Interface Magnetoelectric Coupling Effect And Spin Dependent Transport In M/KNbO₃ （M= Fe, Co And Ni）

Multiferroic materials have wide potential applications in sensors, memories and hard disk due to the magnetoelectric (ME) coupling effect. Only few single-phase multiferroics display ME effect at room temperature and their coupling effects are usually weak. Thus composite multiferroic systems have been highly focused. The topical systems are the ferromagnetic/ferroelectric multiferroic heterostructures. There are many different physical mechanisms that produce the ME effect in such systems. On the other hand, ferromagnetic/ferroelectric heterostructures compose a multiferroic tunnel junction. It is known that M (M=Fe, Co, Ni) and KNbO3 are two "classical" ferroic materials at room temperature. In this thesis, we study the ME effect and spin dependent transport in M/KNbO3 (M= Fe, Co and Ni) systems constructed with M and KNbO3 based on first principle calculations. The theoretical calculations not only provide the atomic and electronic structures of heterostructures but also guide later experimental studies.We construct the M/KNbO3 superlattices with NbO2 or KO terminations. For Fe/KNbO3 with NbO2 terminations, the ME effects originating from interface bonding. The hybridization between the unoccupied Nb Ad band and the unoccupied minority-spin Fe 3d band produces minority-spin bonding states peaked just below the Fermi energy. This leads to an induced magnetic moment on the interfacial Nb atom aligned antiparallel to that of Fe. The interface Fe-0 bond also induces magnetic moments on the interfacial O atoms aligned parallel to that of Fe. Moreover, the bond lengths are controlled by the ferroelectric displacements. The switching of the polarization can change the magnitudes of the induced magnetic moments. The ME coefficient is 2.0 ×10-10G cm2/V. For the KO-terminated Fe/KNbO3 superlattices, we also find such ME effect with 0.1×10-10Gcm2/V. The situation is similar with Co/KNbO3 and Ni/KNbO3. However, the Fe-Nb bond is weaker than the Co-Nb bond since the unoccupied minority-spin Co 3d band is narrower than the corresponding Fe 3d band. As a result, the magnitude of the induced magnetic moment on the interface Nb in Co/KNbO3 is smaller than that in Fe/KNbO3. As the unoccupied minority-spin Ni 3d band is narrowest, the Ni-Nb bond is the weakest. Hence we obtain the smallest magnitude of the induced magnetic moment on the interface Nb in Ni/KNbO3.The M/KNbO3 systems with asymmetric interfaces have the NbO2 termination at the left interface and the KO termination at the right interface. We find that the polarizations of the KNbO3 barrier can pint to the left or the right. There also exists interface bonding in such systems controlled by the ferroelectricity and thus produce induced magnetic moments on interface Nb and O atoms. We find a magnetic reconstruction of Ni atoms near the right interface when the polarization in Ni/KNbO3 pointing to the KO-terminated right interface. The change of total magnetic moments is 2.66 μB due to the switching of the polarization. We obtain a ME effect of 1.9×10-9Gcm2/V.According to the band structures of bcc Fe, Co and Ni along the [001] direction and the complex band structure of P4mm phase bulk KNbO3, states with A5 and Δ1 symmetry play an important role in the electron tunneling. In NbCh-terminated M/KNbO3/M, the electron tunneling depends on magnetization configuration of electrodes and thus leads the tunneling magnetoresistance (TMR). Detail analysis indicates that the conductance is determined by the electrodes, KNbO3 barrier and interface bonding. In addition, the KⅡ-resolved conductance within the two-dimensional Brillouin zone is highly related to the evanescent states in KNbO3 and the density of states for interface atomic layers. We find a TMR of -84% for NbO2-terminated Fe/KNbO3/Fe junction.In the M/KNbO3/M junctions, the tunneling conductance depends on both the magnetization configuration of electrodes and the polarization direction of the KNbO3 barrier. Therefore, the TMR and tunneling electroresistance (TER) can coexistence in the M/KNbO3/M systems. We find a complex tunneling behavior in Ni/KNbOI3/Ni junctions. The magnetic reconstruction of Ni atoms near the KO-terminated interface with the reversal of the ferroelectric polarization filters the spin-dependent current and thus induces large TER effects. We obtain a giant TER up to 104% for the parallel and antiparallel configurations. The TMR effects for two polarization states are also remarkable.The innovation of this thesis is the finding of a magnetic reconstruction in Ni/KNbO3 with the NbO2 termination at the left interface and the KO termination at the right interface when the polarization is switched. This leads large ME effects. Meanwhile, the magnetic reconstruction due to the reversal of the polarization filters the conductance in Ni/KNbO3/Ni MFTJs and thus produces huge TER effects.