Morphology control of volatile resistive switching in La0.67Sr0.33MnO3 thin films on LaAlO3 (001)

The development of in-memory computing hardware components based on different types of resistive materials is an active research area. These materials usually exhibit analog memory states originating from a wide range of physical mechanisms and offer rich prospects for their integration in artificial neural networks. The resistive states are classified as either non-volatile or volatile, and switching occurs when the material properties are triggered by an external stimulus such as temperature, current, voltage, or electric field. The non-volatile resistance state change is typically achieved by the switching layer’s local redox reaction that involves both electronic and ionic movement. In contrast, a volatile change in the resistance state arises due to the transition of the switching layer from an insulator to a metal. Here, we demonstrate volatile resistive switching in twinned LaAlO3 onto which strained thin films of La0.67Sr0.33MnO3 (LSMO) are deposited. An electric current induces phase transition that triggers resistive switching, close to the competing phase transition temperature in LSMO, enabled by the strong correlation between the electronic and magnetic ground states, intrinsic to such materials. This phase transition, characterized by an abrupt resistance change, is typical of a metallic to insulating behavior, due to Joule heating, and manifested as a sharp increase in the voltage with accompanying hysteresis. Our results show that such Joule heating-induced hysteretic resistive switching exhibits different profiles that depend on the substrate texture along the current path, providing an interesting direction toward new multifunctional in-memory computing devices.


Introduction
In modern computers, the computation and memory units are separated, leading to a large amount of memory traffic for data retrieval and storage. In contrast, computation in the brain is performed in the memory itself; this co-location of memory and processing reduces latency and energy consumption. Hence, there is a significant interest in developing architectures that mimic brain-like neuronal networks. Hardware components that can emulate brain-like characteristics and possess analog functionalities are promising building blocks for such architectures. A key functionality associated with these devices is resistive switching that relies on the non-linear electrical transport of the switching material driven by an external stimulus.
In strongly correlated electronic systems (Hwang et al., 2012;Rana et al., 2013;Sulpizio et al., 2014;Roy et al., 2015), correlated oxides are an important material class. Such materials exhibit switching from an insulator to a metal, triggered by various external stimuli such as temperature, pressure, or electric field (Zhang et al., 2020). Mott insulators such as VO 2 , V 2 O 3 , and NbO 2 are examples of such materials (Stoliar et al., 2014;Kumar et al., 2017;Valmianski et al., 2018;Kalcheim et al., 2020). When electrically triggered, they undergo a change in resistance, spanning over an order of magnitude, changing from an insulating behavior below its critical temperature (T C ) to a metallic state above it (Imada et al., 1998), rendering them useful for volatile resistive switching applications. Such switching of resistive states between two phases involves a structural transition that happens at high operational temperatures. These materials are reported to be quite efficient for mimicking certain brain functionalities, including integrate and fire neurons and short-term memory, and involve the requirement of high temperature to drive the structural transformation to derive the distinct resistive phases (Liu et al., 2016;Kumar et al., 2017). In this context, the quest for material systems that exhibit volatile resistive switching but not involving any structural transition is a promising research direction.
In this work, we demonstrate volatile resistive switching in strained thin films of La 0.7 Sr 0.3 MnO 3 (LSMO) deposited on LaAlO 3 (LAO) substrates. LSMO is a mixed-valence doped material in the La 1−x Sr x MnO 3 manganite family, displaying a wide range of electronic and magnetic phases (Izyumskaya et al., 2009) depending on the doping and ionic radii of the divalent cations. Thin films of LSMO are metallic ferromagnets below their T C of approximately 370 K and undergo transition to insulating paramagnets above this temperature. In our earlier works (Burema et al., 2019;Burema and Banerjee, 2021), we have demonstrated that compressively strained thin films of La 0.7 Sr 0.3 MnO 3 on a LaAlO 3 substrate with twin domains lead to the coexistence of different magnetically ordered phases, as revealed from magnetization and electronic transport measurements. This was ascribed to the straininduced Jahn-Teller distortion that favors d 3z2−r2 orbitals over d x2−y2 orbitals at such compressively strained interfaces. The coupling between the electronic and magnetic phases in such strained films, without any accompanying structural transition, makes them good candidates for volatile resistive switching operation. In this work, we electrically induce resistive switching, by Joule heating, in 10-nm strained LSMO films deposited on textured substrates of LAO. We observe volatile resistive switching in LSMO and use the morphology of twin planes in LAO to capture the nature of resistive switching across different connection pathways. By designing contacts that intersect differently oriented twin planes, we demonstrate abrupt changes in the film resistance, with accompanying hysteresis at temperatures close to the phase transition. We foresee that volatile and nonvolatile switching can be realized in such LSMO films deposited on LAO by a careful choice of both the operating temperature and the magnitude of the input stimuli. Such features are useful for charge-based oscillators that can mimic the spiking behavior of neurons in the human brain, relevant for in-memory computing applications.

