Perpendicular Anisotropy Research Articles

Evaluating critical metals contained in spintronic memory with a particular focus on Pt substitution for improved sustainability

Thanks to their unique combination of properties: non-volatility, speed, density and write endurance, spintronic memory called spin transfer-torque Magnetic Random Access Memory (STT-MRAM) is expected to play a major role in the future development of the Internet of Things (IoT) and more generally in information and communication technologies. This type of spintronic device is usually made of materials, some of which can be classified as critical. Recent studies have evaluated critical materials contained in magnetic random access memory (Ku, 2018; European Commission, 2020 [1,2]). However, in those cases the type of memory analyzed belongs to the first generation of MRAMs developed in the early 2000s. Nowadays, the memory devices are magnetized perpendicularly to the plane of the layers and contain a synthetic antiferromagnet (SAF) that provides a high coercivity to the STT-MRAM reference layer with reduced stray field. This SAF is typically made of cobalt (Co) and platinum (Pt) multilayers antiferromagnetically coupled across a thin ruthenium (Ru) layer. Due to the high-embodied energy of platinum group metals (PGMs), a common concern when evaluating these materials is the environmental risk associated with their production. An evaluation of the environmental and economic risks of using such multilayers is first reported here, followed by a discussion of its supply risk. Substitution of Co/Pt multilayers by Co/Ni multilayers can lead to a reduction by 3–4 orders of magnitude in terms of energy requirements or global warming potential (GWP) associated with the use of these multilayers. An alternative concept based on perpendicular shape anisotropy (PSA) can also yield a reduction by 1–2 orders of magnitude in these quantities. However, for the case of STT-MRAM, tiny quantities of PGM layers are used in comparison with the mass of the silicon wafers on which these type of devices are grown. Therefore, the environmental and economic impact of the silicon wafer fabrication is found to be much higher than that of the PGM materials incorporated in the STT-MRAM stacks. Nonetheless, the high supply risk associated with PGMs remains a reason for awareness. One explored possibility is a SAF structure based on Co/Ni multilayers which can have similar performance. A second more challenging alternative is also proposed based on the aforementioned PSA concept. Finally, we address the case of several other metals identified by the European Commission as critical and used in MRAM such as W or Ta, both recently included in the EU's Conflict Minerals Regulation released in January 2021 (https://ec.europa.eu/trade/policy/in-focus/conflict-minerals-regulation/regulation-explained/, 2020 [3]).

Domain-Wall Damping in Ultrathin Nanostripes with Dzyaloshinskii-Moriya Interaction

Asymmetrically sandwiched thin magnetic layers with perpendicular anisotropy and Dzyaloshinskii-Moriya interaction (DMI) is the prospective material science platform for spin-orbitronic technologies that rely on the motion of chiral magnetic textures, like skyrmions or chiral domain walls (DWs). The dynamic performance of a DW-based racetrack is defined by the strength of the DMI and the DW damping. The determination of the latter parameter is typically done based on technically challenging DW motion experiments. Here, we propose a method to access both the DMI constant and DW damping from static experiments by monitoring the tilt of magnetic DWs in nanostripes. We experimentally demonstrate that in perpendicularly magnetized ${\mathrm{//Cr}\mathrm{O}}_{x}\mathrm{/Co/Pt}$ stacks, DWs can be trapped on edge roughness in a metastable tilted state as a result of the DW dynamics driven by an external magnetic field. The measured tilt can be correlated to the DMI strength and DW damping in a self-consistent way in the frame of a theoretical formalism based on the collective coordinate approach.

