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In this article, we present a novel micromagnetic model for L10 ordered FePt grains based on their atomic structure. In this model, each monolayer of Fe atoms is represented by a magnetic moment vector and the entire grain consists of a stack of magnetic moments coupled by magnetostatic and nearest-neighbor exchange fields. The grain model enables simulation of incoherent magnetization switching, especially near the Curie temperature. The model is applied to simulate the heat-assisted recording process in granular L10 FePt thin-film media. One of the unique findings is that grains at transition centers could have a domain wall trapped within, provided sufficient grain height. The existence of these grains, whose magnetic moments could be fractional at transition centers, resulting in significant reduction of medium noise. The noise reduction is effectively equivalent to an increase of the number of grains across the read track width. Enhancing and controlling this mechanism by material engineering could lead to significant increase of area density capability, specifically, increase of track density, without decreasing grain size.

Convolution neural network (CNN) is used as the data detection channel for two-dimensional magnetic recording (TDMR). We demonstrate that by only feeding interfered multireader signals as well as the target binary data at supervised learning, it is possible for CNN to automatically resolve the intertrack interference (ITI) in a noisy environment without any physical modeling. It is worth mentioning that throughout the entire training/learning process, CNN never receives ITI-free waveforms as target nor is explicitly “instructed” to resolve ITI. We illustrate that the nonlinear detection capability plays the most important role for CNN to effectively “learn” the right correlation between multiple read channels under heavy noise. Therefore, ITI can be completely eliminated as long as the noise exhibits similar correlation, while the white electronic noise completely disturbs this correlation and “decorrelates” the channels, hence posing most serious limitations toward ITI mitigation capability for CNN.

The application of a deep neural network (DNN) as the detection channel for hard disk drive (HDD) data recovery at high user bit density and the prominent magnetic transition jitter noise are investigated in this article. Directly trained with the un-equalized readback signals without any prior knowledge of the magnetic recording channel, the DNN can automatically learn the signal characteristics, in particular the correlations between the input signals and the impact from the noise. As a result, the DNN read channel not only adapts the inter-symbol interference (ISI) but also demonstrates strong resilience against the colored magnetic noise. Our simulation results also reveal that to fully harness the learning power of the DNN data detection channel, the neural network inputs must cover the ISI spread. In addition, the training data must be sufficiently representative so that the inductive bias learned by the DNN detection channel can be used as good prior knowledge for actual detection.

In this article, we present a systematic modeling investigation for some important physical insights in microwave-assisted magnetic recording (MAMR). Specifically, the characteristics of the effective write field in the presence of the ac field generated by the embedded spin torque oscillator (STO) is analyzed in terms of field strength and field-gradient gain for recording, as well as the enhancement of reverse erasure field. It is found that the rotating frequency of the field generating layer (FGL) of the STO plays a very important role. The effective write field enhancement for both recording and erasure due to the rotating ac field increase with increasing the ac field frequency with a rate close to 1 T per 28 GHz. Either the recording field gain or erasure field enhancement rolls off at high frequencies. The roll-off frequency for the recording field gain is significantly higher, approximately 20 GHz, than that for erasure field enhancement, leaving a broader frequency range for the ac field over which substantial gains for the recording field strength and gradients can be obtained while avoiding any erasure field enhancement. The understanding also provides guidelines for both head and media designing.

Spin transfer torque magnetoresistive random access memory (STT-MRAM) has shown a great promise to be a main stream novel nonvolatile memory technology. Reduction of state switching current still remains to be a continued research focus for scaling down the size of the addressing transistors. One of the switching characteristics attributing to the relatively high power consumption is the asymmetry of switching current threshold: switching from parallel (P) to antiparallel (AP) state requires more than twice as much current as switching from antiparallel to parallel state. In this paper, we present a design that can significant reduce the switching current threshold for switching from P to AP state to substantially reduce or eliminate the switching asymmetry. By incorporating an antiferromagnetic/ferromagnetic stack to provide a rotating spin transfer torque that matches with the ferromagnetic resonance frequency of the free layer, switching threshold can be significantly reduced.

