Spin-Dependent Electronic Transport in Noncollinear Antiferromagnetic Antiperovskites

Spin-dependent properties are the heart of spintronic devices. Spintronics exploits electron’s spin, in addition to charge, to process and store the information. Recently, antiferromagnetic (AFM) spintronics has emerged as a subfield of spintronics, where an AFM order parameter (the Néel vector) is exploited to control spin-dependent transport properties. Due to being robust against magnetic perturbations, producing no stray fields, and exhibiting ultrafast dynamics, antiferromagnets can serve as promising functional materials for spintronic applications. Among antiferromagnets, high Néel temperature noncollinear antiperovskites ANMn3 (A = Ga, Ni, Sn, and Pt) are interesting due to their magnetic group symmetry supporting non-trivial spin-dependent transport phenomena. These materials have structural similarity to perovskites which allows their epitaxial deposition on perovskite substrates. Using symmetry analyses, first-principles methods based on density-functional theory, tight-binding Hamiltonian models, and magnetization dynamics techniques, this dissertation makes predictions and provides insights into different spin-dependent phenomena in non-collinear AFM antiperovskites. The results are as follow. It is shown that the noncollinear AFM phase of the antiperovskites exhibits sizable anomalous Hall conductivity (AHC), while the phase has zero AHC by symmetry. The Néel vector can be switched on the picosecond timescale using a spin torque generated by a spin polarized charge current. The critical switching current density can be tuned by ANMn3 stoichiometry engineering. It is demonstrated that the noncollinear AFM phase of GaNMn3 exhibits unconventional spin Hall conductivity, in addition to the conventional existing in the paramagnetic phase. Due to its out-of-plane spin polarization, spin Hall current exerts a spin torque that can switch out-of-plane magnetization in an adjacent ferromagnet. This unconventional spin torque has been realized experimentally using spin torque ferromagnetic resonance measurements carried out by our collaborators at University of Wisconsin-Madison. It is shown that noncollinear AFM antiperovskites allow generation of a spin-polarized longitudinal charge current like ferromagnets. The magnitude of the net spin polarization depends on crystallographic direction. These results demonstrate that AFM antiperovskites can be used as a spin source, spin-torque generator, and information carrier in spintronic devices. Advisor: Evgeny Tsymba

relationship of aid and economic growth in Nepal

Thesis(Master) --KDI School:Master of Development Policy,2014masterpublishedGautam, Gurung

Dirac nodal line metal for topological antiferromagnetic spintronics

Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which exploits the N\'eel vector to control the topological electronic states and the associated spin-dependent transport properties. A recently discovered N\'eel spin-orbit torque has been proposed to electrically manipulate Dirac band crossings in antiferromagnets; however, a reliable AFM material to realize these properties in practice is missing. Here, we predict that room temperature AFM metal MnPd$_{2}$ allows the electrical control of the Dirac nodal line by the N\'eel spin-orbit torque. Based on first-principles density functional theory calculations, we show that reorientation of the N\'eel vector leads to switching between the symmetry-protected degenerate state and the gapped state associated with the dispersive Dirac nodal line at the Fermi energy. The calculated spin Hall conductivity strongly depends on the N\'eel vector orientation and can be used to experimentally detect the predicted effect using a proposed spin-orbit torque device. Our results indicate that AFM Dirac nodal line metal MnPd$_{2}$ represents a promising material for topological AFM spintronics

