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Asymmetric (Pt/Co/X) multilayers where X is Ta or W have been investigated. Both structures showed a tailed hysteresis loop indicating the existence of skyrmions. The interfacial Dzyaloshinskii–Moriya interaction (iDMI) extracted from Brillouin light scattering revealed that sample with W has a value of −0.52 mJ/m2, which is ∼2.5 times larger than Ta case. In addition to iDMI, the perpendicular magnetic anisotropy is also stronger for W case due to the orbital hybridization at the interface. From magnetic force microscopy, W sample showed a change from unusual rod-like domains to skyrmions, while the change was from labyrinth domains to skyrmions in Ta-based structure. [ABSTRACT FROM AUTHOR]

Neuromorphic computing (NC) is a crucial step toward realizing power-efficient artificial intelligence systems. Hardware implementation of NC is expected to overcome the challenges associated with the conventional von Neumann computer architecture. Synaptic devices that can emulate the rich functionalities of biological synapses are emerging. Out of several approaches, electrolyte-gated synaptic transistors have attracted enormous scientific interest owing to their similar working mechanism. Here, we report a three-terminal electrolyte-gated synaptic transistor based on Fe3O4 thin films, a half-metallic spinel ferrite. We have realized gate-controllable multilevel, non-volatile, and rewritable states for analog computing. Furthermore, we have emulated essential synaptic functions by applying electrical stimulus to the gate terminal of the synaptic device. This work provides a new candidate and a platform for spinel ferrite-based devices for future NC applications. [ABSTRACT FROM AUTHOR]

Neuromorphic computing (NC) is considered a potential solution for energy-efficient artificial intelligence applications. The development of reliable neural network (NN) hardware with low energy and area footprints plays a crucial role in realizing NC. Even though neurons and synapses have already been investigated using a variety of spintronic devices, the research is still in the primitive stages. Particularly, there is not much experimental research on the self-reset (and leaky) aspect(s) of domain wall (DW) device-based neurons. Here, we have demonstrated an energy-efficient NN using a spintronic DW device-based neuron with self-reset (leaky) and integrate-and-fire functions. An "anisotropy field gradient" provides the self-resetting behavior of auto-leaky, integrate, and fire neurons. The leaky property of the neuron was experimentally demonstrated using a voltage-assisted modification of the anisotropy field. A synapse with a meander wire configuration was used to achieve multiple-resistance states corresponding to the DW position and controlled pinning of the DW. The NN showed an energy efficiency of 0.189 nJ/image/epoch while achieving an accuracy of 92.4%. This study provides a fresh path for developing more energy-efficient DW-based NN systems. [ABSTRACT FROM AUTHOR]

Neuromorphic computing (NC) has been gaining attention as a potential candidate for artificial intelligence. The building blocks for NC are neurons and synapses. Research studies have indicated that domain wall (DW) devices are one of the most energy-efficient contenders for realizing NC. Moreover, synaptic functions can be achieved by obtaining multi-resistance states in DW devices. However, in DW devices with no artificial pinning, it is difficult to control the DW position, and hence achieving multilevel resistance is difficult. Here, we have proposed the concept of nanoscale interfacial Dzyaloshinskii–Moriya interaction (iDMI) for controllably stopping the DWs at specific positions, and hence, realizing multi-resistance states. We show that the nanoscale iDMI forms an energy barrier (well), which can controllably pin the DWs at the pinning sites. Moreover, a tunable depinning current density was achieved by changing the width and iDMI constant of the confinement region. We have also studied pinning in a device with five successive pinning sites. This feature is a proof-of-concept for realizing multi-resistance states in the proposed concept. Based on these observations, a magnetic tunnel junction—where the free layer is made up of the proposed concept—can be fabricated to achieve synapses for NC applications. [ABSTRACT FROM AUTHOR]

Pursuing enhanced spin–orbit torque (SOT) has become a significant focus for achieving energy-efficient spintronics devices. Researchers have explored different materials, layer engineering, and various post-deposition modification methods to realize higher SOT. Here, we have utilized the bombardment of mixed ions (Ar+ and He+ in various ratios) with 0.6 kV bias voltage to enhance the SOT efficiency in a Pt/Co/W stack. The bombardment modifies the physical properties (magnetic, electric, and structural) due to cascade collision, knocking off atoms (from the surface), and ion implantation. The increased percentage of He+ suppresses the drastic behavior of cascade collisions, and hence, the properties are tunable by changing the gas ratio. We exploited this behavior to design a material stack with enhanced SOT efficiency. We fabricated Hall bars to quantify SOT and observed a 4.5× increase in the effective damping-like efficiency. The efficiency and switching current density in the stack could be tuned by the variation of Ar:He ion ratio and the exposure duration. This study offers an easy route to tune the energy efficiency of spin devices, contributing to the advancement of next-generation spintronics. [ABSTRACT FROM AUTHOR]

Recently, brain-inspired neuromorphic computing (NC) has been gaining traction as it is expected to be more power efficient and a more suitable platform for artificial intelligence. Artificial neurons and synapses are the main components of the NC architecture, and there have been many studies on artificial synapses. Experimental studies on artificial neurons that should exhibit the leaky integrate-and-fire properties are lacking due to the challenges in fabricating such a device. In this work, we have fabricated domain wall based devices consisting of (Co/Pt)n free and hard layers without interlayer exchange coupling, whereby the stray field from the hard layer triggers the automatic leakage function in the free layer. In addition, devices of smaller width were able to fully reset, showing the potential to scale down to smaller sizes. This experimental proof of concept provided evidence that the proposed neuron design has potential applications in NC. Further studies were performed via micromagnetic simulations to understand the role of the width of the device, thickness, and saturation magnetization of the hard layer. [ABSTRACT FROM AUTHOR]

