Ferroelectric and Multiferroic Tunnel Junctions

Abstract

The phenomenon of electron tunneling has been known since the advent of quantum mechanics, but it continues to enrich our understanding of many fields of physics, as well as offering a route toward useful devices. A tunnel junction consists of two metal electrodes separated by a nanometer-thick insulating barrier layer, in which an electron is allowed to transverse a potential barrier exceeding the electron’s energy. The electron therefore has a finite probability of being found on the opposite side of the barrier. In the 1970’s, spin-dependent electron tunneling from ferromagnetic metal electrodes across an amorphous Al2O3 film was observed by Tedrow and Meservey(1)(2). Based on this discovery, Julliere proposed and demonstrated that in a magnetic tunnel junction tunnel current depends on the relativemagnetization orientation of the two ferromagnetic electrodes(3). Such a phenomenon nowadays is known as tunneling magnetoresistance(TMR)(4). Magnetic tunnel junctions may be very useful for various technological applications in spintronics devices such as magnetic field sensors and magnetic random access memories. Other insulators are also used for tunnel barriers. For example, epitaxial perovskite SrTiO3 barriers were studied by De Teresa et al. to demonstrate the importance of interfaces in spin-dependent tunneling(5). In tunnel junctions with MgO barriers, Ikeda et al. found large magnetoresitance as high as 604% at room temperature and 1144% at 5 K(6), which approaches the theoretical predictions of Butler et al.(7) and Mathon et al.(8). Despite the diversity of materials used as the barrier of the tunnel junctions, the common feature is that almost all the barriers are nonpolar dielectrics. On the other hand, magnetic insulators, i.e, EuO, EuS and EuSe, are used for tunnel barriers. Spin filtering has been observed in these junctions as were first discussed by Moodera et al.(9). in 1988. They observed that the tunneling current in Au/EuS/Al junction has a spin polarization with the magnitude as high as 80%. and attributed it to the electron tunneling across the spin-dependent barriers (Fig.1). Later, they reported that the tunneling current across Ag/EuSe/Al junctions has an enhanced spin-polarization reaching 97%(10). Recently, using EuO with a higher Curier temperature (69 K) than EuS (16.7 K) and EuSe (4.6 K), Santos et al. obtained 29% spin-polarized tunneling current(11). Naturally, if electrodes are not normal metals, but ferromagnetic materials, both TMR and spin filter effects can be observed(Fig.2)(12). Another important concept is the ferroelectric tunnel junction (FTJ)(13)(14)(15), which take advantage of a ferroelectric as the barrier material. Ferroelectrics possess a spontaneous 2