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In a collinear antiferromagnet with easy-axis anisotropy, symmetry dictates that the spin wave modes must be doubly degenerate. Theses two modes, distinguished by their opposite polarization and available only in antiferromagnets, give rise to a novel degree of freedom to encode and process information. We show that the spin wave polarization can be manipulated by an electric field induced Dzyaloshinskii-Moriya interaction and magnetic anisotropy. We propose a prototype spin wave field-effect transistor which realizes a gate-tunable magnonic analog of the Faraday effect, and demonstrate its application in THz signal modulation. Our findings open up the exciting possibility of digital data processing utilizing antiferromagnetic spin waves and enable the direct projection of optical computing concepts onto the mesoscopic scale.

In this paper, we present an experimental study of L10-FePt granular films with crystalline boron nitride (BN) grain boundary materials for heat assisted magnetic recording (HAMR). It is found that application of a RF substrate bias (VDC = -15 V) yields the formation of hexagonal boron nitride (h-BN) nanosheets in grain boundaries, facilitating the columnar growth of FePt grains during sputtering at high temperatures. The h-BN monolayers conform to the side surfaces of columnar FePt grains, completely encircling individual FePt grains. The resulting core–shell FePt-(h-BN) nanostructures appear to be highly promising for HAMR application. The high thermal stability of h-BN grain boundaries allows the deposition temperature to be as high as 650℃ such that high order parameters of FePt L10 phase have been obtained. For the fabricated FePt-(h-BN) thin film, excellent granular microstructure with FePt grains of 6.5 nm in diameter and 11.5 nm in height has been achieved along with good magnetic hysteresis properties. [ABSTRACT FROM AUTHOR]

Fabricating highly ordered tall L10–FePt with a small grain pitch distance on a commercially available glass substrate is crucial to realize heat-assisted magnetic recording (HAMR) media for industrial manufacture. We have realized tall FePt grains surrounded by crystalized h–BN on Corning NXT™ glass deposited at elevated temperatures in the presence of radio frequency (RF) bias. In this paper, we discuss the effect of deposition temperature on the order parameter of L10–FePt–BN granular media. Well-isolated L10–FePt–BN granular media with a grain diameter of 6.5 nm and height of 11 nm is achieved. These films exhibit a high order parameter of 0.85 with a perpendicular coercivity of 35 kOe. [ABSTRACT FROM AUTHOR]

We studied the effects of medium microstructure on the switching field (SF) and its distribution (SFD). Specifically, quasi-single crystalline and polycrystalline Co/Pt based multilayers, chosen for its high perpendicular anisotropy and high exchange coupling, have been prepared via sputtering technique. The quasi-single crystalline was achieved by utilizing an Ag underlayer. Using electron-beam lithography, films were patterned into arrays of small islands, ranging from 1 [micro]m to 0.050 [micro]m (= 50 nm) in size. The hysteresis loop for the both types of film, as unpatterned, showed similar anisotropy field and coercivity whereas, as patterned, the quasi-single crystalline samples consistently exhibited a higher SF than that of the polycrystalline samples. However, the interpretation on the SFD was rather complex. Additionally, a critical size where remanent magnetization changes a multi domain to a single domain state appeared to be notably smaller for the polycrystalline samples than that for the quasi-single crystal samples, suggesting a different nucleation volume in the films. Index Terms--Co/Pt multilayers, medium microstructure, switching field (SF), switching field distribution (SFD).

In this paper, we present a combined transmission electron microscopy study and micromagnetic modeling investigation of current granular-continuous-composite (CGC) perpendicular thin-film media. Our transmission electron microscopy-EELS elemental analysis on CoCrPt-TiOx layers found evident Cr segregation to the grain boundaries as well as in the middle of a grain where Co and Pt appears to be deficient. Similar localized Co and Pt deficiency is also found in CoCrPt-SiOx media, however, Cr segregation is much less evident. STEM-HAADF analysis reveals that the apparent non-uniform composition is the result of a 'cavity-like' topography of the magnetic grains. It is argued that the 'cavity' grain topography significantly enhances the grain boundary surface area. Micromagnetic modeling investigation was conducted to study the impact of possible surface anisotropy to the corresponding grain switching field. A systematic numerical analysis is presented. Index Terms--Perpendicular recording media, surface anisotropy, switching field, thin-film media.

