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P. K. Muduli

Publications and source records attributed to P. K. Muduli.

At least 19 recordsLinked to original sources

Magnetic Proximity induced efficient charge-to-spin conversion in large area PtSe$_{2}$/Ni$_{80}$Fe$_{20}$ heterostructures

As a topological Dirac semimetal with controllable spin-orbit coupling and conductivity, PtSe$_2$, a transition-metal dichalcogenide, is a promising material for several applications from optoelectric to sensors. However, its potential for spintronics applications is yet to be explored. In this work, we demonstrate that PtSe$_{2}$/Ni$_{80}$Fe$_{20}$ heterostructure can generate a large damping-like current-induced spin-orbit torques (SOT), despite the absence of spin-splitting in bulk PtSe$_{2}$. The efficiency of charge-to-spin conversion is found to be $(-0.1 \pm 0.02)$~nm$^{-1}$ in PtSe$_{2}$/Ni$_{80}$Fe$_{20}$, which is three times that of the control sample, Ni$_{80}$Fe$_{20}$/Pt. Our band structure calculations show that the SOT due to the PtSe$_2$ arises from an unexpectedly large spin splitting in the interfacial region of PtSe$_2$ introduced by the proximity magnetic field of the Ni$_{80}$Fe$_{20}$ layer. Our results open up the possibilities of using large-area PtSe$_{2}$ for energy-efficient nanoscale devices by utilizing the proximity-induced SOT.

cond-mat.mtrl-sci

Large spin-to-charge conversion at the two-dimensional interface of transition metal dichalcogenides and permalloy

Spin-to-charge conversion is an essential requirement for the implementation of spintronic devices. Recently, monolayers of semiconducting transition metal dichalcogenides (TMDs) have attracted considerable interest for spin-to-charge conversion due to their high spin-orbit coupling and lack of inversion symmetry in their crystal structure. However, reports of direct measurement of spin-to-charge conversion at TMD-based interfaces are very much limited. Here, we report on the room temperature observation of a large spin-to-charge conversion arising from the interface of Ni$_{80}$Fe$_{20}$ (Py) and four distinct large area ($\sim 5\times2$~mm$^2$) monolayer (ML) TMDs namely, MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$. We show that both spin mixing conductance and the Rashba efficiency parameter ($λ_{IREE}$) scales with the spin-orbit coupling strength of the ML TMD layers. The $λ_{IREE}$ parameter is found to range between $-0.54$ and $-0.76$ nm for the four monolayer TMDs, demonstrating a large spin-to-charge conversion. Our findings reveal that TMD/ferromagnet interface can be used for efficient generation and detection of spin current, opening new opportunities for novel spintronic devices.

physics.app-ph

Direct measurement of interfacial Dzyaloshinskii-Moriya interaction at the MoS$_{\rm 2}$/Ni$_{80}$Fe$_{20}$ interface

We report on a direct measurement of sizable interfacial Dzyaloshinskii-Moriya interaction (iDMI) at the interface of two-dimensional transition metal dichalcogenide (2D-TMD), MoS$_{\rm 2}$ and Ni$_{80}$Fe$_{20}$ (Py) using Brillouin light scattering spectroscopy. A clear asymmetry in spin-wave dispersion is measured in MoS$_{\rm 2}$/Py/Ta, while no such asymmetry is detected in the reference Py/Ta system. A linear scaling of the DMI constant with the inverse of Py thickness indicates the interfacial origin of the observed DMI. We further observe an enhancement of DMI constant in three to four layer MoS$_{\rm 2}$/Py system (by 56$\%$) as compared to 2 layer MoS$_{\rm 2}$/Py which is caused by a higher density of MoO$_{\rm 3}$ defect species in the case of three to four layer MoS$_{\rm 2}$. The results open possibilities of spin-orbitronic applications utilizing the 2D-TMD based heterostructures.

physics.app-ph

Evaluation of spin diffusion length and spin Hall angle of antiferromagnetic Weyl semimetal Mn$_3$Sn

