Electric field-induced switching for magnetic memory devices

The field of magnetism has witnessed a remarkable series of discoveries over the past few decades, and has given rise to new mechanisms to enable interactions between electrical and magnetic properties of materials and devices at the nanoscale. This has resulted in the emergence of various spintronic devices, where electrical currents and voltages can directly interact with the magnetization in nanostructures [1-10]. The discoveries of giant magnetoresistance (GMR) [8, 9] and tunneling magnetoresistance (TMR) [3-7] effects allowed for electrical reading of the relative magnetization orientations of different layers in spin valves and magnetic tunnel junctions (MTJs), respectively. This resulted in major advances in the field of magnetic sensors [1, 11-15], with applications in hard disk drives, magnetoresistive random access memory (MRAM), and biomedical sensing devices. New spintronic devices were further developed over the past decade, following the theoretical prediction and experimental realization of the spin-transfer torque (STT) effect [16-20], which allows for manipulation and switching of magnetic moments using spin-polarized electric currents. The STT effect offers many potential applications, most notably in random access memory (STT-MRAM) [2, 10, 19, 21-28] and microwave nano-oscillators [18, 20, 22, 29-37]. The appeal of STT for use in MRAM arises from the fact that unlike previous generations of MRAM, where bits are switched by magnetic fields induced by currents, in the case of STT-MRAM, writing is performed by passing currents directly through the MTJ memory bit. This results in advantages in terms of energy efficiency, density, and scalability over fieldswitched toggle MRAM. In addition, when combined with complementary metal-oxide silicon (CMOS) logic circuits, STT devices also offer the potential for the realization of nonvolatile and programmable logic circuits [2, 21, 38-40]. These circuits take advantage of the fact that standby power can be eliminated and intermediate computation steps are stored in a nonvolatile manner using the integrated nonvolatile memory, hence allowing for instant on/off capability. However, as will be outlined below, these logic solutions suffer from limitations due to the dynamic (i.e., switching) energy dissipation of STT devices.