Quantifying spin torques in CoO(001)/Pt bilayers by comparing field- and current-induced switching

Abstract

Antiferromagnets (AFMs) are promising materials for spintronics, that compared to ferromagnets exhibit faster dynamics, enhanced stability with respect to interfering magnetic fields and higher bit packing density, thanks to the absence of long-range magnetic coupling via stray fields. However, the absence of a net magnetic moment makes manipulation using conventional magnetic fields challenging [1]. For the use of AFMs, one requires efficient electrical writing and reading. Recently, there have been reports on the current-induced switching of the Néel vector orientation [2-5]. In particular, we demonstrated that in the insulating antiferromagnet NiO/heavy metal Pt thin film system there are multiple effects contributing to the electrical signal [6]. These include the switching of the magnetic Néel order, whose mechanism is debated in terms of origin and efficiency [2-6], but also non-magnetic signals possibly related to annealing or electromigration in the Pt layer [7]. While NiO has been intensively studied, key information such as the torque strength is missing so far. Using a different AFM such as CoO, that we study here [8], entails a number of advantages: we first show that, due to the compressive strain by the MgO substrate, a fourfold in-plane magnetic anisotropy of the CoO layer with two easy axes of the Néel order in the (001) plane is favored, and the spin flop field is accessible (around 7 T at 200 K). Such a system with two orthogonal stable states (along the [110] and [-110] directions) is ideal for applications where the orientation of n is read by spin Hall magnetoresistance (SMR) [4, 9, 10]. Furthermore, we achieve electrical switching and probe its symmetry, where the accessible spin flop field allows us to directly compare the effects of fields and currents. By looking at the switching above and below the Neél temperature in the CoO/Pt bilayer, we can show that this switching is of magnetic origin, as the signal related to the antiferromagnetism disappears above the Néel temperature, which in CoO is easily accessible around room temperature. For the switching, 8-arms Hall stars devices are used, made of Pt 2 nm thick, with the pulsing arms orientated along the [110] and [-110] easy axes directions (Fig.1). Before the pulses, the Néel vector n is aligned along [110]. When current pulses are applied along the orientation given by the contact combination 3-2 (initial state n∥jpulse), the transverse resistance drops in a step-like fashion, indicating a current-induced 90° n rotation (Fig. 1a). Performing a MR scan with field along the contact combination 4-1 after the current pulses, shown in Fig. 1b, yields a field-induced spin flop transition of n back to the initial state (along [110]). No background was subtracted to the data in Fig. 1, meaning that at this temperature (200 K), pulse length (1 ms), average current density (jpulse = 1.15x1012 A m−2) and in this switching configuration (straight pulses), where the effect of hot corners is not important, the non-magnetic effects are negligible and the electrical signal is dominated by a step-like magnetic contribution. At higher current densities the non-magnetic signal, possibly related to a local annealing process and electromigration in the Pt layer, can be observed and does not disappear above the Néel temperature, in contrast to the magnetic one. Finally, one can see how the threshold and saturation current density of the switching vary as a function of the magnetic field applied during the switching. By doing this, we find a linear relation between the current and the magnetic field and we ascertain a current-field equivalence ratio of value 4x10−11 T A−1 m2, that is orders of magnitude higher than for ferromagnets. The Néel vector final state (n⊥j) in the center of the device and the linear current-field dependence are in line with a thermomagnetoelastic switching mechanism for a negative magnetoelastic constant of the CoO, whose sign is at the moment not known for CoO thin films. With this we show that, for the switching of AFMs, currents are much more efficient than magnetic fields. **