Pivotal role of magnetic ordering and strain in lattice thermal conductivity of chromium-trihalide monolayers

Understanding the coupling between spin and phonons is critical for controlling the lattice thermal conductivity (κl) in magnetic materials, as we demonstrate here for CrX3 (X = Br and I) monolayers. We show that these compounds exhibit large spin-phonon coupling (SPC), dominated by out-ofplane vibrations of Cr atoms, resulting in significantly different phonon dispersions in ferromagnetic (FM) and paramagnetic (PM) phases. Lattice thermal conductivity calculations provide additional evidence for strong SPC, where particularly large κlis found for the FM phase. Most strikingly, PM and FM phases exhibit radically different behavior with tensile strain, where κl increases with strain for the PM phase, and strongly decreases for the FM phase — as we explain through analysis of phonon lifetimes and scattering rates. Taken all together, we uncover the very high significance of SPC on the phonon transport in CrX3 monolayers, a result extendable to other 2D magnetic materials, that will be useful in further design of thermal spin devices.

Epitaxial growth of bilayer Bi(110) on two-dimensional ferromagnetic Fe3GeTe2

Heterostructures of two-dimensional (2D) layered materials with selective compositions play an important role in creating novel functionalities. Effective interface coupling between 2D ferromagnet and electronic materials would enable the generation of exotic physical phenomena caused by intrinsic symmetry breaking and proximity effect at interfaces. Here, epitaxial growth of bilayer Bi(110) on 2D ferromagnetic Fe3GeTe2 (FGT) with large magnetic anisotropy has been reported. Bilayer Bi(110) islands are found to extend along fixed lattice directions of FGT. The six preferred orientations could be divided into two groups of three-fold symmetry axes with the difference approximately to 26°. Moreover, dI/dV measurements confirm the existence of interface coupling between bilayer Bi(110) and FGT. A variation of the energy gap at the edges of bilayer Bi(110) is also observed which is modulated by the interface coupling strengths associated with its buckled atomic structure. This system provides a good platform for further study of the exotic electronic properties of epitaxial Bi(110) on 2D ferromagnetic substrate and promotes potential applications in the field of spin devices.

Ferromagnetic Schottky Contact for GaN Based Spin Devices

In this paper, ferromagnetic Schottky contacts for GaN based spin injection are being studied. The electrical characterization of this Co/n-GaN and Fe/n-GaN Schottky contacts showing the zero-bias barrier height comes closer to unity as the temperature is increased. Also, the Richardson constant is extracted for this Schottky contact. Both the zero-bias barrier height and the Richardson constant are verified both experimentally as well as theoretically. Thus, this Schottky contacts will serve as spin injector for GaN based spin devices specifically for GaCrN based devices

Investigation of the thermal tolerance of silicon-based lateral spin valves

Improvement in the thermal tolerance of Si-based spin devices is realized by employing thermally stable nonmagnetic (NM) electrodes. For Au/Ta/Al electrodes, intermixing between Al atoms and Au atoms occurs at approximately 300 °C, resulting in the formation of a Au/Si interface. The Au–Si liquid phase is formed and diffuses mainly along an in-plane direction between the Si and AlN capping layers, eventually breaking the MgO layer of the ferromagnetic (FM) metal/MgO electrodes, which is located 7 µm away from the NM electrodes. By changing the layer structure of the NM electrode from Au/Ta/Al to Au/Ta, the thermal tolerance is clearly enhanced. Clear spin transport signals are obtained even after annealing at 400 °C. To investigate the effects of Mg insertion in FM electrodes on thermal tolerance, we also compare the thermal tolerance among Fe/Co/MgO, Fe/Co/Mg/MgO and Fe/Co/MgO/Mg contacts. Although a highly efficient spin injection has been reported by insertion of a thin Mg layer below or above the MgO layer, these thermal tolerances decrease obviously.

Improved Thermal Tolerance of Silicon-based Lateral Spin Valves

Improvement in the thermal tolerance of Si-based spin devices is realized by employing thermally stable nonmagnetic (NM) electrodes. For Au/Ta/Al electrodes, intermixing between Al atoms and Au atoms occurs at approximately 300°C, resulting in the formation of a Au/Si interface. The Au-Si liquid phase is formed and diffuses mainly along an in-plane direction between the Si and AlN capping layers, eventually breaking the MgO layer of the ferromagnetic (FM) metal/MgO electrodes, which is located 7 mm away from the NM electrodes. By changing the layer structure of the NM electrode from Au/Ta/Al to Au/Ta, the thermal tolerance is clearly enhanced. Clear spin transport signals are obtained even after annealing at 400°C. To investigate the effects of Mg insertion in FM electrodes on thermal tolerance, we also compare the thermal tolerance among Fe/Co/MgO, Fe/Co/Mg/MgO and Fe/Co/MgO/Mg contacts. Although a highly efficient spin injection has been reported by insertion of a thin Mg layer below or above the MgO layer, these thermal tolerances decrease obviously.

Energy-Efficient Multiferroic Spin-Devices and Spin-Circuits

Spin-devices are switched by flipping spins without moving charge in space and this can lead to ultra-low-energy switching replacing traditional transistors in beyond Moore’s law era. In particular, the electric field-induced magnetization switching has emerged to be an energy-efficient paradigm. Here, we review the recent developments on ultra-low-energy, area-efficient, and fast spin-devices using multiferroic magnetoelectric composites. It is shown that both digital logic gates and analog computing with transistor-like high-gain region in the input-output characteristics of multiferroic composites are feasible. We also review the equivalent spin-circuit representation of spin-devices by considering spin potential and spin current similar to the charge-based counterparts using Kirchhoff’s voltage/current laws, which is necessary for the development of large-scale circuits. We review the spin-circuit representation of spin pumping, which happens anyway when there is a material adjacent to a rotating magnetization and therefore it is particularly necessary to be incorporated in device modeling. Such representation is also useful for understanding and proposing experiments. In spin-circuit representation, spin diffusion length is an important parameter and it is shown that a thickness-dependent spin diffusion length reflecting Elliott–Yafet spin relaxation mechanism in platinum is necessary to match the experimental results.