arXiv subjects

Jaivardhan Sinha

Publications and source records attributed to Jaivardhan Sinha.

At least 19 recordsLinked to original sources

arXiv2019

Observation of angle-dependent mode conversion and mode hopping in 2D annular antidot lattice

We report spin-wave excitations in annular antidot lattice fabricated from 15 nm-thin Ni80Fe20 flm. The nanodots of 170 nm diameters are embedded in the 350 nm (diameter) antidot lattice to form the annular antidot lattice, which is arranged in a square lattice with edge-to-edge separation of 120 nm. A strong anisotropy in the spin-wave modes are observed with the change in orientation angle (ϕ) of the in-plane bias magnetic field by using Time-resolved Magneto-optic Kerr microscope. A fattened fourfold rotational symmetry, mode hopping and mode conversion leading to mode quenching for three prominent spin-wave modes are observed in this lattice with the variation of the bias field orientation. Micromagnetic simulations enable us to successfully reproduce the measured evolution of frequencies with the orientation of bias magnetic field, as well as to identify the spatial profles of the modes. The magnetostatic field analysis, suggest the existence of magnetostatic coupling between the dot and antidot in annular antidot sample. Further local excitations of some selective spin-wave modes using numerical simulations showed the anisotropic spin-wave propagation through the lattice.

cond-mat.mes-hall↗

arXiv2019

Observation of band narrowing and mode conversion in two-dimensional binary magnonic crystal

We introduce a new type of binary magnonic crystal, where Ni$_{80}$Fe$_{20}$ nanodots of two different sizes are diagonally connected forming a unit and those units are arranged in a square lattice. The magnetization dynamics of the sample is measured by using time-resolved magneto-optical Kerr effect microscope with varying magnitude and in-plane orientation ($ϕ$) of the bias magnetic field. Interestingly, at $ϕ=0^{\circ}$, the spin-wave mode profiles show frequency selective spatial localization of spin-wave power within the array. With the variation of $ϕ$ in the range $0^{\circ}<ϕ\leq 45^{\circ}$, we observe band narrowing due to localized to extended spin-wave mode conversion. Upon further increase of $ϕ$, the spin-wave modes slowly lose the extended nature and become fully localized again at 90$^{\circ}$. We have extensively demonstrated the role of magnetostatic stray field distribution on the rotational symmetries obtained for the spin-wave modes. From micromagnetic simulations, we find that the dipole-exchange coupling between the nano-dots leads to remarkable modifications of the spin-wave mode profiles when compared with arrays of individual small and large dots. Numerically, we also show that the physical connection between the nano-dots provides more control points over the spin-wave propagation in the lattice at different orientations of bias magnetic field. This new type of binary magnonic crystal may find potential applications in magnonic devices such as spin-wave waveguide, filter, coupler, and other on-chip microwave communication devices.

cond-mat.mes-hall↗

arXiv2019

Effects of Nanodots Shape and Lattice Constants on the Spin Wave Dynamics of Patterned Permalloy Dots

Micromagnetic simulations studies on Permalloy (Ni80Fe20) nanodot with different shape and edge-to-edge separation (s) down to 25nm arranged in square lattice are reported. We observe the significant variation of spin-wave (SW) dynamics of nanodots of different shapes (triangular, diamond and hexagon) and of fixed dot diameter 100nm with varying s. Modes for single dot are transformed in an array into multiple quantized, edge and centre modes for different shapes and edge-to-edge separations, with different spin wave frequencies and peak intensities. Specifically, in the triangular dot sample, a broad range of mode frequencies is observed with highest SW frequency 14.7 GHz. For separation less than 100nm, the SW frequencies undergoes significant modification due to the varying nature of the magnetostatic and dipolar interaction in the array while for separation above 100nm, the SW frequency mostly remain constant. The power profiles confirm the nature of the observed modes. The spatial profiles of magnetostatic field are determined by a combination of internal magnetic-field profiles within the nanodots and the magnetostatic fields within the lattice. The inter-dots interaction of magnetostatic field shows dipolar and quadrupole contributions for all the shapes. Interestingly, vortex states with shifted core and polarity are observed in the array for all the shapes at Hbias = 0. Our results provide important understanding about the tunability of SW spectra in the array of triangular, diamond and hexagon shaped nanoelements.

cond-mat.mes-hall↗

arXiv2019

All optical detection of picosecond spin-wave dynamics in two-dimensional annular antidot lattice

