High-density Spintronic Devices Research Articles

Observation of the multiple magnetic phases in double perovskite Pr1.8La0.2CoFeO6

The structural and magnetic properties of the double perovskite Pr1.8La0.2CoFeO6 have been investigated. The X-ray diffraction study shows that the system acquires room-temperature orthorhombic phase with the Pnma space group. The X-ray photoemission spectroscopy (XPS) measurement confirmed that the B-site ions are present in mixed valence states i.e. Co has been found in Co2+ and Co3+ valance states whereas Fe has been found in Fe3+/Fe4+. Magnetic measurements of the system confirm the existence of several fascinating magnetic behaviors, such as long-range canted antiferromagnetism withTN ∼ 271 K, giant exchange bias, and re-entrant cluster glass phase (TG ∼ 33 K). Field-dependent magnetization shows a large spontaneous exchange bias; HSEB∼ 1.8 kOe and giant conventional exchange bias, HCEB∼ 2.4 kOe at 5 K. Antiferromagnetic materials with large exchange bias can be utilized in high-density spintronic devices. Temperature-dependent Raman study demonstrates that the observed phonon mode exhibits anomalous behavior close to the magnetic transition temperature. Analysis of anomalous softening of phonon mode below TN, clarifies the presence of significant spin-phonon coupling while the magnetostriction effect does not play any significant role in the observed phonon anomaly.

Giant Spin-Orbit Torque in Antiferromagnetic-Coupled Pt/[Co/Gd]N Multilayers with Suppressed Spin Dephasing and Robust Thermal Stability.

Manipulating magnetization via power-efficient spin-orbit torque (SOT) has garnered significant attention in the field of spin-based memory and logic devices. However, the damping-like SOT efficiency (ξDL) in heavy metal (HM)/ferromagnetic metal (FM) bilayers is relatively small due to the strong spin dephasing accompanied by additional spin polarization decay. Furthermore, the perpendicular magnetic anisotropy (PMA) originating from the HM/FM interface is constrained by the thickness of FM, which is unfavorable for thermal stability in practical applications. Consequently, it is valuable to develop systems that not only exhibit large ξDL but also balance thermal stability. In this work, we designed antiferromagnetic-coupled [Co/Gd]N multilayers, where staggered Co and Gd magnetic moments effectively suppress the spin dephasing and additional spin polarization decay. The ordered Co-Gd arrangements along the out-of-plane direction provide bulk PMA, endowing Pt/[Co/Gd]N high thermal stability. The SOT of Pt/[Co/Gd]N was systematically studied with N, demonstrating a significantly large ξDL of up to 0.66. The ξDL of Pt/[Co/Gd]N is greater than those of Pt/Co and Pt/ferrimagnetic alloys. This significant enhancement relies on the effective suppression of spin dephasing in [Co/Gd]N. Our work highlights that the antiferromagnetic-coupled [Co/Gd]N multilayer is a promising candidate for low-consumption and high-density spintronic devices.

Manipulating exchange bias in 2D magnetic heterojunction for high-performance robust memory applications

The exchange bias (EB) effect plays an undisputed role in the development of highly sensitive, robust, and high-density spintronic devices in magnetic data storage. However, the weak EB field, low blocking temperature, as well as the lack of modulation methods, seriously limit the application of EB in van der Waals (vdW) spintronic devices. Here, we utilized pressure engineering to tune the vdW spacing of the two-dimensional (2D) FePSe3/Fe3GeTe2 heterostructures. The EB field (HEB, from 29.2 mT to 111.2 mT) and blocking temperature (Tb, from 20 K to 110 K) are significantly enhanced, and a highly sensitive and robust spin valve is demonstrated. Interestingly, this enhancement of the EB effect was extended to exposed Fe3GeTe2, due to the single-domain nature of Fe3GeTe2. Our findings provide opportunities for the producing, exploring, and tuning of magnetic vdW heterostructures with strong interlayer coupling, thereby enabling customized 2D spintronic devices in the future.

