This chapter deals with diffusional phase transformations. We first define a phase and move on to discuss various ways of classifying phase transformations. We then discuss in detail the energetics (thermodynamics) and kinetics of diffusional phase transformations. Transformations discussed include: precipitation, atomic ordering, spinodal decomposition, massive and cellular transformations in the light of the sections on energetics and kinetics. A final section of the role of symmetry in developing microstructure concludes the chapter.

Heat-assisted magnetic recording (HAMR) requires a high thermal gradient in the recording media to suppress jitter noises. The FePt-(h-BN) granular film has been proposed as a promising medium due to the formation of columnar grains in the film structure. It is also expected to have a low in-plane (IP) thermal conductivity for enhanced thermal gradient. Accurate measurements of the thermal conductance of nanometer-scale thin films remain challenging, but multilayer structures can provide an important platform for studying the thermal transport in nanocomposite materials. Here, we report on the fabrication and thermal characterization of [h-BN/L 1(0) -FePt]x N multilayers, where h-BN nanosheets (2.5 nm thick) and L 1(0) -FePt layers (6.5 nm thick) alternate periodically. Differential 3 omega measurements reveal an ultralow effective thermal conductivity of 0.60 +/- 0.05 W center dot m(-1) K-1 across the multilayer films and a low thermal boundary conductance (TBC) of 67.9 +/- 6.6 MW center dot m(-2) K-1 for the [FePt/h-BN(2.5 nm)/FePt] interface at room temperature. The weak van der Waals bonding at the h-BN/FePt interface is considered the primary contributor to its high thermal resistance. This work can advance the understanding of thermal transport across the 2-D-material/metal contacts and provide valuable insights for the design of HAMR media with thermally isolated FePt grains.

Recently hexagonal-BN (h-BN)/L $1_{0}$ -FePt granular media has been proposed as an ideal candidate for next generation heat assisted magnetic recording (HAMR) media. The formation of crystalline h-BN at the FePt grain boundaries has resulted in significant enhancements to the sputtered media's microstructural and magnetic properties. While it has been shown that high temperature and RF substrate bias are necessary to achieve highly crystalline h-BN at the grain boundaries, limited discussion has been given to the broader impact of substrate bias on the chemical ordering of FePt within this system. To further evaluate the influence of RF substrate bias on sputtered FePt/h-BN granular media, we have prepared several samples using different RF bias powers, 3 W/-21 VDC to 8 W/-70 VDC, on Corning NXT glass substrates. It was observed that the L $1_{0}$ chemical ordering of the system decreased with increased substrate bias over the above range. This behavior was confirmed in the magnetization data which also reflected a reduction of the out-of-plane coercivity from 32 to 10 kOe for the -21 and -70 VDC substrate bias samples respectively. In addition, energy dispersive spectroscopy (EDX) was used to probe the composition of the deposited h-BN/FePt films. This revealed a reduction in the Fe at% observed for all samples deposited with increased substrate bias. Relevant structural, microstructural, magnetic, and compositional data are presented below.

Granular L1_0-FePt thin films with small columnar grains are essential for heat-assisted magnetic recording media. While hexagonal boron nitride(h-BN) has proven effective for promoting columnar FePt grains, we explored multilayer graphene as an alternative grain boundary material leveraging its structural similarity to h-BN. The FePt granular thin films with carbon-based grain boundary materials(GBMs) were deposited by cosputtering on Si/SiO2 substrates with substrate bias at 650C. The RF bias and high temperature facilitated formation of interlinked graphene nanoribbons wrapping around FePt grains, yielding 7.5 nm diameter, 8 nm height grains with an order parameter of 0.78 and a perpendicular coercivity of 40 kOe. However, the formation of graphene nanoribbons could not effectively promote columnar structures, likely due to co-existing amorphous carbon in grain boundaries. Optimizing deposition to improve graphene grain boundary quality is necessary to realize this 2D material's potential for achieving desirable microstructures for HAMR media.

Heat transfer in nanocomposite materials has attracted great interest for various applications. Multilayer structures provide an important platform to study interfacial thermal transport and to engineer materials with ultralow thermal conductivity. Here we report on the fabrication and thermal characterization of [h-BN/L1_0-FePt]xN multilayers, where hexagonal boron nitride (h-BN) nanosheets (2.5 nm thick) and L1_0-FePt layers (6.5 nm thick) alternate periodically. Differential three-omega(3ω) measurements reveal an ultralow effective thermal conductivity of 0.60 ± 0.05 W · m^-1K^-1 across the multilayer films, and a low thermal boundary conductance (TBC) of 67.9 ± 6.6 MW · m^-2K^-1 for the [FePt/h-BN(2.5nm)/FePt] interface at room temperature. We attribute the ultralow thermal conductivity to the weak van der Waals bonding at h-BN/FePt interfaces, which dominates the thermal resistance of the multilayer structure. These findings provide insights into the thermal transport in 2D-material/metal multilayer nanostructures and suggest the [h-BN/FePt] superlattice as a promising material for nanoscale thermal barrier coating. Furthermore, the obtained TBC lays the foundation for analyzing heat transfer in FePt-(h-BN) nanogranular films, a promising magnetic recording media which can potentially provide high thermal gradient for heat-assisted magnetic recording (HAMR). This work advances the understanding of thermal transport in 2D-material/metal nanocomposites and demonstrates interface engineering as an effective approach to achieve materials with ultralow thermal conductivity.