Quantum Sensing of Spin Fluctuations of Magnetic Insulator Films with Perpendicular Anisotropy

Nitrogen vacancy (NV) centers, optically active atomic defects in diamond, have been widely applied to emerging quantum sensing, imaging, and network efforts, showing unprecedented field sensitivity and nanoscale spatial resolution. Many of these advantages derive from their excellent quantum-coherence, controllable entanglement, and high fidelity of operations, enabling opportunities to outperform the classical counterpart. Exploiting this cutting-edge quantum metrology, we report noninvasive measurement of intrinsic spin fluctuations of magnetic insulator thin films with a spontaneous out-of-plane magnetization. The measured field dependence of NV relaxation rates is well correlated to the variation of magnon density and band structure of the magnetic samples, which are challenging to access by the conventional magnetometry methods. Our results highlight the significant opportunities offered by NV centers in diagnosing the noise environment of functional magnetic elements, providing valuable information to design next-generation, high-density, and scalable spintronic devices.

Domain wall mobility engineering by a perpendicular magnetic field in microwires with a gradient of perpendicular anisotropy

Fast domain wall propagation is a typical feature of amorphous glass-coated microwires with positive magnetostriction. The high domain wall velocity can be effectively tailored either by sample postprocessing or temperature. In this work, we show that the domain wall dynamics can be engineered by a perpendicular magnetic field. We perform the domain wall mobility measurements in microwires with a varying gradient of the perpendicular magnetic anisotropy. It is shown that domain wall mobility is mainly determined by counterplay between the amplitude of perpendicular anisotropy and externally applied perpendicular field. The presence of perpendicular magnetic anisotropy is observed by Scanning electron microscopy. The relation between the maximum applied perpendicular magnetic field, and the wire dimensions are provided based on the measured data.

Long-Time-Scale Magnetization Ordering Induced by an Adsorbed Chiral Monolayer on Ferromagnets

When an electron passes through a chiral molecule, there is a high probability for correlation between the momentum and spin of the charge, thus leading to a spin polarized current. This phenomenon is known as the chiral-induced spin selectivity (CISS) effect. One of the most surprising experimental results recently demonstrated is that magnetization reversal in a ferromagnet with perpendicular anisotropy can be realized solely by chemisorbing a chiral molecular monolayer without applying any current or external magnetic field. This result raises the currently open question of whether this effect is due to the bonding event, held by the ferromagnet, or a long-time-scale effect stabilized by exchange interactions. In this work we have performed vectorial magnetic field measurements of the magnetization reorientation of a ferromagnetic layer exhibiting perpendicular anisotropy due to CISS using nitrogen-vacancy centers in diamond and followed the time dynamics of this effect. In parallel, we have measured the molecular monolayer tilt angle in order to find a correlation between the time dependence of the magnetization reorientation and the change of the tilt angle of the molecular monolayer. We have identified that changes in the magnetization direction correspond to changes of the molecular monolayer tilt angle, providing evidence for a long-time-scale characteristic of the induced magnetization reorientation. This suggests that the CISS effect has an effect over long time scales which we attribute to exchange interactions. These results offer significant insights into the fundamental processes underlying the CISS effect, contributing to the implementation of CISS in state-of-the-art applications such as spintronic and magnetic memory devices.

Systematic survey of magnetic configurations in multilayer ferromagnet system with dipole-dipole interaction

We investigate various magnetic configurations caused by the dipole--dipole interaction in thin-film magnets with perpendicular anisotropy under open boundary conditions. Two different approaches are simulated: one starts from a random magnetic configuration and decreases temperatures stepwisely; the other starts from the saturated out-of-plane ferromagnetic state to evaluate its metastability. As typical patterns of magnetic configuration, five typical configurations are found: an out-of-plane ferromagnetic, in-plane ferromagnetic, vortex, multidomain, and canted multidomain states. Notably, the canted multidomain forms a concentric magnetic-domain pattern with an in-plane vortex structure, resulting from the open boundary conditions. Concerning the coercivity, a comparison of the magnetic configurations in both processes reveals that the out-of-plane ferromagnetic state exhibits metastability in the multidomain state, but not in the vortex state. We also confirm that the so-called Neel-cap magnetic-domain-wall structure, which is originally discussed in the in-plane anisotropy system, appears at the multidomain state.

