Electric-field control of nonvolatile resistance state of perpendicular magnetic tunnel junction via magnetoelectric coupling.

Magnetic tunnel junctions (MTJs) are the core elements of spintronic devices. Now, the mainstream writing operation of MTJs mainly relies on electric current with high energy dissipation, which can be greatly reduced if an electric field is used instead. In this regard, strain-mediated multiferroic heterostructure composed of MTJ and ferroelectrics are promising with the advantages of room temperature and magnetic field-free as already demonstrated by MTJ with in-plane magnetic anisotropy. However, there is no such report on the perpendicular MTJs (p-MTJs), which have been commercialized. Here, we investigate electric-field control of resistance state of MgO-based p-MTJs in multiferroic heterostructures. A remarkable and nonvolatile manipulation of resistance is demonstrated at room temperature without magnetic field assistance. Through various characterizations and micromagnetic simulation, the manipulation mechanism is uncovered. Our work provides an effective avenue for manipulating p-MTJ resistance by electric fields and is notable for high density and ultralow power spintronic devices.

Switching magnetic strip orientation using electric fields.

In spintronics, ordered magnetic domains are important for magnetic microdevices and controlling the orientation of ordered magnetic domains is important for applications such as domain wall resistance and spin wave propagation. Although a magnetic field or a current can reorient ordered magnetic domains, an energy-efficient electric-field-driven rotation of the ordered magnetic domains remains elusive. Here, using a nanotrenched polymeric layer, we obtain ordered magnetic strip domains in Ni films on a ferroelectric substrate. By applying electric fields to the ferroelectric substrate, we demonstrate that the ordered magnetic strip domains in Ni films are switched between the y- and x-axes driven by electric-fields. This switching of magnetic strip orientation is attributed to the electric-field-modulated in-plane magnetic anisotropies along the x- and y-axes of the Ni films, which are caused by the anisotropic biaxial strain of the ferroelectric substrate via strain-mediated magnetoelectric coupling. These results provide an energy-efficient approach for manipulating the ordered magnetic domains using electric fields.

Highly Efficient Magnetic Propulsion of NiFe Nanorod-Based Miniature Swimmers in Three Dimensions.

Magnetically actuated miniature robots have attracted the attention of the scientific community over the past two decades, but the confined workspace of their manipulation system (typically a tri-axial coil or eight electromagnetic coils) and the low efficiency of propulsion have limited their utility. Here, we describe a highly efficient NiFe nanorod-based magnetic miniature swimmer that can be manipulated in 3D spaces using two pairs of coils placed in the x-y horizontal plane. In the new swimmer, the shape symmetry is broken along its body, and the asymmetry in magnetizations is introduced perpendicular to the long axis of its body simultaneously. Such a combined asymmetry design offers favorable controllability in planar magnetic fields, which relaxes the multi-axial coil requirement of the commonly used manipulation system and thus reduces the restriction on the shape and size of the workspaces. The new swimmers display efficient 3D propulsion, with a speed of over 5000 µm s-1 (∼3 body length s-1) and powerful locomotion in biological media such as raw human blood. The fuel utilization efficiency of the swimmer, defined as the ratio of the distance to the net input work in one period, was estimated to be approximately from 10-2 to 10-3 m/J, which is significantly higher than that of magnetic motors with a slender body. Moreover, to provide practical support for further potential use, we demonstrated that the swimmer is able to perform incision operations as a minimally invasive microsurgical tool. Such a swimmer actuation strategy provides a simple and efficient way for 3D manipulation of magnetic miniature robots, offering great potential for future biomedical and other applications.

Using Dipole Interaction to Achieve Nonvolatile Voltage Control of Magnetism in Multiferroic Heterostructures.

Nonvolatile electrical control of magnetism is crucial for developing energy-efficient magnetic memory. Based on strain-mediated magnetoelectric coupling, a multiferroic heterostructure containing an isolated magnet requires nonvolatile strain to achieve this control. However, the magnetization response of an interacting magnet to strain remains elusive. Herein, Co/MgO/CoFeB magnetic tunnel junctions (MTJs) exhibiting dipole interaction on ferroelectric substrates are fabricated. Remarkably, nonvolatile voltage control of the resistance in the MTJs is demonstrated, which originates from the nonvolatile magnetization rotation of an interacting CoFeB magnet driven by volatile voltage-generated strain. Conversely, for an isolated CoFeB magnet, this volatile strain induces volatile control of magnetism. These results reveal that the magnetization response to volatile strain among interacting magnets is different from that among isolated magnets. The findings highlight the role of dipole interaction in multiferroic heterostructures and can stimulate future research on nonvolatile electrical control of magnetism with additional interactions.

Nano-magnetic tunnel junctions controlled by electric field for straintronics.

The magnetic tunneling junction (MTJ) controlled by electric field as an alternate approach for energy efficiency is the highlight for nonvolatile RAM, while there is still a lack of research on resistance manipulation with the electric field in nanoscale MTJs. In this study, we integrated nanoscale MTJs on the (011) orientated Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) ferroelectric substrates and systematically investigated the magnetoresistance as a function of the magnetic field and electric field. A single domain state of the nanoscale MTJ was demonstrated by the experimental result and theoretical simulation. Afterward, the obvious electric field control of R-H curves was obtained and explained by the competition between magnetoelastic energy and shape anisotropy. More importantly, simulation results also predicted that the switching pathway of magnetic moments under the magnetic field is strongly dependent on the applied electric field, displaying the electric field control of chiral switching in the nano-MTJ. Our work is a milestone in the realization of the emerging dubbed straintronics field.

