Search

We report reliable 5 ns switching of spin-transfer torque magnetoresistive random-access memory devices of nominal size 43 nm and a resistance area product of 11 Ω·µm2. We measured 256 devices with a 100% write-error-rate (WER) yield at a WER floor of $10^{-6}$ and a steep WER slope as a function of voltage. A single device had a WER less than $10^{-10}$ for 5 ns write pulses. We show promising 3 ns switching performance, with a 94% WER yield at a $10^{-6}$ WER floor, for 64 devices of nominal size 50 nm.

We demonstrate spin-transfer torque magnetoresistive random access memory (STT-MRAM) arrays achieving 2.8e-10 write error rate (WER) performance at 3 ns write duration at a magnetic tunnel junction (MTJ) diameter of 40 nm. The bit-to-bit distribution of the write voltage at a WER of 1e-6 is characterized by a relative standard deviation of 3.7% for W0 and 4.5% for W1, sufficient to meet the write voltage distribution requirement for last-level cache (LLC) applications at 1x nm nodes.

Magnetic tunnel junctions operating in the superparamagnetic regime are promising devices in the field of probabilistic computing, which is suitable for applications like high-dimensional optimization or sampling problems. Further, random number generation is of interest in the field of cryptography. For such applications, a device's uncorrelated fluctuation time-scale can determine the effective system speed. It has been theoretically proposed that a magnetic tunnel junction designed to have only easy-plane anisotropy provides fluctuation rates determined by its easy-plane anisotropy field and can perform on a nanosecond or faster time-scale as measured by its magnetoresistance's autocorrelation in time. Here, we provide experimental evidence of nanosecond scale fluctuations in a circular-shaped easy-plane magnetic tunnel junction, consistent with finite-temperature coupled macrospin simulation results and prior theoretical expectations. We further assess the degree of stochasticity of such a signal.

We report on the fabrication of nanoscale, three-terminal in-plane spin–orbit torque switching devices with low switching current densities. Critical parameters in the fabrication process, including the ion beam etching angle and time, were optimized to avoid fabrication defects and improve device yield. Measurements of the magnetic field and current-induced switching behavior of the tunnel junctions demonstrate a sensitivity to the nanopillar aspect ratio, which dictates the nanopillars’ anisotropy and thermal stability. Additionally, we show that the current density required for switching can be reduced and the device thermal stability increased by inserting Hf interlayers into the heterostructure. Micromagnetic simulations are generally consistent with the experimentally observed switching behavior, suggesting an increase in the interfacial perpendicular anisotropy at the CoFeB/MgO interface and the reduction in the Dzyaloshinskii–Moriya interaction at the W/CoFeB interface by the Hf interlayers.

The Hi-C experiment can capture the genome-wide spatial proximities of the DNA, based on which it is possible to computationally reconstruct the three-dimensional (3D) structures of chromosomes. The transcripts of the long non-coding RNA (lncRNA) Xist spread throughout the entire X-chromosome and alter the 3D structure of the X-chromosome, which also inactivates one copy of the two X-chromosomes in a cell. The Hi-C experiments are expensive and time-consuming to conduct, but the Hi-C data of the active and inactive X-chromosomes are available. However, the Hi-C data of the X-chromosome during the process of X-chromosome inactivation (XCI) are not available. Therefore, the 3D structure of the X-chromosome during the process of X-chromosome inactivation (XCI) remains to be unknown. We have developed a new approach to reconstruct the 3D structure of the X-chromosome during XCI, in which the chain of DNA beads representing a chromosome is stored and simulated inside a 3D cubic lattice. A 2D Gaussian function is used to model the zero values in the 2D Hi-C contact matrices. By applying simulated annealing and Metropolis-Hastings simulations, we first generated the 3D structures of the X-chromosome before and after XCI. Then, we used Xist localization intensities on the X-chromosome (RAP data) to model the traveling speeds or acceleration between all bead pairs during the process of XCI. The 3D structures of the X-chromosome at 3 hours, 6 hours, and 24 hours after the start of the Xist expression, which initiates the XCI process, have been reconstructed. The source code and the reconstructed 3D structures of the X-chromosome can be downloaded from http://dna.cs.miami.edu/3D-XCI/.