Compared to conventional perpendicular magnetic recording, heat-assisted magnetic recording occurs at temperatures more than twice as high in degrees Kelvin. The ability of head field to align the medium magnetization over the short duration in recording a bit is limited by the ratio of magnetic potential energy to the thermal energy. In this letter, we present a systematic micromagnetic modeling study of the transition jitter resulting from thermal agitation during the recording process. As we continue to decrease grain size in order to increase recording areal density, the height of grains in the recording medium needs to increase accordingly to keep the number of magnetization-misaligned grains to a minimum.

Grain size and size distribution reduction in L10 FePt granular thin film is becoming critical to increase the areal storage density and realize the full potential of heat-assisted magnetic recording media. FePt and amorphous carbon or mixture with other segregants are often deposited at elevated temperature in order to promote the L10 ordering in FePt. Due to the materials system and high temperature involved in the fabrication process, the bimodal distribution of the grains is often observed with a significant amount of small grains with diameter

In the quest for novel, scalable and energy-efficient computing technologies, spin-based logic devices are being extensively explored due to their potential for nonvolatility, small cell area, and low operational power. Spin torque majority gate (STMG) is one of the most promising options for beyond CMOS nonvolatile logic circuits for normally-off computing. However, significant problems arose with cascade-ability, signal nonreciprocity, and complicated circuit configurations based on STMG. In this paper, a novel magnetic majority gate (MMG) logic has been proposed, utilizing both spin transfer torque and spin–orbit torque effects. A logic family including and/nand and or/nor functions can be achieved with an easy configuration and under a stable operation. Communication between logic units is realized by spin current injection through a nonferromagnetic metal wire to ensure its cascade-ability and nonreciprocity to design multiple logic-depth circuits. With all of these advantages, the proposed cascadable MMGs can be utilized to design logic functions such as the BUFFER/ NOT, XOR/ XNOR, and complicated logic gates, which pave the pathway for designing robust and comprehensive logic circuits using full spintronic devices.

FeCo (60%-70% Fe, 30%-40% Co at.%) alloys have been used as write transducer materials due to their large saturation magnetization at room temperature (μ0M = 2.45 T). In this research, we investigate the influence of oxidation on the magnetic moment of a Fe70Co30 material at elevated temperatures in the application of heat assisted magnetic recording. We find that the moment of Fe70Co30 decreases after oxidation for all temperatures explored and that Co does not take part in the oxidation process. The moment loss is mainly due to the formation of ferrimagnetic Fe3O4 or γ-Fe2O3 as well as the change of the Fe/Co ratio in the FeCo alloy. A 2 nm Cr underlayer has been employed to change the FeCo texture from (1 1 0) to (0 0 2). This also increases the grain size of the FeCo thin film. In the case of the (0 0 2) textured sample, voids form between the FeCo film and the oxide layer, which could be the direct result of Fe cations diffusion.

Transition curvature in heat-assisted magnetic recording can be eliminated by appropriately varying head field amplitude across recording track width. In this paper, we present a systematic micromagnetic study on the effects of non-vanishing field beyond track edges in practical head designs, focusing on the recording characteristics beyond track edges. It is found that a uniform field amplitude beyond track edges will create wing-like patterns beyond straightened transitions, forming erase bands at high linear densities. The width of the erase band decreases if the head field diminishes beyond track edges. However, for practical field reduction gradients beyond track edges, the width of the erase band still remains significant.

A hybrid model incorporating two methods, i.e., the Potts model Monte Carlo and the level set method, is developed to simulate the templated growth of a two-phase CoPt/SiO2 thin film. Previously it was demonstrated experimentally that the film grown on the prefabricated template followed the pattern of the template, forming a highly ordered microstructure. The simulation in this study investigates the physical mechanism by which such film growth behavior occurs and which parameters dictate the resultant microstructure. To correctly represent the physical process, the interfacial energies between different phases and the rate of each microevent to occur were established through the simulated film deposition on a flat substrate and subsequent comparison to the experimental observations. With the established interfacial energy and rate of microevents, the resultant film microstructure grown on the templated substrate is found to strongly depend on the geometries of domes fabricated on the template. The domin...

