This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

https://link.aps.org/accepted/10.1103/RevModPhys.89.025006

Interface-induced phenomena in magnetism

Organic thermoelectric materials are emerging as low-cost, versatile alternatives to more established inorganic ones. Avery et al. report carbon nanotube-based materials with selected properties that exhibit enhanced thermoelectric performance.

Tailored semiconducting carbon nanotube networks with enhanced thermoelectric properties

Inorganic chalcogenides are traditional high-performance thermoelectric materials. However, they suffer from intrinsic brittleness and it is very difficult to obtain materials with both high thermoelectric ability and good flexibility. Here, we report a flexible thermoelectric material comprising highly ordered Bi2Te3 nanocrystals anchored on a single-walled carbon nanotube (SWCNT) network, where a crystallographic relationship exists between the Bi2Te3 <
 $$\bar{1}2\bar{1}0$$
 > orientation and SWCNT bundle axis. This material has a power factor of ~1,600 μW m−1 K−2 at room temperature, decreasing to 1,100 μW m−1 K−2 at 473 K. With a low in-plane lattice thermal conductivity of 0.26 ± 0.03 W m−1 K−1, a maximum thermoelectric figure of merit (ZT) of 0.89 at room temperature is achieved, originating from a strong phonon scattering effect. The origin of the excellent flexibility and thermoelectric performance of the Bi2Te3–SWCNT material is attributed, by experimental and computational evidence, to its crystal orientation, interface and nanopore structure. Our results provide insight into the design and fabrication of high-performance flexible thermoelectric materials. Bi2Te3 materials suffer from brittleness, limiting their application for thermoelectric harvesting. By depositing ordered nanocrystals onto single-wall carbon nanotubes, a flexible material is formed that achieves ZT of 0.89 at room temperature.

Flexible layer-structured Bi2Te3 thermoelectric on a carbon nanotube scaffold

Controlling thermal transport is commonly achieved by introducing scattering centres. Here, the authors demonstrate that coherent band structure effects can also be used to control phonon transport, via the use of periodically nanostructured phononic crystals.

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Engineering thermal conductance using a two-dimensional phononic crystal

Controlling thermal transport has become relevant in recent years. Traditionally, this control has been achieved by tuning the scattering of phonons by including various types of scattering centres in the material (nanoparticles, impurities, etc). Here we take another approach and demonstrate that one can also use coherent band structure effects to control phonon thermal conductance, with the help of periodically nanostructured phononic crystals. We perform the experiments at low temperatures below 1 K, which not only leads to negligible bulk phonon scattering, but also increases the wavelength of the dominant thermal phonons by more than two orders of magnitude compared to room temperature. Thus, phononic crystals with lattice constants ≥1 μm are shown to strongly reduce the thermal conduction. The observed effect is in quantitative agreement with the theoretical calculation presented, which accurately determined the ballistic thermal conductance in a phononic crystal device.

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We present thermal conductivity measurements of micromachined 500 nm thick silicon-nitride (Si–N) beams suspended between two Si–N islands, in the temperature range from 77 to 325 K. The measured thermal conductivity, k, of Si–N at high temperatures is in good agreement with previously measured values for Si–N grown by low-pressure chemical vapor deposition, but behaves much differently as temperature is lowered, showing a dependence more similar to polycrystalline materials. Preliminary structural characterization by x-ray diffraction suggests that the material is likely nano- or polycrystalline. The micromachined suspended platform structure is designed to allow highly accurate measurements of the thermal conductivity of deposited metallic, semiconducting, or insulating thin films. As a demonstration, we present measurements of a 200 nm thick sputtered molybdenum film. In the entire temperature range the measured thermal conductivity matches the prediction of the Wiedemann–Franz thermal conductivity det...

