H. Jiang

Electrical detection of the spin resonance of a single electron in a silicon field-effect transistor

The ability to manipulate and monitor a single-electron spin using electron spin resonance is a long-sought goal. Such control would be invaluable for nanoscopic spin electronics, quantum information processing using individual electron spin qubits and magnetic resonance imaging of single molecules. There have been several examples of magnetic resonance detection of a single-electron spin in solids. Spin resonance of a nitrogen-vacancy defect centre in diamond has been detected optically, and spin precession of a localized electron spin on a surface was detected using scanning tunnelling microscopy. Spins in semiconductors are particularly attractive for study because of their very long decoherence times. Here we demonstrate electrical sensing of the magnetic resonance spin-flips of a single electron paramagnetic spin centre, formed by a defect in the gate oxide of a standard silicon transistor. The spin orientation is converted to electric charge, which we measure as a change in the source/drain channel current. Our set-up may facilitate the direct study of the physics of spin decoherence, and has the practical advantage of being composed of test transistors in a conventional, commercial, silicon integrated circuit. It is well known from the rich literature of magnetic resonance studies that there sometimes exist structural paramagnetic defects near the Si/SiO2 interface. For a small transistor, there might be only one isolated trap state that is within a tunnelling distance of the channel, and that has a charging energy close to the Fermi level.

Switching current reduction using perpendicular anisotropy in CoFeB–MgO magnetic tunnel junctions

We present in-plane CoFeB–MgO magnetic tunnel junctions with perpendicular magnetic anisotropy in the free layer to reduce the spin transfer induced switching current. The tunneling magnetoresistance ratio, resistance-area product, and switching current densities are compared in magnetic tunnel junctions with different CoFeB compositions. The effects of CoFeB free layer thickness on its magnetic anisotropy and current-induced switching characteristics are studied by vibrating sample magnetometry and electrical transport measurements on patterned elliptical nanopillar devices. Switching current densities ∼4 MA/cm2 are obtained at 10 ns write times.

Ultralow-current-density and bias-field-free spin-transfer nano-oscillator.

The spin-transfer nano-oscillator (STNO) offers the possibility of using the transfer of spin angular momentum via spin-polarized currents to generate microwave signals. However, at present STNO microwave emission mainly relies on both large drive currents and external magnetic fields. These issues hinder the implementation of STNOs for practical applications in terms of power dissipation and size. Here, we report microwave measurements on STNOs built with MgO-based magnetic tunnel junctions having a planar polarizer and a perpendicular free layer, where microwave emission with large output power, excited at ultralow current densities, and in the absence of any bias magnetic fields is observed. The measured critical current density is over one order of magnitude smaller than previously reported. These results suggest the possibility of improved integration of STNOs with complementary metal-oxide-semiconductor technology, and could represent a new route for the development of the next-generation of on-chip oscillators.

Low writing energy and sub nanosecond spin torque transfer switching of in-plane magnetic tunnel junction for spin torque transfer random access memory

This work investigated in-plane MgO-based magnetic tunnel junctions (MTJs) for the application of spin torque transfer random access memory (STT-RAM). The MTJ in this work had an resistance area product (RA) = 4.3 Ω·μm2, tunneling magnetoresistance ratio ∼135%, thermal stability factor Δ(H)=68 (by field measurement), and Δ(I) = 50 (by current measurement). The optimal writing energy was found to be 0.286 pJ per bit at 1.54 ns for antiparallel (AP) state to parallel (P) state switching, and 0.706 pJ per bit at 0.68 ns for P state to AP state switching. Ultra fast spin torque transfer (STT) switching was also observed in this sample at 580 ps (AP to P) and 560 ps (P to AP). As a result, 0.6–1.3 GHz was determined to be the optimal writing rate from writing energy consumption of view. These results show that in-plane MgO MTJs are still a viable candidate as the fast memory cell for STT-RAM.

High-Power Coherent Microwave Emission from Magnetic Tunnel Junction Nano-oscillators with Perpendicular Anisotropy

The excitation of the steady-state precessions of magnetization opens a new way for nanoscale microwave oscillators by exploiting the transfer of spin angular momentum from a spin-polarized current to a ferromagnet, referred to as spin-transfer nano-oscillators (STNOs). For STNOs to be practical, however, their relatively low output power and their relatively large line width must be improved. Here we demonstrate that microwave signals with maximum measured power of 0.28 μW and simultaneously narrow line width of 25 MHz can be generated from CoFeB-MgO-based magnetic tunnel junctions having an in-plane magnetized reference layer and a free layer with strong perpendicular anisotropy. Moreover, the generation efficiency is substantially higher than previously reported STNOs. The results will be of importance for the design of nanoscale alternatives to traditional silicon oscillators used in radio frequency integrated circuits.

A graphene quantum dot with a single electron transistor as an integrated charge sensor

A quantum dot (QD) with an integrated charge sensor is becoming a common architecture for a spin or charge based solid state qubit. To implement such a structure in graphene, we have fabricated a twin-dot structure in which the larger dot serves as a single electron transistor (SET) to read out the charge state of the nearby gate controlled small QD. A high SET sensitivity of 10−3e/Hz allowed us to probe Coulomb charging as well as excited state spectra of the QD, even in the regime where the current through the QD is too small to be measured by conventional transport means.

Deep subnanosecond spin torque switching in magnetic tunnel junctions with combined in-plane and perpendicular polarizers

We show that adding a perpendicular polarizer to a conventional spin torque memory element with an in-plane free layer and an in-plane polarizer can significantly increase the write speed and decrease the write energy of the element. We demonstrate the operation of such spin torque memory elements with write energies of 0.4 pJ and write times of 0.12 ns.

Gate-controlled electron spin resonance in G a A s / A l x Ga 1 − x As heterostructures

The electron spin resonance (ESR) of two-dimensional electrons is investigated in a gated GaAs/AlGaAs heterostructure. We found that the ESR resonance frequency can be turned by means of a gate voltage. The front and back gates of the heterostructure produce opposite g-factor shift, suggesting that electron g-factor is being electrostatically controlled by shifting the equilibrium position of the electron wave function from one epitaxial layer to another with different g-factors.

Thermodynamic signature of a two-dimensional metal-insulator transition

We present a study of the compressibility kappa of a two-dimensional hole system which exhibits a metal-insulator phase transition at zero magnetic field. It has been observed that dkappa/dp changes sign at the critical density for the metal-insulator transition. Measurements also indicate that the insulating phase is incompressible for all values of B. Finally, we show how the phase transition evolves as the magnetic field is varied and construct a phase diagram in the density-magnetic field plane for this system.

Ultrafast universal quantum control of a quantum-dot charge qubit using Landau–Zener–Stückelberg interference

A basic requirement for quantum information processing is the ability to universally control the state of a single qubit on timescales much shorter than the coherence time. Although ultrafast optical control of a single spin has been achieved in quantum dots, scaling up such methods remains a challenge. Here we demonstrate complete control of the quantum-dot charge qubit on the picosecond scale, orders of magnitude faster than the previously measured electrically controlled charge- or spin-based qubits. We observe tunable qubit dynamics in a charge-stability diagram, in a time domain, and in a pulse amplitude space of the driven pulse. The observations are well described by Landau–Zener–Stückelberg interference. These results establish the feasibility of a full set of all-electrical single-qubit operations. Although our experiment is carried out in a solid-state architecture, the technique is independent of the physical encoding of the quantum information and has the potential for wider applications.