An important goal of spintronics is to covert a charge current into a spin current with a controlled spin polarization that can exert torques on an adjacent magnetic layer. Here we demonstrate such torques in a two ferromagnet system. A CoNi multilayer is used as a spin current source in a sample with structure $\mathrm{CoNi}/\mathrm{Au}/\mathrm{CoFeB}$. Spin torque ferromagnetic resonance is used to measure the torque on the CoFeB layer. The response as a function of the applied field angle and current is consistent with the symmetry expected for a torque produced by the planar Hall effect originating in CoNi. We find the strength of this effect to be comparable to that of the spin Hall effect in platinum, indicating that the planar Hall effect holds potential as a spin current source with a controllable polarization direction.

A practical problem for memory applications involving perpendicularly magnetized magnetic tunnel junctions is the reliability of switching characteristics at high-bias voltage. Often it has been observed that at high-bias, additional error processes are present that cause a decrease in switching probability upon further increase of bias voltage. We identify the main cause of such error-rise process through examination of switching statistics as a function of bias voltage and applied field, and the junction switching dynamics in real time. These experiments show a coincidental onset of error-rise and the presence of a new low-frequency microwave emission well below that dictated by the anisotropy field. We show that in a few-macrospin coupled numerical model, this is consistent with an interface region with concentrated perpendicular anisotropy, and where the magnetic moment has limited exchange coupling to the rest of the layers. These results point to the important role high-frequency interface magnetic moment dynamics play in determining the switching characteristics of these tunnel junction devices.

Binary stochastic neurons (BSNs) form an integral part of many machine learning algorithms, motivating the development of hardware accelerators for this complex function. It has been recognized that hardware BSNs can be implemented using low-barrier magnets (LBMs) by minimally modifying present-day magnetoresistive random-access memory (MRAM) devices. A crucial parameter that determines the response of these LBM-based BSN designs is the correlation time of magnetization $\tau _c$ . In this letter, we show that, for magnets with low-energy barriers ( $\Delta \approx k_BT$ and below), circular disk magnets with in-plane magnetic anisotropy (IMA) lead to $\tau _c$ values that are two orders of magnitude smaller than $\tau _c$ of magnets with perpendicular magnetic anisotropy (PMA). Analytical descriptions demonstrate that this striking difference in $\tau _c$ is due to a precessionlike fluctuation mechanism that is enabled by the large demagnetization field in IMA magnets. We provide a detailed energy-delay performance evaluation of previously proposed BSN designs based on spin-orbit torque MRAM and spin-transfer torque MRAM employing low-barrier circular IMA magnets by SPICE simulations. The designs exhibit subnanosecond response times leading to energy requirements of approximately a few femtojoules to evaluate the BSN function, orders of magnitude lower than digital CMOS implementations with a much larger surface area. While modern MRAM technology is based on PMA magnets, results in this letter suggest that low-barrier circular IMA magnets may be more suitable for this application.

Fast magnetic fluctuations due to thermal torques have useful technological functionality ranging from cryptography to probabilistic computing. The characteristic time of fluctuations in typical uniaxial anisotropy magnets studied so far is bounded from below by the well-known energy relaxation mechanism. This time scales as $\alpha^{-1}$, where $\alpha$ parameterizes the strength of dissipative processes. Here, we theoretically analyze the fluctuating dynamics in easy-plane and antiferromagnetically coupled nanomagnets. We find in such magnets, the dynamics are strongly influenced by fluctuating intrinsic fields, which give rise to an additional dephasing-type mechanism for washing out correlations. In particular, we establish two time scales for characterizing fluctuations (i) the average time for a nanomagnet to reverse|which for the experimentally relevant regime of low damping is governed primarily by dephasing and becomes independent of $\alpha$, (ii) the time scale for memory loss of a single nanomagnet|which scales as $\alpha^{-1/3}$ and is governed by a combination of energy dissipation and dephasing mechanism. For typical experimentally accessible values of intrinsic fields, the resultant thermal-fluctuation rate is increased by multiple orders of magnitude when compared with the bound set solely by the energy relaxation mechanism in uniaxial magnets. This could lead to higher operating speeds of emerging devices exploiting magnetic fluctuations.

