Probing magnetism with magnetic field, current and heat

Systems exhibiting long-range order, for example, superconductivity, magnetism, ferroelectrics, liquid crystals, etc have been of significant interest in condensed matter physics. Probing and manipulating the response of the ensuing order parameters of such systems with external stimuli not only reveals novel physics but also can have a major technological impact. In this talk, I will concentrate on magnetism and address three different ways of manipulating the magnetic order parameter of various systems by magnetic field, (charge and spin) current and heat while delving into the underlying physics and discussing their potential towards new and promising spin based technologies.
In the first part, using the theory of topological defects, I will discuss how one can understand and control the trajectory of domain walls in in-plane magnetized branched networks, composed of connected nanowires, by a consideration of their fractional elementary topological defects and how they interact with those innate to the network. Using these concepts, I'll unravel the microscopic origin of the one-dimensional (1D) nature of magnetization reversal of artificial spin ice systems that have been observed in the form of Dirac strings.
In the second part, I will show a novel way of injecting domain walls in out-of-plane magnetized nanowires using spin transfer torque (STT) from nanosecond current pulses that cross a 90° magnetization boundary, which can be created, for example, by local ion irradiation at specific sites along the nanowire.
Recently, by taking advantage of the spin-dependent thermoelectric properties of magnetic materials, novel means of generating spin currents from temperature gradients have been realized, but so far their associated thermal spin torques (TSTs) have not been large enough to influence magnetic tunnel junction (MTJ) switching. In the last part of my talk, I will show evidence for significant TSTs in MTJs by generating large temperature gradients across ultrathin MgO tunnel barriers. I will show that the TST strongly depends on the relative orientation of the free and the reference layers of the MTJ and can be attributed to an asymmetry of the tunneling conductance across the zero bias voltage of the MTJ.