Magnetic skyrmions are promising candidates for computing and memory applications. The static and dynamic behaviors of skyrmions are tunable by altering the interfacial magnetic properties. These interfacial magnetic properties are alterable by modifying the interface structure of thin films. However, the relationship between the structural properties of the interface and the skyrmions properties is not straightforward, and a comprehensive insight is required to facilitate better controllability of the skyrmions' behaviors. Here, we comprehensively understand the relationship between atomic displacements at the interface and skyrmions' static behavior. In this study, we used ion irradiation to achieve inter-atomic displacements. We observed that the inter-atomic displacements could tailor the physical properties of skyrmions. We noticed a peculiar increase in the magnetization, Dzyaloshinskii–Moriya interaction, and exchange stiffness. The modifications in magnetic properties reduced the domain wall energy, which enhanced the skyrmion density (by six-folds) and reduced the average skyrmion diameter (by 50%). Furthermore, we compared the observed results of ion irradiation with those from the annealing process (a well-studied method for modifying magnetic properties) to better understand the effect of atomic displacements. Our study provides a route to achieve a highly-dense skyrmion state, and it can be explored further to suppress the skyrmion Hall effect for skyrmion-based applications. [ABSTRACT FROM AUTHOR]

Magnetization reversal of antiferromagnetically coupled (AFC) soft and hard (Co/Pd) multilayers was studied as a function of temperature. While the hard [Co(0.3?nm)/Pd(0.8?nm)]×10 was kept unchanged, the softness of the [Co(t)/Pd(0.8?nm)]×3 was controlled by varying the thickness t of the Co sublayer. Clear two-step hysteresis loops were observed for all the investigated multilayers with t ranging between 0.4 and 1?nm. The spin reorientation of the soft layer magnetization from in-plane direction to out-of-plane direction was investigated from 50 to 300?K. The antiferromagnetic field HAFC measured from the shift of the minor hysteresis loop reveals a good agreement to the quantum-well model. From the out-of-plane hysteresis loop of the uncoupled soft layer, its magnetization shows an in-plane orientation for t?≥?0.6?nm. The strong HAFC helps to induce an out-of plane orientation of the soft layer with a linear decrease of its coercivity with temperature. These investigated structures show the possibility to reduce the unwanted stray field and improving the out-of-plane anisotropy even for relatively thicker soft layer. [ABSTRACT FROM AUTHOR]

First order reversal curves (FORC) method has been reported to be an efficient tool to study interaction between grains and layers of magnetic materials. Although a few studies have been carried out on perpendicular recording media in the past, a study on the effect of systematic variation of exchange interaction in granular perpendicular magnetic recording media on FORC contours has not been carried out in detail. Such a study will help to understand the use of FORC better. In this paper, we have made a systematic set of samples in order to study the variation in exchange coupling and its effect on FORC contours. The pressure during the deposition of the second ruthenium layer and the magnetic layer was varied to alter the separation between the grains and hence the exchange interaction between the grains in the CoCrPt-oxide recording layer. In addition, the thickness of Co-alloy cap layer was used as an additional tool to control the exchange interaction between the magnetic grains. The results indicated that the interaction field obtained from the FORC does not vary in a significant manner when the changes in exchange interaction are small. In comparison, the peak intensity of the FORC shows a clear trend as the exchange coupling is varied, making it a more suitable parameter to study the exchange and magnetostatic interactions in systems such as magnetic recording media. [ABSTRACT FROM AUTHOR]

In magnetic recording media, the grains or clusters reverse their magnetization over a range of reversal field, resulting in a switching field distribution. In order to achieve high areal densities, it is desirable to understand and minimize such a distribution. Clusters of grains which contain stacking faults (SF) or fcc phase have lower anisotropy, an order lower than those without them. It is believed that such low anisotropy regions reverse their magnetization at a much lower reversal field than the rest of the material with a larger anisotropy. Such clusters/grains cause recording performance deterioration, such as adjacent track erasure and dc noise. Therefore, the observation of clusters that reverse at very low reversal fields (nucleation sites, NS) could give information on the noise and the adjacent track erasure. Potentially, the observed clusters could also provide information on the SF. In this paper, we study the reversal of nucleation sites in granular perpendicular media based on a magnetic force microscope (MFM) methodology and validate the observations with high resolution cross-section transmission electron microscopy (HRTEM) measurements. Samples, wherein a high anisotropy CoPt layer was introduced to control the NS or SF in a systematic way, were evaluated by MFM, TEM, and magnetometry. The magnetic properties indicated that the thickness of the CoPt layer results in an increase of nucleation sites. TEM measurements indicated a correlation between the thickness of CoPt layer and the stacking fault density. A clear correlation was also observed between the MFM results, TEM observations, and the coercivity and nucleation field of the samples, validating the effectiveness of the proposed method in evaluating the nucleation sites which potentially arise from stacking faults. [ABSTRACT FROM AUTHOR]

CoPt films have been grown in the past with a high anisotropy in L11 or L10 phase, and a high coercivity is observed only in L10 CoPt films. Recently, we have grown CoPt films which exhibited a high coercivity without exhibiting an ordered phase. In this study, high resolution transmission electron microscopy (HRTEM) investigations have been carried out to understand the strong thickness and deposition pressure dependent magnetic properties. HRTEM studies revealed the formation of an initial growth layer in a metastable hexagonal (hcp) CoPt with high anisotropy. This phase is believed to be aided by the heteroepitaxial growth on Ru as well as the formation of Ru-doped CoPt phase. As the films grew thicker, transformation from hcp phase to an energetically favourable face-centered cubic (fcc) phase was observed. Stacking faults were found predominantly at the hcp-fcc transformation region of the CoPt film. The higher coercivity of thinner CoPt film is attributed to relatively less fcc fraction, less stacking faults, and to the isolated grain structure of these films compared to the thicker films. [ABSTRACT FROM AUTHOR]

