arXiv subjects

Braj Bhusan Singh

Publications and source records attributed to Braj Bhusan Singh.

17 recordsLinked to original sources

arXiv2022

Degenerate skyrmionic states in synthetic antiferromagnets

Topological magnetic textures, characterized by integer topological charge $S$, are potential candidates in future magnetic logic and memory devices, due to their smaller size and expected low threshold current density for their motion. An essential requirement to stabilize them is the Dzyaloshinskii-Moriya interaction (DMI) which promotes a particular chirality, leading to a unique value of $S$ in a given material. However, recently coexistence of skyrmions and antiskyrmions, with opposite topological charge, in frustrated ferromagnets has been predicted using $J_1$--$J_2$--$J_3$ classical Heisenberg model, which opens new perspectives, to use the topological charge as an additional degree of freedom. In this work, we propose another approach of using a synthetic antiferromagnetic (SAF) system, where one of the ferromagnetic (FM) layer has isotropic and the other FM layer has anisotropic DMI to promote the existence of skyrmions and antiskyrmions, respectively. A frustrated interaction arises due to the coupling between the magnetic textures in the FM layers, which enables the stabilization and coexistence of 6 novel elliptical topological textures.

cond-mat.mtrl-sci↗

arXiv2022

Magnetization reversal and domain structures in perpendicular synthetic antiferromagnets prepared on rigid and flexible substrates

Ferromagnetic (FM) layers separated by nonmagnetic metallic spacer layers can exhibit Ruderman Kittel Kasuya Yosida (RKKY) coupling which may lead to a stable synthetic antiferromagnetic (SAF) phase. In this article we study magnetization reversal in [Co/Pt] layers by varying the number of bilayer stacks (Pt/Co) as well as thickness of Ir space layer tIr on rigid Si(100) and flexible polyimide substrates. The samples with tIr = 1.0 nm shows a FM coupling whereas samples with tIr = 1.5 nm shows an AFM coupling between the FM layers. At tIr = 2.0 nm, it shows a bow-tie shaped hysteresis loop indicating a canting of magnetization at the reversal. Higher anisotropy energy as compared to the interlayer exchange coupling (IEC) energy is an indication of the smaller relative angle between the magnetization of lower and upper FM layers. We have also demonstrated the strain induced modification of IEC as well as magnetization reversal phenomena. The IEC shows a slight decrease upon application of compressive strain and increase upon application of tensile strain which indicates the potential of SAFs in flexible spintronics.

cond-mat.mtrl-sci↗

arXiv2021

Spin pumping and inverse spin Hall effect in CoFeB/C$_{60}$ bilayers

Pure spin current based research is mostly focused on ferromagnet (FM)/heavy metal (HM) system. Because of the high spin orbit coupling (SOC) these HMs exhibit short spin diffusion length and therefore possess challenges for device application. Low SOC (elements of light weight) and large spin diffusion length make the organic semiconductors (OSCs) suitable for future spintronic applications. From theoretical model it is explained that, due to $π$ - $σ$ hybridization the curvature of the C$_{60}$ molecules may increase the SOC strength. Here, we have investigated spin pumping and inverse spin hall effect (ISHE) in CoFeB/C$_{60}$ bilayer system using coplanar wave guide based ferromagnetic resonance (CPW-FMR) set-up. We have performed angle dependent ISHE measurement to disentangle the spin rectification effects for example anisotropic magnetoresistance, anomalous Hall effect etc. Further, effective spin mixing conductance (g$_{eff}^{\uparrow\downarrow}$) and spin Hall angle ($θ_{SH}$) for C$_{60}$ have been reported here. The evaluated value for $θ_{SH}$ is 0.055.

cond-mat.mes-hall↗

arXiv2021

Spin pumping and inverse spin Hall effect in CoFeB/IrMn heterostructures

High spin to charge conversion efficiency is the requirement for the spintronics devices which is governed by spin pumping and inverse spin Hall effect (ISHE). In last one decade, ISHE and spin pumping are heavily investigated in ferromagnet/ heavy metal (HM) heterostructures. Recently antiferromagnetic (AFM) materials are found to be good replacement of HMs because AFMs exhibit terahertz spin dynamics, high spin-orbit coupling, and absence of stray field. In this context we have performed the ISHE in CoFeB/ IrMn heterostructures. Spin pumping study is carried out for $Co_{40}Fe_{40}B_{20} (12\ nm)/ Cu (3\ nm)/ Ir_{50}Mn_{50} (t\ nm)/ AlO_{x} (3\ nm)$ samples where \textit{t} value varies from 0 to 10 nm. Damping of all the samples are higher than the single layer CoFeB which indicates that spin pumping due to IrMn is the underneath mechanism. Further the spin pumping in the samples are confirmed by angle dependent ISHE measurements. We have also disentangled other spin rectifications effects and found that the spin pumping is dominant in all the samples. From the ISHE analysis the real part of spin mixing conductance (\textit{$g_{r}^{\uparrow \downarrow}$}) is found to be 0.704 $\pm$ 0.003 $\times$ $10^{18}$ $m^{-2}$.

