Hall effect of asymmetric La0.7Sr0.3MnO3/SrTiO3/SrRuO3 and La0.7Sr0.3MnO3/BaTiO3/SrRuO3 superlattices

The Hall resistivity and magnetization of high quality asymmetric La0.7Sr0.3MnO3/SrTiO3/SrRuO3 and La0.7Sr0.3MnO3/BaTiO3/SrRuO3 superlattices were investigated. The Hall resistivity hysteresis curves have a complex magnetic field dependence in an intermediate temperature regime that is not reflected in the global magnetization. We interpret this as arising either from the formation of complex magnetization textures that lead to the appearance of a topological Hall effect or from the shift of the Weyl nodes in SrRuO3 by the coupling to the adjacent La0.7Sr0.3MnO3 magnetization. Published by AIP Publishing

Unconventional anomalous Hall effect driven by oxygen-octahedra-tailoring of the SrRuO3 structure

The anomalous Hall effect of SrRuO3 is of special interest, since Weyl nodes appear in the band structure and lead to an unconventional temperature dependence of the anomalous Hall constant. Moreover, it has been proposed that coupling of SrRuO3 films to materials with strong spin-orbit coupling or with ferroelectric or ferromagnetic order might lead to the formation of skyrmions and a topological contribution to the Hall effect. This latter conjecture is strongly debated. We probed this proposal by interfacing thin SrRuO3 layers to Pr0.7Ca0.3MnO3, since it is known that the strong antiferromagnetic coupling between these two ferromagnets leads to complex magnetization states. Superlattices with sharp interfaces were grown by pulsed-laser deposition. The epitaxial interfacing with the Pr0.7Ca0.3MnO3 layers led to major modifications of the structural symmetry of the SrRuO3 layers. High resolution scanning transmission electron microscopy revealed that the individual SrRuO3 layers of the superlattices had heterogeneous structure with varying oxygen octahedral tilt angles across the layers, turning their structure to be tetragonal-like, with largely suppressed octahedral tilts when the thickness of the neighboring Pr0.7Ca0.3MnO3 layers was increased. These structural modifications were accompanied by major changes in the field dependence of the Hall signal with the mainly tetragonal SrRuO3 layers showing features strongly reminiscent of a topological Hall effect. However, since there was an intimate link between Hall effect and structure, the Hall data were interpreted as arising from a superposition of Hall effect contributions from tetragonal and orthorhombic SrRuO3 sub-layers

Ultrafast nondestructive pyroelectric reading of FeRAM memories

FeRAM memories constitute themselves as a robust counterpart to the mainstream NVRAM memories. Their unique properties, such as radiation resilience and low voltage operation, together with high speed writing and reading, high and long lasting remanence and very good endurance to cycling endorse them in the data storage domain. However price, poor scalability, the destructive way of reading and the necessary consequent re-writing represent technical drawbacks that diminish their practical value. This paper presents a new paradigm of completing previous efforts towards fast nondestructive reading of FeRAMs. An intermittent low intensity IR (infrared) laser diode generates a pyroelectric signal in the ferroelectric material. The amplitude and phase of this response depends on the state of polarisation of the ferroelectric. Comparing the phase of the electric response with the phase of the illuminating radiation (on/off), a decisive indication of the poling sense in the ferroelectric is given. Aiming to circumvent the notorious slowness of the pyroelectric response the array is continuously illuminated with the pulsating laser, which ensures a prompt answer from every bit of memory. The reading speed is, practically, limited by the electronics

Back-scattered electron visualization of ferroelectric domains in a BiFeO3 epitaxial film

Three-dimensional orientation of the ferroelectric (FE) domain structure of a BiFeO3 epitaxial film was investigated by scanning electron microscopy (SEM) using back-scattered electrons and piezoresponse-force microscopy (PFM). By changing the crystallographic orientation of the sample and the electron collection angle relative to the detector, we establish a link between the orientation of polarization vectors (out-of-plane and in-plane) in the BiFeO3 film and the back-scattered electron image contrast in agreement with PFM investigations. The different FE polarization states in the domains correspond to altered crystalline environments for the impingent primary beam electrons. We postulate that the resultant back-scattered electron domain contrast arises as a result of either differential absorption (through a channelling effect) or through back-diffraction from the sample, which leads to a projected diffraction pattern super-imposed with the diffuse conventional back-scattered electron intensity. We demonstrate that SEM can be sensitive for both out-of-plane and in-plane polarization directions using the back-scattered electron detection mode and can be used as a non-destructive and fast method to determine 3D FE polarization orientation of domains. Published by AIP Publishing

Control of binary states of ferroic orders in bi-domain BiFeO3 nanoislands

Understanding switching mechanisms in multiferroics such as BiFeO3 (BFO) is an important challenge to control ferroic orders (ferroelectric or ferroelastic) as it could lead to the design of non-volatile memories based on magnetoelectric coupling. Here, we demonstrate an alternative way to control the binary states of ferroic orders by locally applying pressure and electric field in ferroelectric bi-domains confined in single BFO nanoislands. The study of the electronic transport properties and domain orientations using atomic force microscopy (AFM) based techniques enabled us to determine the electric and mechanical parameters at which ferroelectric and ferroelastic resistive switching can be observed. Nanoislands exhibited binary high and low resistance states without scaling effect, with high performance switching characteristics. Positive-forward rectifying behavior at high tip force was interpreted by the formation of a subsurface non-conductive interface due to the strain gradient. Ferroelastic switching at the surface was associated with a symmetry-breaking induced by electromechanical coupling between the AFM tip and the BFO thin film. It led to out-of-plane polarization pinning that allows performing only in-plane switching accompanied by nucleation and propagation of a conductive domain wall. The control of ferroic binary states by the electric field and pressure may pave the way for multilevel data storage devices