Spin--orbit torque (SOT) is a promising approach to manipulate the magnetization for high-performance spintronic applications. In conventional SOT heterostructures with heavy metal (HM)/ferromagnet layers, the SOT efficiency is determined by the charge-to-spin conversion, characterized by the spin Hall angle θSH of the HM layer. Researchers have investigated various HMs with different θSH to enhance the SOT efficiency while it is still limited because of the HM's intrinsic properties. In this study, we employ a rare-earth holmium (Ho) layer on top of a ferromagnetic Co layer (Pt/Co/Ho) to enhance the SOT efficiency. An increased damping-like SOT efficiency up to 200% is achieved at an optimized thickness of 2-nm Ho, corresponding to a lower switching current density, which is 60% less compared to the sample without a Ho layer. The damping-like torque efficiency per current density is estimated at around 0.256 for Pt/Co/Ho heterostructures. Our results, herein, demonstrate that inserting a rare-earth metal affords an additional spin current and/or improves the spin transparency to enhance the SOT efficiency, providing a route for energy-efficient spintronic devices. [ABSTRACT FROM AUTHOR]

We present the micromagnetic study of magnonic band structures for exchange spin waves propagating in one-dimensional magnonic crystals. The crystals are of laterally patterned periodic arrays of alternating cobalt and nickel stripes. Large magnonic bandgaps with widths of tens of GHz are observed. It is found that the higher-order transmission bands and bandgaps have wider widths than those of the lower-order bands and bandgaps. Another interesting feature is that the widths of the first two bandgaps are independent of the applied field, in contrast with an earlier report of decreasing bandgap widths with increasing applied field observed for dipolar spin waves. [ABSTRACT FROM AUTHOR]

Co/Pd multilayers, including exchange coupled structures with thin layers of Co with different exchange coupling strengths, were investigated using variable angle magnetometry. Hysteresis loops of perpendicular and in-plane components of magnetization were measured using two-axis vibrating sample magnetometer and variable angle magneto-optical Kerr effect system. Co/Pd multilayer structures have high perpendicular remanence and the magnetization reversal mechanism is mainly domain wall motion. For the systems with Co and Co/Pd multilayers, the magnetization reversals of high coercivity (Co/Pd) multilayer and low coercivity Co single layer were found to be independent, if the soft layer (Co) is thicker than 2 nm. In the samples with a high degree of exchange coupling, the magnetization of the multilayer has two components, normal and in-plane. The proportion between them varies with field strength and orientation of the sample. Taking this into account, it was possible to determine the anisotropy energy of the (Co/Pd) multilayer, which is in the range of 400-420 kJ/m3. [ABSTRACT FROM AUTHOR]

Magnetization reorientation from in-plane to perpendicular direction, observed in Co thin film coupled antiferromagnetically to high perpendicular magnetic anisotropy (Co/Pd) multilayers, is studied systematically for Co thickness ranging from 0 to 2.4 nm. The sample with 0.75 nm thick Co showed an exchange coupling field (Hex) exceeding 15 kOe at room temperature and 17.2 kOe at 5 K. With an increase of Co thickness, Hex decreased as expected and beyond certain thickness, magnetization reorientation was not observed. Indeed, three regions were observed in the thickness dependence of magnetization of the thin layer; one in which the thin layer (in the thickness range up to 0.8 nm) had a perpendicular magnetic anisotropy due to interface effects and antiferromagnetic coupling, another in which the thin layer (0.9-1.2 nm) magnetization had no interface or crystallographic anisotropy but was reoriented in the perpendicular direction due to antiferromagnetic coupling, and the third (above 1.2 nm) in which the magnetization was in-plane. In addition, Hall effect measurements were carried out to observe the anomalous and planar Hall voltages and to quantify the perpendicular and in-plane components of magnetization. The sample with thicker Co layer (2.4 nm) showed an in-plane component of magnetization, whereas the sample with 0.75 nm Co showed no in-plane component. The high value of Hex observed in 0.75 nm Co samples can have important implications in spintronics and bit patterned media. [ABSTRACT FROM AUTHOR]

The magnetic and structural properties of graded media fabricated by ion implantation of nitrogen (14 N+), oxygen (16O+), and cobalt (59Co+) ions in the CoCrPt-SiO2 recording layer of prototype disk have been studied. Ion implantation of the species was controlled at the atomic scale to fabricate the graded media. Magnetometric measurements indicated that the coercivity was reduced with an increasing dose of the implanted species. The observation of an increase in magnetic domain size has been attributed to the reduction in magnetocrystalline anisotropy energy, which is desirable for achieving graded media. The study indicates that the magnetic properties can be tailored by the appropriate selection of the implantation dose and species. [ABSTRACT FROM AUTHOR]

Magnetization reversal in (Co/Pd) multilayers with perpendicular anisotropy for different numbers of bilayers (N) is investigated experimentally and by theoretical modeling. The focus of this study is on the magnetostatic energy in these structures and its effect on the magnetization reversal behavior and the nature of domain formation. For (Co/Pd) multilayers with small N, sharp magnetization switching and large domains were observed. In contrast, (Co/Pd) multilayers with a large N have long tail in the hysteresis loop that gets more pronounced as N increases. The size of domains becomes considerably smaller as N increases. Based on theoretical modeling that takes into account of the magnetostatic energy from the different magnetic layers, the domain size dependence on the number of bilayers is explained. For large N, the tail in the hysteresis loop is revealed to be the result of an increase in the magnetostatic energy, which at the same time leads to a drastic reduction in domain width. [ABSTRACT FROM AUTHOR]

