Simultaneous control of the Dzyaloshinskii-Moriya interaction and magnetic anisotropy in nanomagnetic trilayers.

Magneto-optical Kerr effect (MOKE) microscopy measurements of magnetic bubble domains demonstrate that Ar^{+} irradiation around 100 eV can tune the Dzyaloshinskii-Moriya interaction (DMI) in Pt/Co/Pt trilayers. Varying the irradiation energy and dose changes the DMI sign and magnitude separately from the magnetic anisotropy, allowing tuning of the DMI while holding the coercive field constant. This simultaneous control emphasizes the different physical origins of these effects. To accurately measure the DMI, we propose and apply a physical model for a poorly understood peak in domain wall velocity at zero in-plane field. The ability to tune the DMI with the spatial resolution of the Ar^{+} irradiation enables new fundamental investigations and technological applications of chiral nanomagnetics.

Probing electric field control of magnetism using ferromagnetic resonance.

Exchange coupled CoFe/BiFeO3 thin-film heterostructures show great promise for power-efficient electric field-induced 180° magnetization switching. However, the coupling mechanism and precise qualification of the exchange coupling in CoFe/BiFeO3 heterostructures have been elusive. Here we show direct evidence for electric field control of the magnetic state in exchange coupled CoFe/BiFeO3 through electric field-dependent ferromagnetic resonance spectroscopy and nanoscale spatially resolved magnetic imaging. Scanning electron microscopy with polarization analysis images reveal the coupling of the magnetization in the CoFe layer to the canted moment in the BiFeO3 layer. Electric field-dependent ferromagnetic resonance measurements quantify the exchange coupling strength and reveal that the CoFe magnetization is directly and reversibly modulated by the applied electric field through a ~180° switching of the canted moment in BiFeO3. This constitutes an important step towards robust repeatable and non-volatile voltage-induced 180° magnetization switching in thin-film multiferroic heterostructures and tunable RF/microwave devices.

Simultaneous measurement of magnetic vortex polarity and chirality using scanning electron microscopy with polarization analysis (SEMPA).

The magnetic vortex structure is an equilibrium configuration frequently found in patterned magnetic nanostructures. It is characterized by an in-plane curling of the magnetization with clockwise or anticlockwise chirality and by an out-of-plane vortex core that can have a positive or negative polarity. The small size of the vortex core, on the order of 10nm, makes it technologically interesting due to potential data storage, but also difficult to measure or image directly. In this work, we used Scanning Electron Microscopy with Polarization Analysis (SEMPA) to directly image magnetic vortex cores in patterned NiFe/Ta bilayer structures. With SEMPA we can simultaneously measure the in-plane and the out-of-plane component of the surface magnetization and thereby determine both the vortex chirality and the vortex core polarity in a single measurement. Our magnetic simulation of the vortex core, considering only the exchange and magnetostatic energy, is in good agreement with the SEMPA measurement of the magnetization when other experimental factors are taken into account.

Electronic effects in the length distribution of atom chains.

Gold deposited on Si(553) leads to self-assembly of atomic chains, which are broken into finite segments by defects. Scanning tunneling microscopy is used to investigate the distribution of chain lengths and the correlation between defects separating the chains. The length distribution reveals oscillations that indicate changes in the cohesive energy as a function of chain length. We present a possible interpretation in terms of the electronic scattering vectors at the Fermi surface of the surface states. The pairwise correlation function between defects shows long-range correlations that extend beyond nearest-neighbor defects, indicating coupling between chains.

End states in one-dimensional atom chains.

End states--the zero-dimensional analogs of the two-dimensional states that occur at a crystal surface--were observed at the ends of one-dimensional atom chains that were self-assembled by depositing gold on the vicinal Si(553) surface. Scanning tunneling spectroscopy measurements of the differential conductance along the chains revealed quantized states in isolated segments with differentiated states forming over end atoms. A comparison to a tight-binding model demonstrated how the formation of electronic end states transforms the density of states and the energy levels within the chains.

Scanning electrochemical microscopy. 23. Reaction localization of artificially patterned and tissue-bound enzymes.

The scanning electrochemical microscope (SECM), operating in the feedback mode, was used to image localized surface reactions of redox enzymes at the micrometer level. Surfaces imaged with the SECM included glucose oxidase immobilized within 8-microns-diameter pores of a filtration membrane and individual whole mitochondria with active NADH cytochrome reductase enzymes in their outer membranes. Factors influencing enzyme image resolution and specificity are discussed.

Chemical imaging of surfaces with the scanning electrochemical microscope.

Scanning electrochemical microscopy is a scanning probe technique that is based on faradaic current changes as a small electrode is moved across the surface of a sample. The images obtained depend on the sample topography and surface reactivity. The response of the scanning electrochemical microscope is sensitive to the presence of conducting and electroactive species, which makes it useful for imaging heterogeneous surfaces. The principles and instrumentation used to obtain images and surface reaction-kinetic information are discussed, and examples of applications to the study of electrodes, minerals, and biological samples are given.

Polarized electron probes of magnetic surfaces.

The magnetic properties of surfaces are now being explored with electron spectroscopies that use electron spin polarization techniques. The increased activity in surface magnetic measurements with polarized electron beams is spurred by new scientific and technological challenges and is made feasible by recent advances in the technology of sources and detectors of polarized electrons. The ability to grow thin films and to engineer artificial structures permits new phenomena to be investigated at magnetic surfaces and interfaces. For such investigations, spin-polarized electron techniques-such as polarized electron scattering, polarized photoemission, polarized Auger spectroscopy, and scanning electron microscopy with polarization analysis-have been and will probably continue to be used to great advantage.

Spin and energy analyzed photoemission: a feasibility analysis.

New scientific opportunities, particularly for investigation of surface magnetism, will be provided by spin and energy analyzed photoemission. Electron-optical conservation laws and phase space concepts are summarized and applied to determine the feasiblity of an experiment consisting of a photoemitter in a magnetic field, a photoelectron energy analyzer and an electron spin analyzer. For the example of photoemission from a Ni crystal using He I resonance radiation and typical parameters for the energy and spin analyzers, a final signal count rate of approximately 220 counts/s is calculated. Ways to increase the count rate by orders of magnitude are described. In particular, a new experimental configuration is suggested which may avoid the large reduction in count rate caused by the magnetic field.