Perpendicular Magnetic Anisotropy Field Research Articles

Write error rate analysis of field-free spin-orbit torque switching in conically magnetized free layer nanomagnet

Enhancing the performance of magnetic random access memories (MRAMs) is crucial, considering speed, energy efficiency, and endurance. Spin-orbit torque-based MRAMs offer ultrafast operation and enhanced reliability. Still, the energy efficiency and external magnetic field requirement for deterministic switching of nanomagnets with perpendicular magnetic anisotropy (PMA) are a significant hurdle. To address these issues, one of the proposed solutions employs a conically magnetized free layer magnetic tunnel junction device featuring second-order PMA. This approach can minimize the required switching current density by leveraging the interplay between first- and second-order PMA energies. Additionally, coupling with exchange bias from the antiferromagnet can eliminate the need for an external field entirely. Our analysis investigates the impact of current density (magnitude and rise/fall time), first- and second-order PMA fields, exchange bias, and field-like torque on the device's switching performance. By optimizing the perpendicular magnetic anisotropies, we report improvement in the write error rate from 10−4 to 10−7. Our findings hold promise for magnetic memory applications.

Bifurcation to complex dynamics in largely modulated voltage-controlled parametric oscillator

An experimental demonstration of a parametric oscillation of a magnetization in a ferromagnet was performed recently by applying a microwave voltage, indicating the potential to be applied in a switching method in non-volatile memories. In the previous works, the modulation of a perpendicular magnetic anisotropy field produced by the microwave voltage was small compared with an external magnetic field pointing in an in-plane direction. A recent trend is, however, opposite, where an efficiency of the voltage controlled magnetic anisotropy (VCMA) effect is increased significantly by material research and thus, the modulated magnetic anisotropy field can be larger than the external magnetic field. Here, we solved the Landau–Lifshitz–Gilbert equation numerically and investigated the magnetization dynamics driven under a wide range of the microwave VCMA effect. We evaluated bifurcation diagrams, which summarize local maxima of the magnetization dynamics. For low modulation amplitudes, the local maximum is a single point because the dynamics is the periodic parametric oscillation. The bifurcation diagrams show distributions of the local maxima when the microwave magnetic anisotropy field becomes larger than the external magnetic field. The appearance of this broadened distribution indicates complex dynamics such as chaotic and transient-chaotic behaviors, which were confirmed from an analysis of temporal dynamics.

Perpendicular magnetic anisotropy and magneto-optical properties of Bi,Mn:YIG epitaxial films

Bi,Mn co-doped Y3Fe5O12 films with good crystallinity, great magneto-optical properties and high perpendicular magnetic anisotropy field (>3000 Oe) were prepared via liquid phase epitaxy.

Aluminum doping hexagonal barium ferrite films with large perpendicular magnetic anisotropy and high remanence ratio

Aluminum doping hexagonal barium ferrite film with large perpendicular magnetic anisotropy (PMA) and high remanence ratio was prepared on Al2O3 substrate by post-annealing at high temperature. The crystal structure measurements have shown that the crystallinity and degree of orientation were improved by post-annealing at high temperature, while it happened the Al ions diffusion from the Al2O3 substrate into the ferrite film at 1100 °C of annealing temperature. The diffusion phenomenon is also confirmed by the distribution of elements in the cross-section which displays a wide and slow-changing region of Al element. The Al element diffusion gives rise to a decrease of saturation magnetization, and an increase of coercivity and remanence ratio. Generally, a hexagonal barium ferrite film with large PMA field (HA = 18.1 kOe), relatively small FMR linewidth (540 Oe) and high remanence ratio (86%) was obtained by annealing the film at 1100 °C, which shows potential applied in self-biased microwave devices.

