J-Ph. Ansermet

Perpendicular transport of spin-polarized electrons through magnetic nanostructures

A quasi one-dimensional model of spin transport in heterogeneous media based on the Boltzmann equation is presented in order to define the basic properties characterizing perpendicular spin transport in nanostructures. Experimental results are reviewed, first on the giant magnetoresistance of magnetic multilayers: spin dependent scattering in bulk and at interfaces, spin-diffusion length in the ferromagnetic layers and in the non-magnetic spacers. The observations of magnetoresistance associated with spin-scattering at Bloch walls are summarized. The junction magnetoresistance of ferromagnetic-insulator-ferromagnetic structures are reviewed, including the tunnel junctions produced with materials which present colossal magnetoresistance. An outlook on possible devices and novel structures is given.

Template synthesis and magnetoresistance property of Ni and Co single nanowires electrodeposited into nanopores with a wide range of aspect ratios

Template synthesis and magnetoresistance property of Ni and Co single nanowires electrodeposited into nano-pores with wide range of aspect ratios

Note: stacked rings for terahertz wave-guiding.

We demonstrate the construction of corrugated waveguides using stacked rings to propagate terahertz frequencies. The waveguide allows propagation of the same fundamental mode as an optical-fiber, namely, the HE(11) mode. This simple concept opens the way for corrugated wave-guides up to several terahertz, maintaining beam characteristics as for terahertz applications.

Note: Three-dimensional stereolithography for millimeter wave and terahertz applications

Metal-coated polymers shaped by 3D stereolithography are introduced as a new manufacturing method for passive components for millimeter to terahertz electromagnetic waves. This concept offers increased design capabilities and flexibilities while shortening the manufacturing process of complex shapes, e.g., corrugated horns, mirrors, etc. Tests at 92.5, 140, and 170 GHz are reported.

Ferrimagnetism of the magnetoelectric compound Cu2OSeO3 probed by Se-77 NMR

We present a thorough Se-77 nuclear-magnetic-resonance (NMR) study of a single crystal of the magnetoelectric compound Cu2OSeO3. The temperature dependence of the local electronic moments extracted from the NMR data is fully consistent with a magnetic phase transition from the high-T paramagnetic phase to a low-T ferrimagnetic state with 3/4 of the Cu2+ ions aligned parallel and 1/4 aligned antiparallel to the applied field of 14.09 T. The transition to this 3up-1down magnetic state is not accompanied by any splitting of the NMR lines or any abrupt modification in their broadening, hence there is no observable reduction in the crystal symmetry from its high-T cubic P2(1)3 space group. These results are in agreement with high-resolution x-ray diffraction and magnetization data on powder samples reported previously by Bos et al. [Phys. Rev. B 78, 094416 (2008)]. We also develop a mean-field theory description of the problem based on a microscopic spin Hamiltonian with one antiferromagnetic (J(afm) similar or equal to 68 K) and one ferromagnetic (J(fm) similar or equal to -50 K) nearest-neighbor exchange interaction.

Design of a frequency-tunable gyrotron for DNP-enhanced NMR spectroscopy

We report on the design of a modular low-power (10–50W) high-frequency gyrotron (265–530GHz) for DNP enhanced Solid-State NMR spectroscopy. With the view of covering a wide range of frequencies, a 9.7T helium-free superconducting magnet (SCM) is planned for the gyrotron operation on either the fundamental or second harmonic of the electron cyclotron frequency. The gyrotron design is based on a triode electron gun (Vk=15kV, Ib=100mA, Va= 6–8kV) which is very flexible for adapting the electron beam properties to a wide variety of cavities operating at the fundamental or at the second harmonic. The gyrotron is designed for a lateral output with an internal Vlasov-type converter. The reference parameters for application of DNP-enhanced NMR spectroscopy on a 400MHz (1H) spectrometer are optimized with a RF frequency tunability corresponding to twice the proton NMR frequency. The modularity of the construction of the gyrotron allows for the possibility of changing only some elements like the cavity-uptaper system in order to adapt to the wide range of NMR spectrometers existing at EPFL.

Phase transition in the localized ferromagnet EuO probed by μSR

We report results of muon-spin-rotation measurements performed on the ferromagnetic semiconductor EuO, which is one of the best approximations to a localized ferromagnet. We argue that implanted muons are sensitive to the internal field primarily through a combination of hyperfine and Lorentz fields. The temperature dependences of the internal field and the relaxation rate have been measured and are compared with previous theoretical predictions.

Current susceptibility of magnetization in spin valves

We observed the second harmonic voltage response of pseudo-spin valves subjected to currents oscillating at frequencies in the sub-kilohertz range. Peaks appear in its magnetic field dependence, with a signal-to-noise ratio equal to or greater than the magnetoresistance response of the same samples. This signal is interpreted as arising from the response of the magnetization to the spin torque induced by current when the magnetic layers are in non-collinear orientations. Thus, the method probes the current susceptibility of the magnetization which is shown to be sensitive to non-collinear states undetectable in the magnetoresistance.

The Nd–Mn exchange interaction in Nd0.7Sr0.3MnO3

The Nd-Mn exchange interaction in Nd0.7Sr0.3MnO3 is considered by studying its influence on the Mn-55 spin-echo NMR lineshape and spin- spin relaxation time. It is seen that the interaction is of considerable strength well above the Nd spin ordering temperature (approximate to 20 K), with a significant influence on the Mn (electron) spin dynamics.

Off-Axis Electron Holography of Single Ferromagnetic Nanowires

Abstract Off-axis electron holography has been applied to study the remanent magnetization state of single ferromagnetic Co93Cu7 nanowires a few micrometer in length and a typical radius of 40nm. Because the objective lens of the electron microscope has to be switched off, the spatial resolution of the reconstructed phase images is presently limited to approximately 70nm. The magnetization reversal of an individual nanowire has been followed by observing a series of remanent states, obtained ex situ by applying different external magnetic field sweeps parallel to the nanowire axis. The relation between misoriented crystal grains and nonuniform magnetization states has been studied by the combination of electron holography and conventional transmission electron microscopy.