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ac susceptibility of a ferromagnetic semiconductor (Ga,Mn)As was measured using microscaled Hall bar through the anomalous Hall effect. The synchronous component of the Hall resistance with applied ac magnetic field (∼1 mT) was detected using a lock-in technique. The temperature dependence of the ac Hall resistance shows a clear peak in the vicinity of the Curie temperature TC, when an offset dc field, whose magnitude is comparable to that of the ac field, is superimposed. We applied this method to detect the change of TC as a function of hole concentration by applying electric fields. [ABSTRACT FROM AUTHOR]

We have investigated the pulse-width and magnetic-field dependences of current-induced magnetization switching in a (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction. Critical current to induce magnetization switching shows a linear dependence on pulse width from 10 to 1000 μs. The magnetic-field dependence appears to indicate that the observed current-induced magnetization switching proceeds through metastable magnetization structures. [ABSTRACT FROM AUTHOR]

We have investigated the spin tunneling transport in magnetic tunnel junctions (MTJs) with Co40Fe40B20, Co50Fe50, and Co90Fe10 free layers which were deposited on the lower electrode consisting of the same Co40Fe40B20 reference layer/MgO barrier. The tunnel magnetoresistance (TMR) ratio depends critically on the choice of the free layer; the TMR ratios up to 355% were obtained for the MTJs with Co40Fe40B20 free layers and up to 277% with Co50Fe50 free layers, both of which have highly (001)-oriented bcc structures. No high TMR ratio was observed for the MTJs with Co90Fe10 free layer having a polycrystalline fcc structure. [ABSTRACT FROM AUTHOR]

An array of exchange biased spin-valve giant-magnetoresistance nanopillars was fabricated and the current I dependence of the resistance R was investigated using an electrically conducting atomic-force microscope (AFM) probe contact at room temperature. We observed current induced switching in a MnIr/CoFe/Cu/CoFe/NiFe nanopillar using the AFM probe contact. Current-driven switching using nanoprobe contact is a powerful method for developing nonvolatile and rewritable magnetic memory with high density. [ABSTRACT FROM AUTHOR]

We investigate the magnetization-reversal mode in CoFeB/MgO magnetic tunnel junctions with perpendicular easy axis from the magnetic-field-angle dependence of coercivity. The reversal in a free layer with diameter of ~20 nm is found to be in a good agreement with coherent reversal mode for a device with a reference layer much larger than the free layer. However, the reversal mode is quite different in a junction in which the two layers are almost the same size. From micro-magnetic simulation, the difference is attributed to the reduction of the magnetic anisotropy in the vicinity of the device edge. [ABSTRACT FROM AUTHOR]

Reports the quantum Hall effect of electrons in AlGaAs/Ga[sub1-x]In[subx]As/gallium arsenide pseudomorphic strained heterostructures. Effect of strain in the GaInAs layer; Overview of reciprocal magnetic fields corresponding to the maximum of magnetoresistivity; Number of scattering events by cluster-scattering centers in one orbit area.

We investigated the wire orientation dependence of ensemble electron spin dynamics by using a time-resolved Kerr rotation technique in wires made from a modulation-doped GaAs/AlGaAs quantum well. We observed a strong anisotropy of the electron spin dephasing time of as large as 40 depending on the wire orientation at 5 K by tuning the spin-orbit interaction. The wire width dependence of the spin dynamics is reproduced in a Monte Carlo simulation. In addition, two characteristic spin decay rates were observed in wires whose widths were close to the spin precession length. The dependence of the magnetic field direction on the spin dynamics of the wires was also studied to determine the difference from that of two dimensional electron gas. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions (MTJs) with ferromagnetic electrodes possessing a perpendicular magnetic easy axis are of great interest as they have a potential for realizing next-generation high-density non-volatile memory and logic chips with high thermal stability and low critical current for current-induced magnetization switching. To attain perpendicular anisotropy, a number of material systems have been explored as electrodes, which include rare-earth/transition-metal alloys, L10-ordered (Co, Fe)–Pt alloys and Co/(Pd, Pt) multilayers. However, none of them so far satisfy high thermal stability at reduced dimension, low-current current-induced magnetization switching and high tunnel magnetoresistance ratio all at the same time. Here, we use interfacial perpendicular anisotropy between the ferromagnetic electrodes and the tunnel barrier of the MTJ by employing the material combination of CoFeB–MgO, a system widely adopted to produce a giant tunnel magnetoresistance ratio in MTJs with in-plane anisotropy. This approach requires no material other than those used in conventional in-plane-anisotropy MTJs. The perpendicular MTJs consisting of Ta/CoFeB/MgO/CoFeB/Ta show a high tunnel magnetoresistance ratio, over 120%, high thermal stability at dimension as low as 40 nm diameter and a low switching current of 49 μA. [ABSTRACT FROM AUTHOR]