Peculiar thickness dependence of perpendicular magnetic anisotropy in epitaxial NiCo2O4 spinel films

We reported a peculiar thickness dependence of perpendicular magnetic anisotropy (PMA), stronger in thinner films, in spinel NiCo2O4 (NCO) films deposited epitaxially on MgAl2O4 (001) single crystalline substrates. The effective PMA field is about 3 T in 23.0 nm-thick films but increases dramatically to about 7 T in 4.5 nm-thick films. X-ray absorption spectra reveal an inhomogeneous distribution of Co2+ cations along the film's normal with more Co2+ adjacent to the film/substrate interface. X-ray linear dichroism spectra reveal that the PMA in NCO films is associated with electrons occupying the x2–y2 orbital of Co cations at the tetrahedral sites. This work demonstrates the potential of ultrathin NCO films in all-oxide spintronic devices.

Current induced magnetization switching in ferrimagnetic insulators with perpendicular magnetic anisotropy: A macrospin modeling

Spin-orbit torque (SOT) assisted magnetization switching by in-plane current in heavy-metal (HM)/ferrimagnetic insulators (FMIs) heterostructure with perpendicular magnetic anisotropy (PMA) has attracted more attention than in HM/FM (ferromagnetic) heterostructure due to it’s low magnetic losses and electric insulation. Here, macrospin modeling was employed to study the current-induced magnetization switching in Bi doped Y3Fe5O12 (BiYIG), Tm3Fe5O12 (TmIG) and Eu3Fe5O12 (EuIG) FMIs layers with heavy metal on to explore promising FMIs materials for SOT-based fast and efficient devices. The magnetization switching phase diagram (MSPD) of BiYIG, TmIG, EuIG1 and EuIG2 was calculated. It is found that the distribution of MSPD is strongly dependent both on the PMA field Hk and Gilbert damping parameter α. The larger the PMA field Hk is, the greater the damping-like effective field HDL (proportional to the current density) and in-plane magnetic field Hx required to realize magnetization switching in HM/FMIs. Furthermore, switching trajectories and diagrams of BiYIG, TmIG, EuIG1 and EuIG2 with variable Hx and HDL were calculated. The results show that compared with spin transfer torque (STT) and SOT-assisted magnetization switching in all-metallic heterostructures, the switching speed in HM/FMIs system has great advantages. Besides, presence of an external field-like effective field (HFL) can improve switching reliability for low Gilbert damping heterostructure like HM/BiYIG. It is noteworthy that although small damping coefficient is not conducive to the stability of magnetization switching, a huge advantage of smaller damping coefficient is that it can reduce the critical switching current density, which is of great significance to the application of SOT devices. In the end, the SOT-assisted magnetization switching loops of four kinds of FMIs were calculated. This study provides a new perspective for further exploring the current-induced magnetization switching in HM/FMIs system.

Analysis of spin-orbit torque magnetic tunnel junction model without external magnetic field assistance based on antiferromagnetism

The effect of spin-orbit torque (SOT) provides a new method of implementing ultra-low power spintronic devices. The in-plane exchange bias (EB) field in antiferromagnetic material can effectively assist SOT magnetization switching. Meanwhile, the utilization of voltage-controlled magnetic anisotropy (VCMA) can effectively reduce the switching barrier. Taking advantage of the EB and VCMA effect, it is possible to realize SOT magnetic tunnel junctions without external field assistance. In this work, a spin-orbit torque magnetic tunnel junction model composed of antiferromagnetic/ferromagnetism/oxides without external magnetic field is developed by solving the modified Landau-Lifshitz-Gilbert (LLG) modular equation, and its magnetization dynamics is analyzed and studied. The effective fields in the model include the demagnetization field, thermal noise field, perpendicular magnetic anisotropy field with VCMA effect, and exchange bias field. Taking IrMn/CoFeB/MgO material system for example, the factors affecting the precession of magnetization are investigated, such as the effect of the exchange bias field, the VCMA effect and the mechanism of SOT field-like torque. Considering the practical applications, the effect of the deviation of the fabrication process of magnetic tunnel junctions is also analyzed. The simulation results demonstrate that the combined effect of <inline-formula><tex-math id="M5">\begin{document}$ {{\boldsymbol{H}}_{{\text{EB}}}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20230901_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20230901_M5.png"/></alternatives></inline-formula> with VCMA effect can greatly reduce the critical <i>I</i><sub>SOT</sub>, thus assisting and realizing the complete field-free magnetization reversal; the SOT field-like torque plays a dominant role in realizing the magnetization reversal, and by adjusting the ratio of the SOT field-like torque to the damping-like torque, field free switching can be realized in the device at the ps grade ; and the MTJ can realize effective switching when the deviation of oxide thickness <inline-formula><tex-math id="M6">\begin{document}$ {\gamma _{{\text{tf}}}} \leqslant 10{\text{%}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20230901_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20230901_M6.png"/></alternatives></inline-formula> or the deviation of free layer thickness <inline-formula><tex-math id="M7">\begin{document}$ {\gamma _{{\text{tox}}}} \leqslant 13{\text{%}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20230901_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20230901_M7.png"/></alternatives></inline-formula>. Spin-orbit torque devices based on the antiferromagnetic without external magnetic field will provide highly promising solutions for a new-generation ultra-low power, ultra-high speed, and ultra-high integration devices and circuits.

