Electrical control of metallic heavy-metal/ferromagnet interfacial states

Voltage control effects provide an energy-efficient means of tailoring material properties, especially in highly integrated nanoscale devices. However, only insulating and semiconducting systems can be controlled so far. In metallic systems, there is no electric field due to electron screening effects and thus no such control effect exists. Here we demonstrate that metallic systems can also be controlled electrically through ionic not electronic effects. In a Pt/Co structure, the control of the metallic Pt/Co interface can lead to unprecedented control effects on the magnetic properties of the entire structure. Consequently, the magnetization and perpendicular magnetic anisotropy of the Co layer can be independently manipulated to any desired state, the efficient spin toques can be enhanced about 3.5 times, and the switching current can be reduced about one order of magnitude. This ability to control a metallic system may be extended to control other physical phenomena.Comment: 20 pages, 7 figures, Accepted by Physical Review Applied (2017

Spin-orbit torque switching of synthetic antiferromagnets

We report that synthetic antiferromagnets (SAFs) can be efficiently switched by spin-orbit torques (SOTs) and the switching scheme does not obey the usual SOT switching rule. We show that both the positive and negative spin Hall angle (SHA)-like switching can be observed in Pt/SAF structures with only positive SHA, depending on the strength of applied in-plane fields. A new switching mechanism directly arising from the asymmetric domain expansion is proposed to explain the anomalous switching behaviors. Contrary to the macrospin-based switching model that the SOT switching direction is determined by the sign of SHA, the new switching mechanism suggests that the SOT switching direction is dominated by the field-modulated domain wall motion and can be reversed even with the same sign of SHA. The new switching mechanism is further confirmed by the domain wall motion measurements. The anomalous switching behaviors provide important insights for understanding SOT switching mechanisms and also offer novel features for applications.Comment: 40 pages, 14 figure

Perpendicular Magnetic Tunnel Junctions with Unconventional Tunneling Barriers

Spintronics has become an area of interest for future computing beyond the transistor. Of particular interest is the storage of data in magnetic states with the use of Magnetic Tunneling Junctions (MTJs). An MTJ consists of two ferromagnetic layers seperated by a thin insulating barrier, the standard stucture being CoFeB/MgO/CoFeB. One focus of current spintronics research is lowering the switching energy of nanomagnets in MTJs. A nanomagnet can be switched by a magnetic field governed by Ampere’s law, or by current-induced spin transfer torques and spin-orbit torques. For future spintronic applications, it is highly desirable to accomplish magnetization switching with voltage, which, by eliminating Joule heating, could dramatically reduce the switching energy. In this work two new MTJ barrier materials are investigates with a focus on the unique voltage controllable magnetic properties they bring to MTJs with perpendicular magnetic anisotropy (pMTJs). For the first-time voltage controllable interlayer coupling (VCIC) has been experimentally demonstrated with the use of a GdOX tunneling barrier. Due to the interfacial nature of the magnetism, the ability to move oxygen vacancies within the barrier, and a large proximity-induced magnetization of GdOx, both the magnitude and the sign of the interlayer coupling in these junctions can be directly controlled by voltage. In the final portion of this dissertation pMTJs with an antiferromagnetic CrOx tunneling barrier are explored. Due to the unique properties of CrOX the direction of the exchange bias between it and the bottom CoFeB FM layer can be changed thus modifying the hysteretic properties of the MTJ.Release after 08/27/202