Spin transfer effect in magnetic tunnel junction with a nano-current-channel Layer in free layer

The current-induced magnetization switching (spin transfer effect) in a low resistance-area (RA) product magnetic tunnel junction (MTJ) device with critical current density of 1.4/spl times/10/sup 7/ A/cm/sup 2/ was demonstrated. The RA product of the MTJ is 4.2 /spl Omega//spl mu/m/sup 2/ and the magnetoresistance (MR) ratio induced by current is up to 16%. An MTJ structure with a novel nano-current-channel (NCC) layer inserted into the free layer for the current-induced magnetization switching by lower current density was proposed and prototyped. By using the current confined effect, the local current density in the integrated free layer was sufficiently high to switch the magnetization locally. Such local magnetization reversal helped to reverse the magnetic moments around together with the polarized current and spread out the switching of the entire free layer through the superparamagnetic nano-channels. The critical current density was reduced to 4.2/spl times/10/sup 6/ A/cm/sup 2/, which is only one quarter of that for a pure MTJ structure.     

Current-Induced Helicity Reversal of a Single Skyrmionic Bubble Chain in a Nanostructured Frustrated Magnet

Helicity indicates the in-plane magnetic-moment swirling direction of a skyrmionic configuration. The ability to reverse the helicity of a skyrmionic bubble via purely electrical means has been predicted in frustrated magnetic systems; however, it has been challenging to observe this experimentally. The current-driven helicity reversal of the skyrmionic bubble in a nanostructured frustrated Fe₃Sn₂ magnet is experimentally demonstrated. The critical current density required to trigger the helicity reversal is 10⁹–10¹⁰ A m⁻², with a corresponding pulse-width varying from 1 µs to 100 ns. Computational simulations reveal that both the pinning effect and dipole–dipole interaction play a crucial role in the helicity reversal process.     

Magnetization Reversal by Electrical Spin Injection in Ferromagnetic (Ga,Mn)As-Based Magnetic Tunnel Junctions

We report current-induced magnetization reversal in a ferromagnetic semiconductor-based magnetic tunnel junction (Ga,Mn)As/AlAs/(Ga,Mn)As prepared by molecular beam epitaxy on a p-GaAs(001) substrate. A change in magneto-resistance that is asymmetric with respect to the current direction is found with the excitation current of 10⁶ A/cm². Contributions of both unpolarized and spin-polarized components are examined, and we conclude that the partial magnetization reversal occurs in the (Ga,Mn)As layer having smaller magnetization with the spin-polarized tunneling current of 10⁵ A/cm².     

Ultrafast Magnetization Manipulation Using Single Femtosecond Light and Hot‐Electron Pulses

Current‐induced magnetization manipulation is a key issue for spintronic applications. This manipulation must be fast, deterministic, and nondestructive in order to function in device applications. Therefore, single‐ electronic‐pulse‐driven deterministic switching of the magnetization on the picosecond timescale represents a major step toward future developments of ultrafast spintronic systems. Here, the ultrafast magnetization dynamics in engineered Gd_x [FeCo]_1?x ‐based structures are studied to compare the effect of femtosecond laser and hot‐electron pulses. It is demonstrated that a single femtosecond hot‐electron pulse causes deterministic magnetization reversal in either Gd‐rich and FeCo‐rich alloys similarly to a femtosecond laser pulse. In addition, it is shown that the limiting factor of such manipulation for perpendicular magnetized films arises from the formation of a multidomain state due to dipolar interactions. By performing time‐resolved measurements under various magnetic fields, it is demonstrated that the same magnetization dynamics are observed for both light and hot‐electron excitation, and that the full magnetization reversal takes place within 40 ps. The efficiency of the ultrafast current‐induced magnetization manipulation is enhanced due to the ballistic transport of hot electrons before reaching the GdFeCo magnetic layer.     