cond-mat.mes-hall↗

arXiv2020

Effect of spin glass frustration on exchange bias in NiMn/CoFeB bilayers

Exchange bias in ferromagnetic/antiferromagnetic systems can be explained in terms of various interfacial phenomena. Among these spin glass frustration can affect the magnetic properties in exchange bias systems. Here we have studied a NiMn/CoFeB exchange bias system in which spin glass frustration seems to play a crucial role. In order to account the effect of spin glass frustration on magnetic properties, we have performed the temperature and cooling field dependence of exchange bias. We have observed the decrease of exchange bias field (HEB) with cooling field (HFC) whereas there is not significant effect on coercive field (HC). Exponential decay of HEB and HC is found in these exchange bias systems. Further, training effect measurements have been performed to study the spin relaxation mechanism. We have fitted the training effect data with frozen and rotatable spin relaxation model. We have investigated the ratio of relaxation rate of interfacial rotatable and frozen spins in this study. The training effect data are also fitted with various other models. Further, we observed the shifting of peak temperature towards higher temperature with frequency from the ac susceptibility data.

cond-mat.mtrl-sci↗

arXiv2020

Simultaneous observation of anti-damping and inverse spin Hall effect in La$_{0.67}$Sr$_{0.33}$MnO$_{3}$/Pt bilayer system

Manganites have shown potential in spintronics because they exhibit high spin polarization. Here, by ferromagnetic resonance we have studied the damping properties of La$_{0.67}$Sr$_{0.33}$MnO$_{3}$/Pt bilayers which are prepared by oxide molecular beam epitaxy. The damping coefficient ($α$) of La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) single layer is found to be 0.0104. However the LSMO/Pt bilayers exhibit decrease in $α$ with increase in Pt thickness. This decrease in the value of $α$ is probably due to high anti-damping like torque. Further, we have investigated the angle dependent inverse spin Hall effect (ISHE) to quantify the spin pumping voltage from other spin rectification effects such as anomalous Hall effect and anisotropic magnetoresistance. We have observed high spin pumping voltage ($\sim$~20 $ μV$). The results indicate that both anti-damping and spin pumping phenomena are occuring simultaneously.

cond-mat.mtrl-sci↗

arXiv2020

Skyrmion Racetrack memory with an antidot

Skyrmion racetrack memory has a lots of potential in future non-volatile solid state devices. In general such devices require current to nucleate skyrmions via spin transfer torque (STT) effect. Further the current is also required to drive the skyrmions in the nanowire device. However the current applied during nucleation of successive skyrmions may have unwanted perturbation \emph{viz.} Joule heating and skyrmion Hall effect, on the propagation of previously generated skyrmions. Therefore new methodology is required to decouple the generation and propagation of skyrmions. Here we present a novel route via micromagnetic simulation for generation of skyrmions from triangular antidot structure in a ferromagnetic nanotrack using local oersted field. Antidots are holes in a magnetic nanoelement. Controlled skyrmion injection can be achieved by tuning the dimensions of the antidots that are placed at either end of the nanotrack. Multiple skyrmions can be simultaneously generated by incorporating more number of antidots. Here we propose a novel design to realise skyrmionic racetrcak memory where one can individually generate and manipulate the skyrmions within the nanotrack.

cond-mat.mtrl-sci↗

arXiv2020

Study of magnetic interface and its effect in Fe/NiFe bilayers of alternating order

We present a comprehensive study on the magnetization reversal in Fe/NiFe bilayer system by alternating the order of the magnetic layers. All the samples show growth-induced uniaxial magnetic anisotropy due to oblique angle deposition technique. Strong interfacial exchange coupling between the Fe and NiFe layers leads to the single-phase hysteresis loops in the bilayer system. The strength of coupling being dependent on the interface changes upon alternating the order of magnetic layers. The magnetic parameters such as coercivity HC, and anisotropy field HK become almost doubled when NiFe layer is grown over the Fe layers. This enhancement in the magnetic parameters is primarily dependent on the increase of the thickness and magnetic moment of Fe-NiFe interfacial layer as revealed from the polarized neutron reectivity (PNR) data of the bilayer samples. The difference in the thickness and magnetization of the Fe-NiFe interfacial layer indicates the modification of the microstructure by alternating the order of the magnetic layers of the bilayers. The interfacial magnetic moment increased by almost 18 % when NiFe layer is grown over the Fe layer. In spite of the different values of anisotropy fields and modified interfacial exchange coupling, the Gilbert damping constant values of the ferromagnetic bilayers remain similar to single NiFe layer.