A systematic investigation has been performed to optimize the microstructure of L10-FePt–SiOx granular thin films as recording media for heat-assisted magnetic recording. The FePt–boron nitride (BN) nucleation layer, which is stable even at 700 °C, is used to control the grain sizes and microstructure during the high-temperature processing. The study finds that films of high-aspect-ratio FePt grains with well-formed silicon oxide (SiOx) grain boundaries require the grading of the deposition temperature during film growth as well as the grading of the silicon oxide concentration. Well-isolated columnar grains of L10-FePt with an average height greater than 11 nm and diameters less than 7 nm have been achieved. Transmission electron microscopy analysis of the microstructures of samples produced under a variety of non-optimal conditions is presented to show how the microstructure of the films depends on each of the sputtering parameters. [ABSTRACT FROM AUTHOR]

We present a micromagnetic modeling analysis on recording performance, in particular adjacent track erasure (ATE), in discrete track media. We have compared the ATE performance between the continuous and discrete track media, and we have modeled two types of perpendicular media: one with zero intergranular exchange coupling and the other with moderate intergranular exchange coupling. We found that in the medium with moderate intergranular exchange coupling, the ATE has significantly been suppressed; that is, the track pitch can be significantly smaller with the same recording head and recording condition for the same ATE level. This gain becomes less significant if the medium has zero intergranular exchange coupling. Index Terms--Adjacent track erasure, discrete track media, intergranular exchange coupling, perpendicular recording.

Conventional needle-shaped tape particles utilize shape anisotropy to enable magnetic storage. Developed with near-spherical shape and strong crystalline anisotropy, the nanosized composite advanced particles (NanoCAP) are one of the potential candidates for future tape data storage. However, because of the imperfect shape in fabrication, significant shape anisotropy still exists in NanoCAP, forming a second easy axis in addition to that of the crystalline anisotropy. In this paper, the effect of the dual easy axes is studied by micromagnetic modeling. The analysis shows that the parasitic shape anisotropy degrades media performance. Schemes for reducing the parasitic shape anisotropy are quantitatively evaluated. Index Terms--Longitudinal recording, magnetic nanoparticles, magnetic tape recording, particulate tape media.

Discrete track recording has emerged as a promising candidate for high storage capacity since it reduces adjacent track erasing (ATE) and alleviates narrow head requirements. In this paper, the writability of discrete lines was studied in discrete track media (DTM) fabricated by e-beam lithography and ion-milling on perpendicular magnetic recording (PMR) media. The writability of discrete lines with finite length and three kinds of line width (50, 100, and 150 nm) is compared with continuous media on the same track. When writing current is small (less than 12 mA), the narrowest discrete lines (50 nm) are not easily magnetized, most likely due to deformed magnetic layer created by a shaliow wall angle. Conversely, wider discrete lines ([greater than or equal to] 100 nm) are magnetized like continuous media. No difference in the level of magnetization was observed for the narrowest fines if writing current was large enough (more than 12 mA). This smooth magnetization of the narrowest line at sufficient writing current flow makes it certain that narrow discrete line can be used for high capacity storage. Index Terms--Discrete track media (DTM), electron beam lithography, perpendicular magnetic recording (PMR).

Microwave assisted magnetic recording (MAMR) has been proposed to provide sufficient writability in magnetic recording when the magnetic grain has a large crystalline anisotropy field in high recording density. In this paper, the relationship between the track width and recording head designs in this novel recording scheme is studied using micromagnetic modeling. By controlling the AC field generation layer width, high track density can be obtained without decreasing the main pole width in the writer. A quantitive result shows 770 KTPI can be realized in MAMR with a 120 nm wide main pole and 25 nm wide AC field generation layer. Index Terms--Perpendicular magnetic recording, microwave generation.

Low-resistance magnetic tunnel junctions utilizing perpendicular magnetic anisotropy of Co/Pt multilayer electrodes have been investigated. In these junctions, Al[O.sub.x] tunnel barrier has been prepared by repeated natural oxidation processes. Each natural oxidation process has been optimized by varying the thickness of pre-oxidized Al layer prior to oxidation. The perpendicular magnetic anisotropy of Co/Pt multilayer has been found to be dependent on the degree of oxidation at the interface of tunnel barrier and Co adjacent layer. It has been observed that magnetization of Co/Pt multilayer is tilted when Co layer adjacent to tunnel barrier is relatively thick, which is attributed to the lack of interfacial anisotropy contribution near the interface of Co adjacent layer and Al[O.sub.x] barrier. Interfacial oxidation treated on the monolayer thick Co adjacent layer has enhanced perpendicular magnetic anisotropy of the entire Co/Pt multilayer electrodes, enabling sharp transition between well-defined parallel and anti-parallel states at micro-fabricated perpendicular magnetic tunnel junctions of ~100 [ohm] x [micro][m.sup.2] resistance-area product. Index Terms--Magnetoresistance, magnetic tunnel junction, natural oxidation, perpendicular magnetic anisotropy.

The ability of magnetic annuli to produce various magnetization configurations make them attractive for magnetic memory applications. In this study, the authors present NiFe/CoFe/Cu/CoFe current-perpendicular-to-plane giant magnetoresistive rings with 600 nm outer diameter and 200 nm inner diameter. By directly injecting current, it is shown that single-step vortex-vortex magnetic switching occurs due to contributions from an Oersted field and spin transfer torque. Experimental evidence reveals the spin transfer torque can either assist or act against the Oersted field by as much as 30% depending on the chirality of the reference layer in the CPP-GMR ring. The total switching contributions of the spin torque and the Oersted field are quantified and the mechanisms contributing to the magnitude of each contribution are discussed. Index Terms--CPP-GMR, current induced magnetic switching, magnetic annuli, spin transfer torque, vortex switching.