The dependence of the write-error rate (WER) on the applied write voltage, write pulse width, and device size was examined in individual devices of a spin-transfer torque (STT) magnetic random-access memory (MRAM) 4 kbit chip. We present 10 ns switching data at the ${10^{ - 6}}$ error level for 655 devices, ranging in diameter from 50 nm to 11 nm, to make a statistically significant demonstration that a specific magnetic tunnel junction stack with perpendicular magnetic anisotropy is capable of delivering good write performance in junction diameters range from 50 to 11 nm. Furthermore, write-error-rate data on one 11 nm device down to an error rate of $7{\times}10^{ - 10}$ was demonstrated at 10 ns with a write current of $7.5\;\upmu{\rm A}$ , corresponding to a record low switching energy below 100 fJ.

The spin Hall effect, which arises from the spin–orbit interaction, is expected to be energy dependent, but experiments typically only characterize electrons near the Fermi surface. A tunnelling spectroscopy method has now been developed to probe the energy dependence.

Key design parameters of 64 Mb STT-MRAM at 90-nm technology node are discussed. A design point was developed with adequate TMR for fast read operation, enough energy barrier for data retention and against read disturbs, a write voltage satisfying the long term reliability against dielectric breakdown and a write bit error rate below 10-9. A direct experimental method was developed to determine the data retention lifetime that avoids the discrepancy in the energy barrier values obtained with spin current- and field-driven switching measurements. Other parameters detrimental to write margins such as backhopping and the existence of a low breakdown population are discussed. At low bit-error regime, new phenomenon emerges, suggestive of a bifurcation of switching modes. The dependence of the bifurcated switching threshold on write pulse width, operating temperature, junction dimensions and external field were studied. These show bifurcated switching to be strongly influenced by thermal fluctuation related to the spatially inhomogeneous free layer magnetization. An external field along easy axis direction assisting switching was shown to be effective for significantly reducing the percentage of MTJs showing bifurcated switching.

We report on room temperature ferromagnetic resonance (FMR) studies of [$t$ Co$|2t$ Ni]$\times$N sputtered films, where $0.1 \leq t \leq 0.6$ nm. Two series of films were investigated: films with same number of Co$|$Ni bilayer repeats (N=12), and samples in which the overall magnetic layer thickness is kept constant at 3.6 nm (N=1.2/$t$). The FMR measurements were conducted with a high frequency broadband coplanar waveguide up to 50 GHz using a flip-chip method. The resonance field and the full width at half maximum were measured as a function of frequency for the field in-plane and field normal to the plane, and as a function of angle to the plane for several frequencies. For both sets of films, we find evidence for the presence of first and second order anisotropy constants, $K_1$ and $K_2$. The anisotropy constants are strongly dependent on the thickness $t$, and to a lesser extent on the total thickness of the magnetic multilayer. The Land\'e g-factor increases with decreasing $t$ and is practically independent of the multilayer thickness. The magnetic damping parameter $\alpha$, estimated from the linear dependence of the linewidth, $\triangle H$, on frequency, in the field in-plane geometry, increases with decreasing $t$. This behaviour is attributed to an enhancement of spin-orbit interactions with $t$ decreasing and in thinner films, to a spin-pumping contribution to the damping., Comment: 18 pages, 13 figures

We apply known relations for voltage-driven current and torque to magnetic tunneling junctions (MTJs) symmetric in composition. Postulated are effects of possibly asymmetric microstructures of the two interfaces separating the two electrodes from a barrier composed of (0 0 1)MgO. Our model takes into account effects of (i) dependence of state density on energy, (ii) possible asymmetry of the elastic transfer Hamiltonian, and tunneling enabled by inelastic mechanisms (iii) within the barrier or (iv) within the electrodes. It leads to distinct observable signatures for these effects in the voltage dependences of current and torque. A parameter-free relation connecting torque with conductance makes possible direct experimental tests of the model. Additionally, a derived formula illustrates how coulomb-correlation may contribute to these effects at high voltage.