The crystallographic growth, magnetic properties, microstructure, and recording performance of CoCrPt–SiO2 perpendicular magnetic recording media on RuCoCr type of intermediate layers were systematically investigated. Excellent Co (00•2) crystallographic texture with dispersion Δθ50 as low as 3.2° could be obtained on the RuCoCr as the grain isolation layer, whereas the Δθ50 on the pure Ru grain isolation layer was 3.9°. Upon injecting oxygen into the RuCoCr layer via reactive sputtering and at high sputter pressures, grain center-to-center distance as small as 5.9 nm with a distribution of about 13% was achieved. The Δθ50 deteriorated on the RuCoCr-oxide type of intermediate layers, however, it remained comparable to that on the pure Ru layers. Signal-to-noise ratio also increased on the RuCoCr-oxide grain isolation layers due to the decreased grain sizes. The effectiveness of the RuCoCr-oxide type of intermediate layers toward grain size reduction and in improving the performance of perpendicular magnetic recording media is discussed. [ABSTRACT FROM AUTHOR]

We have used the method of anomalous Hall effect (AHE) to characterize magnetic thin film structures consisting of both perpendicular (normal-to-plane) and longitudinal (in-plane) anisotropies. AHE enables the simultaneous extraction of the perpendicular and in-plane magnetizations. The method is used to obtain the hysteresis loops and study the switching behavior of [Co/Pd] multilayers coupled to different seed layers. Results show that Cu seed layer helps to reduce the switching field distribution (SFD) and increase the coercivity while Co underlayer with an in-plane anisotropy helps to reduce the coercivity and increase the SFD. Furthermore, the magnetization in the Co in-plane layer is found to be pointing out of plane possibly due to exchange coupling with the [Co/Pd] multilayers. [ABSTRACT FROM AUTHOR]

We studied the reversal properties of perpendicular anisotropy (Co/Pd) multilayers with different crystallographic textures. In case of continuous films, an increase in the coercivity and reduction in the switching field distribution (SFD) were observed as the growth is improved. From magnetic force microscopy, a stripe-type domain configuration was observed in films deposited at low gas pressure while a bubble-type domain was observed in high pressure deposited films. In patterned films, the SFD did not vary significantly for samples with different textures although a 2 kOe increase in the switching field was measured. In patterned structures, the controllability of SFD may not be related to the improvement of film crystallographic growth as was observed for unpatterned films. The results from this study indicate that local variation in the intrinsic film properties plays a major role in the SFD. [ABSTRACT FROM AUTHOR]

Current generation of cobalt-oxide-based perpendicular magnetic recording media uses single or dual ruthenium intermediate layers in order to grow crystallographically textured, and magnetically isolated granular media. In this work, the potential advantages of an antiferromagnetic iridium-manganese intermediate layer directly under the recording layer are highlighted. Owing to its close lattice matching with hexagonal cobalt, iridium-manganese which has the L12, or AuCu3-type crystal structure, can support the heteroepitaxial growth of the cobalt-based recording layer. In one of the media schemes described here, (111) textured iridium-manganese thin film was grown on 7.5 nm thick ruthenium layer. On the iridium-manganese as segregation layer, the Co-oxide-based magnetic recording layer showed perpendicular texture with Δθ50 below 4°, coercivity of over 4000 Oe alongside magnetic exchange decoupling, average grain sizes of 6 nm with distributions under 14%, and improved thermal stability. Measurements of the anisotropy constant did not show any significant change and even an IrMn capping layer was observed to improve the thermal stability. The possible mechanisms through which the IrMn layer could affect the thermal stability are hypothesized. The initial layers of the magnetic recording layer on IrMn segregation layers also showed exchange-decoupled and segregated grains, which is unlike that observed on Ru segregation layers. In a second media scheme, (111) textured iridium-manganese thin film was grown on a crystalline soft magnetic underlayer belonging on top of amorphous soft underlayers. In this scheme, partial pinning of the soft underlayer due to exchange-bias interaction with the IrMn layer was observed. This scheme offers the possibility to reduce the intermediate layer thickness, thus improve media writability, and with further optimization, could potentially facilitate the approach toward 1 Tbits/in.2. [ABSTRACT FROM AUTHOR]

Writability in bit-patterned media (BPM) is a critical issue for high areal densities. In this study, magnetization reversal for multilayer of (Co/Pd) nanodots was investigated using magnetic force microscopy. We observed an increase of more than 15 times in switching field (Hsw) in BPM over that of continuous films. An exchange coupled structure made of a thin Co layer with in-plane magnetization and high perpendicular anisotropy layer of (Co/Pd) multilayer to reduce the switching field is proposed. When the Co layer is thinner than 2 nm, its magnetization is aligned perpendicular to the film plane due to the exchange coupling with the 15 nm thick (Co/Pd) multilayer. The thin Co layer helps in reducing the Hsw of (Co/Pd) by almost 50% and also its distribution by 57% as measured from remanence magnetization. [ABSTRACT FROM AUTHOR]

Writability of magnetic recording media with high anisotropy constant (Ku) is one of the major challenges encountered in extremely high density recording. A possible avenue to improve writability in perpendicular magnetic recording is to reduce the spacing between the writing head and the soft magnetic underlayer (SUL) or head-to-keeper spacing (HKS). A hybrid layer structure for reducing the HKS between the SUL and the writing head is investigated. The proposed structure makes use of a crystalline SUL with in-plane anisotropy together with an amorphous SUL to form a hybrid SUL (H-SUL). The H-SUL design helps to achieve better surface smoothness, thinner HKS, and good c-axis dispersion for the recording layer while still maintaining small and exchange-decoupled grains. Micromagnetic simulation demonstrates that the H-SUL scheme helps in improving writing field effectiveness and increasing field gradient along down-track and cross-track directions. [ABSTRACT FROM AUTHOR]

