Daniel T. Pierce

The GaAs spin polarized electron source

The design, construction, operation, and performance of a spin polarized electron source utilizing photoemission from negative electron affinity (NEA) GaAs are presented in detail. A polarization of 43±2% is produced using NEA GaAs (100). The polarization can be easily modulated without affecting other characteristics of the electron beam. The electron beam intensity depends on the intensity of the exciting radiation at 1.6 eV; beam currents of 20 μA/mW are obtained. The source is electron optically bright; the emittance phase space (energy‐area‐solid angle product) is 0.043 eV mm2 sr. The light optics, electron optics, and cathode preparation including the GaAs cleaning and activation to NEA are discussed in depth. The origin of the spin polarization in the photoexcitation process is reviewed and new equations describing the depolarization of photoelectrons in the emission process are derived. Quantum yield and polarization measurements for both NEA and positive electron affinity surfaces are reported. T...

Scanning electron microscopy with polarization analysis (SEMPA)

The high spatial resolution imaging of magnetic microstructure has important ramifications for both fundamental studies of magnetism and the technology surrounding the magnetic recording industry. One technique for imaging surface magnetic microstructure on the 10‐nm‐length scale is scanning electron microscopy with polarization analysis (SEMPA). This technique employs a scanning electron microscope (SEM) electron optical column to form a medium energy (10–50 keV), small probe ( 1 nA) on a ferromagnetic specimen. Secondary electrons excited in the ferromagnet by the high spatial resolution probe retain their spin‐polarization orientation as they leave the sample surface. The spin polarization of the emitted secondary electrons can be related directly to the local magnetization orientation. A surface magnetization map is generated when the spin polarization of the secondary electrons is analyzed as the electron beam is rastered point‐by‐point across the ferromagnet’s surface. In th...

Direct Verification of Hydrogen Termination of the Semiconducting Diamond (111) Surface

Low‐energy, high‐resolution electron energy loss spectroscopy has been used to identify the vibrational modes of hydrogen on the semiconducting diamond surface providing the first direct evidence that the (111) 1×1 surface is terminated by hydrogen. The vibrational loss spectrum from the ’’as‐polished’’ surface shows two major losses near 160 meV (CH3 deformation), a major loss at 360 meV (CH3 stretch), and two minor losses at 520 and 720 meV (combinations and overtones). All of these losses disappear from the spectrum after heating the sample to ∠1000 °C (which has been established by other experiments to be sufficient to reconstruct the surface to 2×2/2×1). The loss spectrum for the reconstructed surface is indicative of a two‐dimensional metallic state of the dangling‐bond surface states for clean diamond. Exposure of this reconstructed surface to atomic hydrogen results in a loss spectrum which is essentially identical to that for the as‐polished surface. Further verification that the loss spectrum re...

Low‐energy diffuse scattering electron‐spin polarization analyzer

A new, compact (approximately fist sized), efficient electron‐spin analyzer is described. It is based on low‐energy (150 eV) diffuse scattering from a high‐Z target, for example, an evaporated polycrystalline Au film opaque to the incident electron beam. By collecting a large solid angle of scattered electrons, a figure of merit S2I/I0=10−4 is achieved with an analyzing power S=0.11. The figure of merit degrades only marginally (<10%) for beams with an energy width of 40 eV or after one month of operation at 10−8 Torr. The electron optical acceptance is of order 100 mm2 sr eV. The details of the design and construction are discussed and its performance is compared to six other spin analyzers. Illustrative results are presented from an application to scanning electron microscopy with polarization analysis (SEMPA) to image magnetic microstructure.

Microscopic aspects of the initial growth of metastable fcc iron on Au(111)

We report on the microscopic aspects of the growth of Fe on the Au(111) surface observed with scanning tunneling microscopy (STM). Nucleation of triangular Fe islands is observed to occur at the corners of the herringbone reconstruction of the Au(111) surface, which are imaged with atomic resolution. STM measurements show the existence of a metastable close‐packed face‐centered‐cubic (fcc) phase of Fe with a 0.288±0.01‐nm in‐plane nearest neighbor distance. Initial fcc film growth proceeds in a nonideal layer‐by‐layer growth mode in which new layers start before preceding layers are complete.

Oscillatory exchange coupling in Fe/Au/Fe(100)

Scanning electron microscopy with polarization analysis was used to investigate the interlayer exchange coupling in Fe/Au/Fe(100) sandwich structures. The films were epitaxially grown on single‐crystal Fe(100) substrates. Electron diffraction measurements revealed that the Au spacer film grew with a surface reconstruction consistent with that observed for bulk Au crystals. The exchange coupling oscillates between primarily ferromagnetic and antiferromagnetic coupling for Au spacer layers up to 65 layers (13 nm) thick, but a significant biquadratic coupling component was also observed. The oscillatory coupling exhibited two components with periods of 2.48±0.05 layers (0.506±0.010 nm) and 8.6±0.3 layers (1.75±0.06 nm). The measured periods are in excellent agreement with those calculated from spanning vectors of the Au Fermi surface.

Realization of Ground-State Artificial Skyrmion Lattices at Room Temperature

Magnetic Skyrmions exhibit topologically protected quantum states, not only offering exciting new mechanisms for ultrahigh density and low dissipation information storage, but also providing an ideal platform for explorations of unique topological phenomena. Prerequisite are systems exhibiting skyrmion lattices at ambient conditions. Here, we demonstrate the realization of artificial Bloch skyrmion lattices over extended areas in their ground state at room temperature by patterning asymmetric magnetic nanodots with controlled circularity on an underlayer film with perpendicular magnetic anisotropy (PMA) [1], shown in Fig. 1.

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.

Interfacial coupling in multiferroic/ferromagnet heterostructures

We report local probe investigations of the magnetic interaction between BiFeO3 films and a ferromagnetic Co0.9Fe0.1 layer. Within the constraints of intralayer exchange coupling in the Co0.9Fe0.1, the multiferroic imprint in the ferromagnet results in a collinear arrangement of the local magnetization and the in-plane BiFeO3 ferroelectric polarization. The magnetic anisotropy is uniaxial, and an in-plane effective coupling field of order 10 mT is derived. Measurements as a function of multiferroic layer thickness show that the influence of the multiferroic layer on the magnetic layer becomes negligible for 3 nm thick BiFeO3 films. We ascribe this breakdown in the exchange coupling to a weakening of the antiferromagnetic order in the ultrathin BiFeO3 film based on our x-ray linear dichroism measurements. These observations are consistent with an interfacial exchange coupling between the CoFe moments and a canted antiferromagnetic moment in the BiFeO3.

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.