Magnetic Films Research
Vision:
Our vision is to address the grand challenges in condensed matter and materials physics via the exploration of the realm of nanomagnetism. Nanomagnetism is connected to fundamental questions of how the energy demands of future generations will be met via the utilization of wind turbines as a viable alternate energy source, and electric vehicles as alternatives to continued fossil-fuel consumption. Nanomagnetism is connected to the question of how the information technology revolution will be extended via the advent of spintronics and the possibilities of communication by means of pure spin currents. Nanomagnetism provides deep issues to explore in the realms of nanoscale confinement, physical proximity, far-from-equilibrium phenomena, and ultrafast and emergent behavior, and can even provide a window on the bio-realm via new therapeutic techniques and insights. While magnetism is regarded as the oldest field in all of science, nanomagnetism is fresh and vibrant and helped usher in the era of nanoscience and nanotechnology.
Research Areas:
Three hot issues in nanomagnetism which our group is poised to address and illuminate encompass:
- spin dynamics,
- spin transport, and
- the creation of new multilayer materials, based on metallic heterostructures
Our program in spin dynamics will provide insights into artificial magnonic materials. The proposed work will advance our fundamental understanding of linear and nonlinear excitations in magnetic nanostructures. Our program in spin transport focuses on the physics of pure spin currents. It is only recently that spin currents have been recognized as a possible means to communicate without charge currents, potentially eliminating some of the wasted heat that impedes further transistor miniaturization. Due to this heat, information technology is becoming an energy technology issue, as well as a U.S. economic competitiveness issue. Finally, the quest for new functional materials via nanoscale multilayering enables us to create systems that possess unusual synergistic properties that may otherwise be mutually exclusive. Such systems include exchange spring nanomagnetic composites with low or no rare-earth content than can still exceed today’s commercial capabilities as used in electric motors and generators. Or ferromagnetic-superconducting multilayers that support an exotic interfacial pairing mechanism even though the individual components can be as simple as elemental layers. Such multilayering also enables us to explore the energetics and transport mechanisms underlying organic spintronic heterostructures.
Recent Progress:
Quantifying Spin Hall Effects: Spin Hall effects interconvert spin and charge currents even in nonmagnetic materials and, ultimately might facilitate spin transport without the need for ferromagnets. We showed how spin Hall effects can be quantified by integrating permalloy/normal-metal bilayers into a coplanar waveguide. A dc spin current in the normal metal can be generated via spin pumping by exciting the ferromagnetic resonance of the Py. Using this approach we could determine the spin Hall angle, which is the ratio of the spin-Hall-to-charge conductivity. This technique is adaptable to various materials, providing trends with the periodic table for charge-to-spin current conversion efficiency. In particular, a giant spin Hall effect had recently been reported for Au. Our studies via spin pumping and also via mesoscopic Au Hall bar structures were used to determine a spin Hall effect value that is more than an order of magnitude lower than the previously reported giant value.
Functionalized Magnetic Microdisks: Most magnetic vortices studied to date involve arrays on a substrate. Here we investigated an aqueous suspension of lithographical Fe-Ni microdisks (50-nm thick, ~1-mm diameter). The hysteresis loop is typical for magnetization reversal due to nucleation, displacement and annihilation of the vortices. A small external magnetic field causes the vortex center to (reversibly) displace, inducing a magnetic moment proportional to the field strength. As a result, the microdisks physically rotate in solution until the plane of the disk is aligned along the field direction. The transmitted light intensity from an external laser, increases when the field is on, and decreases when field is off. The finding could have applications in low-energy consumption magneto-optic devices. Furthermore, we showed (with CNM and Univ. of Chicago) that an ac field induces oscillations of the microdisks when attached, via a linker antibody, to human brain cancer cells in aqueous solution. The oscillations transmit a mechanical force to the nucleus, that compromises the integrity of the cell membrane, and initiates programmed cell death. Weak fields (<100 Oe) applied at low frequency (few tens of Hz) for only 10 min were sufficient to induce cell death in 90% of the cells. The external power in our experiments is 105 times smaller than in hyperthermia usage of magnetic nanoparticles.
Organic Spin Valves: The energy barrier at organic/electrode interfaces has been a source of confusion with respect to charge injection efficiency in organic electronic devices. It also contributes to the present controversy in organic spintronics, where there are claims that, for example, the lowest-unoccupied-molecular-orbital (LUMO) level of the organic semiconductor Alq3 is low enough to support electron injection from ordinary ferromagnetic metals in a spin-valve geometry. We directly measured the electron injection barrier at the interface between Alq3 and metal electrodes using hot electrons injected from a tunnel barrier. We reported the energy barrier for electron injection (2.3 eV at the Fe/Alq3 interface, and 2.2 eV for Al/Alq3.) Our results are consistent with the known Alq3 transport gap derived from inverse photoemission spectroscopy, proving that claims of molecular spin transport in "organic spin valves" are not supported by intrinsic material properties. They also underscore the need to understand the charge transport mechanism in organic spintronic devices before spin transport can be understood.
