S. Urazhdin

Effect of antiferromagnetic interlayer coupling on current-assisted magnetization switching

We compare magnetization switching in Co/Cu/Co nanopillars with uncoupled and dipole-field coupled Co layers. In uncoupled nanopillars, current-driven switching is hysteretic at low magnetic field H and changes to reversible, characterized by telegraph noise, at high H. We show that dipolar coupling both affects the switching current and causes the switching to become reversible at small H. The coupling thus changes the switching to reversible, hysteretic, and then reversible again as H increases. We describe our results in terms of current-assisted thermal activation.

Switching current versus magnetoresistance in magnetic multilayer nanopillars

We study current-driven magnetization switching in nanofabricated magnetic trilayers, varying the magnetoresistance in three different ways. First, we insert a strongly spin-scattering layer between the magnetic trilayer and one of the electrodes, giving increased magnetoresistance. Second, we insert a spacer with a short spin-diffusion length between the magnetic layers, decreasing the magnetoresistance. Third, we vary the angle between layer magnetizations. In all cases, we find an approximately linear dependence between magnetoresistance and inverse switching current. We give a qualitative explanation for the observed behaviors, and suggest some ways in which the switching currents may be reduced.

Controlled Normal and Inverse Current-Induced Magnetization Switching and Magnetoresistance in Magnetic Nanopillars

Combining pairs of ferromagnetic metals with different signs of scattering anisotropies, let us independently invert the magnetoresistance and the direction of current-driven switching in ferromagnetic/non-magnetic/ferromagnetic metal nanopillars. We show all four combinations of normal and inverse behaviors, at both room temperature and 4.2K. In all cases studied, the direction of switching is set by the net scattering anisotropy of the fixed (polarizing) ferromagnet. We provide simple arguments for what we see.

Structural and magnetic properties of triode-sputtered epitaxial γ′-Fe4N films deposited on SrTiO3 (001) substrates

Highly ordered, single-crystal Fe4N films were prepared on single-crystal SrTiO3 (001) substrates by reactive magnetically enhanced dc triode sputtering. Analytical techniques, including x-ray diffraction, reflectivity, ion channeling, and atomic force microscopy, were used to characterize the structure and morphology of the films. The magnetic properties of the films were measured by magnetic force microscopy and superconducting quantum interference device magnetometry. A technique that utilizes anisotropic magnetoresistance was employed for measurement of the magnetocrystalline anisotropy of the Fe4N films.

A simple low-dissipation amplifier for cryogenic STM

A current sensitive preamplifier designed for low-temperature scanning tunneling microscopy applications is presented. It combines the dc current measurement necessary for the feedback loop operation with a low noise ac measurement used for spectroscopy. The active device is a high electron mobility transistor which was chosen for its low input capacitance and excellent low-temperature performance. The power dissipation of the transistor can be kept at about 10 μW making it compatible with a variety of cryogenic systems. The ac current sensitivity is about 4 fA/√Hz at 4.2 K.

High-scan-range cryogenic scanning probe microscope

We have designed and constructed a scanning probe microscope operable at temperatures down to 260 mK within a top-loading helium-3 cryostat. It achieves a large scan range with the sample situated near the bottom of the scanning head—maximizing the cooling efficiency of the liquid helium. The scan head is completely thermally compensated, thus eliminating thermal expansion and contraction on cooling and warm-up, as well as thermal drift during operation. We demonstrate the performance using two distinct scanning probe methods: scanning tunneling microscopy and charge accumulation imaging.

Current-driven excitations in magnetic multilayers: A brief review

In 1996, Berger and Slonczewski independently predicted that a large enough spin–polarized dc current density sent perpendicularly through a ferromagnetic layer could produce magnetic excitations (spin–waves) or reversal of magnetization (switching). In the past few years, both current-driven switching and current-driven excitation of spin–waves have been observed. The switching is of potential technological interest for direct ‘writing’ of magnetic random access memory (MRAM) or magnetic media. The spin–wave generation could provide a new source of dc generated microwave frequency radiation. We will describe what has been learned experimentally about these two related phenomena, and some models being tested to explain these observations. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Modifying the surface electronic properties of YBa2Cu3O7−δ with cryogenic scanning probe microscopy

We report the results of a cryogenic study of the modification of YBa2Cu3O7−δ surface electronic properties with the probe of a scanning tunnelling microscope (STM). A negative voltage applied to the sample during STM tunnelling is found to modify locally the conductance of the native degraded surface layer. When the degraded layer is removed by etching, the effect disappears. An additional surface effect is identified using scanning Kelvin probe microscopy in combination with STM. We observe reversible surface charging for both etched and unetched samples, indicating the presence of a defect layer even on a surface never exposed to air.

Magnetic anisotropy and structure of epitaxial permalloy films sputter-deposited on Nb(110)

Epitaxial Permalloy™ (Py) thin films sputter-deposited onto epitaxial Nb with and without a Cu buffer layer have been investigated. Transmission electron microscopy was used to characterize the morphology of the Nb/Py/Cu and Nb/Cu/Py multilayers. Rotational magnetization measurements indicated significant differences in the magnetic anisotropy of the films. A phenomenological model taking into account uniaxial anisotropy agree well with the data, and provides a simple method for determinating the anisotropy strength. The correlations between the crystal structures and the magnetic properties are discussed.

Studies of Current-Driven Excitations in Co/Cu/Co Trilayer Nanopillars

Summary. We measure the dynamic resistance of a Co/Cu/Co trilayer nanopillar at varied magnetic field H and current I. The resistance displays the usual behavior, almost symmetric in H, both when magnetization switching is hysteretic at small I, H, and reversible at larger I, H. We show differences in the I, H magnetization stability diagram measured by holding I fixed and varying H and vice versa. We also show how the peak in dV /dI associated with telegraph noise in the reversible switching regime, is calculated from the telegraph noise variations with I. Lastly, we show data for a similar sample that displays behavior asymmetric in H ,a nd a negative reversible switching peak instead of a usual positive one.