Long Liu

Permalloy (Py) is a crucial component in spin nano-oscillators due to its excellent soft magnetic properties. Due to orbital angular momentum quenching, Py exhibits very low magnetic damping. It reduces intrinsic energy dissipation during precession, which is beneficial for lowering operational power consumption and enhancing the thermal stability of certain memory devices. But lower magnetic damping limits its application in fast-switching spintronic devices. Thus, in this work, the rare earth element Gd is introduced into Py to further enhance the spintronic performance of Py_100−xGd_x alloys. Through spin-torque ferromagnetic resonance measurements (ST-FMRs), the maximum spin Hall angle of the system was calculated to be 0.149 when x = 20, significantly exceeding that of 0.042 in the pure Py sample. Additionally, Gd doping significantly enhances the ability to modulate the magnitude of the linewidth. Also, as the Gd content in the alloy increased, the magnetic damping coefficient of the device gradually rose, reaching a peak in the sample with 17% Gd content. The maximum magnetic damping coefficient of the Py-Gd alloy was 0.051, representing an approximate 2.4-fold increase compared to that of pure Py. The findings of this study confirm that the use of rare-earth elements is highly effective in tuning the performance of spintronic devices and provide support for the development of highly efficient SOT devices. It is noted that the regulation of magnetic damping by Py-Gd holds significant implications for enhancing the magnetization switching speed of SOT devices and reducing the drive current density for microwave emission in spin nano-oscillators.

Modulations on the microstructure and magnetic properties of Nd–Fe–B thick films are essential for applications in microelectromechanical systems. In Nd–Fe–B/Fe multilayers, strong long-range dipolar interactions between hard and soft magnetic phases lead to the enhancement of remanence but result in lower coercivity. In this work, Tb diffusion layers are integrated into Nd–Fe–B/Ta/Fe multilayer thick films to systematically investigate the magnetic properties and coercivity mechanism. The results show that the introduction of Tb diffusion layers in Nd–Fe–B/Ta/Fe multilayers significantly enhances both remanence and coercivity, increasing the maximum energy product from 25.2 to 36.2 MGOe. Microstructural characterization and magnetization reversal analysis reveal that the multilayer structure improves crystallinity and promotes the c-axis texture of the main magnetic phase, while the long-range dipolar interaction between Fe and Nd–Fe–B layers effectively enhances the overall magnetization. Meanwhile, Tb diffusion strengthens domain wall pinning in the main magnetic phase, suppressing the inhomogeneous magnetization reversal induced by the Fe composite layers. Consequently, a simultaneous enhancement of the remanence and coercivity is achieved. This study provides insights into preparation strategies for high-performance Nd–Fe–B thick films.

A spatially selective Dy co-sputtering strategy reconstructs the microstructure and magnetization reversal behavior of Nd–Fe–B/Fe composite thick films, enabling simultaneous enhancement of magnetization, coercivity, and maximum energy products.

It is crucial to study the materials that could effectively facilitate the inter‐conversion between charge, spin, and orbital degrees of freedom. In this work, the conversion among these three types of degrees of freedom in Pt/CoFeB/rare‐earth (RE, represents Nd, Gd, and Ho)/Ti multilayers is manipulated. Through terahertz (THz) emission measurements, it is found that the spin current induced by a femtosecond laser is converted into an orbital current via the spin‐orbit coupling of the RE layer. Notably, the light RE (Nd) and heavy RE (Gd and Ho) induce the orbital current with opposite polarization directions, ultimately leading to a weakening or enhancement of the THz emission intensity, respectively. Moreover, the fast Fourier transform reveals that Gd exerts the most significant influence on increasing the whole THz spectrum within the Pt/CoFeB/RE/Ti structure. The findings of RE‐modulated spin‐to‐orbital conversion provide valuable insights into the fundamental transport mechanism of the orbital current.

