Himadri Dey

Spin–orbit torque-assisted switching in magnetic insulator thin films with perpendicular magnetic anisotropy

As an in-plane charge current flows in a heavy metal film with spin–orbit coupling, it produces a torque on and thereby switches the magnetization in a neighbouring ferromagnetic metal film. Such spin–orbit torque (SOT)-induced switching has been studied extensively in recent years and has shown higher efficiency than switching using conventional spin-transfer torque. Here we report the SOT-assisted switching in heavy metal/magnetic insulator systems. The experiments used a Pt/BaFe12O19 bilayer where the BaFe12O19 layer exhibits perpendicular magnetic anisotropy. As a charge current is passed through the Pt film, it produces a SOT that can control the up and down states of the remnant magnetization in the BaFe12O19 film when the film is magnetized by an in-plane magnetic field. It can reduce or increase the switching field of the BaFe12O19 film by as much as about 500 Oe when the film is switched with an out-of-plane field.

Switching Behavior of Sharply Pointed Nanomagnets for Logic Applications

We study the switching behavior of variously shaped, near-single-domain nanomagnets for nanomagnet logic (NML) applications. Magnets in the size range of 60 × 100 to 60 × 200 nm, with different lengths and end-shapes are fabricated and characterized, and the results compared to temperature-dependent micromagnetic simulations. In this study, the length of the nanomagnets is more important to the switching properties than is end shape. However, for sharper magnets, temperature fluctuations play a large role in determining the switching field.

Coherent precession in arrays of dipolar-coupled soft magnetic nanodots

Precession modes of rectangular nanodot arrays with diameters of 90 and 150 nm and varied dipolar interaction are systematically studied by vector network analyzer ferromagnetic resonance, and micromagnetic simulations. The large dots reveal two dominant modes: a low frequency edge mode and a high frequency center mode, while in the smaller dots only the edge mode is observed. With increasing dipolar interaction, the precession-mode frequencies increase. The interaction has a stronger influence on the edge mode than on the center mode, resulting in different precession behaviors in different ranges of magnetic fields: well-separated modes in high fields and a merging of edge and center modes at low fields. At low fields and for strong dipolar interaction, coherent precession of the whole dot array is observed.

Econometric modeling and forecasting of natural gas demand for power sector in Bangladesh

Natural gas is the principal indigenous resource for power generation in Bangladesh. Use of other conventional fossil fuels like petroleum and coal are quite insignificant with respect to the use of natural gas. The gas reserve of Bangladesh is depleting fast because of this heavy dependence on gas. Therefore, it is essential to have a realistic assessment of the gas demand pattern in the near future. In this paper the impacts of economic variables like gas price, per capita Gross Domestic Product (GDP) on gas demand were investigated by employing advanced econometric modeling technique. This research uses time series data for modeling the gas demand in Power sector and forecasting the demand up to 2025. Various statistical tests were employed to choose the correct functional form. Results show no significant impact of price on gas demand. Moreover, the forecast obtained from the analysis shows that the gas demand will increase rapidly in future. This research thus concludes that gas demand cannot possibly be controlled by price management and also emphasizes on strengthening the supply side to meet the growing gas demand in the near future.

Spin wave eigenmodes in single and coupled sub-150 nm rectangular permalloy dots

We present the results of a Brillouin light scattering investigation of thermally excited spin wave eigenmodes in square arrays of either isolated rectangular dots of permalloy or twins of dipolarly coupled elements, placed side-by-side or head-to-tail. The nanodots, fabricated by e-beam lithography and lift-off, are 20 nm thick and have the major size D in the range between 90 nm and 150 nm. The experimental spectra show the presence of two main peaks, corresponding to modes localized either at the edges or in the center of the dots. Their frequency dependence on the dot size and on the interaction with adjacent elements has been measured and successfully interpreted on the basis of dynamical micromagnetic simulations. The latter enabled us also to describe the spatial profile of the eigenmodes, putting in evidence the effects induced by the dipolar interaction between coupled dots. In particular, in twinned dots the demagnetizing field is appreciably modified in proximity of the “internal edges” if compared to the “external” ones, leading to a splitting of the edge mode. These results can be relevant for the exploitation of sub-150 nm magnetic dots in new applications, such as magnonic metamaterials, bit-patterned storage media, and nano-magnetic logic devices.

