arXiv subjects

Zhifeng Zhu

Publications and source records attributed to Zhifeng Zhu.

24 records · Page 2Linked to original sources

arXiv2022

Spatially Nonuniform Oscillations in Ferrimagnets Based on an Atomistic Model

The ferrimagnets, such as GdxFeCo(1-x), can produce ultrafast magnetic switching and oscillation due to the strong exchange field. The two-sublattices macrospin model has been widely used to explain the experimental results. However, it fails in describing the spatial nonuniform magnetic dynamics which gives rises to many important phenomenons such as the domain walls and skyrmions. Here we develop the two-dimensional atomistic model and provide a torque analysis method to study the ferrimagnetic oscillation. Under the spin-transfer torque, the magnetization oscillates in the exchange mode or the flipped exchange mode. When the Gd composition is increased, the exchange mode firstly disappears, and then appears again as the magnetization compensation point is reached. We show that these results can only be explained by analyzing the spatial distribution of magnetization and effective fields. In particular, when the sample is small, a spatial nonuniform oscillation is also observed in the square film. Our work reveals the importance of spatial magnetic distributions in understanding the ferrimagnetic dynamics. The method developed in this paper provides an important tool to gain a deeper understanding of ferrimagnets and antiferromagnets. The observed ultrafast dynamics can also stimulate the development of THz oscillators.

cond-mat.mes-hall↗

arXiv2020

Electrical generation and detection of terahertz signal based on spin-wave emission from ferrimagnets

Terahertz (THz) signals, mainly generated by photonic or electronic approaches, are being sought for various applications, whereas the development of magnetic source might be a necessary step to harness the magnetic nature of electromagnetic radiation. We show that the relativistic effect on the current-driven domain-wall motion induces THz spin-wave emission in ferrimagnets. The required current density increases dramatically in materials with strong exchange interaction and rapidly exceeds 1012 A m-2, leading to the device breakdown and thus the lack of experimental evidence. By translating the collective magnetization oscillations into voltage signals, we propose a three-terminal device for the electrical detection of THz spin wave. Through material engineering, wide frequency range from 264 GHz to 1.1 THz and uniform continuous signals with improved output power can be obtained. As a reverse effect, the spin wave generated in this system is able to move ferrimagnetic domain wall. Our work provides guidelines for the experimental verification of THz spin wave, and could stimulate the design of THz spintronic oscillators for wideband applications as well as the all-magnon spintronic devices.

cond-mat.mes-hall↗

arXiv2019

Spin-wave mediated interactions for Majority Computation using Skyrmions and Spin-torque Nano-oscillators

Recent progress in all-electrical nucleation, detection and manipulation of magnetic skyrmions has unlocked the tremendous potential of skyrmion-based spintronic devices. Here, we show via micromagnetic simulations that the stable magnetic oscillations of STNO radiate spin waves (SWs) that can be scattered in the presence of skyrmions in the near vicinity. Interference between SWs emitted by the STNO and SWs scattered by the skyrmion gives rise to interesting dynamics that leads to amplification or attenuation of STNO's magnetic oscillations. In the presence of strong Dzyaloshinskii-Moriya interaction (DMI), the amplified magnetic oscillations evolve into a new skyrmion. These interactions between skyrmions and STNOs are found to be identical for both Neel-type and Bloch-type skyrmions, and are not observed between domain walls and STNOs. These findings offer a novel perspective in processing information using single skyrmions and we propose a 3-bit majority gate for logic applications.

cond-mat.mes-hall↗

arXiv2018

Spin-torque-induced magnetization dynamics in ferrimagnets based on Landau-Lifshitz-Bloch Equation

A theoretical model based on the Landau-Lifshitz-Bloch equation is developed to study the spin-torque effect in ferrimagnets. Experimental findings, such as the temperature dependence, the peak in spin torque, and the angular-momentum compensation, can be well captured. In contrast to the ferromagnet system, the switching trajectory in ferrimagnets is found to be precession free. The two sublattices are not always collinear, which produces large exchange field affecting the magnetization dynamics. The study of material composition shows the existence of an oscillation region at intermediate current density, induced by the nondeterministic switching. Compared to the Landau-Lifshitz-Gilbert model, our developed model based on the Landau-Lifshitz-Bloch equation enables the systematic study of spin-torque effect and the evaluation of ferrimagnet-based devices.

cond-mat.mtrl-sci↗

arXiv2017

Theoretical Proposal for Determining Angular Momentum Compensation in Ferrimagnets

This work demonstrates that the magnetization and angular momentum compensation temperature (TMC and TAMC) in ferrimagnets (FiM) can be unambiguously determined by performing two sets of temperature dependent current switching, with the symmetry reverses at TMC and TAMC, respectively. A theoretical model based on the modified Landau-Lifshitz-Bloch equation is developed to systematically study the spin torque effect under different temperatures, and numerical simulations are performed to corroborate our proposal. Furthermore, we demonstrate that the recently reported linear relation between TAMC and TMC can be explained using the Curie-Weiss theory.

cond-mat.mtrl-sci↗

arXiv2017

Switching based Spin Transfer Torque Oscillator with zero-bias field and large tuning-ratio

We propose a novel concept of obtaining oscillations with frequencies in very-high frequency (VHF) and ultra-high frequency (UHF) bands. A traditional spin torque nano-oscillator (STNO) consists of at least one pinned layer (PL) and one free layer (FL) which precesses in a fixed orbit, defined by a precession angle, which results in magneto-resistance (MR) oscillations. In STNO, with aligned or even orthogonal easy-axis of the magnetic layers and with or without external bias magnetic field, it is not possible to attain full MR swing. The constringed MR swing jeopardizes the extracted output power. Furthermore, the orbit is strongly disturbed by the thermal fluctuations resulting in strong magnetic noise. In stark contrast to the operation principle of a STNO, we theoretically demonstrate, with the practical parameters from the experiments, that with a unidirectional current in a dual asymmetric free-layers (with no pinned layer) based perpendicular magnetic tunnel junction (pMTJ), both of the free layers can attain a complete and out-of-phase self-sustained switching without the aid of any external magnetic field. This design facilitates a switching based spin torque oscillator (SW-STO) with a full MR swing, and hence a larger output power, for stable and more thermally robust free-running oscillations. Furthermore, the integration with the n-type metal-oxide-semiconductor (NMOS) field-effect transistors at 130, 65 and 14 nm node is appraised to expound its effect on the oscillator performance, controllability with DC bias and the design constraints, to demonstrate the viability of the design as a dynamically controllable oscillator for practical on-chip implementation.

cond-mat.mes-hall↗