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The relevance of anisotropic heatsinks for heat-assisted magnetic recording (HAMR) is investigated. First, 3-D thermal modeling is used to demonstrate that the heatsink performance on a glass substrate is determined by the heatsink in-plane thermal conductivity. The out-of-plane thermal conductivity has almost no impact on the heatsink ability to carry heat away from the hot spot. As a result, an anisotropic heatsink can be a good alternative to some conventional isotropic heatsink materials when its in-plane thermal conductivity is larger than that of the isotropic material. Second, time-domain thermo-reflectance (TDTR) measurements are presented to extract both the in-plane and the out-of-plane thermal conductivities of a (Cu/Ta) multilayer heatsink. The sensitivity to in-plane thermal conductivity is achieved by using small laser beam sizes and by combining measurements at multiple modulation frequencies on both a low and a high thermally conductive substrate. The fitting of all TDTR spectra gives for the (Cu[5.1]/Ta[1.4]) multilayers an effective out-of-plane thermal conductivity of 11.5 W/mK and an in-plane thermal conductivity of 69 W/mK. These thermal properties make such multilayer material an interesting candidate as HAMR heatsink, equivalent to a 70 W/mK isotropic material.

The effect of a 1.5 nm thick CoX layer (CAP) of high Curie temperature TC, deposited on top of an L10 Fe50Pt50 layer (MAG) on the properties of the MAG/CAP bilayer is investigated. Two series of samples are studied: “w/o CAP” are single MAG layers of thickness in the 3.8–10.5 nm range and “w/CAP” are MAG/CAP bilayers of MAG thickness in the 3.8–10.5 nm range and 1.5 nm CAP. For both series, magnetization reversal is investigated at TRT = 300 K and is shown to be coherent rotations up to a MAG thickness threshold of tcr ∼ 7.5 nm and incoherent, domain-wall mediated, above tcr. The dependence of d H RC / d T T = T C , where HRC is the remanence coercivity, on the MAG thickness indicates tcr is poorly dependent on temperature from TRT up to the MAG TC. The MAG TC distribution, of average 〈TC〉 and standard deviation σTC, is evaluated for both series. The results indicate 〈TC〉 is higher and σTC/〈TC〉 is lower w/CAP than w/o, particularly at low MAG thicknesses. This behavior is interpreted as a consequence of spin exchange hardening in the MAG layer in the proximity of the MAG/CAP interface. For the w/CAP case, the saturation field of the Thermoremanent magnetization exhibits a non-motononic dependence on the MAG thickness, displaying a maximum. This behavior indicates the CAP assists the applied field to set the MAG layer magnetization during a HAMR process, the assist effect becoming more efficient the lower is the MAG thickness.

Free-standing semiconductor nanowires constitute an ideal material system for the direct manipulation of electrical and optical properties by strain engineering. In this study, we present a direct quantitative correlation between electrical conductivity and nanoscale lattice strain of individual InAs nanowires passivated with a thin epitaxial In0.6Ga0.4As shell. With an in situ electron microscopy electromechanical testing technique, we show that the piezoresistive response of the nanowires is greatly enhanced compared to bulk InAs, and that uniaxial elastic strain leads to increased conductivity, which can be explained by a strain-induced reduction in the band gap. In addition, we observe inhomogeneity in strain distribution, which could have a reverse effect on the conductivity by increasing the scattering of charge carriers. These results provide a direct correlation of nanoscale mechanical strain and electrical transport properties in free-standing nanostructures.

The domain structure of short-range order is directly observed in an aged Ti-6Al alloy., Chemical short-range order (SRO) within a nominally single-phase solid solution is known to affect the mechanical properties of alloys. While SRO has been indirectly related to deformation, direct observation of the SRO domain structure, and its effects on deformation mechanisms at the nanoscale, has remained elusive. Here, we report the direct observation of SRO in relation to deformation using energy-filtered imaging in a transmission electron microscope (TEM). The diffraction contrast is enhanced by reducing the inelastically scattered electrons, revealing subnanometer SRO-enhanced domains. The destruction of these domains by dislocation planar slip is observed after ex situ and in situ TEM mechanical testing. These results confirm the impact of SRO in Ti-Al alloys on the scale of angstroms. The direct confirmation of SRO in relationship to dislocation plasticity in metals can provide insight into how the mechanical behavior of concentrated solid solutions by the material’s thermal history.

To date, there has not yet been a direct observation of the initiation and propagation of individual defects in metallic glasses during deformation at the nanoscale. Here, we show through a combination of in situ nanobeam electron diffraction and large-scale molecular dynamics simulations that we can directly observe changes to the local short to medium range atomic ordering during the formation of a shear band. We observe experimentally a spatially resolved reduction of order prior to shear banding due to increased strain. We compare this to molecular dynamics simulations, in which a similar reduction in local order is seen, and caused by shear transformation zone activation, providing direct experimental evidence for this proposed nucleation mechanism for shear bands in amorphous solids. Our observation serves as a link between the atomistic molecular dynamics simulation and the bulk mechanical properties, providing insight into how one could increase ductility in glassy materials., Observing defect formation during bulk metallic glass deformation remains challenging. Here, the authors combine in situ nanobeam electron diffraction and large-scale molecular dynamics simulations to directly link changes to the local atomic ordering with shear band formation in a metallic glass.

FePt granular films with grain size smaller than 10 nm are promising candidates for storage media used in the next generation heat-assisted magnetic recording technology. However, FePt films show degraded magnetic properties when the grain size is reduced to this scale, which cannot be explained solely by the finite size theory. In this study, we explored the structural cause of property degradation by employing advanced electron microscopy and atomistic modeling. Structural features unique to the nanostructured FePt granular films at significantly reduced grain sizes of 2∼8 nm were studied by high-resolution scanning transmission electron microscopy with geometric aberrations corrected up to the third order. Two critical structural parameters, the threshold grain size corresponding to the upper size limit of the FePt grains with zero chemical ordering and the sub-nanometer thin interfacial impurity at grain boundaries, were identified. A new atomistic model was developed to correlate these structural characteristics with key magnetic properties such as Curie temperature, saturation magnetization, magnetocrystalline anisotropy, and their grain-to-grain variation. The model shows good agreement with the experimental magnetic data and explains the gap in magnetic properties between the bulk and nanostructured FePt.

Nanosized precipitates have been observed in a Nb-lean gum metal-related alloy, Ti–20.0Nb–0.6Ta–1.7Zr–1.1O at.% (Ti–31.9Nb–2.0Ta–2.7Zr–0.3O wt.%) using probe-corrected high-resolution scanning transmission electron microscopy with a high-angle annular dark-field detector (HAADF). This characterization yields three distinct atomic motifs and STEM multislice simulations are semi-quantitatively used to verify that each motif can be attributed to the widely observed “athermal” omega phase. However, the presence of chemical ordering cannot be unambiguously ruled out in this system. Data presented here, demonstrate the complexity of interpreting HAADF images of multiphase, multicomponent alloys when complementary experimental data are unavailable.

Structural and magnetic properties of granular L10 FePt films are investigated as a function of film thickness in the range 3–11 nm. Scanning transmission electron microscopy observations indicate an increasing contribution of in-plane variants to the L10 FePt crystalline structure with increasing film thickness, which may explain the noticed degradation of the perpendicular anisotropy. The temperature dependence of coercivity suggests a progressive deviation of the magnetization reversal mechanism from coherent rotations with increasing film thickness above ~6 nm, corresponding to a domain-wall width. The anisotropy field distribution (〈H K〉, ) is estimated using Monte Carlo (MC) simulations of AC transverse susceptibility (ACTS). The average anisotropy field 〈H K〉 displays a similar temperature dependence to (M S)1.1, where M S is the film saturation magnetization, consistent with the theoretical predictions for L10 FePt. The Curie temperature distribution (〈T C〉, ) is evaluated using two methods: fast (ns) thermal erasure (ThEr) of remanent magnetization and MC modeling of the temperature dependence of AC susceptibility (ACS). The values evaluated by the two methods are in agreement, indicating an increase of in the range 2–6% with decreasing film thickness. The observed increase of 〈T C〉 in the range 620–680 K with increasing film thickness is explained using the finite-size scaling law with the critical exponent set to the theoretical value for L10 FePt. A general procedure to extract 〈T C〉 and using MC modeling of ACS, essentially independent of the magnetization reversal mechanism, is proposed. MC simulations of ACS confirm the deviation of the magnetization reversal mechanism from coherent rotations with increasing film thickness above ~6 nm.

Semiconductor heterostructures are the fundamental platform for many important device applications such as lasers, light-emitting diodes, solar cells and high-electron-mobility transistors. Analogous to traditional heterostructures, layered transition metal dichalcogenide (TMDC) heterostructures can be designed and built by assembling individual single-layers into functional multilayer structures, but in principle with atomically sharp interfaces, no interdiffusion of atoms, digitally controlled layered components and no lattice parameter constraints. Nonetheless, the optoelectronic behavior of this new type of van der Waals (vdW) semiconductor heterostructure is unknown at the single-layer limit. Specifically, it is experimentally unknown whether the optical transitions will be spatially direct or indirect in such hetero-bilayers. Here, we investigate artificial semiconductor heterostructures built from single layer WSe2 and MoS2 building blocks. We observe a large Stokes-like shift of ~100 meV between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment with spatially direct absorption but spatially indirect emission. Notably, the photoluminescence intensity of this spatially indirect transition is strong, suggesting strong interlayer coupling of charge carriers. The coupling at the hetero-interface can be readily tuned by inserting hexagonal BN (h-BN) dielectric layers into the vdW gap. The generic nature of this interlayer coupling consequently provides a new degree of freedom in band engineering and is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties with customized composite layers., http://www.pnas.org/content/early/2014/04/10/1405435111.abstract

GaAs/GaAsP strain-compensated superlattices (SLs) with thickness up to 90-pair were fabricated. Transmission electron microscopy revealed the SLs are of high crystal quality and the introduced strain in SLs layers are fixed in the whole SL layers. With increasing SL pair number, the strain-compensated SLs show a less depolarization than the conventional strained SLs. In spite of the high crystal quality, the strain-compensated SLs also remain slightly depolarized with increasing SL pairs and the decrease in spin-polarization contributes to the spin relaxation time. 24-pair of GaAs/GaAsP strain-compensated SL demonstrates a maximum spin-polarization of 92% with a high quantum efficiency of 1.6%.