Spin Transfer Torque In Magnetic Nanostructures

This dissertation describes a number of research projects with the common theme of manipulating the magnetization of a nanoscale magnet through electrical means, and the major part is devoted to exploring the effect of spin angular momentum transfer from a spin-polarized current to a nanomagnet, which we call spin transfer torque. Spin transfer torque is a promising new mechanism to "write" magnetic storage elements in magnetic random access memory (MRAM) devices with magnesium oxide (MgO)-based magnetic tunnel junction (MTJ) architecture. The first part of our work aims at a quantitative measurement of the spin transfer torque exerted on one of the ferromagnetic electrodes in exactly this type of tunneling structures used for MRAM applications. We use a technique called spin-transfer-driven ferromagnetic resonance (ST-FMR), where we apply a microwave-frequency oscillating current to resonantly excite magnetic precession, and we describe two complementary methods to detect this precession. We resolve previous controversies over the bias dependence of spin transfer torque, and present the first quantitative measurement of spin transfer torque in MgO-based MTJs in full bias range. We also analyze and test the potential to use the ST-FMR technique for microwave detection and microwave amplification. In the second part of the our work, we fabricate ferromagnetic nanoparticles made of CoFeB or Co embedded in the MgO tunnel barrier of a typical magnetic tunnel junction device, and study the spin transfer torque exerted on these nanoparticles 2-3 nm in size. We present the first evidence of spin transfer torque in magnetic nanoparticles insulated from electrodes by mapping out the switching phase diagram of a single nanoparticle. We also study ferromagnetic resonance of a small number of nanoparticles induced by spin transfer torque, with the goal of approaching single electron tunneling regime. The last part of our work explores a dramatically different way to manipulate magnetization electrically. We couple a ferromagnet to a multiferroic material, bismuth ferrite (BiFeO3), by exchange bias interaction, and try to manipulate the ferromagnet by ferroelectric switching of the BiFeO3.