Role of non-thermal electrons in ultrafast spin dynamics of ferromagnetic multilayer.

Understanding of ultrafast spin dynamics is crucial for future spintronic applications. In particular, the role of non-thermal electrons needs further investigation in order to gain a fundamental understanding of photoinduced demagnetization and remagnetization on a femtosecond time scale. We experimentally demonstrate that non-thermal electrons existing in the very early phase of the photoinduced demagnetization process play a key role in governing the overall ultrafast spin dynamics behavior. We simultaneously measured the time-resolved reflectivity (TR-R) and the magneto-optical Kerr effect (TR-MOKE) for a Co/Pt multilayer film. By using an extended three-temperature model (E3TM), the quantitative analysis, including non-thermal electron energy transfer into the subsystem (thermal electron, lattice, and spin), reveals that energy flow from non-thermal electrons plays a decisive role in determining the type I and II photoinduced spin dynamics behavior. Our finding proposes a new mechanism for understanding ultrafast remagnetization dynamics.

The effect of magnetic field pretreatment on the corrosion behavior of carbon steel in static seawater.

The corrosion behavior of carbon steel pretreated with a magnetic field before electrochemical testing was investigated in static seawater using electrochemical methods in the absence of an external magnetic field. The shift in corrosion potential was more significant with increasing pretreating magnetic field strength, and the corrosion current density also decreased. This implies that the carbon steel corrosion was inhibited. The main reason for this inhibition is that the magnetic field affects the formation of intermediate products on the carbon steel surface by both charge transfer and magnetic ion adsorption. The magnetic field pretreatment will likely offer a new approach for marine anti-corrosion technology.

Magnetically Powered Shape-Transformable Liquid Metal Micromotors.

Shape-transformable liquid metal (LM) micromachines have attracted the attention of the scientific community over the past 5 years, but the inconvenience of transfer routes and the use of corrosive fuels have limited their potential applications. In this work, a shape-transformable LM micromotor that is fabricated by a simple, versatile ice-assisted transfer printing method is demonstrated, in which an ice layer is employed as a "sacrificial" substrate that can enable the direct transfer of LM micromotors to arbitrary target substrates conveniently. The resulting LM microswimmers display efficient propulsion of over 60 µm s-1 (≈3 bodylength s-1 ) under elliptically polarized magnetic fields, comparable to that of the common magnetic micro/nanomotors with rigid bodies. Moreover, these LM micromotors can undergo dramatic morphological transformation in an aqueous environment under the irradiation of an alternating magnetic field. The ability to transform the shape and efficiently propel LM microswimmers holds great promise for chemical sensing, controlled cargo transport, materials science, and even artificial intelligence in ways that are not possible with rigid-bodies microrobots.

Magnetically Powered Annelid-Worm-Like Microswimmers.

A bioinspired magnetically powered microswimmer is designed and experimentally demonstrated by mimicking the morphology of annelid worms. The structural parameters of the microswimmer, such as the surface wrinkling, can be controlled by applying prestrain on substrate for the precise fabrication and consistent performance of the microswimmers. The resulting annelid-worm-like microswimmers display efficient propulsion under an oscillating magnetic field, reaching a peak speed of ≈100 µm s-1 . The speed and directionality of the microswimmer can be readily controlled by changing the parameters of the field inputs. Additionally, it is demonstrated that the microswimmers are able to transport microparticles toward a predefined destination, although the translation velocity is inevitably reduced due to the additional hydrodynamic resistance of the microparticles. These annelid-worm-like microswimmers have excellent mobility, good maneuverability, and strong transport capacity, and they hold considerable promise for diverse biomedical, chemical sensing, and environmental applications.

Large, Linear, and Tunable Positive Magnetoresistance of Mechanically Stable Graphene Foam-Toward High-Performance Magnetic Field Sensors.

Here, we present the first observation of magneto-transport properties of graphene foam (GF) composed of a few layers in a wide temperature range of 2-300 K. Large room-temperature linear positive magnetoresistance (PMR ≈ 171% at B ≈ 9 T) has been detected. The largest PMR (∼213%) has been achieved at 2 K under a magnetic field of 9 T, which can be tuned by the addition of poly(methyl methacrylate) to the porous structure of the foam. This remarkable magnetoresistance may be the result of quadratic magnetoresistance. The excellent magneto-transport properties of GF open a way toward three-dimensional graphene-based magnetoelectronic devices.

Spatially Resolved Ferroelectric Domain-Switching-Controlled Magnetism in Co40Fe40B20/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 Multiferroic Heterostructure.

Intrinsic spatial inhomogeneity or phase separation in cuprates, manganites, etc., related to electronic and/or magnetic properties, has attracted much attention due to its significance in fundamental physics and applications. Here we use scanning Kerr microscopy and scanning electron microscopy with polarization analysis with in situ electric fields to reveal the existence of intrinsic spatial inhomogeneity of the magnetic response to an electric field on a mesoscale with the coexistence of looplike (nonvolatile) and butterfly-like (volatile) behaviors in Co40Fe40B20/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 ferromagnetic/ferroelectric (FM/FE) multiferroic heterostructures. Both the experimental results and micromagnetic simulations suggest that these two behaviors come from the 109° and the 71°/180° FE domain switching, respectively, which have a spatial distribution. This FE domain-switching-controlled magnetism is significant for understanding the nature of FM/FE coupling on the mesoscale and provides a path for designing magnetoelectric devices through domain engineering.