Spin-torque switching of noncollinear antiferromagnetic antiperovskites

Antiferromagnetic (AFM) spintronics exploits the Néel vector as a state variable for novel electronic devices. Recent studies have demonstrated that the Néel vector can be switched by a spin-orbit torque. These studies however are largely limited to collinear antiferromagnets of proper magnetic space-group symmetry. There is, however, a large group of high-temperature noncollinear antiferromagnets, which are suitable for such switching. Here, we predict that spin torque can be efficiently used to switch a noncollinear AFM order in antiperovskite materials. Based on first-principles calculations and atomistic spin-dynamics modeling, we show that in antiperovskites ANMn3 (A = Ga, Ni, etc.) with the AFM Γ4g ground state, the AFM order can be switched on the picosecond timescale using a spin torque generated by a spin current. The threshold switching current density can be tuned by the ANMn3 stoichiometry engineering, changing the magnetocrystalline anisotropy. The Γ4g AFM phase supports a sizable anomalous Hall effect, which can be used to detect the spin-torque switching of the AFM order. The predicted ultrafast switching dynamics and the efficient detection of the AFM order state make noncollinear magnetic antiperovskites a promising material platform for AFM spintronics

Access to water, sanitation and hygiene for people living with HIV and AIDS: a cross-sectional study in Nepal

People living with HIV/AIDS (PLHA) are one of the most vulnerable people to WASH associated diseases in Nepal. They are still stigmatized while enjoying WASH services and their risk & vulnerability are further exacerbated due to lack of inadequate WASH. A descriptive cross-sectional study was commissioned by WaterAid in Nepal (WAN) with the objectives to increase understanding of PLHA’s access to WASH and its impact on their daily lives in order to inform health, HIV and WASH sectors. This was a cross-sectional study used mixed methods. Data were collected from 196 PLHAs from different geographic areas. The study does re-emphasize that PLHA have limited access to safe water and improved hygiene & sanitation services, more pronounced in rural areas than in urban. There is an increased need of WASH for them but lacking to meet the needs. Lack of access to WASH and its effect on quality of life invariably call for an urgent action by all stakeholders. The study also revealed some evidence of stigma and discrimination faced by PLHAs

Anomalous Hall Conductivity of a Non-Collinear Magnetic Antiperovskite

The anomalous Hall effect (AHE) is a well-known fundamental property of ferromagnetic metals, commonly associated with the presence of a net magnetization. Recently, an AHE has been discovered in non-collinear antiferromagnetic (AFM) metals. Driven by non-vanishing Berry curvature of AFM materials with certain magnetic space group symmetry, anomalous Hall conductivity (AHC) is very sensitive to the specific type of magnetic ordering. Here, we investigate the appearance of AHC in antiperovskite GaNMn$_{3}$ as a representative of broader materials family ANMn$_{3}$ (A is a main group element), where different types of non-collinear magnetic ordering can emerge. Using symmetry analyses and first-principles density-functional theory calculations, we show that with almost identical band structure, the nearly degenerate non-collinear AFM $\Gamma_{5g}$ and $\Gamma_{4g}$ phases of GaNMn$_{3}$ have zero and finite AHC, respectively. In a non-collinear ferrimagnetic $M$-1 phase, GaNMn$_{3}$ exhibits a large AHC due to the presence of a sizable net magnetic moment. In the non-collinear antiperovskite magnets, transitions between different magnetic phases, exhibiting different AHC states, can be produced by doping, strain, or spin transfer torque, which makes these materials promising for novel spintronic applications

Anomalous Hall conductivity of noncollinear magnetic antiperovskites

The anomalous Hall effect (AHE) is a well-known fundamental property of ferromagnetic metals, commonly associated with the presence of a net magnetization. Recently, an AHE has been discovered in noncollinear antiferromagnetic (AFM) metals. Driven by nonvanishing Berry curvature of AFM materials with certain magnetic space-group symmetry, anomalous Hall conductivity (AHC) is very sensitive to the specific type of magnetic ordering. Here, we investigate the appearance of AHC in antiperovskite materials family ANMn3 (A = Ga, Sn, Ni), where different types of noncollinear magnetic ordering can emerge. Using symmetry analyses and first-principles density-functional theory calculations, we show that with almost identical band structure the nearly degenerate noncollinear AFM Γ5g and Γ4g phases of GaNMn3 have zero and finite AHC, respectively. In a noncollinear ferrimagnetic M-1 phase, GaNMn3 exhibits a large AHC due to the presence of a sizable net magnetic moment. In the noncollinear antiperovskite magnets, transitions between different magnetic phases, exhibiting different AHC states, can be produced by doping, strain, or spin transfer torque, which makes these materials promising for novel spintronic applications