Novel magnetic structures with precisely controlled dimensions and shapes at the nanoscale have potential applications in spin logic, spintronics and other spin-based communication devices. We report the fabrication of two-dimensional bi-structure magnonic crystal in the form of embedded nanodots in a periodic Ni80Fe20 antidot lattice structure (annular antidot) by focused ion-beam lithography. The spin-wave spectra of the annular antidot sample,studied for the first time by a time-resolved magneto-optic Kerr effect microscopy show a remarkable variation with bias field, which is important for the above device applications. The optically induced spin-wave spectra show multiple modes in the frequency range 14.7 GHz to 3.5 GHz due to collective interactions between the dots and antidots as well as the annular elements within the whole array. Numerical simulations qualitatively reproduce the experimental results, and simulated mode profiles reveal the spatial distribution of the spin wave modes and internal magnetic fields responsible for these observations. It is observed that the internal field strength increases by about 200 Oe inside each dot embedded within the hole of annular antidot lattice as compared to pure antidot lattice and pure dot lattice. The stray field for the annular antidot lattice is found to be significant (0.8kOe) as opposed to the negligible values of the same for the pure dot lattice and pure antidot lattice. Our findings open up new possibilities for development of novel artificial crystals.

cond-mat.mes-hall↗

arXiv2017

All-optical Detection of Spin Hall Angle in W/CoFeB/SiO2 Heterostructures by Varying Tungsten Layer Thickness

The development of advanced spintronics devices hinges on the efficient generation and utilization of pure spin current. In materials with large spin-orbit coupling, the spin Hall effect may convert charge current to pure spin current and a large conversion efficiency, which is quantified by spin Hall angle (SHA), is desirable for the realization of miniaturized and energy efficient spintronic devices. Here, we report a giant SHA in beta-tungsten (\b{eta}-W) thin films in Sub/W(t)/Co20Fe60B20(3 nm)/SiO2(2 nm) heterostructures with variable W thickness. We employed an all-optical time-resolved magneto-optical Kerr effect microscope for an unambiguous determination of SHA using the principle of modulation of Gilbert damping of the adjacent ferromagnetic layer by the spin-orbit torque from the W layer. A non-monotonic variation of SHA with W layer thickness (t) is observed with a maximum of about 0.4 at about t = 3 nm, followed by a sudden reduction to a very low value at t = 6 nm. This variation of SHA with W-thickness correlates well with the thickness dependent structural phase transition and resistivity variation of W above the spin diffusion length of W, while below this length the interfacial electronic effect at W/CoFeB influences the estimation of SHA.

cond-mat.mtrl-sci↗

arXiv2016

Spin orbit effects in CoFeB/MgO hetereostructures with heavy metal underlayers

We study effects originating from the strong spin orbit coupling in CoFeB/MgO heterostructures with heavy metal (HM) underlayers. The perpendicular magnetic anisotropy at the CoFeB/MgO interface, the spin Hall angle of the heavy metal layer, current induced torques and the Dzyaloshinskii-Moriya interaction at the HM/CoFeB interfaces are studied for films in which the early 5d transition metals are used as the HM underlayer. We show how the choice of the HM layer influences these intricate spin orbit effects that emerge within the bulk and at interfaces of the heterostructures.

cond-mat.mes-hall↗

arXiv2016

Giant nonreciprocal emission of spin waves in Ta/Py bilayers

Spin waves are propagating disturbances in the magnetization of magnetic materials. One of their interesting properties is the nonreciprocity, exhibiting that their amplitude depends on the magnetization direction. Nonreciprocity in spin waves is of great interest in both fundamental science and applications, as it offers an extra knob to control the flow of waves for the technological fields of logics and switch applications. We show a high nonreciprocity in spin waves from Ta/Py bilayer systems with out-of-plane magnetic fields. The nonreciprocity depends on the thickness of Ta underlayer which is found to induce an interfacial anisotropy. The origin of observed high nonreciprocity is twofold; different polarities of the in-plane magnetization due to different angles of canted out-of-plane anisotropy and the spin pumping effect at the Ta/Py interface. Our findings provide an opportunity to engineer highly efficient nonreciprocal spin wave based applications such as nonreciprocal microwave devices, magnonic logic gates, and information transports.

cond-mat.mtrl-sci↗

arXiv2015

Enhanced orbital magnetic moments in magnetic heterostructures with interface perpendicular magnetic anisotropy

We have studied the magnetic layer thickness dependence of the orbital magnetic moment in magnetic heterostructures to identify contributions from interfaces. Three different heterostructures, Ta/CoFeB/MgO, Pt/Co/AlO$_x$ and Pt/Co/Pt, which possess significant interface contribution to the perpendicular magnetic anisotropy, are studied as model systems. X-ray magnetic circular dichroism spectroscopy is used to evaluate the relative orbital moment, i.e. the ratio of the orbital to spin moments, of the magnetic elements constituting the heterostructures. We find that the relative orbital moment of Co in Pt/Co/Pt remains constant against its thickness whereas the moment increases with decreasing Co layer thickness for Pt/Co/AlO$_x$, suggesting that a non-zero interface orbital moment exists for the latter system. For Ta/CoFeB/MgO, a non-zero interface orbital moment is found only for Fe. X-ray absorption spectra shows that a particular oxidized Co state in Pt/Co/AlO$_x$, absent in other heterosturctures, may give rise to the interface orbital moment in this system. These results show element specific contributions to the interface orbital magnetic moments in ultrathin magnetic heterostructures.

cond-mat.mtrl-sci↗

arXiv2015

Current driven asymmetric magnetization switching in perpendicularly magnetized CoFeB/MgO heterostructures

The flow of in-plane current through ultrathin magnetic heterostructures can cause magnetization switching or domain wall nucleation owing to bulk and interfacial effects. Within the magnetic layer, the current can create magnetic instabilities via spin transfer torques (STT). At interface(s), spin current generated from the spin Hall effect in a neighboring layer can exert torques, referred to as the spin Hall torques, on the magnetic moments. Here, we study current induced magnetization switching in perpendicularly magnetized CoFeB/MgO heterostructures with a heavy metal (HM) underlayer. Depending on the thickness of the HM underlayer, we find distinct differences in the inplane field dependence of the threshold switching current. The STT is likely responsible for the magnetization reversal for the thinner underlayer films whereas the spin Hall torques cause the switching for thicker underlayer films. For the latter, we find differences in the switching current for positive and negative currents and initial magnetization directions. We find that the growth process during the film deposition introduces an anisotropy that breaks the symmetry of the system and causes the asymmetric switching. The presence of such symmetry breaking anisotropy enables deterministic magnetization switching at zero external fields.

cond-mat.mtrl-sci↗

arXiv2014

Anomalous temperature dependence of current induced torques in CoFeB|MgO heterostructures with Ta based underlayers

We have studied the underlayer thickness and temperature dependences of the current induced effective field in CoFeB|MgO heterostructures with Ta based underlayers. The underlayer thickness at which the effective field saturates is found to be different between the two orthogonal components of the effective field, i.e. the damping-like term tends to saturate at smaller underlayer thickness than the field-like term. For large underlayer thickness films in which the effective field saturates, we find that the temperature significantly influences the size of the effective field. A striking difference is found in the temperature dependence of the two components: the damping-like term decreases whereas the field-like term increases with increasing temperature. Using a simple spin diffusion-spin transfer model, we find that all of these results can be accounted for provided the real and imaginary parts of an effective spin mixing conductance are negative. These results imply that either spin transport in this system is different from conventional metallic interfaces or effects other spin diffusion into the magnetic layer need to be taken account in order to model the system accurately.

cond-mat.mes-hall↗

arXiv2014

Interface control of the magnetic chirality in CoFeB|MgO heterosctructures with heavy metal underlayers

Recent advances in the understanding of spin orbital effects in ultrathin magnetic heterostructures have opened new paradigms to control magnetic moments electrically. The Dzyaloshinskii-Moriya interaction (DMI) is said to play a key role in forming a Neel-type domain wall that can be driven by the spin Hall torque, a torque resulting from the spin current generated in a neighboring non-magnetic layer via the spin Hall effect. Here we show that the strength and sign of the DMI can be changed by modifying the adjacent heavy metal underlayer (X) in perpendicularly magnetized X|CoFeB|MgO heterstructures. Albeit the same spin Hall angle, a domain wall moves along or against the electron flow depending on the underlayer. We find that the sense of rotation of a domain wall spiral11 is reversed when the underlayer is changed from Hf to W and the strength of DMI varies as the number of 5d electrons of the heavy metal layer changes. The DMI can even be tuned by adding nitrogen to the underlayer, thus allowing interface engineering of the magnetic texture in ultrathin magnetic heterostructures.

cond-mat.mes-hall↗

arXiv2013

Interface control of the magnetic chirality in TaN|CoFeB|MgO heterosctructures

Recent advances in the understanding of spin orbital effects in ultrathin magnetic heterostructures have opened new paradigms to control magnetic moments electrically. The Dzyaloshinskii-Moriya interaction (DMI) is said to play a key role in forming a Neel-type domain wall that can be driven by the spin Hall torque, a torque resulting from the spin current generated in a neighboring non-magnetic layer via the spin Hall effect. Here we show that the sign of the DMI, which determines the direction to which a domain wall moves with current, can be changed by modifying the adjacent non-magnetic layer. We find that the sense of rotation of a domain wall spiral is reversed when the Ta underlayer is doped with nitrogen in Ta|CoFeB|MgO heterostructures. The spin Hall angle of the Ta and nitrogen doped Ta underlayers carry the same sign, suggesting that the sign of the DMI is defined at the interface. Depending on the sense of rotation, spin transfer torque and spin Hall torque can either compete or assist each other, thus influencing the efficiency of moving domain walls with current.