Antiferromagnetic Weyl semimetal Mn$_3$Sn has shown to generate strong intrinsic anomalous Hall effect (AHE) at room temperature, due to large momentum-space Berry curvature from the time-reversal symmetry breaking electronic bands of the Kagome planes. This prompts us to investigate intrinsic spin Hall effect, a transverse phenomenon with identical origin as the intrinsic AHE. We report inverse spin Hall effect experiments in nanocrystalline Mn$_3$Sn nanowires at room temperature using spin absorption method which enables us to quantitatively derive both the spin diffusion length and the spin Hall angle in the same device. We observed clear absorption of the spin current in the Mn$_3$Sn nanowires when kept in contact with the spin transport channel of a lateral spin-valve device. We estimate spin diffusion length $λ_{s(Mn_3Sn)}$ $\sim$0.75 $\pm$0.67 nm from the comparison of spin signal of an identical reference lateral spin valve without Mn$_3$Sn nanowire. From inverse spin Hall measurements, we evaluate spin Hall angle $θ_{SH}$ $\sim$5.3 $\pm$ 2.4 $\%$ and spin Hall conductivity $σ_{SH}$ $\sim$46.9 $\pm$ 3.4 ($\hbar/e$) ($Ω$ cm)$^{-1}$. The estimated spin Hall conductivity agrees with both in sign and magnitude to the theoretically predicted intrinsic $σ_{SH}^{int}$ $\sim$36-96 ($\hbar/e$) ($Ω$ cm)$^{-1}$. We also observed anomalous Hall effect at room temperature in nano-Hall bars prepared at the same time as the spin Hall devices. Large anomalous Hall conductivity along with adequate spin Hall conductivity makes Mn$_3$Sn a promising material for ultrafast and ultrahigh-density spintronics devices.

cond-mat.mes-hall

Large spin current generation by the spin Hall effect in mixed crystalline phase Ta thin films

Manipulation of the magnetization in heavy-metal/ferromagnetic bilayers via the spin-orbit torque requires high spin Hall conductivity of the heavy metal. We measure inverse spin Hall voltage using a co-planar wave-guide based broadband ferromagnetic resonance set-up in Py/Ta system with varying crystalline phase of Ta. We demonstrate a strong correlation between the measured spin mixing conductance and spin Hall conductivity with the crystalline phase of Ta thin films. We found a large spin Hall conductivity of $-2439~(\hbar/e)~Ω^{-1}$cm$^{-1}$ for low-resistivity (68 $μΩ-$cm) Ta film having mixed crystalline phase, which we attribute to an extrinsic mechanism of the spin Hall effect.

cond-mat.mes-hall

Detection of the interfacial exchange field at a ferromagnetic insulator-nonmagnetic metal interface with pure spin currents

At the interface between a nonmagnetic metal (NM) and a ferromagnetic insulator (FI) spin current can interact with the magnetization, leading to a modulation of the spin current. The interfacial exchange field at these FI-NM interfaces can be probed by placing the interface in contact with the spin transport channel of a lateral spin valve (LSV) device and observing additional spin relaxation processes. We study interfacial exchange field in lateral spin valve devices where Cu spin transport channel is in proximity with ferromagnetic insulator EuS (EuS-LSV) and yttrium iron garnet Y$_3$Fe$_5$O$_{12}$ (YIG-LSV). The spin signals were compared with reference lateral spin valve devices fabricated on nonmagnetic Si/SiO$_2$ substrate with MgO or AlO$_x$ capping. The nonlocal spin valve signal is about 4 and 6 times lower in the EuS-LSV and YIG-LSV, respectively. The suppression in the spin signal has been attributed to enhanced surface spin-flip probability at the Cu-EuS (or Cu-YIG) interface due to interfacial spin-orbit field. Besides spin signal suppression we also found widely observed low temperature peak in the spin signal at $T \sim$30 K is shifted to higher temperature in the case of devices in contact with EuS or YIG. Temperature dependence of spin signal for different injector-detector distances reveal fluctuating exchange field at these interfaces cause additional spin decoherence which limit spin relaxation time in addition to conventional sources of spin relaxation. Our results show that temperature dependent measurement with pure spin current can be used to probe interfacial exchange field at the ferromagnetic insulator-nonmagnetic metal interface.

cond-mat.mes-hall

Time-domain stability of parametric synchronization in a spin-torque nano-oscillator based on a magnetic tunnel junction

We report on a time-domain study of parametric synchronization in a magnetic tunnel junction based spin torque nano-oscillator (STNO). Time-domain measurements of the instantaneous frequency ($f_{i}$) of a parametrically synchronized STNO show random short-term unlocking of the STNO signal for low injected radio-frequency (RF) power, which cannot be revealed in time-averaged frequency domain measurements. Macrospin simulations reproduce the experimental results and reveal that the random unlocking during synchronization is driven by thermal fluctuations. We show that by using a high injected RF power, random unlocking of the STNO can be avoided. However, a perfect synchronization characterized by complete suppression of phase noise, so-called phase noise squeezing, can be obtained only at a significantly higher RF power. Our macrospin simulations suggest that a lower temperature and a higher positive ratio of the field-like torque to the spin transfer torque reduce the threshold RF power required for phase noise squeezing under parametric synchronization.