A near field transducer (NFT) is a key photonics component in heat assisted magnetic recording (HAMR) for the localized heating of the magnetic medium. In this work, we present a novel NFT design through capacitive coupling. In our design, tapered metal bars separated by thin dielectric materials with gap distance G are used to create the plasmonic resonance and focus the electromagnetic field. The design is motivated by the intention to improve thermal stability, which can be achieved through segmentation using thermally stable dielectric material between the plasmonic metal bars. Using COMSOL Multiphysics software, the performance of this capacitive-coupled NFT is systematically modeled. It is shown that the electromagnetic field could gradually be focused through the tapering towards the air bearing surface (ABS). In addition, the focusing effect could be enhanced with a smaller NFT peg size at the resonant wavelength. The material selection for the NFT tip material will be discussed to further address the thermal stability of the device. In conclusion, this capacitive-coupled NFT with dielectric separation gaps and tapering yields an enhanced |E| field intensity at the tip with the potential for an enhanced material thermal stability. Such a design can also exhibit applications in other energy delivery systems as well as plasmonic waveguides and sensors.

In this paper, we present an experimental study of L1 0 -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 ( V DC = -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 L1 0 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.

In this paper, we present an experimental study of L1 0 -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 ( V DC = -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 L1 0 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.

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.

Recently, it has been suggested that relatively long-lived ordered states can develop in reactions involving two or more order parameters. These states are termed pseudostable states and correspond to saddle points in the free energy surface. The Al (FCC) → Ll2 reaction involves the independent growth of three concentration wave amplitudes, corresponding to each of the <100>* k-space points. It is suggested that L10 pseudostable states could form during this reaction, under suitable thermodynamic and kinetic conditions. The range of structures possible by varying the <100>* amplitudes are given. A discussion of the microstructures that are likely to be observed if such a state were to be encountered is given. A fourth-order Landau expansion is developed, using these amplitudes as variables. The stabilities of all L10 states within this model are derived and tabulated. Under the right conditions, this expansion gives saddle points corresponding to L10 states.

Hexagonal boron nitride ( ${h}$ BN) has recently been investigated as a promising grain boundary material (GBM) for the nanogranular structure of L10-FePt-based heat-assisted magnetic recording (HAMR) media. In this study, we systematically investigated FePt- ${h}$ BN granular thin films (7.5 nm) with various ${h}$ BN concentrations to provide a fundamental understanding of the effect of introducing ${h}$ BN nanosheets in the medium film on its microstructure and magnetic properties. Following the optimization of the ${h}$ BN concentrations, we demonstrated a method to fabricate a thicker recording layer while maintaining the desired nanogranular microstructure. We successfully fabricated the 16 nm thick FePt- ${h}$ BN film with well-isolated columnar grains of diameter 6.4 nm, grain aspect ratio 2.5, and decent magnetic properties ( ${H} _{\mathbf {C\bot }}$ = 21.3 kOe and ${H} _{\mathbf {C//}}$ = 5 kOe).

In this paper, we present a novel near field transducer (NFT) design concept for heat assisted recording. In this design, metal bars separated by thin dielectric are used to form a resonance plasmonic grating with distributed feedback (DFB). The motivation for such structure design is the use of dielectric separation for enhancing the material stability at elevated temperature so that best materials can be used without compromising much needed plasmonic properties. COMSOL Multiphysics software is used to simulate the plasmonic excitation and propagate to provide a detailed performance analysis on novel NFT with a nanocomposite structure to create distributed optical feedback (DFB) for maintaining the plasmonic resonance. The NFT is composed of an array of Au rectangular gratings separated by a constant gap, with each grating component embedded in the dielectric medium. It is shown that the grating based NFT could have the same NFT efficiency compared with a solid piece of Au NFT of the same geometric dimensions.

In this article, we present the fabrication of FePt/FePt-BN/FePt-SiO x and conventional FePt/FePt-C for heat-assisted magnetic recording (HAMR) on Corning Lotus NXT glass substrates. For FePt/FePt-BN/FePt-SiO x media, a 2.5 nm FePt/FePt-35vol.%BN was fabricated on polycrystalline MgO underlayers at 700 °C to serve as the templated layer for the growth of 4 nm FePt-35vol.%SiO x at 550 °C. For the FePt/FePt-C media, 6.5 nm FePt/FePt-C media was fabricated at 550 °C with FePt and C co-sputtered on top of 0.5 nm FePt. Both media show well-isolated FePt grains, which are fully surrounded by the grain boundaries materials. Compared with FePt/FePt-C media, the FePt/FePt-BN/FePt-SiO x media show distinctively smaller grain-to-grain pitch distance or higher grain areal density. Moreover, the bimodal grain-size distribution that is often observed in FePt/FePt-C media has been suppressed in the FePt/FePt-BN/FePt-SiO x media. In addition, both media show good magnetic properties on Corning Lotus NXT glass substrate.

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.