Review of Fiber-optic Distributed Acoustic Sensing Technology

A distributed optical-fiber acoustic sensor is an acoustic sensor that uses the optical fiber itself as a photosensitive medium,and is based on Rayleigh backscattering in an optical fiber.The sensor is widely used in the safety monitoring of oil and gas pipelines,the classification of weak acoustic signals,defense,seismic prospecting,and other fields.In the field of seismic prospecting,distributed optical-fiber acoustic sensing(DAS)will gradually replace the use of the traditional geophone.The present paper mainly expounds the recent application of DAS,and summarizes recent research achievements of DAS in resource exploration,intrusion monitoring,pattern recognition,and other fields and various DAS system structures.It is found that the high-sensitivity and long-distance sensing capabilities of DAS play a role in the extensive monitoring applications of DAS in engineering.The future application and development of DAS technology are examined,with the hope of promoting the wider application of the DAS technology,which benefits engineering and society.

Emergent ultra-high temperature ferromagnetism in La_(2)CoO_(4±x) thin films

Searching for novel ferromagnetic oxides with high Curie temperature(TC)has been one of the main goals for oxide spintronics.The well-known perovskite cobaltate LaCoO_(3) is a classical ferromagnet in its thin-film form;however,it suffers from a low TC(~85 K).Here we report a new type of ferromagnetic La-Co-O films with an ultrahigh TC of~820 K.They are fabricated by pulsed laser deposition from a LaCoO_(3) target at low oxygen partial pressures.Detailed structural analysis indicates that they crystallize in terms of the Ruddlesden–Popper phase of La_(2)CoO_(4±x).In sharp contrast to the antiferromagnetism of bulk La_(2)CoO_(4),the strong ferromagnetism in the La_(2)CoO_(4±x) thin films is firmly demonstrated by magnetometry measurements,X-ray magnetic circular dichroism characterization,and magnetotransport experiments.More importantly,density functional theory calculations indicate that the nonstoichiometric oxygen induces an antiferromagnetic-to-ferromagnetic phase transition,accompanied by the orbital reconstruction of Co 3d electrons.Thus,our study provides an attractive strategy for designing or synthesizing exotic magnetic oxides with high ordering temperatures.

Cooperative control of perpendicular magnetic anisotropy via crystal structure and orientation in freestanding SrRuO_(3)membranes

Flexible magnetic materials with robust and controllable perpendicular magnetic anisotropy(PMA)are highly desirable for developing flexible high-performance spintronic devices.However,it is still challenge to fabricate PMA films on polymers directly.Here,we report a facile method for synthesizing single-crystal freestanding SrRuO_(3) membranes with controlled crystal structure and orientation using water-soluble Ca_(3-x)Sr_(x)Al_(2)O_(3)6 sacrificial layers.Through cooperative effect of crystal structure and orientation,flexible membranes reveal highly tunable magnetic anisotropy from in-plane to out-of-plane with a remarkable PMA energy of 7×10^(6) erg·cm^(−3).First-principle calculations reveal that the underlying mechanism of PMA modulation is intimately correlated with structure-controlled Ru 4d-orbital occupation,as well as spin-orbital matrix element differences,dependent on the crystal orientation.In addition,even after 10,000 bending cycles,the PMA keeps stable,indicating a robust magnetism reliability in the prepared films.This work provides a feasible approach to prepare the flexible oxide films with strong and controllable PMA.