CVD Growth of Large‐scale and Highly Crystalline 2D Chromium Telluride Nanoflakes

AbstractRecently, 2D magnetic materials have attracted much attention due to their potential applications in spintronic devices. Chromium telluride (CrTe) flakes have been reported for hard magnetism with a strong perpendicular anisotropy. However, the synthesis of large‐scale CrTe nanoflakes on SiO2/Si substrate is still a challenge. Here, we report the synthesis of large‐scale CrTe nanoflakes with a size of up to ∼250 μm on a SiO2/Si substrate via chemical vapor deposition (CVD). Our work demonstrated an effective pathway to synthesize large‐scale 2D transition metal tellurides.

Giant Perpendicular Magnetic Anisotropy Enhancement in MgO -Based Magnetic Tunnel Junction by Using Co/Fe Composite Layer

Magnetic tunnel junctions with perpendicular anisotropy form the basis of the spin-transfer torque magnetic random-access memory (STT-MRAM), which is non-volatile, fast, dense, and has quasi-infinite write endurance and low power consumption. Based on density functional theory (DFT) calculations, we propose an alternative design of magnetic tunnel junctions comprising Fe(n)Co(m)Fe(n)/MgO storage layers with greatly enhanced perpendicular magnetic anisotropy (PMA) up to several mJ/m2, leveraging the interfacial perpendicular anisotropy of Fe/MgO along with a stress-induced bulk PMA discovered within bcc Co. This giant enhancement dominates the demagnetizing energy when increasing the film thickness. The tunneling magnetoresistance (TMR) estimated from the Julliere model is comparable with that of the pure Fe/MgO case. We discuss the advantages and pitfalls of a real-life fabrication of the structure and propose the Fe(3ML)Co(4ML)Fe(3ML) as a storage layer for MgO-based STT-MRAM cells. The large PMA in strained bcc Co is explained in the framework of Bruno's model by the MgO-imposed strain and consequent changes in the energies of dyz and dz2 minority-spin bands.

Intrinsic spin-dynamical properties of two-dimensional half-metallic FeX2 ( X = Cl, Br, I) ferromagnets: Insight from density functional theory calculations

Ultrathin two-dimensional (2D) ferromagnets with intrinsic half-metallicity are highly prospective in designing nanoscale spintronics devices. In this work, we systematically investigate the spin transport and dynamical properties of one such group of promising 2D ferromagnets---monolayer iron dihalides (${\mathrm{FeX}}_{2}$, X = Cl, Br, I)---using density functional theory (DFT). First, we explore the spin transport properties of these ${\mathrm{FeX}}_{2}$ monolayers by combining the nonequilibrium Green's function (NEGF) technique with DFT. This study shows an inherent half-metallicity with a large spin gap that offers $100%$ spin-polarization over a wide Fermi window (>1 eV). We then focus on understanding their magnetocrystalline anisotropy, Gilbert damping, and exchange interactions, in-depth, which are the key aspects in controlling the spin dynamics. We use force theorem to determine the magnetocrystalline anisotropy and Kambersky's torque-torque correlation model for Gilbert damping. Our calculations reveal a sizable perpendicular anisotropy (0.04 to 0.25 mJ/${\mathrm{m}}^{2}$) along with a relatively low Gilbert damping ($7.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ to $3.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$) in these materials. Using spin-polarized Green's function formalism, we finally explore the effective exchange interactions in these materials and determine their spin-wave stiffness, exchange stiffness constants, and Curie temperatures. All these calculations, collectively, provide significance of these 2D ${\mathrm{FeX}}_{2}$ ferromagnets for next-generation spintronics applications.