In this paper, we present a novel mechanism for recording at a head field significantly below the medium coereivity in a perpendicular recording geometry. By applying a localized ac field at adequate frequency to the perpendicular recording medium, saturation recording can be achieved with recording field amplitudes significantly below the medium coercivity, or the medium perpendicular anisotropy field. A scheme utilizing spin torque to generate a localized ac field at high frequency (tens of gigahertz) with kilo-oersted field amplitude in the medium is proposed along with a systematic modeling analysis. Recording simulations at high linear densities are presented. Index Terms--Microwave, perpendicular recording, spin momentum transfer, spin torque.

We report on fabrication of discrete tracks on perpendicular magnetic recording (PMR) media with an e-beam lithographical process. We studied the recording performance of the e-beam media on a spinstand in parallel with conventional PMR media. Discrete track media show significant reduction in adjacent track erasure (ATE). We studied and quantitatively measured the source of the ATE improvement, and developed a triple track geometrical model to calculate achievable track density for both discrete track recording (DTR) and continuous media. From the model, we identify two factors of DTR that contribute to reaching a higher TPI. Using the same fabrication technique, we also studied servo burst design and its playback waveform quality. At 250 ktpi, we compare DTR servo bursts with servo bursts written with a conventional method. DTR servo bursts show better edge definition, which can translate to better position error signal sensitivity and support higher TPI in the future. Index Terms--Electron beam lithography, patterned media, perpendicular magnetic recording, servo.

In this paper, we present a systematic micromagnetic analysis on the magnetic properties, thermal magnetic stability, and recording performance of percolated perpendicular media (PPM), a novel film microstructure of exchange coupled small-size magnetic grains with densely distributed nonmagnetic pinning sites. The analysis shows that the PPM microstructure can substantially reduce medium noise over the conventional granular perpendicular media while maintaining sufficient magnetic stability. Index Terms--Granular perpendicular media, high density recording, percolated perpendicular media (PPM), superparamagnetism, thermal stability.

In this paper, a new type of perpendicular magnetic media which we have termed percolated perpendicular media is discussed. We present the method we used to fabricate the media as well as an energetic rationale for the driving force to produce the desired microstructure. The microstructures of samples with various amounts of oxide material are presented and the optimum one for our investigation is presented. We also present some preliminary drag test data that can be used to evaluate the recording properties of this media. Index Terms--Domain wall pinning, microstructure, percolated media, perpendicular magnetic media, recording properties.

Return field-induced partial erasure (RFPE) in trailing-edge shielded perpendicular writers has been studied, both by modeling and by experiments. For a given head-media combination, the return field underneath the trailing shield increases with increasing write current. Once exceeding a certain threshold, it will cause partial erasure of the bits that have just been written by the main pole. Recording performance, such as reverse overwrite, spectral signal-to-noise ratio, and bit-error rate are all found degraded at high write currents, due to RFPE. Design optimization of both head and media together is needed, in order to maximize the advantage of a trailing-edge shielded pole head and minimize the impact of RFPE. Index Terms--Partial erasure, perpendicular recording, return field, shielded pole head.

We studied the remanent field from perpendicular writer pole tips by micromagnetic modeling. The pole tip remanence has two causes: 1) the residual flux from the yoke due to undesirable material properties and/or yoke design and 2) the geometry of the pole tip. An optimal yoke design for low remanence includes a wide and short yoke with relatively shallow yoke flare angle around 30[degrees] that favors transverse major domains. Horizontal anisotropy in the yoke and the pole tip also helps suppress the vertical magnetization, mainly in the yoke, which yields less residual flux into the pole tip, hence lower remanence. The remanent field is also a strong function of the throat height, i.e., the pole tip length, due to the shape anisotropy effect. Micromagnetic modeling shows that multilayer lamination, both in the yoke and in the pole tip, is an effective way to achieve low remanence. It also eliminates the sensitivity of remanence to the throat height. To improve the robustness of the remanence against the stray field, an antiferromagnetic coupling (AFC) between the lamination layers may also be necessary. Our results apply to both single pole heads and shielded pole heads. Index Terms--Antiferromagnetic coupling (AFC), head lamination, micromagnetic modeling, perpendicular recording, pole tip remanence, remanent erasure, shielded pole head, single pole head.

To aid understanding the physical mechanisms responsible for the perpendicular write head remanent field and thereby improve the write head design, we studied the remanence of single-pole perpendicular recording heads with trailing shield by using a contact scanning recording tester. We used two different pole-tip size heads in the study. We found that the remanence areas of the heads tested usually locate at the trailing edge corners of the pole tip. We also found that the remanence areas of both types of heads are also the head field easy-rising areas when the heads are driven by low write currents. Additionally, the current level that leads to remanence is the same for both types of heads. However, the remanence of the small pole-tip heads can be demagnetized by a much lower amplitude reverse current than that of the large pole-tip heads. Index Terms--Head remanence, perpendicular recording, pole-tip size, shielded head.