We investigate the current-induced switching of the Néel order in NiO(001)/Pt heterostructures, which is manifested electrically via the spin Hall magnetoresistance. Significant reversible changes in the longitudinal and transverse resistances are found at room temperature for a current threshold lying in the range of 10^{7}  A/cm^{2}. The order-parameter switching is ascribed to the antiferromagnetic dynamics triggered by the (current-induced) antidamping torque, which orients the Néel order towards the direction of the writing current. This is in stark contrast to the case of antiferromagnets such as Mn_{2}Au and CuMnAs, where fieldlike torques induced by the Edelstein effect drive the Néel switching, therefore resulting in an orthogonal alignment between the Néel order and the writing current. Our findings can be readily generalized to other biaxial antiferromagnets, providing broad opportunities for all-electrical writing and readout in antiferromagnetic spintronics.

The generation of very high quality electron bunches (high brightness and low energy spread) from a plasma-based accelerator in the three-dimensional blowout regime using self-injection in tailored plasma density profiles is analyzed theoretically and with particle-in-cell simulations. The underlying physical mechanism that leads to the generation of high quality electrons is uncovered by tracking the trajectories of the electrons in the sheath that are trapped by the wake. Details on how the intensity of the driver and the density scale-length of the plasma control the ultimate beam quality are described. Three-dimensional particle-in-cell simulations indicate that this concept has the potential to produce beams with peak brightnesses between 10 20 and 10 21 A=m 2 =rad 2 and with absolute slice energy spreads of ∼Oð0.1Þ MeV using existing lasers or electron beams to drive nonlinear wakefields. We also show projected energy spreads as low as ∼0.3 MeV for half the charge can be generated at an optimized acceleration distance. DOI: 10.1103/PhysRevAccelBeams.20.111303 Research in plasma-based acceleration (PBA) driven by a laser pulse or a relativistic electron beam is very active [1] because the large accelerating gradients in plasma wave wakefields may lead to compact accelerators. PBA is also capable of self-generating electron bunches that have significant charge (Q), short duration (τ) and low normalized emittance (ϵ n ). A combination of these quantities define the normalized beam brightness B n ¼ 2I=ϵ 2 n where I ¼ Q=τ is the current. While PBA experiments have produced useful beams, they have not produced beams with the necessary brightness and energy spread needed to drive an x-ray free-electron-laser (X-FEL) [2] or the charge and emittance needed as an injector for a future linear collider [3].The electron bunches needed to load plasma wakefields are very short and need to be synchronized with the driver. Therefore, self-injection has been actively investigated. The threshold for self-injection of electrons into nonlinear three-dimensional (3D) plasma waves in uniform plasmas has been studied in simulations and experiments [4][5][6][7][8]. Even in simulations, this process does not appear to be capable of generating the high quality beams needed for X-FELs or a linear collider [9][10][11]. Therefore there has been much recent work on methods for generating high brightness beams through controlled injection. These ideas fall into three categories. In one, electrons are born inside the wake through field ionization where the wake potential is near a maximum which eases the trapping threshold [12][13][14]. There are now numerous variations of this idea in which the injection and wake excitation are separated [15][16][17]. In the second, one or more laser pulses are used to trigger injection inside one plasma wake bucket [18][19][20][21]. In the third, which we consider here, the effective phase velocity of the wake is slowed down either by a density transition from high to low density [22,23], or through ...

Phase space matching between two plasma-accelerator (PA) stages and between a PA and a traditional accelerator component is a critical issue for emittance preservation of beams accelerated by PAs. The drastic differences of the transverse focusing strengths as the beam propagates between different stages and components may lead to a catastrophic emittance growth in the presence of both finite energy spread and lack of proper matching. We propose using the linear focusing forces from nonlinear wakes in longitudinally tailored plasma density profiles to provide exact phase space matching to properly transport the electron beam through two such stages with negligible emittance growth. Theoretical analysis and particle-in-cell simulations show how these structures may work in four different scenarios. Good agreement between theory and simulation is obtained.

