arXiv subjects

Waseem Akhtar

Publications and source records attributed to Waseem Akhtar.

4 recordsLinked to original sources

arXiv2024

Stability and Dynamics of Skyrmion and Skyrmion Bags Explored under the Influence of Out-of-Plane Strain and Its Gradient

Skyrmions as well as skyrmion bags in magnetic thin films are promising candidates for future high-density memory devices. The observation of skyrmion bags in liquid crystals and their predicted existence in ferromagnetic films has sparked theoretical studies on current induced dynamics of these topological charges. Here using micromagnetism, we study the impact of out of plane strain on the stability of skyrmion and skyrmion bags in ferromagnetic thin film. We further studied the current induced dynamics in the presence of out of plane strain gradient. We demonstrate that out of plane strain gradient direction with respect to the electron flow can be an efficient way to control the dynamics of topological charges. Specifically, the deflection of skyrmion bags correlates with their topological degree, and an appropriate strain gradient can counteract skyrmion Hall effects, enabling straight-line movement. Our micromagnetic simulations align well with theoretical predictions from the Thiele equation.

cond-mat.mtrl-sci↗

arXiv2024

Tailoring the Growth of $\beta$-Tungsten Using Substrate Bias and Its Effect on FMR-Driven Spin Pumping in $\beta$-W/Py Heterostructures

$\beta$-Tungsten ($\beta$-W), an A15 cubic phase of tungsten, exhibits a giant spin Hall angle compared to its bcc-phase $\alpha$-Tungsten ($\alpha$-W), making high-quality $\beta$-W films desirable for spintronic applications. We report the controlled growth of $\beta$-W films on SiO$_2$/Si substrates via DC sputtering, where substrate bias serves as a critical factor in stabilizing the $\beta$ phase by regulating the energy of deposited atoms. This approach enables the formation of $\beta$-W films over a wide thickness range. Additionally, we studied the spin pumping phenomena in different tungsten phases achieved through substrate bias. Ferromagnetic resonance measurements reveal an enhancement in the magnetic damping ($ \alpha_{\text{eff}} $) for $\beta$-W/Py compared to $\alpha$-W/Py dominated film. The effective spin mixing conductance ($ g_{\text{eff}}^{\uparrow\downarrow} $) is found to be higher in $\beta$-W/Py than in $\alpha$-W/Py, which is attributed to variations in the interface structures between these phases. Importantly, the use of substrate bias does not deteriorate the interface quality, underscoring its effectiveness. These findings highlight the potential of substrate bias in thin-film engineering, paving the way for its advanced utilization in spintronic applications.

cond-mat.mtrl-sci↗

arXiv2020

Imaging non-collinear antiferromagnetic textures via single spin relaxometry

Antiferromagnetic materials are promising platforms for next-generation spintronics owing to their fast dynamics and high robustness against parasitic magnetic fields. However, nanoscale imaging of the magnetic order in such materials with zero net magnetization remains a major experimental challenge. Here we show that non-collinear antiferromagnetic spin textures can be imaged by probing the magnetic noise they locally produce via thermal populations of magnons. To this end, we perform nanoscale, all-optical relaxometry with a scanning quantum sensor based on a single nitrogen-vacancy (NV) defect in diamond. Magnetic noise is detected through an increase of the spin relaxation rate of the NV defect, which results in an overall reduction of its photoluminescence signal under continuous laser illumination. As a proof-of-concept, the efficiency of the method is demonstrated by imaging various spin textures in synthetic antiferromagnets, including domain walls, spin spirals and antiferromagnetic skyrmions. This imaging procedure could be extended to a large class of intrinsic antiferromagnets and opens up new opportunities for studying the physics of localized spin wave modes for magnonics.

cond-mat.mes-hall↗

arXiv2011

Coherent storage of photoexcited triplet states using 29Si nuclear spins in silicon

Pulsed electron paramagnetic resonance spectroscopy of the photoexcited, metastable triplet state of the oxygen-vacancy center in silicon reveals that the lifetime of the ms = \pm1 sub-levels differ significantly from that of the ms =0 state. We exploit this significant difference in decay rates to the ground singlet state to achieve nearly ~100% electron spin polarization within the triplet. We further demonstrate the transfer of a coherent state of the triplet electron spin to, and from, a hyperfine-coupled, nearest-neighbor 29Si nuclear spin. We measure the coherence time of the 29 Si nuclear spin employed in this operation and find it to be unaffected by the presence of the triplet electron spin and equal to the bulk value measured by nuclear magnetic resonance.

cond-mat.other↗