Jimmy J. Kan

Dynamic switching of the spin circulation in tapered magnetic nanodisks

Magnetic vortices are characterized by the sense of in-plane magnetization circulation and by the polarity of the vortex core. With each having two possible states, there are four possible stable magnetization configurations that can be utilized for a multibit memory cell. Dynamic control of vortex core polarity has been demonstrated using both alternating and pulsed magnetic fields and currents. Here, we show controlled dynamic switching of spin circulation in vortices using nanosecond field pulses by imaging the process with full-field soft X-ray transmission microscopy. The dynamic reversal process is controlled by far-from-equilibrium gyrotropic precession of the vortex core, and the reversal is achieved at significantly reduced field amplitudes when compared with static switching. We further show that both the field pulse amplitude and duration required for efficient circulation reversal can be controlled by appropriate selection of the disk geometry.

Ultralow Thermal Conductivity of Multilayers with Highly Dissimilar Debye Temperatures

Thermal transport in multilayers (MLs) has attracted significant interest and shows promising applications. Unlike their single-component counterparts, MLs exhibit a thermal conductivity that can be effectively engineered by both the number density of the layers and the interfacial thermal resistance between layers, with the latter being highly tunable via the contrast of acoustic properties of each layer. In this work, we experimentally demonstrated an ultralow thermal conductivity of 0.33 ± 0.04 W m(-1) K(-1) at room temperature in MLs made of Au and Si with a high interfacial density of ∼0.2 interface nm(-1). The measured thermal conductivity is significantly lower than the amorphous limit of either Si or Au and is also much lower than previously measured MLs with a similar interfacial density. With a Debye temperature ratio of ∼3.9 for Au and Si, the Au/Si MLs represent the highest mismatched system in inorganic MLs measured to date. In addition, we explore the prior theoretical prediction that full phonon dispersion could better model the interfacial thermal resistance involving materials with low Debye temperatures. Our results demonstrate that MLs with highly dissimilar Debye temperatures represent a rational approach to achieve ultralow thermal conductivity in inorganic materials and can also serve as a platform for investigating interfacial thermal transport.

Scalable and thermally robust perpendicular magnetic tunnel junctions for STT-MRAM

Thermal budget, stack thickness, and dipolar offset field control are crucial for seamless integration of perpendicular magnetic junctions (pMTJ) into semiconductor integrated circuits to build scalable spin-transfer-torque magnetoresistive random access memory. This paper is concerned with materials and process tuning to deliver thermally robust (400 °C, 30 min) and thin (i.e., fewer layers and integration-friendly) pMTJ utilizing Co/Pt-based bottom pinned layers. Interlayer roughness control is identified as a key enabler to achieve high thermal budgets. The dipolar offset fields of the developed film stacks at scaled dimensions are evaluated by micromagnetic simulations. This paper shows a path towards achieving sub-15 nm-thick pMTJ with tunneling magnetoresistance ratio higher than 150% after 30 min of thermal excursion at 400 °C.

Oriented Growth of Single-Crystal Ni Nanowires onto Amorphous SiO2

Highly oriented, single-crystal Ni nanowire arrays have been synthesized atop amorphous SiO2∥Si substrates using a single-step chemical vapor deposition method in the absence of any foreign catalyst. Electron and X-ray diffraction confirm the crystalline quality of the Ni nanowires while magnetoresistance measurements probe the magnetic response and the behavior is explained using simulation results for nanoscale, single-crystal Ni. A growth mechanism involving competing chemical, energetic, and kinetic influences is presented.

Ultra-thin Co/Pd multilayers with enhanced high-temperature annealing stability

Understanding the thermal budget of perpendicular materials is crucial for the potential application perpendicular magnetic tunnel junctions. In this paper, we study the effects of high-temperature rapid thermal annealing on the structural and magnetic properties of ultra-thin Co/Pd multilayers deposited at room temperature. It is shown that perpendicular magnetic anisotropy of ultra-thin Co/Pd multilayers improves with increasing annealing temperature up to 425 °C. This property of ultra-thin Co/Pd multilayers provides increased thermal budgets for CMOS-integrated magnetic devices.

Tunable surface plasmon polaritons in Ag composite films by adding dielectrics or semiconductors

We demonstrate that the surface plasmon polariton (SPP) properties of the silver composite films can be tuned by modest additions of silicon oxide or silicon. The dispersion relations deviate from that of pure silver films, and exhibit the capability to shift the surface plasmon frequency and provide larger SPP wave vectors at longer wavelengths. The effective permittivities are modeled phenomenologically by taking into account both filling ratios and size effects. These types of tunable composite films have various useful applications in areas, such as superlens imaging, SPP based sensing, enhanced photoluminescence, and SPP based photovoltatics.

Unified embedded non-volatile memory for emerging mobile markets

Emerging mobile markets such as wearable electronics and Internet of Things necessitate innovations in embedded non-volatile memory (eNVM) for energy-efficient mobile computing and connectivity. In this paper, we briefly review how conventional eNVM has served current markets and investigate emerging eNVM in light of new mobile applications on the horizon. We propose spin-transfer-torque MRAM as a unified eNVM solution that can realize an eNVM-only memory configuration and enable emerging mobile products with high energy efficiency at low cost.

Systematic optimization of 1 Gbit perpendicular magnetic tunnel junction arrays for 28 nm embedded STT-MRAM and beyond

This paper demonstrates the co-optimization of all critical device parameters of perpendicular magnetic tunnel junctions (pMTJ) in 1 Gbit arrays with an equivalent bitcell size of 22 F2 at the 28 nm logic node for embedded STT-MRAM. Through thin-film tuning and advanced etching of sub-50 nm (diameter) pMTJ, high device performance and reliability were achieved simultaneously, including TMR = 150 %, Hc > 1350 Oe, Heff <; 100 Oe, Δ = 85, Ic (35 ns) = 94 μA, Vbreakdown = 1.5 V, and high endurance (> 1012 write cycles). Reliable switching with small temporal variations (<; 5 %) was obtained down to 10 ns. In addition, tunnel barrier integrity and high temperature device characteristics were investigated in order to ensure reliable STT-MRAM operation.

Paramagnetic FexTa1-x alloys for engineering of perpendicularly magnetized tunnel junctions

Exchange coupling between two magnetic layers through an interlayer is of broad interest for numerous recent applications of nano-magnetic systems. In this letter, we study ferromagnetic exchange coupling through amorphous paramagnetic Fe-Ta alloys. We show that the exchange coupling depends exponentially on spacer thickness and scales with the Fe-Ta susceptibility, which can be tuned via the alloy composition and/or temperature. Such materials are of high interest for the engineering of perpendicularly magnetized CoFeB-MgO based tunnel junctions as it enables ferromagnetic coupling of magnetic layers with differing crystalline lattices, suppresses dead layers, and can act as an inter-diffusion barrier during annealing.

Write error rate slopes of in-plane magnetic tunnel junctions

Understanding bit error rates of magnetic tunnel junctions (MTJs) is critical for designing reliable spin-transfer-torque magnetoresistive random access memory. In this letter, we study the write error rate (WER) of two types of in-plane MTJs with the same film stacks except for the free layer and the capping layer. By comparative analysis of these two MTJ splits that show significantly different WER characteristics, we find that reducing average switching voltages does not necessarily improve WER slopes, resulting in decreased write margins for a sufficiently low WER requirement. Various magnetic measurements suggest that WER slopes and slope asymmetries are more strongly correlated to spin torque efficiencies rather than thermal stability factors.