Z. H. Yuan

Nonlocal magnetoresistance due to Lorentz force in linear transport region in bulk silicon

Magnetoresistance (MR) of lightly doped bulk silicon has been studied in linear transport region. Both nonlocal and local MR results from deflection of in-plane transport of carriers by Lorentz force. However, nonlocal MR is nearly one order of magnitude larger than local one. We ascribe the enhanced nonlocal MR to polarity-conserved charges accumulated on boundaries near anode and cathode, which alters potential distribution and meanwhile increases current flowing in nonlocal region. This mechanism of nonlocal MR can be generalized to other materials with high or moderate mobility.

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

Polarization‐Mediated Thermal Stability of Metal/Oxide Heterointerface

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Perpendicular magnetic anisotropy in Ta|Co40Fe40B20|MgAl2O4 structures and perpendicular CoFeB|MgAl2O4|CoFeB magnetic tunnel junction

-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.

Double-pinned magnetic tunnel junction sensors with spin-valve-like sensing layers

A polarization-mediated heterointerface is designed to research the thermal stability of magnetic metal/oxide interfaces. Using polarization engineering, the thermal stability of the interface between BiFeO3 and CoFeB can be improved by about 100°C. This finding provides new insight into the chemistry of the metal/oxide heterointerface.

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

Magnetic properties of Co40Fe40B20 (CoFeB) thin films sandwiched between Ta and MgAl2O4 layers have been systematically studied. For as-grown state, Ta/CoFeB/MgAl2O4 structures exhibit good perpendicular magnetic anisotropy (PMA) with interface anisotropy Ki = 1.22 erg/cm2, which further increases to 1.30 erg/cm2 after annealing, while MgAl2O4/CoFeB/Ta multilayer shows in-plane magnetic anisotropy and must be annealed in order to achieve PMA. For bottom CoFeB layer, the thickness window for PMA is from 0.6 to 1.0 nm, while that for top CoFeB layer is between 0.8 and 1.4 nm. Perpendicular magnetic tunnel junctions (p-MTJs) with a core structure of CoFeB/MgAl2O4/CoFeB have also been fabricated and tunneling magnetoresistance ratio of about 36% at room temperature and 63% at low temperature have been obtained. The intrinsic excitations in the p-MTJs have been identified by inelastic electron-tunneling spectroscopy.

Influence of epitaxial BiFeO3 on superparamagnetic behavior of CoFeB thin film

MgO magnetic tunnel junction (MTJ) sensors with spin-valve-like sensing layers of Ir22Mn78 (6)/Ni80Fe20 (tNiFe = 20–70)/Ru (0.9)/Co40Fe40B20 (3) (unit: nm) have been fabricated. A linear field dependence of magnetoresistance for these MTJ sensors was obtained by carrying out a two-step field annealing process. The sensitivity and linear field range can be tuned by varying the thickness of NiFe layer and annealing temperature, and a high sensitivity of 37%/mT has been achieved in the MTJ sensors with 70 nm NiFe at the optimum annealing temperature of 230 °C. Combining the spin-valve-like sensing structure and a soft magnetic NiFe layer, MTJ sensors with relatively wide field sensing range have been achieved and could be promising for showing high sensitivity magnetic field sensing applications.

Spin Hall magnetoresistance in CoFe2O4/Pt films

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

Superparamagnetic behavior of ultrathin CoFeB film grown on epitaxial BiFeO3 (BFO) thin film was investigated by using anomalous Hall effect (AHE) at different temperatures. A transition from superparamagnetism to blocked state was observed for CoFeB grown directly on SrTiO3 (STO) substrate, but absent for CoFeB grown on BFO from 30 K to 300 K. By fitting normalized AHE data with Langevin equation, average grain sizes of the superparamagnetic CoFeB grown on STO and BFO were estimated. Considering the relationship between volume of nanoparticle and blocking temperature ( TB), TB of CoFeB grown on BFO was above 30 K which was not observed experimentally, indicating that in addition to surface roughness or oxidation, interfacial exchange coupling also played a role in the suppression of TB. By taking the interfacial exchange coupling between BFO and superparamagnetic CoFeB as an effective field, Bloch equations were introduced to study the relaxation process of superparamagnetic CoFeB. This suppression of TB...