Magnetic Data Storage Technology Research Articles

Three-dimensional structure of hybrid magnetic skyrmions determined by neutron scattering

Magnetic skyrmions are topologically protected chiral spin textures which present opportunities for next-generation magnetic data storage and logic information technologies. The topology of these structures originates in the geometric configuration of the magnetic spins - more generally described as the structure. While the skyrmion structure is most often depicted using a 2D projection of the three-dimensional structure, recent works have emphasized the role of all three dimensions in determining the topology and their response to external stimuli. In this work, grazing-incidence small-angle neutron scattering and polarized neutron reflectometry are used to determine the three-dimensional structure of hybrid skyrmions. The structure of the hybrid skyrmions, which includes a combination of N\'eel-like and Bloch-like components along their length, is expected to significantly contribute to their notable stability, which includes ambient conditions. To interpret the neutron scattering data, micromagnetic simulations of the hybrid skyrmions were performed, and the corresponding diffraction patterns were determined using a Born approximation transformation. The converged magnetic profile reveals the magnetic structure along with the skyrmion depth profile, including the thickness of the Bloch and N\'eel segments and the diameter of the core.

Free Layer Thickness Dependence of the Stability in Co2(Mn0.6Fe0.4)Ge Heusler Based CPP-GMR Read Sensor for Areal Density of 1 Tb/in2.

Current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) read sensors based on Heusler alloys are promising candidates for ultrahigh areal densities of magnetic data storage technology. In particular, the thickness of reader structures is one of the key factors for the development of practical CPP-GMR sensors. In this research, we studied the dependence of the free layer thickness on the stability of the Co2(Mn0.6Fe0.4)Ge Heusler-based CPP-GMR read head for an areal density of 1 Tb/in2, aiming to determine the appropriate layer thickness. The evaluations were done through simulations based on micromagnetic modelling. The reader stability indicators, including the magnetoresistance (MR) ratio, readback signal, dibit response asymmetry parameter, and power spectral density profile, were characterized and discussed. Our analysis demonstrates that the reader with a free layer thickness of 3 nm indicates the best stability performance for this particular head. A reasonably large MR ratio of 26% was obtained by the reader having this suitable layer thickness. The findings can be utilized to improve the design of the CPP-GMR reader for use in ultrahigh magnetic recording densities.

Multibit racetrack memory

We present a hybrid usage of domain walls and skyrmions in the same racetrack memory which can enable developing a platform for many-valued magnetic data storage technology, by numerically solving the Landau–Lifshitz-Gilbert equation. The conversion of domain-walls to magnetic skyrmions in a plain nanotrack without geometrical restrictions allowed us to realize a ternary racetrack memory. Annihilation, conversion, and passing over of the reverse domains are considered as the three-state of ternary logic. We anticipate that multibit racetrack memory will influence the development of data storage platforms for emergent computing technologies.

Laser Controlled Spin Dynamics of Ferromagnetic Thin Film from Femtosecond to Nanosecond Timescale

Permalloy (iron dissolved in nickel, in about a 1:4 ratio) is an important material for magnetic recording, due to its negligible magnetostriction and relatively low damping. This study of ultrafast magnetization dynamics investigates the correlation between phenomena occurring on time scales from femtoseconds to nanoseconds, following irradiation of a permalloy film by femtosecond laser pulses. The intrinsic Gilbert damping experiences a transient change with varying pump fluence, due to the rising ratio of system temperature to Curie temperature. The insight from this work is valuable for developing several varieties of magnetic data-storage technology.

High magnetic coercivity of FePt–Ag/MgO granular nanolayers

L10-FePt ferromagnetic nanoparticles have a hight coercivity of Tesla order. Thus, these nanoparticles, with size of 10 to 15 nm and uniform surface distribution, are suitable in magnetic data storage technology with density of more than 1GB. In order to improve structural and magnetic properties of FePt nanoparticles, some elements and combinations have been added to compound. In this research, we show that due to the presence of the Ag, the phase transition temperature of FePt from fcc to L10-fct phase decreases. The presence of Ag as an additive in FePt–Ag nanocomposite, increases the magnetic coercivity. This nanocomposite, with 10% Ag, was deposited by magnetron sputtering on the MgO heat layer. VSM results of 10 nm nanoparticles show that coercivity has increased up to 1.4 T. XRD and FESEM results confirm that the size of the L10-FePt nanoparticles are 10 nm and their surface distribution are uniform. Ag gradually form nano scale clusters with separate lattice and FePt–Ag nanocomposite appears. The result of this process is emptiness of Ag position in FePt-fcc lattice. So, the mobility of Fe and Pt atoms in this lattice increases and it can be possible for them to move in lower temperature. This mechanism explain the effect of Ag on decreasing the transition temperature to fct-L10 phase, and hight coercivity of FePt nanoparticles.

Head and Media Design for Curvature Reduction in Heat-Assisted Magnetic Recording

Recording curvature in magnetic data-storage technology has long been one of the significant challenges impacting on recording performance. Despite curvature occurrence in the conventional recording techniques such as perpendicular magnetic recording (PMR), heat-assisted magnetic recording (HAMR) is demonstrated to induce much more severe curvature than PMR. HAMR curvature could cause poor bit error rate and limits the maximum areal density capacity. Here we have theoretically predicted and demonstrated various approaches for curvature reduction from the aspect of either altering the near-field transducer head design or recording medium design. Optical and thermal modeling have indicated that by utilizing a crown-shape peg to change the thermal source profile and compensate for thermal expansion and rounding effect, it could potentially improve curvature figure of merit (FOM) and achieve curvature reduction by ~45%. In terms of the recording media design, by altering the heat sink and internal layer media material or geometry, it could also achieve curvature cancellation of ~40% with increased thermal gradient. The combined approach from both HAMR head and media perspectives with balanced recording FOMs, could potentially realize significant curvature reduction to be of similar or better recording curvature level to PMR.

Magnetic alloy nanowire arrays with different lengths: Insights into the crossover angle of magnetization reversal process

Nanoscale magnetic alloy wires are being actively investigated, providing fundamental insights into tuning properties in magnetic data storage and processing technologies. However, previous studies give trivial information about the crossover angle of magnetization reversal process in alloy nanowires (NWs). Here, magnetic alloy NW arrays with different compositions, composed of Fe, Co and Ni have been electrochemically deposited into hard-anodic aluminum oxide templates with a pore diameter of approximately 150nm. Under optimized conditions of alumina barrier layer and deposition bath concentrations, the resulting alloy NWs with aspect ratio and saturation magnetization (Ms) up to 550 and 1900emucm−3, respectively, are systematically investigated in terms of composition, crystalline structure and magnetic properties. Using angular dependence of coercivity extracted from hysteresis loops, the reversal processes are evaluated, indicating non-monotonic behavior. The crossover angle (θc) is found to depend on NW length and Ms. At a constant Ms, increasing NW length decreases θc, thereby decreasing the involvement of vortex mode during the magnetization reversal process. On the other hand, decreasing Ms decreases θc in large aspect ratio (>300) alloy NWs. Phenomenologically, it is newly found that increasing Ni content in the composition decreases θc. The angular first-order reversal curve (AFORC) measurements including the irreversibility of magnetization are also investigated to gain a more detailed insight into θc.

FerrimagneticDyCo5Nanostructures for Bits in Heat-Assisted Magnetic Recording

Increasing the magnetic data recording density requires reducing the size of the individual memory elements of a recording layer as well as employing magnetic materials with temperature-dependent functionalities. Therefore, it is predicted that the near future of magnetic data storage technology involves a combination of energy-assisted recording on nanometer-scale magnetic media. We present the potential of heat-assisted magnetic recording on a patterned sample; a ferrimagnetic alloy composed of a rare earth and a transition metal, DyCo$_5$, which is grown on a hexagonal-ordered nanohole array membrane. The magnetization of the antidot array sample is out-of-plane oriented at room temperature and rotates towards in-plane upon heating above its spin-reorientation temperature (T$_R$) of ~350 K, just above room temperature. Upon cooling back to room temperature (below T$_R$), we observe a well-defined and unexpected in-plane magnetic domain configuration modulating with ~45 nm. We discuss the underlying mechanisms giving rise to this behavior by comparing the magnetic properties of the patterned sample with the ones of its extended thin film counterpart. Our results pave the way for novel applications of ferrimagnetic antidot arrays of superior functionality in magnetic nano-devices near room temperature.

