A novel ferroelectric Rashba semiconductor

Fast, reversible, and low-power manipulation of the spin texture is crucial for next generation spintronic devices like non-volatile bipolar memories, switchable spin current injectors or spin field effect transistors. Ferroelectric Rashba semiconductors (FERSC) are the ideal class of materials for the realization of such devices. Their ferroelectric character enables an electronic control of the Rashba-type spin texture by means of the reversible and switchable polarization. Yet, only very few materials are established to belong to this class of multifunctional materials. Here, Pb$_{1−x}$Ge$_{x}$Te is unraveled as a novel FERSC system down to nanoscale. The ferroelectric phase transition and concomitant lattice distortion are demonstrated by temperature dependent X-ray diffraction, and their effect on electronic properties are measured by angle-resolved photoemission spectroscopy. In few nanometer-thick epitaxial heterostructures, a large Rashba spin-splitting is exhibiting a wide tuning range as a function of temperature and Ge content. This work defines Pb$_{1−x}$Ge$_{x}$Te as a high-potential FERSC system for spintronic applications

Persistence of structural distortion and bulk band Rashba splitting in SnTe above its ferroelectric critical temperature

The ferroelectric semiconductor $\alpha$-SnTe has been regarded as a topological crystalline insulator and the dispersion of its surface states has been intensively measured with angle-resolved photoemission spectroscopy (ARPES) over the last decade. However, much less attention has been given to the impact of the ferroelectric transition on its electronic structure, and in particular on its bulk states. Here, we investigate the low-energy electronic structure of $\alpha$-SnTe with ARPES and follow the evolution of the bulk-state Rashba splitting as a function of temperature, across its ferroelectric critical temperature of about $T_c\sim 110$ K. Unexpectedly, we observe a persistent band splitting up to room temperature, which is consistent with an order-disorder contribution to the phase transition that requires the presence of fluctuating local dipoles above $T_c$. We conclude that no topological surface state can occur at the (111) surface of SnTe, at odds with recent literature.Comment: 26 pages, 8 figure

Robust multiferroicity and magnetic modulation of the ferroelectric imprint field in heterostructures comprising epitaxial Hf0.5Zr0.5O2 and Co

Magnetoelectric multiferroics, either single-phase or composites comprising ferroelectric/ferromagnetic coupled films, are promising candidates for energy efficient memory computing. However, most of the multiferroic magnetoelectric systems studied so far are based on materials that are not compatible with industrial processes. Doped hafnia is emerging as one of the few CMOS-compatible ferroelectric materials. Thus, it is highly relevant to study the integration of ferroelectric hafnia into multiferroic systems. In particular, ferroelectricity in hafnia, and the eventual magnetoelectric coupling when ferromagnetic layers are grown atop of it, are very much dependent on quality of interfaces. Since magnetic metals frequently exhibit noticeable reactivity when grown onto oxides, it is expected that ferroelectricity and magnetoelectricity might be reduced in multiferroic hafnia-based structures. In this article, we present excellent ferroelectric endurance and retention in epitaxial Hf0.5Zr0.5O2 films grown on buffered silicon using Co as the top electrode. The crucial influence of a thin Pt capping layer grown on top of Co on the ferroelectric functional characteristics is revealed by contrasting the utilization of Pt-capped Co, non-capped Co and Pt. Magnetic control of the imprint electric field (up to 40% modulation) is achieved in Pt-capped Co/Hf0.5Zr0.5O2 structures, although this does not lead to appreciable tuning of the ferroelectric polarization, as a result of its high stability. Computation of piezoelectric and flexoelectric strain-mediated mechanisms of the observed magnetoelectric coupling reveal that flexoelectric contributions are likely to be at the origin of the large imprint electric field variation.Financial support from the Spanish Ministry of Science, Innovation and Universities and European Union NextGenerationEU/PRTR (MCIN/AEI/10.13039/501100011033), through the Severo Ochoa FUNFUTURE (CEX2019-000917-S), PID2023-147211OB-C21, PDC2023-145874-I00, PID2020-116844RB-C21, PID2020-112548RB-I00, and PID2019-107727RB-I00 projects program, from Generalitat de Catalunya (2021 SGR 00804 and 2021 SGR 00651) and the European Research Council (2021-ERC-Advanced ‘REMINDS’ Grant No. 101054687) is acknowledged. We also acknowledge projects TED2021-130453B-C21 and TED2021-130453B-C22, funded by MCIN/AEI/10.13039/50110001103. This work was supported by: (i) the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding Contract UIDB/04650/2020; (ii) the exploratory research project 2022.01740.PDTC (https://doi.org/10.54499/2022.01740.PTDC) and (iii) the project M-ERA-NET3/0003/2021 – NanOx4EStor grant agreement No 958174 (https://doi.org/10.54499/M-ERA-NET3/0003/2021). J. P. B. S. also thanks FCT for the contract under the Institutional Call to Scientific Employment Stimulus – 2021 Call (CEECINST/00018/2021). This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020, financed by national funds through the FCT/MCTES (PIDDAC). Guillaume Sauthier from ICN2 is aknowledged for assistance on the XPS characterization. Francisco Javier Campos López is aknowledged for assistance on the X-ray characterization.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe