S. I. Kiselev

Spin-transfer effects in nanoscale magnetic tunnel junctions

We report measurements of magnetic switching and steady-state magnetic precession driven by spin-polarized currents in nanoscale magnetic tunnel junctions with low-resistance, <5Ωμm2, barriers. The current densities required for magnetic switching are similar to values for all-metallic spin-valve devices. In the tunnel junctions, spin-transfer-driven switching can occur at voltages that are high enough to quench the tunnel magnetoresistance, demonstrating that the current remains spin polarized at these voltages.

Current-induced nanomagnet dynamics for magnetic fields perpendicular to the sample plane.

We present electrical measurements of high-frequency magnetic dynamics excited by spin-polarized currents in Co/Cu/Ni(80)Fe20 nanopillar devices, with a magnetic field applied perpendicular to the sample layers. As a function of current and magnetic field, the dynamical phase diagram contains several distinguishable precessional modes and also static magnetic states. Using detailed comparisons with numerical simulations, we provide rigorous tests of the theory of spin-transfer torques.

Temperature Dependence of Spin-Transfer-Induced Switching of Nanomagnets

We measure the temperature, magnetic-field, and current dependence for the switching of nanomagnets by a spin-polarized current. Depending on current bias, switching can occur between either two static magnetic states or a static state and a current-driven precessional mode. In both cases, the switching is thermally activated and governed by the sample temperature, not a higher effective magnetic temperature. The activation barriers for switching between static states depend linearly on current, with a weaker dependence for dynamic to static switching.

Mechanisms limiting the coherence time of spontaneous magnetic oscillations driven by dc spin-polarized currents

The spin-transfer torque from a DC spin-polarized current can generate highly-coherent magnetic precession in nanoscale magnetic-multilayer devices. By measuring linewidths of spectra from the resulting resistance oscillations, we argue that the coherence time can be limited at low temperature by thermal deflections about the equilibrium magnetic trajectory, and at high temperature by thermally-activated transitions between dynamical modes. Surprisingly, the coherence time can be longer than predicted by simple macrospin simulations.

Spin torque, tunnel-current spin polarization, and magnetoresistance in MgO magnetic tunnel junctions.

We employ the spin-torque response of magnetic tunnel junctions with ultrathin MgO tunnel barrier layers to investigate the relationship between spin transfer and tunnel magnetoresistance (TMR) under finite bias, and find that the spin torque per unit current exerted on the free layer decreases by < 10% over a bias range where the TMR decreases by > 40%. This is inconsistent with free-electron-like spin-polarized tunneling and reduced-surface-magnetism models of the TMR bias dependence, but is consistent with magnetic-state-dependent decay lengths in the tunnel barrier.

Spin-transfer excitations of permalloy nanopillars for large applied currents

Using measurements of the spectra of microwave-frequency resistance oscillations, we determine the roomtemperature phase diagram of magnetic excitations caused by torques from dc spin-polarized currents in thin permalloy/copper/thick permalloy multilayer samples. We extend the measurements to larger values of current than have been reported previously. We find several additional modes that we are able to identify with motion of the thick magnetic layer as well as the thin one. Peaks in the microwave spectra at multiple frequencies suggest that spatially nonuniform dynamical states can be important in some circumstances. We compare the experimental phase diagram with simple theoretical models and achieve a good qualitative agreement.

Sound propagation in liquid He in impurity–helium solids

The observed features of the attenuation of ultrasound in Im–He samples created after the introduction of impurity particles (D2,N2,u200aNe,u200aKr) in a volume of helium II show that a porous substance consisting of a loosely interconnected continuous network is created. It is formed by impurity particles encapsulated in solidified helium. The propagation of ordinary sound in these porous samples is similar to the fast sound mode in light aerogels. The temperature dependence of the attenuation for different Im–He samples is investigated. It is established that the character of the attenuation in D2–He samples is considerably different from that in heavier Im–He solids (Im=N2,u200aNe,u200aKr). Analysis of the attenuation leads to the conclusion that Im–He samples have a wide distribution of pores, from 8 nm to 800 nm. The study of ultrasound in helium in Im–He samples near the λ point shows the presence of broadening in the attenuation peak as compared with bulk liquid helium. The suppression of Tc is very small, ⩽ 0.2 mK.

ESR and X-ray Investigations of Deuterium Atoms and Molecules in Impurity-Helium Solids

Impurity-helium solids created by injecting deuterium atoms and molecules into superfluid 4He have been studied via electron spin resonance (ESR) and x-ray diffraction methods. We measured the g-factor, the hyperfine constant and the spin-lattice relaxation time of D atoms in D-D2-He solids. These measurements show that D atoms are mainly stabilized in D2 clusters. Using an x-ray method we found the size of D2 clusters to be ∼90Å in diameter and the densities of D2 molecules in the samples to be of order 2.5⋅1021 cm−3 . The highest average concentration of D atoms achieved in D-D2-He solids was ∼1.5⋅1018 cm−3 . The local concentrations of D atoms within D2 clusters is found to be large (∼2⋅1019 cm−3).

Magnetic Resonance Studies of Impurity-Helium Solids Containing Hydrogen and Deuterium Impurities

Impurity-Helium Solids (Im-He) produced by injecting a mixed beam of helium and impurity gases into superfluid 4He have been investigated by electron spin resonance (ESR) and nuclear magnetic resonance (NMR) techniques. NMR signals from deuterium molecules in a D2-He solid have been studied. Only samples prepared from gaseous mixtures containing high concentrations of D2 molecules gave observable signals. The ESR experiments were performed on H and/or D atomic impurities in Im-He solids containing H, D, H2, and D2 in various combinations. The exchange tunneling reactions D+H2→HD+H and D+HD→D2+H were used to generate high concentrations of H atoms (∼1017/cm3) in Im-He samples.

Investigation of Ultrasound Attenuation in Impurity-Helium Solids Containing Liquid Helium

The attenuation of ultrasound in Impurity-Helium (Im-He) solids created upon introduction of impurity atoms or molecules (D2,xa0N2,xa0Kr) into a volume of superfluid helium has been investigated. The observed features of attenuation show that a porous substance consisting of a loosely interconnected continuous network is created in superfluid helium. This network is formed by impurity particles encapsulated in solidified helium. Analysis of attenuation allows us to conclude that Im-He solid samples have a wide distribution of pores from 8 nm to 800 nm. It was established that the character of attenuation in D2-He samples is considerably different from that in heavier Im-He solids, for which two maxima of attenuation were sometimes observed. A sharp peak was observed at Tc0 very close to the bulk helium lambda transition temperature and a second broad peak occurred at Tc<Tc0. This behavior is similar to that predicted theoretically for liquid helium in restricted fractal porous media.