The performance of CoCrPt–SiO2 perpendicular recording media, which incorporate synthetic nucleation layers for inducing grain size control, was evaluated with regard to the crystallographic, microstructural, magnetic, interfacial, and recording properties. The media structure consisted of a 14 nm thick CoCrPt–SiO2 recording layer sputtered on two stacked Ru intermediate layers. When a submonolayer thick synthetic nucleation layer was inserted between the two Ru intermediate layers, the grain size and grain size dispersion were significantly reduced in both the upper Ru layer and the recording layer. An average grain size of 5.8 nm with 20% dispersion was achieved for the recording layer. The crystallographic growth of the Co (00·2) perpendicular texture remained unaffected with the Δθ50 at ∼4.4°; the coercivity was ∼2800 Oe and the coercivity squareness was ∼0.6. The roughness of the interface between the synthetic nucleation layer and the upper Ru intermediate layer did not significantly change, and the surface roughness was also relatively unchanged at ∼2.5 Å. An improvement in the signal-to-noise ratio by 1.5 dB was observed due to the synthetic nucleation layer. The effectiveness of the synthetic nucleation layer approach toward achieving grain size control and improving the performance of the CoCrPt–SiO2 perpendicular recording media is demonstrated. [ABSTRACT FROM AUTHOR]

The crystallographic growth, interfacial roughness, and magnetic properties of CoCrPt–SiO2 perpendicular magnetic recording media prepared on various types of Ru intermediate growth layers were systematically investigated based on high angle and omega offset x-ray diffraction scans, rocking curve scans, synchrotron radiation based grazing incidence reflectivity scans, and magneto-optical Kerr hysteresis loops. For samples that make use of one Ru growth layer, voltage bias applied on the Ru layer was seen to have two observable effects: (1) the dispersion in the Ru(00·2) perpendicular texture increased, but that of the Co(00·2) remained unchanged, leading to identical layered growth and (2) the in-plane a-lattice parameter of the Ru decreased leading to enhanced heteroepitaxy with the Co. There was no significant change in the Ru–Co interfacial roughness with changing the bias on the Ru layer. The bias effect can be used to optimize the design of the Ru intermediate layers. A scheme that makes use of two Ru growth layers consisting of a bottom Ru layer prepared under zero bias, which is inserted below a second Ru layer prepared under biased conditions, is shown to lead to significant benefits such as improved texture without affecting the magnetic properties. This is due to the different functional roles ascribed to each of the Ru growth layers. [ABSTRACT FROM AUTHOR]

Perpendicular recording technology has recently been introduced in hard disk drives for computer and consumer electronics applications. Although conceptualized in the late 1970s, making a product with perpendicular recording that has competing performance, reliability, and price advantage over the prevalent longitudinal recording technology has taken about three decades. One reason for the late entry of perpendicular recording is that the longitudinal recording technology was quite successful in overcoming many of its problems and in staying competitive. Other reasons are the risks, problems, and investment needed in making a successful transition to perpendicular recording technology. Iwasaki and co-workers came up with many inventions in the late 1970s, such as single-pole head, CoCr alloy media with a perpendicular anisotropy, and recording media with soft magnetic underlayers [S. Iwasaki and K. Takemura, IEEE Trans. Magn. 11, 1173 (1975); S. Iwasaki and Y. Nakamura, ibid. 14, 436 (1978); S. Iwasaki, Y. Nakamura, and K. Ouchi, ibid. 15, 1456 (1979)]. Nevertheless, the research on perpendicular recording media has been intense only in the past five years or so. The main reason for the current interest comes from the need to find an alternative technology to get away from the superparamagnetic limit faced by the longitudinal recording. Out of the several recording media materials investigated in the past, oxide based CoCrPt media have been considered a blessing. The media developed with CoCrPt-oxide or CoCrPt–SiO2 have shown much smaller grain sizes, lower noise, and larger thermal stability than the perpendicular recording media of the past, which is one of the reasons for the success of perpendicular recording. Moreover, oxide-based perpendicular media have also overtaken the current longitudinal recording media in terms of better recording performance. Several issues that were faced with the soft underlayers have also been solved by the use of antiferromagnetically coupled soft underlayers and soft underlayers that are exchange coupled with an antiferromagnetic layer. Significant improvements have also been made in the head design. All these factors now make perpendicular recording more competitive. It is expected that the current materials could theoretically support areal densities of up to 500–600 Gbits/in.2. In this paper, the technologies associated with perpendicular recording media are reviewed. A brief background of magnetic recording and the challenges faced by longitudinal recording technology are presented first, followed by the discussions on perpendicular recording media. Detailed discussions on various layers in the perpendicular recording media and the recent advances in these layers have been made. Some of the future technologies that might help the industry beyond the conventional perpendicular recording technology are discussed at the end of the paper. [ABSTRACT FROM AUTHOR]

The role of substrate bias during the sputter deposition of various layers of double-layered CoCrPt-SiO2 perpendicular recording media has been investigated in order to understand the physical mechanisms behind the various effects observed. Perpendicular recording media with dual Ru intermediate layers were investigated using several magnetic and microstructural characterization techniques. It was observed that, in general, the application of a bias voltage during the deposition of the seedlayer (Ta) and the first intermediate layer (Ru) is helpful in reducing the c-axis dispersion of the recording layer. For the other layers, application of bias voltage leads to deterioration in the magnetic properties. It was also observed that the application of a bias voltage during the deposition of the first intermediate layer (especially Ru) may not enhance the preferred growth of Ru hexagonal-close-packed (00.2) planes parallel to the disk surface, as predicted before. However, the bias voltage on the Ru layer still reduces the c-axis dispersion of the magnetic layer. From the omega-offset x-ray diffraction investigations, it is estimated that the lattice parameter “a” of the Ru layer is reduced slightly with bias voltage, which could probably lead to a reduction in the lattice mismatch between the Ru layer and Co-alloy layer. Bias conditions also could lead to improved interface condition. Such an improvement in the lattice matching or interface conditions could probably be the cause of the reduction of c-axis dispersion of the recording layer. [ABSTRACT FROM AUTHOR]

