TMO) has attracted significant attention as a spin source material in SOT devices due to its large and tunable charge-to-spin conversion efficiency. However, current research on SOT control in SRO primarily focuses on utilizing substrate strain, with limited exploration of other control methods. Crystal orientation can produce various novel physical properties by affecting material symmetry and electronic structure, which is one of the important means to control the properties of TMO materials. Considering the close correlation between the SOT effect and electronic structure as well as surface states, crystal orientation is expected to affect SOT properties by adjusting the electronic band structure of TMO. This work investigates the effect of crystal orientation on the SOT performance of SrRuO<sub>3</sub> film and develops a novel approach for SOT control. The (111)-oriented SRO/CoPt heterostructures and SOT devices are prepared by using pulse laser deposition, magnetron sputtering, and micro-nano processing techniques. Through harmonic Hall voltage(HHV) measurements, we find that the SOT efficiency reaches 0.39, and the spin Hall conductivity attains 2.19×10<sup>5</sup><inline-formula><tex-math id="Z-20240522222523">\begin{document}$\hbar $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240367_Z-20240522222523.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240367_Z-20240522222523.png"/></alternatives></inline-formula>/2<i>e</i> Ω<sup>–1</sup>·m<sup>–1</sup>, which are 86% and 369% higher than those of the (001) orientation, respectively. Furthermore, current-driven perpendicular magnetization switching is achieved in SrRuO<sub>3</sub>(111) device at a low critical current density of 2.4×10<sup>10</sup> A/m<sup>2</sup>, which is 37% lower than that of the (001) orientation. These results demonstrate that the crystal orientation can serve as an effective approach to significantly enhancing the comprehensive performance of SrRuO<sub>3</sub>-based SOT devices, thus providing a new idea for developing high-efficiency spintronic devices.

Field-free spin–orbit torque-induced switching of perpendicular magnetization at room temperature in WTe2/ferromagnet heterostructures

Spin–orbit torque provides an efficient way to achieve switching of perpendicular magnetization, which is essential for designing energy-efficient spintronic devices. An in-plane antidamping torque combined with an out-of-plane antidamping torque can often deterministically switch perpendicular magnetization without an external magnetic field. Encouragingly, field-free perpendicular magnetization switching of a two-dimensional (2D) material WTe2/ferromagnet bilayer has been reported recently, but the working temperature (< 200 K) is quite below room temperature. Here, we demonstrate field-free perpendicular magnetization switching in the Pt/Co/Pt/WTe2 multilayer films at room temperature, which is mainly attributed to the out-of-plane antidamping torque originating from the WTe2 layer. In addition, current-induced perpendicular magnetization switching at zero magnetic field is also accomplished in the [Co/Pt]2/WTe2 multilayer film with a very large perpendicular magnetic anisotropic field (∼13 600 Oe), which is very useful for practical applications. This work offers a potential way to develop spintronic devices based on 2D materials at room temperature.

Quasi‐hexagonal nanopatterned porous cobalt ferrite thin films prepared via block copolymer self‐assembly

AbstractWe present a novel method for fabricating dense and nanopatterned porous cobalt ferrite (CoFe2O4) thin films with a remarkable thickness of 65 nm, achieved through direct block copolymer self‐assembly. By decomposing the block copolymer and inducing crystallization of the spinel phase, we successfully produce films exhibiting quasi‐hexagonal arrays of pores with an average diameter ranging from 60 to 80 nm. The resulting nanopatterned porous thin films demonstrate exceptional orderliness, as the pores are intricately designed by the cobalt ferrite grains, and the platinum substrate offers complete accessibility in the pore area. Our findings reveal high magnetization values, comparable to those of bulk CoFe2O4, in the dense film. Additionally, in the case of the nanopatterned porous film, we observed a distinctive contribution of perpendicular magnetization. This innovative approach holds great promise for advanced magnetization and thin‐film engineering applications.

Field‐Free Spin‐Orbit Torque Driven Perpendicular Magnetization Switching of Ferrimagnetic Layer Based on Noncollinear Antiferromagnetic Spin Source

AbstractThe utilization of novel noncollinear antiferromagnetic materials holds great promise for the development of energy‐efficient spintronic devices. However, only a few studies have reported on the all‐electrical control of perpendicular magnetization switching using noncollinear antiferromagnets as the spin source, and the underlying mechanism behind the unconventional spin‐orbit torque (SOT) is still a topic of debate. In this work, deterministic perpendicular magnetization switching in Mn3Sn/CoTb bilayers is successfully achieved. Compared to the control samples with heavy metal as the spin source, the critical switching current density is over one order of magnitude reduced, indicating an enhanced efficiency of the out‐of‐plane charge‐to‐spin conversion in the textured Mn3Sn films. The influence of film thickness and growth temperature on the efficiency of different spin polarizations suggests potential roles of crystal quality and spin texture in spin diffusion with different spin polarization directions. These findings provide valuable insights into the crystal structure, spin‐orbit torque effects, and charge‐to‐spin conversion in Mn3Sn films, highlighting the importance of understanding interface and bulk contributions in antiferromagnetic spin transport phenomena.

