Seminar Znanstvenog centra izvrsnosti QuantiXLie i Fizičkog odsjeka

Tunnel magnetoresistance (TMR) elements have recently started replacing the existing
sensors in hard disk drives and proved to be excellent angle sensors. In their novel
multiferroic version, TMR junctions are characterized by 4 resistance states, instead of
standard 2 in magnetic and ferroelectric tunnel junctions. On top of that, recent experiments
have shown that in the Co/PbZr_0.2Ti_0.8O_3/La_0.7Sr_0.3MnO_3 multiferroic tunnel
junctions (MTJ) it is possible to reversibly switch the sign of the spin polarization of the
tunneling electrons by an electric field, indicating technologically promising new ways of
spin transport control in spintronic devices. Still, the possible optimization of this effect as
well as the application to other MTJ systems is hindered by the lack of understanding of the
underlying mechanism. So far, despite the theoretical efforts, the spin inversion with
ferroelectric switching has not been explained. In this presentation, I will describe a likely
and realistic mechanism explaining the observed ferroelectric switching of spin polarization
which we identified using density-functional theory. The explanation is based on an Ovacancy
related interface structure, which could be expected at such interfaces. The
mechanism we find is based on drastic and reversible changes in the reactivity and
chemical binding of the interfacial O with Co upon the inversion of the ferroelectric
polarization. The ferroelectric-controllable reactivity of O leads virtually to ON/OFF
switchable O-Co hybridization and a swappable spin polarization in the PZT barrier.