High-temperature thermal stability driven by magnetization dilution in CoFeB free layers for spin-transfer-torque magnetic random access memory

Abstract

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.

Ultrathin perpendicular magnetic anisotropy CoFeB free layers for highly efficient, high speed writing in spin-transfer-torque magnetic random access memory

“…Figure 2b shows that the addition of Mg to CFB results in a room temperature saturation magnetization M s of 1350 emu/cm 3 with M s disappearing at 860 K (). The temperature dependence of the free layer M s is fitted by the T 1/3 power law indicated by the solid line, where and is consistent with previous studies of CFB free layers and other ferromagnets 15,17,18 .

Figure 2Magnetic properties of ultrathin 9 Å CFB free layers. ( a ) Magnetic moment as a function of applied field for a 9 Å CFB free layer with and without a 5 Å Mg insertion layer.

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“…Conventional free layers use thicker CoFeB (CFB) layers that consist of refractory metal interfaces or insertions 13–15 . These free layers can exhibit good thermal stability withstanding 400 °C back end of line process temperatures and a wide range of operating temperatures, but require a high switching voltage.…”

Sub-nanosecond spin-torque switching of perpendicular magnetic tunnel junction nanopillars at cryogenic temperatures

“…Eq. 2 shows that V c depends on several material parameters that can vary with temperature, notably, α, M s , H k,eff and P [19][20][21][22]. Specifically, the magnetization and magnetic anisotropy both increase with decreasing temperature.…”

A Recent Progress of Spintronics Devices for Integrated Circuit Applications

“…To obtain high thermal tolerance, one needs to control diffusion of boron from CoFeB to the adjacent layer because boron concentration plays a crucial role in realizing perpendicular easy axis [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. As shown in Figure 2a,c, all the s-MTJs showed an increase of TMR ratio up to a specific temperature, above which the TMR ratio degraded.…”

“…In the double CoFeB-MgO interface structure, a thin metal layer such as Ta, W, or Mo was inserted to absorb boron from the CoFeB layers (MgO/CoFeB/insertion layer/CoFeB/MgO) for high tunnel magnetoresistance (TMR) ratio and perpendicular anisotropy [15,16,29,[33][34][35][36][37][38][39][40][41][42][43][44]. First, Ta was used as insertion material because Ta has a bcc crystal structure and is a good boron absorber, as mentioned above.…”