Samiran Choudhury

Pure spin current has transformed the research field of conventional spintronics due to its various advantages, including energy efficiency. An efficient mechanism for generation of pure spin current is spin pumping, and high effective spin-mixing conductance (G_eff) and interfacial spin transparency (T) are essential for its higher efficiency. By employing the time-resolved magneto-optical Kerr effect technique, we report here a giant value of T in substrate/W (t)/Co₂₀Fe₆₀B₂₀ (d)/SiO₂ (2 nm) thin-film heterostructures in the beta-tungsten (β-W) phase. We extract the spin diffusion length of W and spin-mixing conductance of the W/CoFeB interface from the variation of damping as a function of W and CoFeB thickness. This leads to a value of T = 0.81 ± 0.03 for the β-W/CoFeB interface. A stark variation of G_eff and T with the thickness of the W layer is obtained in accordance with the structural phase transition and resistivity variation of W with its thickness. Effects such as spin memory loss and two-magnon scattering are found to have minor contributions to damping modulation in comparison to the spin pumping effect which is reconfirmed from the unchanged damping constant with the variation of Cu spacer layer thickness inserted between W and CoFeB. The giant interfacial spin transparency and its strong dependence on crystal structures of W will be important for future spin-orbitronic devices based on pure spin current.

Development of energy-efficient on-demand magnonic nanochannels (MNCs) can revolutionize on-chip data communication and processing. We have developed a dynamic MNC array by periodically tailoring perpendicular magnetic anisotropy using the electric field. Brillouin light scattering spectroscopy is used to probe the spin wave (SW) dispersion of MNCs formed by applying a static electric field at the CoFeB/MgO interface through the one-dimensional stripe-like array of indium tin oxide electrodes placed on top of Ta/CoFeB/MgO/Al₂O₃ heterostructures. Magnonic bands, consisting of two SW frequency modes, appear with a bandgap under the application of moderate gate voltage, which can be switched off by withdrawing the voltage. The experimental results are reproduced by plane wave method–based numerical calculations, and simulated SW mode profiles show propagating SWs through nanochannels with different magnetic properties. The anticrossing between these two modes gives rise to the observed magnonic bandgap.

A hexagonally arranged array of ferromagnetic nanodots is particularly interesting because it offers the highest areal density of features achievable using modern nanofabrication techniques. They are important for high density magnetic storage, memory, logic, sensors and magnonic crystals. However, understanding the collective static and dynamic magnetic properties by varying the inter-dot separations in such a lattice has yet not been fully explored. Here, we demonstrate transition from a strongly collective to a completely isolated magnetization dynamics via various weakly collective dynamics by systematically varying the inter-dot separation of circular Ni₈₀Fe₂₀ nanodots of 100 nm in diameter (d) arranged in hexagonal lattice. Time-resolved Kerr microscopy has been exploited to study the ultrafast magnetization dynamics of the arrays with varying inter-dot separation (S) between 30 nm and 390 nm. The transition between different collective regimes was identified from sudden change in frequency values and number of modes in the frequency spectra. This was further supported by the bias field variation of the frequency of various spin wave modes and simulated mode profiles. The latter clearly showed the variation in the nature of the spatial distribution of the collective modes in the arrays in different collective regimes. The observations are imperative for selection of correct values of inter-dot separation in hexagonal arrays of nanodots for various applications.