49 research outputs found
Reconfigurable neural spiking in bias field-free spin Hall nano oscillator
In this study, we theoretically investigate neuron-like spiking dynamics in
an elliptic ferromagnet/heavy metal bilayer-based spin Hall nano oscillator
(SHNO) in bias field-free condition, much suitable for practical realization of
brain inspired computing schemes. We demonstrate regular periodic spiking with
tunable frequency as well as the leaky-integrate-and-fire (LIF) behavior in a
single SHNO by manipulating the pulse features of input current. The frequency
of regular periodic spiking is tunable in a range of 0.5 GHz to 0.96 GHz (460
MHz bandwidth) through adjusting the magnitude of constant input dc current
density. We further demonstrate the reconfigurability of spiking dynamics in
response to a time varying input accomplished by continuously increasing the
input current density as a linear function of time. Macrospin theory and
micromagnetic simulation provide insights into the origin of bias field-free
auto-oscillation and the spiking phenomena in our SHNO. In addition, we discuss
how the shape anisotropy of the elliptic ferromagnet influence the bias
field-free auto oscillation characteristics, including threshold current,
frequency and transition from in-plane to out-of-plane precession. The SHNO
operates below input current density and exhibits a large
auto-oscillation amplitude, ensuring high output power. We show that the
threshold current density can be reduced by decreasing the ellipticity of the
ferromagnet layer as well as enhancing the perpendicular magnetic anisotropy.
These findings highlight the potential of bias field-free elliptic SHNO in
designing power-efficient spiking neuron-based neuromorphic hardware
Magnetization dynamics due to field interplay in field free spin Hall nano-oscillators
Spin Hall nano oscillators (SHNOs) have shown applications in unconventional
computing schemes and broadband frequency generation in the presence of applied
external magnetic field. However, under field-free conditions, the oscillation
characteristics of SHNOs display a significant dependence on the effective
field, which can be tuned by adjusting the constriction width, thereby
presenting an intriguing area of study. Here we study the effect of nano
constriction width on the magnetization dynamics in anisotropy assisted field
free SHNOs. In uniaxial anisotropy-based field-free SHNOs, either the
anisotropy field or the demagnetization field can dominate the magnetization
dynamics depending on the constriction width. Our findings reveal distinct
auto-oscillation characteristics in narrower constrictions with 20 nm and 30 nm
constriction width compared to their wider counterpart with 100 nm width. The
observed frequency shift variations with input current and constriction widths
stem from the inherent nonlinearity of the system. The interplay between the
B_demag and B_anis, coupled with changes in constriction width, yields rich
dynamics and offers control over frequency tunability, auto oscillation
amplitude, and threshold current. Notably, the spatial configuration of spin
wave wells within the constriction undergoes transformations in response to
changes in both constriction width and anisotropy. The findings highlight the
significant influence of competing fields at the constriction on the field-free
auto oscillations of SHNOs, with this impact intensifying as the constriction
width is varied.Comment: 25 pages, 11 figure
Secondary spin current driven efficient THz spintronic emitters
Femtosecond laser-induced photoexcitation of ferromagnet (FM)/heavy metal
(HM) heterostructures have attracted attention by emitting broadband terahertz
frequencies. The phenomenon relies on the formation of ultrafast spin current,
which is largely attributed to the direct photoexcitation of the FM layer.
However, we reveal that during the process, the FM layer also experiences a
secondary excitation led by the hot electrons from the HM layer that travel
across the FM/HM interface and transfer additional energy in the FM. Thus, the
generated secondary spins enhance the total spin current formation and lead to
amplified spintronic terahertz emission. The results also emphasize the
significance of the secondary spin current, which even exceeds the primary spin
currents when FM/HM heterostructures with thicker HM are used. An analytical
model is developed to provide deeper insights into the microscopic processes
within the individual layers, underlining the generalized ultrafast
superdiffusive spin-transport mechanism.Comment: 20 pages, 3 figure
Unconventional spin polarization at Argon ion milled SrTiO3 Interfaces
Interfacial two-dimensional electron gas (2DEG) formed at the perovskite-type
oxide, such as SrTiO3, has attracted significant attention due to its
properties of ferromagnetism, superconductivity, and its potential application
in oxide-based low-power consumption electronics. Recent studies have
investigated spin-to-charge conversion at the STO interface with different
materials, which could affect the efficiency of this 2DEG interface. In this
report, we presented an Ar^+ ion milling method to create a 2DEG at STO
directly by inducing oxygen vacancies. To quantify the spin-to-charge
conversion of this interface, we measured the angular-dependent spin-torque
ferromagnetic resonance (ST-FMR) spectra, revealing an unconventional spin
polarization at the interface of Argon ion-milled STO and NiFe. Furthermore, a
micromagnetic simulation for angular-dependent spin-torque ferromagnetic
resonance (ST-FMR) has been performed, confirming the large unconventional spin
polarization at the interface
Spin Pumping in Asymmetric Fe50Pt50/Cu/Fe20Ni80 Trilayer Structure