Shape-Dependent Switching Behavior of Exchange-Coupled Nanomagnet Stacks

We explore the switching mechanism of nanomagnet stacks that are patterned from a multilayered film and simultaneously exhibit strong shape anisotropy and antiferromagnetic exchange coupling between the ferromagnetic layers. We study the interplay of these two effects and demonstrate how they determine the switching characteristics of the magnets. Micromagnetic simulations give insight into the details of the magnetization reversal. These findings could be important in spintronic and magnetic logic applications, where both the shape and interlayer interactions can be exploited to tailor the properties of the magnetic building blocks of these devices.

Study of switching behavior of exchange-coupled nanomagnets by transverse magnetization metrology

We investigate the static switching modes of nanomagnets patterned from antiferromagnetically exchange-coupled magnetic multilayers, and compare them to nanomagnets having only dipole coupling between the ferromagnetic layers. Vibrating sample magnetometry experiments, supported by micromagnetic simulations, reveal two distinct switching mechanisms between the exchange-coupled and only dipole-coupled nanomagnets. The exchange-coupled nanomagnets exhibit gradual switching of the layers, dictated by the strong antiferromagnetic exchange coupling present between the layers. However, the layers of the only dipole-coupled nanomagnets show abrupt nucleation/growth type switching. A comprehensive understanding of the switching modes of such layered and patterned systems can add new insight into the reversal mechanisms of similar systems employed for spintronic and magneto-logic device applications.

Ferromagnetic resonance modes of nanomagnetic logic elements

Nanomagnetic logic (NML) has attracted extensive interest as an alternative for digital circuit design due to its advantages of low power consumption and non-volatility. [1-4] NML builds from nano-scale magnets with separation between neighbouring magnets on the order ~20 nm. [3, 4] Through magnetostatic interactions between neighbouring magnets, NML enables the propagation of information and realization of logic functionality. The dynamic properties of NML element are relevant to understand the maximum operation speed and to investigate, e.g., microwave-assisted programming schemes. Note that the precession dynamics of the NML elements are greatly influenced by the elements shape and also their interaction with neighbouring elements [5-7].

Fabrication of pseudo-spin-valve giant magnetoresistance arrays for nanomagnet logic by liftoff and the snow-jet process

In Nanomagnet Logic, an electronic read-out device converts magnetization to electronic signals. A giant magnetoresistance (GMR) stack, with simple deposition procedures, is a good candidate for this purpose. In this paper, the authors propose a way to pattern GMR films with the help of a CO2 snow-jet to simplify device fabrication procedures. A scanning electron microscope and a vibrating sample magnetometer were used to characterize the devices and verify the feasibility of the fabrication method. Results have shown that the CO2 snow-jet process can remove sidewalls of nanopillars that are formed after lift-off of sputtered films, and the nanopillars are uniform in shape and size. This method presents a new way to fabricate electronic readout devices for Nanomagnet Logic circuits.

Edge-Mode Resonance-Assisted Switching of Nanomagnet Logic Elements

We study the ferromagnetic resonance (FMR) modes of arrays consisting of elements of supermalloy-based nanomagnet logic devices: individual rectangular magnets, lengthwise-coupled magnet pairs, and driver-input magnet pairs. In FMR experiments, we observe a center resonance mode for all three samples. Furthermore, edge modes can be resolved for arrays with individual rectangular magnets and lengthwise-coupled magnet pairs. By micromagnetic simulations, the characteristics of the experimentally observed precession modes are elaborated, and distinct edge modes not visible in the experiments are revealed. Based on these numerical investigations, we propose a new addressing scheme to switch a specific element of the driver-input magnet pair based on resonant excitation of the unique edge-mode resonance of the element. This new switching scheme could significantly reduce switching power and increase bit selectivity, and hence programming reliability.