Magnetic Bits Research Articles

Tape in the cloud—Technology developments and roadmaps supporting 80 TB cartridge capacities

Tape, which is the storage of magnetic bits on flexible 0.5” wide media, provides critical data storage needs for the Cloud as evidenced by continued growth in exabyte shipments of tape products. This paper outlines the present storage landscape environment for tape, HDD, and NAND flash, describes the tape roadmap for capacity improvements, and presents recording data on narrow track, 32 channel, TMR tape heads spanning read sensor widths from 1.0 μm to 0.20 μm illustrating the potential for 80 TB cartridge operation. Critically, future improvements in tape areal density and tape cartridge capacity are expected to be achieved from evolutionary developments in head/media technology, not requiring revolutionary developments, such as energy assisted magnetic recording technologies required by HDD strategies.

Fundamental Limits on the Repetition Rate of Photomagnetic Recording

In the last decade it was demonstrated that the fastest way to write information employs ultrashort laser pulses. Naturally such experiments raise questions about the ultimate limit of repetition rate at which light can switch a medium between stable bit states. Here the authors demonstrate that with femtosecond pulses it is possible to write and rewrite magnetic bits in iron garnet with a frequency of up to 20 GHz, with the maximum repetition rate being defined by the frequency of ferromagnetic resonance in the field of photoinduced magnetic anisotropy. This finding reveals the principles to be employed in achieving magnetic recording at frequencies far beyond today's state of the art.

Plasmonic layer-selective all-optical switching of magnetization with nanometer resolution

All-optical magnetization reversal with femtosecond laser pulses facilitates the fastest and least dissipative magnetic recording, but writing magnetic bits with spatial resolution better than the wavelength of light has so far been seen as a major challenge. Here, we demonstrate that a single femtosecond laser pulse of wavelength 800 nm can be used to toggle the magnetization exclusively within one of two 10-nm thick magnetic nanolayers, separated by just 80 nm, without affecting the other one. The choice of the addressed layer is enabled by the excitation of a plasmon-polariton at a targeted interface of the nanostructure, and realized merely by rotating the polarization-axis of the linearly-polarized ultrashort optical pulse by 90°. Our results unveil a robust tool that can be deployed to reliably switch magnetization in targeted nanolayers of heterostructures, and paves the way to increasing the storage density of opto-magnetic recording by a factor of at least 2.

Strain-mediated electric manipulation of magnetic skyrmion and other topological states in geometric confined nanodiscs

Magnetic skyrmions in multilayer stack structures are promising magnetic memory bits for applications in race-track storage devices or random-access memories. For practical implementation in energy efficient memory devices, it is essential to achieve a deterministic switch of skyrmion states, e.g. nucleation and elimination, triggered by electric fields. In this work, by means of micromagnetic simulation, we demonstrate the strain-mediated electric field driven switching of magnetic skyrmion and other topological states without the presence of a magnetic field in a proposed device consisting of nanodiscs of trilayer Pt/Co/Ta stacks on a piezoelectric substrate. It is revealed that the multiple magnetic state switching behaviors, including the nucleation and elimination of skyrmion or labyrinth stripe domain states, as well as topological transitions between skyrmion, vortex, skyrmionium and vertical single domain states, can be triggered by applying an external electric field mediated by strains. The corresponding critical strains and their dependence on the geometric parameters of the nanodiscs (e.g. size and number of stacks) for triggering such magnetic switching are also explored. The stability of these topological domain states and possible switching behaviors (e.g. volatile and non-volatile) between the different states are discussed as well. The capability for controlled switching of these topological states provides a new pathway to energy efficient high-density magnetoelectric memory and logic devices.

Modeling of multilevel cell memory with theBaTiO3/Fenanostructured multiferroic composite material

In order to enhance the performance of calculators, multilevel cell (MLC) memories have been proposed as an alternative solution to improve the communication speed between their processors and memories, and also in order to increase the effective storage density of memory devices. In this Rapid Communication, we propose a model of MLC memory based on a multiferroic (MF) composite material. We introduce a one-dimensional trilayer ferromagnetic/ferroelectric/ferromagnetic MF composite material through which we investigate theoretically the dynamics of an electromagnon excitation displaying a soliton structure with an envelope shape. It emerges from this study based on barium titanate (${\text{BaTiO}}_{3}$) and iron (Fe) that, for suitable values of the parameters of a single initial polarization excitation, some propagation scenarios display the encoding of the states of a system of two magnetic bits, i.e., the 11, 10, 01, and 00 states. In addition to this result, it is also realized that the system exhibits another scenario for which the signal propagating in the MF material is alternately converted from an electrical signal into a magnetic signal and reversely. Such outcomes suggest, on the one hand, the possibility to model a four-level MLC memory, and on the other hand, the modeling of an electromagnetic oscillator, based on a MF composite material.