cond-mat.mtrl-sci↗

arXiv2020

Strain engineered domain structure and their relaxation in perpendicularly magnetized Co/Pt deposited on flexible polyimide

The demand of fast and power efficient spintronics devices with flexibility requires additional energy for magnetization manipulation. Stress/and strain have shown their potentials for tuning magnetic properties to the desired level. Here, we report a systematic study for the effect of both tensile and compressive stresses on the magnetic anisotropy (MA). Further the effect of stress on the domain structure and magnetization relaxation mechanism in a perpendicularly magnetized Co/Pt film has been studied. It is observed that a minimal in-plane tensile strain has increased the coercivity of the film by 38$\%$ of its initial value, while a very small change of coercivity has been found under compressive strain. The size of ferromagnetic domains decreases under tensile strain, while no change is observed under the compressive strain. Magnetization relxation measured at sub-coercive fields yields longer relaxation time in the strained state.

cond-mat.mtrl-sci↗

arXiv2020

Large spin Hall angle and spin mixing conductance in highly resistive antiferromagnetic Mn2Au

Antiferromagnetic (AFM) materials recently have shown interest in the research in spintronics due to its zero stray magnetic field, high anisotropy, and spin orbit coupling. In this context, the bi-metallic AFM Mn2Au has drawn attention because it exhibits unique properties and its Neel temperature is very high. Here, we report spin pumping and inverse spin Hall effect investigations in Mn2Au and CoFeB bilayer system using ferromagnetic resonance. We found large spin Hall angle θ_SH = 0.22

cond-mat.mtrl-sci↗

arXiv2019

High spin mixing conductance and spin interface transparency at $Co_2Fe_{0.4}Mn_{0.6}Si$ Heusler alloy and Pt interface

Ferromagnetic materials exhibiting low magnetic damping ($α$) and moderately high saturation magnetization are required from the viewpoints of generation, transmission and detection of spin wave. Since spin-to-charge conversion efficiency is another important parameter, high spin mixing conductance ($g_{r}^{\uparrow \downarrow}$) is the key for efficient spin-to-charge conversion. Full Heusler alloys e.g. $Co_2Fe_{0.4}Mn_{0.6}Si$ (CFMS), which are predicted to be 100$\%$ spin polarized, possess low $α$. However, the $g_{r}^{\uparrow \downarrow}$ at the interface between CFMS and a paramagnet has not fully been understood. Here, we report the investigations of spin pumping and inverse spin Hall effect in $CFMS/Pt$ bilayers. Damping analysis indicates the presence of significant spin pumping at the interface of CFMS and Pt, which is also confirmed by the detection of inverse spin Hall voltage. We show that in CFMS/Pt the $g_{r}^{\uparrow \downarrow}$ (1.77$\times$10$^{20}$m$^{-2}$) and interface transparency (84$\%$) are higher compared to values reported for other ferromagnet/heavy metal systems.

cond-mat.mtrl-sci↗

arXiv2019

Superconductivity in Ag implanted Au thin film

Au and Ag are known to be metallic and none of them have shown independently superconductivity. Here we show superconductivity in Ag implanted Au thin films. Ag implanted Au films of different thicknesses have been studied using four probe resistance versus temperature measurements. We have also measured the pristine samples (i.e. without any implantation) to compare the transport properties. The superconducting transition occurs around ~2 K (TC) for Ag implanted 20 nm Au film (the lowest resistance observed below TC is ~10-6 Ohm). Change of Tc towards lower temperatures has been observed under the effect of increasing current and magnetic field which endorses the superconducting state. Further the V~I measurements were carried out to verify the superconducting nature. On the other hand, the pristine samples clearly did not exhibit superconductivity down to 1.6 K. Our results open an avenue for further exploring this type of novel superconductivity for future applications.

cond-mat.supr-con↗

arXiv2019

Exchange bias in Fe/Ir20Mn80 bilayers: Role of spin-glass like interface and bulk antiferromagnet spins

We have performed magnetic measurements like temperature (T), cooling field (HFC) dependence of exchange bias (EB) and training effect to investigate the magnetic nature of the interface of the Fe/Ir20Mn80 systems. Thin film bilayer samples of different thicknesses of Ir20Mn80 have been prepared by dc magnetron sputtering at room temperature. The variation of exchange bias field (HEB) with the increase in thickness of Ir20Mn80 predicts the antiferromagnet (AFM) bulk spins contribution to EB. Exponential decay of HEB and coercive field (HC) with temperature reveals the presence of spin glass (SG) like interface. Also, the decrease of HEB with increasing HFC confirms the SG like frustration at the interface. Further, the fitting of training effect experimental data envisages the presence of frozen and rotatable spins at the magnetically frustrated interface of these EB systems.