Magnetic properties and thermal stability of ultrathin TbFeCo films encapsulated by heavy metals Pt and W

Amorphous rare earth (RE)-transition metal (TM) ferrimagnetic alloy films have been intensively studied recently in spintronics and ultrafast information storage due to the large perpendicular magnetic anisotropy (PMA), ultrafast magnetization switching, and the presence of magnetization compensation and angular momentum compensation. In this work, we fabricate <i>X</i>/Tb<sub><i>x</i></sub>(Fe<sub>0.75</sub>Co<sub>0.25</sub>)<sub>1–<i>x</i></sub>/<i>X</i> (0.13 ≤ <i>x</i> ≤ 0.32, <i>X</i> = SiO<sub>2</sub>, Pt and W) trilayers by magnetron sputtering, and systematically investigate the magnetic properties and thermal stabilities of the ultrathin TbFeCo films encapsulated by heavy metals Pt and W at room temperature. The 5–50-nm-thick TbFeCo films sandwiched by SiO<sub>2</sub> exhibit PMA with magnetic compensation occurring in Tb concentration <i>x</i> between 0.21 with 0.24. For 3-nm- and 5- nm-thick TbFeCo ultrathin films encapsulated by Pt, however, there is no magnetic compensation observed throughout the composition range 0.13 ≤ <i>x</i> ≤ 0.32 with the film magnetization dominated by the FeCo moment. Nevertheless, the weakened PMA for the Pt/ultrathin TbFeCo/Pt trilayers is completely destroyed after annealing at 250 ℃. When the buffer layer and capping layer of Pt are replaced by W, the ultrathin TbFeCo films show magnetic compensation at 0.21 < <i>x</i> < 0.24, so do the thick TbFeCo films. The effective PMA field (<i>H</i><sub>K</sub>) exceeds 11.5 T for the W/ultrathin TbFeCo/W films near the compensation composition, and remarkably, the <i>H</i><sub>K</sub> decreases slowly on annealing, with PMA maintained even after annealing at 350–400℃. We further prepare [Pt/TbFeCo]<sub>5</sub>/Pt and [W/TbFeCo]<sub>5</sub>/W multilayers to clarify the origin of the huge difference between Pt/ultrathin TbFeCo/Pt and the W counterpart. It is found that there are partial recrystallization and phase separation for TbFeCo layer around the Pt/TbFeCo interface, leading to the disappearance of magnetic compensation and the deterioration of the PMA in the Pt/ultrathin TbFeCo/Pt films. With large PMA, W/ultrathin TbFeCo/W films show the presence of magnetic compensation, and excellent thermal robustness. The present study provides a promising heavy metal/RE-TM heterostructure for spintronic applications.