We present a systematic study of the structural, magnetic, and magnetotransport properties of Co-doped Fe3O4 films deposited on MgO (100) substrates by cosputtering technique. Transmission electron microscopy images suggest that the undoped and Co-doped Fe3O4 films are polycrystalline in nature and consist of a well defined grain boundary network. The temperature dependence of resistance also shows that the transport mechanism in our films is dominated by electron tunneling across antiferromagnetically coupled grain boundaries. We observed that the magnetic properties of the doped films are markedly sensitive to the Co doping concentration, with the magnetization curves showing drastic changes in coercivity with increasing doping concentration. In-plane magnetoresistance curves show linear magnetic field dependence for the undoped Fe3O4 films while a reduction in magnetoresistance and a departure from linear field dependence are observed for the Co-doped films. [ABSTRACT FROM AUTHOR]

The effect of thermal energy (k[sub B]T), which has been found to play some important roles in the magnetic properties of recently developed antiferromagnetically coupled media, is described. It was observed that the thermal energy helps to obtain an antiparallel configuration of moments at remanence. Therefore, a reduction in the remnant moment–thickness product (M[sub r]δ) is observed, even for smaller values of J (interface coupling constant) than those used in simulations that do not consider thermal energy. The magnetic viscosity measurement helps to distinguish the magnetization decay behavior of the top and bottom layers. The magnetic moments of top and bottom layers show maximum decay at different fields and the decay rates approximately scale with their thickness. Viscosity results also point out that the magnetization reversal of the bottom layer should occur in the first quadrant, in order to obtain a low noise and thermally stable media. Micromagnetic simulation was performed by including thermal effects. In that case, M[sub r]δ reduction could be obtained for smaller values of J than in the case where thermal energy is not included in the simulation. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Domain wall (DW) memory devices such as racetrack memory offer an alternative to the hard disk drive in achieving high capacity storage. In DW memory, the control of domain wall positions and their motion using spin-transfer torque is an important challenge. In this Letter, we demonstrate controlled domain wall motion using spin-transfer torque in staggered wires. The devices, fabricated using electron-beam and laser lithography, were tested using magneto-optical Kerr microscopy and electrical transport measurements. The depinning current is found to depend on the device dimensions of the staggering wires. Thus, the proposed staggering configuration can be utilized to fine-tune the properties of DW devices for memory applications. [ABSTRACT FROM AUTHOR]

In this paper, magnetic and structural properties of near-equiatomic CoPt thin films, which exhibited a high coercivity in the film-normal direction-suitable for perpendicular magnetic recording media applications-are reported. The films exhibited a larger coercivity of about 6.5 kOe at 8 nm. The coercivity showed a monotonous decrease as the film thickness was increased. The transmission electron microscopy images indicated that the as fabricated CoPt film generally consists of a stack of magnetically hard hexagonal-close-packed phase, followed by stacking faults and face-centred-cubic phase. The thickness dependent magnetic properties are explained on the basis of exchange-coupled composite media. Epitaxial growth on Ru layers is a possible factor leading to the unusual observation of magnetically hard hcp-phase at high concentrations of Pt. [ABSTRACT FROM AUTHOR]

Spin reorientation in antiferromagnetically coupled (AFC) Co/Pd multilayers, wherein the thickness of the constituent Co layers was varied, was studied. AFC-Co/Pd multilayers were observed to have perpendicular magnetic anisotropy even for a Co sublayer thickness of 1 nm, much larger than what is usually observed in systems without antiferromagnetic coupling. When similar multilayer structures were prepared without antiferromagnetic coupling, this effect was not observed. The results indicate that the additional anisotropy energy contribution arising from the antiferromagnetic coupling, which is estimated to be around 6×106 ergs/cm3, induces the spin-reorientation. [ABSTRACT FROM AUTHOR]

Magnetostatic interactions in patterned media are responsible for increasing the switching field distribution and reducing the thermal stability. Micromagnetic simulations indicate that a reduced saturation magnetization (Ms) and increased anisotropy constant (Ku) lead to better thermal stability. However, writability will be a concern with this approach. Patterned media with an antiferromagnetically coupled (AFC) recording media structure is studied to obtain high thermal stability even with a larger Ms and larger Ku. With the AFC structure, the reduced remanence magnetization (Mr) leads to a reduced magnetostatic interaction, even though the Ms is larger. Magnetic force microscopy measurements indicate that the AFC media show a narrower switching field distribution. [ABSTRACT FROM AUTHOR]

CoRuCr-oxide based intermediate layers have been introduced as a replacement for pure Ru-based intermediate layers in perpendicular recording media. It was observed that the CoRuCr intermediate layers provide better C-axis orientation than the Ru intermediate layers. The introduction of oxygen during the deposition of CoRuCr increases the C-axis dispersion slightly but it is still comparable to that of Ru intermediate layer. Grain size and grain size distribution were reduced with CoRuCr-oxide intermediate layers. Magnetic properties were also optimized, indicating that the CoRuCr-oxide intermediate layers are good candidates for perpendicular recording media. [ABSTRACT FROM AUTHOR]

Thermally stable, highly textured CoCrPt–SiO2 perpendicular magnetic recording media with ∼6 nm size exchange decoupled grains and tight grain size distribution were prepared on synthetic nucleation layers. The media structure consisted of 14 nm thick CoCrPt–SiO2 recording layer on one Ru intermediate layer. For these films, insertion of a synthetic nucleation layer directly under the recording layer was seen to reduce the grain sizes from 6.7 to 6.2 nm and grain size distribution from 17% to 14%. Significant reduction in the intergranular exchange coupling interactions could be achieved while improving magnetic properties. The crystallographic texture of the recording layer remained strongly (00·2) oriented with the dispersion Δθ50 below 3.7°. Isothermal remanence magnetization and thermal stability behavior were studied. A model that describes the role of the synthetic nucleation layer in affecting the grain growth and magnetic properties of the recording layer is presented to understand these observations. [ABSTRACT FROM AUTHOR]

