Size, dimensionality, and shape play important roles in determining the properties of nanomaterials. So far, most of the nanomaterial researches have been focused on zero-dimensional nanoparticles/nanodots and one-dimensional nanowires/nanorods/nanotubes, but very few studies have been carried out on two-dimensional nano-sheets. Starting from carbon, recently we have succeeded in growing a class of nanostructured two-dimensional materials either in the pure forms or in the form of composites with carbon. In this paper, we will first briefly discuss various types of two-dimensional systems and then focus on the formation mechanism of carbon nanowalls and their field-emission and electron transport properties. The use of carbon nanowalls as templates for the formation of other types of nanomaterials will also be discussed.

Toward magnonic devices: The field of magnonics aims to use spin waves (SWs) and their associated quasiparticles—magnons—as carriers of information. Compared with the movement of charge in conventional electronics, a major advantage of SWs is reduced Joule heating. However, SWs are trickier to direct and control. Two groups now go a step further toward magnon-based devices. Han et al. show that in multilayer films, domain walls can be used to change the phase and magnitude of a spin wave. Wang et al. demonstrate how magnon currents can be used to switch the magnetization of an adjacent layer. Science , this issue p. 1121 , p. 1125

Electrical manipulation of spins is essential to design state-of-the-art spintronic devices and commonly relies on the spin current injected from a second heavy-metal material. The fact that chiral antiferromagnets produce spin current inspires us to explore the magnetization switching of chiral spins using self-generated spin torque. Here, we demonstrate the electric switching of noncollinear antiferromagnetic state in Mn₃Sn by observing a crossover from conventional spin-orbit torque to the self-generated spin torque when increasing the MgO thickness in Ta/MgO/Mn₃Sn polycrystalline films. The spin current injection from the Ta layer can be controlled and even blocked by varying the MgO thickness, but the switching sustains even at a large MgO thickness. Furthermore, the switching polarity reverses when the MgO thickness exceeds around 3 nm, which cannot be explained by the spin-orbit torque scenario due to spin current injection from the Ta layer. Evident current-induced switching is also observed in MgO/Mn₃Sn and Ti/Mn₃Sn bilayers, where external injection of spin Hall current to Mn₃Sn is negligible. The inter-grain spin-transfer torque induced by spin-polarized current explains the experimental observations. Our findings provide an alternative pathway for electrical manipulation of non-collinear antiferromagnetic state without resorting to the conventional bilayer structure.

The anomalous Hall effect, observed in conducting ferromagnets with broken time-reversal symmetry, offers the possibility to couple spin and orbital degrees of freedom of electrons in ferromagnets. In addition to charge, the anomalous Hall effect also leads to spin accumulation at the surfaces perpendicular to both the current and magnetization direction. Here, we experimentally demonstrate that the spin accumulation, subsequent spin backflow, and spin–charge conversion can give rise to a different type of spin current-related spin current related magnetoresistance, dubbed here as the anomalous Hall magnetoresistance, which has the same angular dependence as the recently discovered spin Hall magnetoresistance. The anomalous Hall magnetoresistance is observed in four types of samples: co-sputtered (Fe_1−xMn _x )_0.6Pt_0.4, Fe_1−xMn _x /Pt multilayer, Fe_1−xMn _x with x = 0.17–0.65 and Fe, and analyzed using the drift-diffusion model. Our results provide an alternative route to study charge–spin conversion in ferromagnets and to exploit it for potential spintronic applications.

We report on terahertz (THz) emission from a single-layer ferromagnet, which involves the generation of backflow nonthermal charge current from the ferromagnet/dielectric interface by femtosecond laser excitation and subsequent conversion of the charge current to a transverse transient charge current via the anomalous Hall effect, thereby generating THz radiation. The THz emission can be either enhanced or suppressed or the polarity can be reversed by introducing a magnetization gradient in the thickness direction of the ferromagnet. Unlike spintronic THz emitters reported previously, it does not require an additional nonmagnetic layer or Rashba interface.