In current perpendicular magnetic recording, a high-recording signal-to-noise ratio (SNR) always accompanies wide track width, limiting the area density capability. To overcome this difficulty, we propose a novel media design with notched anisotropy distribution along the film depth for microwave-assisted magnetic recording (MAMR). The micromagnetic modeling presented in this letter shows that MAMR with the notched medium significantly improves recording SNR while maintaining narrow track width. The effective field analysis clearly demonstrates the advantage of combining MAMR with the notched medium. With the notched medium design, the viability of MAMR for substantial area density capability gain is further enhanced.

Magnetization switching by spin-orbit torque (SOT) via spin Hall effect represents as a competitive alternative to that by spin-transfer torque (STT) used for magnetoresistive random access memory (MRAM), as it doesn’t require high-density current to go through the tunnel junction. For perpendicular MRAM, however, SOT driven switching of the free layer requires an external in-plane field, which poses limitation for viability in practical applications. Here we demonstrate field-free magnetization switching of a perpendicular magnet by utilizing an Iridium (Ir) layer. The Ir layer not only provides SOTs via spin Hall effect, but also induce interlayer exchange coupling with an in-plane magnetic layer that eliminates the need for the external field. Such dual functions of the Ir layer allows future build-up of magnetoresistive stacks for memory and logic applications. Experimental observations show that the SOT driven field-free magnetization reversal is characterized as domain nucleation and expansion. Micromagnetic modeling is carried out to provide in-depth understanding of the perpendicular magnetization reversal process in the presence of an in-plane exchange coupling field.

In this experimental study, we present a dual-layer structure of FePt-boron nitride (BN)/FePt-SiOx granular media for the heat-assisted magnetic recording. The boron nitride (BN) was deposited together with FePt at 700 °C for the first 2.5 nm in the initial film growth. After the initial growth, SiOx was used to replace the BN to serve as the grain boundary material for the rest of film growth process. The transmission electron microscopy study on the fully grown FePt-BN/FePt-SiOx film shows well-isolated FePt grains that are fully encircled by the well-defined grain boundaries. The areal density of FePt grains is distinctively higher than that of the conventional granular FePt-C-based film at similar film thickness. The FePt-BN/FePt-SiOx bilayer granular media also demonstrate excellent magnetic properties, which are comparable to that of FePt-C-SiOx granular media in terms of the hysteresis characteristics.

By averaging multiple writes of opposite polarity waveforms captured synchronously from heat-assisted magnetic recording (HAMR) media, we distinguish random noise of the HAMR writing process from the spatially anti-correlated repeatable noise due to media microstructure variations. We show both analytically and experimentally how the normalized cross correlation coefficient, $R$ , of two opposite polarity waveforms quantifies the random and repeatable dc-noise fractions, showing where dc-noise reduction efforts should be concentrated. The value of $R$ also reflects the degree of media saturation. At optimal recording conditions for the head and media studied here, the total dc-noise comprises appreciable fractions of both random and repeatable noise. The experimental results are supplemented with micromagnetic modeling and analytic calculations.

In this work, heat assisted magnetic recording (HAMR) is simulated to assess the areal density advantages of various realistic thermal profiles. Specifically, finite element simulation and micromagnetic modeling are used to assess the areal density consequences of a straighter thermal profile with an associated wider track. This assessment is done by simulating the thermal profiles created by heads with gapped near field optical transducers (NFTs) and split magnetic poles. By varying the gap width of the NFT and the thickness of the medium interlayer, the above variation in transition straightness and associated written transition is achieved. At specific operating points, we find that a wider track with a straighter transition results in a slightly higher recording density (1.2Tb/in2 vs 0.9 Tb/in2). As expected, the former case finds its optimum at higher linear bit densities (1370 kFCI) and lower track densities (850 kTPI) than the latter case (870 kFCI and 1000 kTPI).In this work, heat assisted magnetic recording (HAMR) is simulated to assess the areal density advantages of various realistic thermal profiles. Specifically, finite element simulation and micromagnetic modeling are used to assess the areal density consequences of a straighter thermal profile with an associated wider track. This assessment is done by simulating the thermal profiles created by heads with gapped near field optical transducers (NFTs) and split magnetic poles. By varying the gap width of the NFT and the thickness of the medium interlayer, the above variation in transition straightness and associated written transition is achieved. At specific operating points, we find that a wider track with a straighter transition results in a slightly higher recording density (1.2Tb/in2 vs 0.9 Tb/in2). As expected, the former case finds its optimum at higher linear bit densities (1370 kFCI) and lower track densities (850 kTPI) than the latter case (870 kFCI and 1000 kTPI).