Interfacial roughness driven manipulation of magnetic anisotropy and coercivity in ultrathin thulium iron garnet films

We have investigated the magnetic properties of thulium iron garnet (Tm3Fe5O12, TmIG) thin films grown on {111} gadolinium gallium garnet (Gd3Ga5O12, GGG) substrates fabricated via the facing target sputtering method. The annealing temperature (Ta), thickness (tf), and roughness were systematically controlled in order to obtain the perfect magnetic properties of the film showing perpendicular magnetic anisotropy (PMA) and large coercive field, which are the requisites for non-volatile magnetic memory device applications. Here, we demonstrated that the Ta acts as the crucial parameter both for obtaining the high quality of the film surface and engineering the direction of the magnetic anisotropy from the in-plane to out-of-plane. Through the further optimization process with a function of the tf, we successfully achieved the best condition for the PMA films. Specifically, the sample fabricated at a tf of ≤ 20 nm and Ta of 1000 ℃ showed the perfect PMA. Furthermore, the surface roughness of the substrate mainly contributes to the enhancement of the coercive field of the perpendicular magnetic anisotropic films. We speculate that the drag force from the pinning energy induced by the interfacial roughness retarded the domain wall motion at the TmIG/GGG interface, resulting in enhanced coercivity.

Wide modulation of coercive fields in Mn4N ferrimagnetic thin films caused dominantly by dislocation microstructures

Perpendicular magnetic anisotropy and coercive fields (Hc) are governed by the degree of order of N (S) in ferrimagnetic Mn4N thin films. However, in this study, we observed non-negligible modulation of Hc extending from 10 to 7.5 kOe for three Mn4N films grown at a substrate temperature (Tsub) in the range of 400–500 °C, even with identical S, which has not been discussed to date. The possible mechanisms for this phenomenon were explored by transport properties and fine structural analysis. The results indicated that longitudinal resistivity and anomalous Hall resistivity remain unchanged for three Mn4N films. Conversely, the number of dislocations and in-plane grain size (D) increased, whereas the residual strain (ε) decreased with increasing Tsub. This indicates that Hc of the Mn4N film with the same S is sensitive to the variation in dislocations, D and ε. Tunable Hc of this type can be effectively applied in magnetic and spintronic devices using ferrimagnetic Mn4N films, because Hc is an important determinant of the thermal stability of magnetization and working power consumption.

Large Tunable Perpendicular Magnetic Anisotropy in Y3−xTmxFe5O12 ( x = 0–3) Epitaxial Films with Minor Changes in Switching Current

We show perpendicular magnetic anisotropy (PMA) in substituted rare-earth iron garnet ${\mathrm{Y}}_{3\ensuremath{-}x}{\mathrm{Tm}}_{x}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ (x = 0--3) films epitaxially grown on (111) $({\mathrm{Gd}}_{0.63}{\mathrm{Y}}_{2.37})({\mathrm{Sc}}_{2}{\mathrm{Ga}}_{3}){\mathrm{O}}_{12}$ substrates by magnetron sputtering. With $\mathrm{Tm}$ concentrations varying from x = 0 to x = 3, the effective PMA field of the 10-nm-thick films increases monotonically from 0.3 to 2.7 kOe together with a similar coercivity increase from 2 to 65 Oe. The substituted garnet films with x ranging from 0.6 to 1.2 are structurally much more robust against epitaxial strain relaxation. Magnetotransport measurements in the ${\mathrm{Y}}_{3\ensuremath{-}x}{\mathrm{Tm}}_{x}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$/$\mathrm{Pt}$ heterostructure show that the anomalous Hall effect and spin Seebeck effect are significant and change little with $\mathrm{Tm}$ concentration, indicating that the interface spin-exchange interaction and interface spin-current transmission mainly depend on the net moment of ${\mathrm{Fe}}^{3+}$ sublattices rather than the total moment of ${\mathrm{Y}}_{3\ensuremath{-}x}{\mathrm{Tm}}_{x}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$. Importantly, the critical current density for the spin-orbit-torque-induced switching also changes very little with a minor increase from 1.76 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ to 1.83 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ A m${}^{\ensuremath{-}2}$. We tentatively attribute this weak PMA-dependent current density to modification of the domain-wall depinning by the current. Our results demonstrate that substituted rare-earth iron garnets can be a general approach to tune PMA within a large range with nearly constant switching current, and yet there is plenty of work left to be done for more efficient SOT-based garnet ferrimagnet spintronics devices.