Materials and methods
LSMO films of 10 nm thickness were grown on textured (001) pc oriented LAO substrates using pulsed laser deposition (PLD). A KrF excimer laser (248 nm) was focused onto an LSMO target with a laser fluence of 2 J/cm 2 and a repetition rate of 1 Hz. During the growth, the substrate temperature was kept at 750°C, at an oxygen pressure of 0.35 mbar. After deposition, the films were cooled to room temperature at an oxygen pressure of 100 mbar. Different X-ray diffraction techniques, including X-ray reflectivity (XRR) and reciprocal space mapping (RSM), were used to infer the structure, thickness, and strain of the deposited films. Temperature-dependent magnetic measurements, using a superconducting quantum interference magnetometer (SQUID), were carried out to determine the magnetic phase of the films. For electrical measurements, Ti/Au gold contacts were deposited at the four corners of the film using UV-lithography and electron beam evaporation. Current-and voltage-controlled resistive switching measurements were performed using a Keithley 2451source meter.

Results
The growth of the films is monitored using the high-energy electron diffraction (RHEED) technique. The observed diffraction spots at (00), (01), and (01) and RHEED oscillations are shown in Supplementary Figure S1A. The intensity distribution of the central spot over time suggests a 2D layer-by-layer growth, and their streaky nature suggests the formation of domains (Haeni et al., 2000). Figure 1A shows the θ-2θ scan with the signature Bragg reflection peak of LSMO (002) at 46.4°, confirming the growth of LSMO on the substrate. The out-of-plane lattice parameter is calculated to be 3.90 Å, whereas the bulk lattice parameter of LSMO is 3.88 Å. This suggests an out-of-plane elongation of the MnO 6 octahedra with a strain percentage of 0.51%. Figure 1B shows the 3D RSM around the (103) crystallographic peak of the substrate and the film, confirming the epitaxial growth of LSMO on the LAO substrate. The two peaks are aligned with the same in-plane lattice parameter of 3.79 Å, indicating that the film is strained with an inplane strain percentage of~−0.25%. A slight smearing of the film peak along the upper left shoulder is observed, which indicates that the crystalline direction is not uniform throughout the film due to the presence of twin boundary domains. La 0.7 Sr 0.3 MnO 3 is a ferromagnetic metal below its Curie temperature, T C . To characterize the magnetic properties, the zero-field-cooled (ZFC) magnetization (M) dependence on temperature (T) was measured at a running field of 50 Oersted, along the in-plane direction, while warming up the film. Figure 2A shows that the magnetization follows the Curie-Weiss law, with ferromagnetic behavior at low temperatures (below T C ) and paramagnetic behavior above T C . The T C is found to be 325 K, which is determined from the first derivative of the magnetization with respect to temperature, as shown on the right axis of Figure 2B. The sheet resistance of the film was determined in the van der Pauw configuration. The dependence of resistance (R) on temperature (T) was extracted from the voltage drop recorded across the contacts by applying a direct current (dc) of 1 µA. The shape of the R vs T curve, as shown in Figure 2C, indicates that the film shows metallic behavior at low temperatures Frontiers in Nanotechnology frontiersin.org 02 FIGURE 2 (A) Magnetization (black) and resistance (red) vs temperature. The region in white is the ferromagnetic metallic regime, and the green-shaded region is the paramagnetic insulating regime. (B) Zero-field-cooled magnetization vs temperature plot obtained with a running magnetic field of 50 Oe shows that the film follows the Curie-Weiss law exhibiting ferromagnetic behavior up to T C~3 25 K, determined by the slope change (dM/dT) with respect to temperature. (C) Zero-field-cooled resistivity vs temperature dependence obtained with a fixed direct current of 1 µA in the van der Pauw configuration, which shows metallic behavior up to T MIT~3 25 K, determined by the slope change (dR/dT) with respect to temperature.