A novel hybrid magnetic overcoat (Hy-MOC), consisting of a bilayer, suitable for ultrahigh density recording, has been proposed and investigated. The top layer is a thin layer of carbon (about 1 nm) employed mainly for the purpose of lubrication affinity. The bottom layer is a thin CoCrPt alloy layer with silicon oxide/nitride grain boundary (about 4 nm), which helps as an overcoat to prevent corrosion and also in reducing the magnetic spacing. Media prepared with such an overcoat showed a better recording performace and thermal stability than the media with a-C overcoat. The microstructure of the media with carbon overcoats and the magnetic overcoats obtained by transmission electron microscopy were comparable. Improved corrosion protection properties were also observed. These results indicate that Hy-MOC provides a possible solution for ultrathin overcoat system necessary for ultrahigh density media. [ABSTRACT FROM AUTHOR]

The transfer of spin angular momentum from a polarized current to a ferromagnet can induce reversal of magnetization and generation of spin waves. In this article, we present micromagnetic simulations of magnetization dynamics in spin valve structure under effect of spin-polarized current and external magnetic field. Unlike the previous studies, the magnetizations of pinned layer, and the free layer in this study are orthogonal. Such a configuration, which is closer to that of the spin-valve sensors, shows many interesting results such as the observation of steady and oscillatory states. [ABSTRACT FROM AUTHOR]

Spintronic devices such as magnetic random access memory and domain wall (DW) memory are attracting significant attention. Spin-field effect transistor devices have been proposed and researched for logic applications. In domain wall memory, the information is stored in magnetic domain states, which can be moved with a current above a certain threshold value. So far, the domain wall motion is only determined by the current density for most of the DW devices. Here, we demonstrate experimentally that a significant change in domain wall mobility can be achieved by applying a gate voltage. By applying a positive gate voltage, we show that the threshold current density for DW motion can be reduced by more than 10%. By choosing a suitable operating current, the domain wall motion can be switched on or off by the use of a gate voltage. These results are promising for designing high performance domain wall based transistor devices with faster operation speed and lower power consumption. [ABSTRACT FROM AUTHOR]

The effects of ion implantations through a mask on the structural and magnetic properties of Co80Pt20 films were investigated. The mask was patterned using the self-assembly of diblock copolymers. For implantation, high (40 keV for 14N+ and 100 keV for 40Ar+) and low (7.5 keV for 14N+ and 4.5 keV for 40Ar+) energy 14N+ and 40Ar+ ions were used to modify the structural and magnetic properties of these films. X-ray diffraction and TRIM simulations were performed for understanding the structural changes due to ion implantations. These results revealed the intermixing of Co atoms in lower layers and lattice expansion in Co80Pt20 magnetic and Ru layers. A lateral straggling of Co caused an increase in the exchange coupling in the masked region. Depletion of Co atoms in Co80Pt20 layer caused a decrease in the anisotropy constant, which were further confirmed by the alternating gradient force magnetometer and magnetic force microscopy results. The magnetic force microscopy images showed an increase in domain width and domain wall width confirming the above-mentioned effects. [ABSTRACT FROM AUTHOR]

In this paper, magnetic and magnetization dynamic properties of compositionally patterned Co46Fe40Ta9Zr5 thin films are investigated. A combination of self-assembly and ion-implantation was employed to locally alter the composition of Co46Fe40Ta9Zr5 thin film in a periodic manner. 20 keV O+ and 60 keV N+ ions were implanted at different doses in order to modify the magnetization dynamic properties of the samples in a controlled fashion. Magnetic hysteresis loop measurements revealed significant changes in the coercivity for higher influences of 5 × 1016 ions per cm2. In particular, N+ implantation was observed to induce two phase formation with high and low coercivities. Broadband strip-line ferromagnetic resonance spectroscopy over wide range of frequency (8 – 20 GHz) was used to study the magnetization dynamics as a function of ion-beam dosage. With higher fluences, damping constant showed a continuous increase from 0.0103 to 0.0430. Such control of magnetic properties at nano-scale using this method is believed to be useful for spintronics and microwave device applications. [ABSTRACT FROM AUTHOR]

We present a simple fabrication of FePt and Fe nanocomposite by annealing self-assembled FePt nanoparticles. 5 nm FePt nanoparticles were prepared by simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl in the presence of oleic acid and oleylamine as stabilizers. The as-synthesized FePt nanoparticles can self-organize well on a clean SiO/Si substrate and, then, were annealed in a vacuum using a rapid thermal process at different temperatures for 0.5 h. It is found that the annealing temperature plays a key role in determining the final products. The samples annealed at 550 °C and 700 °C consist of a FePt-face-centered-tetragonal (fct) phase with a coercivity of around 1000 and 8800 Oe, respectively, whereas, the samples annealed at 580 °C are composed of two phases: one is the FePt-fct phase and the other is a Fe-body-centered-cubic phase. The hysteresis loop of the sample annealed at 580 °C is also two-phase like with a kink at low field and its coercivity is around 8000 Oe. That indicates that Fe can be segregated from FePt by proper annealing, forming FePt and Fe nanocomposites. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

