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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($3\omega$) measurements reveal an ultralow effective thermal conductivity of $ 0.60 \pm 0.05 W \cdot m^{-1}K^{-1}$ across the multilayer films, and a low thermal boundary conductance (TBC) of $ 67.9 \pm 6.6 MW \cdot m^{-2}K^{-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., Comment: 22 page, 5 figures

Granular $\text{L}1_0$-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 650{\deg}C. 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., Comment: 13 pages, 5 figures, first presented in MMM 2023 conference at Dallas

In this paper, we present an experimental study of $\text{L1}_0$-FePt granular films with crystalline/amorphous boron nitride (BN) grain boundary materials for heat assisted magnetic recording (HAMR). It is found that an adequate RF substrate bias 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 800oC such that high order parameters of FePt $\text{L1}_0$ phase have been obtained. For the fabricated FePt/h-BN thin film, excellent granular microstructure with FePt grains of 6.5nm in diameter and 11.5nm in height has been achieved along with good magnetic hysteresis properties., Comment: Manuscript: 16 pages, 6 figures attached after references. Supplementary information: page 23-27, 4 figures

A systematic investigation has been performed to optimize the microstructure of $\mathrm{L1_0-FePt-SiO_x}$ granular thin film as recording media for heat-assisted magnetic recording. The FePt-BN nucleation layer, which is stable even at $700^\circ \text{C}$, is used to control the grain sizes and microstructure during 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 $\mathrm{L1_0-FePt}$ with an average height greater than 11 nm and diameters less than 7 nm have been achieved. Transmission electron microscopy (TEM) 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., Comment: 17 pages, 7 figures, first published in joint MMM-intermag 2022

Grain-to-grain Curie temperature (Tc) variation in the media reduces signal-to-noise ratio due to its contribution in transition jitter noise, especially when average grain size is pushing down to increase the area storage capacity. A thermally insulating magnetic grain boundary may suppress such grain-to-grain Tc variation, especially at small grain size. Here we present an experimental study on the effect of adding thermally-insulating magnetic oxide, in particular BaFexOy, as part of the grain boundary materials in granular FePt-C HAMR media. It is found that the BaFexOy is chemically inert to FePt and the chemical ordering of FePt-BaFexOy-C media are similar to that of FePt-C meida. By tuning the volume fraction of BaFexOy and C, well-separated FePt grains (average grain size = 6.8 nm) surrounded by BaFexOy shell with perpendicular Hc above 35 kOe can be obtained. Transmission electron microscopy study with chemical analysis shows that the magnetic oxide appears to be crystalline and surround the FePt grains with immediate full enclosure. Magnetic measurements indicate an effective increase of magnetic grain size at temperatures below FePt Curie temperature. Pulsed laser pump-probe measurement indicates a measurable reduction of Curie temperature variation for the FePt-BaFexOy-C media with carefully comparison with the reference FePt-C media.

In this paper, we investigate CoFeCrAl alloy by means of various experimental techniques and ab-initio calculations to look for half-metallic nature. The alloy is found to exist in the cubic Heusler structure, with presence of B2 ordering. Saturation magnetization (MS) value of about 2 Bohr magneton/f.u. is observed at 8 K under ambient pressure, which is in good agreement with the Slater-Pauling rule. MS values are found to be independent of pressure, which is a prerequisite for half-metals. The ab-initio electronic structure calculations predict half-metallic nature for the alloy with a spin slitting energy of 0.31 eV. Importantly, this system shows a high current spin polarization value of 0.67 [with error of 0.02], as deduced from the point contact Andreev reflection (PCAR) measurements. Linear dependence of electrical resistivity with temperature indicates the possibility of reasonably high spin polarization at elevated temperatures (~150 K) as well. All these suggest that CoFeCrAl is a promising material for the spintronic devices., Comment: 11 pages, 5 figures, regular article

Spin gapless semiconductors (SGS) form a new class of magnetic semiconductors, which has a band gap for one spin sub band and zero band gap for the other, and thus are useful for tunable spin transport based applications. In this paper, we report the first experimental evidence for spin gapless semiconducting behavior in CoFeMnSi Heusler alloy. Such a behavior is also confirmed by first principles band structure calculations. The most stable configuration obtained by the theoretical calculation is verified by experiment. The alloy is found to crystallize in the cubic Heusler structure (LiMgPdSn type) with some amount of disorder and has a saturation magnetization of 3.7 Bohr's magneton/f.u.. and Curie temperature of 620 K. The saturation magnetization is found to follow the Slater-Pauling behavior, one of the prerequisites for SGS. Nearly temperature-independent carrier concentration and electrical conductivity is observed from 5 to 300 K. An anomalous Hall coefficient of 162 S/cm is obtained at 5 K. Point contact Andreev reflection data has yielded the current spin polarization value of 0.64, which is found to be robust against the structural disorder. All these properties are quite promising for the spintronic applications such as spin injection and can bridge a gap between the contrasting behavior of half-metallic ferromagnets and semiconductors., Comment: 14 pages, 7 figures, regular article

We report the structure, magnetic property and spin polarization of CoFeMnGe equiatomic quaternary Heusler alloy. The alloy was found to exist in the L21 structure with considerable amount of DO3 disorder. Thermal analysis result indicated the Curie temperature is about 711K without any other phase transformation up to melting temperature. The magnetization value was close to that predicted by the Slater-Pauling curve. Current spin polarization of P = 0.70 {plus/minus}0.1 was deduced using point contact Andreev reflection (PCAR) measurements. Half-metallic trend in the resistivity has also been observed in the temperature range of 5 K to 300 K. Considering the high spin polarization and Curie temperature, this material appears to be promising for spintronic applications., Comment: 14 pages, 6 figures, regular article

The interplay of light and magnetism has been a topic of interest since the original observations of Faraday and Kerr where magnetic materials affect the light polarization. While these effects have historically been exploited to use light as a probe of magnetic materials there is increasing research on using polarized light to alter or manipulate magnetism. For instance deterministic magnetic switching without any applied magnetic fields using laser pulses of the circular polarized light has been observed for specific ferrimagnetic materials. Here we demonstrate, for the first time, optical control of ferromagnetic materials ranging from magnetic thin films to multilayers and even granular films being explored for ultra-high-density magnetic recording. Our finding shows that optical control of magnetic materials is a much more general phenomenon than previously assumed. These results challenge the current theoretical understanding and will have a major impact on data memory and storage industries via the integration of optical control of ferromagnetic bits., Comment: 21 pages, 11 figures