Bias-free spin-wave propagation in a micrometer-thick ferrimagnetic film with perpendicular magnetic anisotropy

The isotropic transmission of magnetostatic forward volume spin waves in magnetic films with perpendicular magnetic anisotropy (PMA) is shown to be useful in the implementation of magnon-based micro-conduits. However, to our knowledge, non-magnetic-bias-field spin-wave propagation in a PMA magnetic insulator has not been achieved yet, which constrains the development of magnonic information devices and systems. Herein, we demonstrate a robust, bias-free spin-wave transmission in an 18.5-μm-thick bismuth-doped thulium iron garnet film with PMA. This ferrimagnetic film grown by liquid phase epitaxy exhibits high quality in both its crystal structure and its chemical composition and displays a large PMA field of ∼173 mT. The bias-free and reciprocal propagation of spin waves is demonstrated by all-electrical spectroscopy and provides a group velocity of 4.90 km s−1 and a decay length of 20.5 µm at zero magnetic field. Direct imaging of the remnant state indicates that the bias-free spin waves propagate along the oppositely oriented stripe domains with Bloch-type walls, which are formed by in-plane pre-magnetization. Our work contributes to the construction of isotropic charge-free micro-circuits with high levels of integration and nonvolatility.

Single-crystal epitaxial europium iron garnet films with strain-induced perpendicular magnetic anisotropy: Structural, strain, magnetic, and spin transport properties

Single-crystal europium iron garnet (EuIG) thin films epitaxially strain-grown on gadolinium gallium garnet (GGG)(100) substrates using off-axis sputtering have strain-induced perpendicular magnetic anisotropy (PMA). By varying the sputtering conditions, we have tuned the europium/iron (Eu/Fe) composition ratios in the films to tailor the film strains. The films exhibited an extremely smooth, particle-free surface with roughness as low as 0.1 nm as observed using atomic force microscopy. High-resolution x-ray diffraction analysis and reciprocal space maps showed in-plane epitaxial film growth, very smooth film/substrate interface, excellent film crystallinity with a small full width at half maximum of 0.012$^{\circ}$ in the rocking curve scans, and an in-plane compressive strain without relaxation. In addition, spherical aberration-corrected scanning transmission electron microscopy showed an atomically abrupt interface between the EuIG film and GGG. The measured squarish out-of-plane magnetization-field hysteresis loops by vibrating sample magnetometry in conjunction with the measurements from angle-dependent x-ray magnetic dichroism demonstrated the PMA in the films. We have tailored the magnetic properties of the EuIG thin films, including saturation magnetization ranging from 71.91 to 124.51 emu/c.c. (increase with the (Eu/Fe) ratios), coercive field from 27 to 157.64 Oe, and the strength of PMA field ($H_\bot$) increasing from 4.21 to 18.87 kOe with the in-plane compressive strain from -0.774 to -1.044%. We have also investigated spin transport in Pt/EuIG bi-layer structure and evaluated the real part of spin mixing conductance to be $3.48\times10^{14} {\Omega}^{-1}m^{-2}$. We demonstrated the current-induced magnetization switching with a low critical switching current density of $3.5\times10^6 A/cm^2$, showing excellent potential for low-dissipation spintronic devices.