Flipping the bits with current

Applied Physics Using magnetic fields to write information into memory storage limits the physical bit density of magnetic media. Hong et al. present a proof-of-principle demonstration of using spin-polarized currents instead of magnetic fields for the writing process, which could potentially increase this density. The magnetic bit was a multilayer structure with cobalt-iron-boron as the magnetic layer. A scanning tip—through which the spin-polarized current tunneled into the bit—was coated with a similar multilayer. The researchers were able to control the magnetization of the bit with the current by placing the tip only 3 angstroms away—a distance that will need to be increased for commercial applications. Appl. Phys. Lett. 114 , 243101 (2019).

Unoccupied surface and interface states in Pd thin films deposited on Fe/Ir(111) surface

We present a systematic first-principles study of the electronic surface states and resonances occuring in thin films of Pd of various thicknesses deposited on a single ferromagnetic monolayer (ML) of Fe on top of Ir(111) substrate. This system is of interest since one Pd layer deposited on Fe/Ir(111) hosts small magnetic skyrmions. The latter are topological magnetic objects with swirling spin-textures with possible implications in the context of spintronic devices since they have the potential to be used as magnetic bits for information technology. The stabilization, detection and manipulation of such non-collinear magnetic entities require a quantitative investigation and a fundamental understanding of their electronic structure. Here we investigate the nature of the unoccupied electronic states in Pd/Fe/Ir(111), which are essential in the large spin-mixing magnetoresistance signature captured using non spin-polarized scanning tunneling microscopy (Crum et al 2015 Nat. Commun. 6 8541, Hanneken et al 2015 Nat. Nanotechnol. 10 1039). To provide a complete analysis, we investigate bare Fe/Ir(111) and Pdn = 2,7/Fe/Ir(111) surfaces. Our results demonstrate the emergence of surface and interface states after deposition of Pd MLs, which are strongly impacted by the large spin–orbit coupling of Ir surface.

Asymmetric Spin-Orbit-Torque-Induced Magnetization Switching With a Noncollinear In-Plane Assisting Magnetic Field

Using spin-orbit torque (SOT) to flip magnetic bits has emerged as a leading strategy for next-generation digital storage and logic. To achieve deterministic magnetization switching, most SOT studies use an assisting magnetic field in the direction of the applied electric current, but this work examines SOT-driven switching with an assisting field that is not aligned with the current. Asymmetry between ``up-to-down'' and ``down-to-up'' switching processes is observed, and is explained by the transverse SOT effective field. This asymmetry is an important feature in understanding the SOT switching process, and in designing delicate spintronic devices with selective switching capability.

First-Principles Study of Magnetoelectric Coupling at Fe/BiFeO3(001) Interfaces

Multiferroics are important for controlling magnetization via electric field (rather than electric current), which is a main theme of spintronics research. However, single-phase multiferroic materials are not practical, due to their small magnetoelectric couplings. Here first-principles calculations predict giant magnetoelectric coupling at the interface between a single-phase multiferroic and a ferromagnet. The study provides microscopic physical insights into the control of magnetization by the electric field at a multiferroic interface, which should open a route to enhancing the efficiency of spintronic devices through low-power magnetic bit manipulation.

Field-Free Magnetization Switching by an Acoustic Wave

Surface acoustic waves (SAWs) at gigahertz frequencies are well suited to interact resonantly with spin waves in magnetostrictive materials, with the prospect of wavelike control of magnetization reversal in spintronic devices. The authors demonstrate SAW-driven all-acoustic switching over millimeter distances, and toggling between two equilibrium magnetic states for over 20 consecutive acoustic pulses. This proof-of-concept experiment confirms an alternative path to manipulating magnetic bits.

Effect of material optical properties on thermo-plasmonics of heat-assisted magnetic recording devices

Plasmonics enables unparalleled focus of optical energy well beyond the diffraction limit of light. The coupling of light with the electrons of a metal produces a surface wave known as surface plasmon. Surface plasmons exhibit unique characteristics, including strongly enhanced local fields, high sensitivity to changes in the local environment, and energy localization to extremely small volumes not restricted by the wavelength of exciting light. However, a significant fraction of optical energy of surface plasmons is dissipated as heat. A novel application of loss-enabled plasmonics is heat-assisted magnetic recording (HAMR), in which a plasmonic near-field transducer (NFT) is used to heat a sub-diffraction-limit region of the magnetic medium of the hard disk to lower its coercivity and enable the write pole of the magnetic head to write data in closely packed magnetic bits. The NFT must deliver a sufficient fraction of the incident laser power to the magnetic medium to increase the temperature of the heated magnetic bit above its Curie temperature, without causing excessive self-heating that could limit the lifetime of the plasmonic transducer. This investigation was focused on the development of an electromagnetic-thermal model of a typical HAMR head stack that can yield simulation results illustrating the effect of material optical properties of various stack layers on thermo-plasmonic performance. The hybrid analytical/numerical approach derived in this study provides an effective means of tuning the optical properties of stacking layers in HAMR devices to optimize the thermal performance and forms a general computational framework for selecting optimum optical properties for other plasmonic devices.

Curie temperature modulated structure to improve the performance in heat-assisted magnetic recording

We investigate how a temperature reduction in z-direction influences the switching probability and the noise in heat-assisted magnetic recording (HAMR) for a bit in bit-patterned media with dimensions d=5nm and h=10nm. Pure hard magnetic bits are considered and simulations with a continuous laser pulse are performed using the atomistic simulation tool VAMPIRE. The results display that the switching behavior shows a thermally induced exchange spring effect. Simultaneously, both the AC and the DC noise increase. Additionally, we illustrate how an artificial Curie temperature gradient within the material can compensate the HAMR performance loss due to the temperature gradient. Further, due to the graded Curie temperature, DC noise can be reduced compared to a structure where no temperature gradient is considered.

All-optical helicity-dependent magnetic switching by first-order azimuthally polarized vortex beams

In this letter, a method to realize all-optical helicity-dependent magnetic switching (AO-HDS) using a first-order azimuthally polarized vortex (FAPV) beam is demonstrated. Numerical calculations of the focal fields of FAPV beams reveal that left-handed and right-handed circular polarizations are generated due to the interaction between the polarization singularity and the helical wave front. Its feasibility for AO-HDS is experimentally demonstrated in Gd27Fe63.87Co9.13 under low numerical aperture (NA) conditions and within a narrow fluence window. It is numerically predicted that under high NA conditions, the lateral size of magnetic bits recorded by FAPV beams can be nearly 30% smaller than that obtained by circularly polarized beams, which opens a promising route to realize ultrafast and ultrahigh-density magnetic recording.

Deterministic, Reversible, and Nonvolatile Low-Voltage Writing of Magnetic Domains in Epitaxial BaTiO3/Fe3O4 Heterostructure.

The ability to electrically write magnetic bits is highly desirable for future magnetic memories and spintronic devices, though fully deterministic, reversible, and nonvolatile switching of magnetic moments by electric field remains elusive despite extensive research. In this work, we develop a concept to electrically switch magnetization via polarization modulated oxygen vacancies, and we demonstrate the idea in a multiferroic epitaxial heterostructure of BaTiO3/Fe3O4 fabricated by pulsed laser deposition. The piezoelectricity and ferroelectricity of BaTiO3 have been confirmed by macro- and microscale measurements, for which Fe3O4 serves as the top electrode for switching the polarization. X-ray absorption spectroscopy and X-ray magnetic circular dichroism spectra indicate a mixture of Fe2+ and Fe3+ at O h sites and Fe3+ at T d sites in Fe3O4, while the room-temperature magnetic domains of Fe3O4 are revealed by microscopic magnetic force microscopy measurements. It is demonstrated that the magnetic domains of Fe3O4 can be switched by not only magnetic fields but also electric fields in a deterministic, reversible, and nonvolatile manner, wherein polarization reversal by electric field modulates the oxygen vacancy distribution in Fe3O4, and thus its magnetic state, making it attractive for electrically written magnetic memories.

Identification of Antiferromagnetic Domains Via the Optical Magnetoelectric Effect.

The ultimate goal of multiferroic research is the development of a new-generation nonvolatile memory devices, where magnetic bits are controlled via electric fields with low energy consumption. Here, we demonstrate the optical identification of magnetoelectric (ME) antiferromagnetic (AFM) domains in the LiCoPO_{4} exploiting the strong absorption difference between the domains. This unusual contrast, also present in zero magnetic field, is attributed to the dynamic ME effect of the spin-wave excitations, as confirmed by our microscopic model, which also captures the characteristics of the observed static ME effect. The control and the optical readout of AFM/ME domains, demonstrated here, will likely promote the development of ME and spintronic devices based on AFM insulators.

