Janice H. Nickel

Engineering nonlinearity into memristors for passive crossbar applications

Although TaOx memristors have demonstrated encouraging write/erase endurance and nanosecond switching speeds, the linear current-voltage (I-V) characteristic in the low resistance state limits their applications in large passive crossbar arrays. We demonstrate here that a TiO2-x/TaOx oxide heterostructure incorporated into a 50 nm× 50 nm memristor displays a very large nonlinearity such that I(V/2) ≈ I(V)/100 for V ≈ 1 volt, which is caused by current-controlled negative differential resistance in the device.

Magnetic Recording Head Materials

The materials used in magnetic recording heads have recently received a tremendous amount of attention. This has been the result of a fortunate set of circumstances. Ever-increasing demands for information storage, especially for graphics-intensive applications, have necessitated unprecedented increases in disk-drive areal densities. Combined with this are recent discoveries in the area of magnetoresistive materials, enabling the design and fabrication of much more sensitive recording heads. The end result is a flurry of activity that has come to dominate the field of magnetics. This article will explore choices for magnetoresistive read head materials, with an emphasis on the materials challenges. The recording heads that are used in high-performance disk drives typically consist of separate magnetoresistive read and inductive write heads (see Figure 1) where previously a single inductive head performed both functions. Separation of the two heads allows each to be optimized for their individual function, an essential factor in enabling disk drives to contain gigabytes of storage. The write head is the simpler of the two, consisting of a U-shaped ferromagnet surrounding a set of coils. The ends of the ferromagnet are the magnetic poles defining the write gap. When current passes through the coils, a field bridges the gap, setting the orientation of the magnetization in the media. Information is stored by changing the polarity of the current in order to write a pattern of magnetic domains in the media. The materials used in write poles will be reviewed in the section, Write Head Materials.

Integration of 4F2 selector-less crossbar array 2Mb ReRAM based on transition metal oxides for high density memory applications

4F2 selector-less crossbar array 2Mb ReRAM test chip with 54nm technology has been successfully integrated for high cell efficiency and high density memory applications by implementing parts of decoders to row/column lines directly under the cell area. Read/write specifications for memory operation in a chip are presented by minimizing sneak current through unselected cells. The characteristics of memory cell (nonlinearity, Kw >;8, Iop <;10uA, Vop<;60;3V), TiOx/Ta2O5, are modified for its working in a chip by adopting appropriate materials for a resistor stack and spacer. Write condition in a chip makes a critical impact on read margin and read/write operation in a chip has been verified.

Spin-dependent tunneling junctions with AlN and AlON barriers

We report on ferromagnetic spin-dependent tunneling (SDT) junctions with NiFe/AlN/NiFe and NiFe/AlON/NiFe structures. Good barriers were formed by plasma nitridation and oxy-nitridation of Al films. Tunneling magnetoresistance ratios (TMR) up to 18% were observed at room temperature. The devices exhibit lower resistance-area products than those seen in reference junctions with Al2O3 barriers. The degradation in TMR at higher bias voltages is found to be less than that found in standard alumina junctions. AlN and AlON could thus be alternate materials for the tunnel barrier in SDT junctions.

Impact of new magnetoresistive materials on magnetic recording heads (invited)

Advances in magnetoresistive materials have recently enabled magnetic recording heads to achieve higher levels of performance. This article describes why higher signal outputs are necessary for improvements to be made in areal density. The requirements for recording at an areal density of 16 Mb/mm2 (10 Gb/in.2) are discussed with regards to both the channel and the head design. Increased output from new multilayer magnetoresistive materials is required to counteract the decrease in output due to the reduction in the size of the head geometry. An areal density of 16 Mb/mm2 is shown to be feasible with spin valve recording heads using materials with magnetoresistance ratios of 10%. Fabrication issues relating to the manufacturing of these materials are shown to be more stringent than previously required.

A model for predicting heating of magnetoresistive heads

A simplified geometric model is presented for analyzing the thermal conduction problem in magnetoresistive heads. The model leads to an approximate analytical solution for the thermal resistance as a function of the key geometric and material parameters. The model reveals trends that will be helpful in designing the next generation of high resolution recording heads.

Memristor Resistance Modulation for Analog Applications

The resistance modulation (RM) of TaOx-based memristors can be precisely controlled by the SET switching compliance current. After electroforming, switching occurs in a rebuilt oxide between the electroformed conductive filament and the Pt electrode. RM is independent of initial oxide thickness. The switching mechanism is postulated as dielectric breakdown at the sidewall of the conductive channel created within the rebuilt oxide: During a SET operation to a lower resistance state, the conductive channel increases in size, conducting a larger current until limited by the external compliance; during a RESET operation to a higher resistance state, the tip of the oxygen-saturated tantalum conductive channel is oxidized, reforming the rebuilt oxide. The conduction of the rebuilt oxide follows a power law function of voltage, in parallel with the modulated channel conductance. The linear resistance can be randomly programmed with accuracy and reproducibility. Analog circuits of tunable memristive low-pass and high-pass filters demonstrate frequency tuning by RM.

Thermal variations in switching fields for sub-micron MRAM cells

Thermal effects in switching of sub-micron tunnel junctions are investigated. The switching field is shown to he inversely dependent on temperature, and switching field jitter is shown to be strong function of temperature. Micromagnetic modeling is used to understand thermal effects. In some instances a stability factor (defined as KV/kT) of 100 or more may be required for acceptable switching field jitter, while with proper optimization of FM layers, stability factors of 50 or 60 may suffice.

DETERMINATION OF BARRIER OXIDATION STATES IN SPIN DEPENDENT TUNNELING STRUCTURES

X-ray photoelectron spectroscopy (XPS) was used to characterize spin dependent tunneling (SDT) structures using plasma oxidized Ta as the insulating barrier. We are able to determine the relative proportion of the different oxidation states of the insulating barrier material. Information available from this technique includes barrier oxidation states, thickness, and completeness of oxidation. Information on the electrodes is also obtained: specifically, Ta is found to diffuse into NiFe, and oxidation of Ni is observed. XPS is shown to be a powerful tool in characterizing the materials sets that comprise SDT devices.

High-resolution electron microscopy study of tunnelling junctions with AlN and AlON barriers

Spin-dependent tunnel junctions with AlON and AlN barriers have been investigated using high-resolution electron microscopy. Plasma gas composition, nitridation time, and deposited Al thickness were studied independently. Plasma-nitrided Al produces thinner tunnel barriers than those obtained with an oxygen or oxygen+nitrogen plasma, for the same plasma exposure time. Experiments to study an increase in plasma nitridation time for a constant Al deposition thickness showed that overexposure to the plasma resulted in a thinner barrier—possibly due to the competing processes of nitride formation and slight etching of the barrier. The AlN barrier thickness increases monotonically with deposited Al thickness for a constant nitridation time. Electron energy loss spectroscopy compositional mapping has confirmed that nitrogen is present in the barrier layer. The roughness between the barrier layer and the two ferromagnetic layers has also been estimated.