Room-Temperature van der Waals Ferromagnet Switching by Spin-Orbit Torques.

The emerging wide varieties of the van der Waals (vdW) magnets with atomically thin and smooth interfaces hold great promise for next-generation spintronic devices. However, due to the lower Curie temperature of the vdW ferromagnets than room temperature, electrically manipulating its magnetization at room temperature has not been realized. In this work, it is demonstrated that the perpendicular magnetization of the vdW ferromagnet Fe3 GaTe2 can be effectively switched at room temperature in the Fe3 GaTe2 /Pt bilayer by spin-orbit torques (SOTs) with a relatively low current density of 1.3 × 107 A cm-2 . Moreover, the high SOT efficiency of ξDL ≈ 0.28 is quantitatively determined by harmonic measurements, which is higher than those in Pt-based heavy metal/conventional ferromagnet devices. The findings of room-temperature vdW ferromagnet switching by SOTs provide a significant basis for the development of vdW-ferromagnet-based spintronic applications.

Resistive Memristor Coupled with Multilevel Perpendicular Magnetic States

Resistive Memristor Coupled with Multilevel Perpendicular Magnetic States

Room temperature field-free switching of perpendicular magnetization through spin-orbit torque originating from low-symmetry type II Weyl semimetal

Spin-orbit torque (SOT) is a promising strategy to deterministically switch the perpendicular magnetization, but usually requires an in-plane magnetic field for breaking the mirror symmetry, which is not suitable for most advanced industrial applications. Van der Waals (vdW) materials with low crystalline symmetry and topological band structures, e.g., Weyl semimetals (WSMs), potentially serve as an outstanding system that may simultaneously realize field-free switching and high energy efficiency. Yet, the demonstration of these superiorities at room temperature has not been realized. Here, we achieve a field-free switching of perpendicular magnetization by using a layered type II WSM, TaIrTe 4 , in a TaIrTe 4 /Ti/CoFeB system at room temperature with the critical switching current density ~2.4 × 10 6 A cm −2 . The field-free switching is ascribed to the out-of-plane SOT allowed by the low crystal symmetry. Our work suggests that using low-symmetry materials to generate SOT is a promising route for the manipulation of perpendicular magnetization at room temperature.

Field-Free Spin-Orbit Torque Switching of Co/Pd Memory Layer in CPP-GMR With Perpendicularly Magnetized SAF Pinned Layer

For the development of high-capacity spin-orbit torque magnetic memory, a giant magneto-resistive (GMR) device with perpendicularly magnetized Co/Pd memory layer was fabricated, and current-induced magnetization reversal of the memory layer was investigated. A synthetic antiferromagnet (SAF) reference layer consisting of [Co/Pt]/Ru/[Co/Pt] multilayer was used for the GMR device to reduce the stray field in the pillar-shaped Co/Pd memory layer. Spin-transfer torque (STT) magnetization reversal of the memory layer as well as the field-assisted spin-orbit torque (SOT) magnetization reversal of the memory layer was confirmed, by applying a current pulse through the GMR or through the bottom electrode, respectively. Furthermore, by injecting SOT and STT simultaneously, deterministic SOT switching of the perpendicularly magnetized Co/Pd memory layer was achieved in the absence of an in-plane assisting magnetic field to break the symmetry. We confirmed that STT assisting current significantly reduced the SOT switching current. Moreover, the assist efficiency defined by Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SOT</sub> / Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">STT</sub> depended on the magnitude of in-plane magnetic field. Higher assist efficiency was obtained at lower in-plane magnetic field.

Spin–orbit torque induced magnetization switching in the W/CoFeB/Zr/MgO multilayers with high thermal stability

We demonstrate the spin–orbit torque (SOT) induced perpendicular magnetization switching in an annealed W/CoFeB/Zr/MgO multilayer with high thermal stability. It is found that the thermal stability factor can reach 79 after annealing at 540 °C. With an increase in the annealing temperature, the absolute damping-like efficiency almost keeps a high constant value (about 0.3). The tungsten in the W/CoFeB/Zr/MgO multilayer could convert from the high resistive β-W to a mediate resistive amorphous-like structure. Therefore, the absolute spin Hall conductance increases from 765 of β-W to 1420 (ℏ/e)(Ω cm)−1 of the amorphous-like tungsten. These results pave a realistic way for the practical application of tungsten in the SOT-based spintronics devices with high thermal stability and SOT efficiency.