Canted standing spin-wave modes of permalloy thin films observed by ferromagnetic resonance

Non-collinear spin structures in materials that combine perpendicular and in-plane magnetic anisotropies are of great technological interest for microwave and spin wave-assisted magnetization switching. [Co/Pt] multilayers are well-known perpendicular anisotropy materials that have the potential to pin the magnetization of a soft magnetic layer, such as permalloy (Py), that has in-plane anisotropy, thereby forming a magnetic exchange spring. Here we report on multilayered [Co/Pt]/Pt/Py films, where an additional ultrathin Pt spacer has been included to control the coupling between the sub-units with in-plane and perpendicular magnetic anisotropy. Vector network analyser (VNA)-ferromagnetic resonance (FMR) measurements were made to obtain a complete picture of the resonant conditions, while the dynamical response of the sub-units was probed by synchrotron-based element- and phase selective x-ray detected FMR (XFMR). For all samples, only slight pinning of the dynamic magnetization of the Py by the [Co/Pt] was noted, and the FMR results were dominated by the 50 nm thick Py layer. Out-of-plane VNA-FMR maps reveal the presence of additional modes, e.g. a perpendicular standing spin-wave (PSSW) state. However, as the magnetic field is reduced below the saturation field, the PSSW state morphs continuously through a series of canted standing spin-wave (CSSW) states into a horizontal standing spin-wave (HSSW) state. The PSSW, CSSW and HSSW states are well described using a multilayer model of the Py film. The observation of CSSW modes is of particular relevance to microwave assisted magnetic recording, where microwave excitation stimulates precession of a soft layer canted out of plane by a pulsed magnetic field.

Amorphous exchange-spring magnets with crossed perpendicular and in-plane anisotropies

We study the magnetic coupling in a thin film bilayer exchange-spring magnet composed of an amorphous ${\mathrm{Tb}}_{10}{\mathrm{Co}}_{90}$ layer with a large perpendicular anisotropy and an amorphous ${\mathrm{Co}}_{85}{({\mathrm{Al}}_{70}{\mathrm{Zr}}_{30})}_{15}$ layer with uniaxial in-plane anisotropy. The CoAlZr is directly exchange coupled to the TbCo with an interface region of at least 7.5 nm within which the CoAlZr magnetization is perpendicular to the plane and switches with the underlying TbCo. The influence of the coupling extends up to 30 nm into the CoAlZr resulting in an effective tilted anisotropy. The coercivity of the CoAlZr is greatly enhanced due to the coupling and is tuneable over a large range by varying its thickness.

High-yield fabrication of perpendicularly magnetised synthetic antiferromagnetic nanodiscs

Synthetic antiferromagnetic (SAF) particles with perpendicular anisotropy display a number of desirable characteristics for applications in biological and other fluid environments. We present an efficient and effective method for the patterning of ultrathin Ruderman-Kittel-Kasuya-Yoshida coupled, perpendicularly magnetised SAFs using a combination of nanosphere lithography and ion milling. A Ge sacrificial layer is utilised, which provides a clean and simple lift-off process, as well as maintaining the key magnetic properties that are beneficial to target applications. We demonstrate that the method is capable of producing a particularly high yield of well-defined, thin film based nanoparticles.

Experiments and SPICE simulations of double MgO-based perpendicular magnetic tunnel junction* *Project supported by State Grid Corporation of China under the 2018 Science and Technology Project of State Grid Corporation: Research on electromagnetic measurement technology based on EIT and TMR (Grant No. JL71-18-007).

We investigate properties of perpendicular anisotropy magnetic tunnel junctions (pMTJs) with a stack structure MgO/CoFeB/Ta/CoFeB/MgO as the free layer (or recording layer), and obtain the necessary device parameters from the tunneling magnetoresistance (TMR) vs. field loops and current-driven magnetization switching experiments. Based on the experimental results and device parameters, we further estimate current-driven switching performance of pMTJ including switching time and power, and their dependence on perpendicular magnetic anisotropy and damping constant of the free layer by SPICE-based circuit simulations. Our results show that the pMTJ cells exhibit a less than 1 ns switching time and write energies < 1.4 pJ; meanwhile the lower perpendicular magnetic anisotropy (PMA) and damping constant can further reduce the switching time at the studied range of damping constant α < 0.1. Additionally, our results demonstrate that the pMTJs with the thermal stability factor ≃ 73 can be easily transformed into spin-torque nano-oscillators from magnetic memory as microwave sources or detectors for telecommunication devices.