cond-mat.mtrl-sci↗

arXiv2018

A novel chiral spin texture: Antiferromagnetic Skyrmionium

Exotic spin textures viz. chiral domain wall, vortices, skyrmion, skyrmionium, etc. have recently emerged as active field of research because of their potential applications in high density data storage technology and logic gate computing. Magnetic skyrmionium is a skyrmion like soliton, which carries zero topological quantum number. Skyrmioniums are superior to conventional skyrmions in ferromagnets due to their negligible skyrmion hall effect and higher velocity. The physical properties of both skyrmion and skyrmionium have been investigated rigorously in ferromagnetic systems. Recent observations hint that such chiral spin structures in antiferromagnetic (AFM) systems are more promising in comparison to the ferromagnetic ones because of their robustness towards external perturbation, absence of Skyrmion hall effect, etc. However skyrmionium in AFM materials are not reported in literature so far. In this work, we demonstrate that skyrmionium can be created and stabilized in AFM materials by application of spin polarized current in an experimentally feasible geometry. We have further studied the dynamics of AFM skyrmionium by applying spin polarized current.

cond-mat.mes-hall↗

arXiv2018

Size and shape of skyrmions for variable Dzyaloshinskii-Moriya interaction and uniaxial anisotropy

We have performed micromagnetic simulations to study the formation of skyrmions in ferromagnetic elements with different shapes having perpendicular anisotropy. The strength of Dzyaloshinskii-Moriya interaction (D) and uniaxial anisotropy (K) are varied to elucidate the regime in which skyrmion formation can take place. It is found that for a certain combination of D and K skyrmion formation does not happen. Further we also observed that for large D and small K values, finite size effect dominates which in turn hinders formation of typical N'eel (spherical) skyrmions. However the resulting magnetic phase is skyrmionic in nature and has different shape. We also have found that the shape of the magnetic nano element has a significant role in determining the final magnetic state in addition to the competing D and K values.

cond-mat.mes-hall↗

arXiv2017

Study of spin pumping in Co thin film vis-a-vis seed and capping layer using ferromagnetic resonance spectroscopy

We investigated the dependence of the seed [Ta/Pt, Ta/Au] and capping [Pt/Ta, Au/Ta] layers on spin pumping effect in the ferromagnetic 3 nm thick Co thin film using ferromagnetic resonance spectroscopy. The data is fitted with Kittel equation to evaluate damping constant and g-factor. A strong dependence of seed and capping layers on spin pumping has been discussed. The value of damping constant {alpha} is found to be relatively large i.e. 0.0326 for the Ta{3}/Pt{3}/Co{3}/Pt{3}/Ta{3} {nm} multi-layer structure, while it is 0.0104 for Ta{3}/Co{3}/Ta{3} {nm}. Increase in {alpha} is observed due to Pt layer that works as a good sink for spins due to high spin orbit coupling. In addition, we measured the effective spin conductance = 2.0e18 m-2 for the trilayer structure Pt{3}/Co{3}/Pt{3} {nm} as a result of the enhancement in {alpha} relative to its bulk value. We observed that the evaluated g-factor decreases as effective demagnetizing magnetic field increases in all the studied samples. The azimuthal dependence of magnetic resonance field and line width showed relatively high anisotropy in the trilayer Ta{3}/Co{3}/Ta{3} {nm} structure.

cond-mat.mes-hall↗

arXiv2013

Magnetic and Electronic Structure study of Fe/MgO/Fe/Co Multilayer Stack Deposited by E-Beam Evaporation

Present work investigates the magnetic and electronic structure of MgO/Fe/MgO/Fe/Co/Au multilayer stack grown on Si(100) substrates by electron beam evaporation method. X-ray diffraction study depicts polycrystalline nature of the multilayers. Results obtained from vibrating sample magnetometry (VSM) and near-edge X-ray absorption fine structure spectra (NEXAFS) at Fe & Co L- and Mg & O K-edges are applied to understand the magnetic and electronic properties of this stack and its interface properties. While the spectral features of Fe L-edge spectrum recorded by surface sensitive total electron yield (TEY) mode shows the formation of FeOx at the Fe/MgO interface, the bulk sensitive total fluorescence yield (TFY), shows Fe in metallic nature. Co L-edge spectrum reveals the presence of metallic nature of cobalt in both TEY and TFY modes. Above results are well correlated with X-ray reflectometry.

cond-mat.mtrl-sci↗