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Magnetization reversal and domain wall dynamics in Pt/Co/AlOx trilayers have been tuned by He+ ion irradiation. Fluences up to 1.5x10$^{15}$ ions/cm$^2$ strongly decrease the perpendicular magnetic anisotropy (PMA), without affecting neither the spontaneous magnetization nor the strength of the Dzyaloshinskii-Moriya interaction (DMI). This confirms the robustness of the DMI interaction against interfacial chemical intermixing, already predicted by theory. In parallel with the decrease of the PMA in the irradiated samples, a strong decrease of the depinning field is observed. This allows the domain walls to reach large maximum velocities with lower magnetic fields with respect to those needed for the pristine films. Decoupling PMA from DMI can therefore be beneficial for the design of low energy devices based on domain wall dynamics. When the samples are irradiated with larger He+ fluences, the magnetization gets close to the out-of-plane/in-plane reorientation transition where 100nm size magnetic skyrmions are stabilized. We observe that as the He+ fluence increases, the skyrmion size decreases while these magnetic textures become more stable against the application of an external magnetic field., Comment: 10 pages, 4 figures

Magneto-ionics, by which the magnetic properties of a thin layer can be modified through the migration of ions within a liquid or solid electrolyte, is a fast developing research field. This is mainly due to the perspective of energy efficient magnetic devices, in which the magnetization direction is controlled not by a magnetic field or an electrical current, as done in traditional devices, but by an electric field, leading to a considerable reduction of energy consumption. In this work, the interfacial perpendicular magnetic anisotropy (PMA) of a series of Pt/Co/oxide trilayers covered by a ZrO$_2$ layer, acting as a ionic conductor, was finely tuned by a gate voltage at room temperature. The non-volatility and the time evolution of the effect point at oxygen ion migration across the ZrO$_2$ layer as the driving mechanism. A large variation of the PMA is obtained by modifying the degree of oxidation of the cobalt layer with the flux of oxygen ions: the easy magnetization axis can be switched reversibly from in-plane, with an under-oxidized Co, to in-plane, with an over-oxidized Co, passing through an out-of-plane magnetization with maximum PMA. The switching time between the different magnetic states is limited by the oxygen ion drift velocity through the multilayer structure. This was shown to depend exponentially on the applied bias voltage, and could be varied by over 5 orders of magnitude, from several minutes to a few ms. On the other hand, for a fixed gate-voltage, the oxidation of the cobalt layer decreases exponentially as a function of time. This behavior is in agreement with the theoretical model developed by Cabrera and Mott (1949) to explain the growth of very thin oxides at low temperatures. The possibility to explain the observed effect with a relatively simple theoretical model opens the possibility to engineers materials with optimized properties., 8 pages, 6 figures

Magnetic skyrmions are localized chiral spin textures, which offer great promise to store and process information at the nanoscale. In the presence of asymmetric exchange interactions, their chirality, which governs their dynamics, is generally considered as an intrinsic parameter set during the sample deposition. In this work, we experimentally demonstrate that a gate voltage can control this key parameter. We probe the chirality of skyrmions and chiral domain walls by observing the direction of their current-induced motion and show that a gate voltage can reverse it. This local and dynamical reversal of the chirality is due to a sign inversion of the interfacial Dzyaloshinskii-Moriya interaction that we attribute to ionic migration of oxygen under gate voltage. Micromagnetic simulations show that the chirality reversal is a continuous transformation, in which the skyrmion is conserved. This control of chirality with 2 - 3 V gate voltage can be used for skyrmion-based logic devices, yielding new functionalities., Comment: 4 figures

The domain wall dynamics driven by an out of plane magnetic field was measured for a series of magnetic trilayers with different strengths of the interfacial Dzyaloshinskii-Moriya interaction (DMI). The features of the field-driven domain wall velocity curves strongly depend on the amplitude of the HD field stabilising chiral N\'eel walls. The measured Walker velocity, which in systems with large DMI is maintained after the Walker field giving rise to a velocity plateau up to the Slonczewski field HS, can be easily related to the DMI strength. Yet, when the DMI field HD and the domain wall demagnetising field HDW have comparable values, a careful analysis needs to be done in order to evaluate the impact of the DMI on the domain wall velocity. By means of a one-dimensional model and 2D simulations, we successfully extend this method to explain experimental results to cases where HD and HDW are comparable., Comment: 9 pages, 3 Figures

