Electric-field control of ferromagnetism through oxygen ion gating

Electric-field-driven oxygen ion evolution in the metal/oxide heterostructures emerges as an effective approach to achieve the electric-field control of ferromagnetism. However, the involved redox reaction of the metal layer typically requires extended operation time and elevated temperature condition, which greatly hinders its practical applications. Here, we achieve reversible sub-millisecond and room-temperature electric-field control of ferromagnetism in the Co layer of a Co/SrCoO2.5 system accompanied by bipolar resistance switching. In contrast to the previously reported redox reaction scenario, the oxygen ion evolution occurs only within the SrCoO2.5 layer, which serves as an oxygen ion gating layer, leading to modulation of the interfacial oxygen stoichiometry and magnetic state. This work identifies a simple and effective pathway to realize the electric-field control of ferromagnetism at room temperature, and may lead to applications that take advantage of both the resistance switching and magnetoelectric coupling.


Supplementary Note 1. Forming process for the resistance switch.
We note that the forming procedure is relating with the creation of defects in the pristine oxides. A fine scan of the forming process is performed as shown in Supple mentary Figure 3, where the slope of the Log-Log fitting increases suddenly from 1 to 10 at about 1 V during the forming process, which can be attributed to the creation of substantial defect concentration during forming. Naturally, the defect formation and redistribution would greatly modify the conducting mechanism through the Co and SrCoO 2.5 interface.
We note that the pristine SrCoO 2.5 possesses an ordered oxygen vacancy channel, which ensures a pronounced oxygen diffusion coefficient. Thus, with the application of electricfield during the forming process, the oxygen ions in bulk would diffuse towards the interfacial layer and lead to the formation of interfacial oxygen-rich disordered regions.
Thus, the high resistance state (HRS) is an intrinsically disordered system with large amounts of defects, in which the resistance can be notably smaller than the pr istine Co/SrCoO 2.5 state (Supplementary Figure 4). We would like point out that similar changes have been widely discovered in other resistance switching systems as well 1,2 . We further carried out controlled experiments with the voltage scan stopped right before the HRS is fully established (cycle 1 in Supple mentary Figure 3). The result shows that the formed conducting state is not stable, and recovers back to the insulating pristine state when the voltage is turned off. This result can be understood with the facts that the oxygen-rich disordered region formed during the forming process are still in the transition state at the early stage. Hence once the applied field is gradually removed, the resistance state will be relaxed back to the pristine state. However, if the equilibrium state is established with higher voltage (cycle 2 in Supplementary Figure 3), the resistance state would be non-volatile, forming the intriguing property of the resistance switch. structures, the disordered layer created during the forming process can be regarded as a self-constructed asymmetric oxide-based RRAM cell. The cycle 1 demonstrates the I-V behavior when the disordered layer is not fully established. The result shows that the formed conducting state is not stable, and recovers back to the insulating pristine state when the voltage is turned off. This phenomenon can be understood with the facts that the oxygen-rich disordered region formed during the forming process are still in the transition state at the early stage. Hence once the applied field is gradually removed, the resistance state will be relaxed back to the pristine state. However, if the equilibrium state is established with higher voltage (cycle 2), the resistance state would be non-volatile, forming the intriguing property of the resistance switch. Measurements were performed at room-temperature with the reading voltage of 0.1 V. 6

Supplementary Note 2. Intrinsic oxygen diffusion barrier determined switching speed
We note that the formation of the disordered layer and the filaments can be related with the oxygen migration, and thus we speculate that the switching speed might be attributed to the intrinsic oxygen ionic conductivity within the SrCoO 2.5 . According to the Fick's law, the diffusion length (L) can be estimated as 3 :

= 2√
(1) where D and t are diffusion coefficient and time, respectively. When oxygen ion diffuses along the out-of-plane direction as proposed in the reference work 4 with D=1.2×10 -12 cm -2 s -1 at room-temperature 5 , it will take about 3×10 -2 s for oxygen ion to diffuse 4 nm (typical thickness of the disordered layer), which is about two orders of magnitude slower than the experimental results (as shown in Figure 1d). However, we note that the diffusion coefficient for during this estimation is the zero-field parameter, which would be further enhanced with the application of electric field. Thus, the experimental resistance switching speed can be increased as well. Nevertheless, this estimation supports the proposal that the current reported switching speed could be limited by the intrinsic ionic mobility of the oxygen in SrCoO 2.5 . The structural periodicity is determined with local FFT analysis as shown in the inset of (c) and (f).

Supplementary Note 3. Inhomogeneous distribution of conducting filaments.
Conventionally, the filamentary and homogeneous resistance switch mechanisms can be distinguished by the area dependence of the resistance values in high resistance and low resistance states 6 . The extreme cases are single filamentary type and homogeneous interfacial type, where the resistance of the former one is independent of the contact area, while the resistance is in linear relationship with the contact area for the latter one 6 .
However, current studies demonstrate that many resistance switch systems are dominated by the combination of these two cases 7 . Furthermore, recent numerical calculations and experiment researches demonstrate that the area dependency of the resistance states could also exist in multi-filament resistance switch systems, similar as the behavior of homogenous interfacial type switching 8  respectively. When the distance between the laser spot and the probe is increased from 20 μm to 100 μm, the magnetic modulation efficiency drops from ~30% to 0%.
When the measurements performed at different sites with the same distance, we obtained similar magnetic modulation results, indicating that the distance is the key parameter to influence the magnetoelectric coupling. as revealed by the current study.

Supplementary Note 4. Inhomogeneous oxygen-ion distribution at the interface .
To obtain the direct information at the interface, we have carried out extended TEM studies with different distances from the probe within the same electrode (diameter of 200 μm), as shown in Supplementary Figure 10. The disordered layer is defined as the region where superlattice diffraction patterns are destroyed as evaluated by the local FFT analysis. Accordingly, the average thicknesses of the disordered region for 10 μm, 20 μm and 50 μm are 6.1±1.0 nm, 4.8±1.8 nm and 3.2±0.7 nm, respectively, which presents a clear variation of the non-uniform oxygen distribution across the device. More importantly, when the distance is increased to ~100 μm, the disordered region could hardly be resolved at the interface, indicating that the interface remains close to the pristine state despite of the application of the electric field. These results are consistent with the previous studies in Co/GdOx system, where the oxygen ion modulation is pronounced at region close to the probe contact 14 . Finally, it is important to note that, since the pristine state possesses higher resistance than both HRS and LRS, the remained pristine area will not influence the I-V modulation observed in the study.