Micromagnetic simulation on dynamics of spin transfer torque magnetization reversal

I study the magnetization dynamics induced by spin transfer torque in CoFe/Ru/CoFe/Cu/NiFe nano-pillars using LLG micromagnetic simulations. The required current for spin transfer torque magnetization reversal was investigated with the switching speed and the temperature. The required current in rectangular nanopillars is much larger than that in elliptical nanopillers. The temperature dependence is pretty complicated. The antiparallel to parallel magnetization reversal at 77 K requires a smaller current than at 300 K. The parallel to antiparallel magnetization reversal at 77 K requires a smaller current than at 300 K only when the pulse duration time is very short.     

The Paramagnetism Carried by Paramagnetic Ions and its Effect on the Superconductivity in High T C Superconductors

Extended Brillouin function with a revision is applied to describe the paramagnetism carried by the rare-earth Gd^{3 +} ions in GdBa₂Cu₃O_6+δ and the Re²⁺ ions in (Hg_0.9Re_0.1)Ba₂Ca₂Cu₃O _g+δ. We believe that the paramagnetism depends on the internal flux density of the sample instead of the applied field. At the field below H _cl the paramagnetism has no contribution to total magnetization; at the field over H _cl the paramagnetic magnetization has different values when in field-increasing and field-decreasing process. The width of the magnetization hysteresis loop Δ M is broadened by the paramagnetism. The effect of the paramagnetism due to paramagnetic ions on the magnetization relaxation rate and the magnetization critical-current density J _c based on the Bean critical-state model is also discussed. The temperature dependence of the paramagnetism is shown in this paper.     

Temperature increase due to Joule heating in a nanostructured MgO-based magnetic tunnel junction over a wide current-pulse range

The temperature increase due to Joule heating in a nanopillar of a magnetic tunnel junction sandwiched by top and bottom electrodes was calculated by the finite element method. The results for the critical condition for the current-induced magnetization switching measured over a wide current-pulse range were taken from the literature. At long pulse widths, the temperature increase was solely dependent on the magnitude of the critical current density. However, no saturation in the temperature increase occurred for short pulse widths. In this case, the temperature increase additionally depended on the pulse width, so that a broad maximum occurred in the pulse width (or the critical current density) dependence of the temperature increase. The original results for the critical condition were corrected by accounting for the temperature increase and these corrected results, together with the Slonczewski equation, were used to extract an accurate value for the thermal stability factor.     

Spin-transfer switching current distribution and reduction in magnetic tunneling junction-based structures

Spin transfer switching current distribution within a cell and switching current reduction were studied at room temperature for magnetic tunnel junction-based structures with resistance area product (RA) ranged from 10 to 30 /spl Omega/-/spl mu/m/sup 2/ and TMR of 15%-30%. These were patterned into current perpendicular to plane configured nanopillars having elliptical cross sections of area /spl sim/0.02 /spl mu/m/sup 2/. The width of the critical current distribution (sigma/average of distribution), measured using 30 ms current pulse, was found to be 3% for cells with thermal factor (KuV/k/sub B/T) of 65. An analytical expression for probability density function p(I/I/sub c0/) was derived considering a thermally activated spin transfer model, which supports the experimental observation that the thermal factor is the most significant parameter in determining the within-cell critical current distribution. Spin-transfer switching current reduction was investigated through enhancing effective spin polarization factor /spl eta//sub eff/ in magnetic tunnel junction-based dual spin filter (DSF) structures. The intrinsic switching current density (J/sub c0/) was estimated by extrapolating experimental data of critical current density (J/sub c/) versus pulse width (/spl tau/), to a pulse width of 1 ns. A reduction in intrinsic switching current density for a dual spin filter (DSF: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/spacer/CoFe/PtMn/Ta) was observed compared to single magnetic tunnel junctions (MTJ: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/Ta). J/sub c/ at /spl tau/ of 1 ns (/spl sim/J/sub c0/) for the MTJ and DSF samples were 7/spl times/10/sup 6/ and 2.2/spl times/10/sup 6/ A/cm/sup 2/, respectively, for identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structure compared to the single MTJ case.     