IMCS2: Novel Device-to-Architecture Co-Design for Low-Power In-Memory Computing Platform Using Coterminous Spin Switch

Spin switch (SS) is a promising spintronic device which exhibits compactness, low power, non-volatility, input–output isolation leveraging giant spin Hall effect, spin transfer torque, and dipolar coupling. In this paper, we propose a novel device-to-architecture co-design for an in-memory computing platform using coterminous SS (IMCS2), which could simultaneously work as non-volatile memory and reconfigurable in-memory logic (AND/NAND, OR/NOR, and XOR/XNOR) without add-on logic circuits to memory chip. The computed logic output could be simply read out like a normal magnetic random access memory bit cell using the shared memory peripheral circuits. Such intrinsic in-memory logic could be used to process data within memory to greatly reduce power-hungry and long distance data communication in the conventional von Neumann computing system. The IMCS2-based in-memory bulk bitwise Boolean vector operation shows ${\sim }9\times $ energy saving and ${\sim }3\times $ speedup compared with that of DRAM-based in-memory computing platform. We further employ in-memory multiplication to evaluate the performance of the proposed in-memory computing platform for vector–vector multiplication with different vector sizes.

Observation of Anomalous Spin Torque Generated by a Ferromagnet

In this work we report observation of in-plane current induced out-of-plane magnetic field driven torque in spin valve structure. Since ferromagnet has high spin orbit coupling it is expected to be the source of spin-orbit-torque as it possesses anomalous-Hall-effect (AHE: equivalent to spin Hall effect in heavy metal). So we have carried out spin-torque ferromagnetic resonance (ST-FMR) experiment in a spin valve (consists of a fixed magnet and a free magnet which are separated by Cu spacer), passing in-plane radio frequency current and measuring DC voltage. Our experimental results eventually indicate that spin torque exerted on the free magnet is not caused due to the spin current injection by the fixed magnet owing to its AHE, but it is originated from in-plane current driven out-of plane effective magnetic field. This is new class of spin torque which is completely different from Slonczewski-spin transfer torque and Rashba like field like torque. The effective out-of plane magnetic field depends on the direction of current (in-plane) and magnetization (in-plane) of the pinned layer. One possible mechanism behind this unconventional torque could be intefacial spin-scattering which is also origin of current in-plane GMR effect. Most importantly this effective out-of plane torque can be useful to switch out-of plane magnetic bits in spintronic memory application.

Spin torque oscillator for microwave assisted magnetization reversal

A theoretical study is given for the self-oscillation excited in a spin torque oscillator (STO) consisting of an in-plane magnetized free layer and a perpendicularly magnetized pinned layer in the presence of a perpendicular magnetic field. This type of STO is a potential candidate for a microwave source of microwave assisted magnetization reversal (MAMR). It is, however, found that the self-oscillation applicable to MAMR disappears when the perpendicular field is larger than a critical value, which is much smaller than a demagnetization field. This result provides a condition that the reversal field of a magnetic recording bit by MAMR in nanopillar structure should be smaller than the critical value. The analytical formulas of currents determining the critical field are obtained, which indicate that a material with a small damping is not preferable to acheive a wide range of the self-oscillation applicable to MAMR, although such a material is preferable from the viewpoint of the reduction of the power consumption.

Electrical detection of single magnetic skyrmions in metallic multilayers at room temperature.

Magnetic skyrmions are topologically protected whirling spin textures that can be stabilized in magnetic materials by an asymmetric exchange interaction between neighbouring spins that imposes a fixed chirality. Their small size, together with the robustness against external perturbations, make magnetic skyrmions potential storage bits in a novel generation of memory and logic devices. To this aim, their contribution to the electrical transport properties of a device must be characterized-however, the existing demonstrations are limited to low temperatures and mainly in magnetic materials with a B20 crystal structure. Here we combine concomitant magnetic force microscopy and Hall resistivity measurements to demonstrate the electrical detection of sub-100 nm skyrmions in a multilayered thin film at room temperature. Furthermore, we detect and analyse the Hall signal of a single skyrmion, which indicates that it arises from the anomalous Hall effect with a negligible contribution from the topological Hall effect.

Electric field driven magnetic switching in nanoscale multiferroic heterostructures

Recently, there has been a surge of research interest in the electric field control of magnetism due to its promising application in spintronic and memory devices, which has become a hot topic in the field of multiferroic research. In current spintronic technology, magnetic reversal is usually driven by a large electric current via current generated magnetic field or spin-torque effect to write/erase a magnetic bit, and thus producing large power consumption and heat dissipation. While using insulating multiferroic materials, the reversal of magnetization can be triggered by applying an electric field instead of current, hence dramatically reducing the energy consumption and heat dissipation. With the current miniature trend in microelectronic technology, it is very essential to explore the electric field driven magnetic reversal (EFMS) behaviours in a micro/nanometer scale. In this article we briefly review the new progress in the field of EFMS based on multiferroic heterostructures, including some new features arising from size reduction, as well as some recent experimental and theoretical advances towards nanoscale EFMS, e.g. strain-mediated coupling, or spin exchange coupling in BiFeO3-based heterostructures, and their associated mechanisms. Finally, some key challenges in developing future EFMS based magnetoelectric devices, and some prospects for future research are also discussed.