Q. T. Zhang

Field-free spin Hall effect driven magnetization switching in Pd/Co/IrMn exchange coupling system

All electrical manipulation of magnetization is crucial and of great important for spintronics devices for the sake of high speed, reliable operation, and low power consumption. Recently, widespread interests have been aroused to manipulate perpendicular magnetization of a ferromagnetic layer using spin-orbit torque (SOT) without field. We report that a commonly used antiferromagnetic material IrMn can be a promising candidate as a functional layer to realize field-free magnetization switching driven by SOT in which IrMn is employed to act as both the source of effective exchange bias field and SOT source. The critical switching current density within our study is Jc = 2.2 × 107 A/cm2, which is the same magnitude as similar materials such as PtMn. A series of measurements based on anomalous Hall effect was systematically implemented to determine the magnetization switching mechanism. This study offers a possible route for IrMn application in similar structures.

Electrical control over perpendicular magnetization switching driven by spin-orbit torques

Flexible control of magnetization switching by electrical manners is crucial for applications of spin-orbitronics. Besides of a switching current that is parallel to an applied field, a bias current that is normal to the switching current is introduced to tune the magnitude of effective damping-like and field-like torques and further to electrically control magnetization switching. Symmetrical and asymmetrical control over the critical switching current by the bias current with opposite polarities is both realized in Pt/Co/MgO and

MgB2 double-sided thick films fabricated on various metals by electrophoresis method

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Crystal Structure and Superconductivity of MgB2 Thin Films Grown on Various Oxide Substrates

-Ta/CoFeB/MgO systems, respectively. This research not only identifies the influences of field-like and damping-like torques on switching process but also demonstrates an electrical method to control it.

Noise suppression and sensitivity manipulation of magnetic tunnel junction sensors with soft magnetic Co70.5Fe4.5Si15B10 layer

Abstract The paper reports a remarkably simple method for fast fabrication of double-sided MgB 2 thick films by electrophoresing precursor boron powders on various metallic substrates followed by an ex situ annealing in magnesium vapor. Both sides of the films are homogeneous in thickness and composition, and no significant difference in T c and J c is found between the MgB 2 thick films on both sides of metals. Scanning electron microscopy on both sides of the double-sided films show dense and homogeneous structure consisting of MgB 2 grains below 1 μm. The X-ray diffraction and resistance measurement of the MgB 2 thick films on different metals show that the films are random oriented and have the onset superconducting temperature T c in the range of about 32–38 K. Estimates using hysteresis loops and Bean model give the critical current densities of the thick films are above 10 5 A/cm 2 in zero field and 5 K. The irreversibility fields of the thick films are comparable to those obtained from the dense wire.

Influence of organic layer thickness on structure, magnetic, and transport properties of Langmuir-Blodgett ttb-CuPc/CoFe

The crystal structure and superconductivity of MgB2 thin films grown on various oxide substrates were investigated by X-ray diffraction and resistance measurement. The films were prepared by a two-step method, in which precursors B films were annealed in Mg vapor at 900○C. The X-ray diffraction shows that the MgB2 films grown on C–AL2O3, R–AL2O3, and MgO (001) are c-axis oriented while the films grown on SrTiO3 (001), LaAlO3 (001), and ZrO2 (001) are aligned with the (101) direction normal to the substrate planes. All the grown films show superconductivity and their transition temperature varies with the substrates in the range of 34–39 K. We think that the transition temperature variation is probably due to the lattice matching between the film and the substrate, as well as the interdiffusion at the film/substrate interface. The experimental results suggest that if there is no severe interdiffusion at the film/substrate interface in the high temperature annealing process, more substrates could be used for the growth of MgB2 films using the two-step method.

Geometric Dependence of Washer Inductance for NbN DC SQUIDs

The voltage noise characteristic and sensitivity of magnetic tunnel junction sensors are crucial for ultralow field detection. In this work, we used a soft magnetic material electrode Co70.5Fe4.5Si15B10 as a sensing layer to improve the sensitivity. Then, a bias field along the easy axis of a free layer was applied to improve the linearity and manipulate the sensitivity of magnetic tunnel junction sensors. More importantly, random telegraph noise was suppressed by the bias field, resulting in hysteresis-free performance. The highest sensitivity of 3.9%/Oe and the best field detectivity of 4.5 nT/√ Hz at 10 Hz without hysteresis have been achieved. The sensors showed excellent performance with CoFeSiB electrodes, indicating that it is an effective way to improve the performance of sensors by introducing the bias field.The voltage noise characteristic and sensitivity of magnetic tunnel junction sensors are crucial for ultralow field detection. In this work, we used a soft magnetic material electrode Co70.5Fe4.5Si15B10 as a sensing layer to improve the sensitivity. Then, a bias field along the easy axis of a free layer was applied to improve the linearity and manipulate the sensitivity of magnetic tunnel junction sensors. More importantly, random telegraph noise was suppressed by the bias field, resulting in hysteresis-free performance. The highest sensitivity of 3.9%/Oe and the best field detectivity of 4.5 nT/√ Hz at 10 Hz without hysteresis have been achieved. The sensors showed excellent performance with CoFeSiB electrodes, indicating that it is an effective way to improve the performance of sensors by introducing the bias field.

Magneto-Seebeck effect in spin valves

In this letter, the ill-defined layer in organic/metal vertical structure was examined using ttb-CuPc/CoFe films. The ill-defined layer thickness was estimated. Beside the usually observed damages to organic underlayer, it is found that ill-defined layer may also influence film properties by modulating the growth of top metal layer. In our case, granular rather than continuous CoFe film was found as increasing ttb-CuPc underlayer thickness with a corresponding change in films magnetic and transport properties.

Temperature-dependent performance of all-NbN DC-SQUID magnetometers

Planar niobium nitride (NbN) dc superconducting quantum interference devices (SQUIDs) magnetometers were developed based on high-quality epitaxial NbN/AlN/NbN Josephson junctions for SQUID applications at higher operating temperatures. To improve the design of NbN dc SQUIDs, we measured the SQUID washers inductance, which is a key SQUID parameter. A set of NbN dc SQUIDs with washers with different dimensions were implemented. The washer inductances were obtained from the periodic flux variation in the SQUIDs when a control current was injected directly into the SQUID washers; the results agree well with the ones calculated with a numerical simulation in InductEX software. We obtained an empirical expression to estimate the NbN SQUID inductance with washers with different geometric structures. Our results will be useful for designing NbN dc SQUID magnetometers in future.

Scaling relation between anomalous Nernst and Hall effect in[Pt/Co]nmultilayers

The magneto-Seebeck (MS) effect, which is also called magneto-thermo-power, was observed in Co/Cu/Co and NiFe/Cu/Co spin valves. Their Seebeck coefficients in the parallel state were larger than those in the antiparallel state, and the MS ratio defined as (SAP -SP)/SP could reach –9% in our case. The MS effect originated not only from trivial giant magnetoresistance but also from spin current generated due to spin-polarized thermoelectric conductivity of ferromagnetic materials and subsequent modulation of the spin current by different spin configurations in spin valves. A simple Mott two-channel model reproduced a –11% MS effect for the Co/Cu/Co spin valves, qualitatively consistent with our observations. The MS effect could be applied for simultaneously sensing the temperature gradient and the magnetic field and also be possibly applied to determine spin polarization of thermoelectric conductivity and the Seebeck coefficient of ferromagnetic thin films.