growth-electrostaticchemical-properties-metallaalo3srtio3

Growth and Electrostatic/chemical Properties of Metal/LaAlO<sub>3</sub>/SrTiO<sub>3</sub> Heterostructures

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide ...

Université Paris-Saclay

Giant magnetocaloric materials are promising for solid-state refrigeration, as an alternative to hazardous gases used in conventional cooling devices. A giant magnetocaloric effect was discovered near room temperature in near-equiatomic FeRh alloys some years before the benchmark study in Gd5Si2Ge2 that launched the field. However, FeRh has attracted significantly less interest in cooling applications mainly due to irreversibility in magnetocaloric cycles associated with the large hysteresis of its first-order metamagnetic phase transition. Here we overcome the irreversibility via a dual-stimulus magnetic-electric refrigeration cycle in FeRh thin films via coupling to a ferroelectric BaTiO3 substrate. This experimental realization of a multicaloric cycle yields larger reversible caloric effects than either stimulus alone. While magnetic hysteretic losses appear to be reduced by 96% in dual-stimulus loops, we show that the losses are simply transferred into an elastic cycle, contrary to common belief. Nevertheless, we show that these losses do not necessarily prohibit integration of FeRh in practical refrigeration systems. Our demonstration of a multicaloric refrigeration cycle suggests numerous designs for efficient solid-state cooling applications.

Large reversible caloric effect in FeRh thin films via a dual-stimulus multicaloric cycle.

In the brain, learning is achieved through the ability of synapses to reconfigure the strength by which they connect neurons (synaptic plasticity). In promising solid-state synapses called memristors, conductance can be finely tuned by voltage pulses and set to evolve according to a biological learning rule called spike-timing-dependent plasticity (STDP). Future neuromorphic architectures will comprise billions of such nanosynapses, which require a clear understanding of the physical mechanisms responsible for plasticity. Here we report on synapses based on ferroelectric tunnel junctions and show that STDP can be harnessed from inhomogeneous polarization switching. Through combined scanning probe imaging, electrical transport and atomic-scale molecular dynamics, we demonstrate that conductance variations can be modelled by the nucleation-dominated reversal of domains. Based on this physical model, our simulations show that arrays of ferroelectric nanosynapses can autonomously learn to recognize patterns in a predictable way, opening the path towards unsupervised learning in spiking neural networks.

Learning through ferroelectric domain dynamics in solid-state synapses.

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 and SrTiO3 has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LaAlO3 thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. In this paper, the chemical, electronic, and transport properties of LaAlO3 /SrTiO3 samples capped with different metals grown in a system combining pulsed laser deposition, sputtering, and in situ X-ray photoemission spectroscopy are investigated. The results show that for metals with low work function a q2DES forms at 1-2 uc of LaAlO3 and is accompanied by a partial oxidation of the metal, a phenomenon that affects the q2DES properties and triggers the formation of defects. In contrast, for noble metals, the critical thickness is increased above 4 uc. The results are discussed in terms of a hybrid mechanism that incorporates electrostatic and chemical effects.

Article	Year
Large reversible caloric effect in FeRh thin films via a dual-stimulus multicaloric cycle. Nature communications\| PubMed ID: 27192941	2016
Learning through ferroelectric domain dynamics in solid-state synapses. Nature communications\| PubMed ID: 28368007	2017
Tuning Up or Down the Critical Thickness in LaAlO3 /SrTiO3 through In Situ Deposition of Metal Overlayers. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 28505388	2017

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