An ab initio study of the magnetic properties of strontium hexaferrite

The magnetic properties of {text{SrFe}}_{12}{text{O}}_{19}, a paradigmatic hexaferrite for permanent magnet applications, have been addressed in detail combining density functional theory including spin–orbit coupling and a Hubbard U term with Monte Carlo simulations. This multiscale approach allows to estimate the Néel temperature of the material from ab initio exchange constants, and to determine the influence of different computational conditions on the magnetic properties by direct comparison versus available experimental data. It is found that the dominant influence arises from the choice of the Hubbard U term, with a value in the 2–3 eV range as the most adequate to quantitatively reproduce the two most relevant magnetic properties of this material, namely: its large perpendicular magnetocrystalline anisotropy and its elevated Néel temperature.

Chiral droplets and current-driven motion in ferromagnets

We find numerically skyrmionic textures with skyrmion number $Q=0$ in ferromagnets with the Dzyaloshinskii-Moriya interaction, perpendicular anisotropy, and the magnetostatic field. These have a skyrmion part and an antiskyrmion part, and they may be called chiral droplets. They are stable in an infinite film as well as in disk-shaped magnetic elements. Droplets are found in films for values of the parameters close to the transition from the ferromagnetic to the spiral phase. Under spin-transfer torque, they move in the direction of the spin flow and behave as solitary waves of Newtonian character, in stark contrast to the Hall dynamics of the standard $Q=1$ skyrmion.

Statistical Characteristics in the Spectrum of Whistler Waves Near the Diffusion Region of Dayside Magnetopause Reconnection

AbstractUsing Magnetospheric Multiscale (MMS) data, we present a statistical analysis revealing characteristics in the frequency spectrum of whistler waves in the vicinity of the diffusion region of magnetopause reconnection. On the magnetosphere side of reconnection, whistler waves are highly centered around the 1/2 electron cyclotron frequency (Ωe). On the magnetosheath side of reconnection, whistler waves are mainly below 1/2 Ωe and peaked around 0.2 Ωe. The electron temperature anisotropy associated with the waves is evaluated. The occurrence rate of the whistler waves is large in the presence of perpendicular electron temperature anisotropy (Te⊥/Te‖ &gt; 1). However, a large part of the whistler waves events occurs in association with parallel electron temperature anisotropy (Te⊥/Te‖ &lt; 1) on the magnetosheath side of reconnection, reflecting a large number of the total observations with this electron condition. These results shed new lights on the whistler‐related electron kinetic processes near the reconnection diffusion region.

Ferrimagnetic insulators for spintronics: Beyond garnets

Ferrimagnetic insulators have gained much attention as material platforms with efficient magnetization dynamics. To date, epitaxial iron garnet thin films are the most widely used materials in the emerging field of “insulator spintronics.” However, further advances in this field require overcoming the disadvantages of garnets—e.g., their complex structure, high growth temperature, incompatibility with other crystalline materials, and relatively weak perpendicular magnetic anisotropy. In this Perspective, we make the case that epitaxial thin films of spinel ferrites and hexagonal ferrites are viable materials for insulator spintronics with complementary advantages over the oft-used garnets. Specifically, spinel ferrites have a simpler structure, can crystallize at lower temperatures, and are more amenable to coherent integration with various materials; hexagonal ferrites possess enormous perpendicular anisotropy of bulk origin, in contrast to garnets where the strength of anisotropy is restricted by interfacial strain. The expanded repertoire of materials for insulator spintronics will enable new physical insights and potential applications, beyond what is currently possible with garnets.

Magnonic Su-Schrieffer-Heeger model in honeycomb ferromagnets

Topological electronics has extended its richness to non-electronic systems where phonons and magnons can play the role of electrons. In particular, topological phases of magnons can be enabled by the Dzyaloshinskii-Moriya interaction (DMI) which acts as an effective spin-orbit coupling. We show that besides DMI, an alternating arrangement of Heisenberg exchange interactions critically determines the magnon band topology, realizing a magnonic analog of the Su-Schrieffer-Heeger model. On a honeycomb ferromagnet with perpendicular anisotropy, we calculate the topological phase diagram, the chiral edge states, and the associated magnon Hall effect by allowing the relative strength of exchange interactions on different links to be tunable. Including weak phonon-magnon hybridization does not change the result. Candidate materials are discussed.

Epitaxial growth of Ho&lt;sub&gt;3&lt;/sub&gt;Fe&lt;sub&gt;5&lt;/sub&gt;O&lt;sub&gt;12&lt;/sub&gt; films with perpendicular magnetic anisotropy and spin transport properties in Ho&lt;sub&gt;3&lt;/sub&gt;Fe&lt;sub&gt;5&lt;/sub&gt;O&lt;sub&gt;12&lt;/sub&gt;/Pt heterostructures

Rare-earth iron garnet films with perpendicular magnetic anisotropy could open new perspectives for spintronics. Holmium iron garnet (Ho&lt;sub&gt;3&lt;/sub&gt;Fe&lt;sub&gt;5&lt;/sub&gt;O&lt;sub&gt;12&lt;/sub&gt;, HoIG) films with thickness ranging from 2 to 100 nm are epitaxially grown on (111) orientated gadolinium gallium garnet single crystal substrate doped with yttrium and scandium (Gd&lt;sub&gt;0.63&lt;/sub&gt;Y&lt;sub&gt;2.37&lt;/sub&gt;Sc&lt;sub&gt;2&lt;/sub&gt;Ga&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;12&lt;/sub&gt;, GYSGG) by ultra-high vacuum magnetron sputtering. A 3-nm Pt film is further deposited on each of the HoIG films. The magnetic anisotropy and magneto-transport properties of heterostructures at room temperature are investigated. It is shown that the HoIG film as thin as 2 nm (less than two unit cells in thickness) exhibits the ferromagnetic properties at room temperature, and perpendicular magnetic anisotropy is achieved in the 2-60 nm thick films, and a maximum effective perpendicular anisotropy field reaches 350 mT due to the strain induced magnetoelastic anisotropy. The HoIG/Pt heterostructure shows significant anomalous Hall effect (AHE) and appreciable spin-Hall magnetoresistance (SMR) and/or anisotropic magnetoresistance (AMR). Remarkably, the AHE starts to decline gradually when the HoIG thickness is less than 4 nm, but the magnetoresistance decreases rapidly with the HoIG layer becoming less than 7 nm in thickness. The fact that the AHE in the heterostructure is less sensitive to the HoIG thickness suggests that the interface effect is more dominant in the AHE mechanism, whereas the bulk magnetic properties of the HoIG plays a more important role for the observed magnetoresistance. In addition, the spin Seebeck effect decreases exponentially with the decrease of HoIG thickness till the ultrathin limit, which was previously validated in the micrometer-thick YIG/Pt stacks in the frame of thermally excited magnon accumulation and propagation. The present results show that the nanometer HoIG/Pt heterostructure with tunable perpendicular magnetic anisotropy and efficient interfacial spin exchange interaction could be a promising candidate for insulating magnet based spintronic devices.

Surface acoustic waves increase magnetic domain wall velocity

Domain walls in magnetic thin films are being explored for memory applications and the speed at which they move has acquired increasing importance. Magnetic fields and currents have been shown to drive domain walls with speeds exceeding 500 m/s. We investigate another approach to increase domain wall velocities, using high frequency surface acoustic waves to create standing strain waves in a 3 micron wide strip of magnetic film with perpendicular anisotropy. Our measurements, at a resonant frequency of 248.8 MHz, indicate that domain wall velocities increase substantially, even at relatively low applied voltages. Our findings suggest that the strain wave derived effective magnetic field acts as an additional driver for domain wall motion.