The evolution of beam phase space in ionization-induced injection into plasma wakefields is studied using theory and particle-in-cell (PIC) simulations. The injection process causes special longitudinal and transverse phase mixing leading initially to a rapid emittance growth followed by oscillation, decay, and eventual slow growth to saturation. An analytic theory for this evolution is presented that includes the effects of injection distance (time), acceleration distance, wakefield structure, and nonlinear space charge forces. Formulas for the emittance in the low and high space charge regimes are presented. The theory is verified through PIC simulations and a good agreement is obtained. This work shows how ultra-low emittance beams can be produced using ionization-induced injection.The field of plasma based acceleration has experienced significant progress in the past decade [1]. GeV energy gain in centimeter-scale laser driven wakes (LWFA) has been achieved in many recent experiments [2][3][4][5]. In beam driven wakes (PWFA), high gradient acceleration has been sustained over meter-scale distances leading to more than 40GeV energy gain [6][7][8]. For future applications of wakefield accelerators such as FELs and colliders, the quality of the self-injected beams in plasma waves, namely the transverse and longitudinal emittances, need to be improved and controlled. Among the many injection schemes [9,10], ionization-induced injection methods have attracted significant interests due to its simplest and flexibility [5,[11][12][13][14][15][16]. However, the injection process involves complex phase space dynamics, and the achievable final beam quality strongly depends on this evolution process. This area of research is of fundamental importance for achieving beam quality well beyond what is achievable with current technology.In this letter, we examine carefully the effects that affect the beam phase space evolution in ionization-induced injection using a combination of theory and simulations. We found the evolution typically has three stages, and each stage can impact the final beam quality. In typical cases where the injection time is limited to few inverse plasma periods (2πω −1 p ) and the charge is low, the three stages are as follows. First, when ionization is occurring, the emittance of the injected beam grows quickly in time from the initial thermal emittance. Second, immediately following ionization, the emittance slowly decreases to a minimum value. Finally, the emittance again gradually increases to saturated values. If the ionization time is more than ∼ πω −1 p then the emittance grows to the saturated level during the first stage including an oscillatory behavior before it slowly decreases. In the "high" charge limit the emittance evolves monotonically towards the same saturated value.The theory reveals that the evolution in emittance described above is due to special longitudinal and transverse phase mixing of electrons born at different times.The derived expressions clearly show how the emittance dep...

Manipulation of oxygen vacancies (V O ) in single oxide layers by varying the electric field can result in significant modulation of the ground state. However, in many oxide multilayers with strong application potentials, e.g. ferroelectric tunnel junctions and solid-oxide fuel cells, understanding V O behaviour in various layers under an applied electric field remains a challenge, owing to complex V O transport between different layers. By sweeping the external voltage, a reversible manipulation of V O and a corresponding fixed magnetic phase transition sequence in cobaltite/manganite (SrCoO 3-x /La 0.45 Sr 0.55 MnO 3-y ) heterostructures are reported. *The magnetic phase transition sequence confirms that the priority of electric-field-induced V O formation/annihilation in the complex bilayer system is mainly determined by the V O formation energies and Gibbs free energy differences, which is supported by theoretical analysis. We not only realize a reversible manipulation of the magnetic phase transition in an oxide bilayer, but also provide insight into the electric field control of V O engineering in heterostructures.

The interplay between orbital, charge, spin, and lattice degrees of freedom is at the core of correlated oxides. This is extensively studied at the interface of heterostructures constituted of two-layer or multilayer oxide films. Here, we demonstrate the interactions between orbital reconstruction and charge transfer in the surface regime of ultrathin (La,Sr)MnO3, which is a model system of correlated oxides. The interactions are manipulated in a quantitative manner by surface symmetry-breaking and epitaxial strain, both tensile and compressive. The established charge transfer, accompanied by the formation of oxygen vacancies, provides a conceptually novel vision for the long-term problem of manganites—the severe surface/interface magnetization and conductivity deterioration. The oxygen vacancies are then purposefully tuned by cooling oxygen pressure, markedly improving the performances of differently strained films. Our findings offer a broad opportunity to tailor and benefit from the entanglements between orbit, charge, spin, and lattice at the surface of oxide films.

We investigate the electrical manipulation of Co/Ni magnetization through a combination of ionic liquid and oxide gating, where HfO2 with a low O2− ion mobility is employed. A limited oxidation-reduction process at the metal/HfO2 interface can be induced by large electric field, which can greatly affect the saturated magnetization and Curie temperature of Co/Ni bilayer. Besides the oxidation/reduction process, first-principles calculations show that the variation of d electrons is also responsible for the magnetization variation. Our work discloses the role of gate oxides with a relatively low O2− ion mobility in electrical control of magnetism, and might pave the way for the magneto-ionic memory with low power consumption and high endurance performance.

Antiferromagnets with zero net magnetic moment, strong anti-interference and ultrafast switching speed have potential competitiveness in high-density information storage. Body centered tetragonal antiferromagnet Mn 2 Au with opposite spin sub-lattices is a unique metallic material for Néel-order spin-orbit torque (SOT) switching. Here we investigate the SOT switching in quasi-epitaxial (103), (101) and (204) Mn 2 Au films prepared by a simple magnetron sputtering method. We demonstrate current induced antiferromagnetic moment switching in all the prepared Mn 2 Au films by a short current pulse at room temperature, whereas different orientated films exhibit distinguished switching characters. A directionindependent reversible switching is attained in Mn 2 Au (103) films due to negligible magnetocrystalline anisotropy energy, while for Mn 2 Au (101) and (204) films, the switching is invertible with the current applied along the in-plane easy axis and its vertical axis, but becomes attenuated seriously during initially switching circles when the current is applied along hard axis, because of the existence of magnetocrystalline anisotropy energy. Besides the fundamental significance, the strong orientation dependent SOT switching, which was not realized irrespective of ferromagnet and antiferromagnet, provides versatility for spintronics. *