Frontiers in Nanotechnology frontiersin.org and insulating behavior at high temperatures. The metal-toinsulator transition temperature (T MIT ) is found to be 325 K, which is determined from the first derivative of R with T, as shown on the right axis of Figure 2C. The strong correlation between T C and T MIT shows that the electronic and magnetic properties are coupled, as shown in Figure 2A. This arises due to the double exchange interaction (Dörr, 2006), which promotes parallel alignment of the manganese (Mn) magnetic moments by enhancing the hopping probability of the itinerant electrons from Mn 3+ sites to adjacent Mn 4+ sites, leading to a low resistance state. Close to the transition temperature, the thermal energy destabilizes this parallel alignment by reducing the hopping probabilities of the itinerant electrons, yielding a high resistance state in the paramagnetic state (Tokura and Tomioka, 1999;Haghiri-Gosnet and Renard, 2003;Štrbík et al., 2014). Ti/Au gold contacts were deposited at the four corners of the sample for resistive switching experiments. One pair of contact is chosen at each edge, to source the current, and another pair is used to sense the voltage at the opposite edge. This configuration enables us to probe the entire surface of the film in four different combinations and directions. Voltage (V) vs current (I) sweeps were taken in all directions at different temperatures, as shown in Figure 3. Each graph shows a transition from a linear to non-linear transport across the entire substrate with increasing current. At 225 K, below T MIT , the measured voltage is linear up to 3.25 mA (left-vertical), 2.27 mA (right-vertical), 2 mA (top-horizontal), and 3.62 mA (bottom-horizontal). This is followed by an abrupt change in the slope of the resistance as the current is further increased. This is associated with the transition of the film from a ferromagnetic lowresistance state to a paramagnetic high-resistance state accompanied by a small hysteresis due to Joule heating. A further slope change is observed for higher currents, after which the measured voltage levels off near compliance voltage. The film returns to its initial lowresistance state when the sourced current is ramped to zero, indicative of volatile resistance behavior. Furthermore, as the temperature is increased closer to T MIT , the threshold current for switching decreases, as shown in Figure 3. This is consistent with the associated metal-to-insulator transition, as observed in the R-T measurements (shown in Supplementary Figure S6B). Similarly, the voltage-controlled sweeps (shown in Supplementary Figure  S6A) show a rapid increase in current beyond a threshold voltage. Thus, in both measurement types, we identify distinct regimes of changes in resistive states across the entire substrate at each temperature. The corresponding I-V curves measured in all directions show a systematic variation with increasing temperature (Figure 3) (See Supplementary Figure S5 and Supplementary Figure  S13). Furthermore, we have carried out similar measurements on different twinned substrates and also on LSMO films of different thicknesses (Supplementary Figures S11, S12). Schematics of the current conduction path along different directions of the films  Figure S9). The optical image of the two substrates shows that the twin domain density, orientation, and homogeneity are different for substrates A and B. The distribution of twin domains influences the magnitude of the current needed to drive the LSMO phase transition, resulting in differences between the two samples and along the different directions as shown in Figures 4A, C. Furthermore, we see that the resistance post-switching and the current at which the voltage (~15 V) levels off are also different for the vertical and horizontal directions. In uniformly terminated substrates, such as in SrTiO 3 , it is unusual for the dc electrical resistance of LSMO thin films to depend on the measurement direction (Oliveira et al., 2020). In twinned substrates of LAO, however, our findings show that the density, as well as the orientation of the twin domains, yields differences in the resistance when measured along or across the twin boundaries, induced by local changes in strain and electronic properties of the LSMO films.

