Caihua Wan

Geometrical enhancement of low-field magnetoresistance in silicon

Inhomogeneity-induced magnetoresistance (IMR) reported in some non-magnetic semiconductors, particularly silicon, has generated considerable interest owing to the large magnitude of the effect and its linear field dependence (albeit at high magnetic fields). Various theories implicate spatial variation of the carrier mobility as being responsible for IMR. Here we show that IMR in lightly doped silicon can be significantly enhanced through hole injection, and then tuned by an applied current to arise at low magnetic fields. In our devices, the ‘inhomogeneity’ is provided by the p–n boundary formed between regions where conduction is dominated by the minority and majority charge carriers (holes and electrons) respectively; application of a magnetic field distorts the current in the boundary region, resulting in large magnetoresistance. Because this is an intrinsically spatial effect, the geometry of the device can be used to enhance IMR further: we designed an IMR device whose room-temperature field sensitivity at low fields was greatly improved, with magnetoresistance reaching 10% at 0.07 T and 100% at 0.2 T, approaching the performance of commercial giant-magnetoresistance devices. The combination of high sensitivity to low magnetic fields and large high-field response should make this device concept attractive to the magnetic-field sensing industry. Moreover, because our device is based on a conventional silicon platform, it should be possible to integrate it with existing silicon devices and so aid the development of silicon-based magnetoelectronics.

A bias voltage dependent positive magnetoresistance in Cox–C1−x/Si heterostructure

Cox–C1−x granular films were deposited on n-type Si substrates by pulsed laser deposition method. The heterostructure, investigated in current-perpendicular-to-plane geometry, has a bias voltage dependent positive magnetoresistance (MR), and at room temperature, the MR value reaches 16% at magnetic field H=2.5 kOe and bias voltage of 6 V. All MRs have saturated behavior when H>2.5 kOe. The mechanism of this MR is attributed to that the applied magnetic field and local random magnetic field modulate the ratio of singlet and triplet spin states leading to the MR.

Zhang et al . reply

replying to J. Luo et al. , 10.1038/nature12589 (2013). We agree with Luo et al.1 that the magnetoresistance effects that we reported2 were dependent on the method used to measure them. The reason that there is a difference in the results depending on whether method 1 or method 2 is used (adopting the measurement notation of ref. 1) is that there are two voltage-stabilizing diodes in the Keithley 2400 instrument we used. We were unaware that when this instrument was used both as current source and voltmeter, one diode connected the input port of the current source to the input port of the voltmeter, whereas the other diode connected the output port of the current source to the output port of the voltmeter. The diodes caused a crossover of the Hall coefficient from negative to positive when the instrument was used to conduct a Hall measurement in this configuration, leading us to propose an invalid mechanism for the abnormal magnetoresistance. Therefore the mechanism we proposed2—minority injection and an induced p-n boundary—does not provide a correct explanation for the observed geometry-enhanced magnetoresistance. Although such a mechanism does not operate in our samples, we note that a p–n boundary could still enhance magnetoresistance in certain circumstances according to our and others’ theoretical calculations and experiments2,3,4.

Photoconductivity of iron doped amorphous carbon films on n-type silicon substrates

The Fe doped a-C films on n-type silicon substrates were deposited by pulse laser deposition. The Fe doped a-C films are p-type semiconductor and they are rich in sp2 (∼75%). I-V characteristics and photoconductivity of the structures were measured in the current in-plane geometry. The photoconductivity with magnitude of 170∼220 was observed under white light illumination with power of 20 mW/cm2 at room temperature. The photoconductivity is ascribed to the p-n junction formed between the p-type a-C: Fe film and the n-type Si substrate whose reverse-biased saturation current increases intensively under illumination.

Room-temperature nonsaturating magnetoresistance of intrinsic bulk silicon in high pulsed magnetic fields

Nonsaturating positive magnetoresistance (MR) of intrinsic bulk silicon (i-Si) was observed at forward bias, exhibiting an almost linear behavior at high magnetic fields (5 T<B<40 T). The MR reaches 180% at 40 T at room temperature using a bias of 1.5 V, and there is no indication that this MR would saturate at even stronger fields. The nonsaturating large MR of i-Si supports experimentally the hypothesis that the MR of silicon may be induced by inhomogeneous current flows.

Hydrogen Impurity Defects in Rutile TiO2

Hydrogen-related defects play crucial roles in determining physical properties of their host oxides. In this work, we report our systematic experimental and theoretical (based on density functional theory) studies of the defect states formed in hydrogenated-rutile TiO2 in gaseous H2 and atomic H. In gas-hydrogenated TiO2, the incorporated hydrogen tends to occupy the oxygen vacancy site and negatively charged. The incorporated hydrogen takes the interstitial position in atom-hydrogenated TiO2, forming a weak O-H bond with the closest oxygen ion, and becomes positive. Both states of hydrogen affect the electronic structure of TiO2 mainly through changes of Ti 3d and O 2p states instead of the direct contributions of hydrogen. The resulted electronic structures of the hydrogenated TiO2 are manifested in modifications of the electrical and optical properties that will be useful for the design of new materials capable for green energy economy.

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.

Abnormal humidity-dependent electrical properties of amorphous carbon/silicon heterojunctions

Amorphous carbon (a-C) film/n-Si heterojunctions have been fabricated by pulse laser deposition, and their current-voltage characteristics have been investigated. The results show that the atmosphere relative humidity (RH) has a significant effect on the reverse bias I-V characteristics of the heterojunctions. For the low bias voltages, the resistance of the a-C/Si heterojunction decreases with the increase of the RH. However, when the applied voltage is greater than a threshold, the resistance of the a-C/Si heterojunctions increases with the increase of the RH. This humidity-dependent phenomenon is attributed to the charge transfer from the absorbed H2O molecular to a-C film.

Spin-orbit torque in MgO/CoFeB/Ta/CoFeB/MgO symmetric structure with interlayer antiferromagnetic coupling

Spin current generated by the spin Hall effect in a heavy metal that would diffuse up and down to adjacent ferromagnetic layers and exert torque on their magnetization is called spin-orbit torque. Antiferromagnetically coupled trilayers, namely, the so-called synthetic antiferromagnets usually are employed to serve as the pinned layer of spintronic devices based on spin valves and magnetic tunnel junctions to reduce the stray field and/or increase the pinning field. Here we investigate the spin-orbit torque in a MgO/CoFeB/Ta/CoFeB/MgO perpendicularly magnetized multilayer with interlayer antiferromagnetic coupling. It is found that the magnetization of two CoFeB layers can be switched between two antiparallel states simultaneously. This observation is replicated by the theoretical calculations by solving the Stoner-Wohlfarth model and the Landau-Lifshitz-Gilbert equation. Our findings combine spin-orbit torque and interlayer coupling, which might advance the magnetic memories with a low stray field and low power consumption.