This report examines the current status and the future directions of the field of nanomagnetism and assesses the ability of hard X-ray synchrotron facilities to provide new capabilities for making advances in this field. The report first identifies major research challenges that lie ahead in three broadly defined subfields of nanomagnetism: confined systems, clusters and complex oxides. It then examines the relevant experimental capabilities that are currently available at hard X-ray synchrotron light sources, using the Advanced Photon Source (APS) at Argonne as an example. Finally, recommendations are made for future development in X-ray facilities that will enhance the study of nanomagnetism, including new experimental directions, modifications that would enable in situ sample preparation, and measurements at high magnetic fields and/or low temperatures. In particular, in situ sample preparation is of high priority in many experiments, especially those in the area of surface magnetism.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; We present a systematic experimental study of the spin-torque-induced magnetic switching statistics at room temperature, using all-perpendicularly magnetized spin-valves as a model system. Three physical regimes are distinguished: a short-time ballistic limit below a few nanoseconds, where spin-torque dominates the reversal dynamics from a thermal distribution of initial conditions; a long time limit, where the magnetization reversal probability is determined by spin-torque-amplified thermal activation; and a cross-over regime, where the spin-torque and thermal agitation both contribute. For a basic quantitative understanding of the physical processes involved, an analytical macrospin model is presented which contains both spin-torque dynamics and finite temperature effects. The latter was treated rigorously using a Fokker-Plank formalism, and solved numerically for specific sets of parameters relevant to the experiments to determine the switching probability behavior in the short time and cross over regimes. This analysis shows that thermal fluctuations during magnetization reversal greatly affect the switching probability over all the time scales studied, even in the short time limit. (C) 2014 Elsevier B.V. All rights reserved

I examined the consequence of a spin-current-induced angular momentum deposition in a monodomain Stoner-Wohlfarth magnetic body. The magnetic dynamics of the particle are modeled using the Landau-Lifshitz-Gilbert equation with a phenomenological damping coefficient $\ensuremath{\alpha}.$ Two magnetic potential landscapes are studied in detail: One uniaxial, the other uniaxial in combination with an easy-plane potential term that could be used to model a thin-film geometry with demagnetization. Quantitative predictions are obtained for comparison with experiments.

We present evidence for {ital forced topological entanglement } of line-like vortices in the liquid regime of YBa{sub 2}Cu {sub 3}O{sub 7{minus}{delta}} with columnar defects. By splaying the defects within a fixed plane (in-fan) we create a {bold {cflx c}}- axis phase coherent vortex (string) structure which correlates with anisotropic dissipation. The high-field vortex motion is {open_quotes}easy{close_quotes} in fan and {open_quotes}hard{close_quotes} across fan in crystals{emdash}a signature of forced entanglement by a small angle splay. The high-field anisotropy is surprisingly reversed in thin films, where forced entanglement is absent. {copyright} {ital 1997} {ital The American Physical Society}

We have designed and carried out a tricrystal experiment, based on macroscopic coherence effects, flux quantization, and pair tunneling, to study the microscopic phase of the pair wave function in high- T c cuprate superconductors. Using a high-resolution scanning SQUID microscope, we have made the first direct observation of spontaneously generated half-integer flux quanta in controlled-orientation tricrystal rings and blanket films. By varying the grain misorientation and the grain-boundary angles in the tricrystal, we have proved that the order parameter in YBa 2 Cu 3 O 7 and Tl 2 Ba 2 Cu 6 + δ has nodes and lobes consistent with d wave, but not with g wave pairing symmetry.

We have used a high-resolution scanning SQUID microscope to directly image conventional ( $h/2e$) Josephson vortices trapped in grain boundaries, and half-integer ( $h/4e$) Josephson vortices trapped at the tricrystal point, of the high- ${T}_{c}$ superconductor ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ grown on tricrystal substrates of SrTi${\mathrm{O}}_{3}$. Our observation of the half-integer vortex at the tricrystal point is the first direct demonstration of the positive paramagnetic Meissner effect in a polycrystalline high- ${T}_{c}$ film. In addition, our images provide the first direct measurement of the Josephson penetration depth.

The presence or absence of the half-integer flux quantum effect in controlled orientation tricrystal grain boundary rings is a general test of the phase of the superconducting order parameter. One such test proves that this effect is symmetry dependent, and that the order parameter in YBa2Cu3O7-ς has lobes and nodes consistent with d-wave symmetry. Our measurements show that the flux in the 1/2 integer ground state is quantized to Φ0/2 within ± 3%. This puts limits on the imaginary component of the superconducting order parameter in YBa2Cu3O7-ς.

