Single-Shot Laser-Induced Switching of an Exchange Biased Antiferromagnet.

Ultrafast manipulation of magnetic order has challenged the understanding of the fundamental and dynamic properties of magnetic materials. So far single-shot magnetic switching has been limited to ferrimagnetic alloys, multilayers, and designed ferromagnetic (FM) heterostructures. In FM/antiferromagnetic (AFM) bilayers, exchange bias (He ) arises from the interfacial exchange coupling between the two layers and reflects the microscopic orientation of the antiferromagnet. Here the possibility of single-shot switching of the antiferromagnet (change of the sign and amplitude of He ) with a single femtosecond laser pulse in IrMn/CoGd bilayers is demonstrated. The manipulation is demonstrated in a wide range of fluences for different layer thicknesses and compositions. Atomistic simulations predict ultrafast switching and recovery of the AFM magnetization on a timescale of 2 ps. The results provide the fastest and the most energy-efficient method to set the exchange bias and pave the way to potential applications for ultrafast spintronic devices.

An integrated ultra-high vacuum cluster for atomic-scale deposition, interface regulation, and characterization of spintronic multilayers.

The study of interface spin effects in spintronic multilayer films requires distinguishing the effects generated by different interfaces. However, testing in atmospheric conditions requires a capping layer to protect the films, which introduces new interfaces and limits the study of interface spin-dependent effects. To address this challenge, we have developed an integrated ultra-high vacuum cluster system that includes magnetron sputtering equipment, ion irradiation equipment, and time-resolved magneto-optical Kerr effect (TR-MOKE) equipment. Our sputtering system integrates 12 cathodes in a single chamber, allowing the co-sputtering of four targets. The ultimate vacuum can reach 1 × 10-10 mbar, and the deposition resolution of 0.1 nm can be achieved. Ion irradiation equipment can ionize to produce He+, and by screening and accelerating the implantation of He+ into multilayer films, ion scanning is realized, and up to 30 keV energy can be applied to the films. The TR-MOKE equipment can detect ultra-fast magnetic dynamics processes in vacuum conditions, and its external magnetic field can be rotated 360°. Our vacuum cluster system connects the three subsystems, allowing in situ film deposition, regulation, and characterization. By accurately detecting the effects of different layers, the system can distinguish the interface effects of multilayers. Experimental results demonstrate that the three subsystems can work independently or coordinate to observe the interface effects of multilayers.

Optical Creation of Skyrmions by Spin Reorientation Transition in Ferrimagnetic CoHo Alloys.

Manipulating magnetic skyrmions by means of a femtosecond (fs) laser pulse has attracted great interest due to their promising applications in efficient information-storage devices with ultralow energy consumption. However, the mechanism underlying the creation of skyrmions induced by an fs laser is still lacking. As a result, a key challenge is to reveal the pathway for the massive reorientation of magnetization from trivial to nontrivial topological states. Here, we studied a series of ferrimagnetic CoHo alloys and investigated the effect of a single laser pulse on the magnetic states. Thanks to the time-resolved magneto-optical Kerr effect and imaging techniques, we demonstrate that the laser-induced phase transitions from single domains into a topological skyrmion phase are mediated by the transient in-plane magnetization state, in real time and space domains, respectively. Combining experiments and micromagnetic simulations, we propose a two-step process for creating skyrmions through laser pulse irradiation: (i) the electron temperature enhancement induces a spin reorientation transition on a picosecond (ps) timescale due to the suppression of perpendicular magnetic anisotropy (PMA) and (ii) the PMA slowly restores, accompanied by out-of-plane magnetization recovery, leading to the generation of skyrmions with the help of spin fluctuations. This work provides a route to control skyrmion patterns using an fs laser, thereby establishing the foundation for further exploration of topological magnetism at ultrafast timescales.

Laser Irradiation Effect on the p-GaSe/n-HfS2 PN-Heterojunction for High-Performance Phototransistors.

Two-dimensional (2D)-based PN-heterojunction revealed a promising future of atomically thin optoelectronics with diverse functionalities in different environments. Herein, we reported a p-GaSe/n-HfS2 van der Waals (vdW) heterostructure for high-performance photodetectors and investigated the laser irradiation effect on the fabricated device. The fabricated 2D vdW heterostructure revealed a high photoresponsivity of 1 × 104 A W-1 with a photocurrent value of 377 nA due to unique type-II band alignment and enhanced surface potential under light illumination, which is further confirmed by density functional theory (DFT) calculations. Before laser irradiation, the device showed high field-effect mobility (µEF) of 26.37 cm2 V-1 s-1, ON/OFF ratio of ∼105, and threshold voltage swing (SS) of ∼463 mV dec-1. With the exposure of 690 mW cm-2 laser power density, µEF reached 204 cm2 V-1 s-1, although ∼2 V ΔVth shifts are observed along with the SS decreased to 175 mV dec-1. Interestingly, the reduced SS shows better channel control of the fabricated device with laser power. Similarly, the ON/OFF ratio decreased to ∼1.29 × 103. The results indicate that the creation of oxide trap charges at the interface of SiO2 and PN-heterojunction layers was observed with voltage biasing and high laser power density. The degradation of electrical parameters is attributed to fewer interface trap charges per surface area of the device rather than direct damage in PN-heterojunction layers. Considering the excellent 2D electronic properties, these materials are better candidates for future high-radiation environments.

