Abstract

AbstractFerroelectric tunnel junctions combine the phenomena of quantum-mechanical tunnelling and switchable spontaneous polarisation of a nanometre-thick ferroelectric film into novel device functionality. Switching the ferroelectric barrier polarisation direction produces a sizable change in resistance of the junction—a phenomenon known as the tunnelling electroresistance effect. From a fundamental perspective, ferroelectric tunnel junctions and their version with ferromagnetic electrodes, i.e., multiferroic tunnel junctions, are testbeds for studying the underlying mechanisms of tunnelling electroresistance as well as the interplay between electric and magnetic degrees of freedom and their effect on transport. From a practical perspective, ferroelectric tunnel junctions hold promise for disruptive device applications. In a very short time, they have traversed the path from basic model predictions to prototypes for novel non-volatile ferroelectric random access memories with non-destructive readout. This remarkable progress is to a large extent driven by a productive cycle of predictive modelling and innovative experimental effort. In this review article, we outline the development of the ferroelectric tunnel junction concept and the role of theoretical modelling in guiding experimental work. We discuss a wide range of physical phenomena that control the functional properties of ferroelectric tunnel junctions and summarise the state-of-the-art achievements in the field.

Highlights

  • Electron tunnelling refers to the ability of electrons to traverse potential barriers exceeding their energy.[1]

  • Sizable changes in the negative tunnelling magnetoresistance (TMR) were observed depending on the orientation of the ferroelectric polarisation in nanoscale Fe/BTO/LSMO and Co/BTO/LSMO multiferroic tunnel junctions (MFTJs).[98,100]

  • A very large TMR was observed in a LSMO/BTO/LCMO/LSMO MFTJ (Figure 7f).[86]

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Summary

INTRODUCTION

Electron tunnelling refers to the ability of electrons to traverse potential barriers exceeding their energy.[1]. There have been a number of demonstrations of the co-existence of the TMR and TER effects in MFTJs.[98,99,100,101] Sizable changes in the negative TMR were observed depending on the orientation of the ferroelectric polarisation in nanoscale Fe/BTO/LSMO and Co/BTO/LSMO MFTJs.[98,100] X-ray resonant magnetic scattering indicated that in these junctions there is an induced magnetic moment on the Ti atom that is coupled with the Fe electrode.[100] This result, as well as the measured negative spin polarisation of the Fe/BTO interface, is in agreement with first-principles calcuations.[93,95] A very large TMR was observed in a LSMO/BTO/LCMO/LSMO MFTJ (Figure 7f).[86] The value of TMR was found to be much larger for the junction in a low resistance state (100% at T = 80 K) than in a high-resistance state (hardly seen within the noise level). Simultaneous TER and TMR effects and their tunability by the complementary ferroic order parameter in MFTJs are consistent with the original theoretical predictions.[23]

CONCLUSIONS AND OUTLOOK
Findings
11 ACKNOWLEDGEMENTS
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