S. D. Bader

Biofunctionalized magnetic-vortex microdiscs for targeted cancer-cell destruction

Nanomagnetic materials offer exciting avenues for probing cell mechanics and activating mechanosensitive ion channels, as well as for advancing cancer therapies. Most experimental works so far have used superparamagnetic materials. This report describes a first approach based on interfacing cells with lithographically defined microdiscs that possess a spin-vortex ground state. When an alternating magnetic field is applied the microdisc vortices shift, creating an oscillation, which transmits a mechanical force to the cell. Because reduced sensitivity of cancer cells toward apoptosis leads to inappropriate cell survival and malignant progression, selective induction of apoptosis is of great importance for the anticancer therapeutic strategies. We show that the spin-vortex-mediated stimulus creates two dramatic effects: compromised integrity of the cellular membrane, and initiation of programmed cell death. A low-frequency field of a few tens of hertz applied for only ten minutes was sufficient to achieve approximately 90% cancer-cell destruction in vitro.

Quantifying Spin Hall Angles from Spin Pumping: Experiments and Theory

Spin Hall effects intermix spin and charge currents even in nonmagnetic materials and, therefore, ultimately may allow the use of spin transport without the need for ferromagnets. We show how spin Hall effects can be quantified by integrating Ni{80}Fe{20}|normal metal (N) bilayers into a coplanar waveguide. A dc spin current in N can be generated by spin pumping in a controllable way by ferromagnetic resonance. The transverse dc voltage detected along the Ni{80}Fe{20}|N has contributions from both the anisotropic magnetoresistance and the spin Hall effect, which can be distinguished by their symmetries. We developed a theory that accounts for both. In this way, we determine the spin Hall angle quantitatively for Pt, Au, and Mo. This approach can readily be adapted to any conducting material with even very small spin Hall angles.

Surface magneto-optic Kerr effect

The surface magneto-optic Kerr effect (SMOKE) has significantly impacted research on magnetic thin films. This is due to its sensitivity, local probing nature, and experimental simplicity. The polar and longitudinal Kerr effects are characterized by a complex rotation of the plane of polarization of linearly polarized incident light upon reflection from the surface of a ferromagnetic material. The rotation is directly related to the magnetization of the material within the probing region of the light. Light penetrates into metals >20 nm deep, but the SMOKE technique derives its surface sensitivity from the limited thickness of the deposited magnetic film, which can be as thin as one atomic layer. Basic principles, experimental arrangements, and applications of SMOKE are reviewed in order to acquaint the nonspecialist with the technique and place it into perspective.

Hard/soft magnetic heterostructures: model exchange-spring magnets

An overview is provided of research on exchange-spring coupled magnetic films and multilayers, including fabrication methods, and the characterization and modeling of the magnetization reversal processes. For coupled hard/soft bilayers and multilayers the deposition process provides nanometer-scale control of thicknesses and magnetic anisotropy. Such magnetic heterostructures provide model systems for studying the exchange hardening mechanism. Recent work on epitaxial SmCo/Fe and SmCo/Co bilayers and superlattices that display many of the characteristic features of exchange-spring magnets is highlighted. Comparison of the experimental results with numerical simulations indicates that the exchange-spring behavior can be understood from the intrinsic parameters of the hard and soft layers. The simulations are extended to realistically estimate the ultimate gain in performance that can potentially be realized in permanent magnets based on the exchange-spring principle.

Detection and quantification of inverse spin Hall effect from spin pumping in permalloy/normal metal bilayers

Spin pumping is a mechanism that generates spin currents from ferromagnetic resonance over macroscopic interfacial areas, thereby enabling sensitive detection of the inverse spin Hall effect that transforms spin into charge currents in nonmagnetic conductors. Here we study the spin-pumping-induced voltages due to the inverse spin Hall effect in permalloy/normal metal bilayers integrated into coplanar waveguides for different normal metals and as a function of angle of the applied magnetic field direction, as well as microwave frequency and power. We find good agreement between experimental data and a theoretical model that includes contributions from anisotropic magnetoresistance and inverse spin Hall effect. The analysis provides consistent results over a wide range of experimental conditions as long as the precise magnetization trajectory is taken into account. The spin Hall angles for Pt, Pd, Au, and Mo were determined with high precision to be 0.013±0.002, 0.0064±0.001, 0.0035±0.0003, and ?0.0005±0.0001, respectively.

Biological sensors based on Brownian relaxation of magnetic nanoparticles

We experimentally demonstrate a biomagnetic sensor scheme based on Brownian relaxation of magnetic nanoparticles suspended in liquids. The characteristic time scale of the Brownian relaxation can be determined directly by ac susceptibility measurements as a function of frequency. The peak in the imaginary part of the ac susceptibility shifts to lower frequencies upon binding the target molecules to the magnetic nanoparticles. The frequency shift is consistent with an increase of the hydrodynamic radius corresponding to the size of the target molecule.

Universal approach to magneto-optics☆

Magneto-optics is described in a unique framework. Reflection and transmission of a general multilayer system is expressed by means of medium boundary and propagation matrices which are universal and apply to any configuration of films and media. The results for the magneto-optic coefficients are cast in the form of sets of four linear, inhomogeneous equations. It is shown that in the thin-film limit, the Kerr effect obeys an additivity law for a system consisting of any number of magnetic films. Computer simulations were performed on different film-configurations, including an overlayer, a sandwich and a superlattice.

Magnetic Vortex Core Dynamics in Cylindrical Ferromagnetic Dots

We report direct imaging by means of x-ray photoemission electron microscopy of the dynamics of magnetic vortices confined in micron-sized circular permalloy dots that are 30 nm thick. The vortex core positions oscillate on a 10 ns time scale in a self-induced magnetostatic potential well after the in-plane magnetic field is turned off. The measured oscillation frequencies as a function of the aspect ratio of the dots are in agreement with theoretical calculations presented for the same geometry.

Realization of a spin-wave multiplexer

Recent developments in the field of spin dynamics--like the interaction of charge and heat currents with magnons, the quasi-particles of spin waves--opens the perspective for novel information processing concepts and potential applications purely based on magnons without the need of charge transport. The challenges related to the realization of advanced concepts are the spin-wave transport in two-dimensional structures and the transfer of existing demonstrators to the micro- or even nanoscale. Here we present the experimental realization of a microstructured spin-wave multiplexer as a fundamental building block of a magnon-based logic. Our concept relies on the generation of local Oersted fields to control the magnetization configuration as well as the spin-wave dispersion relation to steer the spin-wave propagation in a Y-shaped structure. Thus, the present work illustrates unique features of magnonic transport as well as their possible utilization for potential technical applications.

Metal-Insulator Transition and Its Relation to Magnetic Structure in (LaMnO3)2n/(SrMnO3)n Superlattices

Superlattices of (LaMnO3){2n}/(SrMnO3){n} (1<or=n<or=5), composed of the gapped insulators LaMnO3 and SrMnO3, undergo a metal-insulator transition as a function of n, being metallic for n<or=2 and insulating for n>or=3. Measurements of transport, magnetization, and polarized neutron reflectivity reveal that the ferromagnetism is relatively uniform in the metallic state, and is strongly modulated in the insulating state, being high in LaMnO3 and suppressed in SrMnO3. The modulation is consistent with a Mott transition driven by the proximity between the (LaMnO3)/(SrMnO3) interfaces. The insulating state for n>or=3 obeys variable range hopping at low temperatures. We suggest that this is due to states at the Fermi level that emerge at the (LaMnO3)/(SrMnO3) interfaces and are localized by disorder.