Pinning and vortex dynamics in superconducting (K,Ba)BiO 3 single crystals

Pinning and vortex dynamics have been investigated in the 3-dimensional copper free (K, Ba)BiO₃superconductor (T_c 31K) by magnetization and transport measurements up to 30 Tesla. The magnetization curves present a pronounced fishtail effect which persists for time scales down to 10^–4s (pulsed field measurements). We show that it is an intrinsic feature of the critical current which can in part be well described by the collective pinning theory. Furthermore, this system presents evidence for a vortex liquid /glass transition for vanishingly small currents. As the current density is increased, dissipation in the glass state is dominated by creep effects. The temperature and current dependence of the activation energy is discussed.     

Evaluation of laser drilling of holes in thermal barrier coated superalloys

Abstract

Laser drilling of precise holes in thermal barrier coated Ni based superalloys has been studied. The interplay between various hole geometrical features such as hole shape, taper, barrelling, undercut, etc. and laser parameters such as pulse energy, pulse width and pulse repetition rate have been examined. The hole diameters are seen to follow a linear dependence on the incoming laser power densities for pulse width up to 2·0 ms. However, such a linear dependence was not observed for a pulse width of 3·0 ms. It was found that high pulse energy and short pulse width (high power density) gave crack free recast layer, whereas low pulse energy and longer pulse width (low power density) gave microcracks in the heat affected layer of superalloy. The significant barrelling observed in IN718 material at low power density values is due to multiple reflection of the incident beam from the cavity in combination with plasma formation at the evaporation front and trapping of the incident radiation causing excessive heating in that region.     

Low Temperature Magnetization of Two Dimensional 3He on Three Layers of HD Preplated Graphite

We have measured the low temperature magnetization of submonolayer ³He on three layers of HD preplated graphite. NMR measurement has been performed by progressively adding ⁴He to the system, to prevent ³He atoms from being trapped in substrate heterogeneities and to change the areal density of ³He. The exchange constant (J) is found to have a similar density dependence to that for two layers of HD preplated one. The magnetization in the antiferromagnetic region increases gradually down to 100 K and shows no evidence corresponding to a spin gap.     

The Paramagnetism Carried by Paramagnetic Ions and its Effect on the Superconductivity in High TC Superconductors

Extended Brillouin function with a revision is applied to describe the paramagnetism carried by the rare-earth Gd^{3 +} ions in GdBa₂Cu₃O₆₊ and the Re²⁺ ions in (Hg_0.9Re_0.1)Ba₂Ca₂Cu₃O_g+. We believe that the paramagnetism depends on the internal flux density of the sample instead of the applied field. At the field below H_cl the paramagnetism has no contribution to total magnetization; at the field over H_cl the paramagnetic magnetization has different values when in field-increasing and field-decreasing process. The width of the magnetization hysteresis loop M is broadened by the paramagnetism. The effect of the paramagnetism due to paramagnetic ions on the magnetization relaxation rate and the magnetization critical-current density J_c based on the Bean critical-state model is also discussed. The temperature dependence of the paramagnetism is shown in this paper.     

Magnetization and critical current of high-temperature superconductors with artificial pinning centers

We present new data on magnetization at T = 4.2 and 77 K for polycrystalline high-temperature superconductors of the Bi₂Sr₂Ca₂Cu₃O_10+δ system containing various amounts of nanodimensional inclusions of tantalum carbide, niobium carbide, or niobium nitride. The introduction of these nanoparticles (≤30 nm in size) leads to an increase in the magnetization and the critical current density. It is established for the first time that the dependence of the normalized critical current on the bulk concentration of indicated dopants is described by the same universal curve. The interval of the optimum dopant concentrations is found, in which the additives lead to the maximum increase in the critical current density.     

Field dependence of the magnetization of adsorbed3He films at ultra low temperatures

We present measurements of the magnetization of pure³He films adsorbed on graphite at a density of p = 0.235 atoms/Ų, which corresponds to the 2D Heisenberg ferromagnetic regime. Different NMR frequencies (461.3kHz and 1.004 MHz) were used to study the magnetic field dependence of the nuclear magnetization. Measurements were performed on a Papyex sample to investigate the influence of the platelet size. The results are discussed in the context of theoretical models presented recently to describe these systems.     