It is crucial to better understand the magnetization reversal process between soft and hard magnets and to achieve a high maximum energy product in thick composite multilayers. In this study, we find that the exchange interactions dominate in soft–hard-magnetic composite bilayers, while dipole interactions are predominant in soft–hard-magnetic composite trilayers. Based on the first-order reversal curve, magnetization reversal models are developed for both the thick composite bilayer and trilayer. Dipole interactions play an important role in the long range, resulting in higher coercivity and remanence in the thick trilayer. A multilayer in a stacked trilayer structure is achieved, which is composed of thick films with a perpendicular magnetic anisotropy at a thickness of up to 16 μm. The enhanced dipole interactions lead to a remanent polarization of 1 T and a maximum energy product of 22.5 MGOe. This work contributes to the preparation of thick films with a high maximum energy product for applications in magnetic microelectromechanical systems.

Generally, the addition of the Dy element leads to a decrease of the saturation magnetization and the remanent magnetization in NdFeB films due to its antiferromagnetic coupling with Fe. However, in this study, upon increasing the ratio of Dy in the Nd–Dy diffusion layers of NdFeB thick films, the saturation magnetization has an anomalously slight enhancement, while the coercivity and remanent magnetization have a large enhancement. The increase of coercivity is attributed to the decoupling between Nd₂Fe₁₄B grains and the enhanced pinning effect. Microstructural analysis revealed a layered structure composed of spherical Nd₂Fe₁₄B grains at the location of the Dy diffusion layer, which is attributed to the Dy diffusion layer reacting with the region of Nd element aggregation during the annealing process, facilitating transformation into Nd₂Fe₁₄B grains. The increase of the proportion of Nd₂Fe₁₄B grains results in a slight enhancement of saturation magnetization. By this method, we obtained a high-performance anisotropic NdFeB thick film of 28.7 μm with a coercivity of 2.46 T and a surface field of 163 Oe. This work establishes a microscale growth model for NdFeB thick films and helps to prepare high-performance NdFeB thick films applicable directly to microelectromechanical systems.

It has long been believed that the attachment of two heavy metals such as Ta and Pt with opposite spin Hall angles results in a weakened net torque generation efficiency in magnetization switching devices. Here, we report a giant orbital-to-spin conversion in Ta/Pt/Tm₃Fe₅O₁₂ (TmIG) heterostructures. We show that the torque generation efficiency is enhanced by an order of magnitude in the Ta/Pt/TmIG trilayer compared to that in the Pt/TmIG bilayer. This enhancement is further evidenced by the fact that the critical current density for the magnetization switching of the Ta/Pt/TmIG is an order of magnitude smaller than that of the Pt/TmIG. It is found that the orbital current generated from Ta through the orbital Hall effect (OHE) is converted to the spin current in the interior of Pt. Our discovery offers an extraordinary approach to enhance the torque generation for magnetization switching of insulators and provides an important piece of information for orbitronics.

The ability to efficiently manipulate magnetization is of great significance for practical applications of spin–orbit torque (SOT) devices. In this study, we report the voltage-controlled, field-free SOT switching in perpendicular magnetized Pt/Co/Ir/MgO structures with wedge iridium interlayers. The insertion of a thin iridium interlayer at ferromagnet/oxide can significantly reduce the perpendicular magnetic anisotropy depending on the Ir thickness. The wedging of the iridium layer breaks lateral structural symmetry, resulting in deterministic switching without the assistance of in-plane magnetic fields. In such a structure, the SOT critical switching currents are remarkably decreased by 29% when a positive 6 V gate voltage is applied. Further quantitative analysis shows that multiple factors contribute to the decrease in switching currents, including a 23% reduction in magnetic anisotropy energy, a reduction in nucleation field, and a minor enhancement in damping-like torque under gate voltage. Moreover, the probabilistic hindrance that gate voltage poses to field-free switching is revealed by the decrease in current-induced perpendicular effective fields from symmetry-breaking. Our research shows that energy-efficient SOT switching can be controlled by gating and offers insight into the mechanism behind voltage-gated SOT switching.