Dirac Nodal Line Metal for Topological Antiferromagnetic Spintronics

Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which exploits the N´eel vector to control the topological electronic states and the associated spin-dependent transport properties. A recently discovered N´eel spin-orbit torque has been proposed to electrically manipulate Dirac band crossings in antiferromagnets; however, a reliable AFM material to realize these properties in practice is missing. In this Letter, we predict that room-temperature AFM metal MnPd2 allows the electrical control of the Dirac nodal line by the N´eel spin-orbit torque. Based on first-principles density functional theory calculations, we show that reorientation of the N´eel vector leads to switching between the symmetryprotected degenerate state and the gapped state associated with the dispersive Dirac nodal line at the Fermi energy. The calculated spin Hall conductivity strongly depends on the N´eel vector orientation and can be used to experimentally detect the predicted effect using a proposed spin-orbit torque device. Our results indicate that AFM Dirac nodal line metal MnPd2 represents a promising material for topological AFM spintronics

Socio-economic analysis of coffee growers in Gulmi district of Nepal

Coffee is a major plantation cash crop of hills of Nepal. Gulmi is one of the popular districts for coffee production and export in Nepal. This research is an attempt to assess the production potentiality and profitability of coffee in Gulmi district. This study was carried out in four rural municipalities of Gulmi district namely Ruru rural municipality, Dhurkot rural municipality, Satyawoti rural municipality and Musikot municipality. A total of 100 samples (25 from each rural municipality) were selected using simple random sampling technique. Face to face (FtF) interview method was used to collect primary data using pretested semi-structured questionnaire. The economic indicators of coffee production like gross revenue, gross margin, benefit-cost ratio (BCR) and profitability index (PI) were calculated. BCR and PI were found to be 2.84± 0.59 and 2.50± 1.25, respectively. Gross margin per ropani was calculated to be NRs. 15675.29 ± 7189.72. The contribution of coffee in total household income was 12% in Gulmi showing it to be one of the major influencing commodities. The major production problem was found to be insect attack in the district while the major marketing problem was the low market price. The insights of this research were that coffee has the potentiality to uplift the rural income of Gulmi but at the same time there were poor extension services for coffee growers such that farmers had shown dissatisfaction towards coffee enterprise. Therefore, effective package of production and value chain monitoring should be introduced by the Nepal government in order to address the production and marketing constraints of coffee producers

Switchable anomalous Hall effects in polar-stacked 2D antiferromagnet MnBi\u3csub\u3e2\u3c/sub\u3eTe\u3csub\u3e4\u3c/sub\u3e

Van der Waals (vdW) assembly allows controlling symmetry of two-dimensional (2D) materials that determines their physical properties. Especially interesting is the recently demonstrated breaking inversion symmetry by polar layer stacking to realize novel electronic, magnetic, and transport properties of 2D vdW materials switchable by induced electric polarization. Here, based on symmetry analyses and density-functional calculations, we explore the emergence of the anomalous Hall effect (AHE) in antiferromagnetic MnBi2Te4 films assembled by polar layer stacking. We demonstrate that breaking ̂ ̂ symmetry in an MnBi2Te4 bilayer makes this 2D material magnetoelectric and produces a spontaneous AHE switchable by electric polarization. We find that reversable polarization at one of the interfaces in a three-layer MnBi2Te4 film drives a metal-insulator transition, as well as switching between an AHE and quantum AHE (QAHE). Finally, we predict that engineering an interlayer polarization in a three-layer MnBi2Te4 film allows converting MnBi2Te4 from a trivial insulator to a Chern insulator. Overall, our work emphasizes the emergence of quantum-transport phenomena in 2D vdW antiferromagnets by polar layer stacking, which do not exist in this material in the bulk or bulk-like thin-film forms