Tuning magnetic anisotropy in SrRuO3 thin film by Ru vacancies induced phase transition

Effective control of magnetic anisotropy, including perpendicular magnetic anisotropy (PMA) and lateral magnetic anisotropy, is important for the design of low-power and high-density spintronic devices. However, the rarity of oxide materials with PMA and stringent conditions required to control magnetic anisotropy have prevented its large-scale application. Here, we demonstrate that the magnetic anisotropy of SrRuO3 films can be specified on-demand by adjusting the content of Ru vacancies to control the structure of the films. With the increase in Ru vacancies, the structure of SrRuO3 changes from orthorhombic to tetragonal. The field angle dependence of the Hall resistance confirmed that the uniaxial magnetic easy axis of SrRuO3 thin films continuously rotates in the (100)pc crystallographic plane, which is identical to the continuous phase transition. Our results not only provide a way to continuously tune the physical properties of epitaxial oxide films by continuously changing the composition but also help to provide guidance for the on-demand design of spintronic devices.

Spontaneous Topological States and Their Mutual Transformations in a Rare-Earth Ferrimagnet.

Nontrivial chiral spin textures with nanometric sizes and novel characteristics (e.g., magnetic skyrmions) are promising for encoding information bits in future energy-efficient and high-density spintronic devices. Because of antiferromagnetic exchange coupling, skyrmions in ferrimagnetic materials exhibit many advantages in terms of size and efficient manipulation, which allow them to overcome the limitations of ferromagnetic skyrmions. Despite recent progress, ferrimagnetic skyrmions have been observed only in few films in the presence of external fields, while those in ferrimagnetic bulks remain elusive. This study reports on spontaneously generated zero-field ground-state magnetic skyrmions and their subsequent transformation into traditional magnetic bubbles via intermediate states of (bi-)target bubbles during a magnetic anisotropy change in the rare-earth ferrimagnetic crystal DyFe11 Ti. Spontaneous reversible topological transformation driven by a temperature-induced spin reorientation transition is directly distinguished using Lorentz transmission electron microscopy. The spontaneous generation of magnetic skyrmions and successive topological transformations in ferrimagnetic DyFe11 Ti are expected to advance the design of topological spin textures with versatile properties and potential applications in rare-earth magnets.

Giant anomalous charge-spin conversion at Co/Pb(Mg1/3Nb2/3)O3–Pb0.7Ti0.3O3 interfaces

An efficient out-of-plane anti-damping spin–orbit torque (SOT) is in great demand for high-density spintronic devices with perpendicular magnetic anisotropy. Despite its importance, direct realization of such SOT in a single magnetic layer is scarce and has remained challenging. Here, we present experimental evidence uncovering unconventional out-of-plane anti-damping torques in the Co film deposited on the ferroelectric Pb(Mg1/3Nb2/3)O3–Pb0.7Ti0.3O3 (PMN–PT) substrate. We show via spin-torque ferromagnetic resonance that both Rashba- and unconventional-type SOT give rise to a high-efficient charge-to-spin conversion. The strong magnetoelectric effect at the Co/PMN-PT interface allows further directly electric-field control of the conversion.

Giant exchange bias in antiferromagnetic Pr2CoFe0.5Mn0.5O6: a structural and magnetic properties study

Antiferromagnetic (AFM) materials with a colossal exchange bias (EB) effect find applications as high-density spintronic devices. We report structural (geometrical and electronic) and magnetic studies in the polycrystalline Pr2CoFe0.5Mn0.5O6 double perovskite system. The observed lack of training effect suggests the existence of robust EB. In addition, the detailed magnetic studies and Raman studies unravel the Griffith-like phase along with the spin-phonon coupling in the present system. The x-ray photoemission spectroscopy (XPS) analysis supports more than one valence state of B-site elements, which is accountable for the competition between ferromagnetic (FM) and AFM interactions in addition to the anti-site disorder in the system. The neutron measurement confirms the G-type AFM spin arrangement, accredited by the DFT calculation. The magnetic studies have correlated with the electronic structure, neutron study, and theoretical first principle calculations.

Defect-driven antiferromagnetic domain walls in CuMnAs films

Efficient manipulation of antiferromagnetic (AF) domains and domain walls has opened up new avenues of research towards ultrafast, high-density spintronic devices. AF domain structures are known to be sensitive to magnetoelastic effects, but the microscopic interplay of crystalline defects, strain and magnetic ordering remains largely unknown. Here, we reveal, using photoemission electron microscopy combined with scanning X-ray diffraction imaging and micromagnetic simulations, that the AF domain structure in CuMnAs thin films is dominated by nanoscale structural twin defects. We demonstrate that microtwin defects, which develop across the entire thickness of the film and terminate on the surface as characteristic lines, determine the location and orientation of 180∘ and 90∘ domain walls. The results emphasize the crucial role of nanoscale crystalline defects in determining the AF domains and domain walls, and provide a route to optimizing device performance.