The perpendicular magnetic anisotropy at the $\mathrm{Co}$-oxide interface in $\mathrm{Pt}/\mathrm{Co}/{\mathrm{MO}}_{x}$ (${\mathrm{MO}}_{x}={\mathrm{Mg}\mathrm{O}}_{x}$, $\mathrm{Al}{\mathrm{O}}_{x}$, $\mathrm{Tb}{\mathrm{O}}_{x}$) is modified by an electric field using a 10-nm-thick ${\mathrm{Zr}\mathrm{O}}_{2}$ as a solid electrolyte. The large voltage-driven modification of the interfacial magnetic anisotropy and the nonvolatility of the effect is explained in terms of the migration of oxygen ions toward or away from the $\mathrm{Co}/{\mathrm{MO}}_{x}$ interface. While the effect is reversible in ${\mathrm{Pt}/\mathrm{Co}/\mathrm{Al}\mathrm{O}}_{x}$ and ${\mathrm{Pt}/\mathrm{Co}/\mathrm{Tb}\mathrm{O}}_{x}$, where the $\mathrm{Co}$ layer can be oxidized or reduced, in ${\mathrm{Pt}/\mathrm{Co}/\mathrm{Mg}\mathrm{O}}_{x}$ the effect is found to be irreversible. We propose that these differences may be related to the different nature of the ionic conduction within the ${\mathrm{MO}}_{x}$ layers.

The magnetization reversal of perpendicular exchange-biased [Pt/Co]5/Pt/FeMn multilayers was investigated by magneto-optical Kerr microscopy. The effect of varying the Pt interlayer thickness between the topmost Co layer and the FeMn layer was studied. Our measurements reveal that coercivity (HC) and exchange bias field (HEB) vary with Pt spacer thickness, and have a maximum for a Pt thickness of 0.4 nm. Magnetization reversal measurements show that, for the unbiased multilayer, the reversal is dominated by domain wall propagation starting from a few nucleation centers. For the exchange-biased multilayers, magnetization reversal occurs by nucleation and domain wall propagation with a variation in the nucleation density. For 0 and 0.2 nm Pt spacer, no difference is observed in the nucleation density of the ascending and descending branches unlike for 0.4 and 0.6 nm Pt spacer where the measurements reveal a larger density of nucleation centers in the descending branch.

We have investigated the effect of annealing the buffer layer on the magnetization reversal behavior in Pd/Co/Pd thin films using magneto-optical Kerr microscopy. It was found that annealing the buffer layer at 150 °C for 1 h decreases the coercivity and increases the saturation magnetization and the effective magnetic anisotropy constant. This study also shows that the annealing induces a change of the magnetization reversal from a mixed nucleation and domain wall propagation process to one dominated by domain wall propagation. This result demonstrates that the main effect of annealing the buffer layer is to decrease the domain wall pinning in the Co layer, favoring the domain wall propagation mode.

We present an experimental study of the magnetization reversal in Au/Co layers with perpendicular anisotropy versus variable magnetic field. Changes in domain structure were recorded simultaneously with the measurement of the hysteresis loop by a Kerr microscope in polar configuration. The magnetization reversal is performed by domain wall propagation in the two-way up to down and inversely. In addition, it was shown experimentally that the evolution of the domain structure is almost the same along both branches of the hysteresis loop. The magnetization reversal is always triggered at the same places of the sample independently of the applied magnetic field direction.

The manipulation of magnetism with a gate voltage is expected to lead to the realization of energy- efficient spintronics devices and high-performance magnetic memories. Exploiting magnetoionic effects under micropatterned electrodes in solid-state devices adds the possibility of modifying magnetic prop- erties locally, in a nonvolatile and reversible way. Tuning magnetic anisotropy, magnetization and Dzyaloshinskii-Moriya interaction allows the modification “at will” of the dynamics of nontrivial mag- netic textures such as skyrmions and chiral domain walls in magnetic race tracks. In this work, we illustrate efficient magnetoionic effects in a ferrimagnetic Pt/Co/Tb/AlOxstack using a ZrO2thin layer as a solid- state ionic conductor. When a thin layer of terbium is deposited on top of cobalt, it acquires a magnetic moment that aligns antiparallel to that of cobalt, reducing the effective magnetization. Below the micropat- terned electrodes, the voltage-driven migration of oxygen ions in ZrO2toward the ferrimagnetic stack partially oxidizes the Tb layer, leading to the local variation not only of the magnetization, but also of the magnetic anisotropy and of the Dzyaloshinskii-Moriya interaction. This leads to a huge increase in the domain wall velocity, which varies from 10 m/s in the pristine state to 250 m/s after gating. This non- volatile and reversible tuning of the domain wall dynamics may lead to applications to reprogrammable magnetic memories or other spintronic devices.