Jonathan Z. Sun

Measurement of the spin-transfer-torque vector in magnetic tunnel junctions

The transfer of spin angular momentum from a spin-polarized current to a ferromagnet can generate sufficient torque to reorient the magnet’s moment. This torque could enable the development of efficient electrically actuated magnetic memories and nanoscale microwave oscillators. Yet difficulties in making quantitative measurements of the spin-torque vector have hampered understanding. Here we present direct measurements of both the magnitude and direction of the spin torque in magnetic tunnel junctions, the type of device of primary interest for applications. At low bias V, the differential torque dτ/dV lies in the plane defined by the electrode magnetizations, and its magnitude is in excellent agreement with recent predictions for near-perfect spin-polarized tunnelling. We find that the strength of the in-plane differential torque remains almost constant with increasing bias, despite a substantial decrease in the device magnetoresistance, and that with bias the torque vector also rotates out of the plane.

Colossal magnetoresistance of 1 000 000‐fold magnitude achieved in the antiferromagnetic phase of La1−xCaxMnO3

Record values of colossal magnetoresistance (CMR) have been achieved in the antiferromagnetic phase of the La1−xCaxMnO3 system. At 125 K, the CMR of the La0.5Ca0.5MnO3 reaches nearly 1 000 000%. It increases exponentially to 100 000 000% at 57 K. While the ground state is primarily an antiferromagnet, application of a magnetic field induces a ferromagnetic alignment of spins that is highly beneficial to the electron conduction. Other ferromagnetic samples exhibit very sharp magnetic phase transitions, with which the magnetotransport is closely correlated.

Spin angular momentum transfer in current-perpendicular nanomagnetic junctions

Spin angular momentum transfer, or spin-transfer, describes the transfer of spin angular momentum between a spin-polarized current and a ferromagnetic conductor. The angular momentum transfer exerts a torque (spin-current induced torque, or spin-torque) on the ferromagnetic conductor. When its dimensions are reduced to less than 100 nm, the spin-torque can become comparable to the magnetic damping torque at a spin-polarized current of high current density (above 106 A/cm2), giving rise to a new set of current-induced dynamic excitation and magnetic switching phenomena. This has now been definitively observed in sub-100-nm current-perpendicular spin-valves and magnetic tunnel junctions, and appears promising as a basis for direct write-address of a nanomagnetic bit when the lateral bit size is reduced to well below 100 nm. An overview is presented in this paper of spin-transfer phenomena. The first part of the paper contains a brief introduction to spin-transfer, especially the characteristic dynamics associated with spin-torque. In the second part, several representative experiments are described. In the third part, a set of basic phenomenological models are introduced that describe experimental observations. The models also serve as a bridge for quantitative comparison between experiments and first-principles spin-polarized transport theory. In the last part of the paper, some device concepts based on spin-transfer-induced magnetic excitation and magnetic reversal are described.

Magnetoresistance and spin-transfer torque in magnetic tunnel junctions

We comment on both recent progress and lingering puzzles related to research on magnetic tunnel junctions (MTJs). MTJs are already being used in applications such as magnetic-field sensors in the read heads of disk drives, and they may also be the first device geometry in which spin-torque effects are applied to manipulate magnetic dynamics, in order to make non-volatile magnetic random access memory. However, there remain many unanswered questions about such basic properties as the magnetoresistance of MTJs, how their properties change as a function of tunnel-barrier thickness and applied bias, and what are the magnitude and direction of the spin-transfer-torque vector induced by a tunnel current.

A Study of Write Margin of Spin Torque Transfer Magnetic Random Access Memory Technology

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.

Current-Induced Magnetization Reversal in High Magnetic Fields in Co=Cu=Co Nanopillars

Current-induced magnetization dynamics in Co/Cu/Co trilayer nanopillars (approximately 100 nm in diameter) have been studied experimentally at low temperatures for large applied fields perpendicular to the layers. At 4.2 K an abrupt and hysteretic increase in resistance is observed at high current densities for one polarity of the current, comparable to the giant magnetoresistance effect observed at low fields. A micromagnetic model that includes a spin-transfer torque suggests that the current induces a complete reversal of the thin Co layer to alignment antiparallel to the applied field--that is, to a state of maximum magnetic energy.

Demonstration of Ultralow Bit Error Rates for Spin-Torque Magnetic Random-Access Memory With Perpendicular Magnetic Anisotropy

Bit error rates below 10-11 are reported for a 4-kb magnetic random access memory chip, which utilizes spin transfer torque writing on magnetic tunnel junctions with perpendicular magnetic anisotropy. Tests were performed at wafer level, and error-free operation was achieved with 10 ns write pulses for all nondefective bits during a 66-h test. Yield in the 4-kb array was limited to 99% by the presence of defective cells. Test results for both a 4-kb array and individual devices are consistent and predict practically error-free operation at room temperature.

Design and applications of a scanning SQUID microscope

The scanning SQUID (Superconducting Quantum Interference Device) microscope is an extremely sensitive instrument for imaging local magnetic fields. The authors describe one such instrument which combines a novel pivoting lever mechanism for coarse-scale imaging with a piezoelectric tube scanner for fine-scale scans. The magnetic field sensor is an integrated miniature SQUID magnetometer. This instrument has a demonstrated magnetic field sensitivity of <10{sup {minus}6} gauss/{radical}Hz at a spatial resolution of {approximately}10 {micro}m. The design and operation of this scanning SQUID microscope are described, and several illustrations of the capabilities of this technique are presented. The absolute calibration of this instrument with an ideal point source, a single vortex trapped in a superconducting film, is shown. The use of this instrument for the first observation of half-integer flux quanta, in tricrystal thin-film rings of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, is described. The half-integer flux quantum effect is a general test of the symmetry of the superconducting order parameter. One such test rules out symmetry-independent mechanisms for the half-integer flux quantum effect, and proves that the order parameter in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} has lobes and nodes consistent with d-wave symmetry.

Spin angular momentum transfer in a current-perpendicular spin-valve nanomagnet

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.

Mercury-based cuprate high-transition temperature grain-boundary junctions and SQUIDs operating above 110 kelvin

The superconducting transport characteristics of HgBa2 CaCu2O6+δ (Hg-1212) films and grain-boundary junctions grown on (100)-oriented SrTiO3 bicrystal substrates have been investigated. The films exhibit a zero-resistance temperature of ∼120 kelvin and sustain large critical current densities, with values as high as 106 amperes per square centimeter at around 100 kelvin. On the other hand, the grain boundaries behave as weak links, with substantially lower critical currents, as is observed for other cuprate superconductors. A reduction of three orders of magnitude in critical current was observed for transport across a 36.8� grain boundary. The current-voltage characteristics of bridges across such a grain boundary show weak-link behavior qualitatively resembling that of a resistively shunted junction. Single-level direct-current superconducting quantum interference devices (SQUIDs) have been fabricated with such bicrystal junctions. These SQUIDs show clear periodic voltage modulations when subjected to applied magnetic fields. The SQUIDs operate at temperatures as high as 111.8 kelvin, which makes them attractive for operation in portable sensors and devices that utilize nonconventional cooling methods.