Multi-domain ferromagnetic resonance in magnetic van der Waals crystals CrI3 and CrBr3

Two-dimensional (2D) magnetic van der Waals crystals have received great attentions recently owing to their novel electronic and magnetic properties and rich potentials for low-dimensional and high-density spintronic devices. We investigated the magnetization dynamics of CrI3 and CrBr3 bulk single crystals by utilizing broadband ferromagnetic resonance (FMR) in a frequency range of 1 – 40 GHz and over a wide temperature range of 10 – 300 K. Complex features observed in the FMR spectra are quantitatively described by the multi-domain FMR theory. The minimum resonance frequency fmin in FMR spectra was recognized and analyzed as characteristics of the transition of the domain structure. Micromagnetic simulation on the domain structure and spin dynamics explains that the multi-domain structure exists under strong in-plane magnetic field for CrI3 and CrBr3 and significantly influences the magnetic dynamics properties. Our results also suggest that the linewidths of FMR spectra in CrI3 and CrBr3 are dominated by the magnetic inhomogeneous broadening.

Topological charge analysis of dynamic process of transition to Néel-type skyrmion: Role of domain wall skyrmions

Magnetic skyrmions are intriguing topological spin textures that promise future high-density spintronic devices. The creation of magnetic skyrmions has been understood based on the energetics of skyrmions, but the detailed dynamic process of the skyrmion creation remains unclear. Here topological evolution in conversion from uneven domains to Néel-skyrmions is investigated using micromagnetic simulations. We find that, rather than the overall topological charge, annihilation of novel topological defects, i.e., recently suggested domain wall skyrmions, dominantly govern the skyrmion creation process. Also, the topological charge evolution is interpreted in terms of the number and the combination of such topological defects.

Fabrication of TbFeCo alloy films with tunable perpendicular coercivity evaluated by extraordinary Hall effect measurements

Rare-earth transition metal (RE-TM) TbFeCo alloy films with adjustable perpendicular switching field and anisotropy are desirable for the applications in high-density spintronic devices. A series of TbFeCo alloy films are fabricated by sputtering a composite target method. The perpendicular magnetic properties of the ferrimagnetic TbFeCo films are investigated by Extraordinary Hall effect (EHE) measurements. With increasing either sputtering power or gas flow rate film composition shifting from FeCo-rich to Tb-rich side is observed and the orientation of magnetic easy axis changing from in-plane, tilted to out-of-plane direction is witnessed. Pronounced change in magnetic properties particularly the coercivity of TbFeCo films is demonstrated, in line with the increased Tb-to-FeCo ratio of the alloy films. It shows TbFeCo films with desirable coercivity can be achieved by proper combination of sputtering power and flow rate. Our work provides an efficient way in tuning the PMA of the sputtered TbFeCo films during the growth and may be used as guidance for designing TbFeCo-based spintronic devices.

Ferrimagnetic Skyrmions in Topological Insulator/Ferrimagnet Heterostructures.

Magnetic skyrmions are topologically nontrivial chiral spin textures that have potential applications in next-generation energy-efficient and high-density spintronic devices. In general, the chiral spins of skyrmions are stabilized by the noncollinear Dzyaloshinskii-Moriya interaction (DMI), originating from the inversion symmetry breaking combined with the strong spin-orbit coupling (SOC). Here, the strong SOC from topological insulators (TIs) is utilized to provide a large interfacial DMI in TI/ferrimagnet heterostructures at room temperature, resulting in small-size (radius ≈ 100 nm) skyrmions in the adjacent ferrimagnet. Antiferromagnetically coupled skyrmion sublattices are observed in the ferrimagnet by element-resolved scanning transmission X-ray microscopy, showing the potential of a vanishing skyrmion Hall effect and ultrafast skyrmion dynamics. The line-scan spin profile of the single skyrmion shows a Néel-type domain wall structure and a 120 nm size of the 180° domain wall. This work demonstrates the sizable DMI and small skyrmions in TI-based heterostructures with great promise for low-energy spintronic devices.