Herein, spin transport dynamics across asymmetric Fe50Pt50/Cu/Fe20Ni80 softâmagnetic trilayer structure is reported and thereby modulation of magnetic parameters including damping and effective field is determined by means of the angular dependence of broadband ferromagnetic resonance measurements. At distinct precession of individual magnetic layer, spinâpumping is found to be prevalent with expected linewidth increase. Mutual precession for wide range of resonance configuration reveals a collective reduction in anisotropy field of around 200âmT for both Fe50Pt50 and Fe20Ni80 systems. Subsequent observation of noâexcess interface damping shows the possible control of spinâpumping effect by tuning the net flow of spinâcurrent in a multilayer structure. These experimental findings have significance for microwave devices that require tunable anisotropy field in magnetic multilayers
Room temperature charge-to-spin conversion from q-2DEG at SrTiO3-based interfaces
Interfacial two-dimensional electron gas (2DEG), especially the SrTiO3-based
ones at the unexpected interface of insulators, have emerged to be a promising
candidate for efficient charge-spin current interconversion. In this article,
to gain insight into the mechanism of the charge-spin current interconversion
at the oxide-based 2DEG, we focused on conducting interfaces between insulating
SrTiO3 and two types of aluminium-based amorphous insulators, namely SrTiO3/AlN
and SrTiO3/Al2O3, and estimated their charge-spin conversion efficiency,
{\theta}_cs. The two types of amorphous insulators were selected to explicitly
probe the overlooked contribution of oxygen vacancy to the {\theta}_cs. We
proposed a mechanism to explain results of spin-torque ferromagnetic resonance
(ST-FMR) measurements and developed an analysis protocol to reliably estimate
the {\theta}_cs of the oxide based 2DEG. The resultant {\theta}_cs/t, where t
is the thickness of the 2DEG, were estimated to be 0.244 nm-1 and 0.101 nm-1
for the SrTiO3/AlN and SrTiO3/Al2O3, respectively, and they are strikingly
comparable to their crystalline counterparts. Furthermore, we also observe a
large direct current modulation of resonance linewidth in SrTiO3/AlN samples,
confirming its high {\theta}_cs and attesting an oxygen-vacancy-enabled
charge-spin conversion. Our findings emphasize the defects' contribution to the
charge-spin interconversion, especially in the oxide-based low dimensional
systems, and provide a way to create and enhance charge-spin interconversion
via defect engineering
Dense plasma irradiated platinum with improved spin Hall effect
The impurity incorporation in host high-spin orbit coupling materials like
platinum has shown improved charge-to-spin conversion by modifying the up-spin
and down-spin electron trajectories by bending or skewing them in opposite
directions. This enables efficient generation, manipulation, and transport of
spin currents. In this study, we irradiate the platinum with non-focus dense
plasma to incorporate the oxygen ion species. We systematically analyze the
spin Hall angle of the oxygen plasma irradiated Pt films using spin torque
ferromagnetic resonance. Our results demonstrate a 2.4 times enhancement in the
spin Hall effect after plasma treatment of Pt as compared to pristine Pt. This
improvement is attributed to the introduction of disorder and defects in the Pt
lattice, which enhances the spin-orbit coupling and leads to more efficient
charge-to-spin conversion without breaking the spin-orbit torque symmetries.
Our findings offer a new method of dense plasma-based modification of material
for the development of advanced spintronic devices based on Pt and other heavy
metals
Disentanglement of intrinsic and extrinsic side-jump scattering induced spin Hall effect in N-implanted Pt
The rapidly evolving utilization of spin Hall effect (SHE) arising from
spin-orbit coupling in 5d transition metals and alloys have made giant strides
in the development of designing low-power, robust and non-volatile magnetic
memory. Recent studies, on incorporating non-metallic lighter elements such as
oxygen, nitrogen and sulfur into 5d transition metals, have shown an
enhancement in damping-like torque efficiency {\theta}_DL due to the modified
SHE, but the mechanism behind this enhancement is not clear. In this paper, we
study {\theta}_DL at different temperatures (100-293 K) to disentangle the
intrinsic and extrinsic side-jump scattering induced spin Hall effect in
N-implanted Pt. We observe a crossover of intrinsic to extrinsic side-jump
mechanism as the implantation dose increases from 2*10^16 ions/cm2 to 1*10^17
ions/cm2. A sudden decrease in the intrinsic spin Hall conductivity is
counterbalanced by the increase in the side-jump induced SHE efficiency. These
results conclude that studying {\theta}_DL as a function of implantation dose,
and also as a function of temperature, is important to understand the physical
mechanism contributing to SHE, which has so far been unexplored in
incorporating non-metallic element in 5d transition metals
Ultrafast photo-thermal switching of terahertz spin currents
Dissipationless and scattering-free spin-based terahertz electronics is the futuristic technology for energy-efficient information processing. Femtosecond light pulse provides an ideal pathway for exciting the ferromagnet (FM) out-of-equilibrium, causing ultrafast demagnetization and superdiffusive spin transport at sub-picosecond timescale, giving rise to transient terahertz radiation. Concomitantly, light pulses also deposit thermal energy at short timescales, suggesting the possibility of abrupt change in magnetic anisotropy of the FM that could cause ultrafast photo-thermal switching (PTS) of terahertz spin currents. Here, a single light pulse induced PTS of the terahertz spin current manifested through the phase reversal of the emitted terahertz photons is demonstrated. The switching of the transient spin current is due to the reversal of the magnetization state across the energy barrier of the FM layer. This demonstration opens a new paradigm for on-chip spintronic devices enabling ultralow-power hybrid electronics and photonics fueled by the interplay of charge, spin, thermal, and optical signals