arXiv subjects

W. P. Pratt Jr

Publications and source records attributed to W. P. Pratt Jr.

10 recordsLinked to original sources

arXiv2012

Spin Triplet Supercurrent in Co/Ni Multilayer Josephson Junctions with Perpendicular Anisotropy

We have measured spin-triplet supercurrent in Josephson junctions of the form S/F'/F/F'/S, where S is superconducting Nb, F' is a thin Ni layer with in-plane magnetization, and F is a Ni/[Co/Ni]n multilayer with out-of-plane magnetization. The supercurrent in these junctions decays very slowly with F-layer thickness, and is much larger than in similar junctions not containing the two F' layers. Those two features are the characteristic signatures of spin-triplet supercurrent, which is maximized by the orthogonality of the magnetizations in the F and F' layers. Magnetic measurements confirm the out-of-plane anisotropy of the Co/Ni multilayers. These samples have their critical current optimized in the as-prepared state, which will be useful for future applications.

cond-mat.supr-con↗

arXiv2007

Ballistic vs Diffusive Transport in Current-Induced Magnetization Switching

We test whether current-induced magnetization switching due to spin-transfer-torque in ferromagnetic/non-magnetic/ferromagnetic (F/N/F) trilayers changes significantly when scattering within the N-metal layers is changed from ballistic to diffusive. Here ballistic corresponds to a ratio r = lambda/t greater than or equal to 3 for a Cu spacer layer, and diffusive to r = lambda/t less than or equal to 0.4 for a CuGe alloy spacer layer, where lambda is the mean-free-path in the N-layer of fixed thickness t = 10 nm. The average switching currents for the alloy spacer layer are only modestly larger than those for Cu. The best available model predicts a much greater sensitivity of the switching currents to diffuse scattering in the spacer layer than we see.

cond-mat.mtrl-sci↗

arXiv2004

Detecting domain wall trapping and motion at a constriction in narrow ferromagnetic wires using perpendicular-current giant magnetoresistance

We present a versatile method for detecting the presence and motion of a trapped domain wall in a narrow ferromagnetic layer using current-perpendicular-to-plane (CPP) giant magnetoresistance (MR). The CPP-MR response to small motions of the trapped domain wall is enhanced because the CPP current is restricted to the region of wall trapping. We use a Permalloy/Cu/Permalloy spin valve in the shape of a long, ~500-nm-wide wire with a constriction (notch) near its middle that acts as a trapping site for a head-to-head domain wall. Two different notch shapes were studied, mostly at 4.2 K but also at 295K.

cond-mat.mes-hall↗

arXiv2004

Giant Magnetoresistance in Multilayers with Noncollinear Magnetizations

We study the dependence of perpendicular-current magnetoresistance in magnetic multilayers on the angle between the magnetizations of the layers. This dependence varies with the thickness of one of the layers, and is different for multilayers with two and three magnetic layers. We derive a system of equations representing an extension of the two-current series resistor model, and show that the angular dependence of magnetoresistance gives information about the noncollinear spin-transport in ferromagnets.

cond-mat.mtrl-sci↗

arXiv2003

Inverted current-driven switching in Fe(Cr)/Cr/Fe(Cr) nanopillars

From both theory and experiment, scattering of minority electrons is expected to be weaker than scattering of majority electrons in both dilute Fe(Cr) alloys and at Fe(Cr)/Cr interfaces. We show that Fe(Cr)/Cr/Fe(Cr) trilayer nanopillars display a normal magnetoresistance--i.e., largest resistance at low magnetic fields and smallest at high fields, but an inverted current-driven switching--i.e., positive current flowing from the fixed to the reversing layer switches the trilayer from higher to lower resistance, and negative current switches it from lower to higher.

cond-mat.mtrl-sci↗

arXiv2003

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

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 $H$, and a negative reversible switching peak instead of a usual positive one.

cond-mat.mtrl-sci↗

arXiv2003

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 radiation. We describe what has been learned experimentally about these two related phenomena, and some models being tested to explain these observations.

cond-mat.mtrl-sci↗

arXiv2003

Current-Driven Magnetic Excitations in Permalloy-Based Multilayer Nanopillars

We study current-driven magnetization switching in nanofabricated Ni84Fe16/Cu/Ni84Fe16 trilayers at 295 K and 4.2 K. The shape of the hysteretic switching diagram at low magnetic field changes from 295 K to 4.2 K. The reversible behavior at higher field involves two phenomena, a threshold current for magnetic excitations closely correlated with the switching current, and a peak in differential resistance characterized by telegraph noise, with average period that decreases exponentially with current and shifts with temperature. We interpret both static and dynamic results at 295 K and 4.2 K in terms of thermal activation over a potential barrier, with a current dependent effective magnetic temperature.

cond-mat.mtrl-sci↗

arXiv2003

Effect of Interlayer Coupling on Current-Assisted Magnetization Switching in Nanopillars

We show that dipole-field induced antiferromagnetic coupling, or RKKY ferromagnetic coupling, between Co layers can strongly affect the low magnetic field switching behavior of Co/Cu/Co nanopillars. Whereas current-assisted switching at low fields in uncoupled nanopillars is always hysteretic, strong coupling of either kind can change the switching to non-hysteretic (reversible). These differences can be understood with a simple picture of current-assisted thermal activation over a barrier.

cond-mat.mtrl-sci↗

arXiv2003

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.

cond-mat↗