Electric-field-driven Deterministic and Robust 120° Magnetic Rotation in a Concave Triangular Nanomagnet

Deterministic magnetic switching driven by electric fields rather than power-dissipating currents at the nanoscale is fundamentally challenging yet promising for applications to energy-efficient and high-density spintronic devices. Here, we demonstrate an electric-field-controlled deterministic, robust and repeatable $120{}^{o}$ rotation of ``Y''-like magnetic state in a patterned nanoscale multiferroic heterostructure consisting of a concave triangular nanomagnet deposited on $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$-${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_{3}$ film. Using phase-field simulations, we find that the rotation of ``Y''-like magnetic state is controlled by a co-action of strain-mediated electric-field-induced uniaxial magnetoelastic anisotropy, magnetic in-plane shape anisotropy of the concave-triangle-shaped nanomagnet, and an interfacial exchange-bias field from a juxtaposed antiferromagnetic layer. It is also shown that deterministic magnetic state switching can be accomplished by a pulsed strain, the duration of which can span from ten nanoseconds or longer down to a few nanoseconds, providing great design flexibility. We also discuss the dynamics of electric-field-driven switching of ``Y''-like magnetic state as well as the influence of side length, thickness, and shape variation (i.e., concave radius) of the nanomagnet on the critical strain for the switching. These results offer a technologically viable route to designing nanomagnet-based nonvolatile spin memories with high density and low power.

Giant Topological Hall Effect and Superstable Spontaneous Skyrmions below 330 K in a Centrosymmetric Complex Noncollinear Ferromagnet NdMn2Ge2.

Skyrmions with topologically nontrivial spin textures are promising information carriers in next-generation ultralow power consumption and high-density spintronic devices. To promote their further development and utilization, the search for new room temperature skyrmion-hosting materials is crucial. Considering that most of the previous skyrmion-hosting materials are noncollinear magnets, here, the detection of the topological Hall effect (THE) and the discovery of skyrmions at room temperature are first reported in a centrosymmetric complex noncollinear ferromagnet NdMn2Ge2. Below 330 K, the compound can host stable Bloch-type skyrmions with about 75 nm diameter in a wide window of magnetic field and temperature, including zero magnetic field and room temperature. Moreover, the skyrmions can induce a giant topological Hall effect in a wide temperature range with a maximum value of -2.05 μΩ cm. These features make the compound attractive for both fundamental research and potential application in novel spintronic devices.

Creation of a thermally assisted skyrmion lattice in Pt/Co/Ta multilayer films

Néel-type magnetic skyrmions in multilayer films have recently attracted significant attention due to their stability at room temperature and low threshold for current-driven motion, offering the potential for the construction of high-speed and high-density spintronic devices. However, to date, research studies reported in the literature have rarely examined the effect of temperature on the formation and behavior of Néel-type skyrmions. Here, we investigate the effect of the temperature on the creation of a skyrmion lattice in [Pt/Co/Ta]10 multilayer samples, using in-situ Lorentz transmission electron microscopy. By imaging the magnetization reversal process from a positive (negative) to a negative (positive) saturation, we find that the skyrmions can be created by nucleation from a ferromagnetic state and by breaking the labyrinth domains under certain external fields. More importantly, we demonstrate that the density of skyrmions in the multilayers not only depend on the external magnetic field, but also depend on the temperature and the thermal history of the materials.

Ion beam etching process for high-density spintronic devices and its damage recovery by the oxygen showering post-treatment process

The electric short fail trend of the perpendicular magnetic tunnel junctions (p-MTJs) caused by the ion beam etching (IBE) process is studied at various ion beam angles and cell-to-cell space widths. The number of electric short fails increases markedly at an ion beam angle greater than 35° and a cell-to-cell space width less than 30 nm at the assumed MTJ height including a hard mask (HM) of 20 nm. In order to recover these electric short fails, we propose the selective oxidation process called the oxygen showering post-treatment (OSP). By the OSP process, the number of electric short fails in sub-30-nm-spaced MTJ arrays is reduced from 25 to 0.8%, and the magnetoresistance (MR) is increased from 99 to 120%. By this result, we can verify that the damaged layer is recovered successfully by the OSP, and that the OSP can be a universal post-treatment process even beyond the 20 nm design rule for use in both reactive ion etching and IBE schemes.