Low power spin–orbit torque switching in sputtered BiSb topological insulator/perpendicularly magnetized CoPt/MgO multilayers on oxidized Si substrate

Topological insulators (TIs) are promising for efficient spin current sources in spin–orbit torque (SOT) magnetoresistive random access memory (MRAM). However, TIs are usually deposited by molecular beam epitaxy on single crystalline III–V semiconductor or sapphire substrates, which are not suitable for realistic applications. Here, we studied SOT characteristics in sputtered BiSb topological insulator—Pt/Co/Pt—MgO heterostructures deposited on oxidized Si substrates, where Pt/Co/Pt trilayers have a large perpendicular magnetic anisotropy field of 4.5 kOe. We show that the BiSb layer has a large effective spin Hall angle of θSHeff = 2.4 and a high electrical conductivity of σ = 1.0 × 105 Ω−1 m−1. The magnetization can be switched by a small current density of 2.3 × 106 A cm−2 at a pulse width of 100 µs, which is 1 or 2 orders of magnitudes smaller than those in heavy metals. Our work demonstrates the high efficiency and robustness of BiSb as a spin current source in realistic SOT-MRAM.

Enhancement of spin-to-charge conversion efficiency in topological insulators by interface engineering

Topological insulator (TI) based heterostructure is a prospective candidate for ultrahigh spin-to-charge conversion efficiency due to its unique surface states. We investigate the spin-to-charge conversion in (Bi,Sb)2Te3 (BST)/CoFeB, BST/Ru/CoFeB, and BST/Ti/CoFeB by spin pumping measurement. We find that the inverse Edelstein effect length (λIEE) increases by 60% with a Ru insertion while remains constant with a Ti insertion. This can be potentially explained by the protection of BST surface states due to the high electronegativity of Ru. Such enhancement is independent of the insertion layer thickness once the thickness of Ru is larger than 0.5 nm, and this result suggests that λIEE is very sensitive to the TI interface. In addition, an effectively perpendicular magnetic anisotropy field and additional magnetic damping are observed in the BST/CoFeB sample, which comes from the interfacial spin–orbit coupling between the BST and the CoFeB. Our work provides a method to enhance λIEE and is useful for the understanding of charge-to-spin conversion in TI-based systems.

Large Orbital Magnetic Moment and Strong Perpendicular Magnetic Anisotropy in Heavily Intercalated FexTiS2

Titanium disulfide TiS$_2$, which is a member of the layered transition-metal dichalcogenides with the 1T-CdI$_2$-type crystal structure, is known to exhibit a wide variety of magnetism through intercalating various kinds of transition-metal atoms of different concentrations. Among them, Fe-intercalated titanium disulfide Fe$_x$TiS$_2$ is known to be ferromagnetic with strong perpendicular magnetic anisotropy (PMA) and large coercive fields ($H_\text{c}$). In order to study the microscopic origin of the magnetism of this compound, we have performed X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements on single crystals of heavily intercalated Fe$_x$TiS$_2$ ($x\sim0.5$). The grown single crystals showed a strong PMA with a large $H_\text{c}$ of $\mu_0H_\text{c} \simeq 1.0\ \text{T}$. XAS and XMCD spectra showed that Fe is fully in the valence states of 2+ and that Ti is in an itinerant electronic state, indicating electron transfer from the intercalated Fe atoms to the host TiS$_2$ bands. The Fe$^{2+}$ ions were shown to have a large orbital magnetic moment of $\simeq 0.59\ \mu_\text{B}\text{/Fe}$, to which, combined with the spin-orbit interaction and the trigonal crystal field, we attribute the strong magnetic anisotropy of Fe$_x$TiS$_2$.

Thermal stability for domain wall mediated magnetization reversal in perpendicular STT MRAM cells with W insertion layers

We present an analytical model for calculating the energy barrier for the magnetic field-driven domain wall-mediated magnetization reversal of a magneto-resistive random access memory cell and apply it to study thermal stability factor Δ for various thicknesses of W layers inserted into the free layer (FL) as a function of the cell size and temperature. We find that, by increasing the W thickness, the effective perpendicular magnetic anisotropy (PMA) energy density of the FL film monotonically increases, but at the same time, Δ of the cell mainly decreases. Our analysis shows that, in addition to saturation magnetization Ms and exchange stiffness constant Aex of the FL film, the parameter that quantifies the Δ value of the cell is its coercive field Hc, rather than the net PMA field Hk of the FL film comprising the cell.