Magnetic information storage relies on external magnetic fields to encode logical bits through magnetization reversal. But because the magnetic fields needed to operate ultradense storage devices are too high to generate, magnetization reversal by electrical currents is attracting much interest as a promising alternative encoding method. Indeed, spin-polarized currents can reverse the magnetization direction of nanometre-sized metallic structures through torque; however, the high current densities of 107-108?A?cm-2 that are at present required exceed the threshold values tolerated by the metal interconnects of integrated circuits. Encoding magnetic information in metallic systems has also been achieved by manipulating the domain walls at the boundary between regions with different magnetization directions, but the approach again requires high current densities of about 107?A?cm-2. Here we demonstrate that, in a ferromagnetic semiconductor structure, magnetization reversal through domain-wall switching can be induced in the absence of a magnetic field using current pulses with densities below 105?A?cm-2. The slow switching speed and low ferromagnetic transition temperature of our current system are impractical. But provided these problems can be addressed, magnetic reversal through electric pulses with reduced current densities could provide a route to magnetic information storage applications. [ABSTRACT FROM AUTHOR]

The current-induced effective field in perpendicularly magnetized Ta/CoFeB/MgO wire was investigated. A threshold field decrease of 6.4 kOe/mA was observed by measuring the threshold field of Hall resistance versus the magnetic field curve with various bias currents. The decrease was probably caused by the in-plane effective field, mainly due to the Rashba effect. The effective field of the Ta/CoFeB/MgO wire was smaller and opposite in direction compared to that of Pt/Co/AlOx previously reported. [ABSTRACT FROM AUTHOR]

The authors investigate the current density and temperature dependence of current induced effective magnetic field Heff through spin-orbit interaction in a ferromagnetic semiconductor Ga0.92Mn0.08As having uniaxial magnetic anisotropy. The change of the magnitude of apparent magnetic anisotropy induced by Heff that is dependent on the current direction and density is observed by transport measurements using the planar Hall effect. The authors show the 180° magnetization switching through Heff by applying pulsed current. [ABSTRACT FROM AUTHOR]

Micro-Hall magnetometry is employed to study the magnetization dynamics of a single, micron-size CrO2 grain. With this technique, we track the motion of a single domain wall, which allows us to probe the distribution of imperfections throughout the material. An external magnetic field along the grain’s easy magnetization direction induces magnetization reversal, giving rise to a series of sharp jumps in magnetization. Supported by micromagnetic simulations, we identify the transition to a state with a single cross-tie domain wall, where pinning/depinning of the wall results in stochastic Barkhausen jumps. [ABSTRACT FROM AUTHOR]

We have investigated the effect of applied electric field EG on thickness dependent magnetic anisotropy of sputtered Co40Fe40B20 sandwiched with MgO and Ta. The range of CoFeB thickness explored is 2 nm and below. As the thickness is reduced, the easy axis of magnetization becomes perpendicular from in-plane. We show that perpendicular magnetic anisotropy of in-plane samples and coercivity of perpendicular samples can be modified by applying EG at room temperature. Furthermore, superparamagnetic behavior is observed for CoFeB layers with further reduced thickness below ≈0.9 nm, where electric-field effect is also observed below their blocking temperature. [ABSTRACT FROM AUTHOR]

The authors fabricated double MgO barrier magnetic tunnel junctions (MTJs) with 3-nm-thick Co40Fe40B20 free layer. When annealed at 350 °C, tunnel magnetoresistance (TMR) ratio at room temperature was 130%, much lower than that (297%) of single MgO barrier MTJs processed and annealed under the same condition. The middle CoFeB free layer sandwiched between the two MgO barriers was found to be mostly amorphous. Replacement of the Co40Fe40B20 free layer by a highly oriented Co50Fe50 layer and a composite Co50Fe50/Co40Fe40B20 layer led to the enhanced TMR ratios up to 165% and 212% at annealing temperature of 350 °C, respectively. [ABSTRACT FROM AUTHOR]

We investigated the doping concentration ( ND) dependence of the extrinsic spin Hall effect (SHE) in n-doped GaAs with ND raging from 3×1016 cm−3 to 5×1017 cm−3. By using scanning Kerr microscopy (SKM) measurements, we observed the Kerr rotation signal due to the spin accumulation near the channel edges in all the samples with different ND. Moreover, the position and in-plane magnetic field dependence of the Kerr rotation signal are found to vary with ND. We analyzed the ND dependence of the spin Hall conductivity by taking account of the ND-dependent spin lifetime based on the typical drift-diffusion model. [ABSTRACT FROM AUTHOR]