We have measured the current‐voltage, flux‐voltage, and noise characteristics of 77 K superconducting quantum interference devices made with step edge junctions and multilevel edge junctions on SrTiO3 substrates and with step edge junctions on LaAlO3. We find in each case that the I‐V curves can have excess currents of up to 50%–80%, that the responsivity, ∂V/∂Φ, can be substantially smaller than expected from the resistively shunted junction (RSJ) model, but that the white noise voltage spectral density is about as expected. We discuss the extent to which the reduction of the responsivity is correlated with the excess current. We note that the observed unchanged magnitude of the voltage noise and the reduced responsivity would both be expected if we based the RSJ predictions on a model in which the excess current is not considered part of the Josephson current. Finally and perhaps fundamentally, we find that simulations made assuming a nonsinusoidal current‐phase relationship give rise to an excess curre...

International audience; We present a two-current-pulse temporal correlation experiment to study the intrinsic subnanosecond nonequilibrium magnetic dynamics of a nanomagnet during and following a pulse excitation. This method is applied to a model spin-transfer system, a spin-valve nanopillar with perpendicular magnetic anisotropy. Two pulses separated by a short delay (

A pure spin current generated within a nonlocal spin valve can exert a spin transfer torque on a nanomagnet. This nonlocal torque enables new design schemes for magnetic memory devices that do not require the application of large voltages across tunnel barriers that can suffer electrical breakdown. Here we report a quantitative measurement of this nonlocal spin torque using spin-torque-driven ferromagnetic resonance. Our measurement agrees well with the prediction of an effective circuit model for spin transport. Based on this model, we suggest strategies for optimizing the strength of nonlocal torque., Comment: 21 pages, 3 figures. Supplement included

International audience; The spin-torque switching of metallic nanopillar spin valves showing strong perpendicular anisotropy are studied. The magnetic states of the layers depend on extrinsic parameters such as the magnetic field and the dc current applied to the device. A state diagram presents a comprehensive graph of the role of those parameters on the spin-valve magnetic response. After explaining how state diagrams can be built and the different possible representation, experimental state diagrams are studied for perpendicular devices and the influence of lateral size, temperature, and field orientation are shown. An analytical model of a purely uniaxial system is presented. It is shown that this simple model does not properly reflect the experimental results, whereas if the symmetry is broken a qualitative agreement is obtained. Finally, the possible origins of the symmetry break are discussed in light of an analytical model and numerical simulations.

We have used the concept of flux quantization in superconducting $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ rings with 0, 2, and 3 grain-boundary Josephson junctions to test the pairing symmetry in high-${T}_{c}$ superconductors. The magnetic flux threading these rings at 4.2 K is measured by employing a scanning superconducting quantum interference device microscope. Spontaneous magnetization of a half magnetic flux quantum, $\frac{{\ensuremath{\Phi}}_{0}}{2}=\frac{h}{4e}$ has been observed in the 3-junction ring, but not in the 2-junction rings. These results are consistent with $d$-wave pairing symmetry.

International audience; Magnetization excitation and reversal induced by spin-transfer torques is described in terms of power received or dissipated in a macrospin system. This approach provides a clear and intuitive understanding of the effect of both applied magnetic fields and injected spin-polarized currents on magnetization reversal. It is illustrated by solving the case of magnetization reversal in a nanopillar spin valve with perpendicular magnetizations although the approach can be applied more generally. The appearance of critical currents below which spin-transfer torque is no longer efficient is explained by a break in the uniaxial symmetry of such structures.

We examine the sweep-rate dependence of magnetic switching field, Hs, in submicron magnetic tunnel junctions where shape anisotropy is dominant. Experimental data support the use of a single-domain thermal activation model for description of activated magnetic reversal in junctions 0.2 by 0.5 μm or less in size. A scaling law is obtained for the thermal activation energy which varies as the cube of junction size.

We report data from 4-kbit spin torque MRAM arrays using tunnel junctions (TJs) with magnetization perpendicular to the wafer plane. We show for the first time the switching distribution of perpendicular spin torque junctions. The percentage switching voltage width, σ(V c )/ = 4.4%, is sufficient to yield a 64 Mb chip. Furthermore we report switching probability curves down to error probabilities of 5×10−9 per pulse which do not show the anomalous switching seen in previous studies of in-plane magnetized bits.