Modulation of thermal stability and spin–orbit torque in IrMn/CoFeB/MgO structures through atom thick W insertion

Antiferromagnet (AFM)/ferromagnet (FM) systems such as IrMn/CoFeB/MgO enable spin–orbit-torque- (SOT-) induced switching of perpendicular magnetization in the absence of an external magnetic field. However, the low thermal stability, weak perpendicular magnetic anisotropy (PMA), and indistinctive SOT of these AFM/FM heterostructures pose challenges to the practical application. Here, through the insertion of a thin W layer between the IrMn and CoFeB layers, we show that much larger effective PMA fields are obtained with annealing stability to 300 °C, which is guaranteed by the prevention of Mn diffusion via W insertion as shown in spherical aberration corrected transmission electron microscopy and atomic-resolution electron energy-loss spectroscopy measurement results. Furthermore, the spin–orbit torque is effectively tuned by changing the W layer thickness via modulation of the interfacial spin–orbit coupling at IrMn/W/CoFeB interfaces, which was reported to degrade the interface spin transparency for the spin currents. Finally, field-free magnetization switching was achieved with comparable exchange bias fields to samples without W insertion. This work demonstrates an effective strategy for improving the performance of the thermally robust AFM-based SOT device.

Nanometer-Thick Yttrium Iron Garnet Films with Perpendicular Anisotropy and Low Damping

${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ ($\mathrm{YIG}$) thin films having a thickness of several nanometers and showing both strong perpendicular magnetic anisotropy (PMA) and low magnetic damping are reported. The films are deposited by magnetron sputtering at room temperature first and then annealed in ${\mathrm{O}}_{2}$ at high temperature. The substrates are ${\mathrm{Gd}}_{3}({\mathrm{Sc}}_{2}{\mathrm{Ga}}_{3}){\mathrm{O}}_{12}$, which share the same crystalline structure as $\mathrm{YIG}$, but have a lattice constant slightly larger than that of $\mathrm{YIG}$; the lattice mismatching gives rise to an out-of-plane compressive strain and PMA in the $\mathrm{YIG}$ films. The PMA is confirmed by vibrating sample magnetometer, magneto-optical Kerr effect, anomalous Hall effect, and angle-dependent ferromagnetic resonance (FMR) measurements. The damping of the films is analyzed through frequency-dependent FMR measurements. As an example, an 8-nm-thick $\mathrm{YIG}$ film shows an effective PMA field of about 2800 Oe, a nearly square hysteresis loop, and a damping constant of only $4.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. As an illustration of possible applications of such films in spintronic devices, current-induced switching of the magnetization of the PMA $\mathrm{YIG}$ films is demonstrated by the use of $\mathrm{YIG}$/$\mathrm{Pt}$ bilayered Hall bar devices.

Spin-valve Josephson junctions with perpendicular magnetic anisotropy for cryogenic memory

We demonstrate a Josephson junction with a weak link containing two ferromagnets with perpendicular magnetic anisotropy and independent switching fields in which the critical current can be set by the mutual orientation of the two layers. Such pseudospin-valve Josephson junctions are a candidate cryogenic memory in an all superconducting computational scheme. Here, we use Pt/Co/Pt/CoB/Pt as the weak link of the junction with dCo=0.6 nm, dCoB=0.3 nm, and dPt=5 nm and obtain a 60% change in the critical current for the two magnetization configurations of the pseudospin-valve. Ferromagnets with perpendicular magnetic anisotropy have advantages over magnetization in-plane systems, which have been exclusively considered at this point, as, in principle, the magnetization and magnetic switching of layers in the junction should not affect the in-plane magnetic flux.