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One recent breakthrough in the field of magnonics is the experimental realization of reconfigurable spin-wave nanochannels formed by magnetic domain wall with a width of 10 − 100 nm [Wagner et al., Nat. Nano. 11, 432 (2016)]. This remarkable progress enables an energy-efficient spin-wave propagation with a well-defined wave vector along its propagating path inside the wall. In the mentioned experiment, a micro-focus Brillouin light scattering spectroscopy was taken in a line-scans manner to measure the frequency of the bounded spin wave. Due to their localization nature, the confined spin waves can hardly be detected from outside the wall channel, which guarantees the information security to some extent. In this work, we theoretically propose a scheme to detect/eavesdrop on the spin waves inside the domain-wall nanochannel via nonlinear three-magnon processes. We send a spin wave (ω i , k i ) in one magnetic domain to interact with the bounded mode (ω b , k b ) in the wall, where k b is parallel with the domain-wall channel defined as theẑ axis. Two kinds of threemagnon processes, i.e., confluence and splitting, are expected to occur. The confluence process is conventional: conservation of energy and momentum parallel with the wall indicates a transmitted wave in the opposite domain with ω(k) = ω i + ω b and (k i + k b − k) ·ẑ = 0, while the momentum perpendicular to the domain wall is not necessary to be conserved due to the non-uniform internal field near the wall. We predict a stimulated threemagnon splitting (or "magnon laser") effect: the presence of a bound magnon propagating along the domain wall channel assists the splitting of the incident wave into two modes, one is ω 1 = ω b , k 1 = k b identical to the bound mode in the channel, and the other one is ω 2 = ω i − ω b with (k i − k b − k 2 ) ·ẑ = 0 propagating in the opposite magnetic domain. Micromagnetic simulations confirm our theoretical analysis. These results demonstrate that one is able to uniquely infer the spectrum of the spin-wave in the domain-wall nanochannel once we know both the injection and the transmitted waves.

Coal bed methane (CBM) is a primary clean energy source found in coal seams. The recovery ratio of CBM is very low, especially with ground extraction, due to the strong adsorption of CH 4 on the pores and fissures of coal and low permeability of the coal bed. On the basis of the theory of energy balance, a theory of enhanced CBM (ECBM) recovery by energy stimulation is proposed in this review. The desorption and transportation of CH 4 in coal beds are analyzed from the perspective of energy consumption. Two pacesetting stimulation technologies are proposed to increase the permeability of coal beds: supercritical CO 2 (SCCO 2 ) fracturing and thermal fracturing through steam injection. The experimental study and the related results of SCCO 2 -enhanced CBM recovery are presented and analyzed. The results demonstrate that the permeability of SCCO 2 in the tested four ranked coal specimens decreases in the form of a negative exponent function with the increase of effective stress. The transportation capacity of SCCO 2 in high ranked coal beds deep underground is inferior to that in low ranked coal beds at shallow depth. The experiment of the total permeability of CO 2 with CH 4 in coking coal shows that the permeability decreases in the form of a logarithmic function as the content of CO 2 increases. SCCO 2 has higher transportation capacity than gaseous CO 2 in coal beds, and the total permeability decreases with its content increasing in the synthetical system with CH 4 . The SCCO 2 -enhanced CBM recovery experiment demonstrates that the economic production time is different for the four ranked coal specimens given 50% of each content in production gas being defined as a threshold for economic production. The economic production for gas coal and anthracite takes a longer time than that in weakly caking coal of a lower ranked coal. The study demonstrates that different ranked coals have different inner structures and transportation properties. However, the original structures of different ranked coals can be modified by SCCO 2 by changing the void volume or the structure of the pores and fissures. The challenges and perspectives for the CBM recovery theory and technology are also presented. The study is of importance with the novel dual efficient approach for clean energy CBM recovery ratio improvement and greenhouse gas sequestration underground in deep coal beds.

The Dzyaloshinskii-Moriya interaction (DMI) has attracted considerable recent attention owing to the intriguing physics behind and the fundamental role it played in stabilizing magnetic solitons, such as magnetic skyrmions and chiral domain walls. A number of experimental efforts have been devoted to probe the DMI, among which the most popular method is the Brillouin light scattering spectroscopy (BLS) to measure the frequency difference of spin waves with opposite wave vectors ±k perpendicular to the in-plane magnetization m. Such a technique, however, is not applicable for the cases of k m, since the spin-wave reciprocity is recovered then. For a narrow magnetic strip, it is also difficult to measure the DMI strength using BLS because of the spatial resolution limit of lights. To fill these gaps, we propose to probe the DMI via the propagation of spin waves in ferromagnetic films. We show that the DMI can cause the non-collinearity of the group velocities of spin waves with ±k m. In heterogeneous magnetic thin films with different DMIs, negative refractions of spin waves emerge at the interface under proper conditions. These findings enable us to quantify the DMI strength by measuring the angle between the two spin-wave beams with ±k m in homogeneous film and by measuring the incident and negative refraction angles in heterogeneous films. For a narrow magnetic strip, we propose a nonlocal scheme to determine the DMI strength via nonlinear three-magnon processes. We implement theoretical calculations and micromagnetic simulations to verify our ideas. The results presented here are helpful for future measurement of the DMI and for designing novel spin-wave spintronic devices. arXiv:1807.04025v2 [cond-mat.mes-hall]