Spin-orbit torque magnetization switching controlled by geometry.

Magnetization reversal by an electric current is essential for future magnetic data storage technology, such as magnetic random access memories. Typically, an electric current is injected into a pillar-shaped magnetic element, and switching relies on the transfer of spin momentum from a ferromagnetic reference layer (an approach known as spin-transfer torque). Recently, an alternative technique has emerged that uses spin-orbit torque (SOT) and allows the magnetization to be reversed without a polarizing layer by transferring angular momentum directly from the crystal lattice. With spin-orbit torque, the current is no longer applied perpendicularly, but is in the plane of the magnetic thin film. Therefore, the current flow is no longer restricted to a single direction and can have any orientation within the film plane. Here, we use Kerr microscopy to examine spin-orbit torque-driven domain wall motion in Co/AlOx wires with different shapes and orientations on top of a current-carrying Pt layer. The displacement of the domain walls is found to be highly dependent on the angle between the direction of the current and domain wall motion, and asymmetric and nonlinear with respect to the current polarity. Using these insights, devices are fabricated in which magnetization switching is determined entirely by the geometry of the device.

Reduction and characterizations of iron particles: influence of reduction parameters

Iron particles have some applications as electromagnetic devices in magnetic recording and data storage technology due to their small sizes and high data storage capacity. The devices can be advanced by improving the properties of existing materials according to the production parameters. Thus, the influences of reduction parameters on the properties of iron particles were studied. The iron particles were reduced from superparamagnetic iron oxide nanoparticles by altering reduction parameters under hydrogen atmosphere at high (400 °C) temperature. The structural analysis of the films was carried out using the X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) techniques. The XRD data revealed that the crystal textures changed for the particles reduced at each parameter. And, the crystal structure turns from the cubic spinel structure of magnetite and body centered cubic (bcc) structure of iron to the bcc iron as the reduction time increases from 15 to 240 min. Then, the similar structure change can be seen for the samples reduced at increasing hydrogen flow rates. The HRTEM studies revealed that the surface morphology of the films strongly depend on the flow rate. Finally, magnetite peaks weaken and then disappear as the precursor mass decreases to the lowest value. The average crystallite sizes were found to be consistent with changing crystal structure. Furthermore, the magnetic characteristics studied by a vibrating sample magnetometer were observed to be affected by the parameters. Besides, magnetic differences may arise from the variation of crystal structure and crystal sizes caused by individual reduction parameters of reduction time, hydrogen flow rate and precursor mass.

Design and fabrication of a metallic nanostamp using UV nanoimprinting and electroforming for replicating discrete track media with feature size of 35 nm

The demand for high-density data-storage media is increasing, necessitating the development of novel magnetic data-storage technologies. Among the various types of storage media, discrete track media (DTM) is an emerging technology that is being used to overcome the limitations of conventional continuous magnetic data-storage technology, such as the superparamagnetic effect and medium noise. In this study, the authors propose a method of fabricating a metallic stamp for replicating DTM patterns using ultraviolet (UV) nanoimprinting and electroforming, which are inexpensive processes that can be used to fabricate nanostructures with high precision. First, a silicon nanomaster with a feature size of 35 nm and a pitch of 70 nm was designed and fabricated by electron-beam recording and inductively coupled plasma etching. The measured pitch of the silicon master was 71.6 nm. Then, a polymeric master with a full track of nanoline patterns was then replicated from the silicon nanomaster via UV nanoimprinting. To improve the releasing properties during UV nanoimprinting, the silicon nanomaster was coated with a self-assembled monolayer of fluoroctatrichlorosilane. The measured average pitch and height of the replicated polymer master were 71.5 and 61 nm, respectively. Then, a metallic nanostamp with a thickness of 300 μm and a diameter of 80 mm was fabricated using electroforming. The metallic nanostamp was successfully fabricated, and its geometrical properties were measured and analyzed. The pitch and height of fabricated nickel stamp were 71.2 and 60.3 nm, respectively.