Spin-valve based nanojunctions incorporating Co|Ni multilayers with perpendicular anisotropy were used to study spin-torque driven ferromagnetic resonance (ST-FMR) in a nonlinear regime. Perpendicular field swept resonance lines were measured under a large amplitude microwave current excitation, which produces a large angle precession of the Co|Ni layer magnetization. With increasing rf power the resonance lines broaden and become asymmetric, with their peak shifting to lower applied field. A nonhysteretic step jump in ST-FMR voltage signal was also observed at high powers. The results are analyzed in in terms of the foldover effect of a forced nonlinear oscillator and compared to macrospin simulations. The ST-FMR nonhysteretic step response may have applications in frequency and amplitude tunable nanoscale field sensors., Comment: 4 pages, 3 figures, to appear in Applied Physics Letters

We have demonstrated a robust magnetic tunnel junction (MTJ) with a resistance-area product RA=8 Omega-mum2 that simultaneously satisfies the statistical requirements of high tunneling magnetoresistance TMR > 15sigma(Rp), write threshold spread sigma(Vw)/

We use spin-transfer-driven ferromagnetic resonance (ST-FMR) to measure the spin-transfer torque vector T in MgO-based magnetic tunnel junctions as a function of the offset angle between the magnetic moments of the electrodes and as a function of bias, V. We explain the conflicting conclusions of two previous experiments by accounting for additional terms that contribute to the ST-FMR signal at large |V|. Including the additional terms gives us improved precision in the determination of T(V), allowing us to distinguish among competing predictions. We determine that the in-plane component of has a weak but non-zero dependence on bias, varying by 30-35% over the bias range where the measurements are accurate, and that the perpendicular component can be large enough to be technologically significant. We also make comparisons to other experimental techniques that have been used to try to measure T(V)., 30 pages, 8 figures. Expanded with additional data and discussion. In press at PRB

High current density spin-polarized transport in current-perpendicular nanomagnetic junctions reveals an interaction between the spin-polarized current and the nanomagnet electrodes. This interaction causes a current-induced magnetic excitation in the ferromagnetic electrodes, leading either to persistent magnetic precession or to a complete magnetic reversal, under a dc current bias. This article gives a brief review of the basic understanding of this so-called spin-transfer phenomenon, including the basic mechanism, its early theoretical prediction and experimental discovery, and the significance of this interaction for applications in modern electronics. This review uses a framework of phenomenological analyses, bridging experimental observations and first-principle-based theoretical work. Keywords: Spin angular momentum; spin torque; spin transfer; current-induced magnetic switching; spin polarized transport; spintronics

Spin-polarized currents can transfer spin angular momentum to a ferromagnet, generating a torque that can efficiently reorient its magnetization. Achieving quantitative measurements of the spin-transfer-torque vector in magnetic tunnel junctions (MTJs) is important for understanding fundamental mechanisms affecting spin-dependent tunneling, and for developing magnetic memories and nanoscale microwave oscillators. Here we present direct measurements of both the magnitude and direction of the spin torque in Co60Fe20B20/MgO/Co60Fe20B20 MTJs. At low bias V, the differential torque vector d{tau}/dV lies in the plane defined by the electrode magnetizations, and its magnitude is in excellent agreement with a prediction for highly-spin-polarized tunneling. With increasing bias, the in-plane component d{tau}_{parallel}/dV remains large, in striking contrast to the decreasing magnetoresistance ratio. The differential torque vector also rotates out of the plane under bias; we measure a perpendicular component tau_{perp}(V) with bias dependence proportional to V^2 for low V, that becomes as large as 30% of the in-plane torque., 33 pages, 9 figures

When injected spin polarized electrons interact with the magnetic moment of a free layer, their angular momentum becomes transferred to the free layer. If sufficient current is applied, the exerted torque switches the free layer either parallel or anti-parallel to the pinned layer depending on the direction of flow of the current. This type of localized current switching is attractive for an MRAM application. This paper focuses on the fabrication and characterization of sub-100nm spin-transfer switching devices. These fabricated tunnel junctions were characterized at room temperature with a quasi-static electrical measurement setup.

Based on macroscopic quantum coherence effects arising from pair tunneling and flux quantization, a series of tricrystal experiments have been designed and carried out to test the order parameter symmetry in high-Tc cuprate superconductors. By using a scanning SQUID microscope, we have directly and non-invasively observed the spontaneously generated half-integer flux quantum effect in controlled-orientation tricrystal cuprate superconducting systems. The presence or absence of the half-integer flux quantum effect as a function of the tricrystal geometry allows us to prove that the order parameter symmetry in the YBCO and Tl2201 systems is consistent with that of the dx2−y2 pair state.

