Heat-assisted magnetic recording (HAMR) media status, requirements, and challenges to extend the areal density (AD) of magnetic hard disk drives beyond current records of around 1.4 Tb/in.2 are updated. The structural properties of granular high anisotropy chemically ordered L10 FePtX-Y HAMR media by now are similar to perpendicular CoCrPt-based magnetic recording media. Reasonable average grain diameter ⟨D⟩ = 8–10 nm and distributions σD/D ∼ 18% are possible despite elevated growth temperatures TG = 650–670 °C. A 2× reduction of ⟨D⟩ down to 4–5 nm and lowering σD/D < 10%–15% are ongoing efforts to increase AD to ∼4 Tb/in.2. X = Cu ∼ 10 at. % reduces the Curie temperature TC by ∼100 K below TC,bulk = 750 K, thereby lowering the write head heat energy requirement. Multiple FePtX-Y granular layers with Y = 30–35 vol. % grain-to-grain segregants like carbides, oxides, and/or nitrides are used to fully exchange decouple the grains and achieve cylindrical shape. FePt is typically grown on fcc MgO (100) seedlayers to form well oriented FePt (002). A FePt lattice parameter ratio c/a ∼0.96 and high chemical order S > 0.90 result in magnetic anisotropy KU ∼ 4.5 × 107 erg/cm3, and only 25% below the FePt single crystal value KU = 6.6 × 107 erg/cm3 has been achieved in 7–8 nm diameter grains. Switching field distributions depend on anisotropy field (HK) distributions, which are currently of the order of ΔHK/HK ∼ 10% (ΔHK ∼ 10–12 kOe, HK ∼ 10–11 T) at room temperature. High thermal conductivity heat sink layers, including Ag, Au, Cu, and Cr, are used to optimize the cooling rate and maximize the down- and cross-track thermal gradient, which determines the achievable track density.

A new method for the numerical modelling of the thermal fluctuations in micromagnetic systems is presented. The approach is based on the set of stochastic Langevin equations, which are derived from the energy expression for the system studied. The correlation matrix of the corresponding random forces required to perform numerical simulations is evaluated using the fluctuation-dissipation theorem following a transformation to the normal coordinates. The method is tested for the finite 1D chain of classical magnetic moments. The temperature dependence of the average magnetization deviation Delta M/M(O) exhibits good agreement with analytical theory.

The influence of the thermal agitation on the switching dynamics for a pair of identical uniaxially anisotropic dipoles is studied for the case of the applied field parallel to the bond direction and the common anisotropy axis. A set of Langevin equations was derived from the micromagnetic energy expression and solved numerically. The switching behavior resembles a random walk over the energy barrier arising from the anisotropy of the system. The relaxation time is computed as a function of temperature, applied field, and coupling strength. The temperature dependence of the maximum energy of the fluctuations provides a method of evaluating the energy barrier of reversal. The thermal agitation is shown to reduce the symmetry of the ‘‘fanning’’ reversal mode.

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Magnetic data storage is pervasive in the preservation of digital information and the rapid pace of computer development requires ever more capacity. Increasing the storage density for magnetic hard disk drives requires a reduced bit size, previously thought to be limited by the thermal stability of the constituent magnetic grains. The limiting storage density in magnetic recording is investigated treating the writing of bits as a thermodynamic process. A 'thermal writability' factor is introduced and it is shown that storage densities will be limited to 15 to 20 TBit/in 2 unless technology can move beyond the currently available write field magnitudes.PACS numbers: 85.70.Li,75.50.Ss,75.75+a As a technology, magnetic recording has been in existence since the invention of magnetic tape recording in the 1920s and 1930s. Since the early 1980's, and the introduction of metallic thin film recording media, the industry has seen a rapid increase in storage density; up to the TByte storage available in today's PC hard drives. Because technology has kept pace with demand, magnetic information storage is now ubiquitous. Having been around for some 60 years, magnetic recording is running into difficulties imposed by physical limitations.A previous study of the possible limits of recording density was made by Charap et al 1 . This study predicted an upper limit of 36 Gb/in 2 and, remarkably, current technology has already achieved densities over one order of magnitude beyond this value. The reason for this lies in advances in the 'non-magnetic' aspects of recording technology, including error detection and correction and the mechanical actuator systems used to position the read and write sensors, which were not anticipated by the authors of Ref. 1. The question is; does there exist a physical upper limit to recording density which cannot be exceeded by improved technology? Here we argue that the limitation is essentially determined by the maximum tolerable Bit Error Rate and certain materials parameters which, critically, includes the saturation magnetisation of the recording medium.Magnetic recording relies on the storage of information on media comprised of grains of a material with a high magnetocrystalline anisotropy. The grains can be considered as bistable systems capable of representing bits of information in terms of the polarity of the grains. Stability of the information is provided by an anisotropy energy barrier KV where K is the anisotropy constant and V the grain volume. It has long been realised that the phenomenon of 'superparamagnetism' (SPM) defines the upper limit thermal stability of magnetic materials 2 . In the case of magnetic recording information should be stable for at least 10 years, which leads to an established criterion of KV /kT > 60 for media design.Future advances in magnetic recording density will have to circumvent the magnetic recording trilemma 3 .The key component of the trilemma is the necessary reduction in grain size for signal to noise reasons. For thermal stability the anisotropy...