[variant_greek_epsilon]-cobalt nanoparticles with different shapes and sizes, e.g., rod-shaped Co (2×25 nm) and ball-shaped Co (diameter 4–9 nm), were synthesized by the rapid pyrolysis of octacarbonyldicobalt in o-dichlorobenzene solution in the presence of the surfactant, oleic acid. The size, size distribution, and the shape of the nanoparticles were influenced by changing the preparation conditions, such as the molar ratio of the surfactant (oleic acid) to reactant (cobalt carbonyl). The effects of the preparation conditions and annealing temperature on the shape and magnetic properties of cobalt nanoparticles have been investigated. It was found that as-synthesized rod-shaped cobalt nanoparticles (2×25 nm) show paramagnetic properties, but as-synthesized ball-shaped cobalt nanoparticles exhibit superparamagnetic behavior. After postannealing at 773 K for 30 min, the rod-shaped Co nanoparticles formed hcp structure and show ferromagnetic properties. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this study, we propose a new "atomic wall" approach to underlayer design. In this design, a CrX underlayer is used where X and Cr are not miscible in the bulk phase, resulting in a reduced grain size. We have studied the addition of Zr to Cr layers and found that CrZr gives magnetic properties comparable to those of CrMo underlayers. In addition, a CrZr underlayer gives rise to a smaller grain size of 5.5 nm, in comparison to a grain size of 6.2 nm in the case of CrMo underlayers. To our knowledge, this is the smallest grain size reported for Co-alloy recording media. The recording measurements also show a higher signal to noise ratio for CrZr underlayers than for CrMo underlayers, suggesting that CrZr underlayers have potential for use in ultrahigh areal densities. [ABSTRACT FROM AUTHOR]

In this report, we will present our experimental and micromagnetic simulation studies to show that laminated antiferromagnetically coupled media has potentials to push the areal density beyond 100 Gbit/in². Exchange coupling constant as high as 0.32 erg/cm² has been obtained for basic two layer configuration by inserting an ultrathin Co interface layer between CoCrPtB and Ru layers, which is 3-4 times higher than the reported values in literature. This technique improved the thermal stability factor by more than 40%. A type of sandwich three-layer configuration for the antiferromagnetic coupled media is proposed which may double the antiferromagnetic coupling effect for the recording CoCrPtB layer. It also can further reduce the M[sub r]t value (less than 0.2 memu/cm²) while keeping high thermal stability that may be preferred for future application once extreme high sensitivity read head appears. [ABSTRACT FROM AUTHOR]

This paper describes numerical magnetohydrodynamic (MHD) calculation of heat plasma used in heating and welding processes. Most of the calculation models have been two dimensional and static. Since physical phenomena are three dimensional and nonstatic, three dimensional, dynamic MHD calculation of heat plasma is developed in this paper. Since local heat balance is assumed, the heat plasma is considered as a conductive fluid dynamic material such as molten metal. However, heat plasma's material constants such as density, heat transfer coefficient, specific heat, and electrical conductivity are strong functions of the temperature. Temperature is governed by heat plasma material transfer and heat transfer. Material transfer is driven by the electromagnetic force. Turbulent flow is modeled by the large eddy simulator including heat transfer. The electrical phenomenon is modeled using the Poisson equation in a dc magnetic field. It is assumed that the static electrical force may be ignored. Calculations show that the heat plasma concentrates at the center. The center area is of low density and high electrical conductivity. To verify the usefulness of this 3D dynamic MHD method, a conventional model, which is 2D and static is compared to a k - ε model. Comparison of velocity distributions shows good agreement. [ABSTRACT FROM AUTHOR]

Satisfying the mutually conflicting requirements of easy switchability and high thermal stability still remains a hindrance to achieving ultra-high areal densities in hard disk drives. Exchange coupled composite media used with proper exchange control layers (ECLs) presents a potential solution to circumvent this hindrance. In this work, we have studied the role of MgO and Ta ECLs of different thicknesses in reducing the switching field of FePt media. MgO ECL was found to be more effective than a Ta ECL. For a 2 nm MgO ECL, the switching field could be reduced by 41% and at the cost of only a limited loss in thermal stability. Furthermore, a very high switching efficiency of 1.9 was obtained using 2 nm MgO ECL. So, with a proper choice of ECL material and thickness, the switching field of FePt media can be substantially reduced while ensuring high thermal stability and a better signal-to-noise ratio, thus potentially paving the way for very high areal density media. [ABSTRACT FROM AUTHOR]

Examines the magnetic and conversion electron Mössbauer spectroscopy in iron/tantalum multilayers. Analysis of the spectra; Ways in which the fitting for all spectra were carried out; Factor which indicates the presence at each interface of a dead iron layer.

Studies the change in the temperature coefficient of resistivity with manganese (Mn) addition in iron- and cobalt-rich amorphous alloys. Difficulty of determining the temperature dependence of electrical resistivity in amorphous ferromagnetic materials; Effect of the substitution of iron in the presence of Mn on the temperature coefficients of resistivity; Functional form of positive temperature coefficients of resistivity.

Achieving dense patterns with good resolution is a key step for several applications in micro- and nanoelectronics. Based on the mechanical strength and capillary forces between nanometer scale features, the authors have proposed that the use of thin resist is a solution to achieve dense array of patterns. Therefore, the authors have studied the effect of resist thickness on the resolution of dense patterns for both lines and dots. Based on the experimental results using hydrogen silsesquioxane resist, dense patterns with sub-10-nm half-pitch were achieved. The authors also propose a new method for calculating contrast for nanostructures. [ABSTRACT FROM AUTHOR]

A master mold for nanoimprint lithography was fabricated for discrete track recording (DTR) media using electron beam lithography and conventional etching techniques. The DTR pattern, containing 167 tracks of 120 nm pitch (60 nm land and groove widths) and embedded servo information, was automatically generated using an in-house developed program and was optimized for faster electron beam writing on an x-y stage. A daughter mold was duplicated from the master mold by nanoimprinting, using UV-curable resist and an intermediate polymer stamp technique. Scanning electron microscope images showed that the daughter mold was accurately and completely reproduced from the master mold. [ABSTRACT FROM AUTHOR]