arXiv subjects

Alpha T. NDiaye

Publications and source records attributed to Alpha T. NDiaye.

4 recordsLinked to original sources

arXiv2019

Evidence for metallic 1T phase, 3d1 electronic configuration and charge density wave order in molecular-beam epitaxy grown monolayer VTe2

We present a combined experimental and theoretical study of monolayer VTe2 grown on highly oriented pyrolytic graphite by molecular-beam epitaxy. Using various in-situ microscopic and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X-ray and angle-resolved photoemission, and X-ray absorption, together with theoretical analysis by density functional theory calculations, we demonstrate direct evidence of the metallic 1T phase and 3d1 electronic configuration in monolayer VTe2 that also features a (4 x 4) charge density wave order at low temperatures. In contrast to previous theoretical predictions, our element-specific characterization by X-ray magnetic circular dichroism rules out a ferromagnetic order intrinsic to the monolayer. Our findings provide essential knowledge necessary for understanding this interesting yet less explored metallic monolayer in the emerging family of van der Waals magnets.

cond-mat.mtrl-sci↗

arXiv2017

Tuning Perpendicular Magnetic Anisotropy by Oxygen Octahedral Rotations in (La$_{1-x}$Sr$_{x}$MnO$_{3}$)/(SrIrO$_{3}$) Superlattices

Perpendicular magnetic anisotropy (PMA) plays a critical role in the development of spintronics, thereby demanding new strategies to control PMA. Here we demonstrate a conceptually new type of interface induced PMA that is controlled by oxygen octahedral rotation. In superlattices comprised of La$_{1-x}$Sr$_{x}$MnO$_{3}$ and SrIrO$_{3}$, we find that all superlattices (0$\leq$x$\leq$1) exhibit ferromagnetism despite the fact that La$_{1-x}$Sr$_{x}$MnO$_{3}$ is antiferromagnetic for x$>$0.5. PMA as high as 4$\times$10$^6$ erg/cm$^3$ is observed by increasing x and attributed to a decrease of oxygen octahedral rotation at interfaces. We also demonstrate that oxygen octahedral deformation cannot explain the trend in PMA. These results reveal a new degree of freedom to control PMA, enabling discovery of emergent magnetic textures and topological phenomena.

cond-mat.str-el↗

arXiv2016

Spin-current probe for phase transition in an insulator

Spin fluctuation and transition have always been one of central topics of magnetism and condensed matter science. Experimentally, the spin fluctuation is found transcribed onto scattering intensity in the neutron scattering process, which is represented by dynamical magnetic susceptibility and maximized at phase transitions. Importantly, a neutron carries spin without electric charge, and it can bring spin into a sample without being disturbed by electric energy, although large facilities such as a nuclear reactor is necessary. Here we show that spin pumping, frequently used in nanoscale spintronic devices, provides a desktop micro probe for spin transition; spin current is a flux of spin without an electric charge and its transport reflects spin excitation. We demonstrate detection of antiferromagnetic transition in ultra-thin CoO films via frequency dependent spin-current transmission measurements, which provides a versatile probe for phase transition in an electric manner in minute devices.

cond-mat.mes-hall↗

arXiv2015

180 degree Ferroelectric Stripe Nanodomains in BiFeO3 Thin Films

There is growing evidence that domain walls in ferroics can possess emergent properties that are absent in bulk materials. For example, 180 domain walls in the ferroelectric-antiferromagnetic BiFeO3 are particularly interesting because they have been predicted to possess a range of intriguing behaviors; including electronic conduction and enhanced magnetization. To date, however, ordered arrays of such domain structures have not been reported. Here, we report the observation of 180 stripe nanodomains in (110)-oriented BiFeO3 thin films grown on orthorhombic GdScO3 (010)O substrates, and their impact on exchange coupling to metallic ferromagnets. Nanoscale ferroelectric 180 stripe domains with {112 } domain walls were observed in films < 32 nm thick to compensate for large depolarization fields. With increasing film thickness, we observe a domain structure crossover from the depolarization field-driven 180 stripe nanodomains to 71 domains determined by the elastic energy. Interestingly, these 180 domain walls (which are typically cylindrical or meandering in nature due to a lack of strong anisotropy associated with the energy of such walls) are found to be highly-ordered. Additional studies of Co0.9Fe0.1/BiFeO3 heterostructures reveal exchange bias and exchange enhancement in heterostructures based-on BiFeO3 with 180 domain walls and an absence of exchange bias in heterostructures based-on BiFeO3 with 71 domain walls; suggesting that the 180 domain walls could be the possible source for pinned uncompensated spins that give rise to exchange bias. This is further confirmed by X-ray circular magnetic dichroism studies, which demonstrate that films with predominantly 180 domain walls have larger magnetization than those with primarily 71 domain walls. Our results could be useful to extract the structure of domain walls and to explore domain wall functionalities in BiFeO3.

cond-mat.mtrl-sci↗