The authors studied an effect of ferromagnetic (Co20Fe60B20 or Fe) layer insertion on tunnel magnetoresistance (TMR) properties of MgO-barrier magnetic tunnel junctions (MTJs) with CoFe/Pd multilayer electrodes. TMR ratio in MTJs with CoFeB/MgO/Fe stack reached 67% at annealing temperature (Ta) of 200 °C and then decreased rapidly at Ta over 250 °C. The degradation of the TMR ratio may be related to crystallization of CoFe(B) into fcc(111) or bcc(011) texture resulting from diffusion of B into Pd layers. MTJs which were in situ annealed at 350 °C just after depositing bottom CoFe/Pd multilayer showed TMR ratio of 78% by postannealing at Ta=200 °C. [ABSTRACT FROM AUTHOR]

Electric field enhanced electron spin coherence is characterized using time-resolved Faraday rotation spectroscopy in n-type ZnO epilayers grown by molecular beam epitaxy. An in-plane dc electric field E almost doubles the transverse spin lifetime at 20 K without affecting the effective g factor. This effect persists until high temperatures, but decreases with increasing carrier concentration. Comparisons of the variations in the spin lifetime, the carrier recombination lifetime, and photoluminescence lifetimes indicate that the applied E enhances the radiative recombination rate. All observed effects are independent of crystal directionality and are performed at low magnetic fields (B<0.2 T). [ABSTRACT FROM AUTHOR]

We show that both interband and intersubband transitions in n-InAs/AlSb quantum cascade laser structures can have sufficient gain to enable simultaneous lasing at the two transitions. High electric fields generate holes, which are necessary for interband lasing in InAs. The doping concentration in the injection parts is shown to be critical for controlling the electric fields in the device, enabling the selection of lasing at the intersubband transition alone (high doping), at the interband transition alone (low doping), and simultaneous lasing at both transitions (intermediate doping). [ABSTRACT FROM AUTHOR]

Switching of tunnel magnetoresistance in magnetic tunnel junctions (MTJs) has been achieved by magnetic domain wall motion in one of the electrodes. The fabricated devices have two (Ga,Mn)As-based MTJs with a common bottom (Ga,Mn)As electrode, in which the domain wall motion takes place both by magnetic field and by current. This configuration allows not only the observation of switching but also time-resolved detection of the position of the domain wall by the tunnel magnetoresistance. [ABSTRACT FROM AUTHOR]

Anisotropic spin dynamics of two-dimensional electrons in strained n-InGaAs/AlGaAs (110) quantum wells (QWs) is investigated by a time-resolved Faraday rotation technique. Strong anisotropy of the relaxation time for the electron spins in parallel (τ∥) and perpendicular (τ⊥) to the QWs is observed (τ⊥/τ∥∼60) at 150 K as a result of the enhanced D'yakonov–Perel' (DP) spin relaxation mechanism. At 5 K, an anisotropic feature of the spin relaxation time is also observed in the presence of in-plane magnetic field, suggesting that the DP mechanism is effective for low-temperature spin relaxation. [ABSTRACT FROM AUTHOR]

We have investigated the effect of stack structures on tunnel magnetoresistance (TMR) properties in perpendicular anisotropy MgO-based magnetic tunnel junctions (p-MTJs) with CoFe/Pd multilayer and CoFeB insertion. By adopting Ta and Ru cap-layers, the TMR ratios of 113 and 106% are obtained at annealing temperature (Ta) of 325 °C, respectively. Particularly, the Ru cap-layer is effective in realizing a TMR ratio of 100% at Ta = 350 °C. By replacing (Co25Fe75)80B20 with (Co25Fe75)85B15, the TMR ratio increased quickly at low Ta, reaching a maximum of 120% at Ta = 300 °C. [ABSTRACT FROM AUTHOR]

Electric field-induced magnetization switching through magnetization precession is investigated as a function of in-plane component of external magnetic field for a CoFeB/MgO-based magnetic tunnel junction with perpendicular easy axis. The switching probability is an oscillatory function of the duration of voltage pulses and its magnitude and period depend on the magnitude of in-plane magnetic field. Experimental results are compared with simulated ones by using Landau-Lifshitz-Gilbert-Langevin equation, and possible factors determining the probability are discussed. [ABSTRACT FROM AUTHOR]

The electric field-induced ∼180° magnetization reversal is realized for a sputtered CoFeB/MgO-based magnetic tunnel junction with perpendicular magnetic easy axis in a static external magnetic field. Application of bias voltage with nanoseconds duration results in a temporal change of magnetic easy axis in the free layer CoFeB to in-plane, which induces precessional motion of magnetization in the free layer. The magnetization reversal takes place when the bias voltage pulse duration is adjusted to a half period of the precession. We show that the back and forth magnetization reversal can be observed by using successive application of half-period voltage pulses. [ABSTRACT FROM AUTHOR]