Discussion
In manganites, non-volatile resistive phase transitions involving structural and stoichiometric changes are well studied (Asamitsu et al., 1997;Kalkert et al., 2011;Jeon et al., 2021). In LSMO, for example, it has been shown that oxygen vacancy-driven resistive phase transitions occur and are accompanied by structural transformations from a perovskite to a brownmillerite phase (Brockman et al., 2014). Volatile resistive switching, on the other hand, utilizes the strong coupling between electronic and magnetic phases (Salev et al., 2021) and does not involve structural phase transitions (See Supplementary Figure S3 and Supplementary Figure S8).
In our case, we determine the resistive switching to be volatile in nature and related to a metal-to-insulator transition in LSMO. We demonstrate electrical control of this transition in Figures 3, 4. Joule heating induces a local increase in temperature, triggering a phase transition when the film temperature locally exceeds T MIT . This is manifested as an abrupt increase in voltage across the entire film and measured by a pair of contacts positioned across different corners of the substrate. The film returns to its original state when the current is ramped down, thus representing a volatile change in the resistance state (Supplementary Figure S10A shows no film degradation up to 100 cycles). By increasing the measurement temperature and bringing it closer to T MIT , the current required for the transition to occur is lowered (Figures 3, 4 and Supplementary Figure S4). Due to the substrate's morphology involving the random orientation of twin domains, the resistance is expected to depend on whether it is measured across the horizontal or vertical contacts. Rhombohedral wedge-shaped twin domains are frequently observed in the plane of such LAO substrates (shown in Figure 1C and Supplementary Figure S2A). Although these domains are unidirectional, the Frontiers in Nanotechnology frontiersin.org spacing and angle between the twin boundaries are not equal in all directions. This creates differences in the strain environment, leading to a distribution of ferromagnetic metallic phases with small differences in the oxygen of the MnO 6 octahedra, resulting in a complex structure. In such a network, the electrical conductivity is distinctly controlled by the hopping probability of the itinerant d electrons from one Mn site to another, which varies locally due to local differences in the Mn-O-Mn bond angle across or along the twin domains as shown in Figure 3. The accompanying hysteresis results from the lower current needed to maintain the same voltage during retrace. Bare LAO substrates were measured as shown in Supplementary Figure S7; in this case, the resistance is found to be high, and no hysteretic behavior is observed.

Conclusion
The current-induced Joule heating-triggered resistive switching that our experiments demonstrate is unlike i) the widely studied Mott insulators that transform to a metallic state from an insulating state by forming a conducting filament parallel to the current flow or ii) LSMO films on well-ordered substrates, such as STO, where the switch from a metallic to an insulating state happens by the formation of a transverse barrier perpendicular to the current flow (Salev et al., 2021). By utilizing a substrate possessing twin domains, we are able to build a complex resistive structure based on the electronic phase transition. The local variations in strain results in a distribution of coexisting ferromagnetic metallic states. Given that each of these phases can transition to a paramagnetic phase at a nominally different threshold current value, this further increases the number of states in the complex structure. Additional tuning of the phase transition temperature is possible by controlling the epitaxial strain through film thickness, which allows control over the switching energy. Both volatile and non-volatile switching can be realized in the same film depending on the proximity of the operating temperature to the transition temperature and on the magnitude of the input stimuli. Contrary to the existing Mott-based memories, no structural transition is associated with the resistive phase transition demonstrated here, making the switching highly stable over a large number of cycles. Our work opens new possibilities for studying such coexisting states using nanoscale probing techniques (Banerjee et al., 2005) as well as for incorporating other transition metal oxides that can provide nonlinear strain coupling due to their (pseudo)cubic crystal structure, useful for brain-inspired computing structures. These structures may be exploited in emergent complex learning functionalities ranging from shortest path optimization (Pershin and Di Ventra, 2013) to associative memory (Diaz-Alvarez et al., 2020) or providing a framework for reservoir computing applications.

Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions
AJ and TB conceived the idea, AJ and AG planned the different experiments including fabrication of the devices, JvR helped with the measurements and data analysis along with LvdZ. The paper was written by AJ and TB with useful input from all authors.