Yisong Zhang

In situ formation of superconducting YBa2Cu3O7−x thin films using pure ozone vapor oxidation

Superconducting YBa2Cu3O7−x thin films have been prepared by coevaporation using an ozone vapor jet as an oxygen source. Films exhibiting zero resistance at 82 K have been fabricated in situ under high vacuum conditions using substrate temperatures of 700 °C without a post‐evaporation anneal in oxygen. This process has implications for in situ measurements of fully superconducting surfaces using a variety of probes as well as for the fabrication of devices and structures whose properties are dependent on surfaces and interfaces.

In situ growth of DyBa2Cu3O7−x thin films by molecular beam epitaxy

Films of DyBa2Cu3O7−x with transition temperatures as high as 89 K and with nominal thicknesses down to 35 A have been grown in situ using molecular beam epitaxy employing ozone as a source of reactive oxygen. The process has been successful with a variety of substrates including SrTiO3(100), SrTiO3(110), LaAlO3(100), MgO(100), and yttria‐stabilized zirconia. The films could be imaged with a scanning tunneling microscope at 4.2 K, indicating a conducting surface even at low temperatures.

Magnetic Tunnel Junction-Based Spintronic Logic Units Operated by Spin Transfer Torque

Magnetic tunneling junction (MTJ)-based programmable logic devices have been proposed and studied for future reconfigurable and nonvolatile computation devices and systems. Spin transfer torque (STT)-based switching has advantages in device scaling compared to the field-switching mechanism. However, the previously proposed MTJ logic devices have operated independently and, therefore, are limited to only basic logic operations. Consequently, the MTJ device has only been used as an ancillary device, rather than the main computation device. As a result, the full benefits of MTJ-based computation have not been explored. New designs are needed to accelerate the development of the MTJ-based logic devices. Specifically the realization of direct communication between the MTJ devices is crucial to fully utilize the MTJ devices in the circuits to implement more advanced logic functions. In this paper, new MTJ-based spintronic logic units (building blocks) for spintronic circuits using the STT switching mechanism have been proposed and investigated, which includes the designs of a basic STT-MTJ logic cell, a direct communication between the MTJ logic cells, a three-MTJ logic unit and a spintronic logic circuit acting as an arithmetic logic unit.

In situ growth of DyBa sub 2 Cu sub 3 O sub 7 minus x thin films by molecular beam epitaxy

Films of DyBa2Cu3O7−x with transition temperatures as high as 89 K and with nominal thicknesses down to 35 A have been grown in situ using molecular beam epitaxy employing ozone as a source of reactive oxygen. The process has been successful with a variety of substrates including SrTiO3(100), SrTiO3(110), LaAlO3(100), MgO(100), and yttria‐stabilized zirconia. The films could be imaged with a scanning tunneling microscope at 4.2 K, indicating a conducting surface even at low temperatures.

Sub-200ps spin transfer torque switching in in-plane magnetic tunnel junctions with interface perpendicular anisotropy

Ultrafast spin transfer torque (STT) switching in an in-plane MgO magnetic tunnel junction with 50 nm×150 nm elliptical shape was demonstrated in this paper. Switching speeds as short as 165 ps and 190 ps at 50% and 98% switching probabilities, respectively, were observed without external field assistance in a thermally stable junction with a 101% tunnelling magnetoresistance ratio. The minimum writing energy of P-AP switching for 50% and 98% switching probability are 0.16 pJ and 0.21 pJ, respectively. The observed ultrafast switching is believed to occur because of partially cancelled out-of-plane demagnetizing field in the free layer from interface perpendicular anisotropy between the MgO layer and the Co20Fe60B20 layer. High J/Jc0 ratio and magnetization nucleation at the edge of free layer, which result from the reduced perpendicular demagnetizing field, are possibly two major factors that contribute to the ultrafast STT switching.

Probing dipole coupled nanomagnets using magnetoresistance read

We experimentally demonstrated magnetoresistance (MR) read of dipole coupled nanomagnets using magnetic tunnel junctions. The MR allowed the magnetic state of individual nanomagnets to be electrically measured. The sensitivity of the read scheme enabled a systematic study regarding the nanomagnet spacing and revealed a transition in behavior. Below a spacing of 15 nm the dipole field overcomes the individual shape anisotropy and redefines the individual element easy axis along the direction transmission line. The demonstration of MR electrical read marks a significant step forward for applications such as magnetic quantum cellular automata logic devices.

Improved current switching symmetry of magnetic tunneling junction and giant magnetoresistance devices with nano-current-channel structure

The asymmetry of the switching current in magnetic tunneling junction (MTJ) and giant magnetoresistance (GMR) spin torque transfer devices was reported in both theory and experiment. This is one of the key challenges for future magnetic random access memory applications. In this work, the switching symmetry was greatly improved by inserting a nano-current-channel (NCC) structure in both MTJ and GMR devices. With the NCC structure, the current induced magnetization switching is nonuniform with initiation cites induced by locally high current density. The critical switching current density in both switching directions was successfully reduced while the degree of switching asymmetry {[(JcP-AP−JcAP-P)∕JcAP-P]×100%} was improved as well.

Spin-Torque Driven Switching Probability Density Function Asymmetry

We studied the spin transfer torque (STT) driven switching voltage distribution systematically by characterizing the switching probability density function (PDF) with large statistics (105 trials) across a wide time scale from 5 ns to 1 μs. The skew normal distribution function is found to be a good one to fit the measured switching PDF down to low values, which would be used as a guideline to extrapolate read disturb rate (RDR) and write error rate (WER) in STT-RAM design. Moreover, the asymmetry of switching probability density function is observed to flip when the pulse width decreases. It is related to the fluctuation mechanism transition from the thermal agitation to the initial magnetization trajectory dispersion.

Spin-Transfer Torque Switching Above Ambient Temperature

We report the temperature dependences of tunneling magnetoresistance ratio, coercivity, thermal stability, and switching current distribution of magnetic tunnel junctions (MTJs) in the temperature range 25-80°C, the most probable working environment for spin-transfer torque random access memory (STT-RAM). Two distinct temperature dependence of the switching current density are apparent due to two switching mechanisms: a switching current density decrease with increasing temperature in the long-pulse ( >; 1 μs) regime, a result of thermally activated switching, but no decrease in the short-pulse (>;10 ns) regime, as a result of precessional switching. In the temperature range studied, the switching current density variation is less sensitive to environmental temperature than it is to switching time. Thus, switching time is the more important factor to consider in STT-RAM design.

High power and low critical current spin torque oscillation from a magnetic tunnel junction with a built-in hard axis polarizer

The microwave power and the critical current of spin torque oscillator (STO) devices with a built-in hard axis polarizer have been studied. This design allows for an external-field-free STO. The time domain oscillation signals give direct evidence of higher output from the hard axis polarizer STO, which agrees with spectroscopic results. The hard axis polarizer STO generates a higher power because of the larger slope of the magnetoresistance-angle curve at the lowest energy potential. Furthermore, the hard axis polarizer STO shows a low critical current indicating the large spin torque effect when two magnetic layers are in the 90° configuration.