Atomic Layer Deposition of Perpendicularly Magnetized Co Layers Showing Current-Induced Domain Wall Motion

Successful preparation of perpendicularly magnetized Co thin layers on Pt seed-layers using atomic layer deposition (ALD) technique with Co(PF3)4H as a precursor is reported. The residual phosphorus content in the samples, due to low deposition temperatures to realize ALD, was reduced effectively by tuning ALD parameters. Spin-orbit-torque driven current-induced domain wall motion was observed in the strip containing ALD Co layer prepared with the low phosphorus condition.

Enhanced spin orbit torque efficiency induced by large skew scattering in perpendicular Pt/Co/Ta multilayers with superlattice/alloying Nb (Ir) insertion

Perpendicular magnetization switching driven by spin–orbit torque (SOT) facilitates great potential applications in high-efficient memory and logic. However, SOT-based devices suffer from a relatively low SOT efficiency and ultrahigh current density in the conventional heavy metal/ferromagnet bilayer structure. Here, we report that the SOT behavior can be effectively tuned by inserting the ultrathin Nb superlattice into heavy metal Pt layer compared with Ir insertion or the PtNb alloying layer. A slight change of critical current density (Jc) can be found in the multilayers with Ir insertion. The Jc value for the Pt/Co/Ta multilayer with [Pt/Nb]7 insertion is decreased to 1.4 × 107 A/cm2, approximately 60% lower than that in pure Pt/Co/Ta multilayers. Furthermore, the SOT efficiency is significantly enhanced with increasing the period number due to the tunable spin Hall angle (θSH). Compared with pure Pt layer, the θSH value is increased 47% for the sample with [Pt/Nb]5, which is also significantly larger than that in the one with the PtNb alloying layer. Enhanced skew scattering induced by Nb superlattice plays the main role in these tunable SOT properties. Our findings provide a feasible scheme to engineer high-efficiency SOT-based logic-in-memory.

Switching of Perpendicular Magnetization by Spin-Orbit Torque.

Magnetic materials with strong perpendicular magnetic anisotropy are of great interest for the development of nonvolatile magnetic memory and computing technologies due to their high stabilities at the nanoscale. However, electrical switching of such perpendicular magnetization in an energy-efficient, deterministic, scalable manner has remained a big challenge. This problem has recently attracted enormous efforts in the field of spintronics. Here, recent advances and challenges in the understanding of the electrical generation of spin currents, the switching mechanisms and the switching strategies of perpendicular magnetization, the switching current density by spin-orbit torque of transverse spins, the choice of perpendicular magnetic materials are reviewed, and the progress in prototype perpendicular SOT memory and logic devices toward the goal of energy-efficient, dense, fast perpendicular spin-orbit torque applications is summarized.

Implementing Versatile Programmable Logic Functions Using Two Magnetization Switching Types in a Single Device

AbstractThe efficient manipulation of magnetization using spin‐orbit torques (SOTs) generated by heavy metals and topological insulators has attracted significant attention. However, the symmetry of these conventional materials makes it challenging to achieve deterministic switching of perpendicular magnetization by currents, which prevents the practical application of SOT‐based spintronic devices such as magnetic memories and logics. Here, a composition gradient in a TaxTi1‐x alloy is introduced and the field‐free magnetization switching in TaxTi1‐x/CoFeB heterostructures with controllable SOT efficiencies is realized. Additionally, by engineering the gradient with a tilting angle relative to the current injection arm of the device, highly asymmetric switching loops are achieved, which are attributed to tilted spin polarization. Based on these two switching types, namely field‐free switching and field‐assisted asymmetric switching, five programmable Boolean logic functions are successfully demonstrated using a single device. This work paves the way for high‐density computing‐in‐memory applications with industry compatible artificially‐designed asymmetric SOT materials.

Room-temperature orbit-transfer torque enabling van der Waals magnetoresistive memories

The non-volatile magnetoresistive random access memory (MRAM) is believed to facilitate emerging applications, such as in-memory computing, neuromorphic computing and stochastic computing. Two-dimensional (2D) materials and their van der Waals heterostructures promote the development of MRAM technology, due to their atomically smooth interfaces and tunable physical properties. Here we report the all-2D magnetoresistive memories featuring all-electrical data reading and writing at room temperature based on WTe2/Fe3GaTe2/BN/Fe3GaTe2 heterostructures. The data reading process relies on the tunnel magnetoresistance of Fe3GaTe2/BN/Fe3GaTe2. The data writing is achieved through current induced polarization of orbital magnetic moments in WTe2, which exert torques on Fe3GaTe2, known as the orbit-transfer torque (OTT) effect. In contrast to the conventional reliance on spin moments in spin-transfer torque and spin–orbit torque, the OTT effect leverages the natural out-of-plane orbital moments, facilitating field-free perpendicular magnetization switching through interface currents. Our results indicate that the emerging OTT-MRAM is promising for low-power, high-performance memory applications.

Field-free switching of perpendicular magnetization at room temperature using out-of-plane spins from TaIrTe4

Field-free switching of perpendicular magnetization at room temperature using out-of-plane spins from TaIrTe4