Magnetic materials and devices: Research and applications

The history of magnetic materials dates back more than 2,500 years when the magnetic phenomenon was observed in lodestone, a naturally occurring mineral. At that time the only useful application of magnets was in compass needles. Since then, magnetic materials have found applications in a much wider domain. Their use in traditional areas such as electric motors, generators, loudspeakers, magnetic separators, etc. is well known. Lately they have found applications in more advanced fi elds such as biomedical imaging and drug delivery. However, the one fi eld that has probably been infl uenced the most is magnetic data storage technology. This technology has been advancing with almost the same pace as the semiconductor industry. A very exciting concept being explored is marrying semiconductor technology and the advanced magnetic data storage and processing technology. This emerging hybrid technology is known as spintronics. Spintronics utilizes the charge as well as the spin degrees of freedom of electrons. Further, spintronics requires the development of magnetic materials which can effi ciently inject spin polarized carriers into the semiconductors. Despite considerable efforts, effi cient injection of spins into nonmagnetic semiconductors continues to be a major hurdle in this fi eld. Conventional ferro-magnets such as Fe and Ni are not effi cient for the above application. This has led to an urgent need for developing new kinds of magnetic materials known as diluted magnetic semiconductors and

Advances in Magnetic Data Storage Technologies

The articles in this special issue on magnetic data storage technologies are divided into three main categories: state-of-the-art magnetic recording science and technology; future magnetic storage technologies; and storage systems.

Evidence of giant magnetoresistance effect in heterogeneous nanogranular films produced by ultrashort pulsed laser deposition

Giant magnetoresistance effect is found in films of magnetic nanoparticles uniformly mixed with non-magnetic metallic nanoparticles produced by ultrashort pulsed laser deposition (uPLD). The uPLD, which uses femtosecond laser pulses, has been recently reported as a powerful technique for obtaining nanoparticles and nanogranular films. As-deposited Co–Cu and Fe–Ag films in a moderate volume fraction range of magnetic component (15–25%) present detectable values of this magnetoresistive effect, although the average size of the particles is higher than in typical nanogranular materials for magnetoresistive applications. The determined longitudinal, transverse and perpendicular magnetoresistance behaviours, at the temperatures of 10 and 250 K, confirm the strong influence of the production technique on the complex microstructure of these films and consequently on their peculiar magneto-transport properties. In perspective, by optimizing the production parameters, these nanogranular films appear very promising for potential application in magnetic recording and data storage technology.

Electrical switching of the vortex core in a magnetic disk

A magnetic vortex is a curling magnetic structure realized in a ferromagnetic disk, which is a promising candidate for a memory cell for future non-volatile data-storage devices. Thus, an understanding of the stability and dynamical behaviour of the magnetic vortex is a major requirement for developing magnetic data-storage technology. Since the publication of experimental proof for the existence of a nanometre-scale core with out-of-plane magnetization in a magnetic vortex, the dynamics of vortices have been investigated intensively. However, a way to electrically control the core magnetization, which is a key for constructing a vortex-core memory, has been lacking. Here, we demonstrate the electrical switching of the core magnetization by using the current-driven resonant dynamics of the vortex; the core switching is triggered by a strong dynamic field that is produced locally by a rotational core motion at a high speed of several hundred metres per second. Efficient switching of the vortex core without magnetic-field application is achieved owing to resonance. This opens up the potentiality of a simple magnetic disk as a building block for spintronic devices such as a memory cell where the bit data is stored as the direction of the nanometre-scale core magnetization.

