Spin injection and detection in copper spin valve structures

We report measurements of spin injection and detection in a mesoscopic copper wire from which the electron spin relaxation time and the spin current polarization in copper can be found. Spin injection is realized by applying a voltage to drive a current from a ferromagnet into the normal metal, while spin detection is done using transport measurements. Precession of the spin of the injected electrons due to an external magnetic field is also studied. The existence of a previously unobserved spin signal which vanishes at low temperatures but increases nonlinearly above 100K is reported and a possible explanation for its origin, based on interfacial spin-flip scattering, is suggested. Multiple cross checks to test the possibility of artifacts as an origin of this signal are discussed. An alternative spin detection method using magnetic force microscopy (MFM) is also studied. This method measures the magnetic field produced by the injected spins directly, so the spin coherence length and the spin current polarization can be extracted directly without the need of a particular transport model, avoiding issues like contact resistance and interface scattering. The MFM method can also be useful for measuring the spin polarization of currents in semiconductors and semiconductor heterostructures, which is important for the development of spintronics

Coherent control of nanomagnet dynamics via ultrafast spin torque pulses

The magnetization orientation of a nanoscale ferromagnet can be manipulated using an electric current via the spin transfer effect. Time domain measurements of nanopillar devices at low temperatures have directly shown that magnetization dynamics and reversal occur coherently over a timescale of nanoseconds. By adjusting the shape of a spin torque waveform over a timescale comparable to the free precession period (100-400 ps), control of the magnetization dynamics in nanopillar devices should be possible. Here we report coherent control of the free layer magnetization in nanopillar devices using a pair of current pulses as narrow as 30 ps with adjustable amplitudes and delay. We show that the switching probability can be tuned over a broad range by timing the current pulses with the underlying free-precession orbits, and that the magnetization evolution remains coherent for more than 1 ns even at room temperature. Furthermore, we can selectively induce transitions along free-precession orbits and thereby manipulate the free magnetic moment motion. We expect this technique will be adopted for further elucidating the dynamics and dissipation processes in nanomagnets, and will provide an alternative for spin torque driven spintronic devices, such as resonantly pumping microwave oscillators, and ultimately, for efficient reversal of memory bits in magnetic random access memory (MRAM).Comment: 4 pages, 3 figures, submitted to Nature Physic

Temperature dependent asymmetry of the nonlocal spin-injection resistance: evidence for spin non-conserving interface scattering

We report nonlocal spin injection and detection experiments on mesoscopic Co-Al2O3-Cu spin valves. We have observed a temperature dependent asymmetry in the nonlocal resistance between parallel and antiparallel configurations of the magnetic injector and detector. This strongly supports the existence of a nonequilibrium resistance that depends on the relative orientation of the detector magnetization and the nonequilibrium magnetization in the normal metal providing evidence for increasing interface spin scattering with temperature.Comment: 5 pages, 4 figures, accepted for publication in PRL, minor corrections (affiliation, acknowledgements, typo

Macrospin model to explain the absence of preswitching oscillations in magnetic tunnel junctions: Fieldlike spin-transfer torque

We show that the absence of preswitching oscillations (“incubation delay”) in magnetic tunnel junctions can be explained within the macrospin model by a sizable fieldlike component of the spin-transfer torque. It is further suggested that measurements of the voltage dependence of tunnel junction switching time in the presence of external easy axis magnetic fields can be used to determine the magnitude and voltage dependence of the fieldlike torque

Macrospin model of incubation delay due to the field-like spin transfer torque

We show that the absence of pre-switching oscillations ("incubation delay") in magnetic tunnel junctions can be explained within the macrospin model by a sizable field-like component of the spin-transfer torque. It is further suggested that measurements of the voltage dependence of tunnel junction switching time in the presence of external easy axis magnetic fields can be used to determine the magnitude and voltage dependence of the field-like torque.Comment: 4 pages, 4 figure

Effect of resistance feedback on spin torque-induced switching of nanomagnets

In large magnetoresistance devices spin torque-induced changes in resistance can produce GHz current and voltage oscillations which can affect magnetization reversal. In addition, capacitive shunting in large resistance devices can further reduce the current, adversely affecting spin torque switching. Here, we simultaneously solve the Landau-Lifshitz-Gilbert equation with spin torque and the transmission line telegrapher's equations to study the effects of resistance feedback and capacitance on magnetization reversal of both spin valves and magnetic tunnel junctions. While for spin valves parallel (P) to anti-parallel (AP) switching is adversely affected by the resistance feedback due to saturation of the spin torque, in low resistance magnetic tunnel junctions P-AP switching is enhanced. We study the effect of resistance feedback on the switching time of MTJ's, and show that magnetization switching is only affected by capacitive shunting in the pF range.Comment: 8 page

Leukemia-initiating cells from some acute myeloid leukemia patients with mutated nucleophosmin reside in the CD34− fraction

Leukemia-initiating cells (LICs) in acute myeloid leukemia (AML) are believed to be restricted to the CD34+ fraction. However, one of the most frequently mutated genes in AML is nucleophosmin (NPM), and this is associated with low CD34 expression. We, therefore, investigated whether NPM-mutated AMLs have LICs restricted to the CD34+ fraction. We transplanted sorted fractions of primary NPM-mutated AML into immunodeficient mice to establish which fractions initiate leukemia. Approximately one-half of cases had LICs exclusively within the CD34− fraction, whereas the CD34+ fraction contained normal multilineage hematopoietic repopulating cells. Most of the remaining cases had LICs in both CD34+ and CD34− fractions. When samples were sorted based on CD34 and CD38 expression, multiple fractions initiated leukemia in primary and secondary recipients. The data indicate that the phenotype of LICs is more heterogeneous than previously realized and can vary even within a single sample. This feature of LICs may make them particularly difficult to eradicate using therapies targeted against surface antigens

Time-dependent analysis of extra length of stay and mortality due to ventilator-associated pneumonia in intensive-care units of ten limited-resources countries: findings of the International Nosocomial Infection Control Consortium (INICC)

Ventilator-associated pneumonias (VAPs) are a worldwide problem that significantly increases patient morbidity, mortality, and length of stay (LoS), and their effects should be estimated to account for the timing of infection. The purpose of the study was to estimate extra LoS and mortality in an intensive-care unit (ICU) due to a VAP in a cohort of 69 248 admissions followed for 283 069 days in ICUs from 10 countries. Data were arranged according to the multi-state format. Extra LoS and increased risk of death were estimated independently in each country, and their results were combined using a random-effects meta-analysis. VAP prolonged LoS by an average of 2.03 days (95% CI 1.52-2.54 days), and increased the risk of death by 14% (95% CI 2-27). The increased risk of death due to VAP was explained by confounding with patient morbidity