We observe a transition of domain-wall (DW) dynamics in ferromagnetic wires made of Co/Ni multilayers by use of transport measurement. As the wire width reduces, DW dynamics exhibits a transition from dendrite growth to pure DW motion. The threshold width is found to be about 300 nm and strongly depends on the relative dragging direction of the magnetic field and the current on DW: parallel (antiparallel) direction results in much smaller (larger) threshold width. It should be considered as a building block for DW-motion-based device applications. [ABSTRACT FROM AUTHOR]

Abstract: We implement time-resolved Kerr microscopy (TRKM) system with spatial resolution for observation of coupled electron–nuclear spin dynamics such as dynamic nuclear polarization (DNP) in a local area of the laser spot. From TRKM measurements, Larmor precession of electron spins injected by right or left circular polarized pump pulse is detected. The difference of the precession frequencies between and pumping corresponds to the polarization-dependent nuclear magnetic field caused by DNP. This indicates that we successfully detect coupled electron–nuclear spin dynamics in a local area of the laser spot size of . Furthermore, we show the pump and probe power dependences of DNP in high optical intensity region. All-optical NMR is also demonstrated at arbitrary frequencies by controlling acousto-optical modulator. [ABSTRACT FROM AUTHOR]

Abstract: Non-volatile RAM (NV-RAM) enables instant-on/off computing, which drastically reduces power consumption. One of the most promising candidates for NV-RAM technology is the spin-transfer torque RAM (SPRAM) based on magnetic tunnel junction (MTJ) device technology. This paper reviews the development of MTJ device technology and formulates considerations regarding its memory application, including SPRAM memory cell structure and operation, write voltage limitation, and thermal stability. At the circuit level, a disruptive read operation for future large integration scale is described. A 4F 2 memory cell and a multi-bit cell approach are also presented. Finally, the potential value of instant-on/off computing through NV-RAM and its impact are explored. [Copyright &y& Elsevier]

Abstract: We investigated the effective magnetic field induced by spin–orbit interaction in a gated modulation-doped GaAs/AlGaAs quantum well (QW) structure. We measured the precession of the optically injected electron spins at zero magnetic field by a time-resolved Kerr rotation (TRKR) technique as a function of the gate voltage V g. The V g-dependence of the effective magnetic field extracted from the TRKR data was quantitatively analyzed by considering both Rashba and Dresselhaus spin–orbit interaction in a Monte Carlo simulation. With the Dresselhaus spin–orbit coupling parameter γ and the scattering time as fitting parameters, we reproduced the experimental TRKR data, from which we estimated γ∼13eVÅ3. [ABSTRACT FROM AUTHOR]

Abstract: In-plane magnetic anisotropy of 40-μm-long (Ga,Mn)As wires with different widths (0.4, 1.0, and 20μm) has been investigated between 5 and 75K by measuring anisotropic magneto-resistance (AMR). The wires show in-plane 〈100〉 cubic and [−110] uniaxial anisotropies, and an additional lithography-induced anisotropy along the wire direction in narrow wires with width of 0.4 and 1.0μm. We derive the temperature dependence of the cubic, uniaxial, and lithography-induced anisotropy constants from the results of AMR, and find that a sizable anisotropy can be provided by lithographic means, which allows us to control and detect the magnetization reversal process by choosing the direction of the external magnetic fields. [ABSTRACT FROM AUTHOR]

Abstract: Domain wall (DW) resistance in perpendicularly magnetized (Ga,Mn)As has been investigated. The observed DW resistance is decomposed into extrinsic and intrinsic contributions. The former is explained quantitatively by the zig-zaging current due to an abrupt polarity change of the Hall electric field at DW. The latter is consistent with the disorder-induced mixing of spin channels due to small non-adiabacity of carrier spins subject to spatially varying local magnetic moment and is shown to be an order of magnitude greater than a contribution from anisotropic magnetoresistance. [Copyright &y& Elsevier]

Hall and sheet resistance of 200-nm thick metallic (Ga,Mn)As with compressive and tensile strain has been measured as a function of the magnetic field and temperature. The magnitude of resistance is found to depend rather strongly on relative orientations of magnetization and current and their directions in respect to crystal axes, the configuration corresponding to the highest resistance being different for compressive and tensile strain. Negative magnetoresistance, which is observed even if magnetization becomes saturated, is assigned to weak localization. [Copyright &y& Elsevier]

Electron spin dynamics in strained In0.1Ga0.9As/Al0.4Ga0.6As quantum wells (QWs) on (1 0 0) and (1 1 0)-oriented substrates are investigated by time-resolved Faraday rotation. We find that the spin relaxation time in (1 1 0) QWs is 6 times longer than that in (1 0 0) QWs at low temperatures, which is strongly reduced by applying magnetic fields. The sign of g-factor is found positive and its magnitude decreases with increasing well width, in the well width range investigated here. [Copyright &y& Elsevier]