/pdf/thermal-conductivity-of-micromachined-low-stress-silicon-2kgbcbgbdf.pdf

Thermal conductivity of micromachined low-stress silicon-nitride beams from 77 to 325 K

We present measurements of thermal transport in 500-nm-thick, 35-$\ensuremath{\mu}$m-wide, and 806-$\ensuremath{\mu}$m-long micromachined suspended silicon nitride (Si-N) bridges over the temperature range of 77 to 325 K. The measured thermal conductivity of Si-N (for material grown by low-pressure chemical vapor deposition in two different furnaces) deviates somewhat from previously reported measurements and also shows surprising dependence on surface variation at these relatively high temperatures. Addition of discontinuous gold films causes the thermal conductance of Si-N bridges to drop through the entire measured temperature range, before rising again when thicker, continuous films are added. Similar effects occur when continuous but very low-thermal-conductivity alumina films are deposited. The reduction in thermal conductance upon modification of the Si-N surface is strong evidence that vibrational excitations with long mean free paths carry significant heat even at these high temperatures. By measuring a series of film thicknesses the surface-scattering effects can be mitigated, and the resulting experimental values of the thermal conductivity of alumina and Au thin films compare very well to known values or to predictions of the Wiedemann-Franz law. We also present a modified model for the phonon mean free path in thin-film geometries, and use it along with atomic force microsope scans to show that a very small population of phonons with mean free path on the order of 1 $\ensuremath{\mu}$m and wavelength much longer than the expected thermal wavelengths carry up to $50%$ of the heat in Si-N at room temperature.

/pdf/heat-transport-by-long-mean-free-path-vibrations-in-3obchyxzpa.pdf

Heat transport by long mean free path vibrations in amorphous silicon nitride near room temperature

We present measurements of thermopower (Seebeck coefficient) and electrical resistivity of a wide selection of polycrystalline ferromagnetic films with thicknesses ranging from 60--167 nm. For comparison, a copper film of similar thickness was measured with the same techniques. Both the thermal and electrical measurements, made as a function of temperature from 77--325 K, are made using a micromachined thermal isolation platform consisting of a suspended, patterned silicon-nitride membrane. We observe a strong correlation between the resistivity of the films and the thermopower. Films with higher resistivity and residual resistivity ratios, indicating a higher concentration of static defects such as impurities or grain boundaries, with rare exception show thermopower of the same sign, but with absolute magnitude reduced from the thermopower of the corresponding bulk material. In addition, iron films exhibit the pronounced low-temperature peak in thermopower associated with magnon drag, with a magnitude similar to that seen in bulk iron alloys. These results provide important groundwork for ongoing studies of related thermoelectric effects in nanomagnetic systems, such as the spin Seebeck effect.

https://link.aps.org/accepted/10.1103/PhysRevB.83.100401

Thermopower and resistivity in ferromagnetic thin films near room temperature

Exploring thermoelectric effects and Wiedemann–Franz violation in magnetic nanostructures via micromachined thermal platforms

Metallic magnetic calorimeters, where deposited energy is detected by measuring a temperature-dependent magnetization with a low-noise SQUID, remain a promising potential route to X-ray spectrometers with energy resolution approaching 1 eV. In this paper we describe our recent work toward array-compatible, high-resolution MMCs fabricated entirely using thin-film techniques. We describe a meander-style pickup loop designed for good coupling to high-efficiency, low noise SQUIDs, as well as considering various routes to a thin-film paramagnetic sensor. We also briefly overview the most promising technology for multiplexing arrays of non-dissipative metallic magnetic calorimeters.

Rubina Sultan

Papers

Thermal conductivity of micromachined low-stress silicon-nitride beams from 77 to 325 K

Heat transport by long mean free path vibrations in amorphous silicon nitride near room temperature

Thermopower and resistivity in ferromagnetic thin films near room temperature

Exploring thermoelectric effects and Wiedemann–Franz violation in magnetic nanostructures via micromachined thermal platforms

Design, Fabrication, and Multiplexing of Magnetic Calorimeter X-ray Detectors with High-Efficiency SQUID Readout