Perpendicular magnetic recording: Playback

For the past several years, perpendicular magnetic recording has been under intense scrutiny as the primary alternative to magnetic data storage technologies in place today. Major system components, write heads and media in particular, have been the subject of extensive studies. Less attention, however, has been devoted to the playback processes in perpendicular recording systems. The playback heads used in technology demonstrations remain largely unchanged from their longitudinal recording counterparts. It is an open question whether the longitudinal playback-head design is optimal for perpendicular recording. For example, application of longitudinal playback heads in perpendicular recording leads to undesirable phenomena associated with modified playback response, increased flying height sensitivity, adjacent track interference, and calls for major modifications of the existing read channels. The subject of this work is a detailed discussion of the playback physics, in perpendicular recording systems; the focus being to establish the design guidelines for optimized perpendicular playback heads, which are equivalent or superior in their performance characteristics to conventional shielded readers used in longitudinal recording. Conformal mapping is applied to demonstrate the playback wave form equivalency between a shielded and dual-pole readers when applied in longitudinal and perpendicular recording, respectively. Utilizing extensive three-dimensional modeling and reciprocity principle to evaluate the performance of various playback-head configurations, it is demonstrated that differential reader configurations possess advantageous playback characteristics, such as higher playback amplitude, improved spatial resolution, and reduced dependence on flight-height variations as compared to conventional shielded readers. Modified design of differential readers with a single magnetoresistive sensor is proposed to overcome the manufacturability issues associated with a conventional dual-sensor differential reader.

Slider design for sub-3-nm flying height head–disk systems

It is important to further push slider's gap flying height from the current value of 7–8nm in mass production to 2.5–3nm, in order to increase the areal density of magnetic data storage technology from the current 60–80Gb/in2 in mass production to 1Tb/in2. Furthermore, perpendicular magnetic recording will be the only choice for area densities beyond 300Gb/in2, though it might be more sensitive to the fluctuation of the flying height and needs higher flying height stability. This paper reports the authors’ efforts in both reducing slider's gap flying height and increasing the stability of the flying height. Analysis suggests that heat energy generation in both reading and writing operations affects the stability of the flying height, in addition to the intermolecular forces and the flying height modulations caused by disk waviness. An important design strategy is to have high air pressure and pressure gradient around the read/write element. Results suggest that introducing a sub-shallow step in slider's trailing part is an effective approach of achieving high air pressure and pressure gradient. A new type of slider is designed accordingly. Aerodynamic performance analysis indicates that the proposed slider can fly stably at a flying height below 3nm.

Ultrafast nanomagnets: seeing data storage in a new light.

Magnetic materials provide the most important form of erasable data storage for information technology today. The demand for increased storage capacity has caused bit sizes and features of the read-write transducers to be reduced to the nanoscale. However, increased storage capacity is only useful if there is a commensurate reduction in the time taken to read and write the data. In this article, the basic principles that determine the behaviour of nanomagnetic materials are introduced and their use in data-storage systems is described. Particular attention is paid to processes that limit the speed of operation of the data-storage system. It is shown that optical pump-probe experiments may be used to characterize dynamic magnetic processes with femtosecond temporal resolution. The macroscopic magnetization of a ferromagnet can be made to precess in response to an optically triggered magnetic field pulse, leading to reduced switching times. Alternatively, an ultrashort laser pulse may be used to manipulate the magnitude of the magnetization on femtosecond time-scales, leading to an ultrafast demagnetization in certain ferromagnets, and providing new insight into magnetotransport phenomena. Finally, the outlook for increased record and replay rates is assessed and the prospect of further use of optical techniques within magnetic data-storage technology is discussed.

Magnetic Force Microscopy Applied in Magnetic Data Storage Technology

AbstractFor Abstract see ChemInform Abstract in Full Text.

Magnetic force microscopy applied in magnetic data storage technology

Microstructured thin-film elements with critical dimensions of 1 μm or less play an increasingly important role in magnetic components for information technology applications. Devices that are directly based on such microstructures are key components in magnetoelectronics for storage and sensor applications as well as modern concepts which are likely to substitute today’s hard disk drives. Basic research on magnetic materials as well as industrial applications create an increasing demand for high-resolution magnetic imaging methods. One such method is magnetic force microscopy (MFM). In spite of considerable achievements, MFM also has some serious shortcomings, which have not been overcome to date. Under normal circumstances, the method yields only qualitative information about the magnetic object and it is difficult to improve the resolution to values below 100 nm. In this paper, we will report on advanced MFM probe preparation, based on electron beam methods, and discuss the possibilities for batch fabrication of such advanced MFM tips. We show that the advanced probes allow high-resolution imaging of fine magnetic structures within thin-film permalloy elements without perturbing them. Additionally, we present high-frequency MFM measurements on a hard disk write head.