We present a method to measure the effective field contribution to spin-transfer-induced interactions between the magnetic layers in a bilayer nanostructure, which enables spin current effects to be distinguished from the usual charge-current-induced magnetic fields. This technique is demonstrated on submicron $\mathrm{Co}∕\mathrm{Cu}∕\mathrm{Co}$ nanopillars. The hysteresis loop of one of the magnetic layers in the bilayer is measured as a function of current while the direction of magnetization of the other layer is kept fixed, first in one direction and then in the opposite direction. These measurements show a current-dependent shift of the hysteresis loop which, based on the symmetry of the magnetic response, we associate with spin transfer. The observed loop shift with applied current at room temperature is reduced in measurements at $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We interpret these results both in terms of a spin-current-dependent effective activation barrier for magnetization reversal and a spin-current-dependent effective magnetic field. From data at $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, we estimate the magnitude of the spin-transfer-induced effective field to be $\ensuremath{\sim}1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{A}$, about a factor of 5 less than the spin torque.

Spin-angular momentum transfer (or spin-transfer for short) describes the angular momentum exchange between a spin-polarized current and a ferromagnetic conductor. When the conductor dimensions are reduced to around 100nm or below, the spin-angular momentum transfer effect becomes significant compared to the current-induced magnetic field. This paper describes some recent spin-transfer experimental findings in sub-100nm current-perpendicular spin-valve systems consisting of Co-Cu-Co nanopillars. The spin-transfer current is shown to cause a magnetic reversal of the thinner magnetic layer inside the nanopillar. The reversal is experimentally shown to reach sub-nanosecond speed. The effect of spin-transfer is best understood in terms of its modification to the effective Landau-Lifshiz-Gilbert damping coefficient, either increasing or decreasing its value depending on the direction and magnitude of the spin-polarized current. For sufficiently large spin-current, the net damping coefficient may change sign, resulting in amplification of magnetic precession, leading to a magnetic reversal. At finite temperatures, the effect of spin-transfer is to either increase or decrease the thermal agitation of the nanomagnet. A quantitative model is developed that adequately describes the finite temperature experimental observations of the dynamic spin-transfer effect.

This letter discusses the effects of a moderate magnetic field applied to a SQUID magnetometer and introduces the invention of the ‘‘flux dam.’’ A flux dam is a weak link or Josephson junction in series with the pickup loop which allows magnetic flux to enter and leave the inside of the superconducting loop easily for moderate changes in magnetic field. This largely prevents the flux lines from entering and being pinned in the superconducting parts of the magnetometer which would create extra low frequency noise. SQUID magnetometers with flux dams in the pickup loop can be operated in the Earth’s magnetic field without a significant increase in low frequency flux noise.

We have combined a novel low temperature positioning mechanism with a single‐chip miniature superconducting quantum interference device (SQUID) magnetometer to form an extremely sensitive new magnetic microscope, with a demonstrated spatial resolution of ∼10 μm. The design and operation of this scanning SQUID microscope will be described. The absolute calibration of this instrument with an ideal point source, a single vortex trapped in a superconducting film, will be presented, and a representative application will be discussed.

We study experimentally the current-driven magnetic excitations in symmetric $\mathrm{C}\mathrm{o}/\mathrm{C}\mathrm{u}/\mathrm{C}\mathrm{o}$ nanopillars. In contrast with all the previous observations where the current of only one polarity is capable of exciting a multilayer system saturated by an externally applied magnetic field, we observe that both polarities of the applied current trigger excitations in a symmetric multilayer. This may indicate that in symmetric structures the current propels high-frequency magnetic oscillations in all magnetic layers. We argue, however, that only one layer is excited in our multilayers but, interestingly, currents of opposite polarities excite different layers. This hypothesis is supported by modeling the spin accumulation in symmetric magnetic multilayers.