cond-mat.mes-hall↗

arXiv2013

Enhanced interface perpendicular magnetic anisotropy in Ta|CoFeB|MgO using nitrogen doped Ta underlayers

We show that the magnetic characteristics of Ta|CoFeB|MgO magnetic heterostructures are strongly influenced by doping the Ta underlayer with nitrogen. In particular, the saturation magnetization drops upon doping the Ta underlayer, suggesting that the doped underlayer acts as a boron diffusion barrier. In addition, the thickness of the magnetic dead layer decreases with increasing nitrogen doping. Surprisingly, the interface magnetic anisotropy increases to ~1.8 erg/cm2 when an optimum amount of nitrogen is introduced into the Ta underlayer. These results show that nitrogen doped Ta serves as a good underlayer for Spintronics applications including magnetic tunnel junctions and domain wall devices.

cond-mat.mtrl-sci↗

arXiv2012

Layer thickness dependence of the current induced effective field vector in Ta|CoFeB|MgO

The role of current induced effective magnetic field in ultrathin magnetic heterostructures is increasingly gaining interest since it can provide efficient ways of manipulating magnetization electrically. Two effects, known as the Rashba spin orbit field and the spin Hall spin torque, have been reported to be responsible for the generation of the effective field. However, quantitative understanding of the effective field, including its direction with respect to the current flow, is lacking. Here we show vector measurements of the current induced effective field in Ta|CoFeB|MgO heterostructrures. The effective field shows significant dependence on the Ta and CoFeB layers' thickness. In particular, 1 nm thickness variation of the Ta layer can result in nearly two orders of magnitude difference in the effective field. Moreover, its sign changes when the Ta layer thickness is reduced, indicating that there are two competing effects that contribute to the effective field. The relative size of the effective field vector components, directed transverse and parallel to the current flow, varies as the Ta thickness is changed. Our results illustrate the profound characteristics of just a few atomic layer thick metals and their influence on magnetization dynamics.

cond-mat.mtrl-sci↗

arXiv2011

Large amplitude microwave emission and reduced nonlinear phase noise in Co2Fe(Ge0.5Ga0.5) Heusler alloy based pseudo spin valve nanopillars

We have studied microwave emission from a current-perpendicular-to-plane pseudo spin valve nanopillars with Heusler alloy Co2Fe(Ga0.5Ge0.5) electrodes. Large emission amplitude exceeding 150 nV/Hz^0.5, partly owing to the large magnetoresistance, and narrow generation linewidth below 10 MHz are observed. We also find that the linewidth shows significant dependence on the applied field magnitude and its angle within the film plane. A minimum in the linewidth is observed when the slope of the frequency versus current becomes near zero. This agrees with theoretical prediction that takes into account non-linear phase noise as a source for linewidth broadening.

cond-mat.mes-hall↗

arXiv2011

Stress controlled magnetic properties of Cobalt nanowires

We investigate the magnetic properties of a composite comprising of ferromagnetic Cobalt nanowires embedded in nanoporous anodized alumina template. We observe unusual increase in, the saturation magnetization and the coercive field, of the nanowires below 100 K. We also report the appearance of an unusual exchange bias effect in nanowires below 100 K. We argue our results can be understood on the basis of a competition between different magnetic energy scales induced by significant stresses acting on the nanowires at low temperatures. The composite behaves as an effective medium in which the magnetic anisotropy of nanowires can be conveniently controlled via stress on the nanowires.

cond-mat.mes-hall↗

arXiv2010

Driven weak to strong pinning crossover in partially nanopatterned 2H-NbSe2 single crystal

Investigations into the heterogeneous pinning properties of the vortex state created by partially nano-patterning single crystals of 2H-NbSe2 reveal an atypical magnetization response which is significantly drive dependent. Analysis of the magnetization response shows non-monotonic behavior of the magnetization relaxation rate with varying magnetic field sweep rate. With all the patterned pinning centers saturated with vortices, we find that the pinning force experienced by the vortices continues to increase with increasing drive. Our studies reveal an unconventional dynamic weak to strong pinning crossover where the flow of the vortex state appears to be hindered or jammed as it is driven harder through the interstitial voids in the patterned pinning lattice.

cond-mat.supr-con↗

arXiv2009

Giant low frequency velocity fluctuations in a driven vortex lattice

The driven state of a well-ordered flux line lattice in a single crystal of 2H-NbSe2 in the time domain has revealed the presence of substantial fluctuations in velocity, with large and distinct time periods (~ seconds). A superposition of a periodic drive in the driven vortex lattice causes distinct changes in these fluctuations. We propose that prior to onset of the peak effect there exist hithertofore unexplored regime of coherent dynamics, with unexpected behaviour in velocity fluctuations.

cond-mat.supr-con↗