Controllable field-free switching of perpendicular magnetization through bulk spin-orbit torque in symmetry-broken ferromagnetic films.

Programmable magnetic field-free manipulation of perpendicular magnetization switching is essential for the development of ultralow-power spintronic devices. However, the magnetization in a centrosymmetric single-layer ferromagnetic film cannot be switched directly by passing an electrical current in itself. Here, we demonstrate a repeatable bulk spin-orbit torque (SOT) switching of the perpendicularly magnetized CoPt alloy single-layer films by introducing a composition gradient in the thickness direction to break the inversion symmetry. Experimental results reveal that the bulk SOT-induced effective field on the domain walls leads to the domain walls motion and magnetization switching. Moreover, magnetic field-free perpendicular magnetization switching caused by SOT and its switching polarity (clockwise or counterclockwise) can be reversibly controlled in the IrMn/Co/Ru/CoPt heterojunctions based on the exchange bias and interlayer exchange coupling. This unique composition gradient approach accompanied with electrically controllable SOT magnetization switching provides a promising strategy to access energy-efficient control of memory and logic devices.

Magnetic and Biocompatible Fullerenol/Fe(III) Microcapsules with Antioxidant Activities.

Fullerene C60 (refers to C60 hereafter) has a unique three-dimensional architecture and intriguing physicochemical properties. It has great potential applications in materials chemistry and life science. However, a big obstacle for the widespread application of C60 lies in the limited strategies to make supramolecular structures with diverse morphologies and functions. Herein, we report a strategy to prepare C60-based, magnetic microcapsules which can be used as external antioxidants to effectively attenuate oxidative stress. The microcapsules are composed of fullerenol, a highly water-soluble C60 multiadduct, and iron ions (Fe3+) released from a rusty nail. They can be easily obtained through coordination between the hydrophilic functional groups in fullerenol and Fe3+ with polystyrene microspheres as templates. The fullerenol/Fe3+ microcapsules have good colloidal stability both in water and serum. Their biocompatibility has been confirmed by in vitro tests on HEK293 and Hela cells. Electron spin resonance measurements indicate that the fullerenol/Fe3+ microcapsules can effectively scavenge hydroxyl radicals (OH·-) produced by H2O2, which greatly improves the living environment of the cells. The fullerenol/Fe3+ microcapsules exhibit ferromagnetic properties and can respond to the external magnetic field, enabling magnetic manipulation, and/or separation in practical applications.

Distinguishing Interface Magnetoresistance and Bulk Magnetoresistance through Rectification of Schottky Heterojunctions.

High performance of many spintronic devices strongly depends on the spin-polarized electrical transport, especially the magnetoresistance (MR) in magnetic heterojunctions. However, it has been a great challenge to distinguish the bulk MR and interface MR by transport measurements because the bulk resistance and interface resistance formed a series circuit in magnetic heterojunctions. Here, a unique interface-sensitive rectification MR method is proposed to distinguish the interface MR and bulk MR of nonmagnetic In/GeO x/n-Ge and magnetic Co/GeO x/n-Ge diode-like heterojunctions. It is demonstrated that the low-field "butterfly" hysteresis loop observed only in the conventional MR curve originates from the anisotropic MR of ferromagnetic bulk Co layer, whereas the orbit-related large nonsaturating positive MR contains contributions from both the Schottky interface and bulk Ge substrate. This rectification MR method could be extended to magnetic heterojunctions with asymmetric potential barriers to realize a deeper understanding of the fundamental interface-related functionalities.

Stress-Enhanced Interlayer Exchange Coupling and Optical-Mode FMR Frequency in Self-Bias FeCoB/Ru/FeCoB Trilayers.

Nowadays, the most popular method to increase ferromagnetic resonance (FMR) frequency ( fr) in self-bias soft magnetic films is to improve the anisotropy field HK. However, to push fr to higher frequencies only via raising HK becomes increasingly challenging because fr is already higher than 10 GHz by now. In this study, we fabricated a series of magnetically anisotropic FeCoB/Ru/FeCoB sandwich films possessing antiferromagnetic-like coupling and gradually increased uniaxial stress in the FeCoB sublayers from 52 to 110 MPa. It is quite remarkable that the acoustic mode of FMR gradually disappears, whereas the optical mode is enhanced in these structures. We observed simultaneous enhancement of HK and interlayer coupling field ( JIEC) with the uniaxial stress, which leads to a very pronounced optical-mode frequency increase from 8.67 to 11.62 GHz with a very sensitive stress response of 51 Hz/Pa. In contrast, the fr in a FeCoB single layer (acoustic mode) only varies from 3.47 to 5.05 GHz under similar stress. We believe that the strain-induced electron density variation of the Ru spacer's Fermi surface in the out-of-plane direction is responsible for the enhancement of JIEC. This study demonstrates that the antiferromagnetic coupling is a new route to achieve higher fr and provides the possibility of engineering and manipulating optical-mode resonance simply by controlling the interlayer coupling strength via stress.

Engineering optical mode ferromagnetic resonance in FeCoB films with ultrathin Ru insertion.

Ferromagnetic resonance (FMR) in soft magnetic films (SMFs) to a large extent determines the maximum working frequency of magnetic devices. The FMR frequency (fr) in an optical mode is usually much higher than that in the corresponding acoustic mode for exchange coupled ferromagnet/nonmagnet/ferromagnet (FM/NM/FM) trilayers. In this study, we prepared a 50 nm FeCoB film with uniaxial magnetic anisotropy (UMA), showing a high acoustic mode fr of 4.17 GHz. When an ultrathin Ru spacer was inserted in the very middle of the UMA-FeCoB film, the zero-field FMR was abruptly switched from an acoustic mode to an optical one with fr dramatically enhanced from 4.17 GHz to 11.32 GHz. Furthermore, the FMR mode can be readily tuned to optical mode only, acoustic mode only, or double mode by simply varying the applied filed, which provides a flexible way to design multi-band microwave devices.

Spin memristive magnetic tunnel junctions with CoO-ZnO nano composite barrier.

The spin memristive devices combining memristance and tunneling magnetoresistance have promising applications in multibit nonvolatile data storage and artificial neuronal computing. However, it is a great challenge for simultaneous realization of large memristance and magnetoresistance in one nanoscale junction, because it is very hard to find a proper spacer layer which not only serves as good insulating layer for tunneling magnetoresistance but also easily switches between high and low resistance states under electrical field. Here we firstly propose to use nanon composite barrier layers of CoO-ZnO to fabricate the spin memristive Co/CoO-ZnO/Co magnetic tunnel junctions. The bipolar resistance switching ratio is high up to 90, and the TMR ratio of the high resistance state gets to 8% at room temperature, which leads to three resistance states. The bipolar resistance switching is explained by the metal-insulator transition of CoO(1-v) layer due to the migration of oxygen ions between CoO(1-v) and ZnO(1-v).