Magnetic random access memories based on the spin transfer torque phenomenon (STT-MRAMs) have become one of the leading candidates for next generation memory applications. Among the many attractive features of this technology are its potential for high speed and endurance, read signal margin, low power consumption, scalability, and non-volatility. In this paper, we discuss our recent results on perpendicular STT-MRAM stack designs that show STT efficiency higher than 5 kBT/μA, energy barriers higher than 100 kBT at room temperature for sub-40 nm diameter devices, and tunnel magnetoresistance higher than 150%. We use both single device data and results from 8 Mb array to demonstrate data retention sufficient for automotive applications. Moreover, we also demonstrate for the first time thermal stability up to 400 °C exceeding the requirement of Si CMOS back-end processing, thus opening the realm of non-volatile embedded memory to STT-MRAM technology.

The authors report spin-echo NMR studies of ' 'Pt in small particles of Pt supported on alumina. For the samples studied, the fraction of Pt atoms on the surface (called the dispersion) ranged from 4%%uo to 58%. The studies were at fields Ho of 80 to 85 kG, frequency vo-74 MHz, and at a temperature of 77 K. The lines are broad (3-5 kG), requiring special methods which permit substantial signal averaging (-50000 echoes). In the low-dispersion (large-particle) samples, there is a strong absorption at the position of the ' Pt NMR in bulk Pt metal (Ho/vp=1. 138 kG/MHz) which becomes progressively weaker as the particle size decreases. A peak which is near the ' Pt resonance in typical diamagnetic compounds (Ho/v0-1. 09. kG/MHz) is found in samples which are coated with adsorbed molecules. It disappears when the surface is cleaned. The authors show that this peak arises from the surface layer of Pt atoms, and that its position (Ho/vo) shows that coating the Pt atoms on the surface largely ties up the electron spins of the surface Pt atoms in bonds. The exact position of this peak depends on the chemical species which is adsorbed. The authors show that when Pt is cleaned, then exposed to air for long times, the surface peak reveals that the surface has reconstructed to form Pt(OH) 6.

The authors have used NMR to determine the structure of C 2 H 2 adsorbed at room temperature on small platinum particles by probing the 13 C-13 C, ^C^H, and ^-^H dipolar interactions among the nuclei in the adsorbed molecules. They find a model of (77 ±7)% CCH 2 and (23 ±7)% HCCH. The C-C bond length of the CCH 2 is 1.44 ±0.02 A, midway between a single and double bond, suggesting that both C atoms bond to the surface.

Spin-transfer-torque magnetic random access memory (STT-MRAM) is the most promising emerging non-volatile embedded memory. For most applications, a wide range of operating temperatures is required, for example −40 °C to +150 °C for automotive applications. This presents a challenge for STT-MRAM, because the magnetic anisotropy responsible for data retention decreases rapidly with temperature. In order to compensate for the loss of thermal stability at high temperature, the anisotropy of the devices must be increased. This in turn leads to larger write currents at lower temperatures, thus reducing the efficiency of the memory. Despite the importance of high-temperature performance of STT-MRAM for energy efficient design, thorough physical understanding of the key parameters driving its behavior is still lacking. Here we report on CoFeB free layers diluted with state-of-the-art non-magnetic metallic impurities. By varying the impurity material and concentration to modulate the magnetization, we demonstrate that the magnetization is the primary factor driving the temperature dependence of the anisotropy and thermal stability. We use this understanding to develop a simple model allowing for the prediction of thermal stability of STT-MRAM devices from blanket film properties, and find good agreement with direct measurements of patterned devices.

We present the results of a perpendicular magnetic tunnel junction (MTJ) that displays simultaneously low critical switching current and voltage, as well as high thermal stability factor. These results were achieved using a free layer of the MgO/CoFeB/MgO structure by increasing the spin torque efficiency to an average of 3.0 kBT/µA for 37-nm-diameter junctions, about three times that of a MgO/CoFeB/Ta free layer, which makes it the highest value reported to date. By comparing two films with different RA, hence different switching voltage and power, we explore the contributions of heating and voltage-modulated anisotropy change to the switching properties.