Collinear antiferromagnets (AFs) support two degenerate magnon excitations carrying opposite spin polarizations, by which magnons can function as electrons in various spin-related phenomena. In an insulating ferromagnet(F)/AF/F trilayer, we explore the magnon-mediated interlayer coupling by calculating the magnon thermal energy in the AF as a function of the orientations of the Fs. The effect manifests as an interlayer exchange interaction and a perpendicular magnetic anisotropy; they both depend on temperature and the AF thickness. In particular, the exchange interaction turns out to be antiferromagnetic at low temperatures and ferromagnetic at high temperatures, whose magnitude can be $10-100$ $\mu$eV for nanoscale separations, allowing experimental verification., Comment: Physical Review Letters 121, 207202 (2018)

A lattice-based Monte Carlo model is used to investigate the ordering conditions necessary to produce the ordered Co3Pt phase (mm2). In this phase, Co and Co50Pt50 atomic layers alternate along the c-axis of the parent hexagonal structure. It is found that perpendicular ordering where the Co and Co50Pt50 layers alternates, is strongly correlated with in-plane ordering of the Co50Pt50 layers. In these layers the Co atoms and Pt atoms form alternating rows in parallel. Such atomic ordering within the plane is only facilitated by a deformation of the hexagonal lattice such that the ratio between the atomic spacing transverse to the row and along the row, b/a, becomes significantly smaller and the structure becomes orthorhombic. At adequate lattice spacings and b/a ratio, both in-plane ordering and perpendicular ordering occur simultaneously. In addition, the Co50Pt50 layers are also staggered alternately, forming a structure with the Imm2 space group. The ordered Co3Pt with Imm2 symmetry is also found to be sensitive to changes in the Pt composition.

In an antiferromagnet (AF) with uniaxial anisotropy, spin-up and spin-down magnons coexist and form an intrinsic degree of freedom resembling electrons. When polarized by an adjacent ferromagnet (F), a magnonic pure spin current can be thermally generated in an AF. We explore thermal magnon transport in an insulating F/AF/F trilayer where propagating magnons inside the AF spacer can transfer angular momenta between the two Fs. We find that a sufficiently large temperature gradient can switch the downstream F via a magnonic spin-transfer torque if it is initially antiparallel with the upstream F. A reciprocal switching is achievable by reversing the temperature gradient. In typical materials, we estimate the threshold to be 1 K/nm at room temperature, which can be reduced by enhancing the interfacial exchange coupling and by increasing the temperature.

In this paper, a compact quarter-mode substrate integrated waveguide bandpass filter with extremely wide stopband based on a novel integrated face-to-face complementary split ring resonators (CSRRs) is studied and developed. A combination of complementary split ring resonator and quarter-mode substrate integrated waveguide (QMSIW) is used in this development. Equivalent circuit model simulations from ADS Momentum are in good agreement with HFSS electromagnetic analysis for the QMSIW filter unit, demonstrating the feasibility of QMSIW resonators. The center frequency of the designed filter is 1.54GHz with return loss better than 19dB and insertion loss less than 0.6dB. Its first spurious appears at about 16.8GHz (about 10.9 times the central frequency) with high rejection level of 20dB, providing an ultra wide out-of-band rejection performance. The QMSIW filter was fabricated and the measured results agree very well with simulated ones.

In this paper, we present a micromagnetic analysis on two key noise mechanisms for current FePt-L10-based heat-assisted magnetic recording (HAMR) media: grain-to-grain variations of anisotropy field and Curie temperature. We found that the signal-to-noise ratio (SNR) versus heating power measurement could be used to distinguish the noise arising from these two different origins. We also found that in today's HAMR media, the dominate noise origin is likely to be the combination of Curie temperature distribution and heating, or temperature, variation from grain to grain. As one of the possible mechanisms for grain-to-grain heating variation, the effect of grain boundary thickness distribution is studied in terms of its impact to recording SNR. It is concluded that reducing the combined heating and Curie temperature distribution is critical for the realization of the area density capability gain promised by the technology.