Angular momentum transfer in magnetic nanostructures mediated by an electric current has become the subject of intense research. Slonscewski and Berger first considered this theoretically in 1996 [1,2]. Its experimental observation a few years later [3–7] has boosted efforts to understand the influence of the conduction electron spin on the magnetization dynamics of ferromagnetic nanostructures in the presence of high electric currents. A spin current has been demonstrated to switch the magnetization direction of a small magnetic element. Reversible changes in multilayer resistance have also been observed and associated with magnetic excitations or the generation of spin waves. However, an in-depth understanding of the relationship between these two phenomena — reversible excitations of the magnetization and irreversible switching of the magnetization—is still missing. In the initial experiments a point contact was employed to inject high current densities into a magnetic multilayer [3,6,7]. Subsequent experiments have concentrated on the reduction of the lateral size of Co=Cu=Co trilayers to the submicron scale, resulting in the fabrication of nanopillar devices [8–14]. In the point-contact experiments the applied magnetic field was oriented perpendicular to the thin film plane and was larger than the film demagnetization fields (H � 2T ). In this high field regime a peak structure in the differential resistance (dV=dI) at a critical current was interpreted as the onset of current induced excitation of spin waves in which the current induced spin-transfer torque leads to the uniform precession of the magnetization [3,6,15]. Spin-transfer torque studies on pillar devices concentrated on magnetic fields applied in the thin film plane. In the low-field regime, a hysteretic jump in the differential resistance was observed; clear evidence for current induced magnetization reversal of one of the magnetic layers [8–14]. Hence, the effect of the spin-polarized current on the magnetization seemed to be quite distinct in the low and high field regimes. Experimentally, a current induced hysteretic magnetization reversal was observed only at low in-plane fields in nanopillar devices. In this Letter we report detailed studies of current induced dynamics of the magnetization in submicron size pillar devices at high magnetic fields in the field perpendicular to the plane geometry. For sufficiently large currents of one polarity hysteretic magnetic switching of the layers is observed at high magnetic fields. In contrast to previous results in this geometry with mechanical point contacts, these results cannot be understood as small amplitude excitations of the magnetization. Micromagnetic modeling suggests that spin-transfer torques induce precessional states which evolve into a static state of antiparallel alignment of the layers.

Time-resolved measurements of spin-transfer-induced (STI) magnetization reversal were made in current-perpendicular spin-valve nanomagnetic junctions subject to a pulsed current bias. These results can be understood within the framework of a Landau-Lifshitz-Gilbert equation that includes STI corrections and a Langevin random field for finite temperature. Comparison of these measurements with model calculations demonstrates that spin-transfer induced excitation is responsible for the observed magnetic reversal in these samples.

The effects of radio frequency radiation on the dc SQUID are examined. Simulations show how the shape of the SQUID transfer characteristic is distorted by radio frequency interference (RFI). How this affects three commonly used SQUID modulation methods is discussed, and the results explain why we experimentally observe the bias current reversing readout method to be the least susceptible to RFI. The commonly seen increase in the low frequency flux noise power spectrum of dc SQUIDs in unshielded environments is also explained.

We have invented a three superconducting quantum interference device (SQUID) gradiometer (TSG) that uses three SQUID magnetometers and a novel feedback method to measure magnetic field gradients. One SQUID, designated the reference SQUID, operates normally except that its feedback loop output is directed to all three SQUIDs through identical nonsuperconducting coils around each SQUID. The feedback loops for the remaining two SQUIDs, the sensor SQUIDs, measure the differences between the magnetic field at the reference SQUID location and those at the sensor SQUID locations. The voltage difference between the two sensor SQUID outputs divided by the gradiometer base line, the distance between the sensor SQUIDs, represents the average magnetic field gradient. We have measured gradient sensitivities of 10−12 and 10−10 T/m√Hz for TSGs made from bare low‐Tc and high‐Tc SQUIDs. An advantage of a TSG is that a sensitive gradiometer, free of hysteresis error, can be made using relatively small substrates.

The circuit element behavior during an active read-out approach is analyzed for the detection of the bit-state of a spin-transfer-torque (STT) switchable magnetic tunnel junction (MTJ) as a storage bit in a random access memory. Fundamentally, such read-out schemes detect the presence or absence of a hysteresis in the current-voltage characteristic of the MTJ depending on its magnetic configuration and the bias direction. A quantitative assessment is given in terms of the range of threshold distribution to be expected, and the read-out time one can achieve as it depends on various device parameters. The quantitative results given are based on an approximate macrospin model for the STT-switched MTJ.

Magnetic-memory devices of the future could be based on 'spintronics', through switching the directions of electron spins. New work confirms the physics behind a spin-switching mechanism.