Spintronics, which is the basis of a low-power, beyond-CMOS technology for computational and memory devices, remains up to now entirely based on critical materials such as Co, heavy metals and rare-earths. Here, we show that Mn4N, a rare-earth free ferrimagnet made of abundant elements, is an exciting candidate for the development of sustainable spintronics devices. Mn4N thin films grown epitaxially on SrTiO3 substrates possess remarkable properties, such as a perpendicular magnetisation, a very high extraordinary Hall angle (2%) and smooth domain walls, at the millimeter scale. Moreover, domain walls can be moved at record speeds by spin polarised currents, in absence of spin-orbit torques. This can be explained by the large efficiency of the adiabatic spin transfer torque, due to the conjunction of a reduced magnetisation and a large spin polarisation. Finally, we show that the application of gate voltages through the SrTiO3 substrates allows modulating the Mn4N coercive field with a large efficiency.

We study field-driven domain wall (DW) velocities in asymmetric multilayer stacks with perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction (DMI), both experimentally and by micromagnetic simulations. Using magneto-optical Kerr microscopy under intense and nanoseconds-long fields, we show that DWs in these films propagate at velocities up to hundreds of m/s and that, instead of the expected decrease of velocity after the Walker field, a long plateau with constant velocity is observed, before breakdown. Both the maximum speed and the field extent of the velocity plateau strongly depend on the values of the spontaneous magnetization and the DMI strength, as well as on the magnetic anisotropy. Micromagnetic simulations reproduce these features in sufficiently wide strips, even for perfect samples. A physical model explaining the microscopic origin of the velocity plateau is proposed. I.arXiv:1908.08282v1 [cond-mat.mes-hall]

interaction, like Pt/Co, but surprisingly also at FM-oxide interfaces. [2] This is attributed to the hybridization between the metal and the oxygen electronic orbitals across the interfaces, as confirmed by ab initio calculations. [3] The amplitude of this interaction is strongly sensitive to the degree of oxidation of the metal/oxide interface. Previous studies carried out for several Pt/Co/oxide stacks show that the PMA evolves with the cobalt oxidation rate following a typical bell-like curve. [2,[4][5][6][7] The maximum PMA is obtained for the optimal oxidation of the FM/oxide interface, when all the metal interfacial FM atoms are bonded to oxygen atoms. If the contribution of the Pt/FM interface to the PMA is not sufficient to promote out-ofplane magnetization, an increase of the FM oxidation may help driving the magnetization from in-plane (IP) to out-plane (OOP) (see Figure 3b)The interfacial anisotropy at the FMoxide interface can be manipulated by an electric field via electronic [8][9][10] or magneto-ionic effects. [1] The largest effects of the electric field on the PMA were obtained by triggering the oxidation of cobalt via the migration of oxygen ions. [11][12][13] Up to recent years, this mechanism was observed to be slow and to require high temperatures, since the voltage-driven switching between IP and OOP magnetization was shown to require several minutes at room temperature. [11] A voltage-driven mechanism in which the modification of interfacial PMA is attributed to the migration of hydrogen, rather than to the less mobile oxygen ions, was recently proposed as a more efficient [14,15] and faster (down to 2 ms) [16] way to switch the magnetization easy axis direction.In this work, we show that by depositing on top of the magnetic stack a thin ZrO 2 layer acting as an oxygen ion conductor, a huge voltage-driven variation of the PMA can be obtained for two model systems for spintronic applications, Pt/Co/AlO x and Pt/Co/TbO x trilayers. While Pt/Co/AlO x was the first system in which chiral domain walls could be driven efficiently by spin-orbit torque, [17] Pt/Co/TbO x may become ferrimagnetic, with reduced magnetization, and acquire interesting domain wall dynamics properties, if terbium is sufficiently reduced. [18] As reported in our previous work, [19] the PMA of these systems Magneto-ionics is a fast developing research field which opens the perspective of energy efficient magnetic devices, where the magnetization direction is controlled by an electric field which drives the migration of ionic species. In this work, the interfacial perpendicular magnetic anisotropy (PMA) of Pt/ Co/oxide stacks covered by ZrO 2 , acting as a ionic conductor, is tuned by a gate voltage at room temperature. A large variation of the PMA is obtained by modifying the oxidation of the cobalt layer through the migration of oxygen ions: the easy magnetization axis can be switched reversibly from in-plane, with under-oxidized Co, to in-plane, with over-oxidized Co, passing through an out-of-plane magnetization state. T...

Spin-transfer torque (STT) and spin–orbit torque (SOT) are spintronic phenomena allowing magnetization manipulation using electrical currents. Beyond their fundamental interest, they allow developing new classes of magnetic memories and logic devices, in particular based on domain wall (DW) motion. In this work, we report the study of STT-driven DW motion in ferrimagnetic manganese nickel nitride (Mn4–x Ni x N) films, in which magnetization and angular momentum compensation can be obtained by the fine adjustment of the Ni content. Large domain wall velocities, approaching 3000 m/s, are measured for Ni compositions close to the angular momentum compensation point. The reversal of the DW motion direction, observed when the compensation composition is crossed, is related to the change of direction of the angular momentum with respect to that of the spin polarization. This is confirmed by the results of ab initio band structure calculations.

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The actual mechanisms occurring at interfaces underlying the Dzyaloshinskii-Moriya interaction (DMI) remain a question in nanomagnetism. In this study, we investigate the origin of the interfacial DMI, aiming at estimating how independent the DMI contributions of the two interfaces of a FM layer are and what their relative weight in the effective DMI amplitude is. To this aim, we explore the correlation between the effective interfacial DMI and the metal properties, namely, atomic number, electronegativity, and the work function of the metal M. A clear linear relationship is found between the interfacial DMI and the work function difference at the Co/M interface. This result is strong evidence of the independent DMI contributions of the two interfaces for the chosen Co thickness(1 nm). It also suggests that the Co/Cu interface bears no interfacial DMI. These findings can guide the optimization of the magnetic properties of future devices.

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We studied the symmetry of magnetic properties and the resulting magnetic textures in ultra-thin epitaxial Au0.67Pt0.33/Co/W(110), a model system exhibiting perpendicular magnetic anisotropy and interface Dzyaloshinskii–Moriya interaction (DMI). As a peculiar feature, the C2v crystal symmetry induced by the Co/W interface results in an additional uniaxial in-plane magnetic anisotropy in the cobalt layer. Photo-emission electron microscopy with magnetic sensitivity reveals the formation of self-organised magnetic stripe domains oriented parallel to the hard in-plane magnetisation axis. We attribute this behavior to the lower domain wall energy when oriented along this axis, where both the DMI and the in-plane magnetic anisotropy favor a Néel domain wall configuration. The anisotropic domain wall energy also leads to the formation of elliptical skyrmion bubbles under a weak out-of-plane magnetic field.