arXiv subjects

Nian X. Sun

Publications and source records attributed to Nian X. Sun.

7 recordsLinked to original sources

arXiv2026

Theoretical framework of passive ME antenna arrays enabling in-vivo monitoring: A pathway to smart implants

A new brain-computer interface (BCI) technology, deployed through minimally invasive surgery, is changing the way we think about treating severe neurological conditions. The central idea is to place a device called Stentrode in the brain's vasculature, which enables neuromodulation and helps patients regain the ability to communicate. However, in such devices, the battery and electronics are wired and could introduce damage or implant malfunction. In these cases, a Stentrode integrated with magnetoelectric (ME) antennas could be of great interest. ME antennas offer significant advantages over traditional antennas, leveraging acoustic resonance rather than electromagnetic resonance to achieve a size reduction of up to five orders of magnitude. In addition to their compactness and immunity to ground-plane interference, ME antennas could be adopted for use in vascular implants, such as coronary stents, potentially enabling minimally invasive monitoring and communication. Despite these advantages, a single antenna embedded in the implant may be constrained by the limited volume of magnetostrictive material, which could result in low output gain. To address this gain limitation, we propose using antenna arrays designed to produce constructive interference at a designated far-field point, ideally located outside the patient, to enhance signal transmission and receiving capabilities. We develop a mathematical model to represent the antennas and optimize their spatial arrangement and phase synchronization. Simulations based on this model demonstrate promising high-gain performance at the prescribed far-field location through phase manipulation.

eess.SP↗

arXiv2024

Hybrid magnon-phonon cavity for large-amplitude terahertz spin-wave excitation

Terahertz (THz) spin waves or their quanta, magnons, can be efficiently excited by acoustic phonons because these excitations have similar wavevectors in the THz regime. THz acoustic phonons can be produced using photoacoustic phenomena but typically have a low population and thus a relatively low displacement amplitude. The magnetization amplitude and population of the acoustically excited THz magnons are thus usually small. Using analytical calculations and dynamical phase-field simulations, we show that a freestanding metal/magnetic-insulator (MI)/dielectric multilayer can be designed to produce large-amplitude THz spin wave via cavity-enhanced magnon-phonon interaction. The amplitude of the acoustically excited THz spin wave in the freestanding multilayer is predicted to be more than ten times larger than in a substrate-supported multilayer. Acoustically excited nonlinear magnon-magnon interaction is demonstrated in the freestanding multilayer. The simulations also indicate that the magnon modes can be detected by probing the charge current in the metal layer generated via spin-charge conversion across the MI/metal interface and the resulting THz radiation. Applications of the freestanding multilayer in THz optoelectronic transduction are computationally demonstrated.

cond-mat.mes-hall↗

arXiv2019

Sub-terahertz ferrimagnetic spin-transfer torque oscillator

A theory of magnetization dynamics in ferrimagnetic materials with antiparallel aligned spin sub-lattices under action of spin-transfer torques (STT) is developed. We consider magnetization dynamics in GdFeCo layers in two cases of magnetic anisotropy: easy plane and easy axis. We demonstrate that, (i) for the easy plane anisotropy the precession of the Néel vector is conical and the cone angle depends on the STT strength and the value of spin non-compensation, while the frequency of precession can reach sub-THz frequencies; (ii) for the easy axis anisotropy two regimes are possible: deterministic switching of the net magnetization and a conical sub-THz precession depending on the STT strength.

cond-mat.mes-hall↗

arXiv2018

Spin-orbit torque and spin pumping in YIG/Pt with interfacial insertion layers

We experimentally investigate spin-orbit torque and spin pumping in Y$_3$Fe$_5$O$_{12}$(YIG)/Pt bilayers with ultrathin insertion layers at the interface. An insertion layer of Cu suppresses both spin-orbit torque and spin pumping, whereas an insertion layer of Ni$_{80}$Fe$_{20}$ (permalloy, Py) enhances them, in a quantitatively consistent manner with the reciprocity of the two spin transmission processes. However, we observe a large enhancement of Gilbert damping with the insertion of Py that cannot be accounted for solely by spin pumping, suggesting significant spin-memory loss due to the interfacial magnetic layer. Our findings indicate that the magnetization at the YIG-metal interface strongly influences the transmission and depolarization of pure spin current.

cond-mat.mtrl-sci↗

arXiv2016

Comparison of spin-orbit torques and spin pumping across NiFe/Pt and NiFe/Cu/Pt interfaces

We experimentally investigate spin-orbit torques and spin pumping in NiFe/Pt bilayers with direct and interrupted interfaces. The damping-like and field-like torques are simultaneously measured with spin-torque ferromagnetic resonance tuned by a dc bias current, whereas spin pumping is measured electrically through the inverse spin Hall effect using a microwave cavity. Insertion of an atomically thin Cu dusting layer at the interface reduces the damping-like torque, field-like torque, and spin pumping by nearly the same factor of ~1.4. This finding confirms that the observed spin-orbit torques predominantly arise from diffusive transport of spin current generated by the spin Hall effect. We also find that spin-current scattering at the NiFe/Pt interface contributes to additional enhancement in magnetization damping that is distinct from spin pumping.

cond-mat.mes-hall↗

arXiv2016

Interfacial spin-orbit torque without bulk spin-orbit coupling

An electric current in the presence of spin-orbit coupling can generate a spin accumulation that exerts torques on a nearby magnetization. We demonstrate that, even in the absence of materials with strong bulk spin-orbit coupling, a torque can arise solely due to interfacial spin-orbit coupling, namely Rashba-Eldestein effects at metal/insulator interfaces. In magnetically soft NiFe sandwiched between a weak spin-orbit metal (Ti) and insulator (Al$_2$O$_3$), this torque appears as an effective field, which is significantly larger than the Oersted field and sensitive to insertion of an additional layer between NiFe and Al$_2$O$_3$. Our findings point to new routes for tuning spin-orbit torques by engineering interfacial electric dipoles.

cond-mat.mtrl-sci↗

arXiv2014

Quantification of the spin-Hall anti-damping torque with a resonance spectrometer

We present a simple technique using a cavity-based resonance spectrometer to quantify the anti-damping torque due to the spin Hall effect. Modification of ferromagnetic resonance is observed as a function of small DC current in sub-mm-wide strips of bilayers, consisting of magnetically soft FeGaB and strong spin-Hall metal Ta. From the detected current-induced linewidth change, we obtain an effective spin Hall angle of 0.08-0.09 independent of the magnetic layer thickness. Our results demonstrate that a sensitive resonance spectrometer can be a general tool to investigate spin Hall effects in various material systems, even those with vanishingly low conductivity and magnetoresistance.

cond-mat.mtrl-sci↗