Microstructural and electromagnetic characterization of La2−xSrxCuO4

A microstructural electromagnetic characterization of samples of La_2xSr_xCuO₄ has been made. Evidence of percolative behaviour was seen in a.c. and d.c. magnetization measurements. Transport current densities were of the order of 1 A cm ², about ten times smaller than the magnetization current density. The superconducting cluster size was estimated to be of the order of 1 1 μm. A detailed microchemical analysis was made over multiple regions of the sample, both within grains and at grain boundaries. No evidence was seen for the second phase, nor was there any large variation in chemical composition. Isolated dislocations and sub-grain boundaries were seen within the grains. We were not able to find any convincing microstructural evidence for percolative behaviour.     

Possible Spin Pumping Effects on Spin Torque Induced Magnetization Switching in Magnetic Tunneling Junctions

Dependence of spin torque induced magnetization switching upon interfacial insulating layers properties of magnetic tunneling junctions (MTJ) are studied. For the same magnetic properties and patterning geometric dimensions, changes in MTJ interfacial insulating layers properties reveal interesting magnetization switching behaviors. These behaviors cannot be explained by conventional Landau-Lifshitz-Gilbert equation with a spin torque term and an intrinsic ferromagnetic relaxation damping. However the magnetization switching dynamics can be understood through assumption of spin pumping effects in magnetic tunneling junctions. This is not only important for fundamental understanding of spin and electronic transport in MTJ but also important for practical trade-offs between critical switching current and MTJ resistance for spin torque random access memory.     

Investigations on the Magnetization Switching Dynamics in Spin Valve Multilayers Under Periodic Applied Magnetic Field

Magnetization switching dynamics in a spin valve nanopillar, induced by spin transfer torque in the presence of a periodic applied field is investigated by solving the Landau–Lifshitz–Gilbert–Slonczewski equation. Under steady state conditions, the switching of magnetization occurs in the system, above a threshold current density value J _c. A general expression for the critical current density is derived and it is shown that this further reduces when there is magnetic interface anisotropy present in the free layer of the spin valve. We also investigated the chaotic behavior of the free layer magnetization vector in a periodically varying applied magnetic field, in the presence of a constant DC magnetic field and spin current. Further, it is found that in the presence of a nonzero interfacial anisotropy, chaotic behavior is observed even at much smaller values of the spin current and DC applied field.     

Thermal Reversal of Magnetic Grains Under Time Varying Pulse Field

Media thermal magnetization switching under an arbitrary time varying field, pulse shape, and duration is an important issue in many recording applications. This paper shows a scale dilemma in using the time dependent energy barrier approach to describe long time thermal reversal of magnetic grains under a time varying pulse field. A new model approach based on an optimal reversal path is proposed to characterize the thermal reversal behavior of magnetic grains under an arbitrary time varying pulse field. Using this new model approach, the effects of pulse field duration, width, and amplitude on magnetic grain thermal switching behavior are studied.     

Generation of subnanosecond electron beams in air at atmospheric pressure

Optimum conditions for the generation of runaway electron beams with maximum current amplitudes and densities in nanosecond pulsed discharges in air at atmospheric pressure are determined. A supershort avalanche electron beam (SAEB) with a current amplitude of ∼30 A, a current density of ∼20 A/cm², and a pulse full width at half maximum (FWHM) of ∼100 ps has been observed behind the output foil of an air-filled diode. It is shown that the position of the SAEB current maximum relative to the voltage pulse front exhibits a time shift that varies when the small-size collector is moved over the foil surface.     

Secondary Peak on Asymmetric Magnetization Loop of Type-II Superconductors

Asymmetric magnetization loops with a second peak effect were parameterized by the extended critical-state model. The magnetic field distribution in a sample is considered. An expression is suggested for a peak of the critical current density and corresponding depression on field dependence of the depth of surface layer with equilibrium magnetization. These functions determine the width and the asymmetry of a magnetization loop. The asymmetry of the secondary peak height on magnetization branches for increasing and decreasing field is reproduced on the computed magnetization curves.