Dynamic Symmetry Breaking in Chiral Magnetic Systems.

The Dzyaloshinskii-Moriya interaction (DMI) in magnetic systems stabilizes spin textures with preferred chirality, applicable to next-generation memory and computing architectures. In perpendicularly magnetized heavy-metal/ferromagnet films, the interfacial DMI originating from structural inversion asymmetry and strong spin-orbit coupling favors chiral Néel-type domain walls (DWs) whose energetics and mobility remain at issue. Here, a new effect is characterized in which domains expand unidirectionally in response to a combination of out-of-plane and in-plane magnetic fields, with the growth direction controlled by the in-plane field strength. These growth directionalities and symmetries with applied fields cannot be understood from static treatments alone. The authors theoretically demonstrate that perpendicular field torques stabilize steady-state magnetization profiles highly asymmetric in elastic energy, resulting in a dynamic symmetry breaking consistent with the experimental findings. This phenomenon sheds light on the mechanisms governing the dynamics of Néel-type DWs and expands the utility of field-driven DW motion to probe and control chiral DWs.

Current-Induced Spin Torques on Single GdFeCo Magnetic Layers.

Spintronics exploit spin-orbit coupling (SOC) to generate spin currents, spin torques, and, in the absence of inversion symmetry, Rashba and Dzyaloshinskii-Moriya interactions. The widely used magnetic materials, based on 3d metals such as Fe and Co, possess a small SOC. To circumvent this shortcoming, the common practice has been to utilize the large SOC of nonmagnetic layers of 5d heavy metals (HMs), such as Pt, to generate spin currents and, in turn, exert spin torques on the magnetic layers. Here, a new class of material architectures is introduced, excluding nonmagnetic 5d HMs, for high-performance spintronics operations. Very strong current-induced torques exerted on single ferrimagnetic GdFeCo layers, due to the combination of large SOC of the Gd 5d states and inversion symmetry breaking mainly engineered by interfaces, are demonstrated. These "self-torques" are enhanced around the magnetization compensation temperature and can be tuned by adjusting the spin absorption outside the GdFeCo layer. In other measurements, the very large emission of spin current from GdFeCo, 80% (20%) of spin anomalous Hall effect (spin Hall effect) symmetry is determined. This material platform opens new perspectives to exert "self-torques" on single magnetic layers as well as to generate spin currents from a magnetic layer.

Controlling All-Optical Helicity-Dependent Switching in Engineered Rare-Earth Free Synthetic Ferrimagnets.

All-optical helicity-dependent switching in ferromagnetic layers has revealed an unprecedented route to manipulate magnetic configurations by circularly polarized femtosecond laser pulses. In this work, rare-earth free synthetic ferrimagnetic heterostructures made from two antiferromagnetically exchange coupled ferromagnetic layers are studied. Experimental results, supported by numerical simulations, show that the designed structures enable all-optical switching which is controlled, not only by light helicity, but also by the relative Curie temperature of each ferromagnetic layer. Indeed, through the antiferromagnetic exchange coupling, the layer with the larger Curie temperature determines the final orientation of the other layer and so the synthetic ferrimagnet. For similar Curie temperatures, helicity-independent back switching is observed and the final magnetic configuration is solely determined by the initial magnetic state. This demonstration of electrically-detected, optical control of engineered rare-earth free heterostructures opens a novel route toward practical opto-spintronics.

Creation of Magnetic Skyrmion Bubble Lattices by Ultrafast Laser in Ultrathin Films.

Magnetic skyrmions are topologically nontrivial spin textures which hold great promise as stable information carriers in spintronic devices at the nanoscale. One of the major challenges for developing novel skyrmion-based memory and logic devices is fast and controlled creation of magnetic skyrmions at ambient conditions. Here we demonstrate controlled generation of skyrmion bubbles and skyrmion bubble lattices from a ferromagnetic state in sputtered ultrathin magnetic films at room temperature by a single ultrafast (35 fs) laser pulse. The skyrmion bubble density increases with the laser fluence, and it finally becomes saturated, forming disordered hexagonal lattices. Moreover, we present that the skyrmion bubble lattice configuration leads to enhanced topological stability as compared to isolated skyrmions, suggesting its promising use in data storage. Our findings shed light on the optical approach to the skyrmion bubble lattice in commonly accessible materials, paving the road toward the emerging skyrmion-based memory and synaptic devices.