cond-mat.mes-hall

Spin-polarized quasiparticle control in a double spin-filter tunnel junction

Spin-polarized quasiparticles can be easily created during spin-filtering through a ferromagnetic insulator (FI) in contact with a superconductor due to pair breaking effects at the interface. A combination FI-N-FI sandwiched between two superconductors can be used to create and analyze such spin-polarized quasiparticles through their nonequilibrium accumulation in the middle metallic (N) layer. We report spin-polarized quasiparticle regulation in a double spin-filter tunnel junction in the configuration NbN-GdN1-Ti-GdN2-NbN. The middle Ti layer provides magnetic decoupling between two ferromagnetic GdN and a place for nonequilibrium quasiparticle accumulation. The two GdN(1,2) layers were deposited under different conditions to introduce coercive contrast. The quasiparticle tunneling spectra has been measured at different temperatures to understand the tunneling mechanism in these double spin-filter junctions. The conductance spectra were found to be comparable to an asymmetric SINI'S-type tunnel junction. A hysteretic R-H loop with higher resistance for the antiparallel configuration compared to parallel state was observed asserting the spin-polarized nature of quasiparticles. The hysteresis in the R-H loop was found to disappear for sub-gap bias current. This difference can be understood by considering suppression of the interlayer coupling due to nonequilibrium spin-polarized quasiparticle accumulation in the Ti layer.

cond-mat.mes-hall

Spin-polarized quasiparticle tunneling in spin-filter pseudospin-valve devices

Spin selective nature of spin-filter tunnel junctions can be integrated with conventional metallic ferromagnets to regulate spin polarized quasiparticles in superconducting devices. We report fabrication of pseudo spin-valve device made with a bilayer of nitride spin-filter tunnel barrier (DyN or GdN) and a transition metal ferromagnet (Co and Gd). We show resistance switching in these devices corresponding to parallel and antiparallel configuration of their mutual magnetization direction. With optimal deposition process partial nitridation of the Co layer can be achieved. The magnetically dead native CoN$_x$ layer at the Co-DyN interface acts the role of the barrier in these devices. In pseudo spin-valve with Co, lower resistance was found for antiparallel state compared to parallel configuration. Reverse resistance switching behavior was observed for the pseudo spin-valves with Gd. Presence of resistance switching in these devices further confirm the spin-filtering nature of DyN and GdN tunnel barrier. Quasiparticle transport at different temperatures in these devices was found to be compatible with conventional N-I-S tunnelling model. These devices can be further engineered to regulate spin polarized supercurrent in superconducting spintronics devices.

cond-mat.mes-hall

Magnetic field-enhanced spin filtering in rare-earth mononitride tunnel junctions

Spin filter tunnel junctions are based on selective tunneling of up and down spin electrons controlled through exchange splitting of the band structure of a ferromagnetic insulator. Therefore, spin filter efficiency can be tuned by adjusting exchange strength of the tunnel barrier. We have observed that magnetic field and bias voltage (current) can be used to regulate exchange strength and consequently spin-filter efficiency in tunnel junctions with ferromagnetic DyN and GdN tunnel barrier. In tunnel junctions with DyN barrier we obtained $\sim$37$\%$ spin polarization of tunneling electrons at 11 K due to a small exchange splitting ($ E_{ex}$) $\approx$5.6 meV of the barrier height ($Φ_0$) $\approx$60 meV. Huge spin-filter efficiency $\sim$97$\%$ was found for tunnel junctions with GdN barrier due to larger $E_{ex}$ $\approx$47 meV. In the presence of an applied magnetic field, barrier height can further split due to magnetic field dependent exchange splitting $ E_{ex}(H)$. The spin filter efficiency in DyN tunnel junctions can be increased up to $\sim$87$\%$ with magnetic field. Electric and magnetic field tuned spin-filter efficiency of these tunnel junctions gives opportunity for practical application of these devices with additional functionality.

cond-mat.mes-hall

Generation linewidth of mode-hopping spin torque oscillators

Experiments on spin torque oscillators commonly observe multi-mode signals. Recent theoretical works have ascribed the multi-mode signal generation to coupling between energy-separated spin wave modes. Here, we analyze in detail the dynamics generated by such mode coupling. We show analytically that the mode-hopping dynamics broaden the generation linewidth and makes it generally well described by a Voigt lineshape. Furthermore, we show that the mode-hopping contribution to the linewidth can dominate in which case it provides a direct measure of the mode-hopping rate. Due to the thermal drive of mode-hopping events, the mode-hopping rate also provides information on the energy barrier separating modes and temperature-dependent linewidth broadening. Our results are in good agreement with experiments, revealing the physical mechanism behind the linewidth broadening in multi-mode spin torque oscillators.

cond-mat.mes-hall

Temperature dependence of linewidth in nano-contact based spin torque oscillators: effect of multiple oscillatory modes

We discuss the effect of mode transitions on the current (I) and temperature (T) dependent linewidth (Δf) in nanocontact based spin torque oscillators (STOs). At constant I, Δf exhibits an anomalous temperature dependence near the mode transitions; Δf may either increase or decrease with T depending on the position w.r.t. the mode transition. We show that the behavior of Δf as a function of I can be fitted by the single mode analytical theory of STOs, even though there are two modes present near the mode transition, if the nonlinear amplification is determined directly from the experiment. Using a recently developed theory of two coupled modes, we show that the linewidth near mode transition can be described by an "effective" single-oscillator theory with an enhanced nonlinear amplification that carries additional temperature dependence, which thus qualitatively explain the experimental results.

cond-mat.mtrl-sci

Large local Hall effect in pin-hole dominated multigraphene spin-valves

We report local and non-local measurements in pin-hole dominated mesoscopic multigraphene spin-valves. Local spin-valve measurements show spurious switching behavior in resistance during magnetic field sweeping similar to the signal observed due to spin-injection into multigraphene. The switching behavior has been explained in terms of local Hall effect due to thickness irregularity of the tunnel barrier. Local Hall effect appears due to large local magnetostatic field produced at the roughness in the AlO$_x$ tunnel barrier. The effect of this local Hall effect is found to reduce as temperature is increased above 75 K. The strong local Hall effect hinders spin-injection into multigraphene resulting in no spin signal in non-local measurements.

cond-mat.mes-hall

Low-field microwave absorption in epitaxial La-Sr-Mn-O films resulting from the angle-tuned ferromagnetic resonance in the multidomain state

We studied magnetic-field induced microwave absorption in 100-200 nm thick La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ films on SrTiO$_{3}$ substrate and found a low-field absorption with a very peculiar angular dependence: it appears only in the oblique field and is absent both in the parallel and in the perpendicular orientations. We demonstrate that this low-field absorption results from the ferromagnetic resonance in the multidomain state (domain-mode resonance). Its unusual angular dependence arises from the interplay between the parallel component of the magnetic field that drives the film into multidomain state and the perpendicular field component that controls the domain width through its effect on domain wall energy. The low-field microwave absorption in the multidomain state can be a tool to probe domain structure in magnetic films with in-plane magnetization.

cond-mat.mtrl-sci

Decoherence and mode-hopping in a magnetic tunnel junction-based spin-torque oscillator

We discuss the coherence of magnetic oscillations in a magnetic tunnel junction-based spin-torque oscillator as a function of external field angle. Time-frequency analysis shows mode-hopping between distinct oscillator modes, which arises from linear and nonlinear couplings in the Landau-Lifshitz-Gilbert equation, analogous to mode-hopping observed in semiconductor ring lasers. These couplings and therefore mode-hopping are minimized near the current threshold for antiparallel (AP) alignment of free layer with reference layer magnetization. Away from the AP alignment, mode-hopping limits oscillator coherence.

cond-mat.mtrl-sci

Bias dependence of perpendicular spin torque and of free and fixed layer eigenmodes in MgO-based nanopillars

We have measured the bias voltage and field dependence of eigenmode frequencies in a magnetic tunnel junction with MgO barrier. We show that both free layer (FL) and reference layer (RL) modes are excited, and that a cross-over between these modes is observed by varying external field and bias voltage. The bias voltage dependence of the FL and RL modes are shown to be dramatically different. The bias dependence of the FL modes is linear in bias voltage, whereas that of the RL mode is strongly quadratic. Using modeling and micromagnetic simulations, we show that the linear bias dependence of FL frequencies is primarily due to a linear dependence of the perpendicular spin torque on bias voltage, whereas the quadratic dependence of the RL on bias voltage is dominated by the reduction of exchange bias due to Joule heating, and is not attributable to a quadratic dependence of the perpendicular spin torque on bias voltage.

cond-mat.mtrl-sci

Frequency modulation of spin torque oscillator pairs

The current controlled modulation of nano-contact based spin torque oscillator (STO) pairs is studied in both the synchronized and non-synchronized states. The synchronized state shows a well behaved modulation and demonstrates robust mutual locking even under strong modulation. The power distribution of the modulation sidebands can be quantitatively described by assuming a single oscillator model. However, in the non-synchronized state, the modulation sidebands are not well described by the model, indicating interactions between the two individual nano-contact STOs. These findings are promising for potential applications requiring the modulation of large synchronized STO arrays.

cond-mat.mtrl-sci

Spin-torque oscillator linewidth narrowing under current modulation

We study the behavior of the linewidth of a nano-contact based spin torque oscillator (STO) under application of a radio frequency (100 MHz) modulating current. We achieve a significant (up to 85%) reduction of the STO linewidth when it is modulated across a region of high nonlinearity. The mechanism responsible for the linewidth reduction is the nonlinear frequency shift under the influence of current modulation, which reduces the nonlinear amplification of the linewidth. The reduction of the linewidth during modulation can be